A cold-rolled low-alloy high-strength steel and a method for producing the same

Abstract

The application discloses a kind of cold-rolled low-alloy high-strength steel and its production method, the chemical composition of the high-strength steel is as follows in mass fraction (wt%): C: 0.07~0.12%, Si: 0.2~0.5%, Mn: 1.2~1.9%, Al: 0.02~0.1%, Nb: 0.02~0.05%, Ti: 0.02~0.05%, Mo: 0.1~0.15%, S≤0.007%, P: 0.015~0.04%, N≤0.005%, wherein Mo / (Nb+Ti) is between 1~3.5, and the balance is Fe and unavoidable impurities;Its production method includes smelting, continuous casting, hot rolling, pickling, annealing process.The cold-rolled low-alloy high-strength steel provided by the application has excellent strength and ductility, and realizes the stable control of uniformization of microstructure and performance.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting, specifically relating to a cold-rolled low-alloy high-strength steel and its production method. Background Technology

[0002] With the deepening development of the automotive industry, the requirements for automotive steel are showing a trend of differentiation and diversification. Compared with traditional phase transformation strengthened high-strength steel, low-alloy high-strength steel has a relatively simple matrix structure, mainly composed of ferrite and a small amount of carbides. It has good formability and is suitable for use in some parts with high requirements for local forming performance. It can reduce the use of laser-welded plates, reduce production costs and carbon emissions.

[0003] Generally, cold-rolled low-alloy high-strength steel is mainly composed of niobium-titanium composites or single niobium, achieving high strength and good weldability through grain refinement and precipitation strengthening. The presence forms of carbonitrides such as niobium and titanium at different stages have a significant impact on the microstructure and properties of the material, which are mainly affected by the production process control. This increases the difficulty of controlling the stability of material properties during the production process. To meet the development of lightweight automobiles, it is necessary to develop high-grade low-alloy steels to meet the manufacturing needs of high-precision and high-strength requirements for seat rails and automotive longitudinal beams. At the same time, achieving the strengthening effect of microalloying elements and uniform microstructure and properties makes the control of the production process increasingly difficult.

[0004] Chinese patent CN 112680655A discloses a 700MPa grade low-alloy high-strength cold-rolled steel sheet for automobiles and its preparation method. It utilizes Nb and Ti composite microalloying to achieve the development of 700MPa grade cold-rolled low-alloy high-strength steel. However, its performance control range is large, and it does not involve a method for uniform control of microstructure and properties, which is not conducive to the running-in of the material and the stamping mold, and reduces the service life of the mold.

[0005] Chinese patent CN 116065095 A discloses a method for producing 500-800 MPa grade multi-stage cold-rolled low-alloy high-strength steel strips. It uses the same chemical composition and hot rolling process, but obtains low-alloy high-strength steel strips with different yield strengths (500-800 MPa) by adjusting different cold rolling annealing process curves. However, it does not consider the influence of microstructure inheritance on the uniformity of the final product, nor does it account for the precipitation of microalloying at different stages, and it does not address methods for controlling the uniformity of the steel strip microstructure.

[0006] Chinese patent CN 113930598 A discloses a manufacturing method for improving the uniformity of microstructure in continuously annealed HSLA. While proposing a method to improve the uniformity of microstructure in continuously annealed HSLA, it does not address the problem of excessively rapid temperature drop at the edges and the beginning and end of the strip along the length direction during hot rolling, which leads to excessive differences in microstructure and properties and uneven distribution of precipitates. In addition, the natural slow cooling process after coiling exacerbates the maturation of precipitated microalloyed carbonitrides, making it unsuitable for controlling the stability of subsequent rolling, especially for the production of high-strength low-alloy high-strength steel with high alloy content. Furthermore, this invention does not cover the production method of low-alloy steel with a strength of 700MPa.

[0007] Therefore, developing a high-strength cold-rolled low-alloy steel with good uniformity of microstructure and properties and its production method has become an urgent technical problem to be solved. Summary of the Invention

[0008] The purpose of this invention is to provide a cold-rolled low-alloy high-strength steel and its production method. The cold-rolled low-alloy high-strength steel has a yield strength of 750MPa~820MPa, a tensile strength of 810~870MPa, an elongation after fracture (A80) ≥8%, a yield strength difference ≤45MPa, a tensile strength difference ≤60MPa, a hardness difference between the upper and lower surfaces of the strip ≤15HV, and can withstand a 180° bend without cracking. The thickness accuracy control range is ±2% of the target thickness.

[0009] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0010] A cold-rolled low-alloy high-strength steel has the following chemical composition by mass fraction (wt%): C: 0.07-0.12%, Si: 0.2-0.5%, Mn: 1.2-1.9%, Al: 0.02-0.1%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, Mo: 0.1-0.15%, S≤0.007%, P: 0.015-0.04%, N≤0.005%, wherein Mo / (Nb+Ti) is between 1 and 3.5, and the balance is Fe and unavoidable impurities.

[0011] The production method of cold-rolled low-alloy high-strength steel according to the present invention includes: smelting, continuous casting, hot rolling, pickling, and annealing processes.

[0012] The smelting and continuous casting process of this invention is as follows: molten steel is smelted in a converter and refined by LF, and then the target billet is obtained by continuous casting; wherein the alloying temperature of the LF refining process is 1600-1650℃, the alloying time is 7min-12min, the static blowing time is 10min-20min, the dynamic light pressure is reduced by 5-8mm and the superheat is controlled at 10-30℃ during the continuous casting process.

[0013] The hot rolling process of this invention involves sequentially heating, descaling, rough rolling, finish rolling, laminar flow cooling, coiling, and cooling to obtain a hot-rolled coil with a thickness of 1.8–4.5 mm. Specifically, the heating temperature is 1250–1280℃, and the holding time is 180–240 min. The rough rolling process uses a 3+3 pass rolling method, with a 30℃–60℃ temperature compensation at the edge of the intermediate slab. The finish rolling entry temperature is 1020–1080℃, and the final rolling temperature is 880–920℃. The laminar flow cooling uses a post-cooling + U-shaped cooling mode, with a water flow ratio of 0.60–0.75 between the upper and lower cooling manifolds in the post-rolling cooling section, and edge shielding of 30–120 mm.

[0014] The winding process involves controlling the winding temperature and compensation temperature based on the thickness T of the hot-rolled plate, as detailed below:

[0015] If 1.8≤T<3mm, the winding temperature is 460~500℃, and the temperature compensation for the first and last 100m is 70~80℃.

[0016] If 3≤T<4.5mm, the winding temperature is 400~460℃, and the head and tail temperatures are compensated by 60~70℃ for 100m.

[0017] The cooling process employs centralized slow cooling via a windbreak wall for ≥72 hours.

[0018] The pickling and rolling process of this invention involves straightening, pickling, and rolling the hot-rolled coil to obtain a cold-rolled hardened coil. During rolling, the surface roughness of the F1 work roll satisfies: upper roll roughness - lower roll roughness = 0.05–0.15 μm; the surface roughness of the F3 work roll satisfies: upper roll roughness - lower roll roughness = 0.1–0.15 μm; the surface roughness of the F5 work roll satisfies: upper roll roughness - lower roll roughness = 0.1–0.25 μm; the sheet shape IU value is ≤4; and the cold rolling reduction rate is 30–63%.

[0019] Preferably, the cold rolling reduction rate is controlled based on the target thickness of the steel strip, specifically as follows:

[0020] If the thickness of the finished steel strip is 1.9 ≤ t ≤ 2.5 mm, the cold rolling reduction rate is 30-40%;

[0021] If the thickness of the finished steel strip is 1.3 ≤ t < 1.9 mm, the cold rolling reduction rate is 40-50%;

[0022] If the thickness of the finished steel strip is 0.7 ≤ t < 1.3 mm, the cold rolling reduction rate is 51-63%.

[0023] Furthermore, the elongation of the tension straightening is controlled within the range of 0.3% to 2.0%.

[0024] Furthermore, in the pickling process, the acid tank temperature is 80-90℃, and the free acid concentrations in the three acid tanks are as follows: acid concentration in acid tank No. 1 is 60-100g / L, acid concentration in acid tank No. 2 is 100-150g / L, and acid concentration in acid tank No. 3 is 150-200g / L; the pickling speed is controlled at 60-180m / min.

[0025] The annealing process of this invention involves subjecting the cold-rolled coil to uniform heating and heat preservation, slow cooling, rapid cooling, aging, leveling, and edge trimming to obtain cold-rolled low-alloy high-strength steel with uniform microstructure and properties; wherein: the uniform heating temperature is 695-730℃, the heating rate is 2-5℃ / s, the annealing time is 100-180s, the slow cooling temperature is 620-650℃, the rapid cooling rate is 30-60℃ / s, the rapid cooling temperature is 410-445℃, the over-aging end temperature is 250-330℃, and the aging time is 200-350s.

[0026] As a preferred embodiment, the annealing process parameters are controlled as follows, based on the target thickness t of the steel strip:

[0027] If 1.9≤t≤2.5mm, the soaking temperature is 695~720℃, the heating rate is 2~3.5℃ / s, the annealing time is 130~180s, the slow cooling temperature is 625~650℃, the rapid cooling rate is 30~40℃ / s, the rapid cooling temperature is 410~435℃, and the over-aging end temperature is 250~310℃.

[0028] If 1.3≤t<1.9mm, the soaking temperature is 705~730℃, the heating rate is 2~4℃ / s, the annealing time is 120~160s, the slow cooling temperature is 640~655℃, the rapid cooling rate is 35~45℃ / s, the rapid cooling temperature is 415~440℃, and the over-aging end temperature is 260~320℃.

[0029] If 0.7≤t<1.3mm, the soaking temperature is 715~730℃, the heating rate is 2.5~5℃ / s, the annealing time is 100~150s, the slow cooling temperature is 635~665℃, the rapid cooling rate is 35~60℃ / s, the rapid cooling temperature is 420~445℃, and the end temperature of over-aging is 265~330℃.

[0030] The annealing process has a flattening elongation of 0.7-0.9% and a trimming allowance of 20-30 mm.

[0031] The method of this invention is applicable to the production of cold-rolled low-alloy high-strength steel with a thickness of 0.7 to 2.5 mm.

[0032] The reasons for adopting the above-described process in this invention are as follows:

[0033] (1) The use of converter smelting + LF refining + continuous casting improves smelting production efficiency and reduces smelting process costs; strictly controlling alloying temperature and static blowing time, and using a combination of low superheat and dynamic light pressing reduces the formation and coarsening of liquid-precipitated TiN, which helps to improve the uniformity of alloying elements in steel and enhance the contribution of precipitation strengthening, reduces the formation of segregation and banded structure, and facilitates the adjustment of the uniformity of subsequent microstructure and properties.

[0034] (2) High-temperature heating of the slab ensures the full solid solution of microalloying elements such as niobium and titanium. High-temperature final rolling ensures that hot rolling is carried out in a fully austenitic state, which is conducive to the formation of Nb and Ti, Mo microalloyed carbonitride nanoprecipitates, thereby inhibiting the recrystallization of austenite and refining the austenite grains. The subsequent cooling and low-temperature coiling promotes the precipitation of microalloying elements in the form of fine carbides and also inhibits the ripening of carbides. At the same time, the addition of Mo helps to inhibit the precipitation of Nb carbonitrides in austenite, thereby achieving the control of Nb precipitation at each process stage, avoiding performance fluctuations caused by uneven precipitation of strengthening phase elements, reducing the risk of large thickness fluctuations in the cold rolling process, and improving rolling stability.

[0035] (3) Edge heating and edge shielding reduce the excessive differences in microstructure and properties caused by inconsistent cooling in the transverse direction of the strip, while improving edge plasticity and avoiding problems such as edge cracking and strip breakage during subsequent cold rolling. U-shaped coiling achieves head and tail temperature compensation, which improves the problem of excessive microstructure and property deviation caused by excessively rapid temperature drop at the head and tail. The control of the water volume ratio of the upper and lower cooling manifolds in the post-rolling cooling section achieves uniformity of microstructure on the upper and lower surfaces, avoids waviness caused by uneven stress, and is conducive to improving the stability and controlling the thickness accuracy of subsequent cold rolling.

[0036] (4) A large cold rolling compression ratio will exacerbate the anisotropy of the microstructure, thereby increasing the performance differences between the transverse and longitudinal directions of the strip. A small cold rolling compression ratio helps to reduce the anisotropy of the strip microstructure, thereby reducing the performance differences in different directions of the strip. A larger cold rolling compression ratio, using a higher homogenization temperature for annealing, helps to promote the recovery and recrystallization of the microstructure and improve the anisotropic differences in strip performance. At the same time, in order to avoid excessive transverse thickness differences caused by a large cold rolling compression ratio, the cold rolling compression ratio of this invention is controlled between 30% and 63%. Differential control of the roughness of the upper and lower rolls of the pickling and rolling process is beneficial to improving the lubrication effect of the upper and lower surfaces of the strip and the shape adjustment, achieving uniform deformation of the cross-sectional grains, thereby helping to reduce the uniformity of cross-sectional microstructure properties in the subsequent annealing process and avoiding surface coarsening and hardness reduction.

[0037] (5) The soaking temperature is selected at 695-730℃ to achieve the recovery recrystallization of ferrite grains, suppress abnormal grain growth, and retain some subgrain boundary strengthening and dislocation strengthening effects, which helps to improve the strength and plasticity of the material. The combination of high-temperature final rolling, low-temperature coiling, low cold rolling reduction rate and low soaking temperature, and the control of cold rolling reduction rate according to the target steel strip thickness, and the use of differentiated annealing temperature control, helps to refine grains and form dislocations, vacancies and subgrain boundaries, and also provides enough nucleation sites and activation energy for subsequent grain recrystallization, which is conducive to increasing the recovery recrystallization ratio of the microstructure and reducing the anisotropy of the strip. During the annealing process, Mo can also promote the precipitation of nano-sized niobium titanium carbides in ferrite, and at the same time, it can prevent the ripening of niobium titanium carbonitrides, which helps to improve the contribution of niobium titanium microalloys to precipitation strengthening during the annealing process.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] This invention improves smelting efficiency and facilitates compositional homogenization through a process of converter smelting + LF refining + continuous casting with strict control over alloying, continuous casting superheat, and light reduction. Differential control of the water volume in the upper and lower cooling pipes and the roughness of the upper and lower rolls during hot rolling effectively controls the uniformity of the strip's cross-sectional microstructure and adjusts its shape. Simultaneously, the rational matching of controlled rolling and cooling processes with cold rolling annealing ensures sufficient precipitation at appropriate temperatures for solid solution microalloying, fully leveraging the grain refinement and precipitation strengthening effects of microalloying elements while retaining some subgrain boundary and dislocation strengthening effects. Combined with the excellent plasticity and toughness of ferrite, this invention achieves the development of high-strength, high-strength low-alloy steel with excellent plasticity.

[0040] The cold-rolled low-alloy high-strength steel provided by this invention has a yield strength of 750MPa~820MPa, a tensile strength of 810~870MPa, an elongation after fracture (A80) ≥8%, a yield strength difference of ≤45MPa and a tensile strength difference of ≤60MPa in through-coiling performance, a hardness difference of ≤15HV between the upper and lower surfaces of the strip, and can withstand 180° bending at 0t without cracking. The thickness accuracy control range is ±2% of the target thickness. Attached Figure Description

[0041] Figure 1 The edge structure of the steel strip in Example 1;

[0042] Figure 2 The structure of the middle section of the steel strip in Example 1;

[0043] Figure 3 The structure of the head and tail portions of the steel strip in Example 1. Detailed Implementation

[0044] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0045] Examples 1-6

[0046] A cold-rolled low-alloy high-strength steel, the production process of which includes smelting, continuous casting, hot rolling, pickling, and annealing, as detailed below:

[0047] (1) Smelting and continuous casting process: The molten steel is smelted in a converter and refined by LF, and then the target billet is obtained by continuous casting. The chemical composition of the high-strength steel in each embodiment is shown in Table 1; the smelting and continuous casting process parameters are shown in Table 2.

[0048] Table 1. Main chemical composition (wt%) of high-strength steel in each embodiment.

[0049]

[0050] Table 2. Smelting and continuous casting process parameters for each embodiment.

[0051]

[0052] (2) Hot rolling process: The billet is sequentially heated, rough rolled, finish rolled, laminar flow cooled, coiled, and cooled to obtain a hot-rolled coil with a thickness of 1.8 to 4.5 mm. The parameters of the heating, rough rolling, finish rolling, and laminar flow cooling processes in each embodiment are shown in Table 3; the parameters of the coiling and cooling processes are shown in Table 4.

[0053] Table 3. Heating, roughing, finishing, and laminar flow cooling parameters in the hot rolling process of each embodiment.

[0054]

[0055]

[0056] Table 4. Coiling and cooling parameters in the hot rolling process of each embodiment.

[0057]

[0058] (3) The pickling and rolling process: the hot-rolled coil is straightened, pickled and rolled to obtain a cold-hardened coil; the straightening and pickling parameters in the pickling and rolling process of each embodiment are shown in Table 5; the rolling parameters are shown in Table 6.

[0059] Table 5. Parameters for tension leveling and pickling in the pickling and rolling process of each embodiment.

[0060]

[0061] Table 6 Rolling parameters in the pickling and rolling process of each embodiment

[0062]

[0063]

[0064] (4) Annealing process: The cold-rolled coil is subjected to uniform heat treatment, slow cooling, rapid cooling, aging, leveling, and edge trimming to obtain cold-rolled low-alloy high-strength steel with uniform microstructure and properties; the annealing process parameters for each embodiment are shown in Table 7. The mechanical properties and thickness control of the cold-rolled low-alloy high-strength steel provided in each embodiment are shown in Table 8.

[0065] Table 7 Annealing process parameters for each embodiment

[0066]

[0067] Table 8. Mechanical properties and thickness control of high-strength steel in each embodiment.

[0068]

[0069]

[0070] As shown in Table 8, the cold-rolled low-alloy high-strength steel provided by this invention has a yield strength of 750MPa~820MPa, a tensile strength of 810MPa~870MPa, an elongation after fracture A80≥8%, a yield strength difference of ≤45MPa throughout the roll, a tensile strength difference of ≤60MPa, a hardness difference of ≤15HV between the upper and lower surfaces of the strip, and does not crack when bent at 0t in 180°. The thickness accuracy is controlled within ±2% of the target thickness, thus achieving high precision control of strip dimensions and uniform performance.

[0071] The microstructures of the edge, middle, and head / tail portions of the cold-rolled low-alloy high-strength material provided in Example 1 are shown below. Figures 1-3 It can be observed that the microstructure in different parts consists of ferrite and precipitated carbides, with the ferrite mainly in the recrystallized state, and the microstructure has high uniformity.

Claims

1. A cold rolled low alloy high strength steel, characterized in that, Its chemical composition, by mass fraction (wt%), is as follows: C: 0.07–0.12%, Si: 0.2–0.5%, Mn: 1.2–1.9%, Al: 0.02–0.1%, Nb: 0.02–0.05%, Ti: 0.02–0.05%, Mo: 0.1–0.15%, S≤0.007%, P: 0.015–0.04%, N≤0.005%, wherein the Mo / (Nb+Ti) ratio is between 1 and 3.5, and the balance is Fe and unavoidable impurities; The cold-rolled low-alloy high-strength steel has a yield strength of 750MPa~820MPa, a tensile strength of 810~870MPa, an elongation after fracture (A80) ≥8%, a yield strength difference of ≤45MPa and a tensile strength difference of ≤60MPa, a hardness difference between the upper and lower surfaces of the strip and sheet of ≤15HV, and does not crack when bent at 180° for 0t.

2. The cold rolled low alloy high strength steel of claim 1, wherein, The production method includes smelting, continuous casting, hot rolling, pickling, and annealing processes; wherein, in the annealing process, the cold-hardened coil obtained from the pickling process is subjected to homogenization and heat preservation, slow cooling, rapid cooling, aging, leveling, and edge trimming; wherein: the homogenization temperature is 695-730℃, the heating rate is 2-5℃ / s, the annealing time is 100-180s, the slow cooling temperature is 620-650℃, the rapid cooling rate is 30-60℃ / s, the rapid cooling temperature is 410-445℃, the over-aging end temperature is 250-330℃, and the aging time is 200-350s.

3. The cold rolled low alloy high strength steel of claim 2, wherein, The pickling and rolling process involves straightening, pickling, and rolling the hot-rolled coil obtained in the hot rolling process to obtain a cold-rolled coil. During rolling, the surface roughness of the F1 work roll satisfies: upper roll roughness - lower roll roughness = 0.05–0.15 μm; the surface roughness of the F3 work roll satisfies: upper roll roughness - lower roll roughness = 0.1–0.15 μm; the surface roughness of the F5 work roll satisfies: upper roll roughness - lower roll roughness = 0.1–0.25 μm; the sheet shape IU value is ≤4; and the cold rolling reduction rate is 30–63%.

4. The cold rolled low alloy high strength steel of claim 3, wherein, The pickling and rolling process controls the cold rolling reduction rate based on the target steel strip thickness t, specifically as follows: If the thickness of the finished steel strip is 1.9 ≤ t ≤ 2.5 mm, the cold rolling reduction rate is 30-40%. If the thickness of the finished steel strip is 1.3 ≤ t < 1.9 mm, the cold rolling reduction rate is 40-50%. If the thickness of the finished steel strip is 0.7 ≤ t < 1.3 mm, the cold rolling reduction rate is 51-63%.

5. The cold rolled low alloy high strength steel of claim 2, wherein, The smelting and continuous casting process involves smelting molten steel in a converter and refining it using an LF furnace, followed by continuous casting to obtain the target billet. The LF refining process has an alloying temperature of 1600–1650°C, an alloying time of 7–12 min, a static blowing time of 10–20 min, and a dynamic light reduction of 5–8 mm during continuous casting, with superheat control of 10–30°C.

6. The cold rolled low alloy high strength steel of claim 2, wherein, The hot rolling process involves sequentially heating, rough rolling, finish rolling, laminar flow cooling, and coiling the cast billet to obtain a hot-rolled coil with a thickness of 1.8–4.5 mm. Specifically, the coiling temperature and compensation temperature are controlled according to the hot-rolled plate thickness T, as follows: If 1.8≤T<3mm, the winding temperature is 460~500℃, and the temperature compensation for the first and last 100m is 70~80℃. If 3≤T<4.5mm, the winding temperature is 400~460℃, and the temperature compensation for the first and last 100m is 60~70℃.

7. The cold rolled low alloy high strength steel of claim 6, wherein, The heating temperature is 1250-1280℃, and the holding time is 180-240 min; the rough rolling adopts 3+3 passes, and the heating temperature compensation of the intermediate billet edge is 30℃-60℃; the finishing rolling inlet temperature is 1020-1080℃, and the finishing rolling temperature is 880-920℃.

8. The cold-rolled low-alloy high-strength steel according to claim 6, characterized in that, The laminar flow cooling adopts a rear-section cooling + U-shaped cooling mode, with a water flow ratio of 0.60 to 0.75 between the upper and lower cooling manifolds in the post-rolling cooling section and a side shielding of 30 to 120 mm.

9. The cold rolled low alloy high strength steel of claim 2, wherein, The annealing process has a flattening elongation of 0.7-0.9% and a trimming allowance of 20-30 mm.

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