A hot-formed steel and a method for producing the same

Abstract

The application provides a hot-formed steel and a preparation method thereof. The method comprises the following steps: S1, obtaining a billet with a set chemical composition; S2, heating the billet under a set temperature and a set time, then rolling and annealing to obtain a steel plate; S3, testing the high-temperature mechanical properties of the steel plate to obtain a high-temperature mechanical property model of the steel plate; S4, according to the high-temperature mechanical property model, obtaining a yield platform length L of the steel plate in the forming process; S5, judging whether the yield platform length L satisfies not more than a set yield platform length value, if yes, hot-forming the steel plate to obtain the hot-formed steel, if not, adjusting the set temperature and the set time in the step S2 according to the yield platform length L and a Larson-Miller parameter, and circulating the steps S2-S5 until the yield platform length L satisfies not more than the set yield platform length value, then hot-forming the steel plate to obtain the hot-formed steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hot forming steel preparation, and particularly relates to a hot forming steel and a preparation method thereof. BACKGROUND

[0002] The hot forming steel has high high-temperature forming capacity, and the strength of a part after cooling is high, and the application of the hot forming steel can improve the light weight level and the crash performance of an automobile and achieve energy saving and emission reduction.

[0003] The hot forming steel needs to be formed and controlled by a hot stamping process. In the hot stamping process, the material needs to be placed in a heating furnace and heated to a certain temperature and kept for a period of time. During the heating process, the heating furnace needs to be maintained at a certain temperature, and maintaining the high temperature will cause a certain energy consumption. In the hot stamping forming process, the stress-strain curve of the material has a certain yield platform. When the material increases with the strain, the stress increases, and when the stress increases to the yield strength, the stress does not continue to increase with the strain, but is maintained near the strength, forming a yield platform, and when the strain value continues to increase, the stress will exceed the yield platform and continue to increase. In order to exceed the yield platform, excessive increase of the heating temperature and excessive prolongation of the holding time will also cause waste of energy and even overburning. Therefore, it is urgent to develop a hot forming steel which can effectively control the yield platform length of the hot forming steel in the high-temperature forming process or avoid the yield platform to improve the formability. SUMMARY

[0004] The present application provides a hot forming steel and a preparation method thereof, which effectively controls the yield platform length of the hot forming steel in the high-temperature forming process or avoids the yield platform, so as to solve the technical problem of poor formability of the existing hot forming steel.

[0005] In a first aspect, the present application provides a preparation method of a hot forming steel, and the method comprises the following steps:

[0006] S1, obtaining a billet with a set chemical composition;

[0007] S2, heating the billet under a set temperature and a set time, then rolling and annealing to obtain a steel plate;

[0008] S3, performing high-temperature mechanical property testing on the steel plate to obtain a high-temperature mechanical property model of the steel plate;

[0009] S4, obtaining a yield platform length L of the steel plate in the forming process according to the high-temperature mechanical property model;

[0010] S5, judging whether the yield platform length L satisfies not greater than a yield platform length setting value, if yes, then hot forming the steel plate to obtain a hot formed steel; if no, then adjusting the setting temperature and the setting time in the step S2 according to the yield platform length L and a Larson-Miller parameter, and circulating the steps S2-S5 until the yield platform length L satisfies not greater than the yield platform length setting value, and then hot forming the steel plate to obtain a hot formed steel.

[0011] Optionally, the Larson-Miller parameter and the setting temperature and the setting time satisfy the following relationship: LMP=T(20+logt),

[0012] In the formula, LMP represents the numerical value of the Larson-Miller parameter, T represents the setting temperature, the unit of T is K, and t represents the setting time, the unit of t is h.

[0013] The numerical value of the Larson-Miller parameter LMP and the yield platform length L satisfy the following relationship:

[0014] If L=0, then LMP=29729;

[0015] If 0

[0016] If 0.02

[0017] If 0.04

[0018] If L>0.07, then LMP=30194.

[0019] Optionally, the yield platform length setting value is 0.02.

[0020] Optionally, the setting temperature is 1180-1220℃, and the setting time is 1.5-3h.

[0021] Optionally, the rolling temperature is 1160-1180℃, and the thickness of the rolled steel plate is 3-8mm.

[0022] Optionally, the annealing temperature is 550-650℃, and the holding time of the annealing is 0.5-1.5h.

[0023] Optionally, the high-temperature mechanical property test is specifically as follows: the temperature of the steel plate is heated to be greater than an austenitizing completion temperature AC3, is kept, is then decreased to a deformation temperature, and tensile deformation is performed at a deformation rate of 0.01s-1 to 10s-1 to obtain a high-temperature mechanical property model of the steel plate.

[0024] Optionally, the chemical composition includes C, Si, Mn, Cr, Al, B, S, P, Ti, Nb, Ni, Cu, Mo, V, N and Fe, and the content of each element is as follows: the content of C is 0.05% to 0.36%, the content of Si is 0.08% to 1.2%, the content of Mn is 5.0% to 9.0%, the content of Cr is ≤0.5%, the content of Al is 0.6% to 2.6%, the content of B is 0.001% to 0.005%, the content of S is ≤0.01%, the content of P is ≤0.01%, the content of Ti is ≤0.1%, the content of Nb is ≤0.1%, the content of Ni is ≤1.0%, the content of Cu is ≤0.5%, the content of Mo is ≤1.0%, the content of V is ≤0.25%, and the content of N is ≤0.02%.

[0025] C is 0.05% to 0.36%, Si is 0.08% to 1.2%, Mn is 5.0% to 9.0%, Cr is ≤0.5%, Al is 0.6% to 2.6%, B is 0.001% to 0.005%, S is ≤0.01%, P is ≤0.01%, Ti is ≤0.1%, Nb is ≤0.1%, Ni is ≤1.0%, Cu is ≤0.5%, Mo is ≤1.0%, V is ≤0.25%, and N is ≤0.02%.

[0026] In a second aspect, the present application provides a hot forming steel prepared by the method of any one of the first aspect, and the hot forming steel satisfies at least one of the following properties: the yield strength is 950MPa to 1300MPa, the tensile strength is 1350MPa to 1700MPa, and the elongation is ≥5%.

[0027] Compared with the prior art, the above technical solution provided by the embodiments of the present application has the following advantages:

[0028] By establishing the Larson-Miller parameter and the high-temperature mechanical property relationship model, the yield platform length of the hot forming steel in the high-temperature forming process can be effectively controlled, the yield platform can be avoided, and the formability is improved. In the hot stamping process, complete austenitization and martensite transformation of the material can be realized by using a lower austenitizing temperature, and the energy consumption is reduced. The yield strength of the hot forming steel obtained by the present application is 950MPa to 1300MPa, the tensile strength is 1350MPa to 1700MPa, and the elongation is ≥5%. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one embodiment consistent with the present application and, together with the description, serve to explain the principles of the application.

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced as follows. Obviously, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

[0031] Figure 1 A flowchart of a preparation method of a hot-formed steel provided by an embodiment of the present application.

[0032] Figure 2 A stress-strain curve of a steel sheet provided by Embodiment 1 of the present application. DETAILED DESCRIPTION

[0033] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort fall within the scope of the present application.

[0034] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is merely for the convenience and brevity, and should not be understood as a hard limitation on the scope of the present application; therefore, it should be considered that the described range has specifically disclosed all possible sub-ranges and single values within the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the described range, such as 1, 2, 3, 4, 5 and 6, which applies to any range. In addition, whenever a numerical range is indicated in this document, it refers to any cited number (fraction or integer) within the indicated range.

[0035] In addition, in the description of the present application, the terms "comprise", "contain" and the like are intended to mean "including but not limited to". In this document, relational terms such as "first" and "second", and the like can be used solely to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions. In this document, "and / or" describes the associated relationship of associated objects, which means that there can be three kinds of relationships, for example, A and / or B can represent: A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural. In this document, "at least one" means one or more, and "multiple" means two or more. "At least one", "at least one of the following" or the like means any combination of the items, including single item or combination of multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can mean a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, where a, b, and c can be single or multiple.

[0036] Unless otherwise specifically stated, the various raw materials, reagents, instruments and equipment used in the present application can be purchased on the market or can be prepared by existing methods.

[0037] In a first aspect, the present application provides a method for preparing a hot-formed steel, please refer to Figure 1 , the method comprises the following steps:

[0038] S1, obtaining a billet with a set chemical composition;

[0039] In some embodiments, the above S1 step further comprises molten iron pretreatment, converter smelting, refining, continuous casting, to obtain a billet.

[0040] In some embodiments, the set chemical composition comprises C, Si, Mn, Cr, Al, B, S, P, Ti, Nb, Ni, Cu, Mo, V, N and Fe; in terms of mass fraction,

[0041] The content of C is 0.05% to 0.36%, the content of Si is 0.08% to 1.2%, the content of Mn is 5.0% to 9.0%, the content of Cr is ≤0.5%, the content of Al is 0.6% to 2.6%, the content of B is 0.001% to 0.005%, the content of S is ≤0.01%, the content of P is ≤0.01%, the content of Ti is ≤0.1%, the content of Nb is ≤0.1%, the content of Ni is ≤1.0%, the content of Cu is ≤0.5%, the content of Mo is ≤1.0%, the content of V is ≤0.25%, and the content of N is ≤0.02%.

[0042] The content of C can be 0.05%, 0.10%, 0.20%, 0.30%, 0.36%, etc.; the content of Si can be 0.08%, 0.20%, 0.40%, 0.80%, 1.0%, 1.1%, 1.2%, etc.; the content of Mn can be 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, etc.; the content of Cr can be 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc.; the content of Al can be 0.6%, 1.0%, 1.5%, 2.0%, 2.6%, etc.; the content of B can be 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.; the content of S can be 0%, 0.002%, 0.006%, 0.008%, 0.01%, etc.; the content of P can be 0%, 0.002%, 0.006%, 0.008%, 0.01%, etc.; the content of Ti can be 0%, 0.05%, 0.1%, etc.; the content of Nb can be 0%, 0.02%, 0.06%, 0.08%, 0.1%, etc.; the content of Ni can be 0%, 0.1%, 0.5%, 0.7%, 1.0%, etc.; the content of Cu can be 0%, 0.1%, 0.3%, 0.4%, 0.5%, etc.; the content of Mo can be 0%, 0.1%, 0.3%, 0.6%, 0.8%, 1.0%, etc.; the content of V can be 0%, 0.05%, 0.10%, 0.20%, 0.25%, etc.; the content of N can be 0.005%, 0.01%, 0.015%, 0.02%, etc.

[0043] S2, heating the billet under a set temperature and a set time, then rolling and annealing to obtain a steel plate;

[0044] In some embodiments, the set temperature is 1180-1220°C, and the set time is 1.5-3h.

[0045] The heating temperature and the holding time directly affect the grain size. Increasing the holding temperature and prolonging the holding time will promote the growth of the grain, and the size of the grain directly affects the length of the yield platform L. The set temperature of heating can be 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, etc., and the set time of heating can be 1.5h, 1.8h, 2.0h, 2.5h, 2.8h, 3h, etc.

[0046] In some embodiments, the rolling temperature is 1160-800°C, and the thickness of the rolled steel plate is 3-8mm.

[0047] The rolling temperature can be 1160℃, 1170℃, 1175℃, 1180℃, etc., and the rolling steel plate thickness can be 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, etc.

[0048] In some embodiments, the annealing temperature is 550℃-650℃, and the annealing holding time is 0.5h-1.5h.

[0049] The hot-rolled steel plate is placed in an annealing furnace for annealing treatment. The annealing temperature can be 550℃, 570℃, 600℃, 620℃, 635℃, 650℃, etc., and the annealing holding time can be 0.5h, 0.8h, 1.0h, 1.2h, 1.5h, etc.

[0050] S3, high-temperature mechanical property testing is performed on the steel plate to obtain a high-temperature mechanical property model of the steel plate;

[0051] In some embodiments, the high-temperature mechanical property testing specifically comprises: heating the temperature of the steel plate to a temperature greater than the austenitization completion temperature AC3, holding, then reducing to a deformation temperature, and performing tensile deformation at a deformation rate of 0.01s -1 -10s -1 . The deformation rate can be 0.01s -1 , 1s -1 , 3s -1 , 5s -1 , 7s -1 , 10s -1 , etc.

[0052] In order to determine the austenitization completion temperature AC3 of the steel plate, phase transition point testing needs to be performed on the steel plate. The phase transition point testing of the material can be performed by a thermal dilatometer. Through thermal dilatometer testing, the austenitization start temperature AC1 and completion temperature AC3 of the steel plate during temperature rising can be obtained.

[0053] The annealed steel plate is subjected to high-temperature mechanical property testing. High-temperature mechanical property testing is usually performed on a Gleeble thermal simulation testing machine. Because the models of Gleeble testing machines are different, the sizes and shapes of the test samples are also different. During thermal simulation testing, the sample is first heated to a temperature above AC3 and held for a period of time to complete the austenitization transformation. Then, it is reduced to a deformation temperature and stretched at a certain deformation rate to obtain a high-temperature mechanical property model of the material.

[0054] S4, according to the high-temperature mechanical property model, the yield platform length L of the steel plate during the forming process is obtained;

[0055] The stress-strain curve of the steel plate is obtained through a high-temperature mechanical property model of the steel plate, and the yield platform length L, that is, the strain length in the stress-strain curve, can be directly measured.

[0056] S5, determining whether the yield platform length L satisfies a condition of being not greater than a yield platform length set value, if the condition is satisfied, performing hot forming on the steel plate to obtain a hot-formed steel, and if the condition is not satisfied, adjusting the set temperature and the set time in step S2 according to the yield platform length L and a Larson-Miller parameter, and repeating steps S2-S5 until the yield platform length L satisfies the condition of being not greater than the yield platform length set value, and then performing hot forming on the steel plate to obtain a hot-formed steel.

[0057] In some embodiments, the yield platform length set value is 0.02.

[0058] In some embodiments, the Larson-Miller parameter and the set temperature and the set time satisfy the following relationship: LMP = T(20 + logt),

[0059] In the formula, LMP represents the numerical value of the Larson-Miller parameter, T represents the set temperature, the unit of T is K, and t represents the set time, the unit of t is h.

[0060] The numerical value LMP of the Larson-Miller parameter and the yield platform length L satisfy the following relationship:

[0061] If L = 0, then LMP = 29729;

[0062] If 0 < L ≤ 0.02, then LMP = 9250L + 29729;

[0063] If 0.02 < L ≤ 0.04, then LMP = 7100L + 29772;

[0064] If 0.04 < L ≤ 0.07, then LMP = 3900L + 29900;

[0065] If L > 0.07, then LMP = 30194.

[0066] Specifically, the specific calculation process of the set temperature and the set time can be: obtaining the numerical value LMP of the Larson-Miller parameter through the yield platform length L, and then selecting T and t according to actual experience and historical data, which can change T and t at the same time, or change a single variable.

[0067] It is found through a large number of studies that the grain size has a significant effect on the yield platform of the high-temperature mechanical model. The larger the grain size, the shorter the yield platform, and even the yield platform disappears. Therefore, it is necessary to control the grain size to affect the length of the yield platform. The grain size is mainly affected by the heating temperature and the holding time. When the heating temperature is increased and the holding time is prolonged, the grain size is promoted to grow, thereby affecting the length of the yield platform L.

[0068] The Larson-Miller parameter is a main parameter affecting the grain size, and the value of the parameter affects the energy absorption of the material, thereby affecting the grain size of the material. At the same time, the grain size directly affects the high-temperature mechanical properties, mainly affecting the length of the yield platform. The yield platform needs to be controlled and optimized.

[0069] The Larson-Miller parameter considers both the heating temperature and the holding time. Therefore, the present application proposes a "reciprocal influence formula of the Larson-Miller parameter and the length of the yield platform". By establishing the adjustment relationship between the Larson-Miller parameter and the yield platform, the Larson-Miller parameter is adjusted, thereby controlling the length of the yield platform. Because excessive increase of the heating temperature and excessive prolongation of the holding time will also cause waste of energy and even overburning, the heating temperature and the holding time and the Larson-Miller parameter need to be controlled within a certain range.

[0070] The steel plate can be completely austenitized and martensite transformed using a lower austenitizing temperature in the hot stamping forming process.

[0071] In the present application, the hot-formed steel is obtained, and the process parameters for subsequent production of the hot-formed steel are determined.

[0072] In a second aspect, the present application provides a hot-formed steel, which is prepared by the method of any one of the embodiments of the first aspect, and the hot-formed steel satisfies at least one of the following properties: the yield strength is 950 MPa to 1300 MPa, the tensile strength is 1350 MPa to 1700 MPa, and the elongation is ≥5%.

[0073] The low-temperature hot-formed steel prepared by the method provided in the present application has excellent high-temperature formability, the strength after forming meets the requirements, and has excellent crash performance. The yield strength can be 950 MPa, 1050 MPa, 1150 MPa, 1250 MPa, 1300 MPa, etc., the tensile strength can be 1350 MPa, 1450 MPa, 1550 MPa, 1600 MPa, 1640 MPa, 1660 MPa, 1700 MPa, etc., and the elongation can be 5%, 6%, 7%, 8%, 9%, etc.

[0074] The application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the application and not used to limit the scope of the application. The experimental methods in the following examples, if no specific conditions are noted, are generally determined according to national standards. If there is no corresponding national standard, the methods are determined according to the general international standards, conventional conditions, or the conditions suggested by the manufacturers.

[0075] Example 1

[0076] The hot-formed steel prepared in this example includes the following steps:

[0077] S11, preparing a steel blank. The chemical composition and mass content of the steel blank are as follows: C: 0.18; Si: 0.31; Mn: 8.0; Cr: 0.26; Al: 1.8; B: 0.0016; S: 0.0051; P: 0.0039; Ti: 0.071; Nb: 0.007; Ni: 0.001; Cu: 0.006; Mo: 0.056; and V: 0.015.

[0078] S21, heating, rolling, and annealing the steel blank to obtain a steel plate. The heating temperature of the heating is 1200℃, the holding time of the heating is 2h; the temperature of the rolling is 1160℃, the thickness of the rolled steel plate is 3mm; the temperature of the annealing is 600℃, and the holding time of the annealing is 1h.

[0079] S31, testing the high-temperature mechanical properties of the steel plate. The phase transition point of the steel plate is tested to obtain an AC3 temperature of 755.4℃. In the thermal simulation test, the sample is heated to 780℃, held for 5min, then cooled to the deformation temperature, and stretched to deform. The deformation temperature is 500℃, and the deformation rate is 1 / s, to obtain the stress-strain curve of the steel plate, please refer to Figure 2 .

[0080] S41, the yield platform length L is measured to be 0.02 from the stress-strain curve.

[0081] S51, the yield platform length L satisfies the condition that it is not greater than the set value of the yield platform length. The steel plate is hot-formed to obtain a hot-formed steel, and the process parameters in the preparation of the hot-formed steel are determined.

[0082] The mechanical properties of the hot-formed steel obtained in this example are detected, and the yield strength of the sample is 1100MPa, the tensile strength is 1600MPa, and the elongation is 8.2%. The performance is better than that of the conventional hot-formed steel, and the elongation of the conventional hot-formed steel is usually 5%.

[0083] The detailed description of the drawings is as follows: Figure 2 As

[0084] Figure 2 ​As shown, the stress-strain curve of the steel sheet has no obvious yield platform, and the forming of the hot-stamped part can be effectively carried out.

[0085] The foregoing is considered as illustrative of the principles of the application. Numerous modifications and changes will readily occur to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, the scope of the application is indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims

1. A method for the production of a hot-formed steel, characterized in that, The method comprises the following steps: S1, obtaining a billet with a set chemical composition; S2, heating the billet under a set temperature and a set time, then rolling and annealing to obtain a steel plate; S3, testing the high-temperature mechanical properties of the steel plate to obtain a high-temperature mechanical property model of the steel plate; S4, obtaining the yield platform length L of the steel plate in the forming process according to the high-temperature mechanical property model; S5, judging whether the yield platform length L satisfies not greater than a set yield platform length value, if yes, hot forming the steel plate to obtain a hot-formed steel, if not, adjusting the set temperature and the set time in step S2 according to the yield platform length L and a Larson-Miller parameter, and circulating steps S2-S5 until the yield platform length L satisfies not greater than the set yield platform length value, then hot forming the steel plate to obtain a hot-formed steel; The Larson-Miller parameter and the set temperature and the set time satisfy the following relationship: LMP=T(20+logt), wherein LMP represents the numerical value of the Larson-Miller parameter, T represents the set temperature, the unit of T is K, and t represents the set time, the unit of t is h; wherein the numerical value of the Larson-Miller parameter LMP and the yield platform length L satisfy the following relationship: if L=0, then LMP=29729; if 0 if 0.02 if 0.04 if 0.04 if L>0.07, then LMP=30194; The set chemical composition comprises C, Si, Mn, Cr, Al, B, S, P, Ti, Nb, Ni, Cu, Mo, V, N, Fe and inevitable impurities, and the mass fraction is:

2. The method of claim 1, wherein, the content of C is 0.05%-0.36%, the content of Si is 0.08%-1.2%, the content of Mn is 5.0%-9.0%, the content of Cr is ≤0.5%, the content of Al is 0.6%-2.6%, the content of B is 0.001%-0.005%, the content of S is ≤0.01%, the content of P is ≤0.01%, the content of Ti is ≤0.1%, the content of Nb is ≤0.1%, the content of Ni is ≤1.0%, the content of Cu is ≤0.5%, the content of Mo is ≤1.0%, the content of V is ≤0.25%, and the content of N is ≤0.02%.

3. The method of claim 1, wherein, The set yield platform length value is 0.

02.

4. The method of claim 1, wherein, The set temperature of step S2 is 1180°C-1220°C, and the set time of step S2 is 1.5h-3h.

5. The method of claim 1, wherein, The rolling temperature is 1160°C-1180°C, and the thickness of the rolled steel plate is 3mm-8mm. The annealing temperature is 550°C-650°C, and the holding time of the annealing is 0.5h-1.5h.

6. The method of claim 1, wherein, The high-temperature mechanical property test is specifically: heating the temperature of the steel plate to greater than the austenitization completion temperature AC3, holding, then reducing to the deformation temperature, and performing tensile deformation at a deformation rate of 0.01 s -1 -10 s -1 to obtain a high-temperature mechanical property model of the steel plate.

7. A hot-formed steel, characterized in that, The hot-formed steel is prepared by the method according to any one of claims 1-6, and the hot-formed steel satisfies at least one of the following properties: yield strength is 950-1300 MPa, tensile strength is 1350-1700 MPa, and elongation is ≥5%.

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