Ultrahigh-strength steel plate as well as preparation method and application thereof

Abstract

The invention discloses an ultrahigh-strength steel plate as well as a preparation method and application thereof, and belongs to the technical field of high-strength steel welding. The ultrahigh-strength steel plate comprises the following chemical components in percentage by mass: 0.17%-0.23% of C, 0.90%-1.55% of Si, 1.50%-2.80% of Mn, less than or equal to 0.009% of P, less than or equal to 0.009% of S, 0.01%-0.15% of Als, 0.08%-0.70% of Cr, 0.02%-0.20% of Nb + Ti, less than or equal to 0.0065% of N and the balance of Fe and impurities. According to the ultrahigh-strength steel plate, bainite is adopted as a matrix structure, the ultrahigh-strength steel plate is composed of fine and regular carbide-free bainite lath bundles and retained austenite distributed between the lath bundles on a bainite matrix, and meanwhile a very small number of tempered martensite structures exist; according to the invention, a bainite-based product is obtained by regulating and controlling a production process, so that excellent local forming performance can be achieved while excellent strength and plasticity can be ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel welding, and particularly relates to an ultra-high-strength steel plate, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, energy conservation and emission reduction work has been carried out in various industries. Especially in the automotive field, in order to reduce vehicle fuel consumption and cut CO2 emissions, lightweight work has been continuously promoted. Along with the improvement of safety regulations requirements, the work of replacing ordinary steel with high-strength steel plates has brought remarkable effects. Spot welding is the most widely used welding method in the automotive industry. However, the alloy element content in high-strength steel is relatively high, which has a great impact on its weldability. The spot welding performance of high-strength steel has become one of the factors restricting its development.

[0003] For the spot welding performance of high-strength steel for automobiles, the tensile strength of the spot weld is an important index. Usually, TSS (tensile shear strength) and CTS (cross tensile strength) are used for evaluation. TSS refers to the anti-tensile shear force measured by loading a tensile load in the shear direction of the spot weld, and CTS refers to the cross tensile force measured by loading a tensile load in the peeling direction of the spot weld. The measurement methods are specified in JIS Z 3136 and JIS Z 3137 standards.

[0004] The patent with the publication number CN110724877A discloses a 1180MPa grade high-plasticity bainite dual-phase steel plate for automobiles and its preparation method. Specifically, the chemical composition of the 1180MPa grade high-plasticity bainite dual-phase steel plate for automobiles by weight percentage is: C: 0.03% - 0.20%, Si: 0.05% - 1.50%, Mn: 0.5% - 2.5%, P ≤ 0.05%, S ≤ 0.05%, Nb: 0.01% - 0.04%, Ti: 0.01% - 0.04%, V: 0.01% - 0.15%, Al: 0.015% - 1.5%, and the alternative components contain, by weight percentage: Cr: 0 - 2%, Mo: 0 - 0.5%, Cu: 0.05 - 0.5%, and the balance is Fe and inevitable impurities; the rolling process is: the hot continuous rolling slab is heated into the furnace at a temperature between 400 - 750°C, the heating temperature is between 1150 - 1250°C, the starting rolling temperature is between 1150 - 1250°C, the final rolling temperature is above 900°C, and the coiling temperature is between 600 - 750°C; the annealing process is: the annealing temperature is between 780 - 880°C, the annealing time is 1 - 5min, the cooling rate is greater than 30°C / s, cooled to the aging temperature, the aging temperature is between 300 - 500°C, the aging time is 300 - 600s, and then cooled to room temperature. However, the ranges of C, Si, and Mn contents in this patent are very broad. In the low C, low Si, and low Mn states, it is not sufficient to achieve the formation of the strength grade of the product, and the hot rolling process only provides the heating temperature, finish rolling temperature, and coiling temperature, without clearly defining the cooling path and intermediate temperature control, which is not conducive to the stable control of the product's tissue properties and surface quality.

[0005] The patent with the publication number CN108136535A discloses a spot welding joint and a spot welding method. Specifically, it discloses a spot welding joint and its welding method that can obtain a high CTS even when including more than 1 sheet of high-strength steel plate. The material used has at least one side as a high-strength steel plate, and the effect of regulating the tissue is achieved by controlling the heat input, thereby improving the CTS strength. However, this patent does not describe the spot welding process.

[0006] The patent with the publication number CN112338334A discloses a resistance spot welding joint of aluminum materials. Specifically, it discloses a resistance spot welding joint of aluminum materials with a weld nugget shape that can reduce the deviation of CTS strength. However, this patent achieves the effect of improving the CTS performance by regulating the pores of the spot welding joint, and the material and process are quite different from those of the present invention. Summary of the Invention

[0007] The purpose of the present invention is to provide an ultra-high strength steel plate, its preparation method, and application, which are used to solve the technical problem of poor spot welding performance of high-strength steel in the prior art.

[0008] To achieve the above object, in one embodiment of the present invention, a super high strength steel plate is provided, which comprises the following chemical components in mass percentage ratio:

[0009] C: 0.17% - 0.23%, Si: 0.90% - 1.55%, Mn: 1.50% - 2.80%, P ≤ 0.009%, S ≤ 0.009%, Als: 0.01% - 0.15%, Cr: 0.08% - 0.70%, Nb + Ti: 0.02% - 0.20%, N ≤ 0.0065%, and the balance is Fe and impurities.

[0010] One of the preferred solutions of the present invention is that the microstructure of the super high strength steel plate consists of 75% - 90% bainite, 5% - 10% martensite, and 2% - 8% retained austenite.

[0011] One of the preferred solutions of the present invention is that the yield strength of the super high strength steel plate is 850MP - 940MPa, the tensile strength is 1200MP - 1350MPa, the elongation at break A 50 value is 16% - 23%, and the hole expansion rate is 25% - 38%.

[0012] The present invention also discloses a preparation method of the super high strength steel plate, which comprises the following steps:

[0013] Smelt the molten steel according to the chemical components of the super high strength steel plate;

[0014] The molten steel after smelting is continuously cast into a slab;

[0015] The slab is heated, descaled, rough rolled, finish rolled, and laminar cooled to obtain a hot rolled coil;

[0016] After pickling the hot rolled coil, cold rolling is carried out to obtain a cold rolled thin strip steel coil;

[0017] The cold rolled thin strip steel coil is continuously annealed to obtain the super high strength steel plate.

[0018] One of the preferred solutions of the present invention is that smelting the molten steel according to the chemical components of the super high strength steel plate includes: during the smelting process, the sulfur in the furnace is controlled below 0.015%, the tapping temperature is 1600°C - 1650°C, aluminum iron is used for deoxidation, and the addition amount of aluminum iron is 450kg / furnace - 480kg / furnace.

[0019] One of the preferred solutions of the present invention is that medium carbon ferromanganese is added when 30% - 38% of the steel is tapped and added completely when 65% - 75% of the steel is tapped; argon blowing is carried out on the small platform for 4.5min - 5min and then the oxygen is determined, and aluminum wire is fed according to the oxygen content; the tapping temperature of LF refining is 1580°C - 1590°C.

[0020] One of the preferred embodiments of the present invention is that the molten steel after smelting is continuously cast into a slab, including: using a special protective slag for low-carbon aluminum-killed steel during continuous casting, controlling the tundish temperature at 1500°C - 1550°C, controlling the liquidus temperature at 1490°C - 1510°C, and the target casting speed at 1.5 m / min - 1.7 m / min.

[0021] One of the preferred embodiments of the present invention is that the slab is heated, descaled, rough rolled, finish rolled, and laminar cooled to obtain a hot rolled coil, including: the finish rolling start temperature is 1150°C - 1230°C, the coiling temperature is 650°C - 670°C, and the laminar cooling adopts the front-segment centralized cooling method.

[0022] One of the preferred embodiments of the present invention is that the cold rolled thin strip steel coil is continuously annealed to obtain an ultra-high strength steel plate, including: the overaging temperature is 350°C - 370°C, and the overaging holding time is 1200 s - 1500 s.

[0023] The present invention also discloses an application of the ultra-high strength steel plate, using the ultra-high strength steel plate in automobile manufacturing.

[0024] One of the preferred embodiments of the present invention is that the ultra-high strength steel plate for automobile manufacturing is pretreated before spot welding.

[0025] One of the preferred embodiments of the present invention is that the baking temperature of the pretreatment is 200°C - 350°C, and the baking time is 20 min - 120 min.

[0026] In summary, the beneficial effects of the present invention are as follows:

[0027] 1. The ultra-high strength steel plate of the present invention uses bainite as the matrix structure, which is composed of fine and regular carbide-free bainite lath bundles and retained austenite distributed between the lath bundles in the bainite matrix. At the same time, there is also a very small amount of tempered martensite structure; the present invention obtains a bainite-based product by regulating the production process, which can not only ensure excellent strength and plasticity but also has excellent local forming performance (such as flanging and hole expansion).

[0028] 2. During the production process of automobile manufacturing with the ultra-high strength steel plate of the present invention, pretreating the ultra-high strength steel plate before spot welding can recover dislocations, eliminate the residual stress of the sheet, and regulate part of the martensite and retained austenite into tempered martensite structure. This method can reduce the martensite content in the heat affected zone, improve the toughness of the spot welded joint, and thus improve the CTS performance, which is of great significance for the popularization and application of this product in the automobile manufacturing industry.

[0029] Other features and advantages of the present invention will be described in the following specification, and, in part, will be obvious from the specification, or will be understood by implementing the present invention. The objectives and other advantages of the present invention can be illustrated by the effects described in the specification and the accompanying drawings. Description of the Drawings

[0030] Figure 1 It is a schematic flow chart of the preparation method of the ultra-high strength steel plate in the embodiment of the present invention;

[0031] Figure 2 It is a schematic diagram of the microstructure of the ultra-high strength steel plate in the embodiment of the present invention. Detailed Embodiments

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0033] In the present invention, the endpoints and any values within the disclosed ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0034] The present invention provides an ultra-high strength steel plate, including the following chemical components in mass percentage:

[0035] C: 0.17%-0.23%, Si: 0.90%-1.55%, Mn: 1.50%-2.80%, P≤0.009%, S≤0.009%, Als: 0.01%-0.15%, Cr: 0.08%-0.70%, Nb+Ti: 0.02%-0.20%, N≤0.0065%, and the balance is Fe and impurities.

[0036] Among them, the functions of each element are as follows:

[0037] C: Forms cementite (Fe3C) or dissolves in ferrite, improving the strength and hardness of the steel. For every 0.1% increase in C, the tensile strength increases by about 80-100 MPa. However, too high a C content will reduce weldability;

[0038] Si: Easily reacts with oxygen to form SiO2, having a deoxidizing effect, and can play a role in solid solution strengthening in the steel. Every 1% Si increases the yield strength by about 80 MPa. However, too high a content will reduce plasticity and weldability, and can balance the toughness of the steel when combined with Mn and Cr;

[0039] Mn: Improves the hardenability of steel and reduces the critical cooling rate. Reacts with S to form high-melting-point MnS, which has the function of desulfurization, thus avoiding the formation of FeS. The presence of FeS will increase the brittleness of steel;

[0040] Cr: Improves the hardenability of steel. After quenching, steel can obtain uniform structure and deeper hardened layer. It can also cooperate with Mn and Si elements to improve the strength of steel and has good plasticity and toughness;

[0041] Nb + Ti: The synergistic effect of the two can produce the effects of grain boundary pinning and precipitation strengthening. Nb(C,N) can inhibit the growth of austenite grains and play a role in refining grains. Ti can fix N to form TiN and improve the toughness of the heat-affected zone of welding.

[0042] The microstructure of the ultra-high strength steel plate consists of 75%-90% bainite, 5%-10% martensite and 2%-8% retained austenite.

[0043] The yield strength of the ultra-high strength steel plate is 850MP-940MPa, the tensile strength is 1200MP-1350MPa, the elongation at break A 50 value is 16%-23%, and the hole expansion rate is 25%-38%.

[0044] The present invention also discloses a preparation method of the ultra-high strength steel plate, as Figure 1 shown, including the following steps:

[0045] Step (1): Smelt the molten steel according to the chemical composition of the ultra-high strength steel plate. Specifically, the sulfur content in the furnace during smelting is controlled not to exceed 0.015%, the tapping temperature is 1600℃-1650℃, the oxygen activity condition during tapping is [a0]220-440PPm, and the converter smelting time is 35-40min; Use ferrosilicon aluminum for deoxidation, 450-480kg / furnace, the temperature of bottom blowing argon is 1600-1630℃, the pressure is 300-450 Pa, and the time is 5.5-8.5min; Add medium-carbon ferromanganese when 30%-38% of the steel is tapped, and finish adding when 65%-75% of the steel is tapped. During the tapping process, 200-240kg of active lime and 50-70kg of fluorite are added to each furnace of steel. After tapping, 200-230kg of slag conditioner is added; After blowing argon on the small platform for 4.5-5min, determine the oxygen content, and supplement and feed aluminum wire according to the oxygen content. The oxygen activity of the molten steel is controlled at 30-45ppm by secondary feeding of Al wire. After secondary feeding of Al wire, argon with a pressure of 320-560Pa is introduced into the bottom of the molten steel ladle for 8-10min, and the argon flow rate is based on the condition that the molten steel does not turn over greatly; After electric heating and supplementary feeding of Al wire, Ca-Si wire is fed again, where Ca≥20%, and the addition amount is 850-1100m / furnace. The tapping temperature of LF refining is controlled at 1580-1590℃;

[0046] Step (2): The molten steel after smelting is continuously cast into slabs; specifically, full protection casting is carried out using a special protective slag for low-carbon aluminum-killed steel. A sealing gasket and appropriate argon blowing are used for the long nozzle, and the exposure of the molten steel is strictly prohibited. The liquid level in the mold is kept stable to prevent secondary oxidation, nitrogen absorption, and the entrainment of the protective slag of the molten steel. The tundish temperature is controlled at 1500°C - 1550°C, and the liquidus temperature is controlled at 1490°C - 1510°C. The protective slag used is a peritectic steel protective slag, and the secondary cooling system adopts a weak cooling method. The target casting speed is 1.5 m / min - 1.7 m / min. Constant speed casting is adopted during production, and the cast slabs are required to have chamfers on both sides.

[0047] Step (3): The slabs are heated, descaled, rough rolled, finish rolled, and laminar cooled to obtain hot-rolled coils; specifically, the finish rolling starting temperature is 1150°C - 1230°C, and the coiling temperature is 650°C - 670°C. The laminar cooling adopts a front-segment centralized cooling method; specifically, the heating temperature is 1230 - 1250°C. Soft blow water is used for pre-cooling the scale during rough rolling. The entire length and all of the rolling line are descaled, and the descaling water pressure is ≥24 MPa; the descaling temperature of the steel billet is ≥1150°C to ensure the removal of the low-melting-point FeO·Fe2SiO4 eutectic compound. The finish rolling starting temperature is 1150 - 1230°C to avoid the formation of Fe2O3. The number of finish rolling passes is 7 passes, the finishing temperature is 900 ± 15°C, the reduction rate of the last pass is ≤15% and lubricated rolling is adopted to avoid the breakage of the third oxidation scale formed during the finish rolling process in the last pass; to avoid excessive cracks in the oxide layer during cooling due to excessive internal stress in the oxide layer, which affects the forming performance of the oxide layer. The intermediate temperature is 750 ± 10°C, and the coiling temperature is 650 - 670°C. The laminar cooling adopts a front-segment centralized cooling method;

[0048] Step (4): After pickling the hot-rolled coils, cold rolling is carried out to obtain cold-rolled thin strip coils; specifically, after the hot-rolled strip is uncoiled, tension levelled and descaled, and the strip shape is adjusted, i-BOX (shallow trough turbulent flow pickling) is used. Among them, the threading speed of the uncoiler is 60 ± 5 m / min, the tailing-off speed is 110 ± 10 m / min, and the rest are uncoiled at a speed of 150 - 170 m / min. The six-high straightener is used to pull the strip head out of the uncoiler and straighten the strip head and tail to make the strip easy to thread. The speed of the six-high tension leveler for descaling treatment is kept at 150 - 170 m / min, and the tension of the tension leveler is 40 - 50 tons; after using an acid solution to remove the scale on the surface of the hot-rolled coil, a pickled sheet is obtained. The pickling adopts an immersion acid tank (i-Box) pickling with low acid consumption and high efficiency, three-stage pickling, and four-stage rinsing. Among them, the acid solution temperature in the three-stage pickling process is 80 - 90°C, and the free HCl concentrations in the three-stage pickling are 3.5 - 4.5%, 6.8 - 10.5%, 10.8 - 15.7% respectively. The Fe in the three-stage pickling 2+They are 120±10g / l, 110±15g / l, 60±15g / l. The temperatures for the four-stage rinsing are 40±10℃, 50±5℃, 50±10℃, 70±15℃ respectively, and the pH values are controlled at 1.0 - 2.5, 3.0 - 4.0, 4.0 - 5.5, 5.0 - 6.5 respectively, while the Cl -1 concentration requirements are all ≤3.8; the strip steel after rinsing is dried by a dryer (the air supply temperature of the dryer is 80 - 140℃), ensuring that the grain boundary oxide layer is pickled clean. After 5 passes of rolling, the cold rolling reduction rate is 50 - 80%, preferably 55 - 65%, to obtain cold-rolled thin strip steel;

[0049] Step (5): The cold-rolled thin strip steel coil is continuously annealed to obtain an ultra-high strength steel plate. Specifically, the soaking temperature is 850±10℃, heating the steel plate at the complete austenitization temperature to obtain the appropriate microstructure requirements in the soaking section. The slow cooling end temperature is 740±5℃, the rapid cooling end temperature is 355±5℃, and the overaging temperature is 350℃ - 370℃, ensuring the formation of bainite in this stage, and then controlling the phase ratios of bainite, martensite and retained austenite in the final microstructure to ensure the balance of strength, plasticity and formability. The unit speed is 40 - 60m / min, and for every 0.2mm increase in the thickness of the cold-rolled thin strip steel, the unit speed decreases by 5m / min, to ensure the formation of ferrite and tempered martensite due to the reverse temperature during the heating and heat preservation processes, and at the same time avoid the excessive growth of the cold-rolled steel plate grains. The skin pass elongation is 0.4±0.1%, and for every 0.1mm increase in the thickness of the cold-rolled thin strip steel, the skin pass elongation decreases by 0.03%;

[0050] Among them, the continuous annealing includes a preheating section, an oxidation section, a heating section, a soaking section, a slow cooling section, a rapid cooling section, and an overaging section:

[0051] The preheating section includes: heating from room temperature to 180 - 200℃ at a rate of 3 - 5℃ / s;

[0052] The oxidation section includes: heating from 180 - 200℃ to 640 - 670℃ at a rate of 1.0 - 1.3℃ / s;

[0053] The heating section includes: heating from 640 - 670℃ to 850 - 870℃ at a rate of 0.8 - 1.0℃ / s;

[0054] The soaking section includes: maintaining at 850 - 870℃ and holding for 200 - 300s;

[0055] The slow cooling section includes: cooling from 850 - 870℃ to 740±5℃ at a rate of 0.6℃ / s - 1.0℃ / s;

[0056] The rapid cooling section includes: cooling from 740 - 760℃ to 355±5 at a rate of 16℃ / s - 23℃ / s;

[0057] The overaging stage includes: maintaining at 360 ± 10 °C and holding for 1200 - 1500 s.

[0058] The present invention also discloses an application of the ultra-high strength steel plate, using the ultra-high strength steel plate in automobile manufacturing.

[0059] The ultra-high strength steel plate for automobile manufacturing is pretreated before spot welding. The baking temperature of the pretreatment is 200 °C - 350 °C, and the baking time is 20 min - 120 min.

[0060] The ultra-high strength steel plate for automobile manufacturing of the present invention is pretreated before spot welding, so as to recover dislocations, eliminate the residual stress of the steel plate, and regulate part of the martensite and retained austenite into tempered martensite structure, achieving the effect of improving the CTS performance of the spot welding joint.

[0061] The technical solutions and effects of the present invention will be further described below through actual examples.

[0062] Example

[0063] The present invention provides three groups of Examples 1 - 3 of ultra-high strength steel plates prepared by the preparation method of the present invention, and their chemical compositions are shown in Table 1.

[0064] Table 1: Chemical composition of ultra-high strength steel plate (wt.%)

[0065]

[0066] The preparation method of the above ultra-high strength steel plate is as follows specifically:

[0067] (a) Smelting process: Smelt the molten steel according to the chemical composition of the ultra-high strength steel plate shown in Table 1; The main process parameters of the smelting process are shown in Table 2:

[0068] Table 2: Main process parameters of the smelting process of ultra-high strength steel plate

[0069]

[0070] (b) Continuous casting process: Continuously cast the molten steel treated in step (a) into a slab;

[0071] (c) Hot rolling process: After heating the slab obtained in step (b) by hot charging or cold charging, perform descaling, rough rolling, finish rolling and laminar cooling;

[0072] (d) Pickling and cold rolling process: After pickling the hot rolled coil obtained in step (c) to remove the surface oxide layer, cold roll it into a cold rolled thin strip coil;

[0073] (e) Continuous annealing process: The cold-rolled thin strip steel coil obtained in step (d) is continuously annealed to produce an ultra-high strength steel plate;

[0074] The process parameters of the continuous casting, hot rolling, and continuous annealing processes are shown in Table 3:

[0075] Table 3: Main process parameters of the continuous casting, hot rolling, and continuous annealing processes of ultra-high strength steel plates

[0076]

[0077] The microstructure of the ultra-high strength steel prepared by the above process is as Figure 1 shown, Figure 1 showing that the microstructure of the ultra-high strength steel plate consists of 87% bainite, 8% martensite, and 5% retained austenite.

[0078] The mechanical properties of the above ultra-high strength steel plate were tested in accordance with GB / T 228-2010 "Metallic materials - Tensile testing at ambient temperature" and GB / T 24524-2021 "Metallic materials - Test method for hole flanging of sheet and strip", and the test results are shown in Table 4:

[0079] Table 4: Mechanical properties of ultra-high strength steel plates

[0080]

[0081] It can be seen from Table 4 that the ultra-high strength steel plates prepared in Examples 1-3 of the present invention have excellent strength plasticity and formability. However, for the welded joints after spot welding, when there is no splash under the spot welding process of an electrode pressure of 4 kN, a welding time of 200 ms, and a spot welding current of 7 kA - 9 kA, the maximum CTS strength is 5.42 kN. This is mainly due to the tissue stress caused by the tissue difference between the base material and the spot welded joint, resulting in a relatively low CTS performance of the spot welded joint. Therefore, the present invention also provides an application of the ultra-high strength steel plate, using the ultra-high strength steel plate in automobile manufacturing to improve the CTS strength of the ultra-high strength steel, that is, pre-treating the ultra-high strength steel plate before spot welding. The pre-treatment process includes a baking temperature (200°C - 350°C) and a baking time (20 min - 120 min). After spot welding in the same process as above, the CTS performance of the spot welded joint is detected. The pre-treatment process and the CTS detection results are shown in Table 5.

[0082] Table 5: Pre-treatment process and CTS detection results

[0083] Pretreatment process Baking temperature / °C Baking time / min Maximum CTS / kN 1# 200 20 9.38 2# 200 120 10.12 3# 250 40 10.98 4# 350 20 10.76 5# 350 120 11.35

[0084] Comparative example

[0085] Using different pre-treatment processes, after spot welding in the same process as above, the CTS performance of the spot welded joint was detected, and the results are shown in Table 6.

[0086] Table 6: Pretreatment process and CTS test results of comparative examples

[0087] Pretreatment process Baking temperature / °C Baking time / min Maximum CTS / kN Comparative example 1 175 120 5.15 Comparative example 2 250 10 6.22 Comparative example 3 380 10 6.35 Comparative example 4 380 140 6.74

[0088] From the data results in Table 5 and Table 6, it can be seen that: before spot welding, the ultra-high strength steel plate of the present invention is pretreated, and the CTS performance of the ultra-high strength steel plate prepared by the pretreatment process within the range of baking temperature (200°C - 350°C) and baking time (20 min - 120 min) is far superior to that of the comparative example. Therefore, it shows that by pretreating the ultra-high strength steel plate before spot welding, and controlling the baking temperature to 200°C - 350°C and the baking time to 20 min - 120 min in the pretreatment process, the CTS performance can be improved.

[0089] In summary, the ultra-high strength steel plate of the present invention obtains a bainite-based product by regulating the production process, which can not only ensure excellent strength and plasticity but also has excellent local forming performance.

[0090] The above-described embodiments are only used to describe the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. An ultra-high strength steel plate, characterized in that, It includes the following chemical components in mass percentage ratio: C: 0.17% - 0.23%, Si: 0.90% - 1.55%, Mn: 1.50% - 2.80%, P ≤ 0.009%, S ≤ 0.009%, Als: 0.01% - 0.15%, Cr: 0.08% - 0.70%, Nb + Ti: 0.02% - 0.20%, N ≤ 0.0065%, and the balance is Fe and impurities.

2. The ultra-high strength steel plate according to claim 1, characterized in that: The microstructure of the ultra-high strength steel plate consists of 75% - 90% bainite, 5% - 10% martensite, and 2% - 8% retained austenite.

3. The ultra-high strength steel plate according to claim 1, characterized in that: The yield strength of the ultra-high strength steel plate is 850MP - 940MPa, the tensile strength is 1200MP - 1350MPa, and the elongation A 50 value is 16% - 23%, and the hole expansion rate is 25% - 38%.

4. A method for preparing an ultra-high strength steel plate according to any one of claims 1-3, characterized in that, It includes the following steps: Smelt the molten steel according to the chemical composition of the ultra-high strength steel plate; The smelted molten steel is continuously cast into a slab; The slab is heated, descaled, rough rolled, finish rolled, and laminar cooled to obtain a hot rolled coil; After pickling the hot rolled coil, it is cold rolled to obtain a cold rolled thin strip coil; The cold rolled thin strip coil is continuously annealed to obtain the ultra-high strength steel plate.

5. The preparation method of a super high-strength steel plate according to claim 4, characterized in that: The smelting of the molten steel according to the chemical composition of the ultra-high strength steel plate includes: during the smelting process, the sulfur input to the furnace is controlled below 0.015%, the tapping temperature is 1600°C - 1650°C, aluminum iron is used for deoxidation, and the addition amount of aluminum iron is 450 kg / furnace - 480 kg / furnace.

6. The preparation method of an ultra-high strength steel plate according to claim 5, characterized in that: Medium carbon ferromanganese is added when 30% - 38% of the steel is tapped and added completely when 65% - 75% of the steel is tapped; argon blowing on the small platform is carried out for 4.5 min - 5 min and then the oxygen is determined, and aluminum wire is supplemented according to the oxygen content; the tapping temperature of LF refining is 1580°C - 1590°C.

7. The preparation method of an ultra-high strength steel plate according to claim 4, characterized in that: The continuous casting of the smelted molten steel into a slab includes: using a special protective slag for low-carbon aluminum killed steel during the continuous casting process, controlling the tundish temperature at 1500°C - 1550°C, controlling the liquidus temperature at 1490°C - 1510°C, and the target casting speed is 1.5 m / min - 1.7 m / min.

8. The preparation method of an ultra-high strength steel plate according to claim 4, characterized in that: The slab is heated, descaled, rough rolled, finish rolled, and laminar cooled to obtain a hot rolled coil, including: the finish rolling starting temperature is 1150°C - 1230°C, the coiling temperature is 650°C - 670°C, and the laminar cooling adopts the front-stage centralized cooling method.

9. The preparation method of an ultra-high strength steel plate according to claim 4, characterized in that: The cold rolled thin strip coil is continuously annealed to obtain the ultra-high strength steel plate, including: the overaging temperature is 350°C - 370°C, and the overaging holding time is 1200 s - 1500 s.

10. An application of an ultra-high strength steel plate according to any one of claims 1-3, characterized in that: The ultra-high strength steel plate is used in automobile manufacturing.

11. The application of a super high-strength steel plate as described in claim 10, characterized in that: The ultra-high strength steel plate for automobile manufacturing is pretreated before spot welding.

12. The application of an ultra-high strength steel plate according to claim 11, characterized in that: The baking temperature of the pretreatment is 200°C - 350°C, and the baking time is 20 min - 120 min.

Images