High-flatness water-quenched ultra-high-strength cold-rolled steel strip and manufacturing method therefor

Abstract

Disclosed in the present invention is a high-flatness water-quenched ultra-high-strength cold-rolled steel strip, the microstructure of which contains tempered martensite having a volume phase proportion of 85-90%, and low-carbon bainite having a volume phase proportion of 8-13%. Further disclosed in the present invention is a manufacturing method for the water-quenched ultra-high-strength cold-rolled steel strip, the method comprising the steps of: smelting and casting; hot rolling; pickling; cold rolling; and annealing. The annealing step comprises: heating and thermal insulation; three-stage cooling; using a straightening machine to improve the strip shape; and tempering. In the three-stage cooling, the first-stage cooling has a starting temperature of 750-820°C, a finish temperature of 450-500°C, and a cooling rate of ≥70°C / s; the second-stage cooling has a finish temperature of 420-480°C and a cooling rate of ≤10°C / s; and the third-stage cooling has a finish temperature of ≤100°C and a cooling rate of ≥300°C / s.

Description

A high-flatness water-quenched ultra-high strength cold-rolled steel strip and its manufacturing method Technical Field

[0001] This invention relates to a steel strip and a method for manufacturing the same, and more particularly to a cold-rolled steel strip and a method for manufacturing the same. Background Technology

[0002] Steel plates with a tensile strength ≥1470MPa are generally hardened by water quenching (rapid cooling). The advantage of water quenching is its fast cooling rate (average cooling rate ≥500℃ / s for 1mm thick steel plates), which allows for achieving high strength with relatively low alloy content.

[0003] However, water cooling can cause a phase change, leading to a volume change and severe deformation of the steel plate, commonly known as hardening warping. Warping-induced poor plate shape increases friction between the steel plate and the mold during processing, resulting in surface defects and mold wear, and further contributing to poor part shape.

[0004] Furthermore, the higher the strength of the steel plate, the higher its susceptibility to hydrogen-induced cracking. When the steel plate inevitably comes into contact with hydrogen during processing and use, it is prone to brittle fracture under stress, severely reducing the safety protection effect of ultra-high strength steel.

[0005] In the prior art, existing patent literature covers the above-mentioned fields, including:

[0006] For example, Chinese patent document CN110684932A, published on January 14, 2020, entitled "A 1500MPa Grade Cold-Formed Strip Steel and Its Production Method," discloses a 1500MPa grade cold-formed strip steel and its production method. Its chemical composition is designed as follows: C: 0.25-0.4%, Si: 0.1-0.3%, Mn: 1.1-1.7%, Cr: 0.2-0.4%, P: ≤0.02%, S: ≤0.012%, Al: 0.03-0.05%, Ti: 0.035-0.05%, B: 0.001-0.003%, V: 0.15-0.3%, N: ≤0.003%. The production method of this technical solution includes hot metal pretreatment, converter steelmaking, LF furnace refining, RH refining, continuous casting, hot rolling, cold rolling, continuous annealing, and leveling processes. The continuous annealing process includes: heating and soaking temperature of 820–860℃, soaking time of 50–100s, rapid cooling start temperature of 660–680℃, cooling rate of 80–100℃ / s, and over-aging temperature of 260–300℃. The resulting strip steel has a microstructure consisting of uniformly distributed Mao islands and a very small amount of ferrite, exhibiting ultra-high strength and excellent weldability and cold formability. However, the cooling method used in the aforementioned patent literature, judging from the cooling rate, is not water quenching but air cooling.

[0007] For example, patent document WO2016129550(A1), published on August 18, 2016, entitled "Ultra-high strength steel with good resistance to delayed fracture at the shear edge," discloses an ultra-high strength steel plate whose composition includes specific amounts of C, Mn, and Al, as well as residual iron and unavoidable impurities, wherein the content of P, S, and N among the unavoidable impurities is limited to specific amounts. The ultra-high strength steel plate contains 90% or more martensite and 0.5% or more retained austenite in an area ratio. The local manganese concentration in the austenite region is at least 1.1 times the manganese content of the entire steel plate. The tensile strength of the ultra-high strength steel plate is 1470 MPa or higher. The microstructure of the steel of this invention is mainly martensite, and the retained austenite is rich in Mn, thus maintaining the hydrogen capture capacity of the cut portion even after the steel plate is cut. This provides an ultra-high strength steel plate with excellent resistance to delayed fracture at the cut edge. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high-flatness water-quenched ultra-high strength cold-rolled steel strip, which has good resistance to delayed cracking while having high flatness.

[0009] To achieve the above objectives, the present invention provides a water-cooled quenched ultra-high strength cold-rolled steel strip with high flatness, the microstructure of which contains 85-90% tempered martensite and 8-13% bainite by volume.

[0010] In some embodiments, the bainite contained in the microstructure of the high-flatness water-quenched ultra-high strength cold-rolled steel strip of the present invention is all low-carbon bainite.

[0011] In some embodiments, the microstructure of the high-flatness water-quenched ultra-high strength cold-rolled steel strip of the present invention contains 85-90% tempered martensite by volume and 8-13% low-carbon bainite by volume.

[0012] Furthermore, the total volume of precipitates in the microstructure of the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention is <0.3%.

[0013] Furthermore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the precipitates include one or more of Fe3C, TiN, Ti(C,N), Nb(C,N), VC, and TiMoC, which are uniformly dispersed in the matrix, with TiN particles having a diameter of no more than 10 micrometers and carbide precipitates having a particle diameter of no more than 0.2 micrometers.

[0014] Furthermore, the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0015] C: 0.20-0.30 wt%;

[0016] Mo: 0.05-0.15 wt%;

[0017] Si: 0.03-1.00 wt%;

[0018] Al: 0.01-0.08 wt%;

[0019] N≤0.005wt%;

[0020] Ti: 3.4×Nwt%+(0.01-0.04wt%), where “N” is the value before the percentage sign of the mass percentage of N element;

[0021] B ≤ 0.0005 wt% or B: 0.0015-0.0035 wt%;

[0022] Mn: When B ≤ 0.0005 wt%, the Mn content is 1.4-1.7 wt%; when B content is 0.0015-0.0035 wt%, the Mn content is 0.8-1.4 wt%.

[0023] Furthermore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage of each chemical element is as follows:

[0024] C: 0.20-0.30 wt%;

[0025] Mo: 0.05-0.15 wt%;

[0026] Si: 0.03-1.00 wt%;

[0027] Al: 0.01-0.08 wt%;

[0028] N≤0.005wt%;

[0029] Ti: 3.4×Nwt%+(0.01-0.04wt%), where “N” is the value before the percentage sign of the mass percentage of N element;

[0030] B ≤ 0.0005 wt% or B: 0.0015-0.0035 wt%;

[0031] Mn: When B ≤ 0.0005 wt%, the Mn content is 1.4-1.7 wt%; when B content is 0.0015-0.0035 wt%, the Mn content is 0.8-1.4 wt%.

[0032] The balance consists of Fe and other unavoidable impurities.

[0033] Because the cooling process used in this invention differs from both conventional water quenching and conventional jet cooling, and because this invention aims to generate a certain amount of low-carbon bainite (distributed along the original austenite grain boundaries) during the second stage of rapid cooling, the design principles of the chemical elements in the water-cooled quenched ultra-high strength cold-rolled steel strip of this invention are specifically described below:

[0034] C: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, carbon (C) can increase the strength of the steel by affecting the hardness of martensite. Therefore, to achieve a certain strength level, the steel strip must have a corresponding carbon content. Thus, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, according to the commonly used empirical formula for calculating the strength of martensitic steel: TS (MPa) = 2880C + 800, the mass percentage of carbon is controlled between 0.20-0.30 wt%.

[0035] Mo: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, Mo can improve the hardenability of the steel and slow down the bainite transformation. Mo can also form dispersed fine TiMoC precipitates with Ti and C, which is beneficial for dispersing hydrogen accumulation in local areas. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage of Mo is controlled between 0.05-0.15 wt%. In some specific embodiments, the mass percentage of Mo can be further controlled between 0.07-0.12 wt%.

[0036] Si: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, Si element can ensure the stability and uniformity of temperature during the annealing process and the surface quality after pickling. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage content of Si element is controlled between 0.03-1.00 wt%. In some specific embodiments, the mass percentage content of Si element can be further controlled between 0.03-0.40 wt%.

[0037] Al: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, Al element is used as a deoxidizer, and the mass percentage content of Al element is controlled between 0.01-0.08 wt%.

[0038] N: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, nitrogen (N) can react with titanium (Ti) in the steel to precipitate large TiN particles. When these large TiN particles are located near the surface of the steel plate, they easily become areas of hydrogen accumulation, thus forming crack initiation sites. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage of nitrogen (N) is controlled to be N ≤ 0.005 wt% to minimize the TiN content. In some specific embodiments, the mass percentage of nitrogen (N) can be between 0.001 and 0.005 wt%.

[0039] Ti: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, Ti can fix N element, which is beneficial to fully utilizing the hardenability-enhancing effect of B element. Furthermore, TiC is beneficial for dispersing hydrogen aggregation, and TiMoC precipitates have a similar effect. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of Ti element is controlled at 3.4 × N% + (0.01-0.04 wt%). In some specific embodiments, the mass percentage content of Ti element can be controlled at 3.4 × N% + (0.015-0.04 wt%). Here, "N" is the value before the percentage sign of the N element mass percentage content.

[0040] B: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, element B can improve the hardenability of the steel. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage content of element B is controlled to B ≤ 0.0005 wt% (e.g., B: 0.0001-0.0005 wt%) or B: 0.0015-0.0035 wt%.

[0041] Mn: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, Mn is the most commonly used alloying element to ensure hardenability. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the content of Mn that can be matched with the process conditions of this invention to ensure the hardenability of the steel plate can be selected. When B ≤ 0.0005 wt% (e.g., B: 0.0001-0.0005 wt%), the mass percentage content of Mn is controlled at 1.4-1.7 wt%; when the B content is 0.0015-0.0035 wt%, the mass percentage content of Mn is controlled at 0.8-1.4 wt%.

[0042] Furthermore, among the other unavoidable impurities in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, S≤0.003wt% and P≤0.015wt%.

[0043] In this invention, the unavoidable impurities are mainly S and P, and their content is preferred to be as low as possible, given the available process conditions. Wherein:

[0044] S: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, sulfur (S) is an impurity element in the steel, which will form MnS, seriously affecting plasticity and toughness. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of sulfur is controlled to S≤0.003wt%.

[0045] P: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, phosphorus (P) is an impurity element in the steel, which reduces the toughness of the steel and is detrimental to delayed cracking. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of phosphorus is controlled to P ≤ 0.015 wt%.

[0046] Furthermore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, its chemical elements also contain at least one of the following:

[0047] Cr: 0.1-1.0 wt%;

[0048] Cu: 0.05-0.25 wt%;

[0049] Ni: 0.05-0.15 wt%;

[0050] Ca: 0.0005-0.0035 wt%;

[0051] Nb: 0.01-0.05 wt%;

[0052] V: 0.01-0.02wt%.

[0053] In this invention, precipitation strengthening elements Nb and V, as well as corrosion-resistant elements Cu and Ni, can be added to further improve the strength of water-quenched ultra-high strength cold-rolled steel strip. Wherein:

[0054] Cr: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage content of Cr element is controlled between 0.1-1.0 wt%. In some embodiments, when adding Cr element, Mn element is simultaneously reduced, with Cr element replacing an equal amount of Mn element; the mass percentage content of Cr element is further controlled between 0.1-0.7 wt%.

[0055] Cu: In the water-quenched ultra-high strength cold-rolled steel strip of the present invention, Cu can improve the corrosion resistance of the steel and help inhibit stress corrosion cracking. Therefore, in the water-quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of Cu is controlled between 0.05-0.25 wt%. In some embodiments, the mass percentage content of Cu is further controlled between 0.12-0.22 wt%.

[0056] Ni: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, Ni element is beneficial to improving the corrosion resistance of steel and to alleviating the brittleness caused by Cu. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of Ni element is controlled between 0.05-0.15 wt%.

[0057] Ca: In the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the element Ca can improve the aspect ratio of inclusions. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention, the mass percentage content of the element Ca is controlled between 0.0005-0.0035 wt%.

[0058] Nb and V: In the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, Nb and V elements can refine the grains and disperse their precipitation, which is beneficial for the aggregation of dispersed hydrogen. Therefore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the mass percentage content of Nb element is controlled between 0.01-0.05 wt%, and the mass percentage content of V element is controlled between 0.01-0.02 wt%.

[0059] Furthermore, in the water-cooled quenched ultra-high strength cold-rolled steel strip described in this invention, the wave height of the steel plate is ≤8mm.

[0060] Furthermore, the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention has a tensile strength (TS) ≥ 1470 MPa, a yield strength (YP) ≥ 1150 MPa, a surface residual stress range ≤ 130 MPa, and a surface average residual stress ≤ 100 MPa.

[0061] In some embodiments, the tensile strength of the water-quenched ultra-high strength cold-rolled steel strip of the present invention is 1470-1670 MPa.

[0062] In some embodiments, the yield strength of the water-cooled quenched ultra-high strength cold-rolled steel strip of the present invention is 1150-1400 MPa.

[0063] In some embodiments, the elongation (E1) of the water-quenched ultra-high strength cold-rolled steel strip of the present invention is ≥5%, for example, 5-8%.

[0064] Another objective of this invention is to provide a method for manufacturing water-quenched ultra-high strength cold-rolled steel strip, which can provide a water-quenched ultra-high strength cold-rolled steel strip with good resistance to delayed cracking and high flatness.

[0065] To achieve the above objectives, the present invention provides a method for manufacturing water-cooled quenched ultra-high strength cold-rolled steel strip, comprising the steps of: smelting and casting; hot rolling; pickling; cold rolling; annealing; wherein the annealing step comprises: heating and holding, three-stage cooling, using a straightening machine to improve the strip shape, and tempering;

[0066] The first stage of the three-stage cooling system has an initial temperature of 750-820℃, an end temperature of 450-500℃, and a cooling rate of ≥70℃ / s.

[0067] The termination temperature of the second stage of the three-stage cooling system is 420-480℃, and the cooling rate is ≤10℃ / s.

[0068] The termination temperature of the third stage of the three-stage cooling system is ≤100℃, and the cooling rate is ≥300℃ / s.

[0069] In some implementations, the annealing step includes, in sequence, heating and holding, three-stage cooling, using a straightening machine to improve the plate shape, and tempering.

[0070] In some implementations, the cooling rate of the first stage of the three-stage cooling system is 70-110°C / s.

[0071] In some implementations, the cooling rate of the second stage of the three-stage cooling system is 1-10°C / s.

[0072] In some implementations, the termination temperature of the third stage of the three-stage cooling is 50-100°C.

[0073] In some implementations, the cooling rate of the third stage of the three-stage cooling system is 300-650°C / s.

[0074] This invention employs a three-stage cooling process and a centrally located tension leveling machine. Wherein:

[0075] For the three-stage cooling process: The first stage is a high-temperature stage, using high-speed air cooling to suppress the precipitation of ferrite, pearlite, and bainite. After the first stage, the microstructure of the steel is still austenite. The second stage is a medium-temperature stage, using controlled cooling. This stage is in the bainite transformation zone, where the strip is cooled at a slower rate, which is conducive to the formation of a small amount of low-carbon bainite near the austenite grain boundaries. After this stage, the microstructure of the steel is a small amount of grain boundary bainite + austenite. The third stage uses water quenching, rapidly cooling from the medium-temperature stage to below 100°C. In this stage, the untransformed supercooled austenite in the steel plate transforms into martensite. After the third stage, the microstructure of the steel plate is grain boundary bainite + untempered martensite.

[0076] This invention employs a centrally located straightening machine layout. In this invention, a centrally located straightening machine means that the straightening machine is placed between the three-stage cooling device and the tempering device; that is, the straightening process is set between the three-stage cooling and tempering processes.

[0077] After the strip steel is quenched in water, it enters a centrally located straightening machine for straightening and shape improvement. Then it undergoes pickling, rinsing, squeezing, and drying before being heated to the over-aging temperature for tempering.

[0078] In other embodiments, a process of first acid washing, alkali washing, cleaning, squeezing dry, drying, and then leveling can be adopted.

[0079] In some specific implementations, the straightener configuration can be as follows: using a smaller straightening roller diameter (e.g., a working roller diameter of 60mm), a straightening roller number of ≥11, and a bending insertion amount during straightening aimed at ensuring the plate shape, with a larger bending insertion amount on the inlet working roller (e.g., a bending insertion amount of 4mm on the inlet working roller) and a zero bending insertion amount on the outlet working roller.

[0080] This process is significantly different from the most commonly used method of quenching, tempering, and then leveling. It also differs from the common method of quenching, tempering, and then offline straightening, as well as the method of quenching, tempering, and then online straightening.

[0081] Of the latter three options, flattening cannot effectively improve the shape of water-quenched steel plates, and the hardening and warping of the steel plates are obvious; offline or online straightening after quenching and tempering can improve the shape, but it impairs the toughness and plasticity of tempered martensite and introduces large residual stress and a large number of vacancy and dislocation defects into the steel plate, which is not conducive to the material and the delayed cracking resistance of the parts made from the material.

[0082] In the present invention, straightening immediately after quenching can improve the plate shape, and then tempering can improve the toughness and plasticity of the material, while also reducing the residual stress and density of vacancy and dislocation defects caused by straightening. Thus, from the three dimensions of material properties, residual stress and diffusible hydrogen content, it can improve the anti-delayed cracking performance of steel plates and steel plates after they are processed into parts.

[0083] Furthermore, in the manufacturing method described in this invention, after three-stage cooling, a straightening machine is used to improve the plate shape, followed by pickling and desqueezing, and then tempering.

[0084] Furthermore, in the manufacturing method described in this invention, after three-stage cooling, the plate is first pickled and squeezed dry, then a straightening machine is used to improve the plate shape, and then tempering is performed.

[0085] Furthermore, in the manufacturing method described in this invention, the tempering process involves holding at 200-300°C for at least 150 seconds. In some embodiments, the tempering process involves holding at 200-300°C for 150-400 seconds. In some embodiments, the tempering temperature can be 220-270°C. In some embodiments, tempering can be performed using induction heating.

[0086] In some embodiments, leveling is not required after tempering. In other embodiments, 0.1-0.3% leveling may be applied.

[0087] Furthermore, in the manufacturing method described in this invention, the first stage of cooling is air cooling, and the third stage of cooling is water quenching.

[0088] Furthermore, in the manufacturing method described in this invention, a slow cooling process (which is not a necessary step) is included between the heat preservation and the three-stage cooling process, with a cooling rate of 3-10°C / s.

[0089] Furthermore, in the heating and holding steps of the manufacturing method described in this invention, the temperature is increased to the austenitic single-phase region at a heating rate of ≥1℃ / s and held for 30-200s. In some embodiments, in the heating and holding steps of the manufacturing method described in this invention, the temperature is increased to the austenitic single-phase region at a heating rate of 1-30℃ / s and held for 30-200s. The temperature of the austenitic single-phase region can be ≥780℃, for example, 780-1200℃ or 780-1000℃.

[0090] In some embodiments, during the heating and holding steps, the temperature is raised to the austenitic single-phase region at a heating rate of ≥1℃ / s and held for 40-180s; during the slow cooling step, the cooling rate is further controlled to be between 3-7℃ / s.

[0091] In some implementations, during the hot rolling step, the heating temperature is 1150-1250℃, and after reaching the target temperature, the holding time is controlled at 40-50 minutes; the final rolling temperature is 870-920℃, followed by rapid cooling at a rate of 20-70℃ / s, and the coiling temperature is 500-650℃; after coiling, the coils are allowed to cool naturally.

[0092] In some implementations, the cold rolling reduction rate is controlled to be 35-65% during the cold rolling step.

[0093] The high-flatness water-cooled quenched ultra-high strength cold-rolled steel strip and its manufacturing method described in this invention have the following advantages and beneficial effects compared with the prior art:

[0094] The high-flatness water-cooled quenched ultra-high strength cold-rolled steel strip and its manufacturing method described in this invention, through reasonable component selection, process design and equipment layout, can obtain ultra-high strength cold-rolled steel plates, especially high flatness (wave height ≤ 8mm), which have good resistance to delayed cracking.

[0095] In some embodiments, the high-flatness water-quenched ultra-high strength cold-rolled steel strip of the present invention has a tensile strength ≥1470MPa, a yield strength ≥1150MPa, a surface residual stress range ≤130MPa, and an average residual stress ≤100MPa.

[0096] In some embodiments, the high-flatness water-quenched ultra-high-strength cold-rolled steel strip of the present invention exhibits excellent resistance to hydrogen-induced cracking. When retaining the original sheared edges from mechanical shearing, a four-point bending specimen, under a prestress equal to one times the tensile strength, can withstand immersion in 1 mol / L hydrochloric acid for 300 hours with cracks extending from the surface along the thickness direction of the steel sheet to ≤1 / 3 of the sheet thickness; the crack propagation length from the edge to the interior in the width direction of the steel sheet is ≤1 mm. In some embodiments, after simulating an automotive painting process by heating and holding at 170°C for 20 minutes, the hydrogen-induced cracking resistance of the high-flatness water-quenched ultra-high-strength cold-rolled steel strip of the present invention is further enhanced: when retaining the original sheared edges from mechanical shearing, a four-point bending specimen, under a prestress equal to one times the tensile strength, can withstand immersion in 1 mol / L hydrochloric acid for 300 hours without cracking, demonstrating good service performance when used in the manufacture of automotive safety structural components. Attached Figure Description

[0097] Figure 1 schematically shows the annealing process curve of the manufacturing method of the high-flatness water-quenched ultra-high strength cold-rolled steel strip of the present invention in one embodiment.

[0098] Figure 2 schematically shows a three-stage cooling layout diagram of the manufacturing method of high-flatness water-quenched ultra-high strength cold-rolled steel strip according to the present invention.

[0099] Figure 3 schematically shows the layout of the straightening machine in the manufacturing method of high-flatness water-quenched ultra-high strength cold-rolled steel strip according to the present invention.

[0100] Figure 4 schematically shows the evolution of the main microstructure during the continuous annealing process of the manufacturing method of high-flatness water-quenched ultra-high strength cold-rolled steel according to the present invention. Detailed Implementation

[0101] The following will further explain and illustrate the high-flatness water-cooled quenched ultra-high strength cold-rolled steel strip and its manufacturing method according to the present invention, in conjunction with the accompanying drawings and specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0102] Examples 1-10 and Comparative Examples 1-10

[0103] The high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10 and the comparative steels of Comparative Examples 1-10 of this invention were all prepared by the following steps:

[0104] (1) Smelting and casting: The determined steel plate composition is smelted and cast to obtain a continuous casting billet;

[0105] (2) Hot rolling: Heat the continuous casting billet to 1150-1250℃. After reaching the target temperature, the holding time is controlled at 40-50 minutes. The final rolling temperature is 870-920℃. After rolling, the billet is rapidly cooled at a rate of 20-70℃ / s. The coiling temperature is 500-650℃. After coiling, the billet is allowed to cool naturally.

[0106] (3) Pickling: Pickling hot-rolled steel coils;

[0107] (4) Cold rolling: The steel coil is cold rolled, and the cold rolling reduction rate is controlled at 35-65%;

[0108] (5) Annealing, which includes:

[0109] As shown in Figure 1, the heating zone is heated to the austenitic single-phase region at a heating rate of ≥1℃ / s, and the heat is held in the heat-holding zone for 30-200s.

[0110] Then, a three-stage cooling process is performed, as shown in Figures 1 and 2: the first stage is a rapid air cooling stage, the second stage is a controlled cooling stage, and the third stage is a water quenching stage.

[0111] The initial temperature of the first stage of cooling is 750-820℃, the final temperature is 450-500℃, and the cooling rate is ≥70℃ / s.

[0112] The termination temperature of the second stage of cooling is 420-480℃, and the cooling rate is ≤10℃ / s;

[0113] The termination temperature of the third stage of cooling is ≤100℃, and the cooling rate is ≥300℃ / s;

[0114] In Examples 1-10, the first stage of cooling uses air cooling, and the third stage of cooling uses water quenching.

[0115] In Examples 1-10, as shown in Figure 1, a slow cooling process (which is not a necessary step) is included between the heat preservation and the three-stage cooling process, with a cooling rate of 3-10°C / s.

[0116] In Examples 2-5 and Examples 7-9, the temperature was raised to the austenitic single-phase region at a heating rate of ≥1℃ / s and held for 40-180s, and the cooling rate was further controlled to be between 3-7℃ / s.

[0117] (5) Straightening: As shown in Figures 1 and 3, after water quenching, a straightening machine is used to improve the plate shape, and then step (6) tempering is carried out.

[0118] In Examples 1-5, after water quenching, a straightening machine is first used to improve the plate shape, then pickling and squeezing are performed, followed by step (6) tempering;

[0119] In Examples 6-10, after water quenching, the plate is first pickled and squeezed dry, then a straightening machine is used to improve the plate shape, and then step (6) tempering is performed;

[0120] The straightening machine is configured as follows: it uses a smaller straightening roller diameter (the working roller diameter is 60mm), the number of straightening rollers is 11, and the bending insertion amount during straightening is aimed at ensuring the plate shape. The bending insertion amount of the inlet working roller is 4mm and the bending insertion amount of the outlet working roller is zero.

[0121] (6) Tempering: Tempering is performed by heating to 200-300℃ and holding for more than 150 seconds using induction heating; in Examples 2-4 and 7-9, tempering is performed by heating to a tempering temperature between 220-270℃ using induction heating.

[0122] In Examples 1-7, no leveling was performed after tempering. In Examples 8-10, 0.1-0.3% leveling was applied.

[0123] It should be noted that Comparative Examples 1-10 use the usual continuous annealing process, which is a process scheme in which quenching and tempering are completed, followed by leveling and / or straightening.

[0124] Table 1 lists the mass percentage of chemical elements in the high-flatness water-quenched ultra-high-strength cold-rolled steel of Examples 1-10 of the present invention and the comparative steel plates of Comparative Examples 1-10.

[0125] Table 1. (wt%, balance Fe and other unavoidable impurities besides S and P)

[0126] Tables 2-1, 2-2, and 2-3 list the specific process parameters for the high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10 of the present invention.

[0127] Table 2-1.

[0128] Table 2-2.

[0129] Table 2-3.

[0130] Tables 2-4 and 2-5 list the specific process parameters of the comparative steel plates of Comparative Examples 1-10 of the present invention.

[0131] Table 2-4.

[0132] Table 2-5.

[0133] To verify the effectiveness of the present invention, samples of Examples 1-10 and Comparative Examples 1-10 were taken and their microstructures were observed. The results of the microstructure observations are listed in Table 3. Tempered martensite volume fraction and bainite volume fraction: Grain boundary distribution maps were obtained using scanning electron backscatter diffraction (EBSD). Then, the average value was obtained by manually measuring five fields of view near the surface of the steel plate, one-quarter of its thickness, and the center using a grid method. In this invention, if there is no obvious carbide precipitation in the bainite, or only granular carbide precipitation (aspect ratio < 3), or long rod-shaped or strip-shaped carbide precipitation (aspect ratio ≥ 3) with the long rod-shaped or strip-shaped carbide accounting for less than half of the total carbide volume fraction, then the bainite is low-carbon bainite; if there is long rod-shaped or strip-shaped carbide precipitation (aspect ratio ≥ 3) in the bainite with the long rod-shaped or strip-shaped carbide accounting for half or more of the total carbide volume fraction, then the bainite is not low-carbon bainite; here, carbide refers to cementite. Observation and test results show that the microstructure of the high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10 includes bainite, and all bainite is low-carbon bainite.

[0134] Total volume of precipitates, diameter of TiN-containing particles, and diameter of carbide precipitates: Field emission scanning electron microscopy with AMICS intelligent mineral analysis system was used to evaluate the type and size of precipitates and particles. Each sample was analyzed for ≥50 fields of view, and the values ​​were taken as the average particle diameter.

[0135] Table 3 lists the microstructure observation results of the high-flatness water-quenched ultra-high strength cold-rolled steel of Examples 1-10 and the comparative steel plates of Comparative Examples 1-10.

[0136] Table 3.

[0137] As can be seen from Table 3 above, the microstructure of the high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10, prepared by the manufacturing method described in this invention, contains 85-90% tempered martensite and 8-13% low-carbon bainite by volume. Furthermore, the total volume of precipitates in the microstructure is <0.3%, the diameter of TiN-containing particles does not exceed 10 micrometers, and the diameter of carbide precipitates does not exceed 0.2 micrometers.

[0138] In addition, Figure 4 schematically shows the evolution of the main microstructure during the continuous annealing process of the manufacturing method of high-flatness water-quenched ultra-high strength cold-rolled steel according to the present invention.

[0139] As shown in Figure 4, the initial microstructure of the steel plate during continuous annealing is mainly ferrite + cementite. After the heating and holding stage, it transforms into full austenite (γ), and the austenite does not decompose during the slow cooling stage. After rapid cooling in stage I, the microstructure consists of austenite and a small amount of bainite (B). After slow cooling in stage II, the volume fraction of bainite increases, while the volume fraction of austenite decreases. After rapid cooling in stage III, the austenite transforms into martensite (M), and the main microstructure consists of martensite and bainite. Then, the steel plate is straightened and tempered. After tempering, the final microstructure of the steel plate mainly consists of tempered martensite (Mt) and bainite.

[0140] To verify the effectiveness of the present invention, the samples obtained in Examples 1-10 and Comparative Examples 1-10 were sampled again, and various performance tests were performed on them. The performance test results are listed in Table 4. Wherein:

[0141] Wave height of steel plate: Place the steel plate (dimensions: length 2 meters × width) on the inspection table, measure the gap between the steel plate and the inspection table, and take the maximum value as the wave height.

[0142] Mechanical properties: Tensile tests were performed according to GB / T228.1-2010 to obtain yield strength (YP), tensile strength (TS), and elongation (E1). Surface residual stress was tested using X-ray diffraction, following the operating instructions for the X-350A stress analyzer, to obtain the surface residual stress range and average surface residual stress.

[0143] Delayed cracking sensitivity: A 30×180mm sample was prepared using mechanical shearing, retaining the original sheared edge. The steel plate sample was subjected to four-point bending, with the crown stress controlled at one tensile strength (1×TS), and then immersed in 1mol / L hydrochloric acid for 300 hours. After cleaning and drying, the crack lengths in the thickness and width directions were observed and measured under a low-power microscope, and the maximum value was recorded.

[0144] Delayed cracking sensitivity after simulating automotive paint baking process by holding at 170℃ for 20 minutes: A 30×180mm sample was prepared using mechanical shearing, retaining the original sheared edges, and held at 170℃ for 20 minutes. The steel plate sample was then subjected to four-point bending, with the arch stress controlled at one times the tensile strength (1×TS), and immersed in 1mol / L hydrochloric acid for 300 hours. After cleaning and drying, the crack lengths in the thickness and width directions were observed and measured under a low-power microscope, and the maximum value was recorded.

[0145] Table 4 lists the relevant performance parameters of the high-flatness water-quenched ultra-high strength cold-rolled steels of Examples 1-10 and the comparative steel plates of Comparative Examples 1-10.

[0146] Table 4.

[0147] As can be seen from Table 4 above, the high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10, manufactured using the method described in this invention, have a tensile strength ≥1470MPa, a yield strength ≥1150MPa, a surface residual stress range ≤130MPa, an average surface residual stress ≤100MPa, and a wave height ≤8mm. Furthermore, the high-flatness water-quenched ultra-high-strength cold-rolled steels of Examples 1-10 also exhibit good resistance to delayed cracking, with crack lengths after acid soaking significantly better than (less than) those of the comparative example. It should be noted that, since the high-flatness water-quenched ultra-high-strength cold-rolled steel of Example 2 has the highest strength, although its delayed crack penetrates the plate thickness, the crack width only extends by 3mm. Therefore, the delayed cracking resistance of the steel plate of Example 2 is at a good level, superior to that of the comparative example.

[0148] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0149] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-flatness water-quenched ultra-high strength cold-rolled steel strip, characterized in that, The microstructure of the water-quenched ultra-high strength cold-rolled steel strip contains 85-90% tempered martensite and 8-13% bainite by volume.

2. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 1, characterized in that, The total volume of precipitates in the microstructure of the water-quenched ultra-high strength cold-rolled steel strip is <0.3%.

3. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 2, characterized in that, The precipitates include one or more of Fe3C, TiN, Ti(C,N), Nb(C,N), VC, and TiMoC. The precipitates are uniformly dispersed in the matrix, with TiN particles having a diameter of no more than 10 micrometers and carbide precipitates having a particle diameter of no more than 0.2 micrometers.

4. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 1, characterized in that, The water-quenched ultra-high strength cold-rolled steel strip contains Fe and unavoidable impurity elements, as well as the following chemical elements in the following mass percentages: C: 0.20-0.30 wt%; Mo: 0.05-0.15 wt%; Si: 0.03-1.00 wt%; Al: 0.01-0.08 wt%; N≤0.005wt%; Ti: 3.4×Nwt%+(0.01-0.04wt%), where "N" is the value before the percentage sign of the mass percentage of N element; B ≤ 0.0005 wt% or B: 0.0015-0.0035 wt%; Mn: When B ≤ 0.0005 wt%, the Mn content is 1.4-1.7 wt%; when B content is 0.0015-0.0035 wt%, the Mn content is 0.8-1.4 wt%.

5. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 4, characterized in that, The chemical element mass percentages of the water-cooled quenched ultra-high strength cold-rolled steel strip are as follows: C: 0.20-0.30 wt%; Mo: 0.05-0.15 wt%; Si: 0.03-1.00 wt%; Al: 0.01-0.08 wt%; N≤0.005wt%; Ti: 3.4×Nwt%+(0.01-0.04wt%), where "N" is the value before the percentage sign of the mass percentage of N element; B ≤ 0.0005 wt% or B: 0.0015-0.0035 wt%; Mn: When B ≤ 0.0005 wt%, the Mn content is 1.4-1.7 wt%; when B content is 0.0015-0.0035 wt%, the Mn content is 0.8-1.4 wt%. The balance consists of Fe and other unavoidable impurities.

6. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 4, characterized in that, In other unavoidable impurities, S ≤ 0.003 wt%, P ≤ 0.015 wt%.

7. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 4, characterized in that, The water-quenched ultra-high strength cold-rolled steel strip also contains at least one of the following chemical elements: Cr: 0.1-1.0 wt%; Cu: 0.05-0.25 wt%; Ni: 0.05-0.15 wt%; Ca: 0.0005-0.0035 wt%; Nb: 0.01-0.05 wt%; V: 0.01-0.02wt%.

8. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 1, characterized in that, The wave height of the water-cooled quenched ultra-high strength cold-rolled steel strip is ≤8mm.

9. The water-cooled quenched ultra-high strength cold-rolled steel strip as described in claim 1, characterized in that, The water-quenched ultra-high strength cold-rolled steel strip has a tensile strength ≥1470MPa, a yield strength ≥1150MPa, a surface residual stress range ≤130MPa, and a surface average residual stress ≤100MPa.

10. The method for manufacturing water-cooled quenched ultra-high strength cold-rolled steel strip as described in any one of claims 1-9, characterized in that, The manufacturing method includes the steps of: smelting and casting; hot rolling; pickling; cold rolling; annealing; wherein the annealing step includes: Heating and heat preservation, three-stage cooling, straightening machine to improve plate shape, tempering; The first stage of the three-stage cooling system has an initial temperature of 750-820℃, an end temperature of 450-500℃, and a cooling rate of ≥70℃ / s. The termination temperature of the second stage of the three-stage cooling system is 420-480℃, and the cooling rate is ≤10℃ / s. The termination temperature of the third stage of the three-stage cooling system is ≤100℃, and the cooling rate is ≥300℃ / s.

11. The manufacturing method as described in claim 10, characterized in that, After three-stage cooling, a straightener is used to improve the plate shape, followed by pickling and desqueezing, and then tempering; or, after three-stage cooling, pickling and desqueezing are performed first, followed by straightener to improve the plate shape, and then tempering.

12. The manufacturing method as described in claim 10, characterized in that, The tempering process involves holding the temperature at 200-300℃ for at least 150 seconds.

13. The manufacturing method as described in claim 10, characterized in that, The first stage of cooling uses air cooling, and the third stage of cooling uses water quenching.

14. The manufacturing method as described in claim 10, characterized in that, Between the insulation and three-stage cooling, there is also a slow cooling process with a cooling rate of 3-10℃ / s.

15. The manufacturing method as described in claim 10, characterized in that, During the heating and holding steps, the temperature is increased to the austenitic single-phase region at a heating rate of ≥1℃ / s and held for 30-200s.

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