High-yield-strength hot-rolled galvanized high-hole-expansion steel and preparation method thereof

Abstract

The invention relates to high-yield-strength hot-rolled galvanized high-hole-expansion steel and a preparation method, and belongs to the technical field of steel manufacturing. The high-hole-expansion steel comprises the following chemical components in percentage by mass: 0.18%-0.25% of C, 0.05%-0.3% of Si, 1.0%-1.4% of Mn, 0.02%-0.5% of Al, less than or equal to 0.01% of P, less than or equal to 0.003% of S, 0.1%-0.3% of Cr, less than or equal to 0.05% of Mo, less than or equal to 0.05% of Cu, less than or equal to 0.05% of Ni, 0.01%-0.05% of Ti, 0.002%-0.004% of B and a matrix element Fe, si + Al is greater than or equal to 0.3%, and in the formula, Si + Al represents the sum of the mass fractions of Si and Al. Through reasonable chemical component design, microstructure control and preparation process optimization, the hole expansion rate of the hot-rolled galvanized steel is improved on the basis of improving the strength. The high-yield-strength hot-rolled galvanized high-hole-expansion steel has a wide application prospect, and can meet the requirements of the fields of automobiles, machine manufacturing and the like on high-strength and high-hole-expansion-performance materials.

Description

Technical Field

[0001] The present application relates to the technical field of steel manufacturing, and in particular to a high yield strength hot-rolled galvanized high hole expansion steel and a preparation method thereof. Background Art

[0002] Hot-rolled galvanized sheet has excellent corrosion resistance and formability, and its application in automobile chassis and structural parts is gradually increasing. With the continuous improvement of automobile lightweight and collision safety requirements, higher requirements are placed on the strength level of hot-rolled galvanized sheet, and some reinforcements require a yield strength of more than 1000MPa. Increasing strength usually requires the addition of a large amount of alloy elements, which not only increases costs, but also easily forms element segregation, resulting in uneven organization, resulting in poor and unstable local forming performance such as hole expansion and bending. Therefore, it is urgent to develop a hot-rolled galvanized steel sheet with high yield strength and good local forming performance. Summary of the invention

[0003] The present application provides a high yield strength hot-rolled galvanized high hole expansion steel and a preparation method to solve the following technical problem: how to increase the hole expansion rate of hot-rolled galvanized steel while improving its strength.

[0004] In a first aspect, an embodiment of the present application provides a high yield strength hot-rolled galvanized high hole expansion steel, wherein the chemical composition of the high hole expansion steel comprises, by mass fraction: C: 0.18% to 0.25%, Si: 0.05% to 0.3%, Mn: 1.0% to 1.4%, Al: 0.02% to 0.5%, P≤0.01%, S≤0.003%, Cr: 0.1% to 0.3%, Mo≤0.05%, Cu≤0.05%, Ni≤0.05%, Ti: 0.01% to 0.05%, B: 0.002% to 0.004%, and matrix element Fe;

[0005] Si+Al≥0.3%, wherein Si+Al represents the sum of the mass fractions of Si and Al.

[0006] Optionally, the banded structure of the high hole expansion steel is rated as level 0; and in terms of area fraction, the microstructure of the high hole expansion steel includes: tempered martensite: 85% to 95%, retained austenite: 5% to 10%, and ferrite ≤5%.

[0007] Optionally, the high hole expansion steel meets at least one of the following properties: yield strength>1050MPa, tensile strength>1250MPa, elongation after fracture>9%, and more than 95% of the high hole expansion steel has a hole expansion rate of ≥75%.

[0008] In a second aspect, an embodiment of the present application provides a method for preparing the high yield strength hot-rolled galvanized high hole expansion steel described in the first aspect, the method comprising:

[0009] obtaining molten steel having the chemical composition, and continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab;

[0010] The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0011] The hot-rolled coil is pickled, continuously hot-dip galvanized and skin-passed in sequence to obtain hot-rolled galvanized high hole expansion steel; wherein the continuous hot-dip galvanizing includes: a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an aging section and a galvanizing section.

[0012] Optionally, the temperature of the heat treatment is 1170°C to 1220°C.

[0013] Optionally, the inlet temperature of the rough rolling is 1080°C to 1120°C.

[0014] Optionally, the outlet temperature of the electromagnetic induction heating is ≥1250°C.

[0015] Optionally, the inlet temperature of the finishing rolling is 1180°C to 1220°C, and the final rolling temperature of the finishing rolling is 870°C to 900°C.

[0016] Optionally, the coiling temperature is 150°C to 200°C.

[0017] Optionally, the thickness of the hot-rolled coil is 1.0 mm to 1.8 mm.

[0018] Optionally, the heating section of the continuous hot-dip galvanizing includes a heating section 1, a pre-oxidation section and a heating section 2.

[0019] Optionally, the starting temperature of the heating stage 1 is 210°C to 230°C, the ending temperature of the heating stage 1 is 650°C to 700°C, and the heating rate of the heating stage 1 is ≥4°C / s.

[0020] Optionally, the temperature of the pre-oxidation section is 650°C to 700°C, the time of the pre-oxidation section is 13s to 20s, and the dew point temperature of the pre-oxidation section is ≥0°C.

[0021] Optionally, the starting temperature of the heating stage 2 is 650°C to 700°C, the ending temperature of the heating stage 2 is 840°C to 880°C, and the heating rate of the heating stage 2 is 0.5°C / s to 1.5°C / s.

[0022] Optionally, the temperature of the soaking section is 840° C. to 880° C., and the soaking time of the soaking section is 80s to 150s.

[0023] Optionally, the temperature of the slow cooling section is 760°C to 800°C.

[0024] Optionally, the temperature of the rapid cooling section is 200°C to 280°C.

[0025] Optionally, the temperature of the aging section is 280° C. to 350° C., and the time of the aging section is 15s to 30s.

[0026] Optionally, there is an induction heater 1 between the rapid cooling section and the aging section, and there is an induction heater 2 between the aging section and the zinc pot section; the heating rate of the induction heater 1 is 30℃ / s~50℃ / s, and the heating rate of the induction heater 2 is 20℃ / s~40℃ / s.

[0027] Optionally, the finishing elongation of the finishing is 0.1% to 0.3%.

[0028] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0029] An embodiment of the present application provides a high yield strength hot-rolled galvanized high hole expansion steel. The chemical composition of the high hole expansion steel, measured by mass fraction, includes: C: 0.18% to 0.25%, Si: 0.05% to 0.3%, Mn: 1.0% to 1.4%, Al: 0.02% to 0.5%, P≤0.01%, S≤0.003%, Cr: 0.1% to 0.3%, Mo≤0.05%, Cu≤0.05%, Ni≤0.05%, Ti: 0.01% to 0.05%, B: 0.002% to 0.004%, and matrix element Fe; Si+Al≥0.3%, wherein Si+Al represents the sum of the mass fractions of Si and Al. By precisely controlling the chemical composition and reducing the addition of elements such as Mn, Cr, and Mo, the strength is improved mainly by C through solid solution strengthening and phase transformation strengthening to promote the formation of hard phase structures such as bainite and martensite. The appropriate amount of Si and Al are added to jointly inhibit the precipitation of cementite and improve the forming performance and tensile strength. The hole expansion rate of hot-rolled galvanized steel is increased on the basis of improving strength. This high yield strength hot-rolled galvanized high hole expansion steel has a wide range of application prospects and can meet the needs of high-strength and high hole expansion performance materials in the fields of automobiles and machinery manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] 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.

[0031] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces 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.

[0032] Figure 1 A schematic flow chart of a method for preparing a high yield strength hot-rolled galvanized high hole expansion steel provided in an embodiment of the present application;

[0033] Figure 2 The microstructure of the high yield strength hot-rolled galvanized high hole expansion steel provided in the embodiments of the present application. DETAILED DESCRIPTION

[0034] 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.

[0035] 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, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical 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 range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0036] In this document, the terms including "including" and "including" mean "including but not limited to". 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 any such actual relationship or order between these entities or operations. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" means 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 single or plural items; for example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. 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, if 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 to 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.

[0037] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0038] In a first aspect, an embodiment of the present application provides a high yield strength hot-rolled galvanized high hole expansion steel, wherein the chemical composition of the high hole expansion steel comprises, by mass fraction: C: 0.18% to 0.25%, Si: 0.05% to 0.3%, Mn: 1.0% to 1.4%, Al: 0.02% to 0.5%, P≤0.01%, S≤0.003%, Cr: 0.1% to 0.3%, Mo≤0.05%, Cu≤0.05%, Ni≤0.05%, Ti: 0.01% to 0.05%, B: 0.002% to 0.004%, and matrix element Fe;

[0039] Si+Al≥0.3%, wherein Si+Al represents the sum of the mass fractions of Si and Al.

[0040] C is an element that improves hardenability and can promote the formation of hard phase structures such as bainite and martensite. It mainly improves strength through solid solution strengthening and phase transformation strengthening. The embodiment of the present application reduces the addition of elements such as Mn, Cr, and Mo, and mainly relies on C strengthening. Therefore, in order to obtain a yield strength of not less than 1000MPa, the mass fraction of C should not be less than 0.18%; however, the mass fraction of C should not exceed 0.25%, otherwise the plasticity and hole expansion performance will deteriorate, and the welding performance will also be impaired. Exemplarily, the mass fraction of C can be 0.18%, 0.20%, 0.22%, 0.24%, 0.25%, etc.

[0041] Si is a solid solution strengthening element and also a non-carbide forming element. During the aging process of continuous hot-dip galvanizing, it can inhibit the formation of cementite, which is beneficial to improving the forming performance and tensile strength. In order to better form the above effects, the mass fraction of Si should not be less than 0.05%. However, when the mass fraction of Si is too high, the surface quality of the hot-rolled coil and the subsequent galvanized coil will deteriorate. Therefore, in the embodiment of the present application, the mass fraction of Si is controlled at 0.05% to 0.3%. Exemplarily, the mass fraction of Si can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, etc.

[0042] Al is a deoxidizing element that can reduce oxide inclusions in steel, so the mass fraction is not less than 0.02%. In addition, Al is also a non-carbide forming element that can inhibit the precipitation of cementite during the galvanizing aging process, so Al can be used to replace part of Si. However, too high a mass fraction of Al will increase the difficulty of continuous casting and the cost of the alloy, so the mass fraction of Al is controlled to be 0.02% to 0.5%. Exemplarily, the mass fraction of Al can be 0.02%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0043] Si and Al are both non-carbide forming elements. In order to effectively inhibit the precipitation of cementite during the galvanizing and aging process, the sum of their mass fractions should not be less than 0.3%.

[0044] Mn has a strong solid solution strengthening effect, and can also promote the phase transformation of bainite and martensite. It is an important component to ensure high yield and tensile strength. Therefore, the mass fraction of Mn is controlled at 1.0% and above. However, when the mass fraction of Mn is higher than 1.4%, element segregation is easily formed, which has a great influence on the uniformity of organizational properties. Therefore, in the embodiment of the present application, the mass fraction of Mn is controlled at 1.0% to 1.4%. Exemplarily, the mass fraction of Mn can be 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, etc.

[0045] As an interstitial solid solution atom, P can appropriately improve the strength of the steel plate, but it is also easy to segregate at the grain boundary and deteriorate the plasticity and formability, so the mass fraction of P is set to ≤ 0.01%. For example, the mass fraction of P can be 0.002%, 0.004%, 0.006%, 0.008%, 0.01%, etc.

[0046] S is easy to combine with Mn to form coarse MnS inclusions, which deteriorates the forming properties of the steel plate such as hole expansion and flanging, so the mass fraction of S is set to ≤ 0.003%. For example, the mass fraction of S can be 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.

[0047] Cr is an element that improves hardenability, promotes martensite formation, and improves yield and tensile strength. However, when the mass fraction of Cr exceeds 0.3%, the effect is no longer obvious and the cost will increase, and element segregation may also occur. Therefore, the mass fraction of Cr is controlled at 0.1% to 0.3%. For example, the mass fraction of Cr can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, etc.

[0048] Mo is also an element that improves hardenability, but Mo is an expensive alloy element. In order to reduce alloy costs, the mass fraction of Mo is controlled to be ≤ 0.05%. For example, the mass fraction of Mo can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0049] Cu has the effect of improving the strength and corrosion resistance of steel, and can significantly eliminate the harmful effects of S in steel. However, the cost of Cu alloy is relatively high. In order to reduce the cost, the mass fraction of Cu is controlled to be ≤ 0.05%. For example, the mass fraction of Cu can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0050] Ni and Cu have an impact on the quality and performance of steel. Generally, both of them exist in steel at the same time. Ni is also an element that improves the atmospheric corrosion resistance of steel. Due to the high cost of Ni alloy, the upper limit is limited to 0.05%. In order to improve the corrosion resistance of the steel matrix, at least one of Cu≤0.05% and Ni≤0.05% can be added. Of course, the chemical composition of high hole expansion steel includes the case where Cu and Ni are both 0, that is, the above elements can be omitted. Exemplarily, the mass fraction of Ni can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0051] Ti is a strong carbonitride-forming element, which can refine grains and strengthen precipitation, improve the yield strength of steel plates and local forming properties such as hole expansion and bending. In order to obtain excellent strength and forming properties, it is selected to add not less than 0.01% Ti. However, since the embodiment of the present application adopts an induction heating process, the yield strength can be appropriately improved, and it is not necessary to rely on the addition of a large amount of micro-alloying elements to improve the strength, so the upper limit of the mass fraction of Ti is set to 0.05%. Exemplarily, the mass fraction of Ti can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0052] B can significantly improve the hardenability of steel. In order to reduce the ferrite content, the embodiment of the present application sets a higher slow cooling temperature, and in order to increase the martensite content, a lower rapid cooling temperature is set. Therefore, there is a huge temperature difference between the slow cooling temperature and the rapid cooling temperature. If the strip steel is to be cooled quickly in a limited time, its hardenability must be improved. Therefore, the mass fraction of B is not less than 0.002%. However, when the mass fraction of B exceeds 0.004%, the toughness will deteriorate, so the upper limit of the mass fraction of B is set to 0.004%. Exemplarily, the mass fraction of B can be 0.002%, 0.0025%, 0.003%, 0.0035%, 0.004%, etc.

[0053] Fe is a matrix element. The specific content / content range of Fe can be obtained by the upper and lower limit formula of the component, that is:

[0054] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges 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.

[0055] In some embodiments, the banded structure of the high hole expansion steel is rated as level 0; in terms of area fraction, the microstructure of the high hole expansion steel includes: tempered martensite: 85% to 95%, retained austenite: 5% to 10%, and ferrite ≤5%.

[0056] Banded structures are likely to appear in steel due to element segregation or rolling deformation, which will significantly damage the uniformity of steel performance. In order to ensure that the hole expansion rate of more than 95% of high hole expansion steels is ≥75%, the banded structure needs to be controlled to level 0.

[0057] Tempered martensite is an important component phase for ensuring the strength of the high hole expansion steel in the embodiment of the present application. In order to obtain a tensile strength of not less than 1250 MPa, the area fraction of tempered martensite should not be less than 85%; however, when the area fraction of tempered martensite is higher than 95%, the plasticity of the steel plate is insufficient. Exemplarily, the area fraction of tempered martensite can be 85%, 87%, 89%, 91%, 93%, 95%, etc.

[0058] During the rapid cooling and aging process of galvanizing, austenite stabilizing elements gradually enrich the untransformed austenite, resulting in improved stability and eventually the formation of retained austenite. Retained austenite can appropriately improve the elongation after fracture through the TRIP effect, but when the area fraction is too high, it will deteriorate the hole expansion performance and increase the risk of delayed cracking. Therefore, the embodiment of the present application limits the area fraction of retained austenite to 5% to 10%. Exemplarily, the area fraction of retained austenite can be 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0059] Ferrite is a soft phase that is easily deformed and causes the steel to yield prematurely. Therefore, in order to obtain a high yield strength, the area fraction of ferrite must be controlled. In order to obtain a yield strength of more than 1000 MPa in the embodiment of the present application, it is necessary to limit the area fraction of ferrite to no more than 5%. For example, the area fraction of ferrite can be 1%, 2%, 3%, 4%, 5%, etc.

[0060] In some embodiments, the high hole expansion steel meets at least one of the following properties: yield strength>1050MPa, tensile strength>1250MPa, elongation after fracture>9%, and more than 95% of the high hole expansion steel has a hole expansion rate of ≥75%.

[0061] Figure 1 A schematic flow chart of a method for preparing high yield strength hot-rolled galvanized high hole expansion steel provided in an embodiment of the present application.

[0062] See also Figure 1 In a second aspect, an embodiment of the present application provides a method for preparing the high yield strength hot-rolled galvanized high hole expansion steel described in the first aspect, the method comprising:

[0063] S1, obtaining molten steel having the chemical composition, and continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab;

[0064] S2, sequentially subjecting the slab to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0065] In some embodiments, the soaking treatment is performed at a temperature of 1170°C to 1220°C.

[0066] When the soaking temperature is lower than 1170°C, the homogenization effect is poor and the rough rolling inlet temperature is low; when the soaking temperature is higher than 1220°C, the ingot microstructure tends to grow significantly. For example, the soaking temperature may be 1170°C, 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, etc.

[0067] In some embodiments, the inlet temperature of the rough rolling is 1080°C to 1120°C.

[0068] When the rough rolling inlet temperature is lower than 1080°C, the rolling resistance is too large, resulting in rolling difficulties; when the rough rolling inlet temperature is higher than 1120°C, the structure is coarse. For example, the rough rolling inlet temperature may be 1080°C, 1090°C, 1100°C, 1110°C, 1120°C, etc.

[0069] In some embodiments, the outlet temperature of the electromagnetic induction heating is ≥ 1250°C.

[0070] When the outlet temperature of induction heating is lower than 1250°C, the finishing inlet temperature greater than 1180°C cannot be guaranteed, which leads to a low finishing rolling temperature and increased structural inhomogeneity. For example, the outlet temperature of electromagnetic induction heating can be 1250°C, 1260°C, 1270°C, 1280°C, etc.

[0071] In some embodiments, the inlet temperature of the finishing rolling is 1180°C to 1220°C, and the final rolling temperature of the finishing rolling is 870°C to 900°C.

[0072] When the finishing rolling inlet temperature is lower than 1180°C, the finishing rolling temperature is too low, which may easily lead to uneven structure; when the finishing rolling inlet temperature is higher than 1220°C, the grain size is too large, which may easily lead to insufficient tensile strength of the final steel plate. For example, the finishing rolling inlet temperature may be 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, etc.

[0073] When the final rolling temperature of the finishing rolling is greater than 900°C, the austenite grains are coarse, which increases the grain size after cooling phase transformation, and easily leads to insufficient yield and tensile strength of the steel plate; when the final rolling temperature of the finishing rolling is lower than 870°C, the structure is prone to striping, and the uneven structure leads to a decrease in the hole expansion performance of the steel plate; in addition, too low a final rolling temperature will also increase the deformation resistance of the hot-rolled plate, which will increase the rolling load of the hot rolling mill. For example, the final rolling temperature of the finishing rolling can be 870°C, 875°C, 880°C, 885°C, 890°C, 895°C, 900°C, etc.

[0074] In some embodiments, the coiling temperature is 150°C to 200°C.

[0075] When the coiling temperature is greater than 200°C, the cooling rate of the hot rolled coil after coiling is slow, the tempering characteristics are obvious, and the driving force of recrystallization decreases during subsequent annealing, which is not conducive to grain refinement; when the coiling temperature is less than 150°C, the shape of the hot rolled plate deteriorates, resulting in difficulty in coiling. For example, the coiling temperature can be 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, etc.

[0076] In some embodiments, the hot rolled coil has a thickness of 1.0 mm to 1.8 mm.

[0077] S3. The hot-rolled coil is pickled, continuously hot-dip galvanized and skin-passed in sequence to obtain hot-rolled galvanized high hole expansion steel; wherein the continuous hot-dip galvanizing includes: a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an aging section and a galvanizing section.

[0078] In some embodiments, the heating section of the continuous hot-dip galvanizing includes a heating section 1, a pre-oxidation section and a heating section 2.

[0079] In some embodiments, the starting temperature of the heating stage 1 is 210°C to 230°C, the ending temperature of the heating stage 1 is 650°C to 700°C, and the heating rate of the heating stage 1 is ≥4°C / s.

[0080] The hot rolled coil structure after coiling in the embodiment of the present application is mainly martensite, retaining a large number of defects such as energy storage and dislocation. When the heating rate is lower than 4°C / s, the hot rolled structure stays in the low temperature zone for a long time, and the recovery characteristics are detailed, which is not conducive to subsequent phase transformation and recrystallization. Exemplarily, the heating rate of heating stage 1 can be 4°C / s, 4.2°C / s, 4.4°C / s, 4.6°C / s, 4.8°C / s, 5°C / s, etc.

[0081] In some embodiments, the temperature of the pre-oxidation stage is 650°C to 700°C, the time of the pre-oxidation stage is 13s to 20s, and the dew point temperature of the pre-oxidation stage is ≥ 0°C.

[0082] When the pre-oxidation temperature is lower than 650°C, the Fe oxidation on the surface of the steel plate is uneven; when the pre-oxidation temperature is higher than 700°C, the oxidation of Si and Mn in the substrate is enhanced, and it is easy to diffuse to the surface, resulting in defects such as plating leakage on the steel plate. For example, the temperature of the pre-oxidation stage can be 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, etc.

[0083] When the pre-oxidation time is less than 13s, the Fe oxidation on the surface of the steel plate is uneven; when the pre-oxidation time is greater than 20s, the Fe oxide layer on the surface of the steel plate thickens, and the subsequent reduction effect becomes poor, resulting in defects such as zinc flow and plating leakage on the surface. For example, the time of the pre-oxidation stage can be 13s, 14s, 15s, 16s, 17s, 18s, 19s, 20s, etc.

[0084] The use of a high atmosphere dew point is conducive to inhibiting the selective oxidation of alloy elements such as silicon and manganese. When the dew point temperature is ≥ 0°C, the corresponding atmospheric oxygen partial pressure is higher than the critical oxygen partial pressure for internal oxidation of alloy elements such as silicon and manganese. Alloy elements such as silicon and manganese undergo internal oxidation on the secondary surface of the steel plate, while only iron oxidation occurs on the surface of the steel plate, which is conducive to obtaining good surface quality. For example, the dew point temperature of the pre-oxidation stage can be 0°C, 2°C, 4°C, 6°C, 8°C, 10°C, etc.

[0085] In some embodiments, the starting temperature of the heating stage 2 is 650°C to 700°C, the ending temperature of the heating stage 2 is 840°C to 880°C, and the heating rate of the heating stage 2 is 0.5°C / s to 1.5°C / s.

[0086] The heating rate of 0.5℃ / s to 1.5℃ / s in the second heating stage is to increase the residence time of the strip in the high temperature zone and promote the homogenization of elements such as Mn and Cr. However, when the heating rate is lower than 0.5℃ / s, the grains grow significantly, resulting in a decrease in strength; when the heating rate is higher than 1.5℃ / s, the homogenization effect of the elements is weakened, segregation is not easy to eliminate, and the uniformity of hole expansion is affected. Exemplarily, the heating rate of the second heating stage can be 0.5℃ / s, 0.8℃ / s, 1.1℃ / s, 1.4℃ / s, 1.5℃ / s, etc.

[0087] In some embodiments, the temperature of the soaking section is 840° C. to 880° C., and the soaking time of the soaking section is 80 s to 150 s.

[0088] When the soaking temperature is lower than 840°C, the homogenization effect of Mn and Cr elements in the steel plate is insufficient, which is not conducive to improving the hole expansion performance; when the soaking temperature is higher than 880°C, the austenite grains grow and coarsen, and the surface of the strip is prone to decarburization, resulting in a decrease in strength. For example, the temperature of the soaking section can be 840°C, 850°C, 860°C, 870°C, 880°C, etc.

[0089] When the soaking time is less than 80 seconds, the uniformity of the microstructure in the high temperature zone is low, which may easily lead to uneven performance; when the soaking time exceeds 150 seconds, the grains grow excessively, which may easily lead to insufficient tensile strength. For example, the soaking time of the soaking section may be 80 seconds, 90 seconds, 100 seconds, 110 seconds, 120 seconds, 130 seconds, 140 seconds, 150 seconds, etc.

[0090] In some embodiments, the temperature of the slow cooling section is 760°C to 800°C.

[0091] When the slow cooling temperature is lower than 760°C, a large amount of ferrite is easily formed during the slow cooling process, resulting in insufficient strength of the steel plate; when the slow cooling temperature is higher than 800°C, poor plate shape is easily caused by overcooling during the rapid cooling process and it is difficult to quickly cool to the terminal temperature. Exemplarily, the temperature of the slow cooling section can be 760°C, 770°C, 780°C, 790°C, 800°C, etc.

[0092] In some embodiments, the temperature of the rapid cooling section is 200°C to 280°C.

[0093] When the rapid cooling temperature is lower than 200°C, the untransformed austenite content is very small, which will lead to a decrease in the final retained austenite content and a decrease in plasticity; when the rapid cooling temperature is higher than 280°C, the untransformed austenite content is high, and the stability is improved during the aging stage, resulting in an increase in the room temperature retained austenite content, which reduces the hole expansion performance of high hole expansion steel. For example, the temperature of the rapid cooling stage can be 200°C, 220°C, 240°C, 260°C, 280°C, etc.

[0094] In some embodiments, the temperature of the aging stage is 280° C. to 350° C., and the time of the aging stage is 15 s to 30 s.

[0095] When the aging temperature is lower than 280°C, the C partitioning effect becomes worse, the stability of the untransformed austenite after rapid cooling is insufficient, and it will be transformed into fresh martensite after galvanizing, resulting in decreased hole expansion and bending performance; when the aging temperature is higher than 350°C, the martensite recovery and decomposition are obvious, resulting in a tensile strength of less than 1250MPa. For example, the temperature of the aging stage can be 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, etc.

[0096] When the aging time is less than 15s, the C distribution is insufficient, the stability of the untransformed austenite after rapid cooling deteriorates, and it will be transformed into fresh martensite after galvanizing, resulting in decreased hole expansion and bending performance; in addition, when the aging time is less than 15s, the internal stress of the martensite is not fully eliminated, and the forming performance is poor; when the aging time is greater than 30s, the martensite is over-tempered, and the tensile strength decreases significantly. For example, the aging period can be 15s, 18s, 21s, 24s, 27s, 30s, etc.

[0097] In some embodiments, there is an induction heater 1 between the rapid cooling section and the aging section, and there is an induction heater 2 between the aging section and the zinc pot section; the heating rate of the induction heater 1 is 30°C / s to 50°C / s, and the heating rate of the induction heater 2 is 20°C / s to 40°C / s.

[0098] If the heating rate of the induction heater 1 is lower than 30°C / s, it cannot be quickly heated to the aging temperature in a short time; if the heating rate of the induction heater 1 exceeds 50°C / s, it will cause temperature overshoot. For example, the heating rate of the induction heater 1 can be 30°C / s, 34°C / s, 38°C / s, 42°C / s, 46°C / s, 50°C / s, etc.

[0099] If the heating rate of the induction heater 2 is lower than 20°C / s, it cannot be quickly heated to the galvanizing temperature in a short time; if the heating rate of the induction heater 2 exceeds 40°C / s, the strip will reach the galvanizing temperature early but not actually enter the zinc pot, resulting in poor surface quality. For example, the heating rate of the induction heater 2 can be 20°C / s, 24°C / s, 28°C / s, 32°C / s, 36°C / s, 40°C / s, etc.

[0100] The aging stage (280℃~350℃) and the galvanizing stage (450℃~470℃) are controlled separately through the galvanizing induction heating technology. Different from the conventional rapid cooling to 450℃~470℃ for aging + galvanizing process, the low-temperature aging process eliminates the internal stress of martensite and improves the stability of residual austenite, avoiding the transformation of unstable austenite into fresh martensite after galvanizing, which leads to a decrease in yield strength and hole expansion rate.

[0101] After continuous hot-dip galvanizing, the steel plate is cooled to room temperature and subjected to skin pass to obtain hot-rolled galvanized high-expansion steel.

[0102] In some embodiments, the finishing elongation of the finishing is 0.1% to 0.3%.

[0103] The embodiment of the present application adopts the design concept of low Mn and low Cr composition, and cooperates with hot-rolled low-temperature coiling and galvanized induction heating technology to improve the yield and tensile strength of the steel plate, avoiding the element segregation caused by high alloy content; the thin slab continuous casting and rolling technology is innovatively adopted, and the thickness of the cast slab is half of the thickness of the conventional cast slab in the prior art. At the same time, electromagnetic induction heating is introduced after the rough rolling is completed to increase the finishing rolling temperature and eliminate the rolling deformation band caused by low-temperature final rolling. The above measures greatly improve the uniformity of the structure of thin-gauge hot-rolled galvanized steel sheets, and improve the stability of the hole expansion rate while obtaining a high hole expansion rate.

[0104] The product prepared by the method for preparing high yield strength hot-rolled galvanized high hole expansion steel is the above-mentioned high yield strength hot-rolled galvanized high hole expansion steel. Since the method for preparing high yield strength hot-rolled galvanized high hole expansion steel adopts part or all of the technical solutions of the high yield strength hot-rolled galvanized high hole expansion steel embodiment, it at least has all the beneficial effects brought by the technical solutions of the high yield strength hot-rolled galvanized high hole expansion steel embodiment, which will not be repeated here one by one.

[0105] The present application is further described below in conjunction with specific examples. The experimental methods in the following examples where specific conditions are not specified are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions or conditions recommended by the manufacturer.

[0106] The molten steels of Examples 1 to 8 and Comparative Examples 1 to 4 were obtained and cast into slabs with a slab thickness of 110 mm. The chemical composition of the molten steels is shown in Table 1.

[0107] Table 1 Chemical composition of molten steel (wt%)

[0108]

[0109]

[0110] Based on the chemical composition of the molten steel in the embodiment and the comparative example, this embodiment also provides a method for preparing high yield strength hot-rolled galvanized high hole expansion steel, comprising the following steps:

[0111] obtaining molten steel having the chemical composition, and continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab;

[0112] The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil;

[0113] The hot-rolled coil is pickled, continuously hot-dip galvanized and skin-passed in sequence to obtain hot-rolled galvanized high hole expansion steel; wherein the continuous hot-dip galvanizing includes: a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an aging section and a galvanizing section. The process parameters of the preparation are shown in Tables 2 and 3.

[0114] Table 2

[0115]

[0116]

[0117] Table 3 Process parameters of continuous hot-dip galvanizing

[0118]

[0119] The microstructure of the steel plate was detected and analyzed by metallographic microscope, scanning electron microscope and electron backscatter diffraction (EBSD); the mechanical properties and hole expansion rate of the steel plate were measured by tensile testing machine and forming testing machine, wherein the hole expansion rate of each embodiment or comparative example was randomly sampled from the steel coil and tested 100 times in parallel. The microstructure and properties of each embodiment and comparative example are shown in Table 4.

[0120] Table 4 Microstructure and properties of each embodiment and comparative example

[0121]

[0122]

[0123] It can be seen from Tables 1 to 4 that the yield strength of the hot-rolled galvanized high hole expansion steel prepared in the embodiment is >1050MPa, the tensile strength is >1250MPa, the elongation after fracture is >9%, and more than 95% of the hole expansion ratios reach ≥75%.

[0124] In Comparative Example 1, the mass fraction of Mn is not within the range of the embodiments of the present application, and the obtained high hole expansion steel has a banded structure, and the elongation after fracture is only 8%, and only 83% reaches a hole expansion rate ≥ 75%;

[0125] In Comparative Example 2, the sum of the mass fractions of Si+Al is not within the scope of the embodiments of the present application, and the elongation after fracture of the high hole expansion steel obtained is only 5%, and only 76% reaches a hole expansion rate ≥ 75%;

[0126] In Comparative Example 3, the final rolling temperature was not within the range of the embodiments of the present application, and the obtained high hole expansion steel had a banded structure, and only 80% reached a hole expansion rate of ≥75%;

[0127] In Comparative Example 4, the aging temperature is not within the range of the embodiments of the present application, and the yield strength of the obtained high hole expansion steel is only 995 MPa, and the tensile strength is only 1183 MPa.

[0128] Attached Figure 2 Detailed description:

[0129] Figure 2 The microstructure of the high yield strength hot-rolled galvanized high hole expansion steel provided in the embodiment of the present application; Figure 2 As shown, the microstructure of the high yield strength hot-rolled galvanized high hole expansion steel provided in the embodiment of the present application includes tempered martensite, residual austenite and ferrite, and the structure is uniform without obvious banding.

[0130] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0131] The contents of Mn, Cr, Mo, Cu, Ni and Nb in the steel of the embodiment of the present invention are lower than those of steel plates of the same strength level, and the cost advantage is obvious; the preparation process does not involve cold rolling, which further reduces the cost and improves the efficiency.

[0132] The above is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application 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 in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A high yield strength hot-rolled galvanized high hole expansion steel, wherein the chemical composition of the high hole expansion steel comprises, by mass fraction: C: 0.18% to 0.25%, Si: 0.05% to 0.3%, Mn: 1.0% to 1.4%, Al: 0.02% to 0.5%, P≤0.01%, S≤0.003%, Cr: 0.1% to 0.3%, Mo≤0.05%, Cu≤0.05%, Ni≤0.05%, Ti: 0.01% to 0.05%, B: 0.002% to 0.004%, and matrix element Fe; Si+Al≥0.3%, wherein Si+Al represents the sum of the mass fractions of Si and Al.

2. The high hole expansion steel according to claim 1, characterized in that: The banded structure of the high hole expansion steel is rated as level 0; in terms of area fraction, the microstructure of the high hole expansion steel includes: tempered martensite: 85% to 95%, retained austenite: 5% to 10%, and ferrite ≤5%.

3. The high hole expansion steel according to claim 1, characterized in that: The high hole expansion steel meets at least one of the following properties: yield strength>1050MPa, tensile strength>1250MPa, elongation after fracture>9%, and more than 95% of the high hole expansion steel has a hole expansion rate of ≥75%.

4. A method for preparing the high hole expansion steel according to any one of claims 1 to 3, the method comprising: obtaining molten steel having the chemical composition, and continuously casting the molten steel by a thin slab continuous casting and rolling process to obtain a slab; The slab is sequentially subjected to soaking treatment, rough rolling, electromagnetic induction heating, finish rolling, cooling and coiling to obtain a hot-rolled coil; The hot-rolled coil is pickled, continuously hot-dip galvanized and skin-passed in sequence to obtain hot-rolled galvanized high hole expansion steel; wherein the continuous hot-dip galvanizing includes: a preheating section, a heating section, a soaking section, a slow cooling section, a fast cooling section, an aging section and a galvanizing section.

5. The method according to claim 4, characterized in that The soaking treatment temperature is 1170°C to 1220°C; and / or, The inlet temperature of the rough rolling is 1080°C to 1120°C; and / or, The outlet temperature of the electromagnetic induction heating is ≥1250°C; and / or, The inlet temperature of the finishing rolling is 1180°C to 1220°C, and the final rolling temperature of the finishing rolling is 870°C to 900°C; and / or, The coiling temperature is 150°C to 200°C; and / or, The thickness of the hot rolled coil is 1.0 mm to 1.8 mm.

6. The method according to claim 4, characterized in that The heating section of the continuous hot-dip galvanizing includes a heating section 1, a pre-oxidation section and a heating section 2; and / or, The starting temperature of the heating stage 1 is 210°C to 230°C, the ending temperature of the heating stage 1 is 650°C to 700°C, and the heating rate of the heating stage 1 is ≥4°C / s; and / or, The temperature of the pre-oxidation stage is 650°C to 700°C, the time of the pre-oxidation stage is 13s to 20s, and the dew point temperature of the pre-oxidation stage is ≥ 0°C; and / or, The starting temperature of the second heating stage is 650°C to 700°C, the ending temperature of the second heating stage is 840°C to 880°C, and the heating rate of the second heating stage is 0.5°C / s to 1.5°C / s.

7. The method according to claim 4, characterized in that The temperature of the soaking section is 840° C. to 880° C., and the soaking time of the soaking section is 80s to 150s; and / or, The temperature of the slow cooling section is 760°C to 800°C; and / or, The temperature of the rapid cooling section is 200°C to 280°C.

8. The method according to claim 4, characterized in that The temperature of the aging stage is 280° C. to 350° C., and the time of the aging stage is 15s to 30s.

9. The method according to claim 4, characterized in that An induction heater 1 is arranged between the rapid cooling section and the aging section, and an induction heater 2 is arranged between the aging section and the zinc pot section; the heating rate of the induction heater 1 is 30°C / s to 50°C / s, and the heating rate of the induction heater 2 is 20°C / s to 40°C / s.

10. The finishing elongation of the finishing is 0.1% to 0.3%.

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