Flexible high-strength steel with both room temperature stamping and high temperature hot forming properties and its manufacturing method

Abstract

This invention discloses a flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties, and its manufacturing method, belonging to the field of metallic materials technology. The flexible high-strength steel of this invention comprises the following chemical composition by weight percentage: 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%, B: 0.001%–0.003%, Ti: 0.035%–0.055%, Nb: 0.04%–0.08%, with the remainder being Fe and unavoidable impurities. The production process of this flexible high-strength steel is as follows: slab smelting and continuous casting, hot rolling and coiling, heat preservation, pickling and cold rolling, continuous annealing, and leveling. The flexible high-strength steel sheet prepared using the components and production process of this invention can be subjected to complex stamping at room temperature and hot forming at high temperature, effectively meeting the flexible processing requirements of "one material for two uses". Its yield strength at room temperature is 780MPa~880MPa, tensile strength is 1180MPa~1280MPa, and elongation is ≥18%. After being heated and quenched at 900℃~950℃, the tensile strength is ≥1500MPa and the hardness is ≥45HRC. The grain size in the cold-rolled state reaches level 9 or above.

Description

Technical Field

[0001] This invention belongs to the field of metal materials technology, specifically relating to a flexible high-strength steel that can be complexly stamped at room temperature and hot-formed at high temperature, and its manufacturing method, which is suitable for the multi-process forming requirements of lightweight automotive structural parts (such as A-pillars, B-pillars, and anti-collision beams). Background Technology

[0002] In today's automotive industry, safety performance has become paramount, leading to a continuous increase in the application of high-strength steel in vehicle bodies. Currently, QP980 and QP1180 steel grades, capable of large-scale industrial production, are receiving significant attention in automotive manufacturing. QP980 boasts a strength of up to 1000 MPa and a plasticity of 20%; QP1180 achieves a strength of up to 1200 MPa and a plasticity of 14%. These steel grades primarily utilize retained austenite for reinforcement and plasticity enhancement, which is their fundamental strengthening and toughening mechanism. However, retained austenite has significant limitations. During processing, due to external forces and temperature changes, its stability is poor, and it is highly susceptible to transformation into ferrite. Especially in some complex cold stamping processes, the transformation of retained austenite can lead to changes in local material properties, thereby affecting the overall quality and performance of parts. During the long-term service of automobiles, the combined effects of vibration, fatigue loads, and environmental factors can also cause retained austenite to transform, reducing the material's plasticity and toughness, posing a potential threat to the vehicle's safety performance.

[0003] Another type of steel widely used in automobile manufacturing is hot-stamped steel. This steel is produced by improving its hardenability and using hot stamping technology to obtain an ultra-high-strength martensitic structure. While hot-stamped parts can currently achieve very high strength, meeting the safety requirements of automobiles in collisions, their plasticity is relatively low, typically only 5-6%. This may result in parts lacking sufficient deformation capacity to absorb energy upon impact, thus limiting their application range to some extent.

[0004] A search revealed a domestic patent, CN103805869B, entitled "A High-Strength Hot-Rolled Q&P Steel and Its Manufacturing Method," which describes a high-strength hot-rolled Q&P steel with a tensile strength >1500MPa and an elongation >10%. The composition is as follows: C: 0.20%–0.50%, Si: 0.8%–2.0%, Mn: 1.5%–3.0%, P≤0.015%, S≤0.005%, Al: 0.02%–0.08%, N≤0.006%, Ti: 0.005%–0.015%, Cr: 0.5%–1.5%, with the remainder being Fe and unavoidable impurities. The final microstructure is primarily martensite plus retained austenite. Although it possesses high strength and good elongation, it still relies on retained austenite as a strengthening mechanism, making it unstable during production and processing.

[0005] For example, the domestic patent CN103882323B, entitled "MnCr Alloyed Hot Forming Steel and Its Production Method," provides a MnCr alloyed hot forming steel characterized by the following main chemical elements in weight percentage: C: 0.15–0.25%, Si: 0.10–0.25%, Mn: 1.5%–2.2%, Cr: 2.0%–2.5%. The hot forming steel, after hot forming, has a microstructure entirely of martensite, and the hot-formed part has a yield strength Rp = 1030 MPa, a tensile strength Rm = 1510 MPa, and a uniform elongation of 5%. It is mainly used for hot forming steel, with a predominantly martensitic microstructure and poor plasticity.

[0006] In existing technologies, the properties of steel grades that are formed at room temperature and those that are formed at high temperature 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 type of steel grade with optimized chemical composition and process parameters to achieve the dual properties of "room temperature stamping preforming + high temperature hot forming strengthening". Summary of the Invention

[0007] The purpose of this invention is to provide a flexible high-strength steel that can be complexly stamped at room temperature and also achieve ultra-high strength through high-temperature hot forming, as well as a method for manufacturing the same. The flexible high-strength steel obtained by this invention possesses the following properties:

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

[0009] Thermoforming properties: After heating and quenching at 900℃~950℃, tensile strength ≥1500MPa, hardness ≥45HRC; cold-rolled grain size (ASTM grade) reaches grade 9 or above;

[0010] Organizational compatibility: In the cold-rolled state, it is ferrite + bainite, and after high-temperature hot forming, it transforms into fine-grained martensite (ensuring strength), realizing flexible processing of "one material for two purposes".

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] Firstly, this invention provides a flexible high-strength steel that combines room-temperature stamping and high-temperature hot-forming properties, comprising the following chemical composition by 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%, with the remainder being Fe and unavoidable impurities.

[0014] It should be noted that the functions of each component and the content control of each component in the flexible high-strength steel provided by this invention are as follows:

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

[0016] Si: Si, as a strengthening element in steel, helps to improve the strength of steel. In this invention, Si plays the role of solid solution strengthening, which effectively improves the strength of steel. However, excessive Si will make it difficult to remove iron oxide scale on the surface of steel plate, and microcracks due to oxide indentation are easily formed on the surface. Therefore, the range of Si in this invention is controlled at 0.10% to 0.30%.

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

[0018] Cr: Cr can significantly improve the strength and hardenability of steel, but excessive Cr content has a significant impact on the material's plasticity and toughness. Therefore, the Cr content in this invention is controlled within the range of 1.5% to 2.0%.

[0019] P: Excessive P in steel is prone to segregation, which is detrimental to the formability and aging resistance of steel plates. Therefore, the content of P element in this invention is controlled within the range of ≤0.010%.

[0020] Sulfur (S): Sulfur is a harmful element in steel, causing hot brittleness, reducing ductility and toughness, and making it prone to cracking during rolling. Furthermore, S is detrimental to weldability and reduces corrosion resistance. Therefore, this invention controls the S content in steel to within the range of S ≤ 0.012%.

[0021] N: N is an impurity element in steel. Since free N will cause aging and is not conducive to resisting aging, the present invention controls the nitrogen content to N≤0.003%.

[0022] Ti: Ti is a strong carbide and nitride forming element. It forms titanium compounds with carbon and nitrogen, which strengthen the material by causing second-phase precipitation. Furthermore, it is not easily soluble in austenite at high temperatures, thus hindering austenite grain growth and refining the grain size. However, excessive Ti can lead to excessive second-phase precipitates, affecting the material's formability. Therefore, the titanium content in this invention is controlled within the range of 0.035% to 0.055%.

[0023] Niobium (Nb): Niobium acts similarly to Ti in steel, serving as a second-phase precipitation strengthening agent. However, excessive Nb significantly affects plasticity and is also a costly alloying element. Therefore, this invention controls the niobium content within the range of 0.04% to 0.08%.

[0024] This invention addresses two main issues. First, by using Ti-Nb composites to form nanoscale TiC and NbC particles (5-20 nm in size), the grains are refined at room temperature (grain size ≥ 9), improving plasticity. During high-temperature hot forming, the austenite grain boundaries are pinned, inhibiting grain coarsening (grain size ≥ 8). Second, by using Si-Mn strengthening: Si solid solution strengthening improves room temperature strength, while Mn expands the austenite region, ensuring that the austenite is fully dissolved during high-temperature hot forming, resulting in a uniform martensitic structure after quenching.

[0025] As a further improvement of the present invention, while increasing the strength of flexible high-strength steel, the stability and plasticity of the steel structure are improved as much as possible. The present invention achieves a dual improvement in strength and plasticity by strictly controlling the elemental composition and optimizing the key components. By controlling the synergistic combination of 200[C]+50[Mn]+80[Cr]≥65 and [Ti]+[Nb]≥0.075%, the application range of the steel is effectively expanded, and it is not easy to deform under different environmental conditions.

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

[0027] (1) Slab smelting and continuous casting: Taking pre-slag removal and post-slag removal measures during the hot metal pretreatment process to adjust the [S] element can reduce harmful elements in the molten steel. Adding self-circulating scrap steel in the early and middle stages of decarburization to modify the ladle top slag 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 initial rolling temperature is 1150℃~1200℃, and the final rolling temperature is controlled at 850℃~890℃. This temperature ensures that the final rolling temperature is above the austenitic temperature, avoiding the mixed crystal phenomenon caused by rolling in the two-phase region. Laminar flow cooling is selected with less water at the rear end, and the coiling temperature is set at 560℃~650℃.

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

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

[0031] (5) Leveling: Laser texturing rollers are used for leveling, and the leveling elongation is controlled at 0.6-1.2%.

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

[0033] Exemplary embodiments of the present invention are described in detail below. While these exemplary embodiments have been 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 various changes may be made to the invention without departing from its spirit and scope. The more detailed description of embodiments of the invention below is not intended to limit the scope of the claimed invention, but is merely illustrative and does not limit the description of the features and characteristics of the invention, in order to suggest the best mode for carrying out the invention and to enable those skilled in the art to practice it. Therefore, the scope of the invention is defined only by the appended claims.

[0034] Examples 1 to 7 of this invention are 7 heats of steel produced using specific components and specific smelting processes of this 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 product is 1.20 mm.

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

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

[0037]

[0038]

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

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

[0041]

[0042] As can be seen from Tables 1 and 2, Examples 1-7 all meet the requirements when implemented according to this patent. However, Comparative Examples 1-7 fail to meet the requirements due to the following defects, the main reasons being:

[0043] In Comparative Example 1, the low Mn / Cr content resulted in insufficient hardenability. During hot forming, the martensite transformation rate was <90%, and the residual ferrite / bainite structure reduced the tensile strength to only 1400 MPa (below the target of 1500 MPa). At the same time, the elongation in the cold-rolled state decreased to 15% (<18%). The root cause was the lack of core reinforcing elements, which led to the disruption of the strength-ductility balance.

[0044] In Comparative Example 2, the insufficient Ti / Nb composition prevented the formation of enough nanoscale carbonitride pinning grain boundaries. The cold-rolled state resulted in grain coarsening to grade 7 (ASTM) and an elongation of 16% (close to the lower limit). After hot forming, the grains further coarsened, and the impact toughness decreased, verifying the crucial role of "Ti+Nb≥0.075%" in grain refinement as stated in the claim.

[0045] In Comparative Example 3, the carbon content was below the lower limit. Although it met some of the plasticity requirements (elongation of 17%), as a core reinforcing element, the lack of carbon led to a decrease in martensitic hardness. The tensile strength in the cold-rolled state was only 1100 MPa (<1180 MPa), and the strength after hot forming was only 1450 MPa. This proves that the lower limit of carbon 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 surge to 950 MPa (exceeding the limit), while the elongation dropped to 14% (<18%). The excessively high carbon content led to severe work hardening, and the martensitic laths coarsened (width > 0.5 μm) after hot forming, resulting in a significant decrease in toughness, thus verifying the protective effect of the upper limit of carbon content on plasticity.

[0047] In Comparative Example 5, the total Ti / Nb content was insufficient: Ti (0.030%) + Nb (0.035%) = 0.065% < 0.075%. Although the composition did not completely exceed the limit, the synergistic grain refinement effect failed. The cold-rolled state had a grain size of grade 9 (barely meeting the standard), but the elongation was 16% (too low). After hot forming, the grain size was grade 8 (close to the lower limit), proving that the synergistic effect of Ti / Nb requires meeting a minimum total requirement.

[0048] In Comparative Example 6, the Ti / Nb ratio exceeded the upper limit, with both Ti (0.060%) and Nb (0.090%) exceeding the upper limit of the claims (0.055% / 0.08%). This resulted in excessive aggregation of nanoscale precipitates (size > 30 nm), abnormally high cold-rolled strength (890 MPa), and an elongation of 15% (< 18%). After thermoforming, excessive grain boundary pinning hindered dislocation movement, leading to decreased toughness (impact energy < 25 J), demonstrating the crucial influence of the upper limit of element content on plasticity.

[0049] In Comparative Example 7, the process parameters were abnormal. The annealing temperature was 700℃ (<800℃ lower limit). Although the composition met the requirements (Ti+Nb=0.100%, CMCr_index=104), the low-temperature annealing resulted in insufficient recrystallization, leading to grain coarsening to grade 8 and a cold-rolled elongation of 17% (too low). This demonstrates that the "annealing temperature of 800℃~840℃" in the claims is a necessary process condition to ensure grain refinement, and simply meeting the composition requirements cannot compensate for process deviations.

[0050] The above description is only a specific example of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the technical concept and technical solution of the present invention, or the direct application of the technical concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A flexible high-strength steel that combines room-temperature stamping and high-temperature hot-forming properties, characterized in that: The chemical composition includes the following weight percentages: 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%, B: 0.001%~0.003%, Ti: 0.035%~0.055%, Nb: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; Its composition satisfies the following formula: 200[C] + 50[Mn] + 80[Cr] ≥ 65%, [Ti] + [Nb] ≥ 0.075%. The prepared flexible high-strength steel has a yield strength of 780MPa~880MPa at room temperature, a tensile strength of 1180MPa~1280MPa, an elongation of ≥18%, and a tensile strength of ≥1500MPa and a hardness of ≥45HRC after heating and quenching at 900℃~950℃. The grain size in the cold-rolled state reaches level 9 or above.

2. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties according to claim 1, 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 leveling.

3. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties according to claim 2, characterized in that, The slab smelting and continuous casting process specifically involves: taking pre-slag removal and post-slag removal measures during the hot metal pretreatment process to adjust the [S] element, which can reduce harmful elements in the molten steel; adding self-circulating scrap steel in the early and middle stages of decarburization to modify the ladle top slag, which helps to reduce impurity elements in the molten steel.

4. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties as described in claim 2, characterized in that, Before hot rolling, the material is heated to a temperature of 1230℃~1280℃ and held for ≥180min. The initial rolling temperature is 1150℃~1200℃, the final rolling temperature is 850℃~890℃, and the coiling temperature is 560℃~650℃.

5. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties as described in claim 3, characterized in that, During pickling and rolling, the pickling reduction rate is ≥70%.

6. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties as described in claim 3, characterized in that, During annealing, the annealing temperature is 800℃~840℃. In the slow cooling section, the speed of the strip in the furnace is ≤150m / min. The cooling time in the heating section + soaking section is ≥200s. The temperature cooling rate in the rapid cooling section is 20℃ / s~30℃ / s.

7. The method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties as described in claim 3, characterized in that, The aging temperature is 360℃~380℃.

8. A method for manufacturing flexible high-strength steel with both room-temperature stamping and high-temperature hot-forming properties according to any one of claims 1-7, characterized in that, The flatness elongation rate is controlled between 0.6% and 1.2%.

Images