Preparation method of 1800MPa-grade short-process hot forming steel and hot forming steel

Abstract

The invention provides a preparation method of 1800MPa-grade short-process hot forming steel and the hot forming steel, and belongs to the field of steel preparation. The method comprises the following steps: continuously casting molten steel to obtain a casting blank; the casting blank is sequentially subjected to first heating, rough rolling, second heating, finish rolling, coiling and slow cooling, the second heating temperature is controlled to be a first set temperature, the coiling temperature is controlled to be a second set temperature, and a hot-rolled plate is obtained; and the hot-rolled plate is subjected to surface treatment and hot stamping, and the hot-formed steel is obtained. Through combination of reasonable component design and optimized technological process, the 1800MPa-grade hot forming steel with excellent strength and plasticity combination and low hydrogen embrittlement sensitivity is obtained. Therefore, the technical problem that the hydrogen embrittlement sensitivity is difficult to reduce on the basis of meeting the tensile strength of 1800 MPa in the prior art is solved.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to an 1800MPa grade short-process hot-formed steel and a preparation method thereof. Background Art

[0002] The application of hot-formed steel is an effective means to achieve lightweight and safety in automobiles. At present, the most widely used hot-formed steel is 1500MPa hot-formed steel, but with the increasingly urgent demand for lightweight and safety, the demand for higher-strength hot-formed steel is increasing. 1800MPa hot-formed steel is one of the hot-formed steel products with the highest strength. Compared with the commonly used 1500MPa hot-formed steel, it can achieve a weight reduction of 10% to 15% for parts. At present, most 1800MPa hot-formed steels are cold-rolled and annealed products, with long processes, high carbon emissions and high costs. Short-process technology is a low-carbon green preparation process. It produces thin plates with a thickness of 0.8mm to 2.0mm based on the thin-cast billet continuous casting and rolling production line. The cooling and reheating process of the billet is omitted during the production process, saving energy consumption. Compared with cold-rolled products of the same thickness specification, the products of the short-process production line save the cold rolling and annealing processes, and have lower energy consumption and costs.

[0003] However, when the tensile strength exceeds 1000MPa, there will be a serious risk of hydrogen embrittlement. Therefore, reducing the sensitivity to hydrogen embrittlement while meeting the tensile strength of 1800MPa is a technical problem that needs to be solved urgently. Summary of the invention

[0004] The present application provides a preparation method of 1800MPa-grade short-process hot-formed steel and hot-formed steel to solve the technical problem in the prior art that it is difficult to reduce the hydrogen embrittlement sensitivity while meeting the tensile strength of 1800MPa.

[0005] In a first aspect, the present application provides a method for preparing 1800MPa grade short process hot-formed steel, the method comprising:

[0006] Continuously casting molten steel to obtain a cast billet; the chemical composition of the cast billet includes, by mass fraction, C: 0.28% to 0.35%, Si: 0.2% to 1.0%, Mn: 1.0% to 3.0%, Alt: 0.02% to 0.10%, Cr: 0.2% to 1.5%, P≤0.015%, S≤0.015%, B: 0.001% to 0.010%, Ti: 0.01% to 0.08%, V: 0.02% to 0.15%, and the balance is Fe and unavoidable impurities;

[0007] The cast slab is sequentially subjected to first heating, rough rolling, second heating, finish rolling, coiling and slow cooling, and the temperature of the second heating is controlled to be a first set temperature, and the temperature of the coiling is controlled to be a second set temperature, to obtain a hot-rolled plate;

[0008] The hot rolled sheet is subjected to surface treatment and hot stamping to obtain hot formed steel.

[0009] Optionally, the first set temperature is 1050°C to 1200°C, and the second set temperature is 600°C to 720°C.

[0010] Optionally, the inlet temperature of the rough rolling is ≥1150°C, and the final rolling temperature of the finish rolling is 800°C to 860°C.

[0011] Optionally, the first heating temperature is 1140° C. to 1200° C.; the slow cooling is performed in a slow cooling pit, and the residence time in the slow cooling pit is 24 h to 72 h.

[0012] Optionally, the continuous casting speed is 4.0 m / min to 6.0 m / min, and the thickness of the ingot is 110 mm to 115 mm.

[0013] Optionally, the hot-rolled plate is subjected to surface treatment and hot stamping to obtain hot-formed steel, comprising:

[0014] Pickling and oiling the hot-rolled plate to obtain a pickled plate;

[0015] Blanking the pickled plate to obtain a sheet;

[0016] The sheet is heated and then transferred to a hot stamping device; the transfer time is 10s to 20s;

[0017] The blank transferred to the hot stamping device is hot stamped to obtain hot formed steel.

[0018] Optionally, the pickling speed is ≥100m / min; the oiling amount is 600mg / m 2 ~2000mg / m 2 The pickled plate meets at least one of the following properties: yield strength of 450MPa to 650MPa, tensile strength of 700MPa to 850MPa, and elongation A50 of 10% to 22%.

[0019] Optionally, the temperature of the soaking section of the heating is 900° C. to 950° C., and the soaking section time is 3 min to 10 min.

[0020] In a second aspect, the present application provides a hot-formed steel prepared by the method described in any one of the embodiments of the first aspect, wherein the metallographic structure of the hot-formed steel comprises: martensite, and the thickness of the hot-formed steel is 0.8 mm to 2.0 mm.

[0021] Optionally, the hot-formed steel satisfies at least one of the following properties: yield strength of 1200MPa to 1400MPa, tensile strength of 1800MPa to 2000MPa, elongation A50 of 5.0% to 10.0%, and hardness of 500HV to 600HV.

[0022] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0023] The present application provides a method for preparing 1800MPa-grade short-process hot-formed steel. By combining reasonable component design with optimized process flow, an 1800MPa-grade hot-formed steel with excellent strength and plasticity and low hydrogen embrittlement sensitivity is obtained. The idea of ​​V microalloying is adopted to ensure low hydrogen embrittlement sensitivity while ensuring rolling stability; the C content is controlled to be 0.28% to 0.35% to ensure that the tensile strength of the hot-formed steel reaches 1800MPa; the temperature of the second heating and coiling is controlled to avoid abnormal growth of the original austenite grains, which affects the mechanical properties and hydrogen embrittlement resistance. The yield strength of the obtained hot-formed steel is 1200MPa to 1400MPa, the tensile strength is 1800MPa to 2000MPa, and the elongation A50 is 5.0% to 10.0%. This solves the technical problem in the prior art that it is difficult to reduce the hydrogen embrittlement sensitivity on the basis of meeting the tensile strength of 1800MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] 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.

[0025] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0026] Figure 1 A flow chart of a method for preparing 1800MPa grade short-process hot-formed steel provided in an embodiment of the present application;

[0027] Figure 2 A schematic diagram of temperature changes during the hot stamping process in the preparation method provided in an embodiment of the present application;

[0028] Figure 3 The microstructure of the edge of the hot-rolled plate provided in Example 3 of the present application;

[0029] Figure 4 This is the microstructure of the core of the hot-rolled plate provided in Example 3 of the present application. DETAILED DESCRIPTION

[0030] 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.

[0031] 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.

[0032] In addition, in the description of the specification of the present application, the terms "including", "comprising", etc. mean "including but not limited to". In this article, relational terms such as "first" and "second", etc. 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. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" means one or more, and "plurality" means two or more. "At least one", "the following at least one item (items)" or similar expressions refer to any combination of these items, including any combination of single items (items) or plural items (items). For example, "at least one of a, b, or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e. a and b), ac, bc or abc, where a, b, c can be single or plural, respectively.

[0033] 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.

[0034] Figure 1 A flow chart of a method for preparing 1800MPa grade short-process hot-formed steel provided in an embodiment of the present application.

[0035] See also Figure 1 The present application provides a method for preparing 1800MPa grade short process hot-formed steel, the method comprising:

[0036] S1. Continuously casting molten steel to obtain a cast billet; the chemical composition of the cast billet includes, by mass fraction: C: 0.28%-0.35%, Si: 0.2%-1.0%, Mn: 1.0%-3.0%, Alt: 0.02%-0.10%, Cr: 0.2%-1.5%, P≤0.015%, S≤0.015%, B: 0.001-0.010%, Ti: 0.01%-0.08%, V: 0.02%-0.15%, and the balance is Fe and unavoidable impurities;

[0037] The positive effect of controlling the mass fraction of C to 0.28% to 0.35% is that the C element is the core element of hot-formed steel, which improves hardenability and ensures strength and hardness after hot stamping. However, the C content cannot be too high, as too high a content will increase brittleness and reduce hydrogen embrittlement resistance. For example, the mass fraction of C can be 0.28%, 0.29%, 0.30%, 0.31%, 0.32%, 0.33%, 0.34%, 0.35%, etc.

[0038] The positive effect of controlling the mass fraction of Si to 0.2% to 1.0% is that the Si element can ensure strength, but it cannot be too high, otherwise it will affect the surface quality. For example, the mass fraction of Si can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0039] The positive effect of controlling the mass fraction of Mn to 1.0% to 3.0% is that the Mn element improves hardenability and ensures strength, but it cannot be too high. Too high will cause the banding level to be too high, and edge cracking will occur when it is too high, especially for thin-gauge products. For example, the mass fraction of Mn can be 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.5%, 3.0%, etc.

[0040] The positive effect of controlling the mass fraction of Alt to 0.02% to 0.10% is that the Al element is added as a deoxidizer, which can refine the grains and improve the strength. The Al element should not be too high, as it will affect the plasticity and welding performance. For example, the mass fraction of Alt can be 0.02%, 0.03%, 0.04%, 0.05%, 0.07%, 0.08%, 0.09%, 0.10%, etc.

[0041] The positive effects of controlling the mass fraction of Cr to 0.2% to 1.5% are as follows: Cr can improve hardenability, strength, etc. For example, the mass fraction of Cr can be 0.2%, 0.3%, 0.5%, 0.7%, 1.0%, 1.2%, 1.4%, 1.5%, etc.

[0042] Positive effects of controlling the mass fraction of P ≤ 0.015%, S ≤ 0.015%: P and S are harmful elements in steel, and they are controlled within a suitable range in consideration of cost. For example, the mass fraction of P can be 0.005%, 0.007%, 0.009%, 0.012%, 0.015%, etc.; the mass fraction of S can be 0.005%, 0.007%, 0.009%, 0.012%, 0.015%, etc.

[0043] The positive effect of controlling the mass fraction of B to 0.001-0.010% is that the main function of boron in steel is to increase the hardenability of steel, thereby saving other rare and precious metals. For example, the mass fraction of B can be 0.001, 0.002%, 0.004%, 0.005%, 0.007%, 0.008%, 0.010%, etc.

[0044] The positive effect of controlling the mass fraction of Ti to 0.01% to 0.08% is that the Ti element combines with the N element to avoid the combination of the B element and the N element to form BN. The formation of BN will weaken the effect of the B element addition to ensure hardenability. In addition, the Ti element also has the effect of refining grains. Exemplarily, the mass fraction of Ti can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.07%, 0.08%, etc.

[0045] The positive effect of controlling the mass fraction of V to 0.02% to 0.15% is that the addition of V can refine the grains (hydrogen element distribution is more uniform, avoiding local hydrogen aggregation and causing hydrogen embrittlement), form V precipitates (forming hydrogen traps, capturing hydrogen, and avoiding hydrogen movement), and improve the hydrogen embrittlement resistance. Especially for the 1800MPa grade hot-formed steel of the present invention, the hydrogen embrittlement resistance is particularly critical, but the V element cannot be added too much. The V element will combine with the C element, weakening the role of the C element in ensuring strength, resulting in low strength of the parts after hot stamping. Another consideration for the present invention to select the addition of the V element is to ensure the stability of the rolling of thin-gauge products. The effect of the V element on improving the rolling resistance after the addition of the V element is very small, ensuring that the rolling resistance is within a suitable range during rolling. Exemplarily, the mass fraction of V can be 0.02%, 0.04%, 0.06%, 0.08%, 0.10%, 0.12%, 0.14%, 0.15%, etc.

[0046] In some embodiments, before the S1 step, the process further includes: subjecting the molten iron to KR desulfurization, converter, LF refining, and VD refining in sequence to obtain molten steel.

[0047] In some embodiments, the continuous casting speed is 4.0 m / min to 6.0 m / min, and the thickness of the cast billet is 110 mm to 115 mm.

[0048] The reason why the continuous casting pulling speed is limited to 4.0m / min to 6.0m / min is that in the short-process continuous casting and rolling production line, a high pulling speed is conducive to the control of banding and segregation, the uniformity of mechanics and hardness after hot stamping, and the improvement of hydrogen embrittlement resistance. Exemplarily, the continuous casting pulling speed can be 4.0m / min, 4.5m / min, 5.0m / min, 5.2m / min, 5.5m / min, 6.0m / min, etc. The thickness of the ingot can be 110mm, 111mm, 112mm, 114mm, 115mm, etc.

[0049] S2, sequentially subjecting the cast billet to first heating, rough rolling, second heating, finish rolling, coiling and slow cooling, and controlling the temperature of the second heating to be a first set temperature, and the temperature of the coiling to be a second set temperature, to obtain a hot-rolled plate;

[0050] In some embodiments, the first set temperature is 1050°C to 1200°C, and the second set temperature is 600°C to 720°C.

[0051] In some embodiments, the second heating is performed by induction heating.

[0052] The second heating temperature is limited to the first set temperature, and the first set temperature is limited to 1050℃~1200℃, in order to: ensure sufficient temperature in the finishing rolling stage, ensure rolling stability; and avoid abnormal growth of original austenite grains, thereby affecting mechanical properties and hydrogen embrittlement resistance. Exemplarily, the first set temperature can be 1050℃, 1080℃, 1100℃, 1130℃, 1150℃, 1180℃, 1200℃, etc.

[0053] The coiling temperature is limited to the second set temperature, and the second set temperature is limited to 600℃~720℃ in order to: ensure the good coil shape of thin hot coils, ensure smooth unloading, rolling stability, avoid bruises, etc.; at the same time, ensure good control of the strip shape, which is conducive to the stability of mechanics and hardness after hot stamping, and good hydrogen embrittlement resistance. Exemplarily, the second set temperature can be 600℃, 620℃, 650℃, 680℃, 700℃, 720℃, etc.

[0054] In some embodiments, the inlet temperature of the rough rolling is ≥1150°C, and the final rolling temperature of the finish rolling is 800°C to 860°C.

[0055] The inlet temperature of the rough rolling is limited to ≥1150°C in order to ensure the stability of the rolling under the condition of a large reduction ratio in the rough rolling stage. For example, the inlet temperature of the rough rolling can be 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, etc.

[0056] The purpose of limiting the final rolling temperature of the finishing rolling to 800°C to 860°C is to ensure stable rolling. For example, the final rolling temperature of the finishing rolling can be 800°C, 810°C, 820°C, 830°C, 840°C, 850°C, 860°C, etc.

[0057] In some embodiments, the rough rolling is performed 3 to 5 times, and the finish rolling is performed 5 to 7 times.

[0058] In some embodiments, a heat-insulating cover is placed on the roller table after the finish rolling.

[0059] In some embodiments, the first heating temperature is 1140° C. to 1200° C.; the slow cooling is performed in a slow cooling pit, and the residence time in the slow cooling pit is 24 h to 72 h.

[0060] The first heating temperature is limited to 1140°C to 1200°C in order to: facilitate the temperature uniformity of the billet and avoid the billet corner temperature being too low. Exemplarily, the first heating temperature may be 1140°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, etc.

[0061] The slow cooling is limited to the slow cooling pit, and the residence time in the slow cooling pit is limited to 24h~72h, in order to ensure the uniformity of the whole hot coil and avoid defects such as edge cracks caused by too fast cooling of the edge. For example, the residence time in the slow cooling pit can be 24h, 30h, 36h, 40h, 46h, 50h, 60h, 72h, etc.

[0062] S3. Surface treatment and hot stamping are performed on the hot rolled plate to obtain hot formed steel.

[0063] Figure 2 Schematic diagram of temperature change during hot stamping process in the preparation method provided in an embodiment of the present application.

[0064] See also Figure 2 In some embodiments, the hot-rolled plate is subjected to surface treatment and hot stamping to obtain hot-formed steel, comprising:

[0065] Pickling and oiling the hot-rolled plate to obtain a pickled plate;

[0066] Blanking the pickled plate to obtain a sheet;

[0067] The sheet is heated and then transferred to a hot stamping device; the transfer time is 10s to 20s;

[0068] The blank transferred to the hot stamping device is hot stamped to obtain hot formed steel.

[0069] The transfer time is limited to 10s to 20s in order to: avoid the austenitized sheet temperature being too low, thereby preventing the inability to perform normal high-precision hot stamping, resulting in low precision after stamping and stamping cracking defects. At the same time, timely transfer avoids excessive temperature drop of the sheet, ensures a stable stamping rhythm, and the upper and lower molds are quickly closed after the sheet is positioned, and formed in a mold with a water channel. Exemplarily, the transfer time can be 10s, 12s, 14s, 16s, 18s, 20s, etc.

[0070] In some embodiments, the pickling speed is ≥100 m / min; the oiling amount is 600 mg / m 2 ~2000mg / m 2 The pickled plate meets at least one of the following properties: yield strength of 450MPa to 650MPa, tensile strength of 700MPa to 850MPa, and elongation A50 of 10% to 22%.

[0071] The pickling speed is limited to ≥100 m / min in order to: ensure a high enough pickling speed to ensure high surface quality after pickling and avoid surface defects such as yellowing. In addition, ensure the effect of rinsing and squeezing the rollers to avoid metal ion residue. Exemplarily, the pickling speed can be 100 m / min, 110 m / min, 120 m / min, 130 m / min, 140 m / min, 150 m / min, etc.

[0072] The oiling amount is limited to 600mg / m 2 ~2000mg / m 2 , is to: apply enough oil to avoid the risk of rust on the steel coil. For example, the amount of oil applied can be 600mg / m 2 , 700mg / m 2 , 900mg / m 2 , 1000mg / m 2 , 1200mg / m 2 , 1600mg / m 2 , 2000mg / m 2 wait.

[0073] In some embodiments, the temperature of the heated soaking section is 900° C. to 950° C., and the soaking section time is 3 min to 10 min.

[0074] The purpose of limiting the temperature of the soaking section of heating to 900℃~950℃ and limiting the soaking section time to 3min~10min is to: ensure that the sheet is completely and evenly austenitized at a sufficiently high temperature and a sufficiently long time, so as to avoid taking too long to complete austenitization and affecting production efficiency; and at the same time, avoid abnormal growth of the original austenite grains, resulting in too large grains and poor uniformity, affecting mechanical and hydrogen embrittlement resistance. Exemplarily, the temperature of the soaking section of heating is 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, etc., and the soaking section time can be 3min, 4min, 5min, 6min, 7min, 8min, 9min, 10min, etc.

[0075] In some embodiments, the method further comprises: laser cutting, shot peening, welding, electrophoresis and baking paint; the baking paint temperature is 175° C. and the time is 20 minutes.

[0076] Specifically, it can be: laser cutting to obtain precise dimensions, in addition to drilling and other processes; shot peening is performed after the parts have obtained precise dimensions to remove the oxide scale on the surface; hot-formed parts and other contacting parts are connected by spot welding; the body in white is electrophoresed and then painted.

[0077] Based on a general inventive concept, the present application provides a hot-formed steel, the metallographic structure of the hot-formed steel includes: martensite, and the thickness of the hot-formed steel is 0.8 mm to 2.0 mm.

[0078] Compared with cold-rolled annealed products, the present invention omits cold rolling and annealing processes, achieving lower carbon emissions. Compared with traditional hot rolling production lines, the headless mode of the continuous casting and rolling production line based on the present invention can achieve a thinnest 0.8mm. Limited by rolling stability, traditional hot rolling production lines have difficulty in specifications below 1.8mm. Currently, most hot-formed steel specifications are ≤1.8mm, and the present invention can cover almost all specifications. Exemplarily, the thickness of the hot-formed steel can be 0.8mm, 1.0mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm, 2.0mm, etc.

[0079] In some embodiments, the hot-formed steel satisfies at least one of the following properties: yield strength of 1200MPa to 1400MPa, tensile strength of 1800MPa to 2000MPa, elongation A50 of 5.0% to 10.0%, and hardness of 500HV to 600HV.

[0080] This application relies on a short-process production line, combines reasonable component design with optimized process flow, and obtains a combination of excellent strength and plasticity, and 1800MPa-grade hot-formed steel with low hydrogen embrittlement sensitivity while reducing production costs and carbon emissions. Exemplarily, the yield strength can be 1200MPa, 1250MPa, 1300MPa, 1350MPa, 1400MPa, etc., the tensile strength can be 1800MPa, 1850MPa, 1900MPa, 1950MPa, 2000MPa, etc., the elongation A50 can be 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, 10.0%, etc., and the hardness can be 500HV, 510HV, 520HV, 530HV, 540HV, 560HV, 580HV, 600HV, etc.

[0081] The hot-formed steel is realized based on the preparation method of the hot-formed steel mentioned above. The specific steps of the preparation method of the hot-formed steel can refer to the above-mentioned embodiment. Since the hot-formed steel adopts part or all of the technical solutions of the above-mentioned embodiment, it has at least all the beneficial effects brought by the technical solutions of the above-mentioned embodiment, which will not be described one by one here.

[0082] 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 which specific conditions are not specified in the following examples are usually measured according to industry standards. If there is no corresponding industry standard, then the conditions recommended by the manufacturer are followed.

[0083] Example 1

[0084] The present embodiment provides a method for preparing hot-formed steel, comprising the following steps: molten iron is sequentially subjected to KR desulfurization, converter, LF refining, and VD refining to obtain molten steel; the molten steel is continuously cast, and the pulling speed is controlled at 4.8 m / min to obtain a cast billet with a thickness of 110 mm; the cast billet is fed into a roller bottom tunnel soaking furnace for soaking treatment, and the soaking temperature is controlled at 1190°C, and then rough rolling is performed, and the inlet temperature is 1170°C. The cast billet undergoes 3 rough rollings, and the cast billet is heated by an induction heating device, and 8 groups of induction heating are opened, and the induction heating outlet temperature is controlled at 1190°C. The finishing rolling adopts 5 rolling passes, and the final rolling temperature is controlled at 840°C. After finishing rolling, an insulation cover is put on the roller table, and the coiling temperature is 670°C. Stay in the insulation pit for 72 hours. The hot coil is pickled, and the pickling speed is 120 m / min. The thickness of the steel coil after pickling is 1.2 mm, and the oil coating amount of the steel coil is 1400 mg / m 2 The steel coil undergoes uncoiling, laser blanking, tunnel furnace heating (heating temperature is 930℃, holding time is 5min), hot stamping, sheet transfer time is 15s, laser cutting, shot peening, welding, electrophoresis and baking paint to obtain hot-formed steel.

[0085] Example 2

[0086] The present embodiment provides a method for preparing hot-formed steel, comprising the following steps: molten iron is sequentially subjected to KR desulfurization, converter, LF refining, and VD refining to obtain molten iron; the molten iron is continuously cast, and the pulling speed is controlled at 4.6 m / min to obtain a cast billet with a thickness of 115 mm, and the cast billet is fed into a roller bottom tunnel soaking furnace for soaking treatment, and the soaking temperature is controlled at 1195°C. The inlet temperature of rough rolling is 1160°C, and the cast billet undergoes 3 rough rollings. The cast billet is heated by an induction heating device, 8 groups of induction heating are opened, and the outlet temperature of induction heating is controlled at 1185°C. The finishing rolling adopts 5 rolling passes, and the final rolling temperature is controlled at 837°C. After finishing rolling, an insulation cover is put on the roller table, and the coiling temperature is 685°C. Stay in the insulation pit for 48 hours. The hot coil is pickled, and the pickling speed is 115 m / min. The thickness of the steel coil after pickling is 1.3 mm, and the oil coating amount of the steel coil is 1600 mg / m 2 The steel coil undergoes uncoiling, laser blanking, tunnel furnace heating (heating temperature is 930℃, holding time is 5min), hot stamping, sheet transfer time is 15s, laser cutting, shot peening, welding, electrophoresis and baking paint to obtain hot-formed steel.

[0087] Example 3

[0088] The present embodiment provides a method for preparing hot-formed steel, comprising the following steps: molten iron is sequentially subjected to KR desulfurization, converter, LF refining, and VD refining to obtain molten iron; the molten iron is continuously cast, and the pulling speed is controlled at 4.8 m / min to obtain a cast billet with a thickness of 110 mm, and the cast billet is fed into a roller bottom tunnel soaking furnace for soaking treatment, and the soaking temperature is controlled at 1190°C. The rough rolling inlet temperature is 1165°C, and the cast billet undergoes 3 rough rollings. The cast billet is heated by an induction heating device, 8 groups of induction heating are opened, and the induction heating outlet temperature is controlled at 1188°C. The finishing rolling adopts 5 rolling passes, and the final rolling temperature is controlled at 829°C. After finishing rolling, an insulation cover is put on the roller table, and the coiling temperature is 664°C. Stay in the insulation pit for 48 hours. The hot coil is pickled, and the pickling speed is 130 m / min. The thickness of the steel coil after pickling is 1.5 mm, and the oil coating amount of the steel coil is 1500 mg / m 2 The steel coil undergoes uncoiling, laser blanking, tunnel furnace heating (heating temperature is 930℃, holding time is 5min), hot stamping, sheet transfer time is 15s, laser cutting, shot peening, welding, electrophoresis and baking paint to obtain hot-formed steel.

[0089] Comparative Example 1

[0090] This comparative example provides a method for preparing hot-formed steel, which is the method for preparing 1800MPa cold-rolled hot-formed steel in the article "Organization and Properties of 1800MPa Cold-rolled Hot-formed Steel" in the Journal of Materials Heat Treatment, Volume 38, Issue 7, July 2017, and includes the following steps: heating the ingot to 1200℃ and keeping it for 1h before forging, the final forging temperature is 800℃, and forging into a forging blank with a specification of 40mm*90mm*140mm. The forging is heated to 1200℃ and kept for 1h, the starting rolling temperature is 1100℃, and it is hot-rolled from 40mm to 6mm in 6 passes, the final rolling temperature is 870℃, and it is water-cooled to 660℃ after rolling to simulate coiling. The hot-rolled plate is cold-rolled to 1.5mm after pickling. Hot stamping simulation is carried out on a continuous annealing simulator, and the optimal mechanical properties are obtained by keeping it at 850℃ + quenching to obtain hot-formed steel.

[0091] Comparative Example 2

[0092] This comparative example provides a method for preparing hot-formed steel, which is the method for preparing 1800MPa cold-rolled hot-formed steel in the article "Application Research of 1800MPa Cold-rolled Hot-formed Steel" in the 12th issue of "Automotive Technology and Materials" in 2020, and includes the following steps: the thickness of the cold-rolled steel plate is 1.4mm, and the hot stamping process is: magnetic sheeting, feeding, heating in a roller-type heating furnace, sheet removal, transportation, flat die press quenching, pressure holding, marking, and shot blasting to remove oxide scale. The protective gas is nitrogen, the size of the flat die stamping sheet is 300*600mm, the heating temperature is 910℃, the heating time is 280s, the pressure holding time is 10s, and the pressure holding pressure is 4500KN to obtain hot-formed steel.

[0093] Comparative Example 3

[0094] This comparative example provides a method for preparing hot-formed steel, comprising the following steps: molten iron is sequentially subjected to KR desulfurization, converter, LF refining, and VD refining to obtain molten iron; the molten iron is continuously cast, and the pulling speed is controlled at 4.8 m / min to obtain a cast billet with a thickness of 110 mm, and the cast billet is fed into a roller bottom tunnel soaking furnace for soaking treatment, and the soaking temperature is controlled at 1190°C. The inlet temperature of rough rolling is 1170°C, and the cast billet undergoes 3 rough rollings. The cast billet is heated by an induction heating device, 8 groups of induction heating are opened, and the outlet temperature of induction heating is controlled at 1190°C. The finishing rolling adopts 5 rolling passes, and the final rolling temperature is controlled at 840°C. After finishing rolling, an insulation cover is put on the roller table, and the coiling temperature is 670°C. Stay in the insulation pit for 72 hours. The hot coil is pickled, and the pickling speed is 120 m / min. The thickness of the steel coil after pickling is 1.2 mm, and the oil coating amount of the steel coil is 1400 mg / m 2 The steel coil undergoes uncoiling, laser blanking, tunnel furnace heating (heating temperature is 930℃, holding time is 5min), hot stamping, sheet transfer time is 15s, laser cutting, shot peening, welding, electrophoresis and baking paint to obtain hot-formed steel.

[0095] The chemical compositions of the ingots in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1.

[0096] Table 1 Chemical composition of the cast slabs in Examples 1 to 3 and Comparative Examples 1 to 3 (wt%)

[0097] Group C Si Mn P S Alt Cr B Ti V Nb Example 1 0.31 0.29 1.7 0.01 0.0012 0.03 0.4 0.0020 0.040 0.050 —— Example 2 0.29 0.52 1.2 0.012 0.0015 0.04 0.7 0.0025 0.040 0.070 —— Example 3 0.33 0.46 1.4 0.013 0.0014 0.04 0.2 0.0025 0.050 0.040 —— Comparative Example 1 0.31 0.3 1.52 ≤0.005 0.02 0.03 1.02 0.0023 0.076 —— 0.046 Comparative Example 2 0.33 0.28 1.46 0.010 0.007 0.055 0.22 0.0030 —— —— —— Comparative Example 3 0.31 0.29 1.7 0.01 0.0012 0.03 0.4 0.0020 0.040 —— ——

[0098] The pickled plates of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to mechanical property tests, and the hot-formed steels of Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to mechanical property and hydrogen embrittlement resistance tests, and the results are shown in Table 2. The hydrogen embrittlement resistance of the hot-formed steel was evaluated by a slow strain rate tensile test, using plastic loss as an evaluation index, the hydrogen charging solution was a 0.1 mol / L NaOH solution, the hydrogen charging time was 24 h, and the hydrogen charging current was 1 mA / cm 2The surface of the slow tensile specimen after hydrogen filling is smooth without hydrogen bulging. The calculation formula for plastic loss is shown below.

[0099]

[0100] Table 2 Mechanical properties and hydrogen embrittlement resistance of pickled plates and hot-formed steels of Examples 1 to 3 and Comparative Examples 1 to 3

[0101]

[0102] From Table 1 and Table 2, we can see that:

[0103] (1) The production process of the embodiment of the present invention is shorter, and compared with the comparative example, the steps of pickling, cold rolling and annealing are omitted, which has lower cost and lower carbon emissions;

[0104] (2) The mechanical properties of the hot-formed steel in the embodiment of the present invention are: yield strength after hot forming is 1200MPa-1400MPa, tensile strength is 1800MPa-2000MPa, and elongation is 5.0%-10.0% (A50). The current demand for hot-formed steel in the automotive industry is met, and the beneficial effects of the present invention are achieved;

[0105] (3) The embodiment of the present invention adds V microalloying element, which reduces the risk of hydrogen embrittlement.

[0106] Figure 3 The microstructure of the edge of the hot-rolled plate provided in Example 3 of the present application.

[0107] Depend on Figure 3 It can be seen that there is no decarburization phenomenon at the edge, which can meet the uniformity of the structure and hardness after hot stamping.

[0108] Figure 4 This is the microstructure of the core of the hot-rolled plate provided in Example 3 of the present application.

[0109] Depend on Figure 4 It can be seen that the structure is ferrite + pearlite, the structure is uniform, and the uniformity of the original hot-rolled plate ensures the uniformity of the structure and hardness after hot stamping.

[0110] 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 method for preparing 1800MPa grade short-process hot-formed steel, characterized in that: The method comprises: Continuously casting molten steel to obtain a cast billet; the chemical composition of the cast billet includes, by mass fraction, C: 0.28% to 0.35%, Si: 0.2% to 1.0%, Mn: 1.0% to 3.0%, Alt: 0.02% to 0.10%, Cr: 0.2% to 1.5%, P≤0.015%, S≤0.015%, B: 0.001% to 0.010%, Ti: 0.01% to 0.08%, V: 0.02% to 0.15%, and the balance is Fe and unavoidable impurities; The cast slab is sequentially subjected to first heating, rough rolling, second heating, finish rolling, coiling and slow cooling, and the temperature of the second heating is controlled to be a first set temperature, and the temperature of the coiling is controlled to be a second set temperature, to obtain a hot-rolled plate; The hot rolled sheet is subjected to surface treatment and hot stamping to obtain hot formed steel.

2. The method according to claim 1, characterized in that The first set temperature is 1050°C to 1200°C, and the second set temperature is 600°C to 720°C.

3. The method according to claim 1, characterized in that The inlet temperature of the rough rolling is ≥1150°C, and the final rolling temperature of the finish rolling is 800°C to 860°C.

4. The method according to claim 1, characterized in that: The temperature of the first heating is 1140° C. to 1200° C.; the slow cooling adopts a slow cooling pit, and the residence time in the slow cooling pit is 24 hours to 72 hours.

5. The method according to claim 1, characterized in that The continuous casting speed is 4.0 m / min to 6.0 m / min, and the thickness of the cast billet is 110 mm to 115 mm.

6. The method according to claim 1, characterized in that The hot-rolled plate is subjected to surface treatment and hot stamping to obtain hot-formed steel, comprising: Pickling and oiling the hot-rolled plate to obtain a pickled plate; Blanking the pickled plate to obtain a sheet; The sheet is heated and then transferred to a hot stamping device; the transfer time is 10s to 20s; The blank transferred to the hot stamping device is hot stamped to obtain hot-formed steel.

7. The method according to claim 6, characterized in that The pickling speed is ≥100m / min; the oiling amount is 600mg / m 2 ~2000mg / m 2 The pickled plate meets at least one of the following properties: yield strength of 450MPa to 650MPa, tensile strength of 700MPa to 850MPa, and elongation A50 of 10% to 22%.

8. The method according to claim 6, characterized in that The temperature of the heating soaking section is 900° C. to 950° C., and the soaking time is 3 min to 10 min.

9. A hot-formed steel prepared by the method according to any one of claims 1 to 8, characterized in that: The metallographic structure of the hot-formed steel includes martensite, and the thickness of the hot-formed steel is 0.8 mm to 2.0 mm.

10. The hot-formed steel according to claim 9, characterized in that The hot-formed steel satisfies at least one of the following properties: a yield strength of 1200MPa to 1400MPa, a tensile strength of 1800MPa to 2000MPa, an elongation A50 of 5.0% to 10.0%, and a hardness of 500HV to 600HV.

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