A steel for automobile structure having a yield strength of 1000mpa or more and a manufacturing method thereof

Abstract

The application discloses a kind of automobile structure steel with yield strength 1000MPa and above and its manufacturing method, its component weight percentage is as follows: C 0.15~0.23%;Si 0.12~0.5%;Mn 1.6~2.4%;B 0.001~0.004%;Al 0.01~0.04%;Cr;0.05~0.5%;Mo 0.15~0.5%;P≤0.015%;S≤0.005%;Rest contains Fe and other inevitable impurities, and need to satisfy the following relationship: the eigenvalue Bs of lower bainite formation rate=0.6~1.4, Bs=(Mo+B*100-Mn / 5) / C;Its microstructure lower bainite≥95%.The automobile structure steel of the application has yield strength≥1000MPa, tensile strength≥1180MPa, elongation≥7%, 180° bending performance d≤3.5T.The automobile structure steel obtained by the application not only has ultrahigh strength, but also has excellent forming performance, especially bending forming performance, and is especially suitable for manufacturing automobile chassis system parts.

Description

Technical Field

[0001] This invention relates to automotive steel, and particularly to automotive structural steel with a yield strength of 1000 MPa or higher, and a method for manufacturing the same. Background Technology

[0002] With the development concept of "green and safe" for the new generation of automobiles, the strength required for automotive structural components is getting higher and higher, which allows for a significant reduction in the thickness of automotive structural components and a substantial reduction in the overall vehicle weight. This achieves the green development goal of "reducing carbon emissions" on the one hand, and improves the overall vehicle's handling and braking distance on the other, thereby improving the safety of the vehicle.

[0003] Currently, the strength of hot-rolled or pickled steel sheets or strips used in automotive structural components is not high, with tensile strength generally around 800 MPa. Therefore, there is an urgent need to further improve the strength of the steel used. While the strength of hot-rolled or pickled steel sheets or strips can be further improved by cold rolling and then annealing, many automotive structural components still require thicker hot-rolled or pickled materials. Furthermore, the cold rolling and annealing process increases manufacturing costs and carbon emissions. Therefore, this invention primarily focuses on the design of methods to improve the strength of hot-rolled or pickled materials used in automobiles.

[0004] There are two existing methods to improve the strength of hot-rolled pickled steel plates or strips:

[0005] 1. Introducing a large amount of martensite or retained (metastable) austenite into the steel plate structure causes the retained (metastable) austenite to transform into martensite through deformation during forming.

[0006] For example, Chinese patents CN200610025065.7 and CN201210461655.X disclose hot-rolled high-strength steel with martensitic microstructure and its manufacturing method, with tensile strength reaching 1150MPa or even 1400MPa and above. Although the large-scale introduction of martensite can significantly improve the strength of steel plates or strips, martensite has low plasticity and toughness. Although tempering or the introduction of other soft phases (such as ferrite) can improve the overall average plasticity and toughness of the material, in local areas, especially at the interface between the martensitic phase and other phases, the excessive strength / hardness difference between the martensitic phase and the surrounding phase results in poor local plasticity and toughness. This manifests as poor bending performance and hole expansion and flanging performance of steel plates or strips, and easy cracking at the interface between martensite and the surrounding phase. In addition, in order to obtain a sufficient amount of martensite or retained austenite, high levels of carbon, manganese and silicon are often added to steel plates or strips, which deteriorates the weldability, paintability and surface color of the steel plates.

[0007] 2. Improve the strength of hot-rolled or pickled steel plates or strips by inducing the precipitation of large amounts of microalloyed carbides or carbonitrides.

[0008] For example, Chinese patent CN201610268167.5 proposes a method to obtain hot-rolled steel plates or strips with tensile strengths of 1180 MPa or higher through the precipitation of microalloyed carbides or carbonitrides. However, this method also has two problems. First, the large-scale precipitation of microalloyed carbides or carbonitrides means that a large amount of microalloying elements such as Ti, V, and Nb need to be added to the steel. These alloying elements are extremely expensive, and adding a large amount will increase the mass production cost of the product. On the other hand, the large-scale precipitation of microalloyed carbides or carbonitrides is not conducive to the bending performance of the steel plate. When the material is bent, cracks are prone to occur at the accumulation of microalloyed carbides or carbonitrides, or at the interface between the precipitation and the matrix. In addition, the above-mentioned elements can also form coarse and sharp-edged nitrides (such as TiN) with nitrogen, which makes the steel plate and strip more prone to bending cracks. Summary of the Invention

[0009] The purpose of this invention is to provide a steel for automotive structures with a yield strength of 1000MPa or higher and a method for manufacturing the same. The steel has a tensile strength of 1180MPa or higher and also has good bending and flanging properties. The steel has a yield strength ≥1000MPa, a tensile strength ≥1180MPa, an elongation ≥7%, and a 180° bending performance d≤3.5T, making it particularly suitable for automotive chassis structural parts.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] The automotive structural steel described in this invention is bainitic steel, which neither introduces martensite nor introduces a large amount of carbide precipitation. Instead, it controls the microstructure of the steel plate or strip to be basically pure lower bainite (lower bainite content ≥95%), thereby obtaining a tensile strength of 1180MPa and above while also having good bending and flanging performance.

[0012] To achieve a yield strength ≥1000MPa and a tensile strength of 1180MPa or higher in steel plates or strips, it is necessary to increase both the proportion of lower bainite and the strength or hardness of the lower bainite. However, due to limitations in hot rolling processes and equipment (such as short cooling rollers and high rolling speeds), the time available for lower bainite transformation is relatively short. Therefore, this invention needs to consider accelerating the transformation rate of lower bainite, shortening the lower bainite formation time, and expanding the bainite phase region to form lower bainite as quickly and abundantly as possible.

[0013] Specifically, the automotive structural steel with a yield strength of 1000 MPa and above described in this invention has the following composition by weight percentage:

[0014] C: 0.15–0.23%;

[0015] Si: 0.12–0.5%;

[0016] Mn: 1.6–2.4%;

[0017] B: 0.001~0.004%;

[0018] Al: 0.01–0.04%;

[0019] Cr; 0.05–0.5%;

[0020] Mo: 0.15–0.5%;

[0021] P: ≤0.015%;

[0022] S: ≤0.005%;

[0023] The balance includes Fe and other unavoidable impurities, and must satisfy the following relationship: the characteristic value of the lower bainite formation rate Bs = 0.6~1.4, Bs = (Mo + B*100 - Mn / 5) / C;

[0024] The microstructure of the steel used in automotive structures contains ≥95% lower bainite.

[0025] Furthermore, it also contains at least one of Ti, V, and Nb, and Ti+Nb+V≤0.03%, preferably Ti+Nb+V≤0.006%.

[0026] Preferably, the microstructure of the automotive structural steel of the present invention is: lower bainite ≥ 95%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.2%;

[0027] Preferably, the microstructure of the automotive structural steel of the present invention is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

[0028] The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T, preferably d≤3T.

[0029] In the composition design of the automotive structural steel described in this invention:

[0030] C: Carbon (C) primarily controls the phase transformation in steel microstructure, the hardness / strength of lower bainite, the time required for lower bainite formation, the martensite formation temperature (Ms), and the precipitation of microalloyed carbides or carbonitrides, thus affecting the mechanical properties of the material. When the C content in steel is below 0.15%, the steel's strength will not meet the target requirements, or excessive Ms will lead to the formation of martensite. In this case, although the strength is high, the bending performance will be significantly deteriorated. Conversely, if the C content in steel is above 0.23%, it is easy to cause excessive strength or excessive carbides, resulting in a decrease in the plasticity and bending performance of the steel plate. Based on this, the present invention controls the C content to be between 0.15% and 0.23%, preferably between 0.18% and 0.21%.

[0031] Si: Si has a certain solid solution strengthening effect, but it affects the surface quality of steel sheets. When the Si content in steel is below 0.12%, it is difficult to obtain a sufficient strengthening effect; while when the Si content in steel is above 0.5%, iron oxide scale or tiger-skin stripe-like color differences are easily formed on the surface of the steel sheet after pickling, which is not conducive to the surface quality of automotive steel sheets. Therefore, this invention controls the Si content between 0.12% and 0.5%.

[0032] Mn: Mn affects the hardenability of steel plates and the formation of martensite and lower bainite. Higher Mn content results in a lower Ms point, but Mn also has a dragging effect on carbon atom diffusion, thus prolonging the time required for lower bainite formation. Therefore, the Mn content in steel should not be too high, otherwise a pure lower bainite structure cannot be obtained. However, when the Mn content is low, the Ms point temperature increases, which is also unfavorable for the formation of a pure lower bainite structure. Based on this, the present invention controls the Mn content between 1.6% and 2.4%, preferably between 1.8% and 2.2%.

[0033] Boron (B): Boron can enhance the hardness of lower bainite and, in synergy with molybdenum (Mo), can shorten the formation time of lower bainite. However, when the B content is too high, brittle borides are easily formed, affecting the plasticity and bending properties of the steel plate. Therefore, this invention controls the B content between 0.001% and 0.004%.

[0034] Al: Al is added to steel only as a deoxidizing element, removing oxygen (O) to ensure the steel's performance and quality. However, excessively high Al content leads to increased costs and significantly increases the difficulty of continuous casting production. Therefore, this invention controls the Al content between 0.01% and 0.04%.

[0035] Cr: Cr is mainly used to expand the bainite phase region, making it easier to obtain lower bainite in steel. It can also further improve the strength of steel plates through solid solution strengthening. However, Cr and C can form carbides, and excessively high Cr content is detrimental to the plasticity and bending properties of the steel plate. Therefore, this invention controls the Cr content to 0.05–0.50%, preferably 0.15–0.35%.

[0036] Mo: Mo can expand the bainite phase region, making it easier to obtain lower bainite in steel. Simultaneously, it synergistically with boron (B) to shorten the lower bainite formation time. Furthermore, Mo can further enhance the strength of steel plates through solid solution strengthening or the formation of carbide or carbonitride precipitation. However, excessively high Mo content will result in coarse carbides in the steel, which is detrimental to the bending performance of the steel plate. Therefore, this invention controls the Mo content to 0.15–0.50%, preferably 0.15–0.38%.

[0037] Among the elements mentioned above, Mo, B, and Mn all affect the formation rate (or time) of lower bainite. To ensure that the lower bainite content is ≥95%, it is necessary to further control Bs = (Mo + B * 100 - Mn / 5) / C, making 0.6 ≤ Bs ≤ 1.4, where Bs is a characteristic value representing the lower bainite formation rate. When Mo and B are added synergistically, the C-curve of bainite transformation can be shifted significantly to the left, thus these two elements make a positive contribution to accelerating the formation rate of lower bainite. In addition, since the formation of lower bainite is also affected by the diffusion rate of carbon, generally, the faster the carbon atom diffusion rate, the faster the lower bainite formation. Therefore, since Mn has a dragging effect on carbon atom diffusion, it makes a negative contribution to accelerating the formation rate of lower bainite. Similarly, since the carbon atom diffusion rate is positively correlated with the bainite formation rate, the above formula also needs to be divided by the carbon content. If Bs is below 0.6, the lower bainite formation rate is too low and the formation time is too long, making it impossible to effectively guarantee the formation of more than 95% lower bainite within the controlled cooling time. If Bs is above 1.4, although the lower bainite formation rate is sufficient, the degradation or decomposition of lower bainite and the precipitation of carbides are also positively correlated with the diffusion rate of atoms such as carbon atoms. An excessively high Bs value also means that lower bainite is prone to degenerate into granular bainite, and the precipitation rate of carbonitrides is also too fast. This leads to the easy formation, aggregation, or coarse growth of carbides in the steel plate or strip, ultimately resulting in deterioration of bending performance.

[0038] Ti, Nb, and V: While Ti, Nb, and V are optional alloying elements that can be added to steel, their addition is not recommended. Ti, Nb, and V can form microalloyed carbides or carbonitrides, resulting in a large amount of second-phase precipitation, further improving the strength of the steel sheet. However, excessive or overly large microalloyed carbides or carbonitrides can actually worsen bending performance. Furthermore, since nitrogen is inevitably present in steel, these alloying elements can also react with N to form large, sharp-edged nitrides (such as TiN), further deteriorating the bending performance of the steel sheet or strip. In addition, the addition of these alloying elements increases material cost. Considering both performance and cost control, this invention controls the mass percentage of Nb, Ti, and V to: Ti + Nb + V ≤ 0.03%, preferably, Ti + Nb + V ≤ 0.006%.

[0039] The microstructure of the automotive structural steel of the present invention is as follows: lower bainite ≥ 95%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.2%; preferably, lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

[0040] The method for manufacturing automotive structural steel with a yield strength of 1000 MPa and above according to the present invention includes the following steps:

[0041] 1) Smelting and continuous casting

[0042] The slab is smelted according to the above composition and cast into a billet by continuous casting, with a cooling rate of ≥5K / s during continuous casting.

[0043] 2) Hot rolling

[0044] For heating the billet, the heating temperature at the center point of the billet width is 1150~1220℃;

[0045] The total reduction rate of rolling is ≥98%, of which the reduction rate of the first and second passes is ≥60%; the finishing mill exit temperature is 920~980℃; the slab rolling speed is controlled so that the time tp taken for any position of the rolled steel plate or strip to be transferred from the finishing mill exit temperature measurement point to the coiling temperature measurement point is ≥(5 / Bs)+4 seconds.

[0046] 3) Layer cooling and winding

[0047] After rolling, the strip is rapidly cooled to ≤530℃ at a cooling rate of ≥150℃ / s, and then cooled to the coiling temperature at a cooling rate of ≥10℃ / s. The coiling temperature is (MS+20℃)-400℃; where the martensitic transformation temperature of the strip is MS=498.9-333.3*(C)-33.3*(Mn)-27.8*(Cr)-16.7*(Ni)-11.1*(Si+Mo+W), in ℃.

[0048] For steel plates and strips with Bs < 0.9, after coiling, the steel coil should be left on the coiler for ≥ (10 / Bs) + 5 seconds before being uncoiled.

[0049] For steel plates and strips with Bs ≥ 0.9, after coiling, the steel coil should remain on the coiler for ≥ (10 / Bs) + 5 seconds before uncoiling.

[0050] 4) Stack cooling.

[0051] Furthermore, it also includes step 5), where hot-rolled steel plates or strips are pickled to produce pickled plates.

[0052] Preferably, in step 3), the strip is rapidly cooled to ≤530℃ after rolling at a cooling rate of ≥180℃ / s.

[0053] Preferably, for steel plates or strips with Bs > 0.9, the steel coil should remain on the coiler for ≥ (10 / Bs) + 5 seconds after coiling before uncoiling.

[0054] Preferably, in step 4), the steel coils are stacked in an insulated pit with an ambient temperature ≥280℃ after uncoiling, and kept warm for 2 to 6 hours; preferably, the steel coils are stacked with one side laid flat.

[0055] Preferably, the thickness of the rolled strip is ≤4mm.

[0056] In the manufacturing method of automotive structural steel with a yield strength of 1000 MPa and above according to the present invention:

[0057] In step 1), the cooling rate of the slab during continuous casting affects the grain size in the final microstructure of the steel plate and strip. The grain size, in turn, affects the formation rate and time of lower bainite. Smaller grain sizes result in a faster lower bainite formation rate and a shorter formation time. If the cooling rate is below 5 K / s, it not only causes coarse grains in the slab microstructure, making it difficult to obtain a fine-grained microstructure during subsequent rolling, but also easily leads to the formation of central segregation or banded structures in the final finished product. Both central segregation and banded structures readily form martensite, significantly increasing the martensite proportion in the final steel plate or strip and deteriorating its bending properties.

[0058] In step 2), excessively high heating temperatures can lead to coarse grain sizes in the steel, thus slowing down the formation rate of lower bainite and resulting in insufficient lower bainite content in the steel plate or strip. Conversely, excessively low heating temperatures result in insufficient austenitization of the slab, also leading to insufficient lower bainite content in the final steel plate or strip. When the finishing rolling temperature is below 920°C, ferrite will precipitate before finishing rolling, resulting in a low lower bainite content in the final microstructure. However, considering the slab heating temperature, the finishing rolling temperature should not exceed 980°C. To ensure a small grain size in the rolled steel plate or strip, the overall reduction rate of the rolling process must be ≥98%, and the reduction rate for each of the first and second passes must be ≥60%. Insufficient reduction rates will prevent the acquisition of a fine and uniform microstructure, resulting in an excessively long lower bainite formation time and an inability to obtain more than 95% lower bainite. Similarly, to ensure that the lower bainite transformation is completed before uncoiling, the slab rolling speed needs to be controlled so that the tpmin of the rolled steel plate or strip is ≥ (5 / Bs) + 4 seconds. Due to differences in slab size and production line equipment, the specific time may vary, but it is essential to ensure that the lower bainite transformation occurs as fully as possible before the steel plate or strip enters the coiler. Therefore, this time is highly correlated with the characteristic value of the bainite transformation rate, Bs. Insufficient time will prevent the acquisition of more than 95% lower bainite.

[0059] In step 3), to avoid the ferrite and pearlite phase regions, the first stage of cooling requires a cooling rate of ≥150℃ / s, preferably ≥180℃ / s, cooling to below 530℃. Higher cooling temperatures will promote the formation of upper bainite, grainy bainite, and even pearlite and ferrite, preventing the lower bainite content from reaching ≥95%. After cooling to below 530℃, the second stage cooling rate is reduced to ≥10℃ / s until the coiling temperature. However, the cooling rate should not be too low to avoid the formation of grainy bainite and upper bainite. The coiling temperature needs to be controlled between (MS+10) and 400℃ to ensure the formation of ≥95% lower bainite. Too low a coiling temperature will cause martensite formation, while too high a coiling temperature will lead to the precipitation of grainy bainite, upper bainite, and carbides or carbonitrides. Meanwhile, for steel plates or strips with Bs < 0.9, in order to ensure the full formation of lower bainite and avoid the formation of martensite or retained austenite, the steel coil needs to be held on the coiler for (10 / Bs) + 5 seconds before being unloaded.

[0060] Preferably, for steel plates or strips with Bs ≥ 0.9, the coil can be left on the coiler for (10 / Bs) + 5 seconds before uncoiling to ensure sufficient formation of lower bainite and further avoid the formation of martensite or retained austenite.

[0061] Preferably, the thickness of the strip after rolling is ≤4mm; if the strip thickness is too thick, the uniformity of the microstructure in the thickness direction is poor, which is not conducive to obtaining more than 95% lower bainite.

[0062] In step 4), it is preferable to further stack the uncoiled steel coils in a vertical, flat-on-one configuration in an insulated pit at an ambient temperature ≥280℃ for 2–6 hours to further prevent martensite formation. However, if the insulated stacking temperature is too high or the time is too long, it may induce the precipitation of carbides or carbonitrides, and may also cause the lower bainite to decompose into granular bainite, which is detrimental to bending performance.

[0063] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0064] The present invention aims to obtain a steel plate with both high strength and high bending performance by forming almost pure high-strength lower bainite from steel plates and strips.

[0065] Existing martensitic high-strength steels primarily consist of martensite or tempered martensite, sometimes supplemented with small amounts of ferrite, bainite, or carbide precipitation. However, these types of martensitic high-strength steels exhibit poor bending properties. In contrast, the almost pure low-bainitic high-strength steel obtained in this invention possesses the same strength as martensitic high-strength steel while also exhibiting better bending properties. Furthermore, according to the literature (Wear Resistance of Medium Carbon Steel with Different Microstructures-PMC (nih.gov)), steel plates with a predominantly bainitic microstructure also have higher fracture toughness than those with a predominantly martensitic or tempered martensitic microstructure.

[0066] On the other hand, existing precipitation-strengthened ultra-high strength hot-rolled steel plates or strips contain a large amount of microalloying elements such as Nb, Ti, and V, which enhance the strength of the steel plates or strips through the large-scale precipitation of microalloyed carbides or carbonitrides.

[0067] In comparison, this invention avoids the addition of large amounts of expensive alloying elements to form microalloyed carbides or carbonitrides, and also avoids the negative impact of large amounts of microalloyed carbides or carbonitrides on bending performance, so that the steel plates or strips obtained by this invention have better bending forming performance at the same strength level.

[0068] A core problem to be solved in this invention is how to form a nearly pure lower bainite structure with ultra-high strength or hardness in hot-rolled or pickled steel plates or strips. However, due to the short and fast production time of hot rolling, the temperature and cooling control stage of laminar cooling is extremely short, and the precision of temperature and cooling control is also low. Furthermore, the lower bainite formation temperature window is narrow, resulting in a low formation temperature and a slow lower bainite formation rate. This leads to a contradiction between the lower bainite formation temperature, formation rate, and hot rolling production time, making the design and manufacturing extremely difficult.

[0069] Therefore, in terms of composition design, on the one hand, it is necessary to maximize the transformation range of lower bainite to allow it to form at the lowest possible transformation temperature, because the lower the formation temperature, the higher the strength or hardness of lower bainite. However, since the transformation of lower bainite involves the diffusion kinetics of carbon atoms and other related atoms, the lower the phase transformation temperature, the slower the transformation of lower bainite. Therefore, in terms of composition design, on the other hand, it is also necessary to rationally design the proportions of relevant elements that affect diffusion and rate, and optimize the transformation rate of lower bainite, i.e., the Bs value mentioned in this invention, so that as much lower bainite as possible can be formed in a shorter time during hot rolling production.

[0070] Of course, besides composition design, this invention also needs to consider the formation range and formation time in the manufacturing process. On the one hand, it aims to form at the lowest possible temperature to improve product strength, and on the other hand, it aims to form as much lower bainite as possible to improve bending performance. Therefore, on the one hand, it designs manufacturing processes that refine grains, such as increasing the slab cooling rate during continuous casting and using a large reduction rate during hot rolling. Grain refinement accelerates atomic diffusion and lower bainite transformation. On the other hand, it optimizes temperature control to ensure that lower bainite forms at the lowest possible temperature without generating other structures, such as martensite and upper bainite. More importantly, this invention also pays special attention to the matching problem between hot rolling production time and lower bainite formation rate. The corresponding production time is designed for each process step of hot rolling to ensure that ≥95% of the lower bainite is formed in the final microstructure.

[0071] Through the above-mentioned composition and manufacturing process design, the automotive structural steel obtained by this invention not only has ultra-high strength, but also excellent formability, especially bending formability. The yield strength of the automotive structural steel is ≥1000MPa, tensile strength is ≥1180MPa, elongation is ≥7%, and 180° bending performance d≤3.5T, which can meet the lightweight requirements of future automotive structures and is especially suitable for use in chassis system components. Attached Figure Description

[0072] Figure 1 This is a microstructure photograph of steel D1 in embodiment D of the present invention. Detailed Implementation

[0073] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0074] The composition of the steel in the embodiments of the present invention is shown in Table 1, wherein the balance includes Fe and other unavoidable impurities. Table 2 shows the process parameters of the steel in the embodiments of the present invention; data in the comparative examples that do not conform to the present invention are underlined. Table 3 shows the properties and microstructure of the steel plates or strips corresponding to the steel in the embodiments of the present invention. All indicators in the embodiments meet the design scheme of the present invention, and therefore, hot-rolled or pickled steel plates or strips with yield strength ≥1000MPa; tensile strength ≥1180MPa; elongation ≥7%; and 180° bending performance d≤3.5T are obtained.

[0075] Among them, Examples E1 and E2 have high Mn, Cr and Mo content, and the tensile strength of the products reaches 1300MPa, but the elongation at break is relatively low, and the 180° bending performance d=3.5T.

[0076] Similarly, Examples D1 and H1 also promoted the bainite transformation and carbonitride precipitation through slow cooling and stacking after coiling, but the bending performance was not optimal, with a 180° bending performance d = 3.5T. Examples B1 and F1 also had a 180° bending performance d = 3.5T. Among them, Example F1 had relatively high strength, but due to its relatively low Bs value, the lower bainite formation rate was relatively slow, which made it easy for martensite to form after coiling and uncoiling and during stacking cooling. Therefore, an additional 20 seconds was placed on the coiler after coiling to ensure the formation of lower bainite, but ultimately about 0.1% of martensite was still formed, resulting in relatively low bending performance. Example B1 had a similar problem. The Bs value of Example B1 was ≥ 0.9, which was relatively moderate. The additional placement on the coiler after coiling resulted in about 0.15% of martensite being formed, leading to relatively low bending performance. Examples A1, B2, C1, D2, G1, and I1 exhibited a preferred d≤3.0T bending performance at 180°. Except for Example D2, the remaining five examples all had a bainite content >99%, resulting in high bending performance. However, Examples A1 and C1, due to their relatively low strength, achieved a d≤2.5T bending performance at 180°.

[0077] In the comparative examples, the main change of comparative example H2 compared to comparative example H1 is that the winding temperature is too high, which leads to excessive carbonitride precipitation and granular bainite formation in comparative example H2. The lower bainite content is insufficient. Although the strength is increased, the bending performance is significantly deteriorated.

[0078] Comparative Example C2, due to its excessively low winding temperature, almost formed a pure martensitic structure, which significantly increased the strength but also significantly deteriorated the plasticity and bending properties.

[0079] Compared with Example D1, Comparative Example J1 has similar content of individual elements in its composition design. However, the Bs value of Comparative Example J1 is too large. Although the manufacturing process is similar to that of Example D1, the excessive Bs value still leads to the precipitation of carbonitrides and excessive bainite particles, resulting in poor bending performance.

[0080] Compared with Example G1, Comparative Example K1 has similar content of individual elements in its composition design, but the Bs value of K1 is too small, and the time given for the formation of lower bainite in the manufacturing process is very short, and it does not stay in the coiler. As a result, the content of lower bainite formation is insufficient, and more martensite and retained austenite are generated. Although the strength and elongation meet the standards, the bending performance is extremely poor.

[0081] The main difference between Comparative Example G2 and Example G1 is that the time to tap (tp) is insufficient and there is no extra dwell time in the winding machine after winding. Since the Bs value of the composition of Example G is relatively low, the rate of lower bainite formation is slow. The short tp time and the lack of extra dwell time after winding ultimately result in insufficient lower bainite formation time. As a result, the lower bainite content of Comparative Example G2 is insufficient and there is martensitic residual austenite formation. Although the strength and elongation meet the standards, the bending performance is poor.

[0082] See Figure 1 The diagram shows the microstructure of Embodiment D1 of the present invention, with lower bainite content ≥95%, ferrite + carbide + granular bainite <5%, and martensite + tempered martensite + upper bainite ≤0.01%.

[0083] In summary, the automotive structural steel obtained by this invention not only has extremely high strength, but also excellent formability, especially bending formability, which can meet the lightweight requirements of future automotive structures and is particularly suitable for use in chassis system components.

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Claims

1. A structural steel for automobiles with a yield strength of 1000 MPa or higher, wherein the weight percentage of its composition is as follows: C：0.15~0.23%； Si: 0.12~0.5%; Mn: 1.6~2.4%; B：0.001~0.004%； Al：0.01~0.04%； Cr；0.05~0.5%； Mo: 0.15~0.5%; P：≤0.015%； S：≤0.005%； The balance includes Fe and other unavoidable impurities, and must satisfy the following relationship: the characteristic value of the lower bainite formation rate Bs = 0.6~1.4, Bs = (Mo + B * 100 - Mn / 5) / C; The microstructure of the steel used in the automotive structure has ≥95% lower bainite and ≤0.2% retained austenite + martensite + tempered martensite + upper bainite.

2. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, characterized in that, It also contains at least one of Ti, V, and Nb, and Ti+Nb+V≤0.03 wt%.

3. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, characterized in that, Ti+Nb+V≤0.006 wt%.

4. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The C content is 0.18~0.21wt%.

5. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The Mn content is 1.8~2.2wt%.

6. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 4, characterized in that, The Mn content is 1.8~2.2wt%.

7. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The Cr content is 0.15~0.35wt%.

8. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 5, characterized in that, The Cr content is 0.15~0.35wt%.

9. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 6, characterized in that, The Cr content is 0.15~0.35wt%.

10. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The Mo content is 0.15~0.38 wt%.

11. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 4, characterized in that, The Mo content is 0.15~0.38 wt%.

12. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 5, characterized in that, The Mo content is 0.15~0.38 wt%.

13. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 6, characterized in that, The Mo content is 0.15~0.38 wt%.

14. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 7, characterized in that, The Mo content is 0.15~0.38 wt%.

15. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 8, characterized in that, The Mo content is 0.15~0.38 wt%.

16. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 9, characterized in that, The Mo content is 0.15~0.38 wt%.

17. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

18. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 4, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

19. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 5, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

20. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 6, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

21. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 7, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

22. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 8, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

23. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 9, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

24. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 10, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

25. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 11, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

26. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 12, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

27. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 13, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

28. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 14, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

29. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 15, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

30. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 16, characterized in that, The microstructure of the steel used in the automotive structure is as follows: lower bainite ≥ 95%, ferrite + carbonitride precipitation + granular bainite ≤ 5%, and retained austenite + martensite + tempered martensite + upper bainite ≤ 0.01%.

31. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 1, 2, or 3, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

32. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 4, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

33. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 5, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

34. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 6, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

35. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 7, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

36. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 8, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

37. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 9, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

38. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 10, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

39. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 11, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

40. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 12, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

41. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 13, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

42. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 14, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

43. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 15, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

44. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 16, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

45. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 17, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

46. ​​The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 18, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

47. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 19, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

48. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 20, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

49. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 21, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

50. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 22, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

51. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 23, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

52. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 24, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

53. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 25, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

54. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 26, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

55. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 27, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

56. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 28, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

57. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 29, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

58. The automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 30, characterized in that, The steel used in the automotive structure has a yield strength ≥1000MPa, tensile strength ≥1180MPa, elongation ≥7%, and 180° bending performance d≤3.5T.

59. A method for manufacturing automotive structural steel with a yield strength of 1000 MPa or higher as described in any one of claims 1 to 58, characterized in that, Includes the following steps: 1) Smelting and continuous casting Smelting according to the stated composition and casting into a billet by continuous casting, wherein the cooling rate of the billet during continuous casting is ≥5K / s; 2) Hot-rolled The billet is heated to a temperature of 1150~1220℃; The total reduction rate of rolling is ≥98%, of which the reduction rate of the first and second passes is ≥60%; the exit temperature of the finishing mill is 920~980℃; 3) Lamination and winding After rolling, the strip is cooled to ≤530℃ at a cooling rate of ≥150℃ / s, and then cooled to the coiling temperature at a cooling rate of ≥10℃ / s. The coiling temperature is (MS+20℃)-400℃. The time tp taken for any position of the hot-rolled steel plate or strip to be transferred from the temperature measuring point at the finishing mill exit to the temperature measuring point at the coiling temperature is ≥(5 / Bs)+4 seconds. in, The martensitic transformation temperature of the strip steel MS = 498.9-333.3*(C)-33.3*(Mn)-27.8*(Cr)-16.7*(Ni)-11.1*(Si+Mo+W), in °C; For steel plates or strips with Bs < 0.9, after coiling, the steel coil should remain on the coiler for ≥ (10 / Bs) + 5 seconds before uncoiling. 4) Stack cooling.

60. The method for manufacturing automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 59, characterized in that, It also includes step 5), where hot-rolled steel plates or strips are pickled to produce pickled plates.

61. The method for manufacturing automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 59, characterized in that, Step 3) After rolling, the strip is rapidly cooled to ≤530℃ at a cooling rate of ≥180℃ / s.

62. The method for manufacturing automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 59, characterized in that, For steel plates or strips with Bs > 0.9, after coiling, the steel coil should remain on the coiler for ≥ (10 / Bs) + 5 seconds before uncoiling.

63. The method for manufacturing automotive structural steel with a yield strength of 1000 MPa or higher as described in claim 59, characterized in that, Step 4) The stacking method for cooling is to stack the steel coils after uncoiling in an insulated pit with an ambient temperature ≥280℃ for 2~6 hours.

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