Cold-rolled IF steel with low yield ratio, hot-dip galvanized steel sheet and manufacturing method of cold-rolled IF steel

Abstract

The invention discloses cold-rolled IF steel with a low yield ratio, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.0035 to 0.006 percent of C, 0.20 to 0.50 percent of Mn, 0.02 to 0.040 percent of P, 0.016 to 0.04 percent of Nb, 0.015 to 0.025 percent of Ti, 0.002 to 0.006 percent of V, 0.01 to 0.08 percent of Al, 0.015 to 0.04 percent of Zr, 0.0003 to 0.0008 percent of B and the balance of Fe. And (Nb * 12) / (C * 93) + ((Ti-0. 0137) * 12) / (C * 48) is greater than or equal to 1.1 and less than or equal to 1.5, and each chemical element in the formula is substituted into a numerical value before the mass percentage of the chemical element. The invention further discloses a hot-dip galvanized steel plate, a base plate of the hot-dip galvanized steel plate is the cold-rolled IF steel, and a hot-dip galvanized layer is plated on the base plate.

Description

Technical Field

[0001] This invention relates to steel plates and their manufacturing methods, and more particularly to a cold-rolled steel and its manufacturing method. Background Technology

[0002] With the development of new energy vehicles, the requirements for steel forming and strength are also constantly increasing. Not only are high strength requirements required, but also good elongation is required to meet the needs of simplified forming process and complex forming.

[0003] Vehicle exterior panels requiring high and complex deformation typically use deep-drawing or even ultra-deep-drawing steels, which generally have lower strength levels and weaker resistance to deformation, making it difficult to meet users' demands for cost reduction, energy conservation, emission reduction, and safety performance. On the other hand, high-strength steels usually do not have high elongation and low yield strength ratio, resulting in lower design flexibility, stamping stability, processing efficiency, forming accuracy, and yield.

[0004] IF steel, a common raw material for automotive sheet metal, is characterized by low yield strength and high elongation. It primarily utilizes Ti / Nb elements to fix C and N atoms in the intergranular spaces, while adding Mn, Si, and P elements for solid solution strengthening. Because Ti and Nb elements form fine precipitates during the hot rolling and coiling stages of strip preparation, they contribute to the refinement of the strip's microstructure, thereby increasing the elongation of the finished product. Simultaneously, since there is no interstitial atomic solid solution in the matrix phase of this strip, it possesses a low yield strength, which helps reduce the yield-to-tensile ratio, making stamping easier. Therefore, it features high elongation, low yield-to-tensile ratio, good deep-drawing performance, and no aging phenomenon, making it widely used in the production of automotive outer panels.

[0005] Currently, the mainstream steel grades used in manufacturing automotive outer panels are CR3, CR4, and 180YD. Their elongation meets the requirements for large deformation, but their strength is relatively low. On the other hand, steel grades with strength levels of 340 or 390 have relatively low elongation, high yield strength (above 230MPa), and a yield-to-tensile strength ratio of around 0.65. They also have poor ability to coordinate deformation and are more difficult to stamp.

[0006] In practical applications, the aforementioned steel grades cannot simultaneously meet the requirements of lightweight automotive design and complex forming. Therefore, it is necessary to improve the strength after stamping while maintaining the low yield strength ratio and high elongation characteristics of low-strength steel grades. Thus, to ensure that the product meets the user's requirements for vehicle body panel design, stamping, and weight reduction, the steel sheet needs to not only meet material strength standards but also control its elongation and yield strength ratio. In existing technologies:

[0007] For example, Chinese patent document CN110172637A, published on August 27, 2019, entitled "A 340MPa grade high-strength interstitial atomic steel strip for deep drawing and its preparation method", discloses a high-strength IF steel and its production method. Its composition and mass fraction are: C≤0.0035%, Si≤0.03%, Mn:0.08-0.20%, P:0.050-0.080%, S≤0.015%, Als:0.015-0.060%, Ti:0.03-0.09%, B:0.0002-0.0014%, N≤0.0045%, and the weight percentage of the above elements satisfies 0.60%≤Mn+10P≤0.90%, 0.01%≤Ti-(3.4N+1.5S+4C)≤0.045%, with the balance being Fe and unavoidable impurities. The aforementioned patent documents mainly use Ti to fix C and N elements and P as the main solid solution component. It has the characteristics of high strength and elongation, low cost and easy implementation. However, its high yield strength will lead to increased difficulty in material stamping.

[0008] For example, Chinese patent document CN106987771A, published on July 28, 2017, entitled "An Extremely Low Yield Strength Steel Plate and Its Production Method," discloses an extremely low yield strength IF steel and its production method. Its composition and mass fraction are: C≤0.0050%, Si≤0.03%, Mn: 0.10-0.20%, P≤0.013%, S≤0.012%, Als: 0.020~0.050%, Ti: 0.055-0.070%, N≤0.0050%, with the balance being Fe and unavoidable impurities. This patent document only uses Ti as the forming element of IF steel and employs a composition with low C and Mn content to give it the characteristics of an extremely low yield strength ratio and high elongation, but it does not consider the impact of excessively low yield strength on automotive exterior painting, transportation, and automotive lightweighting. Summary of the Invention

[0009] One of the objectives of this invention is to provide a cold-rolled IF steel with a low yield strength ratio, which combines high strength, high elongation and low yield strength ratio, and has good formability and strong deformation capacity.

[0010] To achieve the above objectives, the present invention provides a cold-rolled IF steel with a low yield strength ratio, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages:

[0011] C: 0.0035-0.006%, Mn: 0.20-0.50%, P: 0.02-0.04%, Nb: 0.016-0.04%, Ti: 0.015-0 .025%, V: 0.002%-0.006%, Al: 0.01-0.08%, Zr: 0.015-0.04%, B: 0.0003-0.0008%;

[0012] It also satisfies: 1.1≤(Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48)≤1.5, where each chemical element is represented by the value before the percentage sign of its mass percentage content.

[0013] Furthermore, in the cold-rolled IF steel described in this invention, the mass percentage content of each chemical element is as follows:

[0014] C: 0.0035-0.006%, Mn: 0.20-0.50%, P: 0.02-0.04%, Nb: 0.016-0.04%, Ti: 0.015-0.025%, V: 0.002%-0.006%, Al: 0.01-0.08%, Zr: 0.015-0.04%, B: 0.0003-0.0008%; balance Fe and unavoidable impurities.

[0015] The design principles of each chemical element in the cold-rolled IF steel described in this invention are as follows:

[0016] C: In the cold-rolled IF steel described in this invention, carbon (C) is a constituent element of the precipitated strengthening phase. The carbides formed by C with Ti and Nb can achieve both grain refinement and second-phase strengthening effects. When the mass percentage of C is less than 0.0035%, the precipitated phase content is insufficient, failing to effectively improve the tensile strength of the IF steel and easily leading to a low yield strength, which negatively impacts the dent resistance and deformation resistance of stamped parts. When the mass percentage of C is greater than 0.006%, the precipitated phase tends to coarsen, and C partially dissolves in the matrix, which is detrimental to the grain refinement, aging resistance, and elongation of the IF steel. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage of C is controlled between 0.0035% and 0.006%.

[0017] Mn: In the cold-rolled IF steel described in this invention, Mn acts as a solid solution strengthening element, effectively increasing the strength of the IF steel. When the mass percentage content of Mn is less than 0.2%, its contribution to the yield strength is small, failing to effectively improve the strength of the IF steel. When the mass percentage content of Mn is greater than 0.5%, it is detrimental to reducing the yield strength ratio of the IF steel. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of Mn is controlled between 0.20% and 0.50%.

[0018] P: In the cold-rolled IF steel described in this invention, phosphorus (P) acts as a solid solution strengthening element, effectively improving the yield strength of the IF steel. When the mass percentage content of P is less than 0.02%, its contribution to the yield strength is small, and its improvement on the strength of the IF steel is limited. When the mass percentage content of P is greater than 0.04%, it is not conducive to reducing the yield strength ratio of the IF steel. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of P is controlled between 0.02% and 0.04%.

[0019] Nb: In the cold-rolled IF steel described in this invention, Nb mainly forms carbide precipitation with C. The fine, dispersed carbides precipitated can effectively refine the grains and act as dislocation pins. After annealing, a low-content carbide precipitation band can be formed, which can improve the tensile strength of IF steel while reducing the yield strength ratio. When the mass percentage of Nb is less than 0.016%, it is insufficient to completely fix C atoms. When the mass percentage of Nb is greater than 0.04%, the excess Nb atoms after fixing C atoms dissolve in the IF steel, which has limited effect on grain refinement and strength improvement, and is also costly. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage of Nb is controlled between 0.016% and 0.04%.

[0020] Ti: In the cold-rolled IF steel described in this invention, Ti acts as a carbon, nitrogen, and sulfide-forming element, effectively refining the grain size and improving the material strength. When the mass percentage content of Ti is less than 0.015%, the Nb content needs to be increased to fix the C and N atoms, which is detrimental to cost control. When the mass percentage content of Ti is higher than 0.025%, the role of Nb is weakened, and excessive Ti carbides precipitate at high temperatures, which is not conducive to subsequent grain refinement in IF. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of Ti is controlled between 0.015% and 0.025%.

[0021] V: In the cold-rolled IF steel described in this invention, element V serves as a solid solution strengthening and second-phase strengthening element, while simultaneously optimizing the morphology and quantity of Nb precipitates. When the mass percentage content of element V is less than 0.002%, it has no significant effect on improving the strength of IF steel; when the mass percentage content of element V is greater than 0.006%, it is detrimental to reducing the yield strength ratio and controlling costs. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of element V is controlled between 0.002% and 0.006%.

[0022] Al: In the cold-rolled IF steel described in this invention, Al acts as an impurity element in the deoxidizer and can also fix free nitrogen atoms. When the mass percentage content of Al is less than 0.01%, the nitrogen fixation and deoxidation effects cannot be guaranteed; when the mass percentage content of Al is greater than 0.08%, it easily dissolves in the matrix, reducing the material's plasticity and generating more inclusions, reducing the uniformity of the IF steel grain size, which is detrimental to improving the elongation of the IF steel. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of Al is controlled between 0.01% and 0.08%.

[0023] Zr: In the cold-rolled IF steel described in this invention, Zr plays a role in deoxidation and reducing the quantity and size of oxide inclusions. Its deoxidation constant K value is smaller than that of Al, allowing for faster oxide formation, effectively reducing oxygen content, and simultaneously decreasing the quantity and size of oxide inclusions. Furthermore, its oxide density is close to that of molten steel, making it easier to distribute in a fine, dispersed state within the steel matrix. This effectively optimizes the strength and microstructure uniformity of the steel, improving the elongation of the IF steel. When the mass percentage content of Zr is less than 0.015%, the above effects are not significant; when the mass percentage content of Zr is higher than 0.04%, the improvement in the microstructure and mechanical properties of the IF steel no longer significantly increases, and it is detrimental to cost control. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of Zr is controlled between 0.015% and 0.04%.

[0024] B: In the cold-rolled IF steel described in this invention, the appropriate addition of element B can increase the grain boundary bonding strength, significantly improve the strength of IF steel, and simultaneously inhibit the precipitation of element P at the grain boundaries, reducing secondary processing brittleness. When the mass percentage content of element B is less than 0.0003%, the above effects are not significant; when the mass percentage content of element B is greater than 0.0008%, it is not conducive to the formation of {111} texture. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of element B is controlled between 0.0003% and 0.0008%.

[0025] It should be noted that, while controlling the mass percentage content of individual elements, this invention also needs to control the range of (Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48) to be between 1.1 and 1.5. This is a key factor in ensuring that the steel coil described in this invention has a low yield strength ratio, and also an important factor in ensuring that the steel coil has a high elongation. When the mass percentage content of Nb and C elements is lower than the lower limit of the range of (Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48), it is impossible to ensure that C and N elements are sufficiently fixed, and the grain refinement strengthening and second phase strengthening effects are weakened. When the mass percentage content of Nb and C elements is higher than the upper limit of the range of (Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48), it is not conducive to process control, favorable texture formation, and cost control.

[0026] Furthermore, in the unavoidable impurities of the cold-rolled IF steel described in this invention, S≤0.008%, N≤0.004%, and Si≤0.001%.

[0027] In the cold-rolled IF steel described in this invention, S, N, and Si are unavoidable impurities. To ensure the good performance of the cold-rolled IF steel described in this invention, their content is expected to be as low as possible, where conditions permit. Wherein:

[0028] S: In the cold-rolled IF steel described in this invention, sulfur (S) is controlled as a harmful impurity element. It forms low-melting-point precipitates in the steel, causing hot brittleness and negatively impacting weldability. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of sulfur can be controlled to S ≤ 0.008%.

[0029] N: In the cold-rolled IF steel described in this invention, dissolved nitrogen atoms can cause a decrease in the material's r-value and induce aging, which is detrimental to the material's resistance to aging and its stamping performance. Therefore, in the cold-rolled IF steel described in this invention, the mass percentage content of nitrogen can be controlled to N ≤ 0.004%.

[0030] Si: Si is an impurity element produced during steelmaking. It tends to segregate at grain boundaries, forming a hot-brittle phase. Excessive Si content negatively impacts coating adhesion and affects the material's anisotropy and plasticity. In the cold-rolled IF steel described in this invention, the mass percentage of Si can be controlled to ≤0.001%.

[0031] Furthermore, the microstructure grain size of the cold-rolled IF steel described in this invention is grade 9.0-10.0.

[0032] In this invention, controlling the grain size level between 9.0 and 10.0 ensures that the material possesses high strength and good ductility while avoiding an increase in yield strength that would lead to an increase in the yield ratio. When the grain size level of the cold-rolled IF steel is below 9.0, the material strength is insufficient; when the grain size level is above 10.0, the yield strength increases significantly, which is detrimental to reducing the yield ratio. Therefore, in the cold-rolled IF steel described in this invention, the grain size level can be controlled to 9.0-10.0.

[0033] Furthermore, in the cold-rolled IF steel described in this invention, the B-class inclusion rating image level i is 0.5 or 1.

[0034] In this invention, the B-type inclusion rating level i of cold-rolled IF steel is used to characterize the precipitation size of oxides in the cold-rolled IF steel. Excessively large or numerous inclusions in the steel will reduce the elongation of the IF steel. When the B-type inclusion rating level i in the cold-rolled IF steel is 0.5 or 1, it can alleviate stress concentration in the material, effectively delay necking during tensile deformation, and improve the elongation of the IF steel.

[0035] Furthermore, the cold-rolled IF steel described in this invention meets the following performance requirements: yield strength ≤ 220 MPa, tensile strength ≥ 370 MPa, yield-to-tensile ratio ≤ 0.595, elongation ≥ 40%, and plastic strain ratio r. 90 ≥2.2.

[0036] Another objective of this invention is to provide a hot-dip galvanized steel sheet that possesses high strength, high elongation, low yield strength ratio, and excellent deep-drawing performance, thereby meeting users' requirements for complex material formability, stamping efficiency, and energy saving and weight reduction.

[0037] To achieve the above objectives, the present invention also provides a hot-dip galvanized steel sheet, wherein the substrate is the aforementioned cold-rolled IF steel, and the substrate is coated with a hot-dip galvanized layer.

[0038] Another objective of this invention is to provide a method for manufacturing cold-rolled IF steel. This method is based on the control of the production process and the elemental composition of the steel plate, combined with a series of measures such as microstructure regulation, inclusion refinement, and grain boundary strengthening. This enables the steel to have the characteristics of high elongation, high strength, low yield strength ratio, and excellent deep-drawing performance, thereby enabling the steel to meet the needs of complex forming, high stamping requirements, strong safety, and energy saving and weight reduction.

[0039] To achieve the above objectives, the present invention also provides a method for manufacturing cold-rolled IF steel, comprising the steps of:

[0040] A slab is obtained;

[0041] Hot rolling: The furnace exit temperature of hot-rolled steel billets is 1150℃~1200℃, the finishing rolling temperature is 880℃~910℃, and the coiling temperature is 590℃~630℃.

[0042] Cold rolling;

[0043] Annealing: Control the annealing temperature between 780℃ and 830℃, the holding time between 70s and 120s, and the heating rate between 5-10℃ / min;

[0044] smooth.

[0045] In the hot rolling step of this invention, if the furnace exit temperature of the hot-rolled billet is too low, it is not conducive to uniform heating of the billet; if the hot-rolled billet temperature is too high, it is easy to cause an increase in iron oxide scale and coarsening of the original austenite grains, which is not conducive to improving the grain size level of IF steel. Based on this, in the manufacturing method described in this invention, the furnace exit temperature of the hot-rolled billet is controlled at 1150℃~1200℃.

[0046] In the hot rolling step of this invention, if the finishing rolling temperature is too high, it can easily lead to an increase in grain size, making it impossible to reduce the grain size level of IF steel; if the finishing rolling temperature is too low, it can easily increase the rolling difficulty and cause a significant increase in the yield strength of the finished product, which is not conducive to reducing the yield strength ratio. Based on this, in the manufacturing method described in this invention, the finishing rolling temperature of the hot-rolled steel billet is controlled between 880 and 910°C.

[0047] In the hot rolling step described above in this invention, if the coiling temperature is too low, it can easily lead to deterioration of processing performance, making coiling difficult and increasing the difficulty of precipitate precipitation, which is detrimental to subsequent grain size control. If the coiling temperature is too high, it can easily lead to grain growth and the growth of precipitates, which is detrimental to reducing the yield strength ratio and increasing the strength and elongation of the steel. Based on this, in the manufacturing method described in this invention, the coiling temperature of the steel coil is controlled between 590 and 630°C.

[0048] In the annealing step described above in this invention, if the annealing temperature is too low, recrystallization is insufficient, which is detrimental to improving the elongation and tensile strength of the material; if the annealing temperature is too high, coarsening of the precipitated phase and abnormal grain growth are likely to occur, which is also detrimental to improving the tensile strength of the material. Therefore, in the manufacturing method described in this invention, the annealing temperature is controlled between 780-830°C.

[0049] In the annealing step described above in this invention, if the holding time is too short, it is not conducive to sufficient recrystallization and the formation of fine, dispersed precipitates, resulting in a decrease in the tensile strength and an increase in the yield strength ratio of the steel. If the holding time is too long, coarsening of the precipitates and abnormal grain growth are likely to occur, which is not conducive to improving the strength of the material. Based on this, in the manufacturing method described in this invention, the holding time is between 70 and 120 seconds.

[0050] In the annealing step described above in this invention, when the heating rate is too low, the precipitation motive force is insufficient, which is not conducive to the formation of more precipitate nucleation sites and the improvement of tensile strength, resulting in a high yield strength ratio of the steel. When the heating rate is too high, it is not conducive to the uniformity of the microstructure of the steel coil, resulting in a decrease in the elongation of the steel coil. Based on this, in the manufacturing method described in this invention, the heating rate is between 5-10℃ / min.

[0051] Furthermore, in the hot rolling step of the method for manufacturing cold-rolled IF steel according to the present invention, the hot rolling deformation is controlled to be 80% to 88%.

[0052] In the hot rolling step of the manufacturing method of cold-rolled IF steel of the present invention, when the hot rolling deformation is too low, the rolled grains are coarser, and due to the high B element content, it is not conducive to obtaining a sufficient percentage of ideal texture during subsequent annealing and recrystallization; when the hot rolling deformation is too high, the rolling force increases dramatically, increasing the production difficulty. Based on this, in the manufacturing method of the present invention, it is preferable to control the hot rolling deformation between 80-88%.

[0053] Furthermore, in the cold rolling step of the method for manufacturing cold-rolled IF steel according to the present invention, the cold rolling deformation is controlled to be 80% to 85%.

[0054] In the cold rolling step of the manufacturing method of cold-rolled IF steel of the present invention, when the cold rolling deformation is too low, under the condition of low Ti and Nb element content, coarse grains with uneven size are easily formed after annealing and recrystallization, which is not conducive to improving the strength of the steel; when the cold rolling deformation is too high, processing is difficult, and abnormally large grains are easily formed after annealing and recrystallization, resulting in a decrease in the elongation of the steel. Based on this, in the manufacturing method of the present invention, the cold rolling deformation can preferably be controlled between 80% and 85%.

[0055] Another objective of this invention is to provide a method for manufacturing hot-dip galvanized steel sheet. This method, by controlling the elemental composition and production process of the steel sheet, combined with a series of measures such as microstructure regulation, inclusion refinement, and grain boundary strengthening, enables the steel to possess the characteristics of high elongation, high strength, low yield strength ratio, and excellent deep-drawing performance. This allows the steel to meet the application requirements of complex forming, high stamping requirements, strong safety, and energy saving and weight reduction.

[0056] To achieve the above objectives, the present invention also provides a method for manufacturing hot-dip galvanized steel sheet, comprising the following steps:

[0057] A slab is obtained;

[0058] Hot rolling: The furnace exit temperature of hot-rolled steel billets is 1150℃~1200℃, the finishing rolling temperature is 880℃~910℃, and the coiling temperature is 590℃~630℃.

[0059] Cold rolling;

[0060] Annealing: Control the annealing temperature between 780℃ and 830℃, the holding time between 70s and 120s, and the heating rate between 5-10℃ / min;

[0061] Hot-dip galvanizing;

[0062] Smooth and even.

[0063] Furthermore, in the hot rolling step of the manufacturing method of hot-dip galvanized steel sheet according to the present invention, the hot rolling deformation is controlled to be 80% to 88%.

[0064] Furthermore, in the cold rolling step of the manufacturing method of hot-dip galvanized steel sheet according to the present invention, the cold rolling deformation is controlled to be 80% to 85%.

[0065] The low yield strength ratio cold-rolled IF steel, hot-dip galvanized steel sheet, and their manufacturing method described in this invention have the following advantages and beneficial effects:

[0066] The cold-rolled IF steel described in this invention, by controlling the elemental composition and production process of the steel plate, combined with a series of measures such as microstructure regulation, inclusion refinement, and grain boundary strengthening, can enable the steel to have the characteristics of high elongation, high strength, low yield strength ratio, and excellent deep drawing performance, thereby enabling the steel to meet the needs of complex forming, high stamping requirements, strong safety, and energy saving and weight reduction.

[0067] In some embodiments, the cold-rolled IF steel of the present invention has a yield strength ≤220MPa, tensile strength ≥370MPa, yield ratio ≤0.595, elongation ≥40%, and plastic strain ratio r. 90 ≥2.2.

[0068] The hot-dip galvanized steel sheet described in this invention has high strength, good formability, and strong deformation capacity, which can meet users' needs for improving stamping and production efficiency, reducing vehicle body weight, and performing complex deformation designs. It is suitable for producing automotive exterior body panels and has broad application prospects and value. Attached Figure Description

[0069] Figure 1 The image shows a metallographic microstructure of Embodiment 1 of the present invention.

[0070] Figure 2 The graph shows a comparison of the contribution of grain size to yield strength and grain size level of the present invention. Detailed Implementation

[0071] The following will provide further explanation and description of the low yield strength ratio cold-rolled IF steel, hot-dip galvanized steel sheet and their manufacturing method described in this invention with reference to specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0072] Examples 1-9 and Comparative Examples 1-9

[0073] The cold-rolled IF steel and hot-dip galvanized steel sheets in Examples 1-9 of this invention are all prepared using the following steps:

[0074] (1) Obtain a slab blank;

[0075] (2) Hot rolling: The furnace exit temperature of hot-rolled steel billet is controlled at 1150℃~1200℃, the finishing rolling temperature is controlled at 880℃~910℃, the coiling temperature is controlled at 590℃~630℃, and the hot rolling deformation is controlled at 80%~88%.

[0076] (3) Cold rolling: The cold rolling deformation is controlled at 80% to 85%;

[0077] (4) Annealing: The annealing temperature is controlled between 780℃ and 830℃, the holding time is controlled between 70s and 120s, and the heating rate is controlled between 5-10℃ / min;

[0078] (5) Leveling to obtain the cold-rolled IF steel of Examples 1-5.

[0079] In some embodiments, the hardened steel coils of Examples 6-9 may be further hot-dip galvanized and finished after the annealing step to obtain the hot-dip galvanized steel sheets of Examples 6-9 of the present invention.

[0080] It should be noted that the comparative steel plates of Comparative Examples 1-9 were also prepared using the above steps (1)-(5), but their specific component ratios and process parameters do not meet the design requirements of this invention.

[0081] Tables 1-1 and 1-2 list the chemical composition ratios of the cold-rolled IF steels obtained in Examples 1-9 of the present invention and the comparative steels of Comparative Examples 1-9.

[0082] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides S, N and Si)

[0083] serial number C Mn P Nb Ti V Al Zr B Example 1 0.006 0.3 0.03 0.04 0.025 0.04 0.06 0.025 0.0007 Example 2 0.0035 0.3500 0.0200 0.0160 0.0250 0.0060 0.0800 0.0400 0.0008 Example 3 0.0040 0.4500 0.0300 0.0350 0.0150 0.0050 0.0100 0.0400 0.0006 Example 4 0.0055 0.2000 0.0400 0.0340 0.0210 0.0045 0.0300 0.0300 0.0005 Example 5 0.0051 0.5000 0.0300 0.0310 0.0205 0.0055 0.0800 0.0150 0.0007 Example 6 0.0045 0.2800 0.0390 0.0290 0.0230 0.0035 0.0500 0.0200 0.0006 Example 7 0.0041 0.2800 0.0390 0.0290 0.0230 0.0035 0.0500 0.0200 0.0006 Example 8 0.0039 0.3100 0.0350 0.0293 0.0220 0.0020 0.0700 0.0350 0.0003 Example 9 0.0058 0.4800 0.0390 0.0400 0.0248 0.0055 0.0750 0.0380 0.0008 Comparative Example 1 0.0042 0.5000 0.0400 0.0400 0.0150 0.0010 0.0100 0.0350 0.0006 Comparative Example 2 0.0025 0.4500 0.0450 0.0380 0.0300 0.0070 0.0300 0.0300 0.0004 Comparative Example 3 0.0055 0.3700 0.0330 0.0320 0.0240 0.0010 0.1000 0.0100 0.0007 Comparative Example 4 0.0065 0.1500 0.0200 0.0150 0.0140 0.0040 0.0300 0.0250 0.0002 Comparative Example 5 0.0065 0.1500 0.0210 0.0335 0.0245 0.0040 0.0300 0.0250 0.0002 Comparative Example 6 0.0048 0.1000 0.0500 0.0290 0.0210 0.0050 0.0600 0.0300 0.0005 Comparative Example 7 0.0057 0.7000 0.0100 0.0285 0.0220 0.0060 0.0850 0.0500 0.0010 Comparative Example 8 0.0075 0.4000 0.0450 0.0400 0.0350 0.0010 0.0600 0.0300 0.0003 Comparative Example 9 0.0058 0.4000 0.0400 0.0390 0.0270 0.0040 0.0700 0.0400 0.0008

[0084] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides S, N and Si)

[0085]

[0086]

[0087] Table 2 lists the specific process parameters of the cold-rolled IF steel obtained in Examples 1-9 of the present invention and the comparative steels of Comparative Examples 1-9 in the above process steps.

[0088] Table 2.

[0089]

[0090] Samples were taken from the cold-rolled IF steels obtained in Examples 1-9 and the comparative steels in Comparative Examples 1-9, and grain size and Class B inclusion ratings were performed. The test results are recorded in Table 3. Wherein:

[0091] The grain size was determined by referring to GB / T 6394-2002, Method for Determination of Average Grain Size of Metals, and metallographic preparation was performed on each example and comparative example, and measurements were taken from three observation fields.

[0092] The rating of Class B inclusions is based on the microscopic inspection method of GB / T 10561-2005, which specifies the determination of non-metallic inclusion content in steel. Samples were taken from each example and comparative example, polished, and then a comprehensive rating was obtained by selecting three observation fields.

[0093] It should be noted that the microstructure of the finished hot-dip galvanized steel obtained in Examples 6-9 is the same as that of the cold-rolled IF steel obtained before the hot-dip galvanizing step.

[0094] Table 3 lists the microstructure observation results of the cold-rolled IF steel obtained in Examples 1-9 of the present invention and the comparative steels of Comparative Examples 1-9.

[0095] Table 3.

[0096] serial number Microstructure grain size level (grade) Class B inclusions rating image level i Example 1 9.5 0.5 Example 2 9.5 0.5 Example 3 9 0.5 Example 4 9 1.0 Example 5 10 0.5 Example 6 9 1.0 Example 7 9 1.0 Example 8 10 0.5 Example 9 9.5 0.5 Comparative Example 1 8.5 1.0 Comparative Example 2 9 1.5 Comparative Example 3 9.5 1.5 Comparative Example 4 10 0.5 Comparative Example 5 10 1.0 Comparative Example 6 8.5 0.5 Comparative Example 7 9 1.5 Comparative Example 8 10.5 1.5 Comparative Example 9 9.5 1.0

[0097] As can be seen from Table 3 above, the steel plates of Examples 1-9 of the present invention have obtained ideal microstructure characteristics through reasonable chemical element composition design and optimized process parameters. The microstructure grain size level is between 9 and 10, and the B-type inclusion rating image level i is 0.5 or 1.

[0098] Figure 1 The image shows a metallographic microstructure of Embodiment 1 of the present invention.

[0099] like Figure 1 As shown, Example 1, through reasonable chemical element composition design and optimized process parameters, obtained ideal microstructure characteristics with a grain size level of 9.5, uniform grain size, more uniform tensile deformation, and less susceptibility to crack initiation. It can achieve high elongation while maintaining low yield strength.

[0100] The cold-rolled IF steels of Examples 1-5, the hot-dip galvanized steel sheets of Examples 6-9, and the comparative steels of Comparative Examples 1-9 were sampled again, and their mechanical properties were tested. The test results are listed in Table 4. Among them:

[0101] Mechanical property testing was conducted in accordance with GB / T 228.1-2021 Tensile Testing of Metallic Materials. Tensile specimens were prepared for each example and comparative example, and at least 5 sets of valid data were obtained for each specimen.

[0102] It should also be noted that the mechanical properties and deformation resistance of the hot-dip galvanized steel obtained in Examples 1-9 and the cold-rolled IF steel obtained before the hot-dip galvanizing step are the same. Therefore, the relevant properties of hot-dip galvanized steel can also express the properties of the cold-rolled IF steel in the corresponding examples or comparative examples.

[0103] Table 4 lists the mechanical property test results of the comparative steels of Examples 1-9 and Comparative Examples 1-9 of the present invention.

[0104] Table 4.

[0105] serial number Yield strength (MPa) Tensile strength (MPa) The ratio of yield strength Elongation after fracture (A80%) <![CDATA[Plastic strain ratio r 90 > Example 1 220 385 0.571 41 2.3 Example 2 206 372 0.554 43 2.4 Example 3 215 379 0.567 41.5 2.3 Example 4 214 381 0.562 41.5 2.2 Example 5 218 382 0.571 42 2.3 Example 6 200 374 0.535 43.5 2.3 Example 7 211 384 0.549 42 2.2 Example 8 214 370 0.578 43 2.2 Example 9 220 390 0.564 43 2.4 Comparative Example 1 212 371 0.571 38 2.1 Comparative Example 2 222 365 0.608 37.5 2.0 Comparative Example 3 215 367 0.586 38.5 2.1 Comparative Example 4 235 355 0.662 38 2.0 Comparative Example 5 220 368 0.598 39 2.2 Comparative Example 6 212 365 0.581 41 2.3 Comparative Example 7 238 382 0.623 39 1.8 Comparative Example 8 245 393 0.623 37 2.3 Comparative Example 9 224 384 0.583 40.5 2.2

[0106] As can be seen from Table 4 above, Examples 1-9 of the present invention, through reasonable chemical element composition design and optimized process parameters, obtained high-performance cold-rolled IF steel and hot-dip galvanized steel sheets. Their yield strength is less than or equal to 220 MPa, tensile strength is greater than or equal to 370 MPa, yield-to-tensile ratio is less than 0.595, elongation is greater than 40%, and plastic strain ratio r is... 90 All are greater than or equal to 2.2.

[0107] The difference from Embodiments 1-9 of the present invention is as follows:

[0108] The chemical element composition of Comparative Example 1 does not meet the design specifications of this invention, and its coiling temperature and annealing process parameters do not meet the design specifications of this invention. As a result, the microstructure formed during the rolling stage is not fine enough, the microstructure obtained after annealing is coarser, and the content of ideal orientation texture is lower. Ultimately, its grain size, elongation and plastic strain ratio are lower than the lower limit of the index requirements.

[0109] Although the processing parameters of Comparative Example 2 meet the design specifications of this invention, its element content does not meet the design specifications of this invention, resulting in an excessively high content of solid solution elements and a large amount of precipitated inclusions. Ultimately, its yield strength and yield strength ratio exceed the upper limit of the index requirements, while its tensile strength, elongation after fracture, and plastic strain ratio are lower than the lower limit of the index requirements.

[0110] The chemical element composition design of Comparative Example 3 does not meet the design specifications of this invention. Its Al content exceeds the upper limit of the design specifications. Even with a high Zr content, the size of the inclusions generated is still large, ultimately resulting in its tensile strength and elongation being lower than the lower limit of the requirements of this invention.

[0111] The chemical element composition and processing parameters of Comparative Example 4 did not meet the design specifications in some aspects. The contents of Nb, Ti and B elements were too low, and the annealing time was too short, resulting in a low content of precipitated phases in the matrix, weak grain boundary bonding, and small grain size after recrystallization. Ultimately, its yield strength was higher than the upper limit of the requirements of this invention, while its tensile strength, plastic strain ratio and elongation after fracture were lower than the lower limit of the requirements of this invention.

[0112] The chemical element composition and processing parameters of Comparative Example 5 did not meet the design specifications in some aspects. Its annealing heating rate, Mn and B element content, and (Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48) were lower than the lower limit of the design specifications, resulting in poor solid solution strengthening and second phase strengthening effects, low grain boundary bonding force, and ultimately its tensile strength and elongation were lower than the lower limit of the indicators required by this invention.

[0113] The chemical element composition and processing parameters of Comparative Example 6 did not meet the design specifications in some aspects. The Mn element was lower than the lower limit of the design specifications, the P element was higher than the upper limit of the design specifications, and the winding temperature was higher than the upper limit of the design specifications. As a result, during the winding process, the precipitated phase grew, which reduced the resistance to grain boundary movement. Ultimately, its tensile strength and grain size were lower than the lower limit of the requirements of this invention.

[0114] The chemical element composition and processing parameters of Comparative Example 7 did not meet the design specifications, resulting in the precipitation of more inclusions and a lower content of ideal orientation texture. Ultimately, its elongation after fracture and plastic strain ratio were lower than the lower limit of the index requirements, while its yield strength and yield strength ratio exceeded the upper limit of the index requirements.

[0115] The chemical element composition and processing parameters of Comparative Example 8 do not meet the requirements of this invention in some aspects. The content of C, Ti, S and Si elements exceeds the upper limit of the design specifications of this invention, the furnace exit temperature and finishing rolling temperature are lower than the lower limit of the design specifications, and the hot rolling deformation, cold rolling deformation and annealing temperature exceed the upper limit of the design specifications. This results in a high inclusion content and a finer grain size after annealing. Ultimately, its yield strength and grain size exceed the upper limit of the requirements of this invention, and its elongation is lower than the lower limit of the requirements of this invention.

[0116] The chemical element composition design of Comparative Example 9 does not meet the requirements of this invention. Its Ti element content exceeds the upper limit of the design specification of this invention, resulting in the precipitation of a large number of fine Ti carbonitrides, which ultimately causes its yield strength to exceed the upper limit of the index requirement of this invention.

[0117] also, Figure 2 The diagram also shows a comparison of the contribution of grain size to yield strength and grain size level of the present invention.

[0118] like Figure 2 As shown, the grain size of steel materials has a significant impact on the yield strength of the material; as the grain size decreases, the yield strength increases sharply. Therefore, the grain size of the material of this invention is grade 9.0-10.0, which gives the material a low yield strength ratio.

[0119] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0120] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A cold-rolled IF steel with a low yield ratio, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.0035-0.006%, Mn: 0.20-0.50%, P: 0.02-0.040%, Nb: 0.016-0.04%, Ti: 0.015-0.025%, V: 0.002%-0.006%, Al: 0.01-0.08%, Zr: 0.015-0.04%, B: 0.0003-0.0008%; It also satisfies: 1.1≤(Nb×12) / (C×93)+((Ti-0.0137)×12) / (C×48)≤1.5, wherein each chemical element is substituted with the value before the mass percentage.

2. Cold rolled IF steel according to claim 1, characterized in that, The mass percentages of each chemical element are: C: 0.0035-0.006%, Mn: 0.20-0.50%, P: 0.02-0.040%, Nb: 0.016-0.04%, Ti: 0.015-0.025%, V: 0.002%-0.006%, Al: 0.01-0.08%, Zr: 0.015-0.04%, B: 0.0003-0.0008%; the balance being Fe and unavoidable impurities.

3. Cold rolled IF steel according to claim 1 or 2, c h a r a c t e r i z e d in that, In the unavoidable impurities, S≤0.008%, N≤0.004%, Si≤0.001%.

4. Cold rolled IF steel according to claim 1 or 2, c h a r a c t e r i z e d in that, The microstructure grain size level is 9.0-10.

0.

5. Cold rolled IF steel according to claim 1 or 2, c h a r a c t e r i z e d in that, The B-type inclusion rating picture level i is 0.5 or 1.

6. Cold rolled IF steel according to claim 1 or 2, c h a r a c t e r i z e d in that, The performance satisfies: yield strength ≤ 220 MPa, tensile strength ≥ 370 MPa, yield strength ratio ≤ 0.595, elongation ≥ 40%, plastic strain ratio r 90 ≥ 2.

2.

7. A hot-dip galvanized steel sheet, characterized by, The substrate is a cold-rolled IF steel as claimed in any one of claims 1-6, the substrate being coated with a hot-dip galvanized layer.

8. A method of manufacturing a cold rolled IF steel according to any one of claims 1-6, c h a r a c t e r i z e d in that, It comprises the steps of: Producing a slab; Hot rolling: the hot rolling slab is discharged at a temperature of 1150-1200°C, the finishing rolling temperature is 880-910°C, and the coiling temperature is 590-630°C; Cold rolling; Annealing: the annealing temperature is controlled to be between 780-830°C, the holding time is 70-120s, and the heating rate is 5-10°C / min; Skin passing.

9. The manufacturing method of a cold rolled IF steel according to claim 8, characterized in that, In the hot rolling step, the hot rolling deformation is controlled to be 80-88%.

10. The manufacturing method of a cold rolled IF steel according to claim 8, characterized in that, In the cold rolling step, the cold rolling deformation is controlled to be 80-85%.

11. The method of producing a galvannealed steel sheet according to claim 7, characterized by, It comprises the steps of: Producing a slab; Hot rolling: the hot rolling slab is discharged at a temperature of 1150-1200°C, the finishing rolling temperature is 880-910°C, and the coiling temperature is 590-630°C; Cold rolling; Annealing: the annealing temperature is controlled to be between 780-830°C, the holding time is 70-120s, and the heating rate is 5-10°C / min; Hot-dip galvanizing; Skin passing.

12. The method of producing a galvannealed steel sheet according to claim 11, characterized by, In the hot rolling step, the hot rolling deformation is controlled to be 80-88%.

13. The method of producing a galvannealed steel sheet according to claim 11, characterized by, In the cold rolling step, the cold rolling deformation is controlled to be 80-85%.

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