CN117568703A - Hot stamping part with excellent low-temperature brittleness resistance and manufacturing method thereof - Google Patents

Hot stamping part with excellent low-temperature brittleness resistance and manufacturing method thereof Download PDF

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CN117568703A
CN117568703A CN202210940843.4A CN202210940843A CN117568703A CN 117568703 A CN117568703 A CN 117568703A CN 202210940843 A CN202210940843 A CN 202210940843A CN 117568703 A CN117568703 A CN 117568703A
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temperature
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hot stamped
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刘浩
谭宁
金鑫焱
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Baoshan Iron and Steel Co Ltd
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Baoshan Iron and Steel Co Ltd
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Priority to CN202210940843.4A priority Critical patent/CN117568703A/en
Priority to PCT/CN2023/111404 priority patent/WO2024032528A1/en
Publication of CN117568703A publication Critical patent/CN117568703A/en
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Abstract

The invention discloses a hot stamping part with excellent low-temperature brittleness resistance, which contains Fe, unavoidable impurities and the following chemical elements in percentage by mass: c:0.26 to 0.40 percent, si:0.1 to 1.5 percent, mn+Cr:0.5 to 3.0 percent, al:0.01 to 0.50 percent, nb+Ti:0.04 to 0.25 percent, mo+Ni:0.1 to 1.0 percent, B:0.001 to 0.005 percent; the matrix of the hot stamped component microstructure has a lath-shaped tempered martensite of greater than 85% by volume; the microstructure of the hot stamping part also has a precipitated phase, wherein the precipitation amount of Nb and Ti accounts for more than 50% of the total mass fraction of the Nb and Ti elements, and the precipitated phase of Nb and Ti is in granular dispersion distribution in a matrix. Correspondingly, the invention also discloses a manufacturing method of the hot stamping part.

Description

Hot stamping part with excellent low-temperature brittleness resistance and manufacturing method thereof
Technical Field
The present invention relates to a hot-stamped component and a method for manufacturing the same, and more particularly, to a hot-stamped component resistant to low-temperature brittleness and a method for manufacturing the same.
Background
In recent years, with the increasing amount of global automobile maintenance, environmental pollution and energy consumption caused by the global automobile maintenance become more serious, and the automobile weight reduction technology can effectively alleviate such problems, so that a great deal of research is being conducted on automobile weight reduction by automobile manufacturers at present.
The research shows that when the ultra-high strength steel is adopted to prepare the automobile structure, the aim of reducing the weight of the automobile can be achieved on the premise of ensuring the safety. However, when an ultra-high strength steel product with tensile strength higher than 1180MPa is actually prepared, the cold forming difficulty is extremely high, and the rebound control capability is insufficient; the part prepared by hot stamping has the characteristics of ultrahigh strength, easiness in forming, high dimensional accuracy and the like, so that the hot-formed ultrahigh strength steel product is one of important technical solutions for lightening the vehicle body gradually.
Currently, with the gradual upgrade of the crash safety regulations, the market demand for hot stamping steels having tensile strengths exceeding 1700MPa is also increasing. However, the research shows that the steel products for hot stamping with the relevant tensile strength exceeding 1700MPa have quite insufficient performance in terms of bending and impact performance, especially the low-temperature state of minus 60 ℃ is poor, which leads to the risk of brittle fracture of hot stamping parts in service in extremely cold regions, and the hot stamping parts can cause immeasurable loss of life and property safety of users.
For this reason, in order to ensure the service life of the hot stamped part in an extremely low temperature environment, it is also necessary to consider the low temperature brittleness resistance of the material when preparing the hot stamped part that is extremely high in strength. The presently employed VDA238-100 bend performance detection is relatively close to the actual component crash failure mode, and therefore evaluation of low temperature VDAs (bend angles) is particularly important.
However, there are various disadvantages in terms of toughness obtained by the hot stamping steel of 1700MPa strength level, and the problem of low-temperature brittleness under the plane deformation condition has not been paid attention to and described in the related studies.
For example: chinese patent document with publication No. CN110423953a, publication No. 2019, 11/8, entitled "a thermoformed component with excellent bending properties at a tensile strength of 1800MPa or more" discloses a thermoformed component with excellent bending properties at a tensile strength of 1800MPa or more, comprising the following chemical components in weight percentage: c:0.29-0.35%, si less than or equal to 0.5%, mn:0.5-1.5%, P is less than or equal to 0.020%, S is less than or equal to 0.010%, cr is less than or equal to 0.50%, al:0.01-0.06%, nb:0.01-0.06%, V:0.01-0.06%, mo is less than or equal to 0.5%, and the balance is Fe and unavoidable impurities; the surface layer of the hot forming component is a soft ferrite structure, and the inner layer is a martensite structure. In this solution, the bending properties of the material are improved by means of the surface softening phase, but the surface softening inevitably leads to a reduction in the collision deformation resistance of the component, and the solution does not mention bending properties at low temperatures.
For another example: chinese patent document with publication number CN106460115a, publication date 2017, 2 and 22, entitled "heat treated steel and method for producing same", discloses a steel having tensile strength of 1.8GPa or lessThe heat-treated steel material having excellent toughness and weldability comprises the following chemical components: c:0.05 to 0.30 percent of Mn:2.0 to 10.0 percent of Cr, 0.01 to 1.0 percent of Ti:0.01% -0.1%, B:0.001% -0.01%, si: below 0.08, P: less than 0.05%, S: less than 0.05%, N: less than 0.01%, ni:0% -2.0%, cu, mo, V:0 to 1.0 percent of Fe and unavoidable impurities in balance; when the equivalent of C is [ C]Let Mn equivalent be [ Mn ]]When "4612× [ C ] is satisfied]+102×[Mn]+605 is greater than or equal to 1800). The microstructure of the heat-treated steel material has a martensite structure with a volume ratio of 90% or more, and the dislocation density in martensite is 9.0X10 15 m -2 The above. In the technical scheme, the high reinforcement of the part is realized by means of low carbon and high manganese, but the steel making difficulty is high due to the design of higher manganese, and the toughness is deteriorated due to segregation defects.
In summary, the existing hot stamping steel with the tensile strength of 1700MPa or more mainly obtains ultra-high strength and room temperature toughness, and is not related or relevant to the aspect of resisting low-temperature bending brittle fracture. Accordingly, in order to solve the above-described problem of brittle fracture of the ultra-high strength hot stamped part existing in the current extremely cold region, the inventors have desired to provide a new hot stamped part excellent in low-temperature brittleness resistance and a method for manufacturing the same, so as to effectively meet market demands.
Disclosure of Invention
One of the objects of the present invention is to provide a hot-stamped part excellent in low-temperature brittleness resistance, which has an ultra-high strength and excellent low-temperature brittle fracture resistance, has a room-temperature tensile strength of more than 1700MPa, and has a bending property ratio at-60 ℃ low temperature to room temperature (20 ℃) of more than 0.85 (i.e., a bending angle ratio of-60 ℃ to 20 ℃) and a low-temperature strength and toughness product at-60 ℃ (bending angle of room-temperature tensile strength x-60 ℃) of not less than 8 x 10 4 . The hot stamping part can effectively solve the problem of brittle fracture of the ultrahigh-strength hot stamping part in the extremely cold region at present, has good application prospect, and can be widely applied to industries such as automobiles, ships, machinery and the like.
In order to achieve the above object, the present invention proposes a hot stamping part excellent in low-temperature brittleness resistance, which contains Fe and unavoidable impurities, and which further contains the following chemical elements in mass percent:
C：0.26～0.40％，Si：0.1～1.5％，Mn+Cr：0.5～3.0％，Al：0.01～0.50％，Nb+Ti：0.04～0.25％，Mo+Ni：0.1～1.0％，B：0.001～0.005％；
the matrix of the hot stamped component microstructure has a lath-shaped tempered martensite of greater than 85% by volume;
the microstructure of the hot stamping part also has a precipitated phase, wherein the precipitation amount of Nb and Ti accounts for more than 50% of the total mass fraction of the Nb and Ti elements, and the precipitated phase of Nb and Ti is in granular dispersion distribution in a matrix.
Further, in the hot stamping part with excellent low-temperature brittleness resistance, the mass percentage of each chemical element is as follows:
c:0.26 to 0.40 percent, si:0.1 to 1.5 percent, mn+Cr:0.5 to 3.0 percent, al:0.01 to 0.50 percent, nb+Ti:0.04 to 0.25 percent, mo+Ni:0.1 to 1.0 percent, B:0.001 to 0.005 percent; the balance being Fe and other unavoidable impurities.
In the hot stamping part with excellent low-temperature brittleness resistance, the design principle of each chemical element is as follows:
c: in the hot stamped part excellent in low temperature brittleness resistance according to the present invention, C is a key element for achieving ultra-high strength of the steel for hot stamping, and when the content of C element in the steel is less than 0.26%, it is difficult to achieve 1700MPa strength of the hot stamped part. However, it should be noted that the C content in the steel is not too high, and as the C content in the steel increases, the low-temperature bending property and the welding property of the hot stamped part are significantly deteriorated. Therefore, in the hot stamped member excellent in low-temperature brittleness resistance according to the present invention, the mass percentage of the C element is specifically controlled to be between 0.26 and 0.40% in consideration of the influence of the C element content on the hot stamped member performance.
Si: in the hot stamped component with excellent low-temperature brittleness resistance, a certain amount of Si element is added, so that the strength of the hot stamped component can be effectively improved, and the deoxidization of steel is facilitated. It has been found that when the Si element content in the steel for hot stamping is 0.1% or less, the deoxidizing effect is poor; however, when the Si element content in the steel for hot stamping exceeds 1.5%, it affects the platability of the steel sheet. Therefore, in order to exert the beneficial effects of the Si element, in the hot stamped member of the present invention that is excellent in low-temperature brittleness resistance, the mass percentage of the Si element is controlled to be 0.1 to 1.5%.
Mn, cr: in the hot stamping part with excellent low-temperature brittleness resistance, a certain amount of Mn and Cr are added, so that the hardenability of the steel for hot stamping can be effectively improved, the strength and the hardness of the steel are improved, and the effects of the Mn and the Cr are similar. It has been found that when Mn+Cr in the hot stamping steel is less than 0.5%, the contribution of Mn and Cr elements to the strength of the steel is low, and the object of the invention cannot be achieved; and when Mn+Cr in the steel for hot stamping is more than 3.0%, it deteriorates manufacturability and weldability of the steel. Therefore, in the hot stamping part with excellent low-temperature brittleness resistance, the sum of Mn and Cr elements in percentage by mass is controlled to be between 0.5 and 3.0 percent.
Al: in the hot stamped member excellent in low-temperature brittleness resistance described in the present invention, al is used as a deoxidizing element, which can play a deoxidizing role. Therefore, in order to ensure that the Al element exerts its own deoxidizing effect, 0.01% or more of Al is added to the hot stamping steel. However, it should be noted that the content of Al element in the steel for hot stamping is not too high, and when the steel contains a large amount of Al, coarse inclusions are formed in the steel, and the manufacturability is deteriorated. Therefore, in the hot stamped member excellent in low-temperature brittleness resistance according to the present invention, the content of Al element is controlled to be 0.01 to 0.50% by mass.
Nb, ti: in the hot stamping part with excellent low-temperature brittleness resistance, nb and Ti are important microalloy elements in steel, part of Nb and Ti can play a solid solution strengthening role, and in addition, nb, ti and C, N all have extremely strong binding force and can combine and separate out stable carbide, nitride and carbonitride, so that the growth of austenite grains in hot stamping heating is inhibited, and the grain refinement effect is realized; in addition, the precipitated phase formed by the combination can also be used as a hydrogen trap, which can reduce the hydrogen induced delayed cracking sensitivity of the steel for hot stamping and remarkably improve the low-temperature toughness of the steel. The Ti element is also an effective element for deoxidizing and fixing nitrogen, which not only can reduce oxide inclusion in steel, but also can avoid BN formed by combination of B and N. Therefore, in order to exert the beneficial effects of Nb and Ti elements, in the hot stamping member according to the present invention, the sum "nb+ti" of the mass percentages of Nb and Ti elements is specifically controlled to be 0.04 to 0.25%. When the content of Nb+Ti is lower than 0.04%, the quantity of the formed precipitated phases is insufficient, and the beneficial effects are limited; when the content of Nb+Ti is more than 0.25%, the effect is saturated, and a bulk nitride is easily formed, and the low-temperature bending property of the product is deteriorated.
Mo, ni: in the hot stamping part with excellent low-temperature brittleness resistance, mo and Ni elements can obviously improve the bending performance of the hot stamping part, and especially improve the low-temperature brittleness resistance. Among them, ni element can lower the ductile-brittle transition temperature of steel for hot stamping, which is of great importance for improving the bending toughness of steel. The Mo element can obviously improve the hardenability of the steel, and the carbide of the Mo element not only can refine austenite grains, but also can inhibit the brittleness of tempered martensite, so that the strength and the toughness of the hot stamping part can be comprehensively improved. Therefore, the hot stamping part has excellent low-temperature cracking resistance by adding a certain amount of Mo and Ni, but the alloy cost and the element effect saturation degree of the steel are comprehensively considered, and the sum of the mass percentages of Mo and Ni elements is specifically controlled to be 0.1-1.0%.
B: in the hot stamping part with excellent low-temperature brittleness resistance, the hardenability of steel can be greatly improved by adding a certain amount of B element. However, it should be noted that the B element content in the steel has an optimum range section, and when the B element content in the steel is higher than a certain amount, the effect of increasing hardenability is not significant. For this reason, in the present invention, the content of the element B is specifically controlled to be 0.001 to 0.005% by mass.
The hot stamping part obtained by quenching the steel for medium-high carbon hot stamping has higher dislocation density and internal transformation stress, is easy to form twin martensite, is hard and brittle, and has obviously lower bending property than lath martensite. Although the hot stamping quench to the initial martensitic portion formed in the high temperature zone may undergo autotempering to form tempered martensite and precipitate carbides, the overall performance of the component is still poor. Therefore, the invention studies that the formation of twin martensite in the matrix is suppressed by the composition and the process control, and the matrix thereof has lath tempered martensite with a volume ratio higher than 85%.
Research shows that in the invention, the sizes of the Nb, ti, mo and other element precipitated phases have obvious influence on the bending performance of the hot stamping part, so that the whole process is required to be optimized to promote the Nb+Ti precipitated proportion to be more than 50% of the total mass fraction, and the sizes of the precipitated phases are reduced to be in dispersion distribution. Specifically, the method for detecting the precipitation amount of Nb and Ti comprises the steps of obtaining residues of precipitated carbides, nitrides and carbonitrides by adopting a chemical method and carrying out electrolytic extraction, and then determining the precipitation mass fraction of Nb and Ti by using an inductively coupled plasma test method.
Further, in the hot stamped component excellent in low-temperature brittleness resistance according to the present invention, among unavoidable impurities, P is 0.03% or less, S is 0.01% or less, N is 0.004% or less, and O is 0.004% or less.
In the above technical scheme, the P element, the S element, the N element, the O element and the H element are all impurity elements in steel, and in order to obtain steel with better performance and better quality, the content of the impurity elements in the material should be reduced as much as possible under the condition of technical conditions.
P, S: in the invention, P and S are both harmful elements, and the segregation of the P element can lead to the phenomenon of cold embrittlement of steel; however, sulfide such as S element segregation and MnS reduces the toughness of the steel, and causes hot embrittlement at high temperatures. The invention aims to improve the low-temperature brittleness resistance of a hot stamping part, and the mass percentage of P, S elements in steel must be strictly controlled, and the mass percentage is specifically controlled as follows: p is less than or equal to 0.03 percent, S is less than or equal to 0.01 percent.
N: in the present invention, N is an impurity element in hot stamping steel, and has a strong affinity with elements such as Ti, al, and B, and TiN, alN, BN produced by the combination is a hard phase inclusion, which is a source of brittle fracture. Therefore, the N content in the steel for hot stamping needs to be strictly controlled, and the N element content is ensured to be less than or equal to 0.004%, and preferably, the N content can be further controlled to be less than or equal to 0.003%.
O: in the invention, the O element is extremely easy to form oxide inclusion with nonmetal in steel, which can obviously deteriorate the bending property of the steel, so that the steelmaking deoxidization process must be strictly controlled, and the mass percent of the O element is ensured to be controlled to be less than or equal to 0.004 percent. Of course, in some preferred embodiments, O.ltoreq.0.0025% may be further controlled.
Further, in the hot stamped component of the present invention that is excellent in low temperature brittleness resistance, N is 0.003% or less and/or O is 0.0025% or less.
Further, in the hot stamped component excellent in low-temperature brittleness resistance according to the present invention, the chemical elements thereof further include at least one of the following: cu:0.01 to 1.0 percent, W:0.01 to 0.5 percent, V:0.01 to 0.5 percent.
In the above-described technical solution of the present invention, in order to further optimize the performance of the produced hot stamped part, cu, W, V elements may be further preferably added in designing the chemical composition of the hot stamped part.
Cu, W, V: in the hot stamping part with excellent low-temperature brittleness resistance, trace Cu elements can effectively refine grains, improve the toughness of steel for hot stamping, and Cu has the effect of improving the corrosion resistance of materials; the elements W and V in the steel are refined grains through the fine carbonitride precipitation form, so that the toughness of the steel for hot stamping can be effectively improved. However, considering the alloy cost and the element effect saturation of steel in combination, in the present invention, it is particularly preferable to add one or more of Cu, W and V, and to specifically control the content thereof so as to satisfy: cu:0.01 to 1.0 percent, W:0.01 to 0.5 percent, V:0.01 to 0.5 percent.
Further, in the hot stamped component excellent in low-temperature brittleness resistance according to the present invention, the precipitated phase of the Ti element includes TiN, the aspect ratio value of TiN is less than 6, and the length is calculated in terms of area ratioTiN with axis greater than 2 μm has a density of less than 50/mm 2 。
In the present invention, it has been found that N element content in hot stamped parts affects TiN shape, size and quantity, long-strip TiN particularly having a string-like morphology and TiN inclusions having a long axis of more than 2 μm significantly deteriorate low-temperature bending properties, and therefore the quantity thereof needs to be controlled to be less than 50 pieces/mm 2 。
Further, in the hot stamped steel excellent in low-temperature brittleness resistance according to the present invention, the particle diameter of TiC, nb (C, N), moC in the precipitated phase is less than 80nm.
Further, in the hot stamped component excellent in low-temperature brittleness resistance according to the present invention, the microstructure of the substrate further has one or more of ferrite, pearlite, bainite, martensite, and retained austenite.
Further, in the hot stamping part with excellent low-temperature brittleness resistance, the tensile strength is more than 1700MPa, the bending performance ratio of the hot stamping part at the low temperature of minus 60 ℃ to the room temperature is more than 0.85, and the strength and toughness product at the low temperature of minus 60 ℃ is not less than 8 multiplied by 10 4 。
In the present invention, the bending property of the hot stamping part at a low temperature of-60 ℃ and the bending property at room temperature (20 ℃) are detected by referring to the VDA238-100 detection method. When the bending performance of minus 60 ℃ is detected by a low-temperature test, a sample is specifically placed into an alcohol cooling tank to be cooled to a target temperature, the temperature is kept for 20min, the sample is quickly transferred to VDA bending equipment, and the bending test is ensured to be completed within 1min, so that the corresponding bending performance of minus 60 ℃ at low temperature is obtained.
Further, in the hot stamped member excellent in low-temperature brittleness resistance according to the present invention, the diffusion H content in the hot stamped member is less than 0.15ppm.
Accordingly, another object of the present invention is to provide a method for manufacturing the hot stamping part with excellent low temperature brittleness resistance, which is simple to produce and reasonable in process design, and the obtained hot stamping part has the characteristics of excellent low temperature brittleness resistance cracking while having ultrahigh strength, has good application prospect, and can be widely applied to industries such as automobiles, ships, machinery, etc.
In order to achieve the above object, the present invention proposes a method for manufacturing the above-mentioned hot stamped component excellent in low-temperature brittleness resistance, comprising the steps of:
(1) Smelting and casting;
(2) Hot rolling, coiling and acid washing: wherein the temperature of the plate blank discharged from the heating furnace is controlled to be 1220-1280 ℃ and the finishing temperature is controlled to be 880-940 ℃; controlling the coiling temperature to be 580-680 ℃, and cooling to below 300 ℃ at the speed of 0.3-1 ℃/s after coiling;
(3) Cold rolling and annealing: controlling the total rolling reduction of the cold rolling to be 30-60%; controlling the annealing temperature to 680-750 ℃, and cooling to room temperature at an average speed of 1-15 ℃/s after annealing;
(4) Hot stamping and forming: controlling the reheating temperature of the steel plate to 850-950 ℃ and the reheating time to 2-10 min, and then rapidly transferring the steel plate to a die to finish hot stamping; then cooling to 100-200 ℃ at an average speed of 10-100 ℃/s, and then air cooling to room temperature;
(5) Low temperature tempering: preserving heat for 10-40 min in a baking oven at 150-250 ℃, and then taking out and cooling to room temperature.
In the above technical scheme of the invention, in the smelting process of the step (1), the chemical components designed according to the invention are required to be smelted, and the slab is further cast after denitrification, refining and deoxidization of the converter.
The invention can take away partial nitrogen in the process of smelting oxygen blowing decarburization, and prolong the argon blowing time in the later period, so that the content of N element in steel can be effectively ensured to be lower than 0.004%, more preferably lower than 0.003%; correspondingly, the subsequent refining deoxidation can promote the vacuum reaction of C and O, and the later Al addition is carried out for stabilization, so that the content of O element in the steel can be reduced to less than or equal to 0.004%, and more preferably, the content of O is less than or equal to 0.0025%.
Correspondingly, in the step (2) of the manufacturing method, the tapping temperature of the hot rolled slab is higher to ensure the sufficient dissolution of the microalloying elements so as to be convenient for the sufficient precipitation in the subsequent hot rolling coiling process, and the tapping temperature of the slab out of the heating furnace is controlled between 1220 and 1280 ℃ and the finishing temperature is controlled between 880 and 940 ℃ on the basis of combining the precipitation quantity and avoiding coarsening of a precipitation phase. In addition, the inventors' studies indicate that: in the coiling process, the coiling temperature T is controlled to be between 580 and 680 ℃, so that a precipitation phase is easy to generate. And the steel coil is cooled at a speed of 0.3-1 ℃/s by adopting a reduced cooling rate after coiling, so that the steel coil can be ensured to be between coiling temperature T-300 ℃ for a long time, and the precipitation of carbide, nitride and carbonitride of microalloy elements such as Nb and Ti is further promoted.
In step (3) of the production method of the present invention, it is necessary to control the total rolling reduction to 30% to 60%. If the total rolling reduction of the steel plate is less than 30%, the grain refinement effect of the steel plate is not obvious; if the total cold rolling reduction of the steel plate is more than 60%, the residual stress in the steel plate is larger, the strip structure is more, the subsequent production is not favored, and the toughness of the subsequent hot stamping part is remarkably deteriorated by the strip structure.
In addition, in the step (3), the annealing has an effect of improving the non-equilibrium structure of the rolled steel sheet, which can reduce element segregation such as the strip structure C, mn. In the invention, the annealing temperature is controlled between 680 and 750 ℃, which is beneficial to optimizing the structural uniformity of the components of the steel plate; after annealing, the alloy is cooled to room temperature at an average cooling rate of 1-15 ℃/s, so that on one hand, partial micro-alloy elements can be promoted to be separated out, and in addition, the strip-shaped structure and the component segregation formed by rapid cooling can be avoided.
In the present invention, the annealed steel sheet is subjected to the hot press forming treatment of step (4). In the invention, the steel plate is particularly reheated to 850-950 ℃ and the heating time is controlled to be 2-10 min, and the above-mentioned intervals can ensure the steel plate to be fully austenitized and avoid the coarsening of austenite grains.
Correspondingly, after heating, the steel plate needs to be quickly transferred to a die to finish hot stamping, and then is cooled to 100-200 ℃ at an average speed of 10-100 ℃/s. Wherein, when the average cooling rate is lower than 10 ℃/s, the strength of the prepared hot stamping part cannot be ensured; when the average cooling rate is higher than 100 ℃/s, there is the following problem: firstly, the generation of twin martensite caused by rapid cooling; secondly, the martensite dislocation density is higher and the internal stress is larger; thirdly, the generation of precipitated phases is not facilitated. All three of the above points deteriorate the low-temperature bending property of the hot stamped part produced. In addition, when the prepared hot stamped part is cooled to below 200 ℃, the hot stamped part is already below the low-carbon martensite transformation end point, and if the final cooling temperature is below 100 ℃, the hot stamped part is favorable for the formation of medium-high-carbon twin martensite. Thus, in the present invention, cooling is specifically controlled to between 100 and 200 ℃.
In the invention, the hot stamping part prepared after the hot stamping treatment is finished also needs to be tempered at a low temperature of 150-250 ℃ in a low-temperature baking furnace, and the tempering and heat-preserving time is controlled to be 10-40 min, and the main purposes are as follows: the proportion of martensite of high dislocation density is reduced to ensure that the proportion of lath-like tempered martensite of the final resulting finished hot-stamped part is higher than 85%.
It should be noted that, during tempering, the discharge of supersaturated carbon in martensite promotes precipitation of carbides, including but not limited to micro-alloyed carbide precipitated phases, and that, in addition, the H element of the present invention is a detrimental element, and the aggregation of free diffused H in hot stamped parts may deteriorate toughness and increase the risk of cracking. And the diffusion H in the hot stamping part is discharged through low-temperature tempering treatment, so that the diffusion H content of the finished hot stamping part is less than or equal to 0.15ppm, and the risk of brittle fracture is reduced, thereby further improving the low-temperature bending performance of the hot stamping part.
Of course, in specific implementation, the low-temperature tempering process designed by the invention is not limited to being finished in a baking oven off-line, but can be finished on-line through die induction heating or on-line furnace heat treatment. In addition, the tempering heat preservation time is controlled to be between 10 and 40 minutes specifically because: when the low-temperature tempering time is too short and less than 10min, the short-time tempering cannot achieve the effects; when the low-temperature tempering time is too long and is higher than 40 minutes, the high-temperature tempering for a long time can enter a tempering brittleness interval while reducing the strength, and the strength and the low-temperature bending performance of the final finished hot stamping part can be reduced.
Further, in the manufacturing method according to the present invention, the step of coating the steel sheet with a plating layer or a coating layer is further included after the annealing step of the step (3).
In the technical scheme designed by the invention, the hot stamped steel plate obtained after annealing is an uncoated bare plate, and the steel plate can be further subjected to a step of coating or coating, so that the heating iron scale of the steel plate is reduced and the corrosion resistance of the steel plate is improved.
Compared with the prior art, the hot stamping part with excellent low-temperature brittleness resistance and the manufacturing method thereof have the following advantages and beneficial effects:
the invention develops the hot stamping part with excellent low-temperature brittleness resistance through reasonably designing chemical components and combining with an optimization process, and the hot stamping part has the characteristics of ultrahigh strength and excellent low-temperature brittleness resistance cracking resistance. The hot stamping part has tensile strength of 1700MPa or more, bending performance ratio of-60deg.C to room temperature (20deg.C) of 0.85 or more, and low temperature strength and toughness product (room temperature tensile strength x-60deg.C bending angle) of-60deg.C of 8×10 or more 4 . The hot stamping part can effectively solve the problem of brittle fracture of the ultrahigh-strength hot stamping part in the extremely cold region at present, has good application prospect, and can be widely applied to industries such as automobiles, ships, machinery and the like.
In order to achieve the above advantageous effects, the inventors have made the following improvements:
1. the inventors have improved the chemical composition and process design of hot stamped parts by tightly controlling the N and O content in the steel to inhibit TiN, alN, BN large scale nitride inclusion formation, thereby reducing nonmetallic oxide inclusions in the steel, which are the cracking source points for brittle fracture, which can significantly deteriorate low temperature bending properties. However, the precipitation of hard inclusions such as TiN is unavoidable, so that the invention needs to reduce the content in unit area and simultaneously adopts technological measures to inhibit the growth of the inclusions so as to control the length-axis ratio value of the TiN to be less than 6, andTiN with a major axis greater than 2 μm has a density of less than 50 pieces/mm calculated as area ratio 2 。
2. The inventor can effectively promote Nb+Ti precipitation by controlling hot rolling and annealing processes so that the precipitation amount of Nb and Ti accounts for more than 50 percent of the total mass fraction of the two elements, and the precipitation phases of the Nb and Ti can play a role in inhibiting the increase of hot stamping reheating austenite grains and refining the grains, thereby effectively improving the low-temperature bending performance; in addition, the precipitation phases of Nb and Ti elements can also reduce the risk of hydrogen embrittlement. Therefore, the invention can effectively control the sizes of Nb and Ti precipitated phases while meeting the precipitation quantity, so that the particle diameters of TiC, nb (C, N) and MoC in the precipitated phases are smaller than 80nm, thereby forming dispersed nano-scale precipitated phases and further improving the low-temperature bending performance.
3. According to the invention, the precipitation of microalloy carbide is promoted, the content of matrix C is reduced, and the formation of twin martensite can be inhibited by controlling the quenching cooling rate and the cooling end temperature. In the tempering process, the hot stamping part is also subjected to low-temperature tempering in particular so as to ensure that the volume proportion of lath tempered martensite in the final microstructure is higher than 85%, thereby reducing the dislocation density of martensite and improving the low-temperature bending property.
4. The invention can prevent the low-temperature tempering brittleness and inhibit the low-temperature crack expansion and effectively improve the low-temperature bending performance of the hot stamping part by adding a small amount of Mo and Ni.
Drawings
Fig. 1 is a photograph of a microstructure of a hot stamped part of example 6 under a scanning electron microscope.
Detailed Description
The hot stamping part excellent in low-temperature brittleness resistance and the method of manufacturing the same according to the present invention will be further explained and illustrated with reference to specific examples, but the explanation and illustration do not constitute undue limitations on the technical solution of the present invention.
Examples 1 to 10
The hot stamped parts of examples 1-10 were each made using the following steps:
(1) Smelting and casting were performed according to the chemical compositions shown in the following table 1: in actual operation, smelting is carried out according to the chemical composition designed in the table 1, and then denitrification, refining deoxidation and casting are carried out through a converter to obtain a slab.
(2) Hot rolling, coiling and acid washing: carrying out hot rolling on a slab obtained by casting, controlling the temperature of the slab out of a heating furnace to be 1220-1280 ℃ and controlling the final rolling temperature to be 880-940 ℃; coiling after hot rolling, controlling the coiling temperature to be 580-680 ℃, and cooling to below 300 ℃ at the speed of 0.3-1 ℃/s after coiling; and then pickling to obtain the hot-rolled plate blank.
(3) Cold rolling and annealing: cold rolling the hot rolled plate blank, and controlling the total rolling reduction of the cold rolling to be 30-60%; the cold rolled steel plate is further annealed, the annealing temperature is controlled to be 680-750 ℃, and the steel plate is cooled to room temperature at the average speed of 1-15 ℃/s after annealing; wherein, after the annealing is completed, the steel sheet may be further subjected to a step of plating or coating to obtain a plated or coated steel sheet.
(4) Hot stamping and forming: controlling the reheating temperature of the steel plate to 850-950 ℃ and the reheating time to 2-10 min, and then rapidly transferring the steel plate to a die to finish hot stamping; and then cooled to 100-200 ℃ at an average rate of 10-100 ℃/s, and then air-cooled to room temperature to obtain the corresponding hot stamped part.
(5) Low temperature tempering: and (3) inputting the prepared hot stamping part into a baking furnace at 150-250 ℃ for heat preservation for 10-40 min, and then taking out and air cooling to room temperature to obtain the finished hot stamping part with the final thickness of 1.4 mm.
In this technical solution according to the present invention, the chemical composition design and the related process of the hot stamping parts of examples 1 to 10 prepared as described above according to the present invention meet the design specification requirements of the present invention.
Table 1 lists the mass percentages of the chemical elements of the hot stamped parts of examples 1-10.
Accordingly, tables 2-1 and 2-2 list specific process parameters of the hot stamped parts of examples 1-10 in process steps (1) - (5) above.
Table 2-1.
Table 2-2.
In the present invention, after the completion of the above-mentioned manufacturing process, the hot stamped parts of the finished products of examples 1 to 10 were each sampled, and the microstructure of the hot stamped part samples of examples 1 to 10 was observed, and it was observed that the matrix of the microstructure of the hot stamped part samples of examples 1 to 10 had lath-shaped tempered martensite with a volume ratio higher than 85%; meanwhile, the microstructure of the hot stamped part of each example also has one or more of ferrite, pearlite, bainite, martensite, and retained austenite, and the specific observation and analysis results thereof are shown in table 3 below.
Accordingly, when the microstructure of the hot stamped part samples of examples 1 to 10 was observed, it was found that the hot stamped part microstructure also had a precipitated phase in which the precipitation amounts of Nb and Ti elements were 50% or more of the total mass fraction of these two elements, and the precipitated phases of Nb and Ti were dispersed in the matrix in the form of particles.
Further analysis of these precipitated phases revealed that in examples 1 to 10, the particle diameters of TiC, nb (C, N), and MoC in the precipitated phases were all smaller than 80nm; in the precipitated phase of Ti element, the precipitated phase may specifically comprise TiN, the length axis ratio of the TiN is less than 6, and the density of the TiN with the length axis of more than 2 μm calculated by area ratio is less than 50/mm 2 。
Table 3 sets forth the results of the observed analyses for the microstructures of the hot stamping assemblies of examples 1-10.
Table 3.
Accordingly, after the observation and analysis of the microstructure described above are completed, the hot stamped part prepared for further explanation of the present invention has very excellent mechanical properties. Based on the obtained hot stamped parts of examples 1 to 10, the inventors sampled the hot stamped parts of these examples again, and examined the hot stamped parts of examples 1 to 10 for strength and resistance to low temperature brittleness, and the results of the examination are shown in table 4 below.
In the present invention, the detection means employed for examples 1 to 10 are as follows:
tensile test: room temperature tensile strength was measured under the requirements of the GB/T228 metal material tensile test room temperature test method to measure room temperature tensile strength of the hot stamped parts of examples 1-10.
Bending performance test: the bending properties of the hot stamped parts of examples 1-10 at-60 c and room temperature (20 c) were tested with reference to the VDA238-100 test method, and when the bending properties at-60 c were tested by the low temperature test, the samples were placed in an alcohol cooling tank to cool to the target temperature, kept warm for 20min, and then rapidly transferred to the VDA bending apparatus, and the bending test was ensured to be completed within 1min to obtain the corresponding bending properties at-60 c. For the measured bending properties of the hot stamped parts of examples 1-10 at-60 c low temperature to room temperature (20 c), a bending property ratio (i.e., a ratio of-60 c bending angle to 20 c bending angle) can be further obtained.
Accordingly, based on the above-described test, the hot stamped parts of examples 1 to 10 could be further obtained with a low temperature strength product at-60 ℃ (room temperature tensile strength× -60 ℃ bending angle).
In addition, for the prepared hot stamped parts of examples 1-10, the content of diffusion H in the parts was measured using a brookfield G4 diffusion H analyzer, and the content of diffusion H in the hot stamped parts of each example was further measured.
Table 4 shows the mechanical properties of the hot stamped parts of examples 1-10.
Table 4.
As can be seen from Table 4 above, the hot stamping assemblies of examples 1 to 10 specifically prepared by the technical scheme of the present invention have ultra-high strength and excellent low temperature brittleness resistance characteristics, have tensile strength of 1729 to 2191MPa, have bending performance ratio of 0.86 to 0.93 at-60℃low temperature to room temperature (20 ℃) and have low temperature toughness of-60℃of specifically 8.8X10 4 -11.3×10 4 Between them.
Moreover, the final diffusion H content of the finished hot stamping components of examples 1-10 prepared by the technical scheme of the invention is less than or equal to 0.15ppm, and the final diffusion H content is particularly between 0.04 and 0.15ppm.
In summary, the hot stamping component produced according to the invention has the characteristics of ultrahigh strength and excellent low-temperature brittleness resistance, and the problem of brittle fracture of the ultrahigh-strength hot stamping component in the extremely cold region at present can be effectively solved by adopting the hot stamping component.
Fig. 1 is a photograph of a microstructure of a hot stamped part of example 6 under a scanning electron microscope.
As shown in fig. 1, fig. 1 shows the microstructure of the hot stamped component of example 6, which, as can be readily seen from fig. 1, has a lath tempered martensite ratio of higher than 85%.
It should be noted that the combination of the technical features in the present invention is not limited to the combination described in the claims or the combination described in the specific embodiments, and all the technical features described in the present invention may be freely combined or combined in any manner unless contradiction occurs between them.
It should also be noted that the above-recited embodiments are merely specific examples of the present invention. It is apparent that the present invention is not limited to the above embodiments, and similar changes or modifications will be apparent to those skilled in the art from the present disclosure, and it is intended to be within the scope of the present invention.

Claims (12)

1. A hot stamped component excellent in low-temperature brittleness resistance, which contains Fe and unavoidable impurities, characterized by further containing the following chemical elements in mass percent:
C：0.26～0.40％，Si：0.1～1.5％，Mn+Cr：0.5～3.0％，Al：0.01～0.50％，Nb+Ti：0.04～0.25％，Mo+Ni：0.1～1.0％，B：0.001～0.005％；
the matrix of the hot stamped component microstructure has a lath-shaped tempered martensite of greater than 85% by volume;
the microstructure of the hot stamping part also has a precipitated phase, wherein the precipitation amount of Nb and Ti accounts for more than 50% of the total mass fraction of the Nb and Ti elements, and the precipitated phase of Nb and Ti is in granular dispersion distribution in a matrix.
2. The hot stamped component excellent in low temperature brittleness resistance as claimed in claim 1, wherein the mass percentages of each chemical element are:
c:0.26 to 0.40 percent, si:0.1 to 1.5 percent, mn+Cr:0.5 to 3.0 percent, al:0.01 to 0.50 percent, nb+Ti:0.04 to 0.25 percent, mo+Ni:0.1 to 1.0 percent, B:0.001 to 0.005 percent; the balance being Fe and other unavoidable impurities.
3. The hot stamped component of claim 1 or 2, wherein among the unavoidable impurities, P is 0.03% or less, S is 0.01% or less, N is 0.004% or less, and O is 0.004% or less.
4. A hot stamped component having excellent resistance to low temperature embrittlement as claimed in claim 3 wherein N is equal to or less than 0.003% and/or O is equal to or less than 0.0025%.
5. The excellent low temperature brittleness resistant hot stamped component as set forth in claim 1 or 2, wherein the chemical elements thereof further comprise at least one of: cu:0.01 to 1.0 percent, W:0.01 to 0.5 percent, V:0.01 to 0.5 percent.
6. The hot stamped member excellent in low-temperature brittleness resistance as set forth in claim 1 or 2, wherein the precipitate phase of Ti element includes TiN having a long-to-short axis ratio of less than 6 and a density of less than 50 pieces/mm calculated as area ratio of TiN having a long axis of more than 2 μm 2 。
7. The hot stamped component of claim 1 or 2, wherein TiC, nb (C, N), moC in the precipitate phase have a particle diameter of less than 80nm.
8. The hot stamped component of claim 1 or 2, wherein the microstructure matrix further comprises one or more of ferrite, pearlite, bainite, martensite, and retained austenite.
9. The hot stamped component excellent in low temperature brittleness resistance as set forth in claim 1 or 2, wherein its tensile strength is greater than 1700MPa, its bending performance ratio at-60 ℃ low temperature to room temperature is greater than 0.85, and its low temperature toughness product at-60 ℃ is not less than 8 x 10 4 。
10. The hot stamped component of excellent low temperature brittleness resistance as set forth in claim 1 or 2, wherein the diffusion H content of the hot stamped component is less than 0.15ppm.
11. Method for manufacturing a hot stamped component with excellent resistance to low temperature brittleness according to any of claims 1 to 10, characterized in that it comprises the steps of:
(1) Smelting and casting;
(2) Hot rolling, coiling and acid washing: wherein the temperature of the plate blank discharged from the heating furnace is controlled to be 1220-1280 ℃ and the finishing temperature is controlled to be 880-940 ℃; controlling the coiling temperature to be 580-680 ℃, and cooling to below 300 ℃ at the speed of 0.3-1 ℃/s after coiling;
(3) Cold rolling and annealing: controlling the total rolling reduction of the cold rolling to be 30-60%; controlling the annealing temperature to 680-750 ℃, and cooling to room temperature at an average speed of 1-15 ℃/s after annealing;
(4) Hot stamping and forming: controlling the reheating temperature of the steel plate to 850-950 ℃ and the reheating time to 2-10 min, and then rapidly transferring the steel plate to a die to finish hot stamping; then cooling to 100-200 ℃ at an average speed of 10-100 ℃/s, and then air cooling to room temperature;
(5) Low temperature tempering: preserving heat for 10-40 min in a baking oven at 150-250 ℃, and then taking out and cooling to room temperature.
12. The method of manufacturing according to claim 11, further comprising a step of plating or coating the steel sheet after the annealing step of step (3).
CN202210940843.4A 2022-08-07 2022-08-07 Hot stamping part with excellent low-temperature brittleness resistance and manufacturing method thereof Pending CN117568703A (en)

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JP5145803B2 (en) * 2007-07-26 2013-02-20 Ｊｆｅスチール株式会社 Wear-resistant steel plate with excellent low-temperature toughness and low-temperature tempering embrittlement cracking properties
JP5257062B2 (en) * 2008-12-25 2013-08-07 新日鐵住金株式会社 High strength hot stamping molded article excellent in toughness and hydrogen embrittlement resistance and method for producing the same
US9745639B2 (en) * 2011-06-13 2017-08-29 Kobe Steel, Ltd. High-strength steel sheet excellent in workability and cold brittleness resistance, and manufacturing method thereof
JP6044576B2 (en) * 2014-03-31 2016-12-14 Ｊｆｅスチール株式会社 High-strength thin steel sheet excellent in formability and hydrogen embrittlement resistance and method for producing the same
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CN114107795B (en) * 2020-08-31 2023-05-09 宝山钢铁股份有限公司 1180MPa low-temperature tempered martensite high-reaming steel and manufacturing method thereof

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