Cold-rolled high-yield-ratio steel, preparation method thereof and stamping part

Abstract

The invention relates to cold-rolled high-yield-ratio steel, a preparation method thereof and a stamping part, and belongs to the technical field of steel preparation. The cold-rolled high-yield-ratio steel comprises the following chemical components: C, Si, Mn, Nb, P, S, Al and Fe, the alloy comprises the following components in percentage by mass: 0.06%-0.085% of C, 0.001%-0.20% of Si, 0.25%-0.55% of Mn, 0.01%-0.025% of Nb, 0.010%-0.025% of P, and 1t of S; the weight ratio of the Nb to the C is 0.15-0.30, the content of the Al is 0.02-0.06%, and the weight ratio of the Nb to the C is 0.15-0.30. The microstructure of the cold-rolled high-yield-ratio steel comprises ferrite and pearlite, and the grain size of the ferrite is larger than or equal to 11 grades. The yield point elongation of the cold-rolled high-yield-ratio steel is smaller than or equal to 0.01%, and the Luders effect phenomenon is completely eliminated in the stamping process of the cold-rolled high-yield-ratio steel.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a cold-rolled high-yield ratio steel and a preparation method and stamping parts thereof. Background Art

[0002] Cold-rolled high yield ratio steel (also known as precipitation-strengthened steel) generally adopts the technical route of low C microalloying and controlled rolling and controlled cooling. Its strengthening mechanism is mainly fine grain strengthening, solid solution strengthening, precipitation strengthening and dislocation strengthening. The elements such as Nb and Ti in precipitation-strengthened steel will combine with C and N elements and precipitate in the form of second phase particles NbCN or TiCN during hot rolling and cold rolling. During the tensile deformation process of cold-rolled high yield ratio steel, the pinning movement of the second phase particles (precipitates) and dislocations forms Coriolis gas masses. Due to the existence of Coriolis gas masses, an obvious yield point appears in the yield process. When the stress reaches the upper yield point, plastic deformation begins first at the stress concentration point of the specimen, and a deformation band with an angle of about 45° to the tensile axis is generated on the surface of the specimen—Lüders band (i.e., Lüders strain effect occurs). In particular, parts with large planes and small deformation amounts (thin and wide parts) produce surface defects such as tensile strain marks due to the Lüders effect during the stamping process.

[0003] Therefore, there is an urgent need for a preparation method for reducing the yield point elongation of cold-rolled high yield ratio steel. Summary of the invention

[0004] The present application provides a cold-rolled high-yield ratio steel and a preparation method and stamping parts thereof, in order to solve the following technical problem: how to reduce the yield point elongation of the cold-rolled high-yield ratio steel.

[0005] In a first aspect, the present application provides a cold-rolled high yield ratio steel, wherein the chemical composition of the cold-rolled high yield ratio steel comprises: C, Si, Mn, Nb, P, S, Al, and Fe; wherein, in terms of mass fraction,

[0006] C is 0.06% to 0.085%, Si is 0.001% to 0.20%, Mn is 0.25% to 0.55%, Nb is 0.01% to 0.025%, P is 0.010% to 0.025%, S is <0.025%, Al is 0.02% to 0.06%, and the weight ratio of Nb to C is 0.15 to 0.30;

[0007] The microstructure of the cold-rolled high yield ratio steel includes ferrite and pearlite, and the grain size of the ferrite is ≥11 levels.

[0008] Optionally, in the chemical composition, Mn is 0.25% to 0.4%, Nb is 0.01% to 0.02%, and the weight ratio of Nb to C is 0.15 to 0.23.

[0009] Optionally, in the chemical composition, Mn is 0.35% to 0.55%, Nb is 0.015% to 0.025%, and the weight ratio of Nb to C is 0.20 to 0.30.

[0010] Optionally, in terms of volume fraction, the ferrite is 94% to 96%, and the pearlite is 4% to 6%.

[0011] Optionally, the cold-rolled high yield strength ratio steel meets the following mechanical properties: yield strength ≥ 300 MPa, tensile strength

[0012] ≥380MPa, elongation after fracture ≥21%, elongation at yield point ≤0.01%.

[0013] In a second aspect, the present application provides a method for preparing the cold-rolled high yield ratio steel according to any one of the embodiments of the first aspect, the method comprising:

[0014] The slab is sequentially heated and rolled to obtain a hot-rolled plate;

[0015] Coiling the hot-rolled sheet and controlling the coiling temperature so that the precipitated phase particles in the hot-rolled sheet are dispersed to obtain a hot-rolled coil;

[0016] The hot-rolled coil is pickled and cold-rolled in sequence to obtain a chilled plate;

[0017] Continuously annealing the chilled plate, and controlling the temperature of a soaking section of the continuous annealing so that a semi-coherent ferrite-cementite interface is formed in the structure of the chilled plate, thereby obtaining an annealed plate;

[0018] The annealed sheet is subjected to skin-passing, and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel.

[0019] Optionally, the coiling temperature is 500° C. to 580° C.; and / or,

[0020] The temperature of the soaking section of the continuous annealing is 750°C to 780°C.

[0021] Optionally, the annealed sheet is subjected to skin-passing and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel, comprising:

[0022] The annealed sheet is subjected to skin-passing and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel; wherein,

[0023] According to the thickness of the cold-rolled high-yield ratio steel, the skin-pass elongation is obtained;

[0024] If the thickness of the cold-rolled high yield ratio steel is >0 and ≤1.2 mm, the skin-pass elongation is 1.6% to 1.9%;

[0025] If the thickness of the cold-rolled high yield ratio steel is greater than 1.2 mm, the elongation of the skin pass is 1.4% to 1.7%.

[0026] Optionally, the heating temperature is 1230° C. to 1330° C.; and / or,

[0027] The final rolling temperature of the rolling is 900° C. to 950° C.; and / or,

[0028] The total reduction of the cold rolling is 60% to 80%.

[0029] In a third aspect, the present application provides a stamped part, the raw material of which includes the cold-rolled high yield ratio steel described in any one of the embodiments of the first aspect.

[0030] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0031] In the cold-rolled high yield strength ratio steel provided in the embodiment of the present application, C, Mn, Si and P can play a role in solid solution strengthening, S is an impurity element; Nb can play a role in precipitation strengthening; Al mainly plays a role in deoxidation; and C is 0.06% to 0.085%, Si is 0.001% to 0.20%, Mn is 0.25% to 0.55%, Nb is 0.01% to 0.025%, P is 0.010% to 0.025%, S is <0.02 5%, Al is 0.02% to 0.06%, and the weight ratio of Nb to C is 0.15 to 0.30; the microstructure of the cold-rolled high yield ratio steel includes ferrite and pearlite, and the grain size of the ferrite is ≥ 11 levels; the weight ratio of Nb to C is 0.15 to 0.30, which can regulate the amount and strength of the precipitated phase in the steel, thereby promoting the elimination of the yield platform during the subsequent cold rolling and skin-passing process, reducing the generation of Coriolis gas masses in the subsequent tensile deformation of the steel, and thus reducing the yield point elongation of the steel. In summary, the yield point elongation of the cold-rolled high yield ratio steel is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0033] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0034] Figure 1A typical stress-strain curve of a cold-rolled high yield ratio steel provided in an embodiment of the present application;

[0035] Figure 2 A schematic flow chart of a method for preparing cold-rolled high yield ratio steel provided in an embodiment of the present application;

[0036] Figure 3 A microstructure diagram of a cold-rolled 340MPa grade high yield strength ratio steel provided in an embodiment of the present application;

[0037] Figure 4 A room temperature tensile stress-strain curve of a cold-rolled 340MPa grade high yield ratio steel provided in an embodiment of the present application;

[0038] Figure 5 A photo of a simulated stamping part of a cold-rolled 340MPa grade high yield strength ratio steel provided as a comparative example of this application;

[0039] Figure 6 A photo of a stamping part made of cold-rolled 340MPa grade high yield strength ratio steel provided as a comparative example of this application;

[0040] Figure 7 A room temperature tensile stress-strain curve of a cold-rolled 340MPa grade high yield ratio steel provided as a comparative example of the present application;

[0041] Figure 8 A microstructure diagram of a cold-rolled 300MPa grade high yield strength ratio steel provided in an embodiment of the present application;

[0042] Fig. 9 A room temperature tensile stress-strain curve of a cold-rolled 300MPa grade high yield ratio steel provided in an embodiment of the present application;

[0043] Fig.10 A photo of a simulated stamping part of a cold-rolled 300MPa grade high yield strength ratio steel provided as a comparative example of this application;

[0044] Fig.11 A photo of a stamping part of a cold-rolled 300MPa grade high yield strength ratio steel provided as a comparative example of this application;

[0045] Fig.12 The room temperature tensile stress-strain curve of a cold-rolled 300MPa grade high yield ratio steel is provided as a comparative example of the present application. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0047] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have 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 range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0048] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the present specification, the terms "including", "comprising", etc. refer to "including but not limited to". In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A, B can be singular or plural. In this article, "at least one" refers to one or more, and "plural" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one-to-one with the proportional numbers in the proportional formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0049] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0050] Cold-rolled high yield strength ratio steel (also known as precipitation-strengthened steel) generally adopts the technical route of low C microalloying and controlled rolling and controlled cooling. Its strengthening mechanism is mainly fine grain strengthening, solid solution strengthening, precipitation strengthening and dislocation strengthening. The Nb, Ti and other elements of precipitation-strengthened steel will combine with C and N elements and precipitate in the form of second phase particles NbCN or TiCN during hot rolling and cold rolling. During the tensile deformation of the material, the pinning movement of the two-phase particles (precipitates) and dislocations forms Coriolis gas masses. The deformation of the metal is achieved through the movement of dislocations. However, due to the existence of Coriolis gas masses, the movement of dislocations is resisted. In order for the dislocations to continue to move, there must be a stress higher than the normal movement of the dislocations to separate the dislocations from the gas masses, thus generating an upper yield point. When the dislocation moves a certain distance, it can get rid of the resistance of the gas mass and move under normal stress. This stress is the lower yield point. Figure 1A typical stress-strain curve of a cold-rolled high yield ratio steel provided in the embodiment of the present application; see Figure 1 , it can be seen that an obvious yield point appears in the yield process. When the tensile specimen begins to yield, the stress suddenly drops, and the yield elongation continues to occur when the stress is basically constant, so a stress platform area appears in the tensile curve. During the yield extension stage, the strain of the specimen is uneven. When the stress reaches the upper yield point, plastic deformation first begins at the stress concentration point of the specimen, and a deformation band at an angle of about 45° to the tensile axis is generated on the surface of the specimen—the Lüders band (that is, the Lüders strain effect occurs). In particular, parts with large planes and small deformations produce surface defects such as tensile strain marks due to the Lüders effect during the stamping process. Therefore, it is urgent to reduce the yield point elongation of cold-rolled high yield ratio steel.

[0051] In a first aspect, the present application provides a cold-rolled high yield ratio steel, wherein the chemical composition of the cold-rolled high yield ratio steel comprises: C, Si, Mn, Nb, P, S, Al, and Fe; wherein, in terms of mass fraction,

[0052] C is 0.06% to 0.085%, Si is 0.001% to 0.20%, Mn is 0.25% to 0.55%, Nb is 0.01% to 0.025%, P is 0.010% to 0.025%, S is <0.025%, Al is 0.02% to 0.06%, and the weight ratio of Nb to C is 0.15 to 0.30;

[0053] The microstructure of the cold-rolled high yield ratio steel includes ferrite and pearlite, and the grain size of the ferrite is ≥11 levels.

[0054] In the embodiments of the present application, in the embodiments of the present application, C can be 0.06% to 0.085%. C forms interstitial solid solution in steel, causes lattice distortion, has a significant solid solution strengthening effect, and improves the strength, hardness and wear resistance of steel. Therefore, the higher the C content in the steel, the higher the strength and hardness, but the plasticity and toughness will also decrease. The C element is an element that expands the γ phase region and can stabilize the austenite structure. The selection of the C content range mainly considers the matching of strength, formability and welding performance, as well as factors such as the composition interval of peritectic steel and production process control. The atomic radius of C is much smaller than that of iron. It mainly exists in the form of interstitial solid solution and second phase precipitation phase in steel, among which interstitial solid solution is the fundamental reason for the generation of Luders strain (yield platform) in steel during tensile deformation. Exemplarily, the C can be 0.06%, 0.07%, 0.08%, 0.085%, etc.

[0055] Mn can be 0.25% to 0.55%. Mn is a good deoxidizer and desulfurizer, which can eliminate or weaken the hot brittleness of steel caused by sulfur, thereby improving the hot processing properties of steel. Mn is also an austenite stabilizing element, and its addition reduces the Ms point. Mn can exist in a solid solution state or enter cementite to replace a part of iron atoms, playing the role of solid solution strengthening and strengthening the matrix, as well as preventing brittleness caused by S. Sufficient manganese should be added to form MnS with a higher melting point. However, a Mn content higher than 0.55% may have an adverse effect on the plasticity and impact properties of steel. Illustratively, the Mn may be 0.25%, 0.26%, 0.27%, 0.28%, 0.30%, 0.33%, 0.35%, 0.35%, 0.37%, 0.39%, 0.41%, 0.43%, 0.45%, 0.47%, 0.49%, 0.51%, 0.53%, 0.55%, etc.

[0056] Si can be 0.001% to 0.20%. Si is a solid solution strengthening element, which is dissolved in ferrite. With the increase of Si content, the strength of steel is significantly improved, the plasticity and impact toughness are significantly reduced, and the cold formability and welding performance are reduced; Si content higher than 0.20% may affect the surface quality of hot-rolled steel plates. Exemplarily, the Si can be 0.001%, 0.002%, 0.003, 0.005%, 0.007%, 0.010%, 0.013%, 0.015%, 0.018%, 0.019%, 0.20%, etc.

[0057] P can be 0.015% to 0.025%. P is a solid solution strengthening element that can purify recrystallized ferrite, promote pearlite transformation, and improve the aging resistance of steel. As an alloying element, it is added to low-alloy structural steel to improve the strength and atmospheric corrosion resistance of steel, but it may reduce the cold stamping performance of steel and easily cause brittle cracking during cold processing, which is the so-called "cold brittleness" phenomenon. At the same time, P is higher than 0.025%, which also has an adverse effect on the welding of steel. Exemplarily, the P can be 0.015%, 0.017%, 0.019%, 0.021%, 0.023%, 0.025%, etc.

[0058] S can be <0.025%. S segregates severely in steel, reduces the plasticity of steel at high temperatures, and causes hot brittleness of steel. For example, the S can be 0.024%, 0.023%, 0.022%, 0.021%, 0.020%, etc.

[0059] Nb can be 0.01% to 0.025%. Nb is a microalloying element. Its mechanism of action is mainly to improve the strength of steel by refining grains and precipitation strengthening. It is a strong C and N compound forming element. It mainly exists in the form of NbTi (C, N) in steel, inhibiting the dynamic recrystallization of austenite, increasing the nucleation rate and nucleation rate of ferrite, thereby refining ferrite grains, preventing the growth of austenite grains, and refining the structure. Exemplarily, the Nb can be 0.01%, 0.02%, 0.03%, 0.05%, 0.07%, 0.09%, 0.013%, 0.015%, 0.017%, 0.019%, 0.021%, 0.023%, 0.025%, etc.

[0060] Al can be 0.02-0.06%. Al is mainly used as a deoxidizing element to achieve complete deoxidation. In the controlled rolling and controlled cooling process, an appropriate amount of Al can inhibit the transformation of austenite to pearlite. The formed AlN precipitates play a role in refining grains to a certain extent. However, Al above 0.06% may have an adverse effect on the hot working performance and welding performance of the steel plate. For example, the Al can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0061] The weight ratio of Nb to C is 0.15 to 0.30, which can regulate the amount and strength of the precipitated phase in the steel, thereby promoting the elimination of the yield platform during the subsequent cold rolling and skin-passing process, reducing the generation of Coriolis gas clusters in the subsequent tensile deformation of the steel, and thus reducing the yield point elongation of the steel. If the weight ratio of Nb to C is higher than 0.30, it may lead to a high precipitation phase in the steel and a high strength of the steel, which will make it difficult to eliminate the yield platform due to the large skin-passing elongation in the subsequent skin-passing, and increase the yield point elongation of the steel; if the weight ratio of Nb to C is lower than 0.20, the C atoms are high, and it is easy to form Coriolis gas clusters in the tensile deformation of the steel, resulting in an increase in the yield point elongation of the steel. Illustratively, the weight ratio of Nb to C can be 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, etc.

[0062] In addition, the steel may further include residual elements N: 0.008% and Ti: 0.001% to 0.015%.

[0063] It should be noted that, generally speaking, the above-mentioned set chemical composition also includes inevitable impurities, Fe is a matrix element, and the specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is: the sum of the percentage content of each component in a composition should be equal to 100%, and the content range of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤100; the lower limit value of a component + the upper limit value of other components ≥100.

[0064] In some embodiments, in the chemical composition, Mn is 0.25% to 0.4%, Nb is 0.01% to 0.02%, and the weight ratio of Nb to C is 0.15 to 0.23.

[0065] In the embodiment of the present application, for cold-rolled 300 MPa grade high yield ratio steel, Mn may be 0.25% to 0.4%, Nb may be 0.01% to 0.02%, and the weight ratio of Nb to C may be 0.15 to 0.23.

[0066] In some embodiments, in the chemical composition, Mn is 0.35% to 0.55%, Nb is 0.015% to 0.025%, and the weight ratio of Nb to C is 0.20 to 0.30.

[0067] In the embodiment of the present application, for the cold-rolled 340 MPa grade high yield ratio steel, Mn may be 0.35% to 0.55%, Nb may be 0.015% to 0.025%, and the weight ratio of Nb to C may be 0.20 to 0.30.

[0068] In some embodiments, in terms of volume fraction, the ferrite is 94% to 96%, and the pearlite is 4% to 6%.

[0069] In the embodiment of the present application, the ferrite can be 94% to 96%, and the pearlite can be 4% to 6%, which can achieve good toughness and plasticity of the steel and can withstand large deformation without breaking. For example, the ferrite can be 94, 95, 96, etc.; the pearlite can be 4%, 5%, 6%, etc.

[0070] In some embodiments, the cold-rolled high yield ratio steel meets the following mechanical properties: yield strength ≥300 MPa, tensile strength ≥380 MPa, elongation after fracture ≥21%, and elongation at yield point ≤0.01%.

[0071] In the embodiments of the present application, for cold-rolled 300MPa grade high yield ratio steel, the yield strength is 300-380MPa, the tensile strength is 380-480MPa, the elongation after fracture is ≥23%, and the elongation at the yield point is ≤0.01%; for cold-rolled 340MPa grade high yield ratio steel, the yield strength is 340-420MPa, the tensile strength is 410-510MPa, the elongation after fracture is ≥21%, and the elongation at the yield point is ≤0.01%.

[0072] In a second aspect, the present application provides a method for preparing the cold-rolled high yield ratio steel according to any one of the embodiments of the first aspect, Figure 2 A schematic diagram of a process for preparing a cold-rolled 340MPa grade high yield strength ratio steel provided in an embodiment of the present application; see Figure 2 , the method comprising:

[0073] S1. The slab is heated and rolled in sequence to obtain a hot-rolled plate; step S1 comprises: molten iron desulfurization → converter smelting → refining outside the furnace → continuous casting to obtain a slab with a set chemical composition; wherein, the steel tapping temperature of the smelting converter is ≥1670°C, the refining outside the furnace can adopt an argon station, a ladle furnace or a vacuum process, the continuous casting adopts argon sealing protection casting, the crystallizer adopts special protective slag, and the pulling speed is set to medium and low speed to ensure the surface quality of the ingot.

[0074] S2, coiling the hot-rolled sheet and controlling the coiling temperature so that the precipitated phase particles in the hot-rolled sheet are dispersed to obtain a hot-rolled coil;

[0075] S3, pickling and cold rolling the hot-rolled coil in sequence to obtain a chilled plate;

[0076] S4, continuously annealing the chilled plate, and controlling the temperature of the soaking section of the continuous annealing so that a semi-coherent ferrite-cementite interface is formed in the structure of the chilled plate to obtain an annealed plate;

[0077] S5. Perform skin pass on the annealed sheet and control the elongation of the skin pass to obtain cold-rolled high yield strength ratio steel.

[0078] In the embodiment of the present application, the hot-rolled sheet is coiled and the coiling temperature is controlled, so that the amount of precipitated phases in the steel can be regulated and controlled, and the precipitated phases are dispersed and finely precipitated, thereby playing a role of precipitation strengthening; the hot-rolled coil is pickled, so that the surface state of the hot-rolled coil can be improved; the hot-rolled coil after pickling is cold-rolled, which can further play a role of precipitation strengthening; the chilled sheet is continuously annealed, and the temperature of the soaking section of the continuous annealing is controlled, so that the precipitated phases in the chilled sheet can be dispersed and precipitated, and the recrystallized grains in the chilled sheet can be prevented from fully growing, that is, by controlling the temperature of the soaking section of the continuous annealing, the semi-coherent ferrite-carburized C body interface can be transformed into an incoherent ferrite-carburized C body interface, thereby promoting the generation of dislocations; the annealed sheet is skin-finished, and the elongation of skin-finishing is controlled, so that the dislocation density of the annealed sheet can be increased, and the generation of Coriolis gas clusters in the steel during tensile deformation can be reduced, thereby reducing the yield point elongation of the steel. Therefore, the yield point elongation of the cold-rolled high yield ratio steel is reduced, so that the Luders effect phenomenon is completely eliminated during the stamping process of the cold-rolled high yield ratio steel.

[0079] In some embodiments, the coiling temperature is 500° C. to 580° C.; and / or,

[0080] The temperature of the soaking section of the continuous annealing is 750°C to 780°C.

[0081] In the embodiment of the present application, the coiling temperature can be 500°C to 580°C, which can increase the cooling rate of the hot-rolled coil and avoid the precipitation phase precipitation C point (nose point area: the precipitation phase precipitation changes with temperature and presents a "C" curve / inverse parabola shape, that is, the precipitation phase precipitation first increases and then decreases with temperature. The nose point temperature of the "C" curve in the embodiment of the present application is about 610°C), thereby promoting the precipitation phase to disperse and precipitate finely. Exemplarily, the coiling temperature can be 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, etc. In addition, after coiling, the hot-rolled coil can be air-cooled.

[0082] In the embodiment of the present application, the temperature of the soaking section of the continuous annealing can be 750℃~780℃, which can make the precipitation phase in the cold hard plate dispersed and precipitated, and avoid the recrystallized grains in the cold hard plate from growing fully, that is, by controlling the temperature of the soaking section of the continuous annealing, the semi-coherent ferrite-cementite interface can be transformed into an incoherent ferrite-cementite interface, promoting the generation of dislocations. If the temperature of the soaking section is higher than 780℃, it may cause the recrystallized grains of the steel to grow fully, and the ferrite-cementite interface may all appear as incoherent ferrite-cementite interface; if the temperature of the soaking section is lower than 750℃, it may cause the cold rolled hard microstructure to not recover and recrystallize, the material may present a partially rolled hard structure, the mechanical properties may not meet the standards, it may be difficult to meet the product standard requirements, and the forming performance may be poor. Exemplarily, the temperature of the soaking section of the continuous annealing can be 750°C, 752°C, 755°C, 758°C, 760°C, 762°C, 765°C, 768°C, 770°C, 775°C, 780°C, etc. In addition, the annealed sheet can be hot-dip galvanized, and the temperature of the hot-dip galvanizing pot can be 460-500°C.

[0083] In some embodiments, the annealed sheet is subjected to skin-passing and the elongation of the skin-passing is controlled to obtain a cold-rolled high yield ratio steel, comprising:

[0084] The annealed sheet is subjected to skin-passing and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel; wherein,

[0085] According to the thickness of the cold-rolled high-yield ratio steel, the skin-pass elongation is obtained;

[0086] If the thickness of the cold-rolled high yield ratio steel is >0 and ≤1.2 mm, the skin-pass elongation is 1.6% to 1.9%;

[0087] If the thickness of the cold-rolled high yield ratio steel is greater than 1.2 mm, the elongation of the skin pass is 1.4% to 1.7%.

[0088] In the embodiment of the present application, since the design mechanism of the cold-rolled high yield ratio steel is low-carbon microalloyed steel, skin straightening can be used to introduce dislocations to reduce the yield platform. The smaller the thickness of the cold-rolled high yield ratio steel, the larger the skin straightening elongation is required to increase the dislocation density, so as to cause the dislocations pinned by the intracrystalline precipitation phase to bypass or aggregate to form subgrain boundaries, thereby eliminating the influence of the Coriolis gas group. At the same time, the uniformity of the acceptance and the regularity of the plate shape of the cold-rolled high yield ratio steel are also considered. For example, if the thickness of the high yield ratio steel is >0 and ≤1.2mm, the skin straightening elongation can be 1.6%, 1.65%, 1.7%, 1.75%, 1.8%, 1.85%, 19%, etc.; if the thickness of the cold-rolled high yield ratio steel is >1.2mm, the skin straightening elongation can be 1.4%, 1.45%, 1.5%, 1.55%, 1.6%, 1.65%, 1.7%, etc. In addition, the skin-pass mill runs smoothly and is put into wet skin-passing. The diameter of the skin-pass mill working roll is ≤640mm. In order to achieve uniform performance and high surface quality, the skin-pass mill running speed control is adopted, that is, the running speed is stable at 120m / min~160m / min. The cold-rolled high yield ratio steel can also be straightened, and the straightening elongation can be 0~0.2%.

[0089] In some embodiments, the heating temperature is 1230° C. to 1330° C.; and / or,

[0090] The final rolling temperature of the rolling is 900° C. to 950° C.; and / or,

[0091] The total reduction of the cold rolling is 60% to 80%.

[0092] In the embodiment of the present application, in the embodiment of the present application, heating and rolling can play a role in fine grain strengthening, and the heating temperature can be 1230℃~1330℃, thereby improving the plasticity and uniform structure of the slab; the final rolling temperature of rolling can be 900℃~950℃, which can achieve a high cooling rate after rolling, thereby reducing the precipitation of precipitation phases in the steel. Exemplarily, the heating temperature can be 1230℃, 1250℃, 1270℃, 1290℃, 1310℃, 1330℃, etc.; the final rolling temperature of rolling can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, etc. In addition, the rolling includes rough rolling and finishing rolling. Exemplarily, after the slab is heated, it is rolled by the rough rolling mill for 5 to 12 passes, and then it enters the 7-stand finishing mill group for 7 passes.

[0093] Pickling the hot rolled coil can improve the surface condition of the hot rolled coil; cold rolling the pickled hot rolled coil can further play the role of precipitation strengthening. The total reduction of cold rolling can be 60% to 80%, so that the cold hardened plate has a large deformation energy and promotes recrystallization. Exemplarily, the total reduction of cold rolling can be 60%, 65%, 70%, 75%, 80%, etc.

[0094] Exemplarily, for cold-rolled 300MPa grade high yield ratio steel, the final rolling temperature can be 900℃~940℃, the coiling temperature can be 500℃~560℃, and the temperature of the soaking section of continuous annealing can be 760℃~780℃; if the thickness of the cold-rolled 300MPa grade high yield ratio steel is >0 and ≤1.2mm, the elongation of the skin pass can be 1.6%~1.8%; if the thickness of the cold-rolled 300MPa grade high yield ratio steel is >1.2mm, the elongation of the skin pass can be 1.4%~1.6%.

[0095] For cold-rolled 340MPa grade high yield ratio steel, the final rolling temperature can be 910℃~950℃, the coiling temperature can be 520℃~580℃, and the temperature of the soaking section of continuous annealing can be 750℃~770℃; if the thickness of the cold-rolled 340MPa grade high yield ratio steel is >0 and ≤1.2mm, the elongation of the skin-pass can be 1.7%~1.9%; if the thickness of the cold-rolled 340MPa grade high yield ratio steel is >1.2mm, the elongation of the skin-pass can be 1.5%~1.7%.

[0096] The preparation method of the cold-rolled high yield ratio steel is realized based on the above-mentioned cold-rolled high yield ratio steel. The specific chemical composition of the cold-rolled high yield ratio steel can refer to the above-mentioned embodiment. Since the cold-rolled high yield ratio steel adopts part or all of the technical solutions of the above-mentioned embodiment, it at least has all the beneficial effects brought by the technical solutions of the above-mentioned embodiment, which will not be repeated here one by one.

[0097] In a third aspect, the present application provides a stamped part, the raw material of which includes the cold-rolled high yield ratio steel described in any one of the embodiments of the first aspect.

[0098] In the embodiment of the present application, the raw material of the stamped parts includes the above-mentioned cold-rolled high yield ratio steel, and the yield point elongation of the cold-rolled high yield ratio steel can be ≤0.01%. Therefore, the cold-rolled high yield ratio steel completely eliminates the Luders effect phenomenon during the stamping process, thereby improving the surface quality of the stamped parts.

[0099] The stamped part is realized based on the above-mentioned cold-rolled high yield ratio steel. The specific preparation method of the cold-rolled high yield ratio steel can refer to the above-mentioned embodiment. Since the stamped part adopts part or all of the technical solutions of the above-mentioned embodiment, it at least has all the beneficial effects brought by the technical solutions of the above-mentioned embodiment, which will not be repeated here one by one.

[0100] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for the unrecorded specific conditions in the following examples are usually measured according to national standards. If there is no corresponding national standard, then carry out according to general international standards, normal conditions or according to the conditions recommended by the manufacturer.

[0101] The present application provides a method for preparing cold-rolled grade high yield ratio steel. Please refer to the chemical composition (wt%) of the cold-rolled high yield ratio steel shown in Table 1; the preparation process parameters of the cold-rolled high yield ratio steel shown in Table 2; and the mechanical properties of the cold-rolled high yield ratio steel shown in Table 3.

[0102] Table 1 Chemical composition (wt%) of cold-rolled high yield ratio steel, the remainder is Fe and unavoidable impurities

[0103]

[0104]

[0105] Table 2 Preparation process parameters of cold-rolled high yield ratio steel

[0106]

[0107] Table 3 Mechanical properties of cold rolled high yield ratio steel

[0108]

[0109]

[0110] Table 4 Microstructure of cold rolled high yield ratio steel

[0111]

[0112]

[0113] Examples 1 to 5 and Comparative Examples 1 to 2 are directed to cold-rolled 340MPa grade high yield ratio steel, and Examples 6 to 10 and Comparative Examples 3 to 4 are directed to cold-rolled 300MPa grade high yield ratio steel. As shown in Table 3, the cold-rolled high yield ratio steel prepared in the present application has a yield point elongation of ≤0.01%, which is much lower than that of Comparative Examples 1 to 4. Therefore, the cold-rolled high yield ratio steel can completely eliminate the Luders effect phenomenon during the stamping process, thereby improving the surface quality of the stamped parts.

[0114] The difference between Comparative Example 1 and Examples 1-5 is that the chemical composition NB / C value of 0.373 in Comparative Example 1 is higher than that in the examples, resulting in high strength of the steel, which leads to large subsequent skin-pass rolling force, low actual skin-pass elongation, and difficulty in eliminating the yield platform, and increased yield point elongation of the steel; the difference between Comparative Example 2 and Examples 1-5 is that the chemical composition NB / C value of 0.195 in Comparative Example 2 is lower than that in the examples, and the key production processes are basically the same. Due to the high component C, it is easy to form Koch gas masses during the tensile deformation of the steel, resulting in increased yield point elongation of the steel; Comparative Example 3 and Examples The difference between Examples 6-10 is that the chemical composition NB / C value of 0.124 in Comparative Example 3 is lower than that in the embodiment, and the key production processes are basically the same. Due to the high component C, Coriolis gas agglomerates are easily formed during the tensile deformation of the steel, which increases the yield point elongation of the steel; the difference between Comparative Example 4 and Examples 6-10 is that the chemical composition NB / C value of 0.308 in Comparative Example 4 is slightly higher than that in the embodiment, resulting in high strength of the steel, which leads to large subsequent skin-pass rolling force, and the actual skin-pass elongation is 0.15% lower than that in Example 7, making it difficult to eliminate the yield platform, and the yield point elongation of the steel increases.

[0115] For cold rolled 340MPa grade high yield strength ratio steel, Figure 3 A microstructure diagram of a cold-rolled 340MPa grade high yield strength ratio steel provided in the embodiment of the present application; see Figure 3 , indicating that the microstructure of the cold-rolled 340MPa grade high yield ratio steel is ferrite + pearlite structure, the ferrite grain size is 12.5, the ferrite grains are equiaxed, the recrystallization is sufficient, and Figure 3 It can be seen that cementite is discontinuously dispersed at the ferrite grain boundaries, and part of the ferrite still presents a strip-like / pancake-like structure.

[0116] Figure 4 This is a room temperature tensile stress-strain curve of a cold-rolled 340MPa grade high yield ratio steel provided in the embodiment of the present application; please refer to Figure 4 , indicating that the stress continues to increase with strain and no yield platform exists.

[0117] Figure 5 A simulated stamping part photo of a cold-rolled 340MPa grade high yield strength ratio steel provided for the comparative example of this application; please refer to Figure 5 , indicating the presence of typical 45° twill. Figure 6 A photo of a stamped part of a cold-rolled 340MPa grade high yield strength ratio steel provided for the comparative example of this application; please refer to Figure 6 It can be seen that Lüders strain occurs in the plane strain region, resulting in a typical Lüders strain band defect. Figure 7 The room temperature tensile stress-strain curve of a cold-rolled 340MPa grade high yield ratio steel provided for the comparative example of this application. Figure 7It can be seen that in the initial stage of plastic strain, the stress remains unchanged as the strain increases, that is, there is a typical yield platform.

[0118] Similarly, for cold-rolled 300MPa grade high yield strength ratio steel, Figure 8 A microstructure diagram of a cold-rolled 300MPa grade high yield strength ratio steel provided in an embodiment of the present application; Fig. 9 A room temperature tensile stress-strain curve of a cold-rolled 300MPa grade high yield ratio steel provided in an embodiment of the present application; Fig.10 A photo of a simulated stamping part of a cold-rolled 300MPa grade high yield strength ratio steel provided as a comparative example of this application; Fig.11 A photo of a stamping part of a cold-rolled 300MPa grade high yield strength ratio steel provided as a comparative example of this application; Fig.12 A room temperature tensile stress-strain curve of a cold-rolled 300MPa grade high yield ratio steel provided as a comparative example of this application; see Figures 8 to 12 , compared with the cold rolled 340MPa grade high yield strength ratio steel Figures 3 to 7 The analysis is similar to that of , and will not be analyzed here.

[0119] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:

[0120] (1) The embodiment of the present application realizes the industrial batch trial production of cold-rolled high yield ratio steel without yield platform, breaking through the limitation that the prior art is difficult to produce products with no yield platform characteristics, especially thin and wide specifications. The technical solution is verified through production practice, and the manufactured cold-rolled high yield ratio steel is stably applied to downstream industry users in batches, especially for stamping parts with high-grade surface requirements that do not allow the existence of Luders bands;

[0121] (2) The embodiment of the present application defines the alloy composition design of the weight ratio of Nb to C and promotes the formation of a semi-coherent ferrite-cementite interface, thereby reducing the yield point elongation of cold-rolled high yield ratio steel;

[0122] (3) The large smoothing elongation of the embodiment of the present application increases the dislocation density, which causes the dislocations pinned by the precipitated phase in the crystal to bypass or aggregate to form subgrain boundaries.

[0123] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A cold-rolled high yield ratio steel, the chemical composition of which comprises: C, Si, Mn, Nb, P, S, Al, and Fe; among which, in terms of mass fraction, C is 0.06% to 0.085%, Si is 0.001% to 0.20%, Mn is 0.25% to 0.55%, Nb is 0.01% to 0.025%, P is 0.010% to 0.025%, S is <0.025%, Al is 0.02% to 0.06%, and the weight ratio of Nb to C is 0.15 to 0.30; The microstructure of the cold-rolled high yield ratio steel includes ferrite and pearlite, and the grain size of the ferrite is ≥11 levels.

2. The cold-rolled high yield ratio steel according to claim 1, characterized in that: In the chemical composition, Mn is 0.25% to 0.4%, Nb is 0.01% to 0.02%, and the weight ratio of Nb to C is 0.15 to 0.

23.

3. The cold-rolled high yield ratio steel according to claim 1, characterized in that: In the chemical composition, Mn is 0.35% to 0.55%, Nb is 0.015% to 0.025%, and the weight ratio of Nb to C is 0.20 to 0.

30.

4. The cold-rolled high yield ratio steel according to claim 1, characterized in that: In terms of volume fraction, the ferrite is 94% to 96%, and the pearlite is 4% to 6%.

5. The cold-rolled high yield ratio steel according to any one of claims 1 to 4, characterized in that: The cold-rolled high yield strength ratio steel meets the following mechanical properties: yield strength ≥300MPa, tensile strength ≥380MPa, elongation after fracture ≥21%, and elongation at yield point ≤0.01%.

6. A method for preparing the cold-rolled high yield ratio steel according to any one of claims 1 to 5, the method comprising: The slab is sequentially heated and rolled to obtain a hot-rolled plate; Coiling the hot-rolled sheet and controlling the coiling temperature so that the precipitated phase particles in the hot-rolled sheet are dispersed to obtain a hot-rolled coil; The hot-rolled coil is pickled and cold-rolled in sequence to obtain a chilled plate; Continuously annealing the chilled plate, and controlling the temperature of a soaking section of the continuous annealing so that a semi-coherent ferrite-cementite interface is formed in the structure of the chilled plate, thereby obtaining an annealed plate; The annealed sheet is subjected to skin-passing, and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel.

7. The method according to claim 6, characterized in that The coiling temperature is 500°C to 580°C; and / or, The temperature of the soaking section of the continuous annealing is 750°C to 780°C.

8. The method according to claim 6, characterized in that The annealed sheet is subjected to skin-passing and the elongation of the skin-passing is controlled to obtain cold-rolled high yield ratio steel, comprising: The annealed sheet is subjected to skin-passing and the skin-passing elongation is controlled to obtain a cold-rolled high-yield ratio steel; wherein the skin-passing elongation is obtained according to the thickness of the cold-rolled high-yield ratio steel; If the thickness of the cold-rolled high yield ratio steel is >0 and ≤1.2 mm, the skin-pass elongation is 1.6% to 1.9%; If the thickness of the cold-rolled high yield ratio steel is greater than 1.2 mm, the elongation of the skin pass is 1.4% to 1.7%.

9. The method according to claim 6, characterized in that The heating temperature is 1230° C. to 1330° C.; and / or, The final rolling temperature of the rolling is 900° C. to 950° C.; and / or, The total reduction of the cold rolling is 60% to 80%.

10. A stamped part, wherein the raw material of the stamped part comprises the cold-rolled high yield ratio steel according to any one of claims 1 to 5.

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