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(11) | EP 2 159 296 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | HARDENED AND TEMPERED STEEL AND METHOD FOR PRODUCING PARTS OF SAID STEEL |
| (57) The invention relates to a quench hardened and tempered steel with a specific composition
of the following elements: 0.22% ≤ C ≤ 0.30%, 0.40% ≤ Mn <1.00%, 1.00% ≤ Cr ≤ 2.50%,
1.80,% ≤ Ni ≤ 4.00%, 0.30% ≤ Mo < 0.90% and 0.01 % ≤ V < 0.50%, which with a method
for obtaining said steel achieves low P and S contents, contributes to increasing
the mechanical strength of the steel to values greater than 1200 N/mm2 and with high toughness, KV resilience at low temperature, -20°C, with values greater
than 60 J, when a part for the manufacture of which said steel has been selected is
subjected to a specific quench hardening and tempering treatment. |
Object of the Invention
[0001] The present invention relates to a quench hardened and tempered steel and to a method for obtaining parts of said steel, having an application in the field of the steel industry, allowing its use for metal structures in the construction sector, said parts being especially suitable in the shipbuilding industry, for example for manufacturing chains for vessels and anchor line accessories.
[0002] The invention allows obtaining a quench hardened and tempered steel, from a chemical composition and by means of a metallurgical process, having high mechanical strength and at the same time high toughness at low temperature, in addition to having optimal weldability.
Background of the Invention
[0003] In the field of metallurgy, and more specifically for the case of steels, the industrial application of the elements or parts establishes minimum requirements in relation to the mechanical behavior of the steel.
[0004] To obtain certain mechanical properties in a steel, in addition to selecting its chemical composition, heat treatments which allow modifying the crystal structure of the steel without modifying its chemical composition are performed, which allows classifying the steels according to the crystal structure obtained after the heat treatment.
[0005] A type of steel called quench hardened and tempered steels is currently known, which are used for constructing metal structures and for manufacturing mechanical parts and elements of responsibility, i.e., they are subjected to loads during their work situation, and for which the slightest possibility of failure of said parts during their useful life is not admissible.
[0006] The essential mechanical characteristics which this type of part has to have are high mechanical strength, high toughness and an optimal ratio between the elastic limit and the mechanical strength. Furthermore, high fatigue strength and elongation are also required.
[0007] These characteristics are determined to a great extent by the carbon content of the steel, which is usually between 0.03% and 0.70% by weight, as well as the content of other elements.
[0008] The high tensile strength values of quench hardened and tempered steels vary between 700 N/mm2 and 1700 N/mm2, and are achieved with carbon contents by weight ranging between 0.25% and 0.60%. Furthermore, to improve other properties, the addition of variable amounts of alloying elements, such as for example Mn, Cr, Ni, Mo and V, is known.
[0009] The increase of the carbon content in a steel causes on one hand an increase in the tensile strength and in the cold brittleness of said steel, whereas on the other hand it causes a reduction of its toughness and ductility.
[0010] The toughness is the capacity of a material to absorb energy without causing fissures, being determined as an impact strength, i.e., the resistance offered by a material to the propagation of a crack, or the energy absorption capacity of the material without causing fissures.
[0011] The effect of each of the alloying elements during the process for obtaining the steel, with respect to its response to the heat treatments and on properties such as hardness and quench hardenability, is known by metallurgical technicians.
[0012] The multiple applications of quench hardened and tempered steels include the shipbuilding industry, specifically in the manufacture of chains and other accessory elements and devices for anchor lines, as well as the gas liquefaction and transport industries. In this field, it is essential for the steels to have high values of tensile strength and impact strength or toughness, which involves a combination of opposing mechanical properties, given that both characteristics are inversely proportional. Furthermore, these properties must be maintained even at low temperatures, of an order of magnitude of 20ºC below zero or even lower temperatures, considering the service conditions of these elements and parts given their field of application.
[0013] Furthermore, other requirements of quench hardened and tempered steels is that they have a good response to processes involved in the stages of manufacturing, installing and assembling these parts, such as for example hot forming and welding processes.
[0014] Quench hardening is a heat treatment the objective of which is to harden and increase the strength of the steels at the expense of decreasing the ductility and the resilience.
[0015] The resilience is the amount of energy which a material can absorb in the elastic field, i.e., before the plastic deformation when it is subjected to a load starts. The resilience corresponds to the area which is under the stress-strain diagram of the tensile test of a material, between a zero deformation value and the deformation value corresponding to the creep stress, being an indicator of the brittleness of the material.
[0016] The quench hardening heat treatment consists of heating a steel to a temperature above its transformation point (Ac), also called upper critical temperature or transformation temperature, which depends on the chemical composition of the steel and can be for example between 800ºC and 950ºC, all of this for the purpose of achieving an austenitic crystal structure (γ). A rapid cooling is then performed at a rate greater than the critical one for the purpose of achieving a martensitic crystal structure, or in any case if the cooling is not rapid enough achieving a bainitic crystal structure, which provide the steels with high strengths.
[0017] Thus, what is achieved with the quench hardening is a transformation of austenite (γ) into martensite and/or bainite with high strength. The cooling rate depends on the dimensions of the part or element of steel to be quench hardened, water, oil, air or refrigerated means, such as for example a refrigerated chamber, usually being used to perform said cooling.
[0018] In short, the factors affecting the quench hardening are the chemical composition of the steel, considering both the percentage of carbon and of alloying elements, the temperature and the heating time and the cooling rate.
[0019] Once the quench hardening treatment has been performed, it is highly usual to perform a tempering heat treatment on the steel for the purpose of attenuating the effects and mechanical properties resulting from the quench hardening, allowing maintaining to a great extent the required strength and hardness values while at the same time the toughness and elasticity of the steel are increased. Steels with an optimal combination of mechanical strength, elongation and elastic limit are thus obtained, steels being obtained with an elastic limit value of up to 75% the breaking load value. In addition to the combination between strength and elongation, in quench hardened and tempered steels the elastic limit is greater than the elastic limit of steels in which normalizing and annealing heat treatments have been performed.
[0020] Tempering is a heat treatment consisting of heating to a temperature lower than the starting temperature of the austenitic transformation, which usually occurs between 450ºC and 600ºC. During this process, the carbon contained in the martensite, in forced solution, precipitates as carbides and a transformation of the retained austenite (γ) occurs, whereas the martensite is transformed into extremely small cementite (Fe3C) particles dispersed in a ferrite (α) matrix, whereby the stresses created in the abrupt cooling performed in the quench hardening treatment are eliminated.
[0021] On the other hand, there is a heat treatment called martempering, which is a particular case of the previously described quench hardening and tempering treatment, in which the quench hardening is stopped before the martensitic transformation takes place, for the purpose of homogenizing the temperature of a part of steel, prior to continuing the cooling so that martensite is formed, then proceeding as in the case of a tempering treatment.
[0022] Therefore, for each type of application it is important to consider and to define well both the temperatures and the times of maintenance at the tempering temperature, such that the end part obtains the desired ratio of mechanical characteristics.
[0023] In relation to industrial applications requiring higher levels of strength, the use of steels alloyed with Mn, Cr, Ni, Mo and V is usual, whereby strength values of up to 1000 N/mm2 and high toughness at low temperature, with KV resilience values at -20ºC of about 60 J are achieved.
[0024] Currently there are steels and methods for obtaining them aimed at improving the service characteristics of the steels intended for the previously mentioned applications, in which variable amounts of alloying elements such as for example Mn, Cr, Ni, Mo, V or B are usually added, some examples of which are mentioned below.
[0025] Korean patent number KR 100320959-B describes a method for obtaining a steel with high toughness at very low temperatures with high Mn content, which by weight is between 16% and 22%.
[0026] On the other hand, in the steel described in Korean patent number KR 100325714-B, the toughness at low temperature is also raised by means of a bainitic crystal structure, however, the strength values achieved are about 600 N/mm2.
[0027] There are inventions relating to steels with higher strength values which intend to improve the toughness at low temperatures, such as for example the one described in European patent application number EP 1697552, which relates to a steel wire product for cold forging and to the process for manufacturing it, comprising the addition of elements such as C, Si, Mn, Cr and B.
[0028] Japanese patent number JP 2000256783 describes a steel with high and toughness, with resistance to corrosion under stress, as well as to the method for manufacturing it, in which the elastic limit of said steel exceeds 960 N/mm2 (140 ksi), with contents by weight of C between 0.20% and 0.35%, of Cr between 0.20% and 0.70%, of Mo between 0.10% and 0.50%, and of V between 0.10% and 0.30%.
[0029] The development of the industries in which these steels have an application demands increasingly higher tensile strength values maintaining the toughness at low temperatures, without there being at the moment any known solution combining tensile strength values greater than 1000 N/mm2 and high toughness, with KV resilience values at -20ºC of about 60 J, allowing at the same time the weldability of said steels.
[0030] Therefore, the properties of the parts manufactured with quench hardened and tempered steels aimed at said industries can be optimized.
Description of the Invention
[0031] The present invention relates to a quench hardened and tempered steel and to a method for obtaining parts of said steel, in which as a result of various investigations an optimal combination of two opposing mechanical properties, high tensile strength, with strength values of at least 1200 N/mm2, and high toughness at low temperature, with KV resilience values at -20ºC of at least 60 J.
[0032] The invention allows obtaining a quench hardened and tempered steel, from a novel chemical composition and a certain metallurgical process, having high mechanical strength and at the same time high toughness at low temperature, in addition to having optimal weldability, which is important, for example, in the specific case of manufacturing chains for the shipbuilding industry.
[0033] On the other hand, in addition to the chemical composition, the heat treatment performed in the steel significantly affects the mechanical characteristics of the steel finally obtained, i.e., the initial chemical composition is subjected to a certain quench hardening and tempering method, which must necessarily be performed in specific time and temperature conditions.
[0034] For the manufacture of parts, it is necessary to apply in the manufacture of this steel a specific method in relation to the deoxidation method and an inclusion decanting process in certain special conditions.
[0035] The inventors have verified a synergic effect between a novel combination of chemical elements and a method for obtaining said steel, which contemplates a specific heat treatment, achieving a quench hardened and tempered steel with high strength and toughness, in addition to a good suitability for welding and shaping processes.
[0036] The investigations conducted have resulted in a new quality of NiCrMoV alloyed steel, comprising the following chemical composition in percentage by weight:
the rest of the elements being impurities resulting from obtaining it.
[0037] These alloying elements are used in alloyed steels to improve the tensile strength, the tempering resistance, the toughness or other characteristics, but not with the indicated concentrations by weight, with the proposed combination of elements, nor for obtaining the previously described properties which allow their use in the mentioned applications.
[0038] Each of the alloying elements, in the previously indicated proportions, affects certain parameters and properties of the steel finally obtained.
[0039] Manganese increases the quench hardenability and reduces the transformation temperature, which allows obtaining a fine grain crystal structure, allowing at the same time increasing the strength and improving the toughness.
[0040] The use of chromium allows a marked shift of the curves of the TTT, Temperature-Time-Transformation, diagrams towards the right, whereby it allows increasing to a great extent the quench hardenability in a less expensive manner than other elements, as in the case of steels of the state of the art.
[0041] Nickel is a moderate agent favoring the quench hardenability and which allows reducing the tendency to cracking during the quench hardening. The indicated nickel contents allow obtaining a fine grain, achieving higher impact strength, mainly at low temperatures.
[0042] Molybdenum has a strong effect favoring the quench hardenability, being in turn a strong former of carbides, which provide a considerable secondary hardening effect during the tempering.
[0043] Finally, vanadium is a microalloying element which causes an intense hardening by precipitation and which, when it remains in solid solution, highly increases the quench hardenability, furthermore showing a strong secondary hardening effect during the tempering at high temperature, of an order of magnitude greater than 575ºC.
[0044] Furthermore, the steel proposed by the invention can additionally comprise at least one of the following elements or a combination thereof, with a percentage by weight:
[0045] It is likewise contemplated that the steel of the invention comprises any or several of the following elements, with a percentage by weight:
the rest being residual elements resulting from obtaining the steel.
[0046] It has previously been verified that steels with a similar composition in which a conventional quench hardening and tempering process has been performed did not manage to reach the demanded and previously mentioned mechanical properties, due to the fact that the cleaning degree was lower and the S and P levels were not sufficiently reduced as in the quality presented by the steel of the invention.
[0047] The presence of phosphorus and sulfur is generally detrimental for the applications requiring toughness at low temperature, since they reduce the elongation and the strength of the steel, an attempt being made to eliminate these elements in the manufacturing processes. The general recommendation for ordinary steels of the state of the art is that the S content, as well as the P content does not exceed 0.060%, and 0.030% in the case of quality steels.
[0048] A preferred composition of the steel proposed by the invention comprises, in percentage by weight:
[0049] For this preferred composition, the steel can additionally comprise at least one of the following elements, or a combination thereof, by weight:
[0050] Thus, after various experiments a rigorous method for obtaining the steel according to the following steps has been developed:
- Rigorously controlling the raw materials of the furnace, i.e., scrap and, especially, coke and lime.
- Using between 30% and 50% of maximum quality scrap.
- Performing an oxidizing period in an electric furnace, which is important for the dephosphorization of the steel, prior to the foamed slag.
- Once the foamed slag is over, deslagging is performed until the furnace is left virtually without slag, the objective being a presence of phosphorus, in this step or stage, less than 0.007% by weight.
- Tilting with standard temperature and parts per million (ppm) of 0, according to the clean steel standard, ensuring that slag does not pass from the furnace to the ladle.
- Deoxidizing with Al, to obtain very fluid white slag with a lime-spar base.
- Rigorously controlling the refining raw materials, i.e., ferromanganese, ferrochromium, nickel and lime.
- Performing two vacuums with an intermediate H sample, considering the vacuum time as that which is below 2 mbar and being 50% greater than the conventional vacuum time.
- Ending the second vacuum with a sufficient temperature to perform an inclusion decanting process after it. In said decanting time, the melt is slightly stirred with argon to break the slag and without performing additions or heating of any type.
- Finally, a meticulous tapping process must be followed with a special protection of the stream.
[0051] This entire method for manufacturing steel allows achieving low levels of sulfur levels, below 0.010% by weight, and phosphorus, below 0.015% by weight, in addition to a low inclusion level.
[0052] TTT (Temperature-Time-Transformation) diagrams allow representing the heat treatments for a certain chemical composition when the phase transformations occur in non-equilibrium conditions.
[0053] After various experimental tests it has been verified that after the process for manufacturing steel proposed by the invention, with the chemical composition indicated above, adjusting the temperatures and the times of maintenance of the quench hardening and the tempering, a steel with tensile strength above 1200 N/mm2 and high toughness, KV resilience at -20ºC of 60 J is achieved. Furthermore, said steel has a good response to welding.
[0054] To obtain a part of the previously obtained steel, the invention contemplates performing a method by which said part of steel can be obtained.
[0055] The method for obtaining parts of said steel comprises a quench hardening process which is performed with an austenization at a temperature greater than 800ºC, followed by a subsequent cooling, for example in water.
[0056] The method then comprises a tempering process which is carried out at a temperature greater than 550ºC for about 2 hours, thus achieving the adjustment of the hardness and toughness of the material, in addition to preventing decreases of resilience, which are associated with the brittleness phenomenon of tempering.
[0057] Therefore, the method for obtaining parts of steel comprises the following steps:
- Obtaining the previously described steel of the invention, in which the selected steel comprises the previously defined general composition general or preferred composition.
- Manufacturing a part of said steel, for example by means of forging or machining.
- Performing in the part the previously defined quench hardening treatment.
- Performing in the part the previously defined tempering treatment.
[0058] When a part is quench hardened and tempered after being machined, it is easier to perform the turning or milling work in the tempered state, an excess in the final dimensions of the part being able to be provided to eliminate the deformations occurring during the quench hardening and the tempering, which can later be eliminated, for example, by means of machining, in this case the elimination thereof is simple since they are small amounts of material.
Description of the Drawings
[0059] To complement the description which is being made and for the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description, in which the following has been depicted with an illustrative and non-limiting character:
Figure 1 shows a TTT, Temperature-Time-Transformation, diagram of a eutectoid steel (γ) with 0.77% C, in which a quench hardening and tempering heat treatment has been depicted, in which A is austenite, P is perlite, B is bainite and M is martensite.
Figure 2 shows a TTT, Temperature-Time-Transformation, diagram of a eutectoid steel like that of the previous figure, in which a martempering heat treatment has been depicted.
Figure 3 shows a diagram depicting the KV resilience values at -20ºC, in Joules, for each of the samples of steel investigated in the present invention.
Embodiments of the Invention
Example 1
[0060] By way of example, the tests performed with samples of steels with other compositions different from the chemical composition of the steel of the invention, said samples are steels A-E, steel F is the steel of the invention. Table 1 shows the chemical compositions in percentage by weight:
Table 1
| C | Mn | Si | P | S | Cr | Ni | Mo | V | Cu | Al | |
| A | .17 | .51 | .29 | .008 | .006 | 1.00 | 3.30 | .25 | .014 | .30 | .032 |
| B | .27 | .24 | .12 | .006 | .004 | 1.80 | 1.70 | .59 | .130 | .14 | .005 |
| C | .20 | .81 | .34 | .009 | .007 | 1.53 | 2.72 | .38 | .004 | .23 | .031 |
| D | .26 | 1.45 | .18 | .015 | .003 | 1.28 | 1.06 | .69 | .130 | .24 | .012 |
| E | .27 | .42 | .25 | .011 | .005 | 2.03 | 2.04 | .36 | .010 | .22 | .007 |
| F | .24 | .64 | .24 | .009 | .005 | 1.58 | 2.77 | .49 | .094 | .17 | .028 |
[0061] All these steels have been subjected to quench hardening and tempering treatments in different conditions for the purpose of achieving the most optimal combination of mechanical strength and toughness for each of them.
[0062] Thus, the most optimal results achieved are shown in Table 2.
Table 2
| Strength (N/mm2) | Elastic limit (N/mm2) | KV at -20ºC (J) | |
| A | >1150 | >1100 | ≈ 60 |
| B | >1100 | >1000 | ≈ 60 |
| C | >1100 | >1000 | ≈ 60 |
| D | >1100 | >1000 | ≈ 55 |
| E | >1200 | >1100 | ≈ 55 |
| F | >1200 | >1100 | ≈ 70 |
[0063] As shown in Table 2, steels A, B, C, D and E do not reach a strength of 1200 N/mm2, maintaining a KV resilience at -20ºC of 60 J.
[0064] Steels A and C have low carbon and vanadium contents, such that with the toughness demanded at low temperature, only strength values of about 1100 N/mm2 are achieved.
[0065] In turn, steels B, D and E, despite having a greater carbon content, do not achieve the desired levels of strength, since the combination of alloying elements is not the suitable one for reaching the demanded mechanical characteristics.
[0066] Figure 3 shows the KV resilience values at -20ºC obtained with a strength of 1200 N/mm2 for the different steels A-F.
[0067] However, for steel F, having a chemical composition within the limits object of the invention, i.e., it is the steel proposed by the invention, it has been verified that after being subjected to a quench hardening and tempering treatment it reaches the required mechanical characteristics and furthermore has no welding problem.
1. Quench hardened and tempered steel, with high tensile strength and high impact strength,
characterized in that it comprises the following elements with a percentage by weight:
2. Quench hardened and tempered steel according to claim 1, characterized in that it comprises at least one of the following elements with a percentage by weight:
3. Quench hardened and tempered steel according to claim 2, characterized in that it comprises at least one of the following elements with a percentage by weight:
4. Quench hardened and tempered steel according to claim 1, characterized in that it comprises the following elements with a percentage by weight:
5. Quench hardened and tempered steel according to any of claims 2 and 4, characterized in that it comprises at least one of the following elements with a percentage by weight:
6. Quench hardened and tempered steel according to any of the previous claims, characterized in that it comprises at least one of the following elements with a percentage by weight:
7. Quench hardened and tempered steel according to any of the previous claims, characterized in that it has mechanical tensile strength greater than or equal to approximately 1200 N/mm2 and KV resilience at -20ºC greater than or equal to approximately 60 J.
8. Method for obtaining parts of a quench hardened and tempered steel according to any
of claims 1 to 7, characterized in that it comprises obtaining said steel by means of the following steps:
- controlling the raw materials of the furnace,
- using between 30% and 50% of maximum quality scrap,
- performing an oxidizing period in an electric furnace, prior to the foamed slag,
- deslagging the foamed slag until a presence of phosphorus less than 0.007% by weight,
- tilting with standard temperature and parts per million (ppm) of 0, according to the clean steel standard, ensuring that slag does not pass from the furnace to the ladle,
- deoxidizing with Al, to obtain very fluid white slag with a lime-spar base,
- controlling the refining raw materials,
- performing two vacuums with an intermediate H sample, considering the vacuum time as that which is below 2 mbar,
- ending the second vacuum with a sufficient temperature to perform an inclusion decanting process after it and stirring the melt with argon to break the slag and without performing additions or heating, and
- tapping with special protection of the stream.
9. Method for obtaining parts of quench hardened and tempered steel according to claim
8, characterized in that after obtaining the steel it comprises the following steps:
- manufacturing a part of said steel,
- performing in the part a quench hardening heat treatment which is performed with an austenization at a temperature greater than 800ºC, followed by a subsequent cooling, and
- performing in the part a tempering treatment which is carried out at a temperature greater than 550ºC for about 2 hours.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description