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(11) | EP 3 674 434 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | LOW-NI AUSTENITIC STAINLESS STEEL WITH EXCELLENT HOT WORKABILITY AND HYDROGEN EMBRITTLEMENT RESISTANCE |
| (57) The disclosure discloses low Ni austenitic stainless steel that improves hot workability
and hydrogen embrittlement resistance that may occur due to a decrease in Mn and Ni
content. In accordance with one aspect of the disclosure, an austenitic stainless
steel includes: by weight percent, C: 0.05-0.15%, Si: 0.2-0.7%, Mn: 2.0-5.0%, Ni:
2.0-5.0%, Cr: 17.0-19.0%, P: less than 0.1%, S: less than 0.01% , Cu: 1.0-3.0%, N:
0.15-0.30%, and the remainder of Fe and other inevitable impurities, and a crack resistance
index (CRN) value is 0 or more, and a Md30 value satisfies the range of -30 to 0 °C.\ |
[Technical Field]
[0001] The disclosure relates to low Ni austenitic stainless steel with lowered Mn content, and more particularly, to austenitic stainless steel with improved hot workability and hydrogen embrittlement resistance which may occur due to a decrease in Mn and Ni content.
[Background Art]
[0002] Work hardened metastable austenitic stainless steel, represented by STS304 and STS301, is widely used in various applications due to its excellent workability and corrosion resistance. However, these steel grades have the disadvantage that the raw material cost is high according to the high Ni content. In particular, the supply and demand of raw materials is unstable due to extreme fluctuations in Ni raw material prices, thereby the price of raw materials is fluctuated, and thus it is impossible to secure stability of supply prices. Therefore, the development of Ni-saving austenite stainless steel with reduced Ni content is required from various material users.
[0003] Conventional Ni-saving austenitic stainless steel is basically added 5% by weight or more Mn to reduce the Ni to a certain amount to lower the material price and to secure the austenitic phase stability according to the Ni reduction. However, when a large amount of Mn is added, there is a need for improvement in environmental aspects due to the generation of a large amount of Mn fume during the steelmaking process. In addition, due to the content of high Mn, there is a problem of lowering the productivity in the manufacturing process due to the production of steelmaking inclusions (MnS) and lowering the surface corrosion resistance of the final cold rolled material, as well as mechanical properties such as elongation.
[0004] In order to solve this problem, it is desirable to reduce the content of Mn. However, in the case of reducing the Mn content by more than a certain amount in Ni-saving austenitic stainless steel, as the austenitic phase stability is lowered, a large amount of delta ferrite is formed during casting, which may cause quality problems such as slab edge cracks and surface seam during hot rolling.
[0005] In addition, in the case of products requiring a beautiful surface of high corrosion resistance, it is required to maintain the surface properties formed during the final cold rolling to the final product. In the case of applying the bright annealing process in a hydrogen atmosphere to maintain the final cold rolling quality and surface properties, and to secure good annealing material properties by appropriate annealing, there is a problem that workability may be inferior due to hydrogen embrittlement defects due to the decrease in Mn content.
[Disclosure]
[Technical Problem]
[0006] Embodiments of the disclosure are to provide a low Ni austenitic stainless steel having excellent hot workability and hydrogen embrittlement resistance even if Mn is reduced by solving the above problems.
[0007] In addition, embodiments of the disclosure are to provide a low Ni austenitic stainless steel that can ensure the corrosion resistance of STS304 or STS301 level.
[Technical Solution]
[0008] In accordance with one aspect of the disclosure, a low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance includes: by weight percent, C: 0.05-0.15%, Si: 0.2-0.7%, Mn: 2.0-5.0%, Ni: 2.0-5.0%, Cr: 17.0-19.0%, P: less than 0.1%, S: less than 0.01%, Cu: 1.0-3.0%, N: 0.15-0.30%, and the remainder of Fe and other inevitable impurities, and a crack resistance index (CRN) value represented by the following Equation (1) is 0 or more, and a Md30 value represented by the following Equation (2) satisfies the range of -30 to 0 °C.
[0009] In addition, according to an embodiment of the disclosure, a pitting resistance equivalent number (PREN) value represented by the following Equation (3) may satisfy 18 or more:
[0010] In addition, according to an embodiment of the disclosure, the low Ni austenitic stainless steel may further include: By weight percent, at least one of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
[0011] In addition, according to an embodiment of the disclosure, an elongation of the stainless steel may be 50% or more.
[Advantageous Effects]
[0012] Embodiments of the disclosure can improve the heat dissipation of ferritic stainless steel by introducing Al plating to the ferritic stainless steel to improve the thermal conductivity.
[0013] A low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance according to an embodiment of the disclosure can secure excellent hot workability due to the suppression of delta ferrite generation during slab reheating, as a result can solve the surface and edge cracking and quality problems during hot rolling.
[0014] In addition, by inhibiting the production of deformation induced martensite due to securing the austenitic phase stability, it is possible to secure excellent hydrogen embrittlement resistance and workability even through going through a bright annealing process in a hydrogen atmosphere.
[0015] In addition, it is possible to secure excellent hydrogen embrittlement resistance and workability even though the bright annealing process of hydrogen atmosphere is suppressed by inhibiting formation of deformation induced martensite due to securing austenitic phase stability.
[Description of Drawings]
[0017]
FIG. 1 is a graph showing the change in martensite peak intensity and whether hydrogen embrittlement has occurred according to Md30.
FIG.2 is a graph showing a change in elongation according to Md30.
[Best Modes of the Invention]
[0018] In accordance with one aspect of the disclosure, a low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance includes: by weight percent, C: 0.05-0.15%, Si: 0.2-0.7%, Mn: 2.0-5.0%, Ni: 2.0-5.0%, Cr: 17.0-19.0%, P: less than 0.1%, S: less than 0.01% , Cu: 1.0-3.0%, N: 0.15-0.30%, and the remainder of Fe and other inevitable impurities, and a crack resistance index (CRN) value represented by the following Equation (1) is 0 or more, and a Md30 value represented by the following Equation (2) satisfies the range of -30 to 0 °C.
[Modes of the Invention]
[0019] Hereinafter, exemplary embodiments of the disclosure will be described in detail with reference to accompanying drawings. The following examples are provided to fully deliver the spirit of the disclosure to those of ordinary skill in the art. The disclosure may be specified in different forms without limitation to examples, which will not be described herein. To clarify the disclosure, illustration of parts that are not associated with the explanation will be omitted, and to help in understanding, the sizes of components will be slightly exaggerated.
[0020] In order to solve the above problems, the present inventors derive the correlation analysis between the experimental hot workability evaluation results for various alloy components and the predicted delta (δ) ferrite as crack resistance index (CRN) and have secured hot workability by suppressing the surface or edge crack formation during hot working. At the same time, the present inventors predicted hydrogen embrittlement resistance for bright annealing materials by examining the austenitic phase stability of each alloy component. In addition, an alloy component with excellent hot workability and excellent workability and corrosion resistance was derived by predicting corrosion resistance using the pitting resistance equivalent number (PREN).
[0021] A low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance according to an embodiment of the disclosure includes: by weight percent, C: 0.05-0.15%, Si: 0.2-0.7%, Mn: 2.0-5.0%, Ni: 2.0-5.0%, Cr: 17.0-19.0%, P: less than 0.1%, S: less than 0.01% , Cu: 1.0-3.0%, N: 0.15-0.30%, and the remainder of Fe and other inevitable impurities.
[0022] Hereinafter, the reason for the numerical limitation of the content of the alloying component in the embodiment of the disclosure will be described. In the following, the unit is weight % unless otherwise specified.
[0024] Carbon (C) is an effective element to stabilize the austenitic phase. However, excessive addition of C not only lowers cold workability due to the solid solution strengthening effect, but also induces grain boundary precipitation of Cr carbides due to heat affected zones of welds and latent heat after hot-rolled coiling, thereby adversely affecting ductility, toughness and corrosion resistance. For this reason, an upper limit is made into 0.15%. In addition, as described above, it is preferable to add 0.05% or more for austenitic phase stabilization.
[0026] Si acts as a deoxidizer during the steelmaking process, and is effective to improve corrosion resistance, and its property is effective when added more than 0.2%. However, Si is an effective element for stabilizing the ferritic phase, and when excessively added, it promotes the formation of delta (δ) ferrite in the cast slab, which not only reduces hot workability but also reduces the ductility and toughness of the steel due to the solid solution strengthening effect. For this reason, an upper limit shall be 0.7%.
[0028] Mn is an austenitic phase stabilizing element added as a substitute for Ni, which is effective in improving cold rolling property by suppressing deformation induced martensite formation. The property is effective when added at 2.0% or more. However, when the addition is excessive, the S-based inclusions (MnS) increase, resulting in a decrease in the ductility, toughness and corrosion resistance of the steel, and the upper limit thereof is 5.0%.
[0030] Ni is an austenitic phase stabilizing element and is essential to ensure good hot and cold workability. In particular, even when a certain amount or more of Mn is added, addition of 2.0% or more of Ni is essential. However, since Ni is an expensive element, the addition of a large amount leads to an increase in raw material cost. The upper limit thereof is thus 5.0%.
[0032] Cr is not only an element necessary to secure the corrosion resistance required for stainless steel, but it is also effective for suppressing martensitic phase formation, and its properties are effective when 17.0% or more is added. On the other hand, the addition of a large amount promotes the formation of delta (δ) ferrite in the slab, leading to a decrease in hot workability, so the upper limit is 19.0%.
[0038] Cu is an austenitic phase stabilizing element and is effective in softening the material. To express this soft effect, addition of 1.0% or more is essential. However, the addition of a large amount of Cu not only raises the material cost but also causes hot embrittlement, so the upper limit is 3.0%.
[0040] N is an effective element for stabilizing austenitic phase and improving corrosion resistance, and its property is effective when 0.15% or more is added. On the other hand, the excessive addition of N lowers cold workability due to the effect of solid solution strengthening, so the upper limit is 0.30%.
[0041] In addition, according to an embodiment of the disclosure, it may further include at least one of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
[0042] B is an element that is effective in securing good surface quality by suppressing cracking during casting, and its property is effective when 0.001% is added. On the other hand, when B is excessively added, nitride (BN) is formed on the surface of the product during the annealing / pickling process, resulting in a problem of lowering the surface quality. Therefore, the upper limit is 0.005%.
[0043] Ca suppresses the formation of MnS steelmaking inclusions generated at grain boundaries when high Mn is contained, thereby improving the cleanliness of the product, and its property is effective when 0.001% is added. On the other hand, the excessive addition of Ca causes a decrease in hot workability due to Ca inclusions and a decrease in product surface quality, so the upper limit is 0.003%.
[0044] It is known that the hot workability of these high Mn low Ni containing austenitic stainless steels is correlated with the delta (δ) ferrite fraction distributed in the slab. This is a crack caused by different deformation resistance of each phase in the amount of deformation applied in the rolling process when austenite and ferrite are present in the high temperature region. In order to secure hot workability, it is necessary to design an alloying component that suppresses the formation of delta ferrite and to derive hot working conditions. However, when referring to the component characteristics of the disclosure described above, as a large amount of solid solution strengthening elements such as C and N are added, a large amount of cracks are likely to occur on the surface of the material due to the high hot rolling load during hot working at low temperatures. Therefore, it is preferable to operate at a hot rolling temperature that does not cause an abnormal operation during operation.
[0045] Specifically, the surface and edge quality of the hot rolled material was checked to determine the occurrence of cracks as an index of hot workability. The phase fraction of delta (δ) ferrite was predicted through phase analysis using computer simulation for the alloy component. The experimental results of the hot workability evaluation and the correlation analysis with the predicted delta (δ) ferrite resulted in the crack resistance index (CRN) range represented by Equation (1). By analyzing the correlation between these experimental hot workability evaluation results and predicted delta (δ) ferrite, the range of crack resistance index (CRN) represented by equation (1) was derived.
[0046] A low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement resistance according to an embodiment of the disclosure has a crack resistance index (CRN) value of 0 or more.
[0047] If the crack resistance index (CRN) is 0 or more, cracks may not occur on the surface and edges during hot working.
[0048] On the other hand, as described above, for products requiring a beautiful surface, it is common to perform bright annealing on the cold rolled material. This bright annealing is a heat treatment technology that keeps the surface bright and beautiful without changing the color and appearance of the surface by heat-treating the stainless steel cold rolled material under a reducing atmosphere (Dew point -40 to -60 ° C) using nitrogen (N2), hydrogen (H2), etc., to prevent reoxidation of the stainless steel cold rolled material during heat treatment. Bright annealing using hydrogen as an atmosphere gas used for such bright annealing is the most common because it is most widely used to suppress discoloration of the surface as well as high heat capacity.
[0049] There is a point to consider when applying bright annealing of hydrogen atmosphere to stainless steel in which the content of Ni and Mn is relatively reduced as compared to general austenitic stainless steel. Due to the penetration of hydrogen during bright annealing, the final material is more likely to suffer from workability inferiority due to hydrogen embrittlement defects. In the case of stainless steel with reduced austenitic phase stabilizing elements such as Ni and Mn, during cold rolling before final bright annealing, stress induced or deformation induced martensite is formed around the surface layer, the martensitic phase formed on the surface layer is in contact with a hydrogen atom, an inert gas, before being transformed into an austenitic phase by heat treatment during bright annealing and these hydrogen atoms penetrate into some martensitic phases. As the existing stress induced or deformation induced martensite phase transforms into the austenitic phase by bright annealing, the hydrogen atom penetrated inside cannot be discharged to the outside and is trapped in the state of atom in the surface layer. The hydrogen atom penetrated to the surface layer is naturally bake-out after a certain time at room temperature for ferrite or martensite phase, which is a general BCC and BCT structure, and does not significantly affect physical properties. On the other hand, when the surface martensitic phase is transformed into austenitic phase by bright annealing, in other words, when hydrogen atom exists in the lattice structure of FCC, even after a long time at room temperature, the natural bake out of the hydrogen atom is not smooth and exists in the material for a long time.
[0050] These hydrogen atoms are known to cause hydrogen embrittlement, the hydrogen atoms trapped in the material by some processing or deformation change to the state of hydrogen molecules (gas), and when a certain pressure is reached, act as a starting point of the crack under a certain load, causing deterioration of elongation.
[0051] Therefore, for austenitic stainless steels with relatively low Ni and Mn, in addition to the alloying components, it is necessary to control the amount of martensitic phase formed on the surface by work hardening to ensure beautiful surface quality and excellent workability through bright annealing.
[0052] Accordingly, Md30 value represented by the following Equation (2) of the low Ni austenitic stainless steel according to an embodiment of the disclosure satisfies the range -30 to 0°C.
[0053] In metastable austenitic stainless steel, martensite transformation occurs by plastic working at a temperature above the martensite transformation start temperature (Ms). The upper limit temperature which causes phase transformation by this process is represented by Md value, and in particular, the temperature (°C) at which 50% of the phase transformation to martensite occurs when 30% strain is given is referred to as Md30. Higher Md30 values make it easier to produce deformation induced martensitic phases, while lower Md30 values may be considered to be relatively difficult to produce deformation induced martensitic phases. This Md30 value is used as an index to determine the austenite stability of conventional metastable austenitic stainless steel.
[0054] When performing bright annealing of the hydrogen atmosphere, the experimental results for the correlation between Md30, which shows austenite stabilization, and the amount of martensitic phase introduced during cold rolling, and whether hydrogen embrittlement occurred during bright annealing due to the amount of martensitic phase introduced into the superficial layer are shown in FIG. 1.
[0055] FIG. 1 is a graph showing changes in martensite peak intensity and hydrogen embrittlement according to Md30, ● mark indicates that hydrogen embrittlement has not occurred and x mark indicates that hydrogen embrittlement has occurred. As the Md30 value increases, the peak intensity of the surface martensitic phase due to the deterioration of the phase stability of austenite increases and when the peak intensity increases above a certain value, it can be seen that hydrogen embrittlement occurs during bright annealing in the hydrogen atmosphere. Based on these results, it is confirmed that maintaining the Md30 value below 0 °C is preferable to suppress hydrogen embrittlement.
[0056] In addition, as in the disclosure, in the situation where a large amount of C and N is inevitable to improve the phase stability of austenite due to the decrease in the content of Mn and Ni as compared to the existing STS304 and STS301, although the Md30 value can be reduced, it is difficult to secure the desired elongation due to the increased work hardening ability of the material itself. In particular, it is necessary to control the lower limit of Md30 value considering that it is essential to secure elongation of more than about 50% in general 300 series stainless steel applications.
[0057] FIG. 2 is a graph showing a change in elongation according to Md30, ■ mark indicates elongation of 50% or more and x mark indicates elongation of less than 50%.
[0058] Referring to FIG. 2, in order to secure elongation of 50% or more in the alloy component range, it is confirmed that controlling the Md30 value to -30°C or more is preferable.
[0059] In addition, as it is required to secure corrosion resistance similar to that of the existing STS304 or STS301, it is necessary to maintain a pitting resistance equivalent number (PREN) value for the alloying component to a predetermined level or more.
[0060] Thus, according to an embodiment of the disclosure, a pitting resistance equivalent number (PREN) value represented by the following Equation (3) may satisfy 18 or more.
[0061] Hereinafter will be described in more detail through a preferred embodiment of the disclosure.
Example
[0062] For the various alloy component ranges shown in Table 1 below, a 200t slab was prepared and hot rolled to a thickness of 3t after reheating at 1,230°C for 2 hours. Then, the surface and edge quality of the hot rolled material was confirmed, and whether cracks occurred was used as a judgment index of hot workability. In addition, the phase ratio of the delta (δ) ferrite was predicted through phase analysis of the alloy component. The crack resistance index (CRN) of Equation (1) derived through the correlation analysis between the experimental hot workability evaluation result and the predicted delta (δ) ferrite was derived and shown in Table 2. Elongation was measured using the 5th test piece prescribed by JIS Z 2201 according to the tensile test method of the metal material specified by Japanese Industrial Standard JIS Z 2241.
< Table 1 >
| division | C | Si | Mn | Ni | Cr | Cu | N |
| Comparative example 1 | 0.095 | 0.46 | 2.91 | 3.48 | 17.8 | 1.58 | 0.16 |
| Comparative example 2 | 0.076 | 0.45 | 3.01 | 2.9 | 18.1 | 1.61 | 0.22 |
| Comparative example 3 | 0.102 | 0.43 | 4.37 | 3.54 | 18.1 | 1.44 | 0.25 |
| Comparative example 4 | 0.098 | 0.47 | 3.95 | 3.68 | 18.0 | 1.54 | 0.23 |
| Example 1 | 0.097 | 0.46 | 3.42 | 3.48 | 18.1 | 1.43 | 0.19 |
| Example 2 | 0.096 | 0.47 | 3.83 | 3.45 | 18.2 | 1.47 | 0.23 |
| Example 3 | 0.096 | 0.44 | 3.94 | 3.31 | 18.2 | 1.5 | 0.21 |
| Example 4 | 0.087 | 0.44 | 2.95 | 3.44 | 17.9 | 1.48 | 0.244 |
< Table 2>
| division | Crack occurrence | CRN | Md30 (°C) | hydrogen embrittlement | elongation (%) | PREN |
| Comparative example 1 | × | 10.8 | 14.8 | ○ | 38.0 | 18.9 |
| Comparative example 2 | ○ | -0.47 | 7.0 | ○ | 46.0 | 20.1 |
| Comparative example 3 | × | 64.7 | -43.4 | × | 46.8 | 19.9 |
| Comparative example 4 | × | 61.4 | -34.8 | × | 49.7 | 19.7 |
| Example 1 | × | 12.6 | -3.9 | × | 54.3 | 19.4 |
| Example 2 | × | 35.9 | -27.0 | × | 50.2 | 20.0 |
| Example 3 | × | 15.7 | -15.2 | × | 52.5 | 19.6 |
| Example 4 | × | 58.7 | -17.8 | × | 50.5 | 20.3 |
[0063] In Comparative Example 2, cracks occurred at the surface and the edge during hot rolling, and the crack resistance index (CRN) was -0.47. In Comparative Examples 1, 3, and 4, no crack was generated on the surface and the edge during hot rolling, and thus when the crack resistance index (CRN) derived according to the delta (δ) ferrite phase fraction indicates 0 or more, hot workability was found to be a good indicator.
[0064] Referring to FIG. 1 in conjunction with Tables 1 and 2, in the case of Comparative Examples 1 and 2, it was found that the hydrogen embrittlement occurred because the peak intensity of the surface martensitic phase of the cold rolled material increased because the Md30 value calculated from the component exceeds 0°C, this is indicated by x in FIG. 1. On the other hand, in the case of Comparative Examples 3 and 4, the Md30 value satisfies 0°C or less, so that hydrogen embrittlement does not occur, however the Md30 value indicates less than -30°C, so the elongation was measured to be less than 50%. From this, it can be seen that the range of Md30 values must satisfy the range of -30 to 0°C to obtain workability conditions according to hydrogen embrittlement resistance and elongation of 50% or more.
[0065] On the other hand, the pitting resistance equivalent number (PREN) value according to the Equation (3) in the range of the component according to the disclosure satisfies 18 or more, it was found that excellent corrosion resistance of the STS304 level can also be secured.
[0066] As described above, while the disclosure has been described with reference to embodiments of the disclosure, the disclosure is not limited thereto, and it will be understood by those of ordinary skill in the art that various modifications and alternations can be made without departing from the concept and scope of the accompanying claims.
[Industrial Applicability]
1. A low Ni austenitic stainless steel with excellent hot workability and hydrogen embrittlement
resistance, comprising:
by weight percent, C: 0.05-0.15%, Si: 0.2-0.7%, Mn: 2.0-5.0%, Ni: 2.0-5.0%, Cr: 17.0-19.0%, P: less than 0.1%, S: less than 0.01% , Cu: 1.0-3.0%, N: 0.15-0.30%, and the remainder of Fe and other inevitable impurities,
wherein a crack resistance index (CRN) value represented by the following Equation (1) is 0 or more,
wherein a Md30 value represented by the following Equation (2) satisfies the range
of -30 to 0 °C.
Here, C, Si, Mn, Ni, Cr, Cu, N means the content (wt%) of each element.
2. The low Ni austenitic stainless steel according to claim 1, wherein the pitting resistance
equivalent number (PREN) value represented by the following Equation (3) satisfies
18 or more:
Here, Cr, N, Mn means the content (wt%) of each element.
Here, Cr, N, Mn means the content (wt%) of each element.
3. The low Ni austenitic stainless steel according to claim 1, further comprising:
By weight percent, at least one of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
By weight percent, at least one of B: 0.001 to 0.005% and Ca: 0.001 to 0.003%.
4. The low Ni austenitic stainless steel according to claim 1, wherein an elongation
of the stainless steel is 50% or more.