Cold-rolled steel sheet, stamping member, and manufacturing method for cold-rolled steel sheet

Abstract

Disclosed in the present invention is a cold-rolled steel sheet, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: C: 0.06-0.10wt%, Si: 0.01-0.5wt%, Mn: 1.6-2.5wt%, Al: 0.01-0.07wt%, Cr: 0.5-1.0wt%, Mo: 0.1-0.3wt%, Ti: 0.01-0.05wt%, Nb: 0.001-0.03wt%, B: 0.0001-0.005wt%, and 0<N≤0.006wt%. A microstructure of the cold-rolled steel sheet is that spherical cementite particles are evenly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of spherical cementite is greater than or equal to 90%. Correspondingly, further disclosed in the present invention are a manufacturing method for the cold-rolled steel sheet, and a stamping member manufactured by using the cold-rolled steel sheet.

Description

Technical Field

[0001] The present invention relates to a sheet and a manufacturing method therefor, particularly a cold-rolled steel sheet and a manufacturing method therefor.Background Art

[0002] Automotive steel for stamping is widely used in the manufacturing industry of automotive parts. Since hot-rolled steel materials have coarse pearlite lamellae, high strength, and poor plasticity, they are generally difficult to form directly. Additionally, the surface quality and thickness accuracy of hot-rolled steel are poor. Therefore, hot-rolled steel must undergo subsequent cold rolling and spheroidizing annealing processes before it can be proceeded to a subsequent complex fine stamping process.

[0003] The structure and properties of automotive steel for stamping are basic conditions for realizing the stamping of complex parts. To achieve good stamping performance, it is desirable for a material to have high plasticity, low strength, and uniform properties, which is expected to be achieved by increasing the spheroidization rate. The main factors affecting the effect of spheroidization include the original structure of hot-rolled steel and annealing process.

[0004] In the existing production processes, increasing the number of spheroidizing annealing cycles and extending the spheroidizing annealing time are commonly used to improve a material's spheroidization rate. However, increasing the number of annealing cycles and prolonging the annealing time significantly raise the production costs.

[0005] In addition, for some materials or parts that require heat treatment, especially steel grades such as medium-carbon steel, high-carbon steel, and alloy steel, heat treatment is necessary after stamping. This necessitates a material to have good hardenability; only by quenching in a quenching medium such as oil can full martensite be formed, thereby enhancing the strength of the parts. However, due to the design of their alloy composition systems, currently available steel grades generally require a relatively fast cooling rate (≥30 °C / s) to be quenched into martensite. This imposes high requirements on the thickness of the material and the cooling medium; meanwhile, issues such as insufficient hardenability and subsequent dimensional deformation of the parts also need to be considered.

[0006] Based on this, it is expected to provide a cold-rolled steel sheet and a manufacturing method therefor, so as to obtain a high spheroidization rate through a simple process.Summary of the Invention

[0007] One of the objects of the present invention is to provide a cold-rolled steel sheet, which has a high spheroidization rate, good plasticity and relatively low strength, can meet the requirements of complex stamping, and at the same time has excellent hardenability during heat treatment.

[0008] In order to achieve the above object, the present invention provides a cold-rolled steel sheet comprising Fe and inevitable impurities, and further comprising the following chemical elements in mass percentage: C: 0.06~0.10wt%, Si: 0.01-0.5wt%, Mn: 1.6~2.5wt%, Al: 0.01~0.07wt%, Cr: 0.5~1.0wt%, Mo: 0.1~0.3wt%, Ti: 0.01 -0.05wt%, Nb: 0.001~0.03wt%, B: 0.0001~0.005wt%, 0<N≤0.006wt%; wherein a microstructure of the cold-rolled steel sheet is that spherical cementite particles are uniformly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of spherical cementite is greater than or equal to 90%.

[0009] Correspondingly, the present invention also provides a cold-rolled steel sheet, wherein the mass percentages of each chemical elements are: C: 0.06~0.10wt%, Si: 0.01-0.5wt%, Mn: 1.6~2.5wt%, Al: 0.01-0.07wt%, Cr: 0.5~1.0wt%, Mo: 0.1~0.3wt%, Ti: 0.01~0.05wt%, Nb: 0.001~0.03wt%, B: 0.0001-0.005wt%, 0<N≤0.006wt%, and a balance of Fe and inevitable impurities; wherein a microstructure of the cold-rolled steel sheet is that spherical cementite particles are uniformly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of spherical cementite is greater than or equal to 90%.

[0010] By reasonably controlling carbon, manganese, chromium, molybdenum, boron elements and their contents, and through the coordination between the compositions of these elements, the present invention enables a cold-rolled steel sheet to obtain good hardenability, form a martensite structure at a relatively low cooling rate during heat treatment, and improve the strength and hardness of the material.

[0011] The reasons for the composition design of each chemical component adopted in the present invention are as follows: Carbon: in the technical solution described herein, carbon is an important strengthening element, which can significantly improve the strength of steel through solid solution strengthening. However, an excessively high mass percentage of carbon will not only significantly increase the carbon equivalent of the steel strip and reduce its weldability, but also decrease the plasticity of the steel strip and increase the cold brittleness and aging sensitivity of the steel. Therefore, a mass percentage of carbon should not be too high; thus, a mass percentage of carbon in the cold-rolled steel sheet of the present invention is controlled to be 0.06~0.10wt%. Silicon: silicon is a ferrite solid solution strengthening element that can improve strength. However, an addition of silicon will increase the carbon equivalent, reduce weldability, and also have an adverse effect on phosphating properties. Therefore, a mass percentage of silicon in the cold-rolled steel sheet of the present invention is controlled to be 0.01~0.5wt%. Manganese: manganese has a strong solid solution strengthening effect and can effectively improve the strength of a steel sheet. Therefore, to meet the requirement of high strength after heat treatment, a certain amount of Mn is added to a cold-rolled steel sheet of the present invention. However, an excessively high mass percentage of Mn will significantly increase the carbon equivalent of a steel sheet, reduce its weldability, and also aggravate segregation, which is adverse to formability properties such as plasticity, hole expansion property and bending property. Thus, a mass percentage of Mn in the cold-rolled steel sheet of the present invention is controlled to be 1.6~2.5wt%. Aluminum: Al acts as a deoxidizer and combines with N to form AIN, which helps prevent the coarsening of austenite grains. However, when a content of Al exceeds 0.07wt%, the purity of a steel sheet will decrease. Therefore, a mass percentage of Al in the cold-rolled steel sheet of the present invention is controlled to be 0.01-0.07wt%. Chromium: in steel, chromium is mainly used to delay the incubation time of austenite transformation, improve hardenability, delay the transformation of ferrite and pearlite, and inhibit the formation of such structures during the cooling process, and enabling the steel to directly enter the martensitic transformation zone at a relatively low cooling rate. Therefore, a mass percentage of chromium in the cold-rolled steel sheet of the present invention is controlled to be 0.5~1.0wt%. Molybdenum: the main role of molybdenum in steel is solid solution strengthening effect; at the same time, it also improves the stability of carbides, thereby enhancing the strength of the steel. Additionally, it can increase hardenability and hot strength, and prevent temper brittleness. Therefore, a content of molybdenum in the cold-rolled steel sheet of the present invention is controlled to be 0.1~0.3wt%. Titanium: in the present invention, Ti is a strong carbide and nitride-forming element, with strong grain refinement strengthening and precipitation strengthening effects. An addition of Ti in the present invention also helps to reduce edge and corner cracks in slabs. Furthermore, compared with Nb element, the price of Ti is less than one-tenth of that of Nb, offering a significant cost advantage. However, an excessive addition of Ti will reduce its strengthening effect. Therefore, a mass percentage of Ti in the high-strength cold-rolled steel strip described herein is controlled to be 0.01-0.05wt%. Niobium: niobium is a strong carbide and nitride-forming element with strong grain refinement strengthening and precipitation strengthening effects, contributing to the improvement of the strength of the steel strip. However, an excessively high mass percentage of Nb tends to cause the segregation of carbides and nitrides, which degrades the workability of the steel. Additionally, Nb is very expensive, and excessive addition will increase costs. Therefore, an addition amount of Nb should not be too high; in the technical solution described herein, a mass percentage of Nb is controlled to be 0.001~0.03wt%. Boron: boron is an element that strongly segregates at austenite grain boundaries in steel. It can reduce the grain boundary energy of austenite and inhibit the formation of pro-eutectoid ferrite nuclei. Boron has three characteristics in terms of improving the hardenability of steel: it has a very strong ability to improve hardenability, and only a very small amount of B is needed to save a large amount of precious alloying elements. However, B has an optimal content for improving hardenability. Unlike the effect of general alloying elements on improving hardenability, which increases with their content in steel, to effectively improve hardenability, a content of B in the present invention must be more than 0.0001wt%. But when its content exceeds 0.005wt%, the effect of improving hardenability reaches saturation. Therefore, a content of B in the present invention is controlled to be 0.0001~0.005wt%. Nitrogen: in Ti-containing steel, an appropriate amount of N easily forms TiN with Ti at high temperatures, which is beneficial for strengthening the matrix and improving the weldability of a steel sheet. However, an excessively high mass percentage of N tends to coarsen TiN or cause excessive N to dissolve in solid solution, reducing the plasticity and hole expansion-flanging performance of the steel. Moreover, an excessively high content of N will lead to the formation of AIN during hot rolling, resulting in reducing blanking processability and hardenability of a base steel sheet. Therefore, in the technical solution of the present invention, a mass percentage of N is controlled to be 0<N≤0.006wt%.

[0012] The inevitable impurities in the present invention are mainly S and P. Among them, phosphorus will increase the cold brittleness of steel, reduce the plasticity of steel, and also have an adverse effect on the welding performance. Therefore, in the cold-rolled steel sheet of the present invention, the lower the content of impurity P, the better. However, considering the smelting cost, in some embodiments, a mass percentage of P can be controlled to be P ≤ 0.015wt%.

[0013] Sulfur in steel easily combines with Mn to form MnS, which impairs the mechanical properties, hole expansion property, and other formability properties of the steel. Therefore, in the technical solution described herein, the lower the content of S, the better. Consequently, in some embodiments of the present invention, a mass percentage of S can be controlled to be S ≤ 0.01wt%.

[0014] Further, in the cold-rolled steel sheet described herein, a grain size of ferrite ranges from Grand 7.0 to Grand 11.0.

[0015] Further, in the cold-rolled steel sheet described herein, a spheroidization rate of spherical cementite is greater than or equal to 95%.

[0016] Further, a cold-rolled steel sheet described herein has a yield strength of 280~420 MPa, a tensile strength of 450~600 MPa, and an elongation at break A 50mm of ≥25%.

[0017] In some embodiments, a cold-rolled steel sheet described herein has a yield strength of 380~420 MPa, for example, 389~420 MPa.

[0018] In some embodiments, a cold-rolled steel sheet described herein has a tensile strength of 482~600 MPa, for example, 498~600 MPa, 529~600 MPa, 568~600 MPa, 597~600 MPa.

[0019] Further, a microstructure of the cold-rolled steel sheet in the hot-rolled state described herein is a mixed structure of ferrite + pearlite + martensite.

[0020] Another object of the present invention is to provide a manufacturing method for a cold-rolled steel sheet, wherein the cold-rolled steel sheet produced has a high spheroidization rate, good plasticity and relatively low strength, can meet the requirements of complex stamping, and at the same time has excellent hardenability during heat treatment.

[0021] Based on the above objects, the present invention also provides a manufacturing method for the cold-rolled steel sheet as described above, which comprises the steps of: smelting and casting to obtain a slab; hot rolling: heating, rolling and coiling, wherein a coiling temperature is 500~620 °C; pickling and cold rolling; bell-type annealing: an annealing soaking temperature is 660~710 °C and an annealing soaking time is 5~30 h; leveling.

[0022] In the manufacturing method of the present invention, a hot rolling step is a critical step for increasing the spheroidization rate of spherical cementite in a microstructure of a cold-rolled steel sheet. In the prior art, a medium-to-high coiling temperature of 620~670 °C is generally adopted, and a microstructure of a hot-rolled steel sheet obtained is banded ferrite + massive pearlite, with the presence of banded structures. To obtain a product with spherical cementite with the highest possible spheroidization rate as well as spherical cementite uniformly and dispersedly distributed in the ferrite matrix, in the prior art, processes that increase the number of spheroidizing annealing (two times or more) and extend the spheroidizing annealing time are generally adopted. These processes make the cementite fully diffuse and finally obtain dispersed spherical cementite particles. However, increasing the number of annealing treatments and prolonging the annealing time significantly increase the production cost.

[0023] Unlike the prior art, the manufacturing method of the present invention adopts a low-temperature coiling process of 500 °C~620 °C in the hot rolling step. A uniform microstructure of a hot-rolled steel sheet produced thereby is ferrite + pearlite + bainite / martensite. The carbides are uniformly distributed in these structures. Therefore, the diffusion distance required in the subsequent cold rolling step and spheroidizing annealing step is significantly shortened, and under the same spheroidizing annealing process, a microstructure with a better spheroidization rate can be obtained. Hence, in the manufacturing method of the present invention well-spheroidized spherical cementite particles can be obtained without multiple or prolonged annealing treatments.

[0024] In the present invention, if a coiling temperature is lower than 500 °C, a large amount of martensite will be obtained due to a low-temperature coiling, resulting in excessively high strength of a steel sheet and making it prone to cracking. Additionally, a low coiling temperature leads to significant fluctuations in temperature control, large variations in performance and significant differences in structure. It also tends to generate internal stress, causing poor sheet shape. Therefore, the present invention adopts a low-temperature coiling process of 500 °C~620 °C.

[0025] Annealing is to slowly heat the metal to a certain temperature, keep it for a sufficient time, and then cool it at an appropriate rate. Its purpose is to reduce the increase of hardness of a steel sheet caused by a cold rolling step, so as to improve the fine blanking processability. In the manufacturing method of the present invention, a bell-type annealing furnace is used for annealing, and an annealing temperature is selected to be below the A1 point temperature (i.e., the temperature at which austenite, ferrite, and cementite coexist in equilibrium). This is because: an excessively low annealing temperature requires a further extension of the holding time to achieve a good spheroidization effect; an excessively high annealing temperature will cause the steel strip to enter a two-phase region, which transforms into an undesirable structure of massive pearlite in the subsequent cooling process, and it will result in coarse ferrite grains and significant surface decarburization, affecting the final service performance of the material. Therefore, to improve the spheroidizing annealing effect, the present invention sets the annealing soaking temperature of a cold-rolled steel sheet in the bell-type annealing furnace to 660~710 °C. The soaking time during annealing is also critical. If a soaking time is too short, fine spherical particles cannot be formed and a dispersed distribution of cementite particles cannot be achieved; if a soaking time is too long, the spherical pearlite will grow again and agglomerate, forming lamellar segregation, resulting in a decrease in the plasticity of the material and is unfavorable for subsequent fine blanking. Therefore, to improve the spheroidizing annealing effect, an annealing soaking time of a cold-rolled steel sheet in the bell-type annealing step of the present invention is 5 to 30 hours.

[0026] Compared with the prior art, the above-mentioned annealing process of the present invention greatly saves the manufacturing cost under the conditions of reducing the number of spheroidizing annealing and shortening the spheroidizing annealing time, while retaining the high spheroidization rate of the spherical cementite in the cold-rolled steel sheet obtained after annealing. Therefore, the obtained cold-rolled steel sheet has the advantages of good plasticity and relatively low strength, as well as good stamping performance, and can meet the requirements of complex stamping.

[0027] In some embodiments of the present invention, converter or electric furnace is used for smelting, and the thickness of a slab can be 80~300 mm.

[0028] Further, in the hot rolling step of the manufacturing method described herein, a slab heating temperature is controlled to be 1180~1280 °C, and a finish rolling temperature is controlled to be 870~940 °C.

[0029] In the hot rolling step of the present invention, a slab heating temperature is controlled to be 1180~1280 °C. This is because: to ensure the full dissolution of microalloying elements in the slab, it is beneficial to fully exert the effects of the micro-alloying elements in the subsequent steps. Since finish rolling must be carried out in a single-phase region, that is, it cannot be lower than the Ar3 phase transformation temperature (the starting temperature of the transformation of austenite to ferrite during cooling), therefore, a finish rolling temperature is controlled to be 870~940 °C. If a finish rolling temperature is higher than 940 °C, the generated scale will become thicker, the pickling property will decrease, a decarburized layer may form on the surface layer of a steel sheet, and the ferrite grain size will tend to be coarser; if a finish rolling temperature is lower than the Ar3 phase transformation temperature, it will not only lead to mixed grains but also significantly increase the rolling load of the rolling mill. Therefore, a finish rolling temperature is limited to the range of 870~940 °C to avoid problems such as mixed grains and increased rolling difficulty caused by excessively low finish rolling temperature, or excessive grain size, which impairs the strengthening effect, caused by excessively high temperature.

[0030] Furthermore, in the hot rolling step of the manufacturing method described herein, a slab heating temperature is controlled to be 1200~1250 °C, and a finish rolling temperature is controlled to be 890~930 °C.

[0031] Furthermore, in the hot rolling step of the manufacturing method described herein, a coiling temperature is controlled to be 550~600 °C.

[0032] In some embodiments of the present invention, in the hot rolling step, a slab heating temperature can be selected to be 1200~1250 °C, a finish rolling temperature is 890~930 °C, and a coiling temperature is 500~620 °C. This can generate a bainite structure that is beneficial to improving the spheroidization rate, ultimately achieving the purpose of increasing the spheroidization rate, improving the plasticity of the cold-rolled steel, and reducing strength. Meanwhile, it is beneficial for shortening the subsequent spheroidizing annealing time and reducing the number of subsequent spheroidizing annealing, thereby lowering the manufacturing cost.

[0033] In some embodiments of the present invention, after the finishing rolling step, water spray cooling can be further included to cool the steel sheet at a set cooling rate to the coiling temperature for coiling, wherein a cooling rate may be >30 °C / s.

[0034] Further, in the cold rolling step of the manufacturing method described herein, a cold rolling reduction ratio of the cold-rolled steel sheet is controlled to be 20~60%.

[0035] In the cold rolling step, the main parameter determining the quality of the cold-rolled steel sheet is the cold rolling reduction. An appropriate cold rolling reduction can induce strain energy in the material, providing energy for the pearlite spheroidization transformation. In the present invention, cold rolling the hot-rolled steel sheet can promote grain recrystallization in the subsequent bell-type annealing step, obtain a suitable grain size and meet the requirements of continuous fine blanking. Therefore, it is necessary to set the reduction ratio to more than 20%. On the other hand, if a reduction ratio exceeds 60%, the steel sheet will become hardened due to excessive grain refinement, and the fine blanking performance of the cold-rolled steel sheet will be reduced. Meanwhile, an excessively high deformation will cause excessive load on the cold rolling mill, increase the number of cold rolling passes, and increase the cold rolling cost.

[0036] Further, in the cold rolling step of the manufacturing method described herein, a cold rolling reduction ratio of the cold-rolled steel sheet is controlled to be 30~50%.

[0037] A cold rolling reduction ratio is preferably controlled to be 30~50% to further obtain a product with a target spheroidization rate and grain size.

[0038] Further, in the manufacturing method described herein, a leveling step is further performed after a bell-type annealing step, wherein a leveling elongation is controlled to be ≤2.5%.

[0039] Furthermore, in the manufacturing method described herein, a leveling elongation is controlled to be 0.5~2.0% to further obtain an appropriate yield strength ratio, which is beneficial for subsequent precision stamping.

[0040] In some embodiments of the present invention, the surface of the obtained cold-rolled steel sheet is uncoated.

[0041] Another object of the present invention is to provide a stamping part (such as an automotive stamping part), which can be used for parts of automotive engine systems, transmission systems, automotive chassis systems, etc.

[0042] Based on the above objects, the present invention provides a stamping part (such as an automotive stamping part), which is manufactured by stamping the cold-rolled steel sheet described above.

[0043] Further, a stamping part described herein undergoes a heat treatment with a heating temperature of 800~950 °C and a cooling rate of 10~30 °C / s to obtain a full martensite structure.

[0044] Compared with the stamping parts in the prior art, which require a cooling rate of ≥ 30 °C / s during heat treatment to be quenched into martensite, the present invention can obtain full martensite with a lower quenching cooling rate.

[0045] Furthermore, an automotive stamping part described herein has a yield strength of 650~900 MPa, a tensile strength of 800~1100 MPa, and an elongation at break A 50mm of ≥5.5%, for example, ≥5.8%, ≥5.9%, ≥6.4%, ≥6.8%, ≥6.9%, ≥7.4% or ≥8%.

[0046] In some embodiments, an automotive stamping part described herein has a yield strength of 663~900 MPa, for example, 673~900 MPa, 735~900 MPa, 763~900 MPa, 858~900 MPa, 868~900 MPa.

[0047] In some embodiments, an automotive stamping part described herein has a yield strength tensile strength of 879~1100 MPa, for example, 976~1100 MPa, 998~1100 MPa, 1002~1100 MPa, 1055~1100 MPa, 1078~1100 MPa.

[0048] The cold-rolled steel sheet and manufacturing method therefor described herein have the following advantages and beneficial effects: The cold-rolled steel sheet described herein obtains a uniform mixed structure of ferrite + pearlite + martensite in the hot-rolled steel sheet by optimizing the chemical composition ratio and the optimization design of the hot rolling process, and then a microstructure of the cold-rolled steel sheet after cold rolling and spheroidizing annealing is that spheroidal cementite particles are uniformly distributed on a ferrite matrix, wherein a spheroidization rate of spheroidal cementite is ≥90%. Therefore, the cold-rolled steel has a high spheroidization rate, good plasticity, and relatively low strength, and meets the requirements of precision blanking.

[0049] The manufacturing method described herein adopts a low-temperature coiling process of 500 °C~620 °C in the hot rolling step. A microstructure of a hot-rolled steel sheet produced thereby is a uniform mixed structure of ferrite + pearlite + martensite. The carbides in these structures are uniformly distributed. Therefore, the diffusion distance required in the subsequent cold rolling step and spheroidizing annealing step is significantly shortened, and under the same spheroidizing annealing process, a structure with a better spheroidization rate can be obtained. Hence, in the manufacturing method of the present invention, well-spheroidized spherical cementite particles can be obtained without multiple or prolonged annealing treatments.

[0050] The manufacturing method described herein greatly saves the manufacturing cost under the conditions of reducing the number of spheroidizing annealing processes and shortening the spheroidizing annealing time. Meanwhile, it retains the high spheroidization rate of the spherical cementite in the cold-rolled steel sheet obtained after annealing. Therefore, the cold-rolled steel sheet obtained has the advantages of good plasticity and relatively low strength, as well as good stamping performance, and can meet the requirements of complex stamping.Brief Description of the Drawings

[0051] Figure 1 shows a metallographic image of a microstructure of a hot-rolled steel sheet of Example 1 of the present invention. Figure 2 shows a metallographic image of a microstructure of a cold-rolled steel sheet of Example 1 of the present invention. Figure 3 shows a metallographic image of a microstructure of a stamping part after heat treatment of Example 1 of the present invention. Detailed Description

[0052] The cold-rolled steel sheet, the manufacturing method therefor and the stamping part described herein will be further explained and illustrated below in conjunction with specific examples and drawings. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.Example 1-6 and Comparative Example 1-2

[0053] Table 1 lists the mass percentages of each chemical element in the cold-rolled steel sheets of Examples 1-6 and the comparative steel sheets of Comparative Examples 1-2. Table 1. (wt%, the balance is Fe and other unavoidable impurities except P, S)No.CSiMnAlCrBNMoNbTiPSExample 10.0830.231.950.03660.6850.00390.00460.1790.0150.0310.0080.007Example 20.0920.411.780.02780.5870.00240.00580.2850.0120.0420.0140.008Example 30.0650.331.620.06850.9210.00480.00240.2280.0280.0480.0120.005Example 40.0780.032.450.02670.5240.00430.00020.2890.0030.0240.0070.01Example 50.0970.482.140.0180.8970.00020.00520.1230.0180.0160.0110.01Example 60.0850.0132.320.0230.9790.00140.00130.2760.0230.0320.0050.005Comparative Example10.163 0.0501.05 0.04250.880.00080.00430.003 0.0020.0130.0150.015Comparative Example20.23 0.2341.20 0.03780.19 0.00280.00380.002 0.0020.0260.0160.007

[0054] The cold-rolled steel sheets of Examples 1-6 of the present invention are all prepared by the following steps: (1) Smelting and casting to obtain a slab with a thickness of 80~300 mm according to the chemical composition shown in Table 1. (2) Hot rolling: a slab heating temperature is controlled to be 1180~1280 °C, a final rolling temperature is controlled to be 870~940 °C, and water cooling is performed after final rolling with a cooling rate of >30 °C / s, so that a temperature of steel sheet is cooled to a coiling temperature of 500~620 °C for coiling; preferably, a slab heating temperature is controlled to be 1200~1250 °C, a final rolling temperature is controlled to be 890~930 °C, and a coiling temperature is controlled to be 560~620 °C. (3) Pickling and cold rolling: pickling is performed to remove iron oxide scale on surface, and a cold rolling step is performed after a pickling step, with a cold rolling reduction rate controlled to be 20~60%, preferably 30~50%. (4) Bell-type annealing: an annealing soaking temperature is controlled to be 660~710 °C and an annealing soaking time is controlled to be 5~30h. (5) Leveling: a leveling rate is controlled to be ≤2.5%, and preferably a leveling rate can be controlled to be 0.5~2.0%.

[0055] Although the comparative steels of Comparative Examples 1-2 are also produced using the above-mentioned manufacturing process, there are parameters in their chemical composition design and specific manufacturing process parameters that do not meet the design requirements of the present invention.

[0056] Table 2 lists the specific process parameters of the cold-rolled steel sheets of Examples 1-6 and the comparative steel sheets of Comparative Examples 1-2 in the steps of the above-mentioned manufacturing method. Table 2No.Thickness of Slab (mm)Slab Heating Temperature (°C)Finish Rolling Temperature (°C)Cooling Rate (°C / s)Coiling Temperature (°C)Cold Rolling Reduction Ratio (%)Annealing Temperature (°C)Annealing Time (h)Leveling Elongation (%)Example 124012308805060045700201.6Example 210011809405062060680302.0Example 328012808706050035705251.5Example 43001200890605602071050.5Example 524012309304055030660150.5Example 68012508903560050700251.0Comparative Example1240120088030 440 45620300.8Comparative Example2240126093020 680 40730351.5

[0057] To observe changes of the microstructure of the present invention, the inventors also sample the hot-rolled sheets of Examples 1-6 after a hot rolling step to observe their microstructures. Figure 1 shows a metallographic image of a microstructure of the hot-rolled steel sheet of Example 1 of the present invention.

[0058] As can be seen from Figure 1, a microstructure of a hot-rolled steel sheet in its hot-rolled state is ferrite + pearlite + martensite. The microstructures of the hot-rolled steel sheets of Examples 2-6 after a hot rolling step all are ferrite + pearlite + martensite.

[0059] In addition, the inventors also sample the cold-rolled steel sheets prepared in Examples 1-6 of the present invention and Comparative Examples 1-2 respectively. Furthermore, an optical microscope is used to observe the microstructures of the cold-rolled steel sheets from Examples 1-6 and Comparative Examples 1-2. Under an optical microscope, the spheroidization rate of spherical cementite is measured in accordance with the SEP1520 Series 3 rating standard, and the observed results are listed in Table 3 below. The grain size of ferrite is evaluated in accordance with the GB / T 6394 standard, and the results are also listed in Table 3.

[0060] Table 3 lists the observation results of metallographic microstructure of the cold-rolled steel sheets of Examples 1-6 and the cold-rolled steel sheets of Comparative Examples 1-2. Table 3.No.MicrostructureGrain Size of FerriteSpheroidization Rate of Spherical Cementite (%)Example 1Ferrite8.099Example 2Ferrite8.092Example 3Ferrite9.095Example 4Ferrite8.099Example 5Ferrite9.097Example 6Ferrite8.099Comparative Example1Ferrite + Pearlite6.0 74 Comparative Example2Ferrite6.0 85

[0061] As can be seen from Table 3 above, the microstructures of the cold-rolled steel sheets in Examples 1-6 are all that spherical cementite particles are uniformly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of the spherical cementite is ≥90%.

[0062] In addition, Figure 2 also shows a metallographic image of a microstructure of the cold-rolled steel sheet of Example 1 of the present invention.

[0063] As shown in Figure 2, the microstructure exhibits a uniform distribution of ferrite, accompanied by a dispersed distribution of spherical cementite, wherein a spheroidization rate of the spherical cementite is 99%.

[0064] The inventors also conducted mechanical property tests on the cold-rolled steel sheet samples of each example and comparative example. Herein, the composition measurement standard of the examples and comparative examples is GB / T 223, and the test methods of yield strength, tensile strength and elongation are carried out in accordance with the measurement standard GB / T228-2002, and the elongation gauge length is 50 m; the hole expansion performance test is carried out in accordance with the standard GB / T15825.4-2008, and the final results of the performance tests are listed in the following Table 4.

[0065] Table 4 lists the performance test results of the cold-rolled steel sheets of Examples 1-6 and the cold-rolled steel sheets of Comparative Examples 1-2. Table 4.No.Yield Strength (MPa)Tensile Strength (MPa)Elongation A 50mm (%)Example 132548230.2Example 238056826.2Example 329546232.5Example 430652931.4Example 538959727.5Example 629449829.7Comparative Example1275 46528.4Comparative Example234560523.5

[0066] As can be seen from Table 4, in the present invention, the cold-rolled steel sheets of Examples 1-6 all have a yield strength greater than 280 MPa, a tensile strength greater than 450 MPa, and an elongation at break A 50mm greater than 25%.

[0067] It can be seen from the above examples that in the present invention through appropriate chemical composition design and optimized manufacturing processes, a cold-rolled steel sheets with good plasticity and relatively low strength can be obtained, which can meet the requirements of subsequent complex stamping.

[0068] To further verify the stamping performance of the cold-rolled steel sheet of the present invention, the cold-rolled steel sheets of each Example and Comparative Example are stamped into automotive stamping parts. These stamping parts are then subjected to heat treatment under the process parameters of a heating temperature of 800~950 °C and a cooling rate of 10~30 °C / s (i.e., quenching rate). Tensile property tests are conducted in accordance with GB / T 228-2002 to obtain a yield strength, a tensile strength, and an elongation A 50mm of the stamping parts. Additionally, a microstructure of the obtained stamping part is observed using an optical microscope. The results are listed in Table 5. Table 5.No.Heat Treatment Heating Temperature(°C)Quenching Cooling Rate(°C / s)Microstructure of Stampings PartsYield Strength of Stampings Parts (MPa)Tensile Strength of Stampings Parts (MPa)Elongation A 50mm of Stampings Parts (%)Example 193010Martensite86810786.8Example 285020Martensite7639987.4Example 380030Martensite6738795.8Example 495015Martensite66310026.4Example 587010Martensite7359766.9Example 683010Martensite85810555.9Comparative Example193010Ferrite +Martensite60278513.4Comparative Example293020Ferrite +Martensite63277015.3

[0069] As can be seen from Table 5, all embodiments of the present invention can obtain a full martensite microstructure at a quenching cooling rate of at least 10 °C / s, while Comparative Examples 1 and 2 still cannot obtain a full martensite structure at quenching cooling rates of 10 °C / s and 20 °C / s, which makes the strength of Comparative Examples 1 and 2 lower than that of the embodiments of the present invention.

[0070] Figure 3 shows a microstructure of the stamping part made in Example 1 after heat treatment. As can be seen from Figure 3, the stamping part has a full martensite structure.

[0071] It should be noted that the prior art in the protection scope of the present invention is not limited to the embodiments given in the present application documents, and all prior art that does not contradict the scheme of the present invention, including but not limited to prior patent documents, prior public publications, prior public uses, etc., can be included in the protection scope of the present invention.

[0072] In addition, the combination of the various technical features in this invention is not limited to the combinations described in the claims of this invention or the combinations described in the specific embodiments, and all technical features recorded in this invention can be freely combined or integrated in any way unless there is a contradiction between them.

[0073] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and all similar changes or modifications made therewith, which can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, should belong to the protection scope of the present invention.

Claims

1. A cold-rolled steel sheet, wherein the cold-rolled steel sheet comprises Fe and inevitable impurities, and further comprises the following chemical elements in mass percentage: C: 0.06~0.1 0wt%, Si: 0.01~0.5wt%, Mn: 1.6~2.5wt%, Al: 0.01~0.07wt%, Cr: 0.5~1.0wt%, Mo: 0.1~0.3wt%, Ti: 0.01~0.05wt%, Nb: 0.001 -0.03wt%, B: 0.0001~0.005wt%, 0<N≤0.006wt%; wherein a microstructure of the cold-rolled steel sheet is that spherical cementite particles are uniformly distributed on a single-phase ferrite matrix, wherein a spheroidization rate of spherical cementite is greater than or equal to 90%.

2. The cold-rolled steel sheet according to claim 1, wherein the mass percentages of each chemical elements in the cold-rolled steel sheet are: C: 0.06~0.10wt%, Si: 0.01~0.5wt%, Mn: 1.6~2.5wt%, Al: 0.01~0.07wt%, Cr: 0.5~1.0wt%, Mo: 0.1~0.3wt%, Ti: 0.01~0.05wt%, Nb: 0.001 -0.03wt%, B: 0.0001~0.005wt%, 0<N≤0.006wt%, and a balance of Fe and inevitable impurities.

3. The cold-rolled steel sheet according to claim 1 or 2, wherein a grain size of ferrite ranges from Grade 7.0 to Grade 11.0.

4. The cold-rolled steel sheet according to claim 1 or 2, wherein a spheroidization rate of spherical cementite in the cold-rolled steel sheet is greater than or equal to 95%.

5. The cold-rolled steel sheet according to claim 1 or 2, wherein the cold-rolled steel sheet has a yield strength of 280~420 MPa, a tensile strength of 450~600 MPa, and an elongation at break A50mm of ≥25%.

6. The cold-rolled steel sheet according to claim 1 or 2, wherein a microstructure of the cold-rolled steel sheet in a hot-rolled state is ferrite + pearlite + martensite.

7. A manufacturing method for the cold-rolled steel sheet according to any one of claims 1 to 6, wherein the manufacturing method for the cold-rolled steel sheet comprises the steps of: smelting and casting to obtain a slab; hot rolling: heating, rolling and coiling, wherein a coiling temperature is 500~620 °C; pickling and cold rolling; bell-type annealing: an annealing soaking temperature is 660~710 °C and an annealing soaking time is 5~30 h; leveling.

8. The manufacturing method according to claim 7, wherein in the hot rolling step, a slab heating temperature is controlled to be 1180~1280 °C, and a finish rolling temperature is controlled to be 870~940 °C.

9. The manufacturing method according to claim 7, wherein in the hot rolling step, a coiling temperature is controlled to be 550~600 °C.

10. The manufacturing method according to claim 9, wherein in the hot rolling step, a slab heating temperature is controlled to be 1200~1250 °C, and a finish rolling temperature is controlled to be 890~930 °C.

11. The manufacturing method according to claim 7, wherein in the cold rolling step, a cold rolling reduction ratio is controlled to be 20~60%.

12. The manufacturing method according to claim 7, wherein in the cold rolling step, a cold rolling reduction ratio is controlled to be 30~50%.

13. The manufacturing method according to claim 7, wherein in the leveling step, a leveling elongation is controlled to be ≤2.5%.

14. An automotive stamping part, wherein the automotive stamping part is manufactured by stamping the cold-rolled steel sheet according to any one of claims 1 to 6.

15. The automotive stamping part according to claim 14, wherein the automotive stamping part undergoes a heat treatment process with a heating temperature of 800~950 °C and a cooling rate of 10~30 °C / s to obtain a full martensite structure; and / or the automotive stamping part has a yield strength of 650~900 MPa, a tensile strength of 800~1100 MPa, and an elongation at break A50mm of ≥5.5%.

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