Cold-rolled high-strength steel with a tensile strength of 600-1100 MPa and a multi-stage production method thereof

Abstract

The invention discloses a cold-rolled high-strength steel with a tensile strength of 600-1100 MPa and a one-steel multi-level production method thereof, belonging to the technical field of material heat treatment. The main chemical components and mass percentages are: C: 0.14-0.18%, Mn: 1.6-2.0%, Si: 0.2-0.5%, Al: 0.60-0.80%, Cr: 0.40-0.80%, Nb: 0.01-0.05%, Ti: 0.01-0.03%, P≤0.02%, S≤0.01%, and the balance is Fe; for the above steel components, a simple and feasible heat treatment control process is used to obtain three levels of automobile steel with tensile strengths of 600-720 MPa, 800-900 MPa and 980-1100 MPa, respectively, which is conducive to low-cost large-scale continuous casting production, and the obtained materials have excellent forming properties while meeting high strength.

Description

Technical Field

[0001] The invention belongs to the technical field of material heat treatment, and in particular relates to a cold-rolled dual-phase steel with a tensile strength of 600-1000 MPa and enhanced formability, and a one-steel-multi-grade production method thereof. Background Art

[0002] With the rapid development of automobile lightweight technology, the body is required to further "increase strength and reduce weight", so high-strength steel and ultra-high-strength steel are widely used in the structural parts and safety parts of automobile body-in-white. At present, dual-phase steel (DP steel) is the most rapidly growing and most widely used advanced high-strength steel grade among high-strength steel products. For dual-phase steel with a tensile strength of 600-1000MPa, it does not require high equipment capacity, and the product technology content is relatively low, and it has faced the problem of homogeneous competition.

[0003] In addition, mainstream European automobile companies have gradually realized that traditional DP steel is difficult to form on many high-ductility parts, which directly leads to serious cracking during the stamping process and is difficult to meet the requirements of complex stamping structural parts in automobile design. The use of TRIP steel leads to a high alloy content, large welding problems, and excess elongation, resulting in waste. In the VDA239-100 standard released by the German Association of the Automotive Industry in 2016, high-formability dual-phase steel products were included for the first time, named DH steel (Dual Pha≥e Steel≥with Improved Formability). DH steel is optimized on the basis of traditional dual-phase steel design. By adding a small amount of Si and≥or Al, and introducing a small amount of retained austenite with a reasonable annealing heat treatment process, it has good forming performance. Due to the low addition of Si≥Al elements, this series of steels also has excellent welding and painting performance. Therefore, the development of DH steel products provides new material solutions for automotive safety parts and structural parts with high requirements for formability.

[0004] The tensile strength of DH steel products reported so far mainly includes three strength levels: 590MPa, 780MPa and 980MPa.

[0005] For example, the Chinese patent application number is 202010928566.6, and the application publication date is November 24, 2020. It discloses a high ductility and high formability cold-rolled DH590 steel and its production method. The chemical composition of the steel is as follows by mass percentage: C: 0.08%~0.14%, Mn: 1.2%~1.97%, Si: 0.35%~0.75%, Al: 0.27%~0.50%, Cr: 0.03%~0.24%, Mo: 0.02%~0.15%, P≤0.03%, S≤0.03%, Nb≤0.027%, and Si+Al: 0.5%~1.2%, and the balance is iron and unavoidable impurities.

[0006] The Chinese patent application number is 202010928563.2, and the application publication date is November 24, 2020. It discloses a 780MPa grade high-plasticity cold-rolled DH steel and a preparation method thereof. The steel contains C: 0.11%~0.18%, Mn: 2.04%~2.5%, Si: 0.57%~0.8%, Al: 0.32%~0.7%, Cr: 0.02%~0.50%, P≤0.03%, S≤0.03%, Nb≤0.018%, Ti≤0.1%, and Si+Al: 0.89%~1.5%, and the balance is iron and unavoidable impurities.

[0007] The Chinese patent application number is 202110177643.3, and the application publication date is June 25, 2021. It discloses a DH980 steel with high edge quality and a preparation method thereof. The chemical composition of the steel is calculated by mass fraction: C: 0.19%~0.25%, Mn: 1.9%~2.3%, Si: 0.3%~0.5%, Cr≤0.3%, Al: 0.4%~0.8%, and the balance is Fe and unavoidable impurities.

[0008] It can be found that the chemical compositions of the three strength levels of cold-rolled DH steels, 590MPa, 780MPa and 980MPa, are quite different. The steelmaking process needs to use three different steel grades for production, which brings great difficulties to production scheduling. At the same time, the loss of stripped products (unplanned materials) of steel grades with different compositions during steelmaking and hot rolling production is relatively large.

[0009] There is also research in the prior art on dual-phase steel with multiple grades of steel.

[0010] After searching, the Chinese patent application number is 201810507350.5, and the application publication date is September 11, 2018. It discloses an economical cold-rolled dual-phase steel with multiple grades and a production method thereof. The chemical composition of the steel billet includes C: 0.10-0.15%, Si≤0.05%, Mn: 1.80-2.00%, Alt: 0.030-0.060%, N≤0.004%, Cr: 0.20-0.40%, and P is limited to 0.010%, S≤0.005%, and the balance is Fe and other inevitable impurities; at the same time, the key process parameters of each process section are effectively controlled to achieve the use of a component system to produce cold-rolled dual-phase steel products with three tensile strength levels of 490MPa, 590MPa and 780MPa with a thickness specification of 0.7-1.6mm and good surface quality. However, the Si and Al contents in its composition design are low, which is not conducive to the enrichment of C in austenite. Therefore, there is no residual austenite in the finished product structure, resulting in a low elongation of the final product. In addition, the hot rolling heating temperature, final rolling temperature, coiling temperature, cold rolling reduction rate, continuous annealing heating temperature and over-aging temperature of the corresponding three strength levels of dual-phase steel products are different, which is not conducive to continuous large-scale production.

[0011] The Chinese patent application number is 201911018815.1, and the application publication date is March 6, 2020. It discloses a hot-rolled dual-phase steel with multiple grades and thicknesses and a production method. The chemical composition weight percentage is: C: 0.06% to 0.09%, Si: ≤0.05%, Mn: 1.1% to 1.4%, Cr: 0.45 to 0.65%, P: ≤0.015%, S: ≤0.005%, Al≥: 0.015% to 0.045%, and the balance is Fe and unavoidable impurities. In the production method, the heating process of the ingot, the temperature out of the heating furnace is 1250120℃, and the heating time is ≥180min; the heating of the ingot adopts tail temperature compensation, and the temperature of the tail is 20 to 40℃ higher than that of the head. However, the two grades of steel described therein are 540MPa and 590MPa, respectively, with lower strength and relatively small difference. In addition, the three-stage cooling control after hot rolling to obtain the two strength grades of steel is relatively complex, and has high requirements on equipment and temperature control.

[0012] Therefore, how to design a steel composition and obtain multi-grade high-strength steel with high strength and excellent formability through a simple and feasible heat treatment control process is a problem that needs to be solved urgently. Summary of the invention

[0013] 1. Problems to be solved

[0014] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide a cold-rolled high-strength steel with a tensile strength of 600-1100MPa and a one-steel multi-grade production method thereof by adopting an economically reasonable component system through a flexible annealing process design, that is, using a simple and feasible heat treatment control process to obtain three grades of automobile steel with tensile strengths of 600-720MPa, 800-900MPa and 980-1100MPa, which is conducive to low-cost large-scale continuous casting production, and the obtained material has excellent forming properties while meeting high strength.

[0015] 2. Technical solution

[0016] To solve the above problems, the present invention adopts the following technical solutions.

[0017] The present invention aims to provide a cold-rolled high-strength steel with enhanced formability and a tensile strength of 600-1000MPa and a method for producing the steel in multiple levels. The minimum tensile strength of the final product involves three strength levels of 600MPa, 800MPa and 1000MPa, wherein the 600MPa level has a tensile strength of 600-720MPa and an elongation A80 of ≥25%; the 800MPa level has a tensile strength of 800-900MPa and an elongation A80 of ≥18%; and the 1000MPa level has a tensile strength of 980-1100MPa and an elongation A80 of ≥14%.

[0018] The same steel grade is used in the steelmaking process, and its main chemical components are as follows: C: 0.14-0.18%, Mn: 1.6-2.0%, Si: 0.2-0.5%, Al: 0.60-0.80%, Cr: 0.40-0.80%, Nb: 0.01-0.05%, Ti: 0.01-0.03%, P≤0.02%, S≤0.01%, and the balance is Fe. Among them, the addition of Mn element is reduced in the steel composition, and Nb and Ti elements are appropriately added. The role of each chemical component is as follows:

[0019] C: Carbon plays a role in solid solution strengthening and is an important guarantee for achieving material strength. If the carbon content is too low, the steel plate will be insufficient in strength. At the same time, carbon can stabilize austenite. If there are not enough C atoms to enrich the retained austenite during the annealing process, the retained austenite obtained will be insufficiently stable. However, too high a carbon content will be detrimental to the forming performance and welding performance. Therefore, the higher the carbon content of steel is, the better. In the present invention, the C content is controlled in a relatively low range of 0.14 to 0.18%.

[0020] Mn: It improves the strength of the ferrite matrix through solid solution strengthening. At the same time, manganese is an austenite stabilizing element, which expands the austenite phase area; it shifts the proeutectoid ferrite precipitation line to the right and reduces the critical cooling rate. If the Mn content is too low, on the one hand, the material cannot reach the required strength, and on the other hand, the residual austenite cannot be stabilized, thus affecting the plasticity of the material; if the Mn content is too high, the volume of austenite may increase, the banded structure of the steel is very serious, and the cost is high and the processing is difficult. Therefore, the weight percentage of Mn in the present invention is controlled at 1.6-2.0%.

[0021] Si: ferrite forming element, plays a role in solid solution strengthening; can hinder the precipitation of cementite, plays a role in stabilizing austenite; on the other hand, too high silicon content will cause selective oxidation of the steel plate surface during annealing, produce iron oxide scale during heating, and when hot rolling, when the iron oxide scale is pressed into the plate surface, the surface quality of the plate will be reduced, resulting in difficulty in welding the material and poor surface coating, etc. Therefore, the weight percentage of Si in the present invention is controlled at Si: 0.2-0.5%.

[0022] Al: inhibits the precipitation of carbides and the decomposition of retained austenite, while accelerating the bainite transformation, which is beneficial to the carbon enrichment of retained austenite and enhancing its stability. It is beneficial to improving the 600MPa grade low-strength formability in the present invention. However, too much Al will block the nozzle during continuous casting and increase the difficulty of continuous casting. Therefore, the Al≥ content should be controlled at 0.6-0.8%.

[0023] Cr: significantly improves the hardenability of the material and thus improves the strength of the material. If the content is too low, the high strength of 1000 MPa in the present invention cannot be achieved, and if the content is too high, the production cost will increase. Therefore, the Cr content in the present invention is preferably controlled at 0.4-0.8%.

[0024] Nb: refines annealed austenite grains, improves strength and plasticity. If the content is too low, the effect of grain refinement cannot be achieved, and if the content is too high, the production cost will increase. In the present invention, the Nb content is preferably controlled at 0.01-0.05%.

[0025] Ti: Adding a small amount of Ti to steel can improve the strength of the steel, play a role in precipitation strengthening and grain refinement, wherein TiN can effectively prevent the growth of austenite during heating, play a role in refining austenite grains, and improve the toughness of the heat-affected zone. In the present invention, the Ti content is preferably controlled at 0.01-0.03%.

[0026] P, S: In order to reduce the adverse effects of harmful impurities in steel on the stamping performance of steel, the content of P and S in steel must be strictly controlled.

[0027] The present invention adopts the above chemical composition, and the production process is as follows:

[0028] Smelting → continuous casting → hot rolling → pickling and cold rolling → continuous annealing → finished product.

[0029] The present invention adopts the above chemical composition and process flow to produce a cold-rolled dual-phase steel with enhanced formability and a tensile strength of 600-1000 MPa and a one-steel multi-grade production method thereof. The specific implementation method is as follows:

[0030] (1) Smelting and continuous casting: It is suitable for smelting in converters, electric furnaces and induction furnaces. Continuous casting is used to produce ingots. To ensure the quality of ingots, dynamic soft pressure and electromagnetic stirring systems can be used.

[0031] (2) Hot rolling of the billet or ingot: the billet is heated to 1200-1260°C, kept warm for 2-3 hours, rolled in 5-7 passes by the roughing mill, hot rolled to an intermediate billet of 30-50 mm, rolled in 5-7 passes by the hot rolling mill, and rolled to a final rolling temperature of 870-910°C. After rolling to the target thickness (2.0-6.0 mm), it is coiled into a steel coil at a temperature of 520-600°C to obtain a uniform and fine hot-rolled structure;

[0032] (3) Pickling and cold rolling: The hot-rolled strip is pickled in a hydrochloric acid tank to remove the surface oxide scale, and then cold rolled or cold rolled with a cold rolling reduction rate of 40-70% to a target thickness of 0.6-2.5 mm.

[0033] (4) Continuous annealing: The steel plate treated by the pickling and cold rolling step is heated to 760-820°C for two-phase annealing, slowly cooled to 680-720°C, then cooled to the flash cooling temperature at a cooling rate of ≥20°C≥≥ for quenching and heat preservation for overaging for 100-900°C≥, and finally cooled to room temperature.

[0034] ① For 600MPa grade: the flash cooling end temperature and over-aging temperature are 380-420℃, the rapid cooling rate is ≥20℃≥≥, and the finished product structure is ferrite (F) + bainite (B) + retained austenite (RA), of which ferrite accounts for 40-60%, martensite accounts for 10-30%, bainite accounts for 10-30%, and retained austenite accounts for 5-15%;

[0035] ② For 800MPa grade: the flash cooling end temperature and over-aging temperature are 330-370℃, the rapid cooling rate is ≥25℃≥≥, and the finished product structure is ferrite (F) + martensite (M) + bainite (B) + retained austenite (RA), of which ferrite accounts for 40-60%, martensite accounts for 20-40%, bainite accounts for 5-15%, and retained austenite accounts for 5-10%;

[0036] ③ For 1000MPa grade: the flash cooling end temperature and over-aging temperature are 260-300℃, the rapid cooling rate is ≥30℃≥≥, and the finished product structure is ferrite (F) + martensite (M) + retained austenite (RA), of which ferrite accounts for 40-60%, martensite accounts for 35-55%, and retained austenite accounts for 3-8%.

[0037] It should be noted that the phase change of general materials requires isothermal cooling at a certain temperature or slow cooling within the phase change temperature range. Therefore, in order to avoid the formation of unwanted structures in material design, a critical cooling rate, that is, the minimum cooling rate, is required, and there is generally no restriction on the upper limit of the cooling rate.

[0038] 3. Beneficial effects

[0039] Compared with the prior art, the method of the present invention has achieved the following implementation effects since its use:

[0040] (1) The present invention provides a production method for achieving 600-1000MPa grade enhanced formability cold-rolled high-strength steel by adopting an economically reasonable component system through a flexible annealing process design. The component design is optimized on the basis of 800MPa grade DH steel, reducing the addition of Mn elements, and by appropriately adding Nb and Ti elements, the grains are refined to play a role in grain refinement and strengthening, and the uniformity of the structure can be improved to enhance the cold bending, expansion and hole performance of the finished product;

[0041] (2) By adopting the composition system of the present invention, a 600-1000MPa grade high-formability cold-rolled high-strength steel can be achieved through a flexible annealing process design: the minimum tensile strength of the final product involves 600MPa, 800MPa and 1000MPa There are three strength levels, among which the 600MPa level has a tensile strength of 600-720MPa, an elongation A80≥25%, and a finished product structure of ferrite + bainite + martensite + retained austenite; the 800MPa level has a tensile strength of 800-900MPa, an elongation A80≥18%, and a finished product structure of ferrite + martensite + bainite + retained austenite; the 1000MPa level has a tensile strength of 980-1100MPa, an elongation A80≥16%, and a finished product structure of ferrite + martensite + retained austenite; the one-steel multi-level production and manufacturing method provided by the present invention is conducive to reducing the steelmaking and hot rolling group production costs and difficulties of the series of products, increasing the continuous casting furnace times and reducing the proportion of hot-rolled strip products (transition materials). BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a schematic diagram for comparing the annealing process of 600-1000MPa grade cold-rolled ultra-high strength steel of the present invention;

[0043] Figure 2 This is a typical SEM structure of 600MPa grade ultra-high strength steel produced by the annealing process of the present invention;

[0044] Figure 3 This is a typical SEM structure of 800MPa grade ultra-high strength steel produced by the annealing process of the present invention;

[0045] Figure 4 This is a typical SEM structure of 1000MPa grade ultra-high strength steel produced by the annealing process of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described below in conjunction with specific embodiments and drawings, but they do not limit the present invention.

[0047] Example

[0048] The chemical composition of the steel in each embodiment is shown in Table 1, and its preparation process is as follows:

[0049] (1) Smelting and solidification: It is suitable for smelting in converters, electric furnaces and induction furnaces. Continuous casting is used to produce ingots. Electromagnetic stirring and dynamic soft pressure devices are used during the pouring process to reduce component segregation during the solidification process.

[0050] (2) Hot rolling of billets or ingots: the billets are heated to 1200-1260°C, kept warm for 2-3 hours, rolled in 5-7 passes by a rough rolling mill, hot rolled to an intermediate billet of 30-50 mm, rolled in 5-7 passes by a hot rolling mill, and the final rolling temperature is 870-910°C. After rolling to the target thickness, the billets are coiled at 520-600°C to form steel coils;

[0051] (3) Pickling and cold rolling: The hot-rolled strip is pickled in a hydrochloric acid tank to remove the surface oxide scale, and then cold rolled or cold rolled with a cold rolling reduction rate of 40-70% until the target thickness is reached.

[0052] (3) Pickling and cold rolling: The hot-rolled strip is pickled in a hydrochloric acid tank to remove the surface oxide scale, and then cold rolled or cold rolled with a cold rolling reduction rate of 50-75% to the target thickness;

[0053] (4) Continuous annealing: The acid-rolled samples were annealed using a continuous annealing production line. The continuous annealing process and the corresponding actual product performance implementation examples are shown in Table 1.

[0054] Table 1 Chemical composition formula, process parameters and measured product performance data of each embodiment

[0055]

[0056]

[0057] Figure 1 This is a schematic diagram for comparing the annealing process of 600-1000MPa grade cold-rolled ultra-high strength steel of the present invention;

[0058] Figure 2 This is a typical SEM structure of the 600MPa grade ultra-high strength steel in Example 1, wherein the volume fractions of ferrite, martensite, bainite, and retained austenite are 56.2%, 15.6%, 16.8%, and 11.4%, respectively, as measured by the image + XRD quantitative method;

[0059] Figure 3 This is a typical SEM structure of the 800MPa grade ultra-high strength steel in Example 2, wherein the volume fractions of ferrite, martensite, bainite, and retained austenite are 49.4%, 29.8%, 13.5%, and 7.3%, respectively, as measured by the image + XRD quantitative method;

[0060] Figure 4 This is a typical SEM structure of the 1000MPa grade ultra-high strength steel in Example 3, wherein the image + XRD quantitative method is used to measure that the volume fractions of ferrite, martensite, and retained austenite are 45.6%, 49.6%, and 4.8%, respectively.

[0061] The examples described in the present invention are merely descriptions of the preferred implementation modes of the present invention, and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should all fall within the protection scope of the present invention.

Claims

1. A cold-rolled high-strength steel with a tensile strength of 600 to 1100 MPa, characterized in that: The main chemical components and mass percentages are: C: 0.14-0.18%, Mn: 1.6-2.0%, Si: 0.2-0.5%, Al: 0.60-0.80%, Cr: 0.40-0.80%, Nb: 0.01-0.05%, Ti: 0.01-0.03%, P≤0.02%, S≤0.01%, and the balance is Fe; Cold-rolled high-strength steel with a tensile strength of 600-720 MPa, an elongation of A80 ≥ 25%, and a finished product structure of ferrite, bainite and retained austenite, with the proportions being: ferrite 40-60%, martensite 10-30%, bainite 10-30%, and retained austenite 5-15%; Cold-rolled high-strength steel with a tensile strength of 800-900 MPa, an elongation of A80 ≥ 18%, and a finished product structure of ferrite, martensite, bainite and retained austenite, with the proportions being: ferrite 40-60%, martensite 20-40%, bainite 5-15%, and retained austenite 5-10%; Cold-rolled high-strength steel with a tensile strength of 980-1100MPa and an elongation A80≥14%. The finished product structure is ferrite + martensite + retained austenite, with the proportions being 40-60% ferrite, 35-55% martensite, and 3-8% retained austenite.

2. A method for producing a cold-rolled high-strength steel with a tensile strength of 600 to 1100 MPa in one steel in multiple stages according to claim 1, characterized in that: The steps are: after smelting, continuous casting, hot rolling, pickling and cold rolling, and continuous annealing, a cold-rolled high-strength steel with a tensile strength of 600 to 1100 MPa is obtained, wherein the continuous annealing: (1) For cold-rolled high-strength steel with a tensile strength of 600-720 MPa: the flash cooling temperature and overaging temperature are 380-420 °C, and the flash cooling rate is ≥ 20 °C / s; (2) For cold-rolled high-strength steel with a tensile strength of 800-900 MPa: the flash cooling temperature and overaging temperature are 330-370 °C, and the flash cooling rate is ≥25 °C / s; (3) For cold-rolled high-strength steel with a tensile strength of 980-1100 MPa: the flash cooling temperature and overaging temperature are 260-300 °C, and the flash cooling rate is ≥30 °C / s.

3. The method for producing a cold-rolled high-strength steel with a tensile strength of 600-1100 MPa in multiple stages according to claim 2, characterized in that: In the continuous annealing, the steel plate treated by the pickling and cold rolling step is heated to 760-820° C. for two-phase zone annealing, slowly cooled to 680-720° C., then flash cooled to the flash cooling temperature for quenching and heat preservation for overaging for 100-900 seconds, and finally cooled to room temperature.

4. The method for producing a cold-rolled high-strength steel with a tensile strength of 600-1100 MPa in multiple stages according to claim 3, characterized in that: The hot rolling step comprises: heating the ingot to a temperature of 1200-1260° C., keeping the temperature for 2-3 hours, rolling the ingot for 5-7 passes by a rough rolling mill, hot rolling the ingot to a thickness of 30-50 mm, rolling the ingot for 5-7 passes by a hot rolling mill, final rolling at a temperature of 870-910° C., rolling the ingot to a thickness of 2.0-6.0 mm, and then coiling the ingot at a temperature of 520-600° C. into a steel coil; The pickling and cold rolling step comprises: pickling the hot-rolled strip in a hydrochloric acid tank to remove the surface iron oxide scale, and then performing cold continuous rolling or cold rolling, with a cold rolling reduction rate of 40-70% and rolling to 0.6-2.5 mm.

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