Hot-rolled dual-phase steel with a tensile strength of 800 MPa class and method for producing the same

Abstract

A hot-rolled dual-phase steel with a tensile strength of 800 MPa grade and a method for producing the same, containing C: 0.04 to 0.08%, Si: 0.05 to 0.45%, Mn: 1.4 to 1.8%, N ≤ 0.005%, O ≤ 0.0030%, Ca ≤ 0.004%, Al: 0.02 to 0.1%, Ti: 0.07 to 0.13%, Cr: 0.1 to 0.7%, B ≤ 0.0035%, and the balance being Fe and other inevitable impurities. The final microstructure of the steel is bainite, including a small amount of martensite, retained austenite, and ferrite precipitation-strengthened by nanoscale microalloys. The longitudinal yield strength of the steel is ≥ 680 MPa, the tensile strength is ≥ 780 MPa, the elongation rate A50 is ≥ 15%, [KV(20°C) - KV(-40°C)] / KV(20°C) ≤ 0.35, and KV(-40°C) / thickness ≥ 10 J / mm, and the punching hole expansion rate is ≥ 65%. The hot-rolled dual-phase steel according to the present invention has high fracture resistance and can be used as automotive chassis and structural parts, meeting the technical requirements of flanging, pressing of complex automotive parts, and automotive weight reduction.

Description

Technical Field

[0001] Technical Field The present invention belongs to the field of high-strength hot-rolled steel sheets. In particular, it relates to a hot-rolled dual-phase steel sheet with a tensile strength reaching the 800 MPa grade, having forming characteristics such as good hole expansion property, and having fracture toughness resistance during use, and a manufacturing method thereof.

Background Art

[0002] Background Art Lightweighting has become a trend in the development of the automotive industry, and the proportion of high-strength steel sheets in automotive structural parts is also increasing. With the improvement of strength, in many current automotive models, 80 kg-class steel sheets are used in the production of automotive chassis parts such as control arms, tie rods, and spring seats. The forming process of control arms includes press working, flanging, hole expansion, etc. At the same time, during use, there is also a certain requirement for fracture toughness resistance.

[0003] In Patent Document 1 (CN103602895A), a steel sheet with a tensile strength of 780 MPa grade and high hole expansion property and its manufacturing method are disclosed. Its Si content is as high as 0.5 - 1.5%, and the formation of fayalite (2FeO - SiO2) oxide scale is likely to occur and is difficult to remove, making it difficult to obtain a strip steel with a higher quality surface. Also, since it is difficult to control the red scale on the steel sheet surface, it becomes difficult to accurately measure during the hot-rolling temperature measurement process, leading to unstable product performance.

[0004] In Patent Document 2 (CN108570604A), a 780 MPa grade hot-rolled pickled steel with high hole expansion property and its manufacturing method are disclosed. Its Al content in the composition is as high as 0.2 - 0.6%, making it prone to oxidation during the continuous casting process. At the same time, a three-stage cooling method is adopted, resulting in low production stability.

[0005] In Patent Document 3 (CN105483545A), a hot-rolled high hole-expanding property steel sheet of 800 MPa grade and a manufacturing method thereof are disclosed. Its composition contains 0.2 - 1.0% of Si, with a relatively high Si content. Red scale is likely to form on the surface, which is disadvantageous for controlling the surface and coiling temperature. At the same time, it contains 0.03 - 0.08% of Nb, with a relatively high Nb content, high cost, and stepwise cooling is required after rolling, and the cooling process is complex.

[0006] In Patent Document 4 (CN104513930A), an ultra-high strength hot-rolled dual-phase steel sheet and strip steel with good bending performance and hole-expanding property and a manufacturing method thereof are disclosed. However, the performance design of the hot-rolled pickled steel sheet and its manufacturing method are disclosed with emphasis, and the role of element B is not considered in the chemical composition.

[0007] In the above patent documents, only the hole-expanding property during the forming process is considered, and the characteristics of fracture toughness during the use process are not considered. Therefore, the present invention proposes a novel hot-rolled steel sheet of 800 MPa grade in tensile strength and a manufacturing method thereof.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Content of the Invention The object of the present invention is to propose a hot-rolled dual-phase steel sheet of 800 MPa grade in tensile strength and a manufacturing method thereof. In addition to characteristics such as high hole-expanding rate, high strength, and high elongation rate, the hot-rolled steel sheet further has the characteristic of higher fracture toughness, can be applied to automobile body structure parts and automobile chassis parts, and can also be applied to other application fields where high strength, weight reduction, and impact resistance are required, and has high safety. In this text, the hot-rolled steel sheet of 800 MPa grade in tensile strength means that the tensile strength ≥ 780 MPa.

Means for Solving the Problems

[0009] To achieve the above object, the present invention proposes a hot-rolled dual-phase steel with a tensile strength of 800 MPa grade, which contains, by mass percentage, in addition to Fe and other inevitable impurities, further C: 0.04 to 0.08%, Si: 0.05 to 0.45%, Mn: 1.4 to 1.8%, N ≤ 0.005%, O ≤ 0.0030%, Ca ≤ 0.004%, Al: 0.02 to 0.1%, Ti: 0.07 to 0.13%, Cr: 0.1 to 0.7%, B: ≤ 0.0035%.

[0010] Furthermore, in the above hot-rolled dual-phase steel with a tensile strength of 800 MPa grade, its chemical elements are, by mass percentage, C: 0.04 to 0.08%, Si: 0.05 to 0.45%, Mn: 1.4 to 1.8%, N ≤ 0.005%, O ≤ 0.0030%, Ca ≤ 0.004%, Al: 0.02 to 0.1%, Ti: 0.07 to 0.13%, Cr: 0.1 to 0.7%, B: ≤ 0.0035%, and the balance is Fe and other inevitable impurities.

[0011] Furthermore, the chemical composition of the steel plate satisfies the following: (1) 0.1% ≤ Cr ≤ 0.2% and 0.0020% ≤ B ≤ 0.0035%, or (2) 0.2% < Cr ≤ 0.35% and 0.0010% ≤ B < 0.002%, or (3) 0.35% < Cr ≤ 0.7% and B < 0.0010%.

[0012] Furthermore, for other inevitable impurities in the steel plate, P ≤ 0.02% and S ≤ 0.005%.

[0013] Furthermore, the microstructure of the steel plate includes bainite with an area fraction ≥ 90%, martensite and retained austenite with an area fraction ≤ 5%, and ferrite precipitation-strengthened by nanoscale microalloy with an area fraction ≤ 5%, and the grain size of bainite is ≤ 5 μm, the grain size of martensite and retained austenite is ≤ 2.5 μm, and the grain size of ferrite precipitation-strengthened by nanoscale microalloy is ≤ 7.5 μm.

[0014] Furthermore, the microstructure of the steel sheet contains TiN as an inclusion. In this text, TiN includes non-composite TiN and composite TiN. The composite TiN includes TiN containing CaO and Al2O3. The TiN size is preferably ≤ 8 μm. Furthermore, the size of the inclusions in the steel sheet satisfies the following: the size of the composite TiN containing TiN, CaO, Al2O3, etc. is ≤ 8 μm.

[0015] Furthermore, the steel sheet has good formability, with its longitudinal yield strength ≥ 680 MPa, tensile strength ≥ 780 MPa, elongation at 50% A50 ≥ 15%, and punching hole expansion rate ≥ 65%.

[0016] Furthermore, the steel sheet has good fracture resistance, and its impact toughness satisfies the following: [KV(20°C) - KV(-40°C)] / KV(20°C) ≤ 0.35 and KV(-40°C) / thickness ≥ 10 J / mm.

[0017] In some embodiments, the longitudinal yield strength of the steel sheet is within the range of 680 MPa, 690 MPa, 700 MPa, 710 MPa, 720 MPa, 730 MPa, 740 MPa, or between any two of the above values.

[0018] In some embodiments, the tensile strength of the steel sheet is within the range of 780 MPa, 800 MPa, 820 MPa, 840 MPa, 860 MPa, 880 MPa, 890 MPa, 900 MPa, or between any two of the above values.

[0019] In some embodiments, the elongation at 50% A50 of the steel sheet is within the range of 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or between any two of the above values.

[0020] In some embodiments, the punching hole expansion rate of the steel sheet is within the range of 65%, 70%, 80%, 90%, 100%, 110%, 120%, 130%, 140%, or between any two of the above values.

[0021] In some embodiments, the impact toughness of the steel plate satisfies the following: [KV(20°C) - KV(-40°C)] / KV(20°C) ≤ 0.35, or ≤ 0.3, or ≤ 0.25, or ≤ 0.2, or ≤ 0.15, or ≤ 0.1, or ≤ 0.05.

[0022] In some embodiments, the impact toughness of the steel plate satisfies the following: KV(-40°C) / thickness ≥ 10 J / mm, or ≥ 11 J / mm, or ≥ 12 J / mm, or ≥ 13 J / mm, or ≥ 14 J / mm, or ≥ 15 J / mm, or ≥ 16 J / mm, or ≥ 17 J / mm.

[0023] The mechanism of action of the chemical components of the present invention is as follows. C: In the hot-rolled duplex steel plate according to the present invention, the level of carbon content greatly affects the tensile strength level of the steel plate. Carbon participates in solid solution strengthening and forms sufficient precipitation strengthening phases with Ti, etc., to ensure the strength of the steel. However, considering that when the mass percentage of carbon increases, carbide particles become coarser, and at the same time, excessive martensite and retained austenite are likely to form, which is disadvantageous for hole expansion. In order to ensure high hole expansion performance with the strength of the steel grade and bring good formability and weldability, in the technical solution according to the present invention, the mass percentage of C is controlled to be 0.04 - 0.08%, for example, 0.05%, 0.06%, 0.07%.

[0024] Si: In the hot-rolled dual-phase steel sheet according to the present invention, silicon plays a role in solid-solution strengthening to improve the strength of the steel sheet. At the same time, by adding silicon, the work-hardening rate, uniform elongation rate, and total elongation rate at a predetermined strength can be improved, contributing to the improvement of the elongation rate of the steel sheet. Furthermore, silicon can prevent the precipitation of carbides and reduce the appearance of the pearlite phase. However, when silicon is contained in the steel, the surface of the steel sheet tends to form surface defects of 2FeO-SiO2 oxide scale, which has an adverse effect on the surface quality. Therefore, in the technical solution according to the present invention, the mass percentage of silicon is controlled to be 0.05 - 0.45%, for example, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%.

[0025] Al: In the hot-rolled dual-phase steel sheet according to the present invention, Al is a deoxidizing element of the steel, which reduces the inclusion of oxides in the steel to purify the steel and is beneficial to the improvement of the formability of the steel sheet. However, when the mass percentage of aluminum increases, oxidation occurs, which further affects continuous casting production. Therefore, in the technical solution according to the present invention, the mass percentage of Al is controlled to be 0.02 - 0.1%, for example, 0.04%, 0.06%, 0.08%.

[0026] Mn: In the hot-rolled dual-phase steel sheet according to the present invention, manganese is a solid-solution strengthening element. When the mass percentage of manganese is low, the strength is insufficient, but when the mass percentage of manganese is high, the plasticity of the steel sheet decreases. At the same time, manganese delays the pearlite transformation, improves the hardenability of the steel, lowers the bainite transformation temperature, refines the substructure of the steel tissue, and ensures the acquisition of the lath substructure, bringing good formability on the premise of ensuring the tensile strength of the product. However, if the content of Mn is too high, it will cause centerline segregation, promote peeling during punching or cutting of the steel sheet strip, and ultimately impair the hole expansion formability. Therefore, in the technical solution according to the present invention, the mass percentage of Mn is controlled to be 1.4 - 1.8%, for example, 1.5%, 1.6%, 1.7%.

[0027] Cr: In the hot-rolled dual-phase steel sheet according to the present invention, chromium is an element that suppresses the formation of pearlite and contributes to the formation of bainite structure, and ultimately contributes to the improvement of strength and hole expansion rate. However, when the chromium content decreases, the influence on the transformation curve becomes insignificant. On the other hand, when the mass percentage of Cr increases, it leads to an increase in cost on the one hand and a tendency to generate more martensite structure on the other hand. Therefore, in the technical solution according to the present invention, the mass percentage of Cr is controlled to be 0.1 - 0.7%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%.

[0028] Ti: In the hot-rolled dual-phase steel sheet according to the present invention, titanium is one of the important grain refinement strengthening and precipitation strengthening elements. Titanium can increase the recrystallization temperature and refine the grain size in the hot-rolling process. At the same time, the combination of Ti and C has an excellent strengthening effect. However, if the mass percentage of Ti is too high, it is easy to form large-sized TiN, which is disadvantageous to the impact toughness of the steel, so it is not desirable. Therefore, in the technical solution according to the present invention, the mass percentage of Ti is controlled to be Ti: 0.07 - 0.13%, for example, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%.

[0029] B: In the hot-rolled dual-phase steel sheet according to the present invention, boron contributes to the expansion of the bainite phase region, ensures that a bainite structure can be obtained in the steel sheet during cooling after rolling, significantly improves the strength and hardness of the steel, and can partially replace the chromium content to reduce costs. However, when the B element is excessive, massive martensite structure will excessively occur in the steel sheet, leading to a decrease in the hole expansion rate and elongation rate of the steel. Therefore, in the technical solution according to the present invention, the mass percentage of B is controlled to be B: ≤0.0035%.

[0030] O: In the hot-rolled dual-phase steel sheet according to the present invention, oxygen is an inevitable element in the steelmaking process. For the present invention, after deoxidation, there is a certain amount of residual O content in the steel, and it is easy to form oxides. However, the inclusions themselves do not have an obvious adverse effect on the performance of the steel sheet. At the same time, Al2O3 can become the nucleation of TiN and is likely to promote the growth of TiN. Therefore, in the technical solution according to the present invention, the mass percentage of O is controlled to be O ≦ 0.0030%.

[0031] Ca: In the hot-rolled dual-phase steel sheet according to the present invention, calcium can improve the morphology of sulfides such as MnS, change sulfides such as long-strip MnS into spherical CaS, contribute to the improvement of the inclusion morphology, and thereby reduce the adverse effect of long-strip sulfides on the hole expansion formability. However, if too much calcium is added, the number of calcium oxides will increase, which is disadvantageous for hole expandability. Therefore, the addition amount of calcium in the steel is usually ≦ 0.004%.

[0032] N: In the hot-rolled dual-phase steel sheet according to the present invention, nitrogen belongs to impurity elements, and the lower its content, the better. However, nitrogen is an inevitable element in the steelmaking process. Although its content is small, it combines with strong carbide-forming elements such as Ti. Since TiN has a square shape, there is a large stress concentration between its sharp corners and the matrix, and cracks are likely to be formed due to the stress concentration between TiN and the matrix, which also has a great impact on fracture toughness and hole expandability. Therefore, in the present invention, the nitrogen content should be controlled to be ≦ 0.005%.

[0033] In the hot-rolled dual-phase steel sheet according to the present invention, for other inevitable impurities, P ≦ 0.02% and S ≦ 0.005%.

[0034] From the perspective of composition design, the main purposes of Ti are as follows: three effects of refining the crystal grains and one effect of precipitation strengthening, which are: (1) During the slab heating process, the precipitates of Ti prevent the growth of the prior austenite crystal grains; (2) During the hot rolling process, TiC contributes to the increase in the recrystallization temperature and further refines the austenite crystal grains; (3) The already precipitated Ti(C,N) and Ti(Cr)C contribute to the refinement of the phase-transformed bainite and a small amount of martensite crystal grains; (4) During the laminar flow cooling process, the nano-scale precipitation of TiC and Ti(Cr)C exerts a strong precipitation strengthening effect; The design of the carbon content needs to match the Ti content to ensure sufficient precipitation of Ti, and at the same time, strictly control the N content to ≤0.005% to avoid the consumption of the Ti content by excessive N.

[0035] From the perspective of composition design, the contents of Cr and B further satisfy the following: (1) 0.1% ≤ Cr ≤ 0.2% and 0.0020% ≤ B ≤ 0.0035%, or (2) 0.2% < Cr ≤ 0.35% and 0.0010% ≤ B < 0.002%, or (3) 0.35% < Cr ≤ 0.7% and B < 0.0010%. The addition of appropriate amounts of Cr and B elements can obtain a bainite structure and small-sized martensite-austenite islands during the hot rolling coiling process, and does not contain pearlite and massive martensite that affect the hole expansion property.

[0036] The manufacturing method of the hot-rolled duplex steel sheet with a tensile strength of 800 MPa grade according to the present invention includes the following steps: (1) smelting and continuous casting; (2) hot rolling; (3) pickling.

[0037] In some embodiments, the manufacturing method of the hot-rolled duplex steel sheet with a tensile strength of 800 MPa grade according to the present invention includes the following steps: (1) Smelting and continuous casting Smelting and casting are carried out according to the above chemical composition; the superheat degree in the steelmaking process is preferably controlled at 15 - 60°C.

[0038] (2) Hot rolling The slab obtained through smelting and continuous casting is preferably heated to 1200 - 1300 °C and preferably held for 1 - 3 hours, and then rolling is carried out. The rough rolling exit temperature is preferably 1000 - 1080 °C, and the rolling end temperature (i.e., the finish rolling exit temperature) is preferably 840 - 950 °C. The total reduction ratio is preferably ≥80%, the total reduction ratio of finish rolling is preferably ≥50%, the reduction ratio of the last pass of rolling is preferably ≤15%, and the rolling speed (i.e., the threading speed) is preferably controlled to be 7 - 13 m / s.

[0039] After hot rolling, preferably laminar flow cooling is adopted. Preferably, two-stage cooling is adopted, and the intermediate point temperature and cooling rate of laminar flow cooling are controlled such that the average cooling rate of the first stage is preferably ≥100 °C / s, the intermediate point temperature is preferably 600 - 720 °C, the cooling (e.g., air cooling) time is preferably 5 - 10 s, and the average cooling rate of the second stage is preferably ≥30 °C / s.

[0040] Preferably, after finish rolling, the steel plate is cooled (e.g., water cooled) to 430 - 550 °C and coiled.

[0041] Preferably, after hot rolling coiling, it is cooled to room temperature at a cooling rate of ≤100 °C / h. (3) Pickling The elongation rate of pickling tension leveling is preferably 0.2 - 2%, the pickling speed is preferably controlled to be 60 - 150 m / min, the temperature of the final pickling tank in the pickling process is preferably controlled to be 80 - 90 °C, and the iron ion concentration is preferably controlled to be 30 - 40 g / L.

[0042] The reasons for the design gist of the manufacturing process of the present invention are as follows: In the above step (1), the massive, brittle TiN with sharp edges becomes a potential source of cracks, significantly reducing the impact toughness and hole expansion property of the steel grade. Coarse TiN particles are generally induced to nucleate and grow by inclusions composed of Al2O3, CaO, MgO, etc., which are inevitable during the steelmaking process, and are also greatly affected by the superheat degree of steelmaking. The higher the superheat degree, the more favorable it is for the control of inclusions, but the higher the superheat degree, the more favorable it is for the growth of TiN. Therefore, during the steelmaking process, the contents of Al, Ca, etc. are strictly controlled, and at the same time, the superheat degree is strictly controlled at 15 - 60°C.

[0043] In the above step (2), in the case of Ti-containing steel, the heating temperature of the slab is particularly important for performance. During the continuous casting process, Ti precipitates in large amounts as large-sized Ti(C, N). The main purpose of setting the heating temperature ≥1200°C is to dissolve alloying elements such as Ti as much as possible during the heating process of the slab to ensure the nano-scale precipitation of microalloys such as Ti during the subsequent hot rolling coiling process. On the other hand, when the temperature exceeds 1300°C, there is a tendency for grain coarsening, which is disadvantageous to the toughness of the steel plate. Therefore, the heating temperature is preferably set at 1200 - 1300°C.

[0044] In the above step (2), the control of the rough rolling temperature and the rolling speed during the hot rolling process have a great impact on microalloys such as Ti. At a low rough rolling temperature and in the finish rolling process, Ti precipitates as Ti carbides and carbonitrides. The precipitates in this process are large in size and are disadvantageous to the improvement of the final strength. However, the precipitated TiC, Ti(Cr)N contribute to the refinement of austenite grain size. Therefore, the rough rolling exit temperature is controlled at 1000 - 1080°C.

[0045] In the above step (2), the temperature range with the most drastic precipitation temperature of Ti is 600 - 750°C. However, since the actual coiling temperature is lower than this temperature, in order to better exert the nano-scale precipitation strengthening effect by TiC, two-stage cooling is adopted in the laminar cooling process. However, the intermediate point temperature is 600 - 720°C, for example 600 - 700°C, the air cooling time is 5 - 10 seconds, the average cooling rate of the first stage of laminar cooling is controlled to be ≥100°C / s, and the average cooling rate of the second stage is controlled to be ≥30°C / s.

[0046] In the above step (2), in order to improve the bainite strength, the present invention contains an appropriate amount of Cr and B. Therefore, Cr and B satisfy the following: (1) 0.1% ≤ Cr ≤ 0.2% and 0.0020% ≤ B ≤ 0.0035%, or (2) 0.2% < Cr ≤ 0.35% and 0.0010% ≤ B < 0.002%, or (3) 0.35% < Cr ≤ 0.7% and B < 0.0010%. Although the formation of ferrite and pearlite is well suppressed by the addition of Cr and B, massive secondary martensite and retained austenite are likely to form. Therefore, in the hot rolling laminar cooling process, it has a great influence on the volume fraction of bainite transformation. In the present invention, in order to obtain appropriate bainite transformation and small-sized martensite-austenite islands, it is necessary to strictly control a relatively low hot rolling coiling temperature. When the coiling temperature increases, the size of secondary martensite and retained austenite becomes larger, which is disadvantageous for improving the hole expansion rate. However, when the coiling temperature decreases, there is a risk of the appearance of primary martensite structure and the elongation rate decreases. Therefore, the coiling temperature is controlled to be 430 - 580°C, for example 430 - 550°C.

[0047] In the above step (2), cooling to room temperature at a cooling rate of ≤100°C / h after coiling contributes to the further transformation of bainite, the tempering of martensite, and the further precipitation of microalloy. It contributes to the improvement of strength, hole expansion rate, elongation rate, and impact toughness.

[0048] The advantageous effects of the present invention are (1) The present invention is a low-cost design and invention. Instead of using precious microalloys such as Nb and V, it uses a lower-cost microalloy, Ti element, and instead of using precious microalloys such as Mo, it uses lower-cost Cr and B elements. Therefore, the alloy cost of the present invention is reduced; (2) The present invention adopts precise microstructure control and inclusion level control, and combines it with reasonable steelmaking level control, so that it can produce hot-rolled dual-phase steel sheets with a tensile strength of 800 MPa grade on a conventional hot continuous rolling line or a low-carbon short flow line; (3) The hot-rolled dual-phase steel sheet according to the present invention has characteristics such as high strength, high formability, and high toughness. Its longitudinal yield strength is ≧680 MPa, its tensile strength is ≧780 MPa, its elongation rate A50 is ≧15%, [KV(20°C) - KV(-40°C)] / KV(20°C) ≦0.35, and KV(-40°C) / thickness ≧10 J / mm, and the hole expansion rate of punching is ≧65%. It can also be used as automotive body structural parts and automotive chassis parts, and can also be used in other application fields where high strength and weight reduction are required.

Brief Description of the Drawings

[0049]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0050] Specific Embodiment Hereinafter, the present invention will be further described based on examples.

[0051] After smelting steels with different compositions shown in Table 1, heating, hot rolling, and pickling processes were carried out as shown in Table 2 to obtain steel plates with a thickness of ≤5.5 mm. The manufacturing method of the steel plates specifically includes the following steps: (1) Smelting and continuous casting Smelting and casting were carried out according to the chemical compositions in Table 1; the superheat degree in the steelmaking process was controlled at 15 - 60°C; (2) Hot rolling The slabs obtained through smelting and continuous casting were heated to 1200 - 1300°C, held for 2 hours, and then rolled. The rough rolling exit temperature was 1000 - 1080°C, the rolling end temperature was 840 - 950°C, the total reduction ratio was ≥80%, the finish rolling total reduction ratio was ≥50%, the reduction ratio of the last pass of rolling was ≤15%, and the rolling speed was controlled to be 7 - 13 m / s; After hot rolling, two-stage laminar flow cooling was adopted, and the intermediate point temperature and cooling rate of the laminar flow cooling were controlled such that the average cooling rate of the first stage was ≥100°C / s, the intermediate point temperature was 600 - 720°C, the cooling time was 5 - 10 s, and the average cooling rate of the second stage was ≥30°C / s.

[0052] The steel plates were cooled to 430 - 550°C and coiled; After coiling, they were cooled to room temperature at a cooling rate of ≤100°C / h; (3) Pickling The elongation rate of pickling tension leveling was controlled at 1%, the pickling speed was controlled at 100 m / min, the temperature of the final pickling tank in the pickling process was controlled at 85°C, and the iron ion concentration was controlled at 35 g / L.

[0053] The mechanical properties of the steel plates were measured by the following method, and the results are shown in Table 3: A JIS 5# tensile test piece was taken along the longitudinal direction, and the yield strength, tensile strength, and elongation at 50% (A50) were measured. The mechanical property measurement was carried out in accordance with the GB / T 228.1-2010 standard.

[0054] The hole expansion ratio was measured by a hole expansion test in which a sample with a hole in the center was pushed into a die using a punch die until the hole edge of the plate was constricted or a through crack occurred, expanding the hole in the center of the sample. Since the manufacturing method of the original hole in the center of the sample has a significant impact on the measurement results of the hole expansion ratio, the original holes in the centers of the samples were manufactured by punching and reaming respectively, and the subsequent test and measurement methods were carried out in accordance with the hole expansion ratio measurement method specified in the ISO / DIS 16630 standard. The hole expansion ratio in Table 3 is the punched hole expansion ratio.

[0055] The impact toughness measurement was carried out in accordance with the GB / T 229-2020 standard for the Charpy pendulum impact test method for metallic materials, and the original plate thickness of the test piece was used for the thickness of the impact test piece.

[0056] The microstructure of the steel plate was observed and analyzed by the following methods, and the results are shown in Table 4: Measurement method of area fraction: After the test piece was polished and etched, photographed at a magnification of 2000 times, and then, when performing quantitative analysis with ImagePro, the area fractions of bainite, martensite, austenite, and ferrite were determined by setting a predetermined threshold value to distinguish the tissues in gray scale.

[0057] Measurement method of grain size: The test piece was polished and etched, photographed at a magnification of 2000 times, and measured in accordance with the ASTM E112 standard.

[0058] Measurement method of the size of TiN inclusions: Measured in accordance with the measurement method of the GBT10516-2005 standard.

[0059] Examples 1 to 4, 8 to 10, 15, and 18 to 22 in Table 2 adopted the composition shown in Table 1. At the same time, the addition of Ti enhanced the effects of precipitation strengthening and grain refinement during the hot rolling process. At the same time, the hot rolling process in Table 2 was also satisfied. Therefore, the finally obtained hot rolled dual-phase steel sheet achieved the microstructure and mechanical properties in Table 3. However, the microstructure of the steel sheet in the examples satisfied the following: the area fraction of bainite was ≥90%, the area fractions of martensite and retained austenite were ≤5%, the area fraction of ferrite strengthened by precipitation with nano-scale microalloys was ≤5%, the grain size of bainite was ≤5 μm, the grain sizes of martensite and retained austenite were ≤2.5 μm, and the grain size of ferrite strengthened by precipitation with nano-scale microalloys was ≤7.5 μm. The size of the inclusions in the steel sheet of the examples satisfied the following: the size of the composite TiN containing TiN, CaO, Al2O3, etc. was ≤8 μm. The steel sheet of the examples had good formability, with its longitudinal yield strength ≥680 MPa, tensile strength ≥780 MPa, elongation at 50 mm A50 ≥15%, and hole expansion rate during punching ≥65%. The steel sheet of the examples had good fracture resistance, and its impact toughness satisfied the following: [KV(20°C) - KV(-40°C)] / KV(20°C) ≤ 0.35, and KV(-40°C) / thickness ≥ 10 J / mm.

[0060] Compared with Examples 1 to 4, Comparative Example 5 adopted the method of rapid cooling after rolling. Since the cooling rate after hot rolling and coiling was too fast, the ratio of supercooled austenite that generated martensite transformation after coiling was 13%, so the hole expansion rate decreased. In terms of impact toughness, the impact toughness at -40°C was clearly reduced by 36% compared to room temperature. At the same time, KV(-40°C) / plate thickness = 8.9 J / mm, and the impact toughness was low. Comparative Examples 6 to 7 adopted different hot rolling and coiling temperatures. However, since Comparative Example 6 adopted a low coiling temperature, the content of primary martensite in its microstructure reached 70%. Finally, the tensile strength increased, the elongation rate and the hole expansion rate decreased, and KV(-40°C) / plate thickness = 9.2 J / mm, and the impact toughness was low. Since Comparative Example 7 adopted a high coiling temperature, the content of ferrite in its microstructure was high, and the crystal grains were coarse. The area fraction of ferrite reached 30%, and the crystal grain size reached 8.2 μm. At the same time, the crystal grain size of bainite became relatively large at 7.5 μm. Therefore, KV(-40°C) / plate thickness = 8.2 J / mm, and the impact toughness was low.

[0061] Compared with Examples 8 to 10, since the intermediate point temperature of Comparative Example 11 was low, there was almost no ferrite transformation during the laminar flow cooling process. Finally, the elongation rate was relatively low at 14.5%. In Comparative Example 12, the total reduction rate of finish rolling was low and recrystallization was insufficient. As a result, the bainite and ferrite crystal grains were coarse, with their crystal grain sizes being 6.0 μm and 8.5 μm respectively. Therefore, the yield strength was low, the hole expansion rate was relatively low, and at the same time, due to the coarse crystal grains, the impact toughness was also low, with KV(-40°C) / plate thickness = 8.0 J / mm. In Comparative Example 13, since the rough rolling exit temperature was low and the finish rolling end temperature was low, during the rolling process, a ferrite transformation with large size and high area fraction occurred. Finally, the hole expansion rate and impact toughness became low, with the hole expansion rate being 55% and KV(-40°C) / plate thickness = 8.8 J / mm. In Comparative Example 14, since the heating temperature was low, the solid solution of the Ti content was insufficient. During the continuous casting process, the coarse Ti(C,N) particles could not be completely solidified, resulting in a small contribution to strength and insufficient tensile strength of the steel plate. In Comparative Examples 16 and 17, since the overheat degree during the steelmaking and continuous casting processes could not be controlled within a specific range, large-sized TiN was formed, which also affected the hole expansion rate.

[0062] In Comparative Example 23, since the ratios of Cr and B were not reasonable, the bainite transformation was low, while the ratios of martensite and retained austenite transformations were high, which contributed significantly to the strength. Conversely, it decreased the elongation rate, hole expansion rate, and impact toughness. However, [KV(20°C) - KV(-40°C)] / KV(20°C) = 0.37, and KV(-40°C) / thickness = 8.4 J / mm.

[0063] In Comparative Example 24, since elements such as Al and Ca were high, inevitable inclusions were formed first during the steelmaking process. However, during the solidification process, TiN also grew with particles such as Al2O3 as nucleation sites. The size of TiN reached 12 μm, the number of TiN was large and continuous, which had a significant impact on both the hole expansion rate and impact toughness. However, [KV(20°C) - KV(-40°C)] / KV(20°C) = 0.42%, and KV(-40°C) / thickness = 7.4 J / mm.

[0064] In Comparative Example 25, since the contents of C and Mn were high, the hardenability was strong, and the ratio of martensite was as high as 15%, finally, the elongation, hole expansion rate, and impact toughness decreased. However, the elongation was 14.0%, the hole expansion rate was 51%, [KV(20°C) - KV(-40°C)] / KV(20°C) = 0.39, and KV(-40°C) / thickness = 8.9 J / mm.

[0065] In Comparative Example 26, since the content of N was high, a large amount of Ti element was consumed, and a large amount of massive TiN was precipitated. However, the large TiN particles did not contribute much to the strength, which reduced the strength of the steel sheet. At the same time, when punching, microcracks caused by TiN of a large size existed at the punching edge, which had a great influence on the punching hole expansion rate.

[0066] [Table 1]

[0067] [Table 2] TIFF2025522608000004.tif39151

[0068] [Table 3]

Claims

1. A hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa, characterized in that, by mass percentage, in addition to Fe and other inevitable impurities, it further contains C: 0.04 - 0.08%, Si: 0.05 - 0.45%, Mn: 1.4 - 1.8%, N ≤ 0.005%, O ≤ 0.0030%, Ca ≤ 0.004%, Al: 0.02 - 0.1%, Ti: 0.07 - 0.13%, Cr: 0.1 - 0.7%, B ≤ 0.0035%.

2. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to Claim 1, characterized in that its chemical elements are, by mass percentage, C: 0.04 - 0.08%, Si: 0.05 - 0.45%, Mn: 1.4 - 1.8%, N ≤ 0.005%, O ≤ 0.0030%, Ca ≤ 0.004%, Al: 0.02 - 0.1%, Ti: 0.07 - 0.13%, Cr: 0.1 - 0.7%, B ≤ 0.0035%, and the balance is Fe and other inevitable impurities.

3. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to Claim 1 or 2, characterized in that the chemical composition of the hot-rolled dual-phase steel satisfies (1) 0.1% ≤ Cr ≤ 0.2% and 0.0020% ≤ B ≤ 0.0035%, or (2) 0.2% < Cr ≤ 0.35% and 0.0010% ≤ B < 0.002%, or (3) 0.35% < Cr ≤ 0.7% and B < 0.0010%.

4. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to Claim 1 or 2, characterized in that for other inevitable impurities, P ≤ 0.02% and S ≤ 0.005%.

5. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to Claim 1 or 2, characterized in that the microstructure of the hot-rolled dual-phase steel includes bainite with an area fraction ≥ 90%, martensite and retained austenite with an area fraction ≤ 5%, and ferrite precipitation-strengthened by a nano-scale micro-alloy with an area fraction ≤ 5%, and the grain size of bainite is ≤ 5 μm, the grain size of martensite and retained austenite is ≤ 2.5 μm, and the grain size of ferrite precipitation-strengthened by a nano-scale micro-alloy is ≤ 7.5 μm.

6. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to Claim 1 or 2, characterized in that the microstructure of the hot-rolled dual-phase steel contains TiN, and the size of TiN is ≤ 8 μm.

7. The longitudinal yield strength of the hot-rolled dual-phase steel is ≥ 680 MPa, the tensile strength is ≥ 780 MPa, the elongation A50 is ≥ 15%, [KV(20°C) - KV(-40°C)] / KV(20°C) ≤ 0.35, and KV(-40°C) / thickness ≥ 10 J / mm, and the punching hole expansion rate is ≥ 65%. The hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 1 or 2, characterized in that.

8. (1) Steelmaking, continuous casting; (2) Hot rolling; (3) Pickling. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to any one of claims 1 to 7, characterized in that it includes the above steps.

9. In the steelmaking and continuous casting process, the superheat degree is controlled to be 15 - 60°C. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

10. In the hot rolling process, the heating temperature is 1200 - 1300°C, the rough rolling exit temperature is 1000 - 1080°C, the total rolling reduction rate is ≥ 80%, the finish rolling total rolling reduction rate is ≥ 50% is controlled, the rolling end temperature is 840 - 950°C, and the rolling speed is 7 - 13 m / s. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

11. In the hot rolling process, after hot rolling, two-stage laminar flow cooling is adopted. The intermediate point temperature and cooling rate of the laminar flow cooling are controlled such that the average cooling rate of the first stage is ≥ 100°C / s, the intermediate point temperature is 600 - 720°C, the air cooling time is 5 - 10 s, and the average cooling rate of the second stage is ≥ 30°C / s. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

12. In the hot rolling process, the coiling temperature is 430 - 550°C. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

13. In the hot rolling process, after coiling, it is cooled to room temperature at a cooling rate of ≤ 100°C / h. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

14. In the pickling process, the pickling tension leveling elongation rate is controlled to be 0.2 - 2%, the pickling speed is controlled to be 60 - 150 m / min, the temperature of the final pickling tank in the pickling process is controlled to be 80 - 90°C, and the iron ion concentration is controlled to be 30 - 40 g / L. The manufacturing method of the hot-rolled dual-phase steel with a tensile strength ≥ 780 MPa according to claim 8, characterized in that.

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