Cold-rolled steel sheet for galvanized steel sheet, galvanized steel sheet and its manufacturing method

Abstract

The present invention discloses a cold-rolled steel sheet for galvanized steel sheet, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.18-0.25%, Si: 1.5-2.0%, Mn: 1.5-2.3%, Nb: 0.01-0.06%; the microstructure of the cold-rolled steel sheet is bainite + tempered martensite + retained austenite, and the volume fraction of bainite and tempered martensite is ≧95%. Therefore, in the present invention, a method for producing said galvanized steel sheet is disclosed, which includes the following steps: (1) smelting and casting to obtain a billet; (2) hot rolling; (3) cold rolling; (4) annealing: the annealing soaking temperature is 890-920°C, the soaking holding time is 80-150 seconds, and then cooling to 270-350°C at a cooling rate of 30-100°C / s; (5) overaging: the overaging temperature is 450-475°C, and the overaging time is 40-60 seconds; (6) putting into a zinc pot for galvanization; (7) alloying; (8) leveling.

Description

[Technical field]

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

[0002] In recent years, the demand for weight reduction has increased in the automotive industry, and an increasing number of automakers are entering the research and development of lightweight automobiles, especially lightweight electric automobiles.

[0003] In recent years, electric vehicles have become the most popular vehicle types. However, in the development of electric vehicles, driving range has always been a major issue, and the reduction in the weight of vehicle structural parts plays a very important role in extending the driving range of electric vehicles and improving the practicality and comfort of electric vehicles. Summary of the Invention [Problem to be solved by the invention]

[0004] As is well known, high-tensile steel is very suitable for manufacturing automotive structural parts due to its excellent mechanical properties and ease of use. At the same time, galvanized steel sheets have been increasingly attracting attention from automakers in recent years due to their excellent manufacturability and rust-resistant properties. However, there are currently very few cold-rolled galvanized high-tensile steel sheets with large hole expansion capabilities. [Means for solving the problem]

[0005] Based on this, the present invention aims to develop a new cold-rolled steel sheet for galvanized steel sheet, which is lightweight and has very good properties, and is particularly suitable for forming high-strength automobile parts.

[0006] An object of the present invention is to provide a cold-rolled steel sheet for galvanized steel sheet having a yield strength of ≥ 650 MPa, a tensile strength of ≥ 980 MPa, a uniform elongation of ≥ 7%, an elongation at break of ≥ 13%, and a hole expansion ratio of ≥ 50%. This cold-rolled steel sheet has the characteristics of high ductility and high hole expansion ratio, and has both high overall ductility and good local plastic deformation ability, so that it has a well-balanced performance and is of great economic and social merit.

[0007] In order to achieve the above object, the present invention provides a cold-rolled steel sheet for galvanized steel sheet, which contains, in addition to Fe and unavoidable impurities, the following chemical elements in the following mass percentages: C:0.18~0.25%, Si:1.5~2.0%, Mn:1.5~2.3%, Nb:0.01~0.06%; The microstructure of the cold-rolled steel sheet is bainite+tempered martensite+retained austenite, and the volume fraction of bainite and tempered martensite is ≧95%.

[0008] Furthermore, in the cold rolled steel sheet according to the invention, the mass percentages of each chemical element are as follows: C: 0.18~0.25%, Si: 1.5~2.0%, Mn: 1.5~2.3%, Nb: 0.01~0.06%, and the balance is Fe and unavoidable impurities.

[0009] Furthermore, in the cold rolled steel sheet according to the present invention, the volume fraction of retained austenite is ≧1.0%, for example ≧1.5%.

[0010] Furthermore, in the cold-rolled steel sheet according to the present invention, the volume fraction of bainite and tempered martensite is 95-99%, and the volume fraction of retained austenite is 1-5%. In some embodiments, in the cold-rolled steel sheet according to the present invention, the volume fraction of bainite and tempered martensite is 95-98.5%, and the volume fraction of retained austenite is 1.5-5%.

[0011] Furthermore, in the cold-rolled steel sheet according to the present invention, the size of the prior austenite crystal grains formed by the annealing treatment is 10 um to 30 um, the resulting lath-like microstructure has a lath width of 0.5 um to 1.5 um, and the volume fraction of granular carbides precipitated between the laths is 2 to 5%.

[0012] In the present invention, the design principle of the chemical composition of the cold-rolled steel sheet is described as follows: C: In the cold-rolled steel sheet according to the present invention, the strength of the steel can be improved, martensite or bainite phase transformation can be reliably caused, and the strength can be ensured by adding an appropriate amount of element C. Therefore, in order to exert the beneficial effects of element C, the mass percentage of element C in the cold-rolled steel sheet according to the present invention is controlled to 0.18-0.25%.

[0013] The mass percentage of element C is controlled within the range of 0.18 to 0.25% because, if the mass percentage of element C is lower than 0.18%, the strength of bainite and tempered martensite formed after annealing does not meet the standard, and at the same time, the elongation is insufficient due to a lack of retained austenite, whereas, if the mass percentage of element C exceeds 0.25%, the strength of the produced martensite becomes too high, and the strength of the steel plate may become excessive.

[0014] Si: In the cold-rolled steel sheet according to the present invention, the addition of an appropriate amount of element Si can favorably improve the hardenability, and Si in the steel is beneficial for influencing the dislocation interaction, increasing the work hardening rate, improving the elongation rate of the steel, and obtaining better formability. In addition, Si also helps to suppress the precipitation of carbides in the grain boundaries and ensure a higher hole expansion ratio. For this reason, in order to exert the beneficial effects of element Si, it is necessary to control the mass percentage of element Si in the cold-rolled steel sheet according to the present invention to 1.5-2.0%.

[0015] Mn: In the cold-rolled steel sheet according to the present invention, adding an appropriate amount of element Mn is beneficial to improve the hardenability of the steel and ensure the strength of the steel sheet. However, it should be noted that if the content of element Mn in the steel is lower than 1.5%, the hardenability of the steel becomes insufficient, and it becomes difficult to complete the phase transformation in a short time during the cooling process, which affects the strength of the steel sheet. At the same time, the content of element Mn should not be too high. If the content of element Mn in the steel is higher than 2.3%, the martensite phase transition temperature becomes high, and the content of martensite generated after rapid cooling becomes too high, resulting in excessive strength of the steel sheet and reduced elongation. For this reason, in order to exert the beneficial effects of element Mn, the mass percentage of element Mn is controlled to 1.5-2.3% in the cold-rolled steel sheet according to the present invention.

[0016] Nb: In the cold-rolled steel sheet according to the present invention, the addition of an appropriate amount of element Nb promotes the refinement of crystal grains, improving the elongation and hole expansion ratio of the steel sheet. At the same time, the precipitation of an appropriate amount of diffusible niobium oxide can improve the strength in the bainite phase, reducing the interphase strength difference in the final annealed structure, making it difficult for cracks to occur under stress, and improving the hole expansion ratio. For this reason, in the cold-rolled steel sheet according to the present invention, the mass percent of element Nb is controlled to 0.01 to 0.06%.

[0017] Thus, in the present invention, the addition of C is mainly for martensite and bainite. The design of the Si content is to suppress the precipitation of carbides at the bainite interface. The design of the Mn content is to obtain a certain degree of hardenability and ensure the strength of the steel sheet, but the addition of Mn should not be too high to avoid the formation of hard martensite during cooling, which affects the hole expansion ratio. The element Nb can refine the grains, and at the same time, the precipitation of other diffusible Nb compounds improves the balance of the strength of different microstructures in the steel sheet. Therefore, the present invention can obtain cold rolled steel sheets with high hole expansion, high elongation, and high strength.

[0018] According to the above technical means, only main elements such as C, Si, Mn and a certain amount of element Nb are added, preferably adding element Ti of 0.013% or less, more preferably 0.01% or less, and by reasonably designing the element content ratio, a cold-rolled steel sheet having desired basic properties excellent in manufacturability and economy can be obtained.

[0019] In addition, in the present invention, the microstructure of the prepared cold-rolled steel sheet is bainite + tempered martensite + retained austenite, the phase ratio (volume fraction) of bainite and tempered martensite which are uniform lath-shaped is 95% or more, the retained austenite is 5% or less, and fine carbide precipitation occurs around the lath-shaped tempered martensite.

[0020] In the microstructure of the cold-rolled steel sheet manufactured according to the present invention, bainite and tempered martensite are dominant, and a better hole expansion rate can be obtained as compared with a duplex steel composed of martensite and ferrite.

[0021] Furthermore, the cold-rolled steel sheet according to the present invention further contains 0 < Ti ≤ 0.013%. In the above technical means of the present invention, Ti belongs to an element that is optionally added, and in the present invention, it is necessary to particularly control the content of element Ti. Since the compound of element Ti is likely to precipitate at the grain boundary and cause a significant decrease in the hole expansion rate, in the cold-rolled steel sheet according to the present invention, it is necessary to control the mass percentage of Ti element to 0 < Ti ≤ 0.013%.

[0022] Furthermore, in the cold-rolled steel sheet according to the present invention, among inevitable impurities, P ≤ 0.015%, S ≤ 0.003%, and N ≤ 0.005%.

[0023] In the cold-rolled steel sheet according to the present invention, P, S, and N are all inevitable impurity elements, and in order to ensure the quality of the steel, the lower the content of impurity elements in the steel, the better.

[0024] In the present invention, the contents of the elements P, S, and N in the steel must be strictly controlled, and the mass percentage of the element P must be controlled to P≦0.015%. Compounds such as MnS formed by the coordination of the element S affect the formability of the steel, so the mass percentage of the element S in the steel must be controlled to S≦0.003%.

[0025] In addition, the element N easily causes cracks and bubbles on the surface of slab products, and in order to ensure the quality of the products, it is necessary to control the mass percentage of the element N in the steel so as to satisfy N≦0.005%.

[0026] Furthermore, in the cold rolled steel sheet according to the present invention, the yield strength is ≧650 MPa, the tensile strength is ≧980 MPa, the uniform elongation is ≧7%, the elongation at break is ≧13%, and the hole expansion ratio is ≧50%.

[0027] Furthermore, in the cold rolled steel sheet according to the present invention, the yield strength is ≧700 MPa, the tensile strength is ≧990 MPa, the uniform elongation is ≧7%, the elongation at break is ≧13%, and the hole expansion ratio is ≧50%.

[0028] Furthermore, in the cold-rolled steel sheet according to the present invention, the cold-rolled steel sheet has a thickness of 1.2 to 1.8 mm.

[0029] The cold-rolled steel sheet of the present invention can be used as a base for a galvanized steel sheet to be plated on its surface. Therefore, another object of the present invention is to provide a galvanized steel sheet comprising the above-mentioned cold-rolled steel sheet of the present invention and a galvanized layer covering the surface of the cold-rolled steel sheet.

[0030] In addition, when the cold-rolled steel sheet according to the present invention is used as a base and the surface is further coated with a zinc layer, the galvanized cold-rolled steel sheet can have even better rust resistance, and the galvanized steel sheet obtained from this cold-rolled steel sheet is particularly suitable for forming high-strength automobile parts.

[0031] Another object of the present invention is to provide a method for producing the above cold-rolled steel sheet, comprising the following steps: (1) Obtaining billets through smelting and casting; (2) Hot rolling; (3) Cold rolling; (4) Annealing: The annealing temperature is 890-920℃, the soaking time is 80-150 seconds, and then the cooling rate is 30-100℃ / second to 270-350℃; (5) Overaging: The overaging temperature is 450~475℃, and the overaging time is 40~60 seconds.

[0032] Another object of the present invention is to provide a method for producing the above-mentioned galvanized steel sheet, which is simple and feasible, and can effectively prepare the above-mentioned galvanized steel sheet, and by using the method, the above-mentioned cold-rolled steel sheet of the present invention can be prepared, and the galvanized steel sheet can be further obtained based on the obtained cold-rolled steel sheet.

[0033] The method for producing a galvanized steel sheet of the present invention includes the following steps: (1) Obtaining billets through smelting and casting; (2) Hot rolling; (3) Cold rolling; (4) Annealing: The annealing temperature is 890-920℃, the soaking time is 80-150 seconds, and then the cooling rate is 30-100℃ / second to 270-350℃; (5) Overaging: the overaging temperature is 450~475℃, and the overaging time is 40~60 seconds; (6) Place in a zinc pot for galvanizing; (7) Alloying; (8)Leveling.

[0034] In the above-mentioned manufacturing method of the present invention, a cold-rolled steel sheet having good mechanical properties and usage characteristics can be effectively prepared according to the rational chemical composition design and the optimized manufacturing process, and the cold-rolled steel sheet is subjected to galvanizing, alloying and leveling treatment to obtain the galvanized steel sheet of the present invention.

[0035] The central improvement of the present invention is the optimal control of the parameters of the annealing and overaging processes, which, together with the chemical element composition designed by the inventor, can be used to obtain a cold-rolled steel sheet with a microstructure of bainite + tempered martensite + retained austenite, with a volume fraction of bainite and tempered martensite of ≧95%. The cold-rolled steel sheet prepared by the manufacturing method of the present invention has high ductility and high hole expansion ratio, and combines high overall ductility with good local plastic deformation ability, so that the performance is well-balanced. At the same time, the cold-rolled steel sheet can obtain excellent rust resistance after galvanization, and the galvanized steel sheet made from this cold-rolled steel sheet is particularly suitable for forming high-strength automotive parts.

[0036] In the annealing step of the above step (4) of the present invention, the annealing soaking temperature is set to 890 to 920°C and the soaking holding time is set to 80 to 150 seconds in order to obtain an appropriate austenite size and the size of the lath-shaped structure generated after phase transformation. Otherwise, if the annealing soaking temperature is lower than 890°C or the holding time is shorter than 80 seconds, the microstructure cannot be completely austenitized. Also, if the annealing soaking temperature is higher than 920°C or the holding time is longer than 150 seconds, the high-temperature austenite (also called old austenite) becomes too coarse, which affects the final mechanical properties of the steel.

[0037] In addition, if the above annealing soaking temperature is maintained within the temperature range according to the present invention, the occurrence of ferrite phase transformation is prevented, relatively soft ferrite is not generated, and the hole expansion ratio is reduced. Of course, in some preferred embodiments, in order to obtain a better implementation effect, the annealing soaking temperature can be preferably between 900 and 920°C.

[0038] Here, the size of the prior austenite crystal grains formed by the annealing treatment is 10 um to 30 um, the resulting lath-like microstructure has a lath width of 0.5 um to 1.5 um, and the volume fraction of granular carbides precipitated between the laths is 2 to 5%.

[0039] Therefore, in the annealing step of the above step (4) of the present invention, the cooling rate is limited to 30 to 100°C / s, which is higher than that of the prior art. That is, in the present invention, the cold-rolled steel sheet is rapidly cooled to 270 to 350°C. This rapid cooling can increase the recrystallization temperature, shorten the time to complete recrystallization, refine the crystal grains, and improve the overall mechanical properties of the steel sheet. Here, the heating step can specifically be performed by heating to the soaking temperature by transverse magnetic induction heating.

[0040] In the present invention, in order to maintain the strength of the steel sheet by rapidly generating a part of martensite, the cold-rolled steel sheet can be cooled to 270 to 350°C in the annealing process and held for a predetermined time, for example, 20 seconds. In the present invention, if the end temperature of the quenching is lower than 270°C, excessive martensite is generated and the strength becomes too high, and if the end temperature of the quenching is higher than 350°C, martensite is not generated. Then, the holding time is insufficient, and the bainite phase transformation does not occur completely, and ultimately the performance of the steel sheet cannot be guaranteed.

[0041] Therefore, in step (5) of the manufacturing method according to the present invention, the annealed cold rolled steel sheet is directly subjected to overaging treatment, and the temperature and time of overaging are strictly controlled. Bainite phase transformation occurs at this stage. Although the time required for bainite phase transformation is long, since a certain degree of martensite phase transformation has occurred in the previous stage, bainite phase transformation can be completely caused at this stage. Also, cementite can be obtained with an appropriate volume fraction.

[0042] Furthermore, in the manufacturing method according to the present invention, in step (2), the billet is heated to 1150 to 1250°C and held for 0.5 hours or more, and then hot rolled at a temperature of Ac3 (the end temperature at which all ferrite is converted to austenite during heating) or higher, and then cooled at a rate of 30 to 100°C / s to a coiling temperature of 450 to 750°C, and coiled.

[0043] In the hot rolling step of the above step (2) of the present invention, cooling is required after the hot rolling, and then coiling is performed, and the coiling temperature is controlled to 450 to 750° C. Preferably, in some preferred embodiments, the coiling temperature can be controlled to 450 to 550° C.

[0044] Compared with the prior art, the present invention is carried out at a low coiling temperature of 450-750°C. This is the coiling temperature adapted to the composition ratio of the present invention. In the composition design of the steel sheet of the present invention, if the coiling temperature is too high, the external oxidation of the surface becomes more serious, the shape becomes non-uniform, and it is easy to lead to uneven heating, which affects the surface quality of the final product. On the other hand, if the coiling temperature is too low, it will lead to the formation of martensite, the strength will be too high, and the cold rolling process will be difficult.

[0045] Therefore, it should be noted that in the heating step of the present invention, the billet hot rolling holding time may be more than 0.5 hours, for example between 0.5 and 3 hours, and in some preferred embodiments, may even be between 0.5 and 2 hours.

[0046] In the present invention, in the hot rolling step (2), the microstructure of the steel sheet is mainly ferrite and pearlite.

[0047] Furthermore, in the production method according to the present invention, the cold rolling deformation rate in step (3) is 30 to 65%, preferably 40 to 65%.

[0048] Furthermore, in the production method according to the present invention, the zinc pot temperature in step (6) is 450 to 475°C.

[0049] Furthermore, in the manufacturing method according to the present invention, the alloying temperature in step (7) is 500 to 530° C., and the time is 20 to 40 seconds.

[0050] In the above technical means of the present invention, the alloying temperature may be preferably set to 500 to 530° C. and the alloying time may be set to 20 to 40 seconds. By alloying and martensite tempering treatment of the galvanized steel sheet, an Fe-Zn alloy can be obtained at the coating layer interface, and the martensite structure formed initially is converted to tempered martensite, so that the strength approaches that of bainite and the elongation and toughness of the steel are improved.

[0051] Furthermore, in the production method according to the present invention, in step (8), leveling is performed at a leveling rate of ≦0.3%.

[0052] In the leveling treatment in step (8), the leveling may be performed by a commonly used method, for example, the leveling may be performed so that the leveling rate is 0 to 0.3%. Of course, in some preferred embodiments, the leveling may be further performed at a leveling rate of 0 to 0.2%.

[0053] Compared with the prior art, the cold rolled steel sheet for galvanized steel sheet and its manufacturing method according to the present invention have the following advantages and beneficial effects: In the present invention, the inventors have rationally designed the chemical composition of the cold-rolled steel sheet and optimized the design of the manufacturing process. By carrying out the appropriate annealing process and overaging process, a cold-rolled steel sheet with both high ductility and high hole expansion ratio can be effectively prepared. This cold-rolled steel sheet has both high overall ductility and good local plastic deformation ability. Its yield strength is ≧650MPa, tensile strength is ≧980MPa, uniform elongation is ≧7%, elongation at break is ≧13%, and hole expansion ratio is ≧50%, and its well-balanced performance has great economic and social benefits.

[0054] Based on the above design, the cold rolled steel sheet of the present invention has a microstructure of bainite + tempered martensite + retained austenite, in which the volume fraction of uniform lath bainite and tempered martensite is ≧95%, and fine carbide precipitation is present around the lath tempered martensite. The microstructure of the cold rolled steel sheet is dominated by bainite and tempered martensite, and it has a better hole expansion ratio than the prior art dual phase steels consisting of martensite and ferrite.

[0055] By subjecting the obtained cold-rolled steel sheet to galvanization using the manufacturing method of the present invention, a galvanized steel sheet having excellent rust resistance while retaining the excellent properties of a cold-rolled steel sheet can be obtained. This galvanized steel sheet is particularly suitable for forming high-strength automobile parts and components, and can be effectively used in the manufacture of automobile structural parts to realize weight reduction of automobiles, and has great economic and social benefits. [Brief description of the drawings]

[0056] [Figure 1] FIG. 1 is a photograph of the fine microstructure of the cold-rolled steel sheet of Example 2 observed with a scanning electron microscope (SEM). [Diagram 2] FIG. 2 is a photograph of the fine microscopic structure of the cross-sectional plating layer of the zinc-plated steel sheet of Example 2, observed with a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] The following provides a more detailed description of the cold rolled steel sheet for galvanized steel and the manufacturing method thereof according to the present invention based on specific examples, but the description does not limit the technical solution of the present invention.

[0058] Examples 1 to 6 and Comparative Examples 1 to 5 Table 1 shows the mass percentages of the chemical elements in the galvanized steel sheets of Examples 1-6 and the cold-rolled steel sheets of Comparative Examples 1-5.

[0059] [Table 1]

[0060] The galvanized steel sheets of Examples 1 to 6 according to the present invention were all produced by the following process: (1) Billets were obtained by smelting and casting according to the chemical composition shown in Table 1.

[0061] (2) Hot rolling: The billet is heated to 1150 to 1250°C and held for 0.5 hours or more, and then hot rolled at or above the Ac3 temperature (the end temperature at which all ferrite transforms to austenite during heating). The billet is then cooled to the coiling temperature at a rate of 30 to 100°C / s, and then coiled so that the coiling temperature is 450 to 750°C, preferably 450 to 550°C.

[0062] (3) Cold rolling: The cold rolling deformation rate was set to 30 to 65%. (4) Annealing: The annealing temperature was 890 to 920°C, preferably 9000 to 920°C, the soaking time was 80 to 150 seconds, and then the temperature was cooled to 270 to 350°C at a cooling rate of 30 to 100°C / second.

[0063] (5) Overaging: The overaging temperature was 450 to 475°C, and the overaging time was 40 to 60 seconds.

[0064] (6) Place in a zinc pot for galvanizing: the temperature of the zinc pot was 450-475°C, and the hot-dip galvanizing time was 10-15 seconds.

[0065] (7) Alloying: The alloying temperature was 500 to 530°C, and the time was 20 to 40 seconds. (8) Leveling: Leveling was carried out so that the leveling rate was 0 to 0.3%, preferably 0 to 0.2%.

[0066] In step (1), the smelting and casting steps adopted in Examples 1 to 6 and Comparative Examples 1 to 5 are the same, and all are existing conventional operational steps. For example, in a specific operation, molten steel is subjected to RH vacuum degassing treatment and LF furnace desulfurization treatment, and then continuously cast to obtain a slab of a required length, and obvious defects on the slab surface are removed before hot rolling in the subsequent step (2).

[0067] In the present invention, based on the process flow of steps (1) to (5) above, the cold-rolled steel sheets of Examples 1 to 6 and Comparative Examples 1 to 5 can be obtained with the chemical elements of the cold-rolled steel sheets designed in Table 1. Based on the obtained cold-rolled steel sheets, the corresponding galvanized steel sheets can be obtained by carrying out galvanization, alloying, and leveling through steps (6) to (8).

[0068] The galvanized steel sheets of Examples 1 to 6 according to the present invention were all produced through the above steps (1) to (8), and their chemical compositions and related process parameters met the design specifications of the present invention.

[0069] In addition, unlike Examples 1 to 6, the comparative steels of Comparative Examples 1 to 5 were manufactured by the above-mentioned manufacturing process, but their chemical composition design and specific manufacturing process have parameters that do not satisfy the design specifications of the present invention.

[0070] Tables 2-1 and 2-2 show specific parameters in the steps of the above-mentioned manufacturing method for the galvanized steel sheets of Examples 1 to 6 and the comparative steel sheets of Comparative Examples 1 to 5.

[0071] [Table 2-1]

[0072] [Table 2-2]

[0073] In the present invention, after completing the process flow of steps (1) to (5) above and obtaining cold-rolled steel sheets from the chemical compositions designed in Examples 1 to 6 and Comparative Examples 1 to 5 in Table 1, the cold-rolled steel sheets of Examples 1 to 5 and Comparative Examples 1 to 5 formed in the above steps were sampled, and the microstructures of the sample substrates of Examples 1 to 6 and Comparative Examples 1 to 5 were further observed. After observing the microstructures obtained for each of the Examples and Comparative Examples, performance tests were further performed on the cold-rolled steel sheets formed in Examples 1 to 5 and Comparative Examples 1 to 5.

[0074] In the present invention, the samples of the examples and comparative examples were polished and etched with 4% diluted nitric acid, and then the metallographic structures of the examples and comparative examples were observed using an optical microscope and a scanning electron microscope. Among these, the volume fraction of bainite was determined by the structure morphology, and multiple microstructures were counted and averaged, and the volume fraction of retained austenite was determined by XRD (X-ray diffraction). The observation results are shown in Table 3 below.

[0075] Table 3 shows the results of observation of the metallographic structure of the cold-rolled steel sheets of Examples 1-6 and Comparative Examples 1-6.

[0076] [Table 3]

[0077] As can be seen from Table 3, the microstructures of Examples 1 to 6 are all bainite + tempered martensite + retained austenite, and the volume fractions of bainite and tempered martensite are all ≧95%. Of these, the bainite and tempered martensite are in a lath shape, and fine carbide precipitates are present around the lath-shaped tempered martensite.

[0078] Therefore, the samples of the examples and comparative examples were subjected to performance tests, the component measurements of the examples and comparative examples were in accordance with GB / T 223 standard, the yield strength, tensile strength and elongation were in accordance with GB / T228-2002 standard, and the elongation range was 50m. The hole expansion ratio test was in accordance with GB / T15825.4-2008 standard, and the final results are shown in Table 4 below.

[0079] Table 4 shows the performance test results of the cold-rolled steel sheets of Examples 1-6 and the cold-rolled steel sheets of Comparative Examples 1-5.

[0080] [Table 4]

[0081] As can be seen from Table 4, in the present invention, the cold-rolled steel sheets of Examples 1 to 6 have a yield strength of 719 to 880 MPa, a tensile strength of 998 to 1056 MPa, a uniform elongation of 7.2 to 9.1%, and a fracture elongation of 13.6 to 14.3%. In addition, the cold-rolled steel sheets of Examples 1 to 6 also have a high hole expansion ratio of 51.4 to 56.3%.

[0082] As can be seen from the above examples, the present invention provides a cold-rolled steel sheet with high ductility and high hole expansion ratio characteristics, which has both high overall ductility and good local plastic deformation ability and has a well-balanced performance, through appropriate chemical composition design and optimized manufacturing process.

[0083] At the same time, this cold-rolled steel sheet can obtain excellent rust resistance after galvanization, and the galvanized steel sheet made from this cold-rolled steel sheet is particularly suitable for forming high-strength automobile parts.

[0084] Therefore, unlike Examples 1 to 6, the cold rolled steel sheets prepared in Comparative Examples 1 to 6 have significantly inferior properties to those of Examples 1 to 6 due to the presence of parameters that do not satisfy the design requirements of the present invention.

[0085] In Comparative Example 1, the carbon content is too low, so the tensile strength of the steel sheet is less than 980 MPa, and the C content is low, so the Ac3 temperature increases and it is difficult to completely austenitize at the annealing temperature of 875°C, and some ferrite remains. As a result, a product steel with a relatively soft ferrite phase exists in the final structure and a low hole expansion ratio is obtained.

[0086] In Comparative Example 2, the content of the element Mn in the steel is too low, so that the tensile strength of the steel sheet does not meet the requirements.

[0087] In Comparative Example 3, the content of element Si in the steel is too low, resulting in a low elongation rate of the steel sheet, and the content of element Ti is high, which makes it easy for precipitates such as TiC to form, causing cracks during the hole expansion process and resulting in a low hole expansion rate of the steel sheet.

[0088] In Comparative Example 4, the content of element Nb in the steel is too low, so that a sufficient Nb compound precipitation phase cannot be obtained. As a result, the interphase strength difference in the microstructure of the finally completed cold-rolled steel sheet becomes large, and the hole expansion ratio cannot meet the requirement.

[0089] In Comparative Example 5, since the content of the element Mn in the steel is high, the yield strength and tensile strength of the final cold-rolled steel sheet are too high, while the elongation is low.

[0090] In Comparative Example 6, the composition meets the requirements, but the annealing process does not meet the requirements, and an appropriate microstructure cannot be obtained. As a result, the yield strength and tensile strength of the final cold-rolled steel sheet are low, and the hole expansion ratio is low at less than 50%.

[0091] FIG. 1 is a photograph of the fine microstructure of the cold-rolled steel sheet of Example 2 observed with a scanning electron microscope (SEM).

[0092] FIG. 1 shows the metallographic microstructure of the cold-rolled steel sheet of Example 2 after treatment by the annealing process. As shown in FIG. 1, the microstructure of the obtained cold-rolled steel sheet is bainite + tempered martensite + retained austenite. It can be seen that the volume fraction of lath-shaped bainite and tempered martensite in the cold-rolled steel sheet of Example 2 is 97.3%, and the volume fraction of retained austenite is 2.7%.

[0093] FIG. 2 is a photograph of the fine microscopic structure of the cross-sectional plating layer of the zinc-plated steel sheet of Example 2, observed with a scanning electron microscope (SEM).

[0094] As shown in Figure 2, in the zinc-coated steel sheet of Example 2, the upper shiny layer is the alloyed plating layer A, which is uniform and continuous and has a thickness of about 10 um. The lower dark layer is the substrate B, to which the alloyed plating layer A is well connected.

[0095] It should be explained that the part of the prior art within the protection scope of the present invention is not limited to the examples provided in this application, and any technical solutions that do not contradict the technical solutions of the present invention, including but not limited to prior patent documents, prior disclosure publications, prior disclosure applications, etc., fall within the protection scope of the present invention.

[0096] Furthermore, the combination methods of each technical feature in this application are not limited to the combination methods described in the claims of this application or the combination methods described in the specific examples, and all technical features described in this application may be freely combined or combined in any manner as long as they are not in contradiction with each other.

[0097] Furthermore, it should be noted that the above-mentioned embodiments are merely specific embodiments of the present invention, and the present invention is not limited to the above-mentioned embodiments, and similar changes and modifications thereof can be directly obtained or easily conceived by those skilled in the art from the disclosure of the present invention, and therefore, they are within the protection scope of the present invention.

Claims

1. A cold-rolled steel sheet for galvanized steel sheet containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages: C: 0.18-0.25%, Si: 1.5-2.0%, Mn: 1.5-2.3%, Nb: 0.01-0.06%; the microstructure of the cold-rolled steel sheet is bainite + tempered martensite + retained austenite, and the volume fraction of bainite and tempered martensite is ≧95%.

2. 2. The cold rolled steel sheet according to claim 1, wherein the mass percentages of each chemical element are as follows: C: 0.18-0.25%, Si: 1.5-2.0%, Mn: 1.5-2.3%, Nb: 0.01-0.06%, and the balance is Fe and unavoidable impurities.

3. The cold-rolled steel sheet according to claim 1 or 2, further comprising, in mass percent, 0<Ti≦0.013%.

4. The cold-rolled steel sheet according to claim 1 or 2, characterized in that the unavoidable impurities are, in mass percent, P≦0.015%, S≦0.003%, and N≦0.005%.

5. The cold-rolled steel sheet has a yield strength of ≧650 MPa, a tensile strength of ≧980 MPa, a uniform elongation of ≧7%, a fracture elongation of ≧13%, and a hole expansion ratio of ≧50%, preferably, the cold-rolled steel sheet has a yield strength of ≧700 MPa, a tensile strength of ≧990 MPa, a uniform elongation of ≧7%, a fracture elongation of ≧13%, and a hole expansion ratio of ≧50%.

6. The cold-rolled steel sheet according to claim 1 or 2, characterized in that in the cold-rolled steel sheet, the size of prior austenite crystal grains formed by annealing treatment is 10 um to 30 um, the resulting lath-like microstructure has a lath width of 0.5 um to 1.5 um, and the volume fraction of granular carbides precipitated between the laths is 2 to 5%.

7. The cold-rolled steel sheet according to claim 1 or 2, characterized in that the cold-rolled steel sheet has a volume fraction of retained austenite of ≧1.0%, for example ≧1.5%, preferably the cold-rolled steel sheet has a volume fraction of bainite and tempered martensite of 95-99% and a volume fraction of retained austenite of 1-5%, preferably the cold-rolled steel sheet has a volume fraction of bainite and tempered martensite of 95-98.5% and a volume fraction of retained austenite of 1.5-5%.

8. A galvanized steel sheet comprising the cold-rolled steel sheet according to any one of claims 1 to 7 and a galvanized layer covering at least one side surface of the cold-rolled steel sheet.

9. A method for producing a cold-rolled steel sheet according to any one of claims 1 to 7, comprising the following steps: (1) Obtaining billets through smelting and casting; (2) hot rolling; (3) cold rolling; (4) Annealing: the annealing soaking temperature is 890-920°C, the soaking holding time is 80-150 seconds, and then cooling to 270-350°C at a cooling rate of 30-100°C / s; (5) Overaging: The overaging temperature is 450-475°C, and the overaging time is 40-60 seconds.

10. The method for producing a cold-rolled steel sheet according to claim 8, characterized in that it comprises the following steps: (1) Obtaining billets through smelting and casting; (2) hot rolling; (3) cold rolling; (4) Annealing: the annealing soaking temperature is 890-920°C, the soaking holding time is 80-150 seconds, and then cooling to 270-350°C at a cooling rate of 30-100°C / s; (5) Overaging: the overaging temperature is 450-475°C, and the overaging time is 40-60 seconds; (6) Place in a zinc pot for galvanizing; (7) Alloying; (8) Leveling.

11. The manufacturing method according to claim 9 or 10, characterized in that in the step (2), the billet is heated to 1150 to 1250°C, held for 0.5 hour or more, and then hot rolled at an Ac3 temperature or higher, and then cooled at a rate of 30 to 100°C / sec to a coiling temperature of 450 to 750°C, and then coiled.

12. The method according to claim 9 or 10, characterized in that in the step (3), the cold rolling deformation rate is 30 to 65%.

13. The method according to claim 10, characterized in that in step (6), the zinc pot temperature is 450 to 475°C.

14. The method according to claim 10, characterized in that in step (7), the alloying temperature is 500 to 530°C and the time is 20 to 40 seconds.

15. The method according to claim 10, characterized in that in step (8), the leveling is performed at a leveling rate of ≦0.3%.

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