BAINITE STEEL AND PREPARATION METHOD THEREFOR

Abstract

 Disclosed in the present invention is a bainite steel, which comprises the following chemical components in percentages by mass: 0.10-0.19% of C, 0.05-0.45% of Si, 1.5-2.2% of Mn, 0.001-0.0035% of B, 0.01-0.05% of Al, 0.05-0.40% of Cr, 0.05-0.40% of Mo, and more than or equal to 90% of Fe. By rationally controlling the contents of C, Si, Mn, B, Al, Cr, Mo, and the other elements in the steel, the steel can spontaneously form a phase having a structural gradient during the preparation process. In addition, the hardenability of the steel is also improved, such that the strength and forming performance of the bainite steel can be improved. Further disclosed in the present invention is a method for preparing the bainite steel, which method comprises the steps of: smelting and casting; hot rolling; cooling after rolling and coiling; pickling and cold rolling; and annealing. By using the preparation method of the present invention, the bainite steel having a structural gradient in the thickness direction can be prepared, and the bainite steel has good formability.

Description

Technical Field

[0001] The present disclosure relates to the field of metallurgical technology, in particular to a bainite steel and a method for manufacturing the same.

Background Art

[0002] With the development concept of "green-safety" for the new generation of automobiles, the strength of steel used in automotive structural parts is getting higher and higher, and the requirements for materials with different properties in the thickness direction are gradually put forward. For example, the surface layer of the material is required to be hard, wear-resistant, or the surface layer has a high structural uniformity to meet the needs of flanging forming, but at the same time, the core must be made to have high plasticity, so that the steel as a whole does not shrink and fracture during draw forming; or the surface layer is required to have a layered structure with low hardness to ensure that the material has a certain bending performance, but the subsurface layer still needs to have a uniform hard phase structure to ensure flanging and strength, and the core has a soft structure to ensure plasticity, toughness, etc., so as to ensure that the material not only has high strength, but also has good comprehensive forming ability such as bending, flanging and drawability.

[0003] Facing the increasing requirements of the automobile industry for steel with different structures or properties in the thickness direction, the traditional method is to provide steel materials with gradient structure in the thickness direction by subjecting slabs with different compositions or structures to welding, combined rolling and other methods. For example, in CN201210368300.6 and CN201310724615.4, layered composite materials in the thickness direction are obtained by combined rolling of metals. However, this method is a complex process with slow production rate and extremely high cost.

[0004] There are also patents that try to obtain steel plates or steel strips with different microstructures between the surface layer and core by using surface decarburization. For example, the surface decarburization of strip steel is performed to form a decarburized layer of a few microns to tens of microns, so that the upper and lower surface structures are pure ferrite or ferrite accounting for >50%, and the core is other single-phase or multi-phase structures, such as martensite, tempered martensite or bainite structure. A gradient in the thickness direction is spontaneously formed through this method and a high-strength steel plate with three layers of composite structure can be obtained. However, on the one hand, the difference between the strength or hardness of the surface layer and the core is too large, and the strength or hardness of the surface layer is too low, which not only greatly limits the application range of this type of product (such as the application field with high hardness requirements for the surface layer or fatigue resistance requirements, such as car seat slide rails, chassis torsion beams, etc.), but the elongation and the hole-expansion ratio are not high although the material has good bending performance, that is, the plasticity and flanging performance are poor; on the other hand, only a 3-layer composite structure can be formed through this method, and a structure of more layers cannot be provided.

Summary

[0005] In view of the high cost of obtaining a steel plate with a gradient structure in the prior art and the problem that a composite structure of more than 3 layers cannot be obtained, the present disclosure provides a bainite steel having mechanical properties of a yield strength ≥ 800MPa, a tensile strength ≥ 1000MPa, an elongation at break ≥12% and a hole expansion ratio ≥ 40%. In addition, because the steel plate or steel strip forms a gradient structure in the thickness direction, the material has good comprehensive forming properties, i.e., good drawability and hole expansion flanging, which is reflected in the fact that the elongation at break and the hole expansion ratio are relatively high. In all examples, (elongation at break * 10 + hole expansion ratio) is ≥ 170%.

[0006] The bainite steel according to the present disclosure comprises the following chemical elements in mass percentages: C: 0.10∼0.19%, Si: 0.05~0.45%, Mn: 1.5∼2.2%, B: 0.001∼0.0035%, Al: 0.01∼0.05%, Cr: 0.05∼0.40%, Mo: 0.05∼0.40%, Fe≥90%.

[0007] The principles for designing the various chemical elements will be described in detail as follows:

C: In the bainite steel according to the present disclosure, element C mainly controls the microstructure phase transition, carbide size and bainite substructure morphology in carbon steel, thereby affecting the mechanical properties of the material. If the content of element C in the steel is lower than 0.10%, the strength of the steel will not meet the target requirement; and if the content of element C in the steel is higher than 0.19%, it is easy to form martensite structure and coarse cementite which will deteriorate the performances of the steel plate. In addition, in the present disclosure, the C element also affects the sub-morphological structure of bainite, and the higher the C content, the easier it is to form needle-like bainite. As such, in the present disclosure, the mass percentage of C is controlled at 0.10-0.19%. Preferably, the mass percentage of C is controlled at 0.13-0.17%.

Si: In the bainite steel according to the present disclosure, Si has a certain solid solution strengthening effect on the one hand, and it also influences the surface quality of the steel plate on the other hand. When the content of element Si in the steel is lower than 0.05%, it is difficult to achieve sufficient strengthening effect; and when the content of element Si in the steel is higher than 0.45%, it is easy to form iron oxide scale or tiger stripe color difference, which is not conducive to the surface quality of steel plates for automobiles. In addition, in the present disclosure, the Si element also affects the bainite sub-morphological structure, and the higher the Si content, the easier it is to form polygonal bainite. As such, in the present disclosure, the mass percentage of Si is controlled at 0.05-0.45%. Preferably, the mass percentage of Si is 0.05-0.35%. More preferably, the mass percentage of Si is 0.15-0.3%.

Mn: In the bainite steel according to the present disclosure, element Mn is one of the elements that control the phase transformation of the structure in the steel. It also affects the bainite sub-morphological structure, and the higher the Mn content, the easier it is to form polygonal bainite. It should be noticed that the content of element Mn in the steel should not be too high. When the content of element Mn in the steel is too high, it will deteriorate the corrosion resistance and welding performance. As such, in the present disclosure, the mass percentage of Mn is controlled at 1.5-2.2%. Preferably, the mass percentage of Mn is 1.7-2.1%.

B: In the bainite steel according to the present disclosure, element B is not only beneficial to the formation of bainite in the steel, but also influences the strength and forming performance of the steel plate. It also affects the bainite sub-morphological structure, and the higher the B content, the easier it is to form needle-like bainite. The higher the strength of the steel plate, the easier it is to form brittle borides, which affects the hole expansion ratio of the steel plate. As such, in the present disclosure, the mass percentage of B is controlled at 0.001-0.0035%.

Al: In the bainite steel according to the present disclosure, element Al is only added to the steel as a deoxygenating element. It can remove element O from the steel to ensure the performances and quality of the steel. As such, in the bainite steel according to the present disclosure, the mass percentage of Al is controlled at 0.01-0.05%. In some existing technologies, element Al is added to steel in a large amount (≥0.1%) as an element for forming ferrite and inhibiting carbide precipitation in an attempt to effectuate solid solution strengthening, or to change the phase transformation temperature (such as A1, A3), bainite formation kinetics and carbide precipitation kinetics by adding Al, so as to change the phase transformation of the steel to form retained austenite or carbon-free bainite, thereby improving the strength of the steel ultimately. The composition control and process adjustment proposed according to the present disclosure can already provide a bainite steel having good comprehensive formability. However, the carbon-free bainite induced by the addition of a large amount of Al element will destroy the bainite gradient structure formed in the thickness direction and also lead to an increase in cost and a significant increase in the difficulty of continuous casting production. As such, in the present disclosure, the mass percentage of Al is controlled at 0.01-0.05%, so as to avoid cost increase or greatly increased difficulty in continuous casting manufacturing and ensure the formation of bainite gradient structure in the thickness direction.

Cr and Mo: In the bainite steel of the present disclosure, Cr and Mo can not only form a dispersed fine carbide precipitate phase with C, but also further improve the strength of the steel plate, and affect the incubation period of pearlite and ferrite in the CCT curve, and improve the hardenability of the steel plate, so that it can be designed in conjunction with the cooling rate of the steel plate in the annealing process to control the formation of the gradient structure in the thickness direction and the proportion of different thickness. As such, in the present disclosure, the mass percentage of Cr and Mo are respectively controlled as follows: 0.05%≤Cr≤0.40%, 0.05%≤Mo≤0.40%.

[0008] In the present disclosure, the content of elements such as C, Si, Mn, B, Al, Cr, Mo in the steel are reasonably controlled to make the steel spontaneously form a phase with a gradient structure in the preparation process and also improve the hardenability of the steel, so that the strength and forming properties of bainite steel can be improved.

[0009] Further, the bainite steel also comprises at least one of Ti and Nb, wherein the mass percentages of Ti and Nb satisfy: Nb≤0.1%, Ti≤0.15%.

[0010] Ti and Nb: In the bainite steel according to the present disclosure, Ti and Nb are optional alloy elements that can be added to the steel to form a dispersed fine carbide precipitate phase and refine the grain in the structure, thereby further improving the strength and forming properties of the steel plate. As such, in the bainite steel according to the present disclosure, the mass percentage of Nb and Ti are respectively controlled as follows: Nb≤0.1%, Ti≤0.15%. The addition of the above alloying elements will increase the material cost. Considering performance and cost control comprehensively, in the technical solution of the present disclosure, it is preferable to add at least one of Nb and Ti. In some embodiments, the bainite steel of the present disclosure comprises Nb and Ti. The mass percentage of Nb is 0.001-0.1% and the mass percentage of Ti is 0.001-0.15%.

[0011] In some embodiments, the bainite steel of the present disclosure comprises the following chemical elements in mass percentages: C: 0.10∼0.19%, Si: 0.05∼0.45%, Mn: 1.5∼2.2%, B: 0.001∼0.0035%, Al: 0.01∼0.05%, Cr: 0.05∼0.40%, Mo: 0.05∼0.40%, with a balance of Fe and unavoidable impurities.

[0012] Further, in the above unavoidable impurities, P≤0.015%, S≤0.004%.

[0013] Both P and S are impurity elements in the steel. If the technical conditions permit, in order to obtain a quenched and tempered steel having better performances and better quality, the amount of impurity elements in the steel should be minimized.

[0014] Further, in the bainite steel of the present disclosure, the mass percentage of the chemical element should meet the following relationships: R=(Mn+Si)/(12*C+160*B), wherein 0.9≤R≤1.2, where each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element.

[0015] In the present disclosure, R=(Mn+Si)/(12*C+160*B) is defined. Experiments show that if this formula is used for calculation, the R value needs to be limited to a certain range, i.e., 0.9≤R≤1.2, in order to obtain the desired bainite steel plate/strip structure with a gradient structure. Among these elements, the higher the C and B elements, it is the more conducive to the formation of needle-like bainite. The higher the Mn and Si elements, it is the more conducive to the formation of massive bainite. Therefore, by rationally designing the contents of C and B, Mn and Si, the composition design of steel plate and steel strip can be in a critical state that is conducive to the formation of both needle-like bainite and massive bainite, that is, the state of 0.9≤R≤1.2 in this formula. Then, by matching with an optimized annealing process, a gradient structure in the thickness direction of the steel plate can be finally formed. In addition, although the content of C and B elements are low, the influence on bainite formation and morphology is stronger, so a large coefficient is needed in the formula to balance the high content of Mn and Si. After all, the influence of Mn and Si on bainite formation and morphology is significantly weaker than that of C and B. In the design of the present disclosure, the level of 0.9≤R≤1.2 is the critical level that is most suitable for the formation of the gradient structure. If R is too high, the thickness of the massive layer in the gradient structure is too large and the thickness of the needle-like layer is too small, or even there is no needle-like layer, resulting in no gradient structure in the thickness direction. If R is too low, the thickness of the needle-like layer in the gradient structure is too large and the thickness of the massive layer is too small, or even there is no massive layer, resulting in no gradient structure in the thickness direction. Therefore, in the present disclosure, R can be controlled at 0.9≤R≤1.2, so as to ensure the existence of a gradient structure in the thickness direction and the mechanical properties of the steel.

[0016] Further, in the bainite steel of the present disclosure, the mass percentage of chemical elements should meet the following relationship: Q=(C+Cr+Mo+Mn/2)/R, wherein 1.15≤Q≤1.5, where the value in front of the percentage sign in the mass percentage of each element is used for calculation.

[0017] In the present disclosure, Q=(C+Cr+Mo+Mn/2)/R is defined, which can further guide the composition design of the steel. Experiments show that if 1. 15≤Q≤1.5, the steel has suitable hardenability and the formability of the gradient structure. Because the gradient structure or its layered structure is distributed in the thickness direction of the steel plate and the steel strip, the hardenability of the steel plate and the steel strip is also the most important influencing factor for the formation of the gradient structure in the thickness direction. In the present disclosure, C, Cr, Mo and Mn all affect the hardenability of the steel plate and the steel strip. The higher the content of these elements, the stronger the hardenability. However, since the Mn content is an order of magnitude higher than that of other elements, and its effect on hardenability is relatively weak, a coefficient of 1/2 of Mn is designed for this formula. Due to the slight difference in the formation temperature of needle-like bainite and massive bainite in the annealing process, the needle-like bainite formation temperature is lower and the massive bainite formation temperature is higher. Thus, the higher the hardenability of the steel plate, the more conducive to the formation of needle-like bainite and less conducive to the formation of massive bainite, and vice versa. Therefore, in order to ensure that the ratio of needle-like bainite and massive bainite in the thickness direction of the steel plate and the steel strip can form a suitable "sandwich" ratio, when the composition design of the steel plate is more conducive to the formation of massive bainite, that is, when the R value is high, it is necessary to have higher hardenability to promote the formation of needle-like bainite; however, when the composition design of the steel plate is more conducive to the formation of needle-like bainite, that is, when the R value is low, it is necessary to have lower hardenability to promote the formation of massive bainite. Therefore, the numerator of Q value is the alloy content representing the hardenability of the steel strip. The higher the alloy content, the stronger the hardenability. The denominator is R value that represents the formability of massive bainite and needle-like bainite in the structure. The ratio of numerator to denominator, i.e., Q value, directly affects the formability of the massive layer and the needle-like layer during the annealing process and the final ratio thereof. If the Q value is too small, it means that the formability of massive bainite is too strong, and it is difficult to form needle-like bainite in the final structure and also difficult to form the gradient structure. If the Q value is too large, it means that the formability of needle-like bainite is too strong, and it is difficult to form massive bainite in the final structure, and also difficult to form the gradient structure.

[0018] Further, the bainite steel comprises two layers of surface layer structure and one layer of core structure, wherein the core structure is between the two layers of surface layer structure.

[0019] Further, in the bainite steel, the volume of the core structure accounts for 20%∼50% by volume of the bainite steel, and the rest is the surface layer structure.

[0020] Further, the surface structure comprises needle-like bainite and granular carbide precipitate phase. The core structure comprises massive bainite and granular carbide precipitate phase.

[0021] Further, the needle-like bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the surface structure, and the massive bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the core structure.

[0022] In particular, in the bainite steel of some embodiments of the present disclosure, referring to Fig. 1, there are three layers of structures in the thickness direction of the steel plate or steel strip, and the structures from one side of the surface to the other side of the surface are respectively:

Surface layer structure 2: a needle-like layer, that is, a structure dominated by needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 25%∼40%.

Core structure 1: a massive layer, that is, a structure dominated by massive bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 20%∼50%.

Surface layer structure 2: a needle-like layer, that is, a structure dominated by needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 25%∼40%.

[0023] The sum of the proportions of the 3 layers in the thickness direction of the bainite steel is 100%.

[0024] Further, the bainite steel further comprises two multi-phase layers, wherein the two layers of surface structure and the one layer of core structure form an intermediate layer that is between the two multi-phase layers.

[0025] Further, in the bainite steel, the volume of the multi-phase layer accounts for 2%~10% of the volume of the bainite steel, and the rest is the intermediate layer.

[0026] Further, the multi-phase layer comprises polygonal ferrite, needle-like bainite and granular carbide precipitate phase, wherein the polygonal ferrite accounts for no more than 50% by volume of the multi-phase layer, and the polygonal ferrite, the needle-like bainite and the granular carbide precipitate phase account for no less than 99% by volume of the multi-phase layer.

[0027] In particular, in the bainite steel of some embodiments of the present disclosure, referring to Fig. 2, if there are 5 layers of structures in the thickness direction of the steel plate or steel strip, the structures from one side of the surface to the other side of the surface are respectively:

Multi-phase layer 3: a structure dominated by polygonal ferrite, needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase (wherein the polygonal ferrite is <50%), wherein the polygonal ferrite, the needle-like bainite and the dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase account for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 1%~5%.

Surface structure 2: a needle-like layer, that is, a structure dominated by needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 25%∼40%.

Core structure 1: a massive layer, that is, a structure dominated by massive bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 25%∼40%.

Surface structure 2: a needle-like layer, that is, a structure dominated by needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase, accounting for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 25%∼40%.

Multi-phase layer 3: a structure dominated by polygonal ferrite, needle-like bainite and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase (wherein the polygonal ferrite is <50%), wherein the polygonal ferrite, the needle-like bainite and the dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase account for ≥99% of the total phase in this region. The proportion thereof in the thickness direction is 1%~5%.

[0028] The sum of the proportions of the 5 layers in the region is 100%. Among these layers, the hardness of the needle-like layer is the largest, and the hardness of the multi-phase layer is the smallest.

[0029] In bainite steel of the present disclosure, the diameter of the granular carbide precipitate phase is ≤ 5µm. The reason for limiting the carbide precipitate phase is to avoid deterioration of the hole expansion ratio. When the size of the carbide precipitate phase is >5µm, it is easy to crack at the junction between the carbide and the matrix when the steel plate is subjected to the hole-expanding and flanging deformation or when the hole expansion ratio is detected, resulting in the reduction of the hole expansion ratio of the steel plate and the deterioration of the hole-expanding and flanging performance.

[0030] Further, in the bainite steel according to the present disclosure, the bainite steel has a tensile strength of≥1000MPa, a yield strength of ≥800MPa, a hole expansion ratio of ≥40%, and an elongation at break of ≥12%.

[0031] Further, the bainite steel according to the present disclosure has superior drawability and hole expansion flanging performance, i.e., the elongation at break*10+ the hole expansion ratio ≥170%.

[0032] The present disclosure provides a manufacturing method for the above bainite steel, comprising steps of:

smelting and casting;

hot rolling;

post-rolling cooling and coiling;

pickling and cold rolling;

annealing.

[0033] Because the common surface decarburization method is not used to prepare the bainite steel with a gradient structure, the bainite steel in the present disclosure does not have the problem that the surface strength and hardness are substantially lower than those of the core.

[0034] Further, the process parameters of the above manufacturing method are controlled to meet at least one of the following:

in the step of hot rolling, a heating temperature is controlled at 1100-1230 °C; an initial rolling temperature of finishing rolling is controlled at 1050-1180 °C; and a final rolling temperature of finishing rolling is controlled at 870-930 °C.

in the step of post-rolling cooling and coiling, a cooling rate is controlled at 30-150 °C/s, and a coiling temperature is controlled at 540-620 °C.

in the step of cold rolling, a cold rolling reduction rate is controlled at ≥ 30%.

[0035] In the above manufacturing method, the process steps before annealing are mainly to obtain a steel plate or steel strip with uniform composition and original structure, so as to ensure that the subsequent annealing process can meet the requirement of uniform and stable structure and properties when it is implemented. The annealing process plays a key role in the performance of the steel plate.

[0036] Before introducing the annealing process, the following concepts need to be introduced:

[0037] Because the present disclosure intends to design the gradient structure in the thickness direction of the steel plate/steel strip, the steel plate or steel strip will inevitably or deliberately have different temperature ranges in the thickness direction. But due to the limitation of the continuous production mode of the steel plate or steel strip, the temperature detection and control can only be aimed at the upper and lower surface temperatures, and the temperatures of other positions in the thickness direction cannot be detected. For the upper and lower surface temperatures, they are treated according to the same process without additional distinction and are referred to as surface temperature. The temperature and cooling rate mentioned below refer to the surface temperature and the cooling rate calculated from the surface temperature. It should be noted that during cooling, the temperature profile in the thickness direction of the steel plate or steel strip is controlled according to the surface temperature, cooling rate, the pressure of the jet gas during cooling (which represents the cooling capacity), and the hardenability of the steel plate.

[0038] Further, the annealing process comprises a heating stage, a slow cooling stage, a fast cooling stage, a controlled cooling stage and an air cooling stage. The cooling rates at the slow cooling stage, the fast cooling stage, and the controlled cooling stage are controlled to satisfy: the controlled cooling stage < the slow cooling stage < the fast cooling stage.

[0039] Further, the steel plate is heated to a soaking temperature of 840~950°C at a heating rate of ≤50°C/s at a heating stage, then held for a holding time of 60-180 s.

[0040] At the heating stage, it is required that the bainite steel is controlled to be heated at a heating rate of ≤50°C/s to a soaking temperature of 840~950°C and held for 60∼180s. If the heating rate at the heating stage is > 50°C/s, or the holding time is <60s, the uniformity of the strip structure will be poor, which will affect the subsequent formation of the gradient structure in the thickness direction. In addition, if the temperature is below the lower limit of the above soaking temperature, it will not be able to obtain enough bainite structure (no matter needle-like bainite or massive bainite) in the strip. Further, the heating rate is preferably 5~50°C/s. If the holding time is > 180s, or further, if the soaking temperature is higher than 950°C, the grain of the steel strip will be coarse, resulting in the deterioration of the formability of the steel.

[0041] In the present disclosure, in order to form a bainite steel with a three-layer microstructure gradient in the thickness direction, the steel plate is cooled to a slow cooling temperature of 720~800 °C at a slow cooling rate of Q~10*Q °C/s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤Q≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element. In some embodiments, the slow cooling rate is controlled at 5Q∼10Q °C/s. In some embodiments, the slow cooling rate is controlled at 7Q~10Q °C/s.

[0042] In particular, in Examples of the present disclosure, a method of injecting a cooling gas to the surface of the bainite steel is adopted to achieve slow cooling. For example, during cooling, it is performed by injecting a cooling gas to the surface of the bainite steel, and the cooling gas injection pressure is controlled at 0.2*Q~Q kPa, and the holding time of the cooling gas injection is controlled at 5~20 seconds. Of course, in other possible embodiments, other methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as the bainite steel can be cooled to a slow cooling temperature of 720~800 °C at a slow cooling rate of Q~10*Q °C/s. The main purpose of this stage is to make the temperature of the steel plate or steel strip uniform in the width direction and less uniform in the thickness direction without structure transformation in each position.

[0043] The purpose of controlling the slow cooling rate in this step is to make the steel plate or steel strip have a uniform temperature in the width direction, and the purpose of controlling the temperature is to make sure that there is no phase transformation in each position of the steel strip. If the temperature is too low, the austenite may undergo a phase transformation and decompose to form ferrite or pearlite. If the temperature is too high, it is not conducive to the high-precision control of the next cooling stage, which is in turn not conducive to obtaining the gradient structure in the thickness direction. The pressure of the cooling gas injected to the surface of the steel plate or steel strip and the holding time are controlled to control the uneven cooling in the thickness direction of the steel strip. If the pressure of the cooling gas injected to the surface of the steel plate or steel strip is less than 0.2*Q kPa or the holding time is less than 5 seconds, it means that the cooling capacity is insufficient. Although the surface of the steel strip is cooled to the set temperature, most of the regions below the surface are at a higher temperature, which is not conducive to the formation of the gradient structure in the thickness direction in the next step, or the needle-like bainite region in the gradient structure formed in the next stage is too small. However, if the pressure is higher than Q kPa or the holding time is greater than 20 seconds, the cooling capacity will be too great, so that the core temperature of the steel strip will be close to or even reach the surface temperature, which is not conducive to the formation of the gradient structure in the thickness direction in the next step, or the massive bainite region in the gradient structure formed in the next stage is too small.

[0044] In the present disclosure, in order to form a bainite steel with a 5-layer microstructure gradient in the thickness direction, the steel plate or steel strip is cooled to a slow cooling temperature of 620-700 °C at a slow cooling rate of Q~10*Q °C/s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤Q≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element. In some embodiments, the slow cooling rate is controlled at 5Q∼10Q °C/s. In some embodiments, the slow cooling rate is controlled at 7Q~10Q °C/s.

[0045] In particular, in Examples of the present disclosure, a method of injecting a cooling gas to the surface of the bainite steel is adopted to achieve slow cooling. For example, during cooling, it is performed by injecting the cooling gas to the surface of the bainite steel, and the cooling gas injection pressure is controlled at 0.05*Q~0.15*Q kPa, and the holding time of the cooling gas injection is controlled at 5~15 seconds. Of course, in other possible embodiments, other methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as the bainite steel can be cooled to a slow cooling temperature of 620~700°C at a slow cooling rate of Q~10*Q °C/s.

[0046] In this step, cooling to 620~700°C is to ensure that the surface of the steel plate or steel strip enters the ferrite transition temperature range, and through a certain period of heat preservation, a certain amount of ferrite can be formed in the surface area of the steel plate or steel strip, so as to make a preparation for the final formation of the multi-phase layer in the surface layer. Neither below nor above this temperature guarantees the formation of a certain amount of ferrite on the surface of the strip. Similarly, if the holding time is too short, or the cooling rate is too fast, there is no time to generate ferrite on the surface of the strip, so that the multi-phase layer in the layer cannot be formed at last. In contrast, if the holding time is too long or the cooling rate is too slow, it will lead to too much ferrite content and too thick thickness on the surface of the steel strip, which is not only not conducive to the formation of the multi-phase layer in the surface layer, but also leads to a failure to form sufficient needle-like bainite in superficial layer at the fast cooling stage, i.e., affecting the formation of the subsequent needle-like layer.

[0047] The pressure of the cooling gas injected to the surface of the steel plate or steel strip is 0.05*Q~0.15*Q kPa, which is to control the thickness of polygonal ferrite formed on the surface of the steel strip. When the pressure is within the above range and the holding time is also in line with the predetermined range, only the surface layer area of the steel plate or steel strip is actually cooled to 620~700°C and enters the ferrite phase zone. The temperature of other areas is still higher than 700°C and the ferrite transformation does not occur (because the formation of ferrite will also release the latent heat of phase transformation). However, if the cooling gas injection pressure is too high, the temperature of the superficial layer and even the core of the steel plate or steel strip will also decrease, which is not conducive to the formation of the subsequent needle-like layer and massive layer. However, if the cooling gas injection pressure is too low, it is not conducive to the stable formation of a certain amount of polygonal ferrite on the surface layer, so that a stable multi-phase layer cannot be formed on the surface layer.

[0048] After the slow cooling is completed, at the fast cooling stage, it is necessary to control the steel plate or steel strip to be cooled to a fast cooling temperature of 400-540 °C at a fast cooling rate of 10*Q~20*Q °C/s, no matter for forming a bainite steel with 3 or 5 layers of gradient structure in the thickness direction.

[0049] In particular, in Examples of the present disclosure, a method of injecting a cooling gas to the surface of the bainite steel is adopted to achieve fast cooling. At this stage, during cooling, the cooling gas needs to be injected to the surface of the bainite steel twice. The first injection pressure of the cooling gas is controlled at 0.3*Q~1.5*Q kPa, and the first holding time of the cooling gas is controlled at 1-7 seconds. The second injection pressure of the cooling gas is controlled at 0.08*Q~0.2*Q kPa, and the second holding time of the cooling gas is controlled at 5-10 seconds. Similarly, in other possible embodiments, other methods such as liquid cooling can also be used to achieve the purpose of slow cooling, as long as the bainite steel can be cooled to a fast cooling temperature of 400~540 °C at a fast cooling rate of 10*Q~20*Q °C/s at this stage. All these technical solutions fall within the protection scope of the present disclosure.

[0050] Further, the cooling gas used in the annealing step is a mixture of a reducing gas and an inert gas. Preferably, the volume fraction of the reducing gas in the mixture is 1%~8%. In some embodiments, the reducing gas in the mixture is hydrogen, and its volume fraction is 1%~8%. The temperature of the cooling gas can be controlled at 5~50°C.

[0051] In some embodiments of the present disclosure, the cooling of the steel plate or steel strip is carried out by injecting a cooling gas (i.e., a mixture of a reducing gas and an inert gas) onto its surface, where the reduction can be achieved by hydrogen. In the present disclosure, the inert gas refers to a gas that does not affect the structure of the steel through chemical reaction with the bainite steel under experimental conditions. In particular, for cost saving reasons, all inert gases can be nitrogen. The content and temperature of hydrogen in the cooling gas can be further controlled, as shown in Table 2.

[0052] In the cooling of the bainite steel, the cooling capacity or cooling intensity is controlled by controlling the gas injection pressure, the hydrogen content in the cooling gas and the temperature of the cooling gas, etc., and the specific value needs to be determined according to the hardenability of the steel plate or steel strip. For the same embodiment, under normal circumstances, the hydrogen content in the cooling gas and the temperature of the cooling gas remain unchanged in the annealing process. At this time, the cooling intensity and the cooling rate are positively correlated with the gas injection pressure. For example, in Example 1, at the slow cooling stage, the cooling gas injection pressure is 0.6kPa, and the cooling rate of the slow cooling stage is 12.5°C/s. At the fast cooling stage, the first cooling gas injection pressure is 1kPa, and the corresponding cooling rate is 19.2°C/s. For different examples, the cooling capacity and cooling rate are correlated with the cooling gas injection pressure, the hydrogen content in the cooling gas and the cooling gas temperature. The higher the hydrogen content in the cooling gas, the lower the cooling gas temperature, the greater the cooling gas injection pressure, the stronger the cooling capacity and the faster the cooling rate. For example, in Example 7 and Example 9, the cooling gas temperature is the same. However, in Example 9, the hydrogen content in the cooling gas is higher, the cooling gas injection pressure is greater, and the corresponding cooling capacity and cooling rate are also larger.

[0053] In particular, the fast cooling temperature and fast cooling rate of the reaction at this stage are controlled to make the steel plate and steel strip in the bainite phase region at this stage. Too high or too low temperature does not allow the steel plate or steel strip to form a sufficient amount of bainite. The fast cooling rate is controlled at 10*Q~20*Q °C/s in order to make the fast cooling rate as close to the nasal temperature region of the CCT curve in the bainite phase region as possible, so that the bainite transformation is more complete with faster rate. Because starting from the initial smelting stage, in the long production process, it is inevitable that there will be inhomogeneity of local regional composition and structure in the steel plate or steel strip, so that there will be lower carbon equivalent or smaller austenite supercooling degree in some areas, and higher carbon equivalent or larger austenite supercooling degree in some other areas. If the cooling rate is less than the predetermined range, the areas with lower carbon equivalent or smaller austenite supercooling degree will enter the pearlite transition region due to too slow cooling rate, or the transition will be insufficient due to too slow bainite transition rate. At the same time, if the cooling rate is higher than the predetermined range, the areas with lower carbon equivalent or larger austenite supercooling degree will bypass the bainite phase region and enter the martensite phase region, or the transition will be insufficient due to too slow bainite transition rate. These factors will lead to the fact that a gradient structure in the thickness direction cannot be formed at last.

[0054] Among all the factors that affect the process of the fast cooling stage, the pressure of the cooling gas injected onto the surface of the steel plate or steel strip is more important. First, the pressure is controlled at 0.3*Q~1.5*Q kPa and held for 1~7 seconds, in order to form a needle-like bainite layer outside the core area in the thickness direction of the steel plate or steel strip. With the release of phase transition latent heat in these areas due to the bainite phase transition, the temperature of the core area in the thickness direction of the steel strip will be higher than that of the surface layer and the sub-surface layer, so as to prepare for the formation of massive bainite in the core area. At this time, if the cold gas injection pressure or the holding time is lower than the predetermined range, it is not conducive to the formation of needle-like bainite in the surface layer and the sub-surface layer. If the injection pressure or the holding time is higher than the predetermined range, the cooling capacity will be too strong and the needle-like bainite will be also formed in the core area in the thickness direction of the steel strip. Thus, it is impossible to form a gradient structure in the thickness direction. Then, the injection pressure is further reduced to 0.08*Q~0.2*Q kPa and maintained for 5~10 seconds. On the one hand, the surface layer and the sub-surface layer can still be effectively cooled to continuously form needle-like bainite. On the other hand, through the reduction of the cooling gas pressure and the latent heat released by the phase transition in the surface layer and the sub-surface layer, the temperature in the core area in the thickness direction of the steel strip does not continue to decrease or even increases slightly, so as to ensure the formation of massive bainite in the core of the steel strip. And a steel plate or steel strip with a gradient structure in the thickness direction is finally formed.

[0055] After the end of the fast cooling stage, a controlled cooling step is required to obtain bainite steel with a three- or five-layer gradient structure in the thickness direction. At the controlled cooling stage, the controlled cooling rate is ≤ Q °C/s, the holding time of the controlled cooling is 100~200 seconds, and the controlled cooling temperature of bainite steel is ≥ 350°C at the end of the controlled cooling stage. In some embodiments, the temperature of bainite steel at the end of the controlled cooling stage is 350~410°C.

[0056] Through the long-term controlled cooling of the steel plate or steel strip, the phase transformation of each bainite is fully completed. The microstructure is formed slowly and stably at the predetermined temperature, so as to ensure the formation of the steel plate or steel strip with a gradient structure in the thickness direction. At this stage, if the controlled cooling rate is higher than the set value or the controlled cooling temperature of the final steel plate or steel strip is lower than the set value, it will cause the formation of martensite in the structure and deteriorate the formability of the steel plate or steel strip.

[0057] At the end of the controlled cooling stage, the bainite steel is air-cooled to room temperature. Thus, a steel plate or steel strip with a gradient structure in the thickness direction is obtained. The air-cooled stage has no effect on the microstructure of the bainite steel.

[0058] Based on the above, in some examples of the present disclosure, in order to obtain a bainite steel with a five-layer gradient structure, as long as the cooling parameters of the slow cooling stage are controlled, a steel plate or steel strip with a five-layer gradient structure in the thickness direction can be formed by further forming a multi-phase layer on the surface layer on the basis of the initial three-layer gradient structure. Subsequently, after the fast cooling stage and the controlled cooling stage, needle-like bainite or massive bainite will be also formed in other areas of the bainite steel according to the difference in the position of the thickness direction. Finally, a multi-phase layer containing ferrite in the surface layer, a needle-like layer in the superficial layer and a massive layer in the core can be formed, thereby obtaining a steel plate or steel strip having a 5-layer structure with a gradient.

[0059] The beneficial effects of the present disclosure are:

1. The present disclosure optimizes the hardenability of the steel through the rational element composition design of the bainite steel, especially by reasonably controlling the content of C, Si, Mn and B elements in the steel, and reasonably controlling the content of C, Cr, Mo and Mn elements in the steel, so that the steel can spontaneously form a phase with a gradient structure in the preparation process, and the strength and forming properties of bainite steel are improved.

2. The present disclosure provides a method for manufacturing a bainite steel that enables a steel plate/steel strip with a suitable chemical composition to spontaneously form a three- or five-layer gradient structure under the annealing conditions of the present disclosure through the design of a fine annealing step, in particular the control of the cooling gas pressure and temperature at the cooling stage. The bainite steel obtained by adopting the technical solution of the present disclosure has a tensile strength of ≥ 1000MPa, a yield strength of ≥ 800MPa, a hole expansion ratio of ≥40%, and an elongation at break of ≥12%.

Description of the Drawings

[0060] 

Fig. 1 is a schematic diagram of a steel strip having a three-layer structure in the thickness direction in the Examples of the present disclosure.

Fig. 2 is a schematic diagram of a steel strip having a five-layer structure in the thickness direction in the Examples of the present disclosure.

Fig. 3 is a photograph of the metallographic structure in the transition position between the needle-like layer (upper part) and the multi-phase layer (lower part) in Example 7 of the present disclosure.

Fig. 4 is a photograph of the metallographic structure in the transition position between the needle-like layer (upper part) and the massive layer (lower part) in Example 1 of the present disclosure.

Detailed Description

[0061] The embodiments of the present disclosure are illustrated below by specific examples. Those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents revealed in the present description. Although the description of the present disclosure will be presented together with the preferred embodiment, this does not mean that the features of the disclosure are limited to that embodiment. On the contrary, the purpose of the description of the disclosure in conjunction with the embodiments is to cover other options or modifications that may be extended based on the claims of the present disclosure. In order to provide an in-depth understanding of the disclosure, many specific details will be included in the following description. The present disclosure may also be implemented without these details. In addition, in order to avoid confusion or obscurity of the main points of the present disclosure, some specific details will be omitted from the description. It should be noted that, without conflict, the embodiments in the present disclosure and the features of the embodiments may be combined with each other.

Examples 1-14 and Comparative Examples 1-6

[0062] The bainite steel in each of Examples 1-14 according to the present disclosure was prepared using the following steps:

Step 1 smelting and casting;

Step 2 hot rolling: the heating temperature was controlled at 1100-1230°C; the initial rolling temperature of the finishing rolling was controlled at 1050-1180°C; and the final rolling temperature of the finishing rolling was controlled at 870-930°C;

Step 3 post-rolling cooling and coiling: the cooling rate was controlled at 30-150 °C/s; and the coiling temperature was controlled at 540-620 °C;

Step 4 pickling to remove iron oxide scale;

Step 5 cold rolling: the cold rolling reduction rate was controlled at ≥ 30% to provide the target thickness. In particular, in Examples of the present disclosure, the thickness of the steel plate or steel strip after the cold rolling was ≤ 2.2mm;

Step 6 annealing.

[0063] The bainite steel in each of Comparative Examples 1-6 was also made by the process comprising smelting, continuous casting, hot rolling, post-rolling cooling and coiling, pickling and cold rolling, and annealing. The chemical composition of the steel and the process parameters of the preparation process are detailed in Table 1-2.

[0064] Table 1 shows the mass percentages of each chemical element of the bainite steel in Examples 1-14 and Comparative Examples 1-3.

[0065] Table 2 shows the specific process parameters of the bainite steel in Examples 1-14 and the comparative steel in Comparative Examples 1-6.

Table 1: (%, a balance of Fe and other unavoidable impurities except P and S)

	Steel grade	C	Si	Mn	B	Al	Cr	Mo	Nb	Ti	P	S	R value	Q value
Ex.1	A	0.155	0.22	1.75	0.001	0.01	0.18	0.18	0.002	0.003	0.01	0.001	0.98	1.43
Ex.2	B	0.165	0.35	2.15	0.001	0.03	0.05	0.40	0.004	0.002	0.008	0.001	1.17	1.45
Ex.3	C	0.125	0.05	1.85	0.002	0.02	0.31	0.13	0.003	0.11	0.008	0.001	1.04	1.43
Ex.4	D	0.10	0.1	1.65	0.003	0.02	0.12	0.35	0.08	0.02	0.006	0.001	1.04	1.34
Ex.5	E	0.15	0.29	2.2	0.0025	0.03	0.21	0.11	0.001	0.15	0.009	0.001	1.13	1.39
Ex.6	F	0.13	0.3	1.9	0.0035	0.02	0.27	0.15	0.002	0.004	0.012	0.001	1.04	1.45
Ex.7	G	0.135	0.35	1.6	0.002	0.04	0.25	0.1	0.04	0.002	0.015	0.003	1.01	1.28
Ex.8	H	0.145	0.2	2	0.002	0.01	0.15	0.12	0.003	0.004	0.013	0.002	1.07	1.32
Ex.9	I	0.19	0.42	2.1	0.0015	0.05	0.13	0.08	0.002	0.004	0.011	0.001	1.00	1.45
Ex.10	J	0.18	0.37	2.05	0.001	0.03	0.08	0.19	0.004	0.005	0.007	0.001	1.04	1.41
Ex.11	K	0.14	0.25	1.95	0.003	0.02	0.07	0.23	0.02	0.08	0.009	0.001	1.02	1.39
Ex.12	L	0.145	0.12	1.7	0.001	0.02	0.07	0.22	0.002	0.003	0.008	0.002	0.96	1.34
Ex.13	M	0.12	0.15	1.9	0.0025	0.03	0.40	0.05	0.08	0.04	0.005	0.001	1.11	1.36
Ex.14	N	0.11	0.07	1.75	0.0015	0.01	0.11	0.27	0.1	0.01	0.002	0.001	1.17	1.17
Com. Ex.1	O	0.08	0.3	1.3	0.002	0.02	0.25	0.22	0.003	0.02	0.01	0.002	1.25	0.96
Com. Ex.2	P	0.21	0.22	1.8	0.003	0.03	0.3	0.2	0.004	0.002	0.012	0.003	0.67	2.39
Com. Ex.3	Q	0.17	0.15	1.71	0.002	0.02	0.42	0.05	0.02	0.002	0.008	0.003	0.79	1.90

Table 2: specific process parameters used in the manufacturing method for the bainite steel in Examples 1-14 and Comparative Examples 1-6

No.	Steel grad e	Q valu e	Heating stage			Slow cooling stage						Rapid cooling stage								Controlled cooling stage
			Heatin grate (°C/s)	Soaki ng tempe rature (°C)	Holdi ng time (s)	Slow coolin g rate (°C/s)	Slow coolin g tempe rature (°C)	Holding time (s)	Hydroge n volume fraction (%)	Cooling gas temperat ure (°C)	Cooling gas inj ecting pressure (kPa)	Rapid cooling rate (°C/s)	Rapid coolin g temper ature (°C/s)	First holding time (s)	First injecting pressure of cooling gas (kPa)	Second injecting pressure of cooling gas (kPa)	Second holding time (s)	Hydro gen volum e fractio n (%)	Coolin g gas temper ature (°C)	Contro lied coolin g rate (°C/s)	Contro lied coolin g temper ature (°C)	Contr oiled coolin g time (s)
Ex.1	A	1.43	5	860	120	12.5	760	8	6	25	0.6	19.2	530	5	1	0.13	7	6	25	1.08	400	120
Ex.2	B	1.45	10	840	170	12.0	780	5	6	20	0.5	21.7	520	5	1.2	0.14	7	6	20	0.68	350	200
Ex.3	C	1.43	20	880	100	11.4	720	14	2	10	0.3	15.3	490	7	1	0.12	8	2	10	0.67	370	180
Ex.4	D	1.34	15	900	80	13.3	740	12	8	50	0.4	26.7	420	7	2	0.15	5	8	50	0.50	370	100
Ex.5	E	1.39	30	950	90	10.0	800	15	1	5	1.3	16.3	540	6	2	0.16	10	1	5	1.18	410	110
Ex.6	F	1.45	5	850	140	12.8	620	15	6	35	0.1	16.9	400	4	0.6	0.12	9	6	35	0.50	350	100
Ex.7	G	1.28	25	870	130	11.1	660	14	5	45	0.1	16.4	430	4	0.7	0.17	10	5	45	0.50	360	140
Ex.8	H	1.32	40	930	70	12.5	730	16	3	10	1.3	18.7	450	7	1.9	0.19	8	3	10	0.70	380	100
Ex.9	I	1.45	45	910	125	13.5	680	17	7	45	0.2	23.3	470	1	1.3	0.2	8	7	45	0.67	390	120
Ex.10	J	1.41	50	920	65	13.6	770	11	3	15	1.4	16.3	510	6	1.7	0.14	10	3	15	0.61	400	180
Ex.11	K	1.39	35	850	60	12.0	790	5	1	5	1.3	18.0	520	5	2	0.22	10	1	5	0.58	410	190
Ex.12	L	1.34	15	840	140	10.8	700	13	4	40	0.2	14.3	500	6	0.4	0.15	8	4	40	0.80	380	150
Ex.13	M	1.36	20	890	110	13.0	630	20	6	30	0.2	20.0	410	3	0.8	0.11	8	6	30	0.50	350	120
Ex.14	N	1.17	10	850	150	11.4	640	17	6	40	0.1	22.5	460	2	1.4	0.21	6	6	40	0.53	370	170
Comp. Ex.1	O	0.96	20	870	140	12.7	680	15	4	30	0.2	16.4	500	3	0.4	0.15	8	4	30	0.80	380	150
Comp. Ex.2	P	2.39	20	870	130	11.7	660	18	8	50	0.1	19.2	430	7	0.4	0.17	5	8	50	0.50	380	100
Comp. Ex. 3	Q	1.90	20	930	70	15.0	750	12	5	25	1.5	23.1	450	7	1.8	0.19	6	5	25	0.70	380	100
Comp. Ex.4	A	1.43	10	860	120	10.0	660	20	5	35	1.5	17.3	400	5	3	0.13	10	5	35	0.50	350	100
Comp. Ex. 5	A	1.43	10	880	120	10.7	720	15	6	35	0.3	18.8	400	7	3	0.13	10	6	35	0.50	350	100
Comp. Ex. 6	A	1.43	10	870	120	13.0	740	10	6	20	0.3	11.8	540	7	0.3	0.05	10	6	20	1.90	350	100

[0066] Examples 1-5, 8 and 10-11 all provided a three-layer structure in the thickness direction, wherein the upper and lower surface layers are needle-like layers, and the core is a massive layer. Examples 6-7, 9 and 12-14 all provided a five-layer structure in the thickness direction, wherein the upper and lower surface layers are multi-phase layers, the upper and lower sub-surface layers are needle-like layers, and the core is a massive layer. In the microstructure of the bainite steel in the present disclosure, the hardness of the needle-like layer is the largest, the hardness of the multi-phase layer is the smallest, and the hardness of the massive layer is between the needle-like layer and the multi-phase layer. Therefore, for the three-layer composite material, the needle-like layer of the upper and lower surface layers can ensure that the material has high surface hardness and surface yield strength, while the massive layer in the middle ensures that the material has relatively high toughness and plasticity, so it can be used for auto parts that have high requirements for the surface hardness or fatigue limit of the material, and at the same time have high requirements for the toughness and plasticity of the material as a whole, such as car seat slide rails, chassis torsion beams and other structural parts. For the 5-layer composite material, the relatively soft multi-phase layer in the upper and lower surface layers can make the surface layer have better local formability, and the adjacent harder needle-like layer and the massive layer in the core impart the material higher strength and better toughness, so it can be used to prepare parts that have high requirements for strength and comprehensive formability, such as the control arm and triangle arm of automobile chassis.

[0067] Comparative Examples 1-3 did not obtain a steel plate or steel strip with a gradient structure in the thickness direction since the composition design did not meet the requirements of the present disclosure. In Comparative Example 1, only pure massive layer structure was obtained because the R value was too high. In Comparative Examples 2-3, only pure needle-like layer structure was obtained because the R value was too low. Comparative Examples 4-6 used steel grade A. Although the composition design met the requirements, the annealing process in the manufacturing process did not meet the requirements of the present disclosure. Comparative Examples 4-6 did not obtain a steel plate or steel strip with a gradient structure in the thickness direction. In Comparative Example 4, because the cooling gas pressure in the slow cooling stage was larger than the set value, a large proportion of ferrite was formed in the whole thickness direction of the steel plate or steel strip. At the fast cooling stage, because the cooling gas pressure in the fast cooling stage was larger than the set value, the needle-like bainite was formed in the whole thickness direction of the steel plate or steel strip and the massive bainite could not be formed. And because a certain large proportion of ferrite had been preferentially formed in the steel plate or steel strip, carbon was enriched in partial supercooled austenite and no bainite transformation occurred. On the contrary, it would be transformed into fresh martensite in the final air-cooling stage, so that a gradient structure in the thickness direction cannot be formed in the steel plate or steel strip and the formability was also poor. In Comparative Example 5, because the cooling gas pressure in the fast-cooling stage was stronger than the set value, only pure needle-like bainite structure was obtained. Accordingly, in Comparative Example 6, because the cooling gas pressure in the fast-cooling stage was less than the set value, only pure massive layer structure was obtained.

[0068] Fig. 3 shows the lower surface layer area of Example 7 of the present disclosure. In particular, it is the metallographic structure photograph (taken by scanning electron microscope) of the transition position between the needle-like layer (upper part) and the multi-phase layer (lower part). In the upper part of the figure, i.e., the area closer to the core, the structure is typical needle-like bainite, which represents the region starts to enter the needle-like layer. In the lower part of the figure, i.e., the area closer to the lower surface, polygonal ferrite, needle-like bainite, and dispersively precipitated nano-scale, sub-micron or micron-sized granular carbide precipitate phase are comprised, which represents the region starts to enter the multi-phase of the surface layer.

[0069] Fig. 4 shows the upper surface layer area close to the core of Example 1 of the present disclosure. In particular, it is the metallographic structure photograph (taken by scanning electron microscope) of the transition position between the needle-like layer (upper part) and the massive layer (lower part). In the upper part of the figure, i.e., the area closer to the upper surface, the structure contains a large amount of typical needle-like bainite, which represents the region starts to enter the needle-like layer. In the lower part of the figure, i.e., the area closer to the core, a large amount of bainite is transformed into a morphology of massive polygons, that is, a large amount of massive bainite is formed in this area, which represents the region starts to enter the massive layer.

[0070] Table 3 lists the test results of the mechanical properties of the bainite steel in Examples 1-14 and Comparative Examples 1-6. A transverse JIS 5# tensile sample was taken to determine the yield strength, tensile strength and elongation at break of the steel and the test was carried out by using GB/T 228.1-2010 "Tensile test of metal materials Part 1: Test methods at room temperature". The middle area of the plate was used to determine the hole expansion ratio. The hole expansion ratio was determined in a hole expanding test, wherein a test piece with a hole in the center was pressed into a female die with a male die to expand the central hole of the test piece until the edge of the hole in the plate necked or through-plate cracks appeared. Since the manner for preparing the original hole in the center of the test piece and the quality of the corresponding edge of the original hole have a great influence on the test result of the hole expansion ratio, the test and test method were implemented according to the test method of hole expansion ratio specified in the ISO/DIS 16630 standard. The original hole in the center was in the form of a primary punched and blanked hole (corresponding to the processing method for an original hole having the worst edge quality). Correspondingly, if the original hole in the center was made by secondary punching and blanking, or in the form of a drilled or reamed hole, the corresponding hole expansion ratio would be increased by 20% on the basis of the value in the table. If the original hole in the center was made by wire cutting, the corresponding hole expansion ratio would be increased by 50% on the basis of the value in the table. If the original hole in the center was made by laser blanking, the corresponding hole expansion ratio would be increased by 80% on the basis of the value in the table.

Table 3: Test results of mechanical properties of bainite steels in Examples 1-14 and Comparative Examples 1-6

No.	Yield strength /MPa	Tension strength /MPa	Elongation at Break /%	Hole expansion ratio /%	Elongation at Break *10+ Hole expansion ratio/%
Ex.1	812	1019	14.3	52	195
Ex.2	855	1085	12.5	45	170
Ex.3	803	1011	15.2	55	207
Ex.4	801	1006	16	58	218
Ex.5	860	1075	13	44	174
Ex.6	806	1009	14.5	50	195
Ex.7	818	1002	14.7	55	202
Ex.8	830	1066	13.6	42	178
Ex.9	860	1080	13	45	175
Ex.10	845	1068	13.8	45	183
Ex.11	841	1065	13.7	45	182
Ex.12	822	1033	14.4	40	184
Ex.13	815	1042	14.1	42	183
Ex.14	800	1004	16.2	40	202
Comp. Ex.1	631	946	16.8	45	213
Comp. Ex.2	922	1132	7.2	41	113
Comp. Ex.3	892	1028	8.4	60	144
Comp. Ex.4	652	1065	14.2	17	159
Comp. Ex.5	883	1014	9.1	65	156
Comp. Ex.6	722	1022	14.1	23	164

[0071] As it can be seen from Table 3, when the composition and process of the steel plate or steel strip meet the design requirements, all examples can provide the mechanical properties of a yield strength of >800MPa, a tensile strength of >1000MPa, an elongation at break of >12%, and a hole expansion ratio of ≥ 40%. In addition, because the steel plate or steel strip forms a gradient structure in the thickness direction, the material also has good comprehensive forming properties, that is, both drawability and hole expandability/flanging ability are good, which is reflected in the relatively high elongation at break and hole expansion ratio. In all examples, (elongation at break * 10 + hole expansion ratio) is ≥ 170%.

[0072] When the composition and process of the steel plate or steel strip do not meet the design requirements, it is impossible to obtain the desired mechanical properties. For example, the strength of the material in Comparative Example 1 is poor because the C and Mn content are below the lower limit. In Comparative Example 2, the strength of the material is too large and the formability is very poor because the C content is above the upper limit. In Comparative Example 3, because the R value is below the lower limit of the design, a massive layer of the core cannot be formed in the steel plate or steel strip, and the structures are all needle-like bainite. Although the hole expansion ratio is extremely high, the elongation at break is poor. Similarly, in Comparative Example 5, because the process does not meet the design requirements (see above), the structures are also all needle-like bainite. Thus, the hole expansion ratio is also very high, but the elongation at break is poor. These two comparative examples are poor in comprehensive formability, i.e., (elongation at break * 10 + hole expansion ratio) is < 170% due to the "biased" formability. The gradient structure in the thickness direction cannot be formed in Comparative Examples 4 and 6 either, because the process does not meet the design requirements (see above). It is also manifested that the formability is too "biased", resulting in poor comprehensive forming ability, i.e., (elongation at break* 10 + hole expansion ratio) is <170%.

[0073] The combination of the technical features in the present disclosure is not limited to the combination described in the claims or the specific embodiments, and all the technical features recorded herein may be freely combined or combined in any way, unless there is a contradiction between them.

[0074] It should also be noted that the examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above embodiments, and similar changes or modifications made thereby are directly derived from the contents disclosed in the present disclosure or easily envisaged by those skilled in the art, and shall fall within the protection scope of the present disclosure.

Claims

1. A bainite steel comprising the following chemical elements in mass percentages: C: 0.10∼0.19%, Si: 0.05~0.45%, Mn: 1.5∼2.2%, B: 0.001~0.0035%, Al: 0.01∼0.05%, Cr: 0.05∼0.40%, Mo: 0.05∼0.40%, Fe≥90%.

2. The bainite steel according to claim 1 further comprising at least one of Ti and Nb, wherein Nb is ≤0.1%, Ti is ≤0.15%.

3. A bainite steel comprising the following chemical elements in mass percentages: C: 0.10∼0.19%, Si: 0.05~0.45%, Mn: 1.5∼2.2%, B: 0.001~0.0035%, Al: 0.01∼0.05%, Cr: 0.05∼0.40%, Mo: 0.05∼0.40%, with a balance of Fe and unavoidable impurities.

4. The bainite steel according to claim 3, wherein in the unavoidable impurities, P is ≤0.015%, S is ≤0.004%.

5. The bainite steel according to claim 1 or 3, wherein the mass percentages of the chemical elements meet the following relationships: R=(Mn+Si)/(12*C+160*B), wherein 0.9≤R≤1.2, where each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element.

6. The bainite steel according to claim 5, wherein the mass percentages of chemical elements in the bainite steel meet the following relationship: Q=(C+Cr+Mo+Mn/2)/R, wherein 1.15≤Q≤1.5, where the value in front of the percentage sign in the mass percentage of each element is used for calculation.

7. The bainite steel according to claim 1 or 3, wherein the bainite steel comprises two layers of surface layer structure and one layer of core structure, wherein the core structure is between the two layers of surface layer structure.

8. The bainite steel according to claim 7, wherein in the bainite steel, the volume of the core structure accounts for 20%~50% of the volume of the bainite steel, and the rest is the surface layer structure.

9. The bainite steel according to claim 7, wherein the surface layer structure comprises needle-like bainite and granular carbide precipitate phase, wherein the core structure comprises massive bainite and granular carbide precipitate phase.

10. The bainite steel according to claim 9, wherein the needle-like bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the surface layer structure, and the massive bainite and granular carbide precipitate phase account for equal to or more than 99% by volume of the core structure.

11. The bainite steel according to claim 7, wherein the bainite steel further comprises two multi-phase layers, wherein the two layers of surface layer structure and the one layer of core structure form an intermediate layer that is between the two multi-phase layers.

12. The bainite steel according to claim 11, wherein in the bainite steel, the volume of the multi-phase layers accounts for 2%~10% by volume of the bainite steel, and the rest is the intermediate layer.

13. The bainite steel according to claim 11, wherein the multi-phase layer comprises polygonal ferrite, needle-like bainite and granular carbide precipitate phase, wherein the polygonal ferrite accounts for no more than 50% by volume of the multi-phase layer, and the polygonal ferrite, the needle-like bainite and the granular carbide precipitate phase account for no less than 99% by volume of the multi-phase layer.

14. The bainite steel according to any one of claims 1-13, wherein the bainite steel has a tensile strength of ≥ 1000MPa, a yield strength of ≥ 800MPa, a hole expansion ratio of ≥40%, and an elongation at break of ≥12%

15. A manufacturing method for the bainite steel according to any one of claims 1-14, comprising steps of:

smelting and casting;

hot rolling;

post-rolling cooling and coiling;

pickling and cold rolling;

annealing.

16. The manufacturing method for the bainite steel according to claim 15, wherein the annealing step comprises a heating stage, a slow cooling stage, a fast cooling stage, a controlled cooling stage and an air cooling stage in sequence, wherein the cooling rates at the slow cooling stage, the fast cooling stage, and the controlled cooling stage are controlled to satisfy: the controlled cooling stage < the slow cooling stage < the fast cooling stage.

17. The manufacturing method for the bainite steel according to claim 16, wherein the bainite steel is cooled to a slow cooling temperature of 720-800 °C at a slow cooling rate of Q~10*Q °C/s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤Q≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element.

18. The manufacturing method for the bainite steel according to claim 17, wherein the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel, wherein the cooling gas injection pressure is controlled at 0.2*Q~Q kPa, and the holding time of the cooling gas injection is controlled at 5-20 seconds.

19. The manufacturing method for the bainite steel according to claim 16, wherein the bainite steel is cooled to a slow cooling temperature of 620-700 °C at a slow cooling rate of Q~10*Q °C/s at the slow cooling stage; wherein the mass percentages of chemical elements satisfy the relationship Q=(C+Cr+Mo+Mn/2)/R, 1.15≤Q≤1.5, R=(Mn+Si)/(12*C+160*B), 0.9≤R≤1.2, wherein each chemical element in the formula uses the value in front of the percent sign in the mass percentage of each chemical element.

20. The manufacturing method for the bainite steel according to claim 19, wherein the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel, wherein the cooling gas injection pressure is controlled at 0.05*Q~0.15*Q kPa, and the holding time of the cooling gas injection is controlled at 5-15 seconds.

21. The manufacturing method for the bainite steel according to claim 17 or 19, wherein the bainite steel is cooled to a fast cooling temperature of 400-540 °C at a fast cooling rate of 10*Q~20*Q °C/s at the fast cooling stage.

22. The manufacturing method for the bainite steel according to claim 21, wherein the bainite steel is cooled by injecting a cooling gas to the surface of the bainite steel twice, wherein the first injection pressure of the cooling gas is controlled at 0.3*Q~1.5*Q kPa, and the first holding time of the cooling gas is controlled at 1-7 seconds; the second injection pressure of the cooling gas is controlled at 0.08*Q~0.2*Q kPa, and the second holding time of the cooling gas is controlled at 5-10 seconds.

23. The manufacturing method for the bainite steel according to claim 18, 20 or 22, wherein the cooling gas is a mixture of a reducing gas and an inert gas, wherein the reducing gas is hydrogen with a volume fraction of 1%~8%, and the temperature of the cooling gas is controlled at 5~50°C.

24. The manufacturing method for the bainite steel according to claim 16, wherein at the controlled cooling stage, the controlled cooling rate is controlled at ≤ Q °C/s, the holding time of controlled cooling is 100-200 seconds, and the controlled temperature of the bainite steel is ≥ 350°C at the end of the controlled cooling stage.

25. The manufacturing method for the bainite steel according to claim 16, wherein at the heating stage, the bainite steel is heated at a heating rate of ≤50°C/s to the soaking temperature of 840~950°C and then held for a holding time of 60~180 s.

26. The manufacturing method for the bainite steel according to claim 15, wherein the process parameters of the manufacturing method are controlled to meet at least one of the following:

in the step of hot rolling, a heating temperature is controlled at 1100-1230 °C; an initial rolling temperature of finishing rolling is controlled at 1050-1180 °C; and a final rolling temperature of finishing rolling is controlled at 870-930 °C;

in the step of post-rolling cooling and coiling, a cooling rate is controlled at 30-150 °C/s, and a coiling temperature is controlled at 540-620 °C;

in the step of cold rolling, a cold rolling reduction rate is controlled at ≥ 30%.

Drawing