Flexible high-strength steel with normal-temperature stamping performance and high-temperature thermal forming performance and manufacturing method

Abstract

The invention discloses flexible high-strength steel with normal-temperature stamping performance and high-temperature thermal forming performance and a manufacturing method, and belongs to the technical field of metal materials. The flexible high-strength steel comprises the following chemical components in percentage by weight: 0.18%-0.25% of C, 0.10%-0.30% of Si, 1.80%-2.20% of Mn, 1.5%-2.0% of Cr, less than or equal to 0.010% of P, less than or equal to 0.012% of S, less than or equal to 0.003% of N, 0.001%-0.003% of B, 0.035%-0.055% of Ti, 0.04%-0.08% of Nb and the balance of Fe and inevitable impurities. The production process of the flexible high-strength steel comprises the steps of slab smelting and continuous casting, hot rolling and coiling, heat preservation, acid pickling and cold rolling, continuous annealing and flattening. The flexible high-strength steel plate prepared by adopting the components and the production process disclosed by the invention can be subjected to complex punch forming at normal temperature and can also be subjected to thermal forming at high temperature, so that the flexible processing requirement of'one material for two purposes' is effectively met, the yield strength of the flexible high-strength steel plate at normal temperature is 780-880 MPa, the tensile strength of the flexible high-strength steel plate is 1180-1280 MPa, the ductility of the flexible high-strength steel plate is greater than or equal to 18%, and the tensile strength of the flexible high-strength steel plate is greater than or equal to 1500 MPa after the flexible high-strength steel plate is heated and quenched at 900-950 DEG C; the hardness is greater than or equal to 45 HRC; and the cold-rolled grain size reaches more than 9 grade.

Description

Technical Field

[0001] The present invention belongs to the field of metal material technology, and specifically relates to a flexible high-strength steel that can be subjected to complex stamping forming at room temperature and hot forming at high temperature, and a manufacturing method thereof. The steel is suitable for the multi-process forming requirements of lightweight automotive structural parts (such as A-pillars, B-pillars, and anti-collision beams). Background Art

[0002] Safety performance has become paramount in today's automotive industry, leading to a continued increase in the use of high-strength steel in vehicle bodies. Currently, two steel grades, QP980 and QP1180, which are capable of large-scale industrial production, are attracting significant attention in the automotive industry. QP980 boasts a strength of up to 1000 MPa and a ductility of 20%, while QP1180 boasts a strength of up to 1200 MPa and a ductility of 14%. These steel grades primarily utilize retained austenite for reinforcement and plasticization, which is their fundamental strengthening and toughening mechanism. However, retained austenite has significant limitations. During processing, it is subject to external forces and temperature fluctuations, resulting in poor stability and a high risk of transformation to ferrite. This transformation, particularly in complex cold stamping processes, can lead to localized changes in material properties, impacting the overall quality and performance of the part. Over the long term, subjected to vibration, fatigue loads, and environmental factors, retained austenite can also undergo transformations, reducing the material's ductility and toughness, posing a potential threat to vehicle safety.

[0003] Another type of steel widely used in automotive manufacturing is hot-stamped steel. This process primarily improves the steel's hardenability and creates an ultra-high-strength martensitic structure through hot stamping. While hot-stamped components can currently achieve high strength, meeting crash safety requirements, their plasticity is relatively low, typically only 5-6%. This can result in parts lacking the ability to deform sufficiently to absorb energy during impact, limiting their application.

[0004] A search revealed that domestic patent number CN103805869B, "A High-Strength Hot-Rolled Q&P Steel and Its Manufacturing Method," proposes a high-strength hot-rolled Q&P steel and its manufacturing method, with a tensile strength greater than 1500 MPa and an elongation greater than 10%. The steel comprises C: 0.20% to 0.50%, Si: 0.8% to 2.0%, Mn: 1.5% to 3.0%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.02% to 0.08%, N ≤ 0.006%, Ti: 0.005% to 0.015%, Cr: 0.5% to 1.5%, with the remainder being Fe and unavoidable impurities. The final microstructure is primarily martensite plus retained austenite. While this steel exhibits high strength and good elongation, it still utilizes retained austenite as a strengthening mechanism, making it unstable during production and processing.

[0005] For example, the domestic patent authorization number CN103882323B, "MnCr Alloyed Hot Forming Steel and Its Production Method," provides a MnCr alloyed hot forming steel characterized by the following chemical elements by weight: C: 0.15-0.25%, Si: 0.10-0.25%, Mn: 1.5-2.2%, and Cr: 2.0-2.5%. After hot forming, the hot forming steel exhibits a fully martensite microstructure, with a yield strength Rp of 1030 MPa, a tensile strength Rm of 1510 MPa, and a uniform elongation of 5%. This steel is primarily used for hot forming, and its microstructure is primarily martensite, resulting in poor plasticity.

[0006] In existing technologies, the properties of room-temperature forming and hot-forming steel grades are independent of each other, and there is a lack of "flexible" steel grades that can adapt to both processes. Therefore, there is an urgent need to develop a new steel grade with optimized chemical composition and process parameters to achieve the dual performance of "room-temperature stamping preforming + high-temperature hot forming strengthening." Summary of the Invention

[0007] The object of the present invention is to provide a flexible high-strength steel that can be subjected to complex stamping at room temperature and can also obtain ultra-high strength through high-temperature hot forming, and a manufacturing method thereof. The flexible high-strength steel obtained by the present invention has the following properties:

[0008] Room temperature performance: yield strength 780MPa~880MPa, tensile strength 1180MPa~1280MPa, elongation ≥18%, meeting room temperature complex stamping and forming requirements;

[0009] Hot forming performance: After heating and quenching at 900℃~950℃, the tensile strength is ≥1500MPa and the hardness is ≥45HRC; the cold rolled grain size (ASTM grade) is above grade 9;

[0010] Microstructure compatibility: The cold-rolled state is ferrite + bainite, which is transformed into fine-grained martensite after high-temperature hot forming (to ensure strength), realizing flexible processing of "one material for two uses".

[0011] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] First, the present invention provides a flexible high-strength steel having both room temperature stamping and high temperature hot forming properties, which includes the following chemical components in weight percentage:

[0013] C: 0.18%~0.25%, Si: 0.10%~0.30%, Mn: 1.80%~2.20%, Cr: 1.5%~2.0%, P≤0.010%, S≤0.012%, N≤0.003%, Ti: 0.035%~0.055%, Nb: 0.04%~0.08%, and the rest is Fe and unavoidable impurities.

[0014] It should be noted that the functions and contents of the various components of the flexible high-strength steel provided by the present invention are controlled as follows:

[0015] C: C is the most basic and effective strengthening element in steel, the most effective element affecting hardenability, and has low cost. In order to ensure that the flexible high-strength steel has sufficient strength and sufficient plasticity, the carbon content needs to be optimized. The present invention adopts a low carbon content. At the same time, in order to ensure that the flexible high-strength steel has sufficient plasticity, the carbon content is controlled at 0.18-0.25%.

[0016] Si: Si is also a strengthening element in steel that helps to improve the strength of steel. The Si in the present invention plays a role in solid solution strengthening, effectively improving the strength of the steel. However, too high Si content will make it difficult to remove the iron oxide scale on the surface of the steel plate, and microcracks caused by the indentation of oxides are easily formed on the surface. Therefore, the range of Si in the present invention is controlled to 0.10% to 0.30%.

[0017] Mn: The Mn element can significantly improve the strength of steel, expand the austenite phase region, stabilize the austenite structure, improve hot working stability, and avoid mixed crystals. However, too high a Mn content is detrimental to plasticity, stamping performance, and fatigue performance. Taking all factors into consideration, the Mn percentage control range in the present invention is 1.80% to 2.20%.

[0018] Cr: Cr can significantly increase the strength of steel and also improve its hardenability, but too much Cr has a significant impact on the plasticity and toughness of the material. Therefore, the Cr content in the present invention is controlled within a range of 1.5% to 2.0%.

[0019] P: Excessive P is likely to form segregation in steel, which is detrimental to the formability and aging resistance of the steel plate. Therefore, the content of the P element in the present invention is controlled within a range of ≤0.010%.

[0020] S: S is a harmful element in steel, causing hot brittleness, reducing its ductility and toughness, and easily causing cracks during rolling. S also negatively impacts weldability and reduces corrosion resistance. Therefore, the present invention controls the S content in steel to ≤ 0.012%.

[0021] N: N is an impurity element in steel. Free N can cause aging and is disadvantageous for anti-aging. Therefore, the present invention controls the nitrogen content to N≤0.003%.

[0022] Ti: Ti is a strong carbide and nitride former. It forms titanium compounds with carbon and nitrogen, contributing to second-phase precipitation strengthening. Furthermore, it is insoluble in austenite at high temperatures, hindering austenite grain growth and refining the grain size. However, excessive Ti content can lead to excessive second-phase precipitates, compromising the material's formability. Therefore, the titanium content in this invention is controlled within the range of 0.035% to 0.055%.

[0023] Nb: Nb has a similar effect to Ti in steel, acting as a second-phase precipitation strengthening agent. However, excessive Nb significantly affects plasticity and is an alloying element with high cost. Therefore, the present invention limits the niobium content to 0.04% to 0.08%.

[0024] On the one hand, the present invention forms nano-scale TiC and NbC particles (size 5 to 20 nm) through Ti-Nb composite, refines grains (grain size ≥ 9 levels) and improves plasticity at room temperature, pins austenite grain boundaries during high-temperature hot forming, and inhibits grain coarsening (grain size ≥ 8 levels); on the other hand, through Si-Mn strengthening: Si solid solution strengthening improves room-temperature strength, Mn expands the austenite region, ensures that austenite is fully dissolved during high-temperature hot forming, and obtains a uniform martensite structure after quenching.

[0025] As a further improvement of the present invention, while improving the strength of flexible high-strength steel, the structural stability and plasticity of the steel grade are improved as much as possible. The present invention strictly controls the elemental composition, optimizes the key components therein, and controls the coordinated coordination of 200[C]+50[Mn]+80[Cr]≥65, [Ti]+[Nb]≥0.075% to achieve a dual improvement in strength and plasticity, effectively expanding the application range of the steel grade and making it less likely to deform when serving under different environmental conditions.

[0026] Secondly, the present invention provides a production process for the above-mentioned flexible high-strength steel, including slab smelting and continuous casting, hot rolling and coiling, pickling and cold rolling, continuous annealing, and leveling. Specifically, the process includes the following steps:

[0027] (1) Slab smelting and continuous casting: During the molten iron pretreatment process, pre-slag and post-slag removal measures are adopted to adjust the [S] element, which can reduce harmful elements in the molten steel. In the early and middle stages of decarburization, self-circulating scrap steel is added to modify the ladle top slag. This measure helps to reduce impurity elements in the molten steel.

[0028] (2) Hot rolling and coiling: The heating temperature is controlled at 1230℃~1280℃, and the holding time is ≥180min to ensure sufficient and uniform heating. The starting rolling temperature is 1150℃~1200℃, and the final rolling temperature is controlled at 850℃~890℃. This temperature can ensure that the final rolling temperature is controlled above the austenite temperature to avoid the mixed crystal phenomenon caused by rolling in the two-phase region. Laminar cooling is selected with less water at the rear end, and the coiling temperature is set to 560℃~650℃.

[0029] (3) Pickling and cold rolling: Pickling rolling adopts five-stand continuous rolling, and the pickling rolling reduction rate is ≥70%. A large reduction rate can increase the grain distortion energy in the steel, reduce the recrystallization temperature, and is conducive to grain refinement and improving the mechanical properties of the steel plate.

[0030] (4) Continuous annealing: annealing temperature is 800℃~840℃, slow cooling section, strip furnace speed ≤150mpm, heating section + soaking section cooling time ≥200s; cooling from 650℃ to 350℃, fast cooling section, temperature cooling rate is 20℃ / s~30℃ / s, aging temperature control range is 360℃~380℃.

[0031] (5) Smoothing: Laser texturing roller is used for smoothing, and the smoothing elongation is controlled at 0.6-1.2%.

[0032] The flexible high-strength steel produced using the above technical solution has the dual properties of "room temperature stamping preforming + high-temperature hot forming strengthening". At room temperature, the finished product has a yield strength of 780MPa-880MPa, a tensile strength of 1180MPa-1280MPa, and an elongation of 18%. After heating and quenching at 900℃-950℃, the tensile strength is 1500MPa or more and the hardness is 45HRC or more. The cold-rolled grain size (ASTM grade) reaches 9 or above, which can meet the flexible processing requirements of "one material for two uses", thereby fundamentally solving the problem of the incompatibility between strength and plasticity of high-strength steel plates for automobiles. DETAILED DESCRIPTION

[0033] Exemplary embodiments of the present invention are described in detail below. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be implemented and that various changes may be made to the invention without departing from the spirit and scope of the invention. The following more detailed description of the embodiments of the invention is not intended to limit the scope of the claimed invention, but is merely for illustrative and non-limiting purposes, to describe the features and characteristics of the invention, to set forth the best mode for carrying out the invention, and to enable those skilled in the art to practice the invention. Therefore, the scope of the invention is limited solely by the appended claims.

[0034] Examples 1 to 7 of the present invention are 7 heats of steel produced using the specific composition and specific smelting process of the present invention. The chemical composition of the molten steel is shown in Table 1, and the production process is shown in Table 2. The thickness of the finished products is 1.20 mm.

[0035] The chemical composition of the molten steel of Comparative Examples 1 to 7 is shown in Table 1, and the production process thereof is shown in Table 2. The thickness specification of the finished products manufactured is 1.20 mm.

[0036] Table 1 Chemical composition of steel in the embodiments of the present invention and comparative examples (unit: wt%)

[0037]

[0038]

[0039] The steel rolling production process parameters and final performance values of Examples 1 to 6 of the present invention and Comparative Examples 1 to 7 are shown in Table 2:

[0040] Table 2 Production process parameters and final performance values of the embodiments of the present invention and comparative examples

[0041]

[0042] Combining Table 1 and Table 2, it can be seen that Examples 1 to 7 can meet the requirements according to the implementation of this patent. However, Comparative Examples 1 to 7 cannot meet the requirements due to the following defects. The main reasons are:

[0043] The low Mn / Cr content in Comparative Example 1 resulted in insufficient hardenability. During hot forming, the martensite transformation rate was less than 90%, and the retained ferrite / bainite structure resulted in a tensile strength of only 1400 MPa (below the target of 1500 MPa). Meanwhile, the cold-rolled elongation dropped to 15% (less than 18%). This was due to the lack of core strengthening elements, which disrupted the strength-ductility balance.

[0044] In Comparative Example 2, insufficient Ti / Nb composition prevented the formation of sufficient nanoscale carbonitrides to pin grain boundaries. Cold-rolled grain size coarsened to ASTM grade 7, with an elongation of 16% (near the lower limit). Hot-forming further coarsened the grains, while impact toughness decreased, confirming the critical role of "Ti + Nb ≥ 0.075%" in grain refinement as claimed in the claims.

[0045] In Comparative Example 3, the C content is below the lower limit. Although it meets some plasticity requirements (elongation 17%), the insufficient carbon as a core strengthening element leads to a decrease in the hardness of martensite. The tensile strength in the cold-rolled state is only 1100 MPa (<1180 MPa), and the strength after hot forming is only 1450 MPa, which proves that the lower limit of the C content is a necessary condition to ensure strength.

[0046] In Comparative Example 4, the carbon content exceeded the upper limit, causing the cold-rolled yield strength to soar to 950 MPa (exceeding the limit) and the elongation to drop to 14% (<18%). Excessive carbon content causes severe work hardening, and the martensite laths coarsen after hot forming (width > 0.5 μm), significantly reducing toughness, confirming the protective effect of the upper limit of carbon content on plasticity.

[0047] In Comparative Example 5, the Ti / Nb total is insufficient: Ti (0.030%) + Nb (0.035%) = 0.065% < 0.075%. While the composition does not completely exceed the limit, the synergistic grain refinement effect fails. The cold-rolled grain size is 9 (barely meeting the standard), but the elongation is 16% (low). The hot-formed grain size is 8 (close to the lower limit), demonstrating that the minimum Ti / Nb total must be met for synergy.

[0048] In Comparative Example 6, the Ti / Nb ratio exceeds the upper limit, with both Ti (0.060%) and Nb (0.090%) exceeding the claimed upper limit (0.055% / 0.08%). This results in excessive aggregation of nanoscale precipitates (size > 30 nm), an abnormally high cold-rolled strength (890 MPa), and an elongation of 15% (<18%). After hot forming, excessive grain boundary pinning hinders dislocation motion and reduces toughness (impact energy < 25 J), demonstrating the critical influence of the upper limit of element content on plasticity.

[0049] In Comparative Example 7, the process parameters are abnormal, with an annealing temperature of 700°C (less than the lower limit of 800°C). Although the composition meets the standards (Ti+Nb=0.100%, CMCr_index=104), the low-temperature annealing results in insufficient recrystallization, resulting in grain coarsening to level 8 and a cold-rolled elongation of 17% (on the low side). This proves that the "annealing temperature of 800°C to 840°C" in the claims is a necessary process condition to ensure grain refinement, and meeting the composition standards alone cannot compensate for the process deviation.

[0050] The above description only provides a specific exemplary description of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the technical concept and technical solution of the present invention, or the technical concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the scope of protection of the present invention.

Claims

1. Flexible high-strength steel with both room temperature stamping and high temperature hot forming properties, characterized by: The invention comprises the following chemical components in weight percentage: C: 0.18% to 0.25%, Si: 0.10% to 0.30%, Mn: 1.80% to 2.20%, Cr: 1.5% to 2.0%, P≤0.010%, S≤0.012%, N≤0.003%, B: 0.001% to 0.003%, Ti: 0.035% to 0.055%, Nb: 0.04% to 0.08%, and the rest is Fe and unavoidable impurities.

2. The flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 1, characterized in that: The composition satisfies the following formula: 200[C]+50[Mn]+80[Cr]≥65, [Ti]+[Nb]≥0.075%.

3. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 1 or 2, characterized in that: The process includes the following steps: slab smelting and continuous casting, hot rolling and coiling, heat preservation, pickling and cold rolling, continuous annealing, and skin leveling.

4. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 3, characterized in that: Slab smelting and continuous casting involve implementing pre- and post-slag skimming during hot metal pretreatment to adjust the [S] element, reducing harmful elements in the molten steel. Adding recycled scrap steel during the early and middle stages of decarburization to modify the ladle top slag also helps reduce impurity elements in the molten steel.

5. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 3, characterized in that: Heating is carried out before hot rolling, the heating temperature is 1230℃~1280℃, and the holding time is ≥180min; the starting rolling temperature is 1150℃~1200℃, the finishing rolling temperature is 850℃~890℃, and the coiling temperature is 560℃~650℃.

6. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 4, characterized in that: During pickling, the pickling reduction rate is ≥70%.

7. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 4, characterized in that: During annealing, the annealing temperature is 800℃~840℃, the speed of the strip in the furnace in the slow cooling section is ≤150m / min, the cooling time in the heating section + soaking section is ≥200s; the cooling speed in the fast cooling section is 20℃ / s~30℃ / s.

8. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to claim 4, characterized in that: The aging temperature is 360℃~380℃.

9. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to any one of claims 3 to 8, characterized in that: The flattening elongation is controlled at 0.6-1.2%.

10. The method for manufacturing flexible high-strength steel having both room temperature stamping and high temperature hot forming properties according to any one of claims 3 to 8, characterized in that: The prepared flexible high-strength steel has a yield strength of 780MPa to 880MPa at room temperature, a tensile strength of 1180MPa to 1280MPa, and an elongation of 18%. After heating and quenching at 900°C to 950°C, the tensile strength is 1500MPa or more and the hardness is 45HRC or more. The cold-rolled grain size reaches above grade 9.

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