Cold-rolled if steel, hot-dip galvanized steel sheet and manufacturing method therefor

Abstract

Disclosed in the present invention are a cold-rolled IF steel and a hot-dip galvanized steel sheet, further comprising following chemical elements in mass percentage: 0.004-0.007% of C, 0.50-0.80% of Mn, 0.025-0.040% of P, 0.04-0.08% of Nb, 0.05-0.2% of Cr and 0.01-0.1% of Al, and further satisfying 1.2≤(Nb×12) / (C×93)≤2.0, wherein each chemical element in the formula is substituted by the numerical value before the percent sign of the mass percentage content thereof. Further disclosed in the present invention is a manufacturing method for the steel sheet, the manufacturing method comprising the following steps: slab preparation; hot rolling; cold rolling; and annealing. The cold-rolled IF steel and the hot-dip galvanized steel sheet of the present invention have good stamping performance stability, excellent surface morphology and better resistance to deformation.

Description

Cold-rolled IF steel, hot-dip galvanized steel sheet and manufacturing method thereof Technical Field

[0001] The present invention relates to a steel plate and a manufacturing method thereof, and in particular to a high-strength steel and a manufacturing method thereof. Background Art

[0002] The current coating process for automotive exterior panels significantly reduces energy consumption and VOC emissions by eliminating a coating layer and a baking step. However, this reduction in coating and baking passes reduces the new coating process's ability to conceal the substrate's surface topography, placing higher demands on the material's surface topography, particularly surface waviness. Furthermore, lightweighting vehicles is an imperative for the automotive industry. Improving steel plate strength can reduce the thickness of automotive panels, effectively reducing vehicle weight, fuel consumption, and emissions.

[0003] IF steel, a common automotive sheet material, primarily utilizes Ti / Nb elements to anchor carbon and nitrogen atoms in the interstitial spaces, and is solid-solution strengthened by the addition of Mn, Si, and P. This steel, lacking interstitial atoms in the matrix phase, exhibits a low yield strength, while the addition of solid-solution elements effectively increases the material's tensile strength. Consequently, it exhibits a low yield-to-strength ratio, excellent deep-drawing properties, and resistance to aging, making it widely used in the production of automotive exterior panels.

[0004] However, in actual applications, stamped automotive exterior panels made from thinned IF steel are prone to deformation during handling. Furthermore, when high-strength steel is used to manufacture automotive stampings, its yield strength ratio is often greater than 0.65, significantly shortening the uniform plastic deformation range and making the material less susceptible to deformation, which is not conducive to the stamping of automotive panels. Therefore, while maintaining the excellent stamping properties of IF steel, it is necessary to improve its deformation resistance after stamping.

[0005] Therefore, in order to make the product meet the user's stamping, handling, weight reduction and painting needs, in addition to ensuring that the mechanical properties of the steel plate meet the standards, its yield strength ratio, corrugation after stamping and deformation resistance after stamping should also be controlled.

[0006] For example, the Chinese patent publication CN109023050B, published on December 18, 2018, and titled "A 390 MPa Grade High-Strength IF Steel and Its Production Method," discloses a high-strength IF steel and its production method. Its composition and mass fractions are: C: 0.0-0.004%, Si: 0.0-0.030%, Mn: 0.4%-0.6%, P: 0.06%-0.085%, S: 0.0-0.010%, Al: 0.03%-0.1%, Nb: 0.01%-0.1%, B: 0.00050%-0.0012%, with the remainder being iron and unavoidable impurities. In this invention, the precipitated phase has a diameter greater than 50 nm, and the main precipitated phases are FeNbP and FeTiP. It changes the material texture by changing the precipitated phase content, increases the plastic strain ratio, and improves the stamping performance of the material to a certain extent, but does not consider the coating quality of the material.

[0007] Another example: Chinese patent publication number CN114196882B, published on October 28, 2022, and titled "A Steel Strip Coil for High-Surface Quality and High-Strength Automobile Panels and Its Preparation Method," discloses a method for producing automobile exterior panels. The composition and mass fractions are as follows: 0.002% ≤ C ≤ 0.01%, Si ≤ 0.01%, Mn ≤ 0.8%, P ≤ 0.05%, S ≤ 0.01%, 0.01% ≤ Alt ≤ 0.06%, N ≤ 0.004%, 0.02% ≤ Nb ≤ 0.09%, B ≤ 0.0003%, with the remainder being Fe and unavoidable impurities. This invention primarily controls the composition and process of Nb-IF steel, producing a material with high strength and excellent formability. However, it fails to consider the impact of the material's surface morphology and deformation resistance on the painting and transportation of automobile exterior panels. Summary of the Invention

[0008] One objective of the present invention is to provide a cold-rolled IF steel with excellent stamping performance stability, superior surface morphology, and good deformation resistance. This steel prevents deformation of stamped parts during handling and ensures low surface waviness after electrophoresis, without noticeable orange peel. This cold-rolled IF steel effectively improves the deformation resistance and production stability of stamped parts, enhances the surface quality of automotive exterior panel coatings, and is suitable for the production of automotive exterior coverings, possessing broad application prospects and value.

[0009] To achieve the above object, the present invention provides a cold-rolled IF steel containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages:

[0010] C: 0.0040-0.0070%, Mn: 0.50-0.80%, P: 0.025-0.040%, Nb: 0.040-0.080%, Cr: 0.05-0.20%, Al: 0.010-0.100%;

[0011] It also satisfies: 1.20≤(Nb×12) / (C×93)≤2.00, where each chemical element is substituted into the value before the percentage sign of its mass percentage content.

[0012] Furthermore, in the cold-rolled IF steel of the present invention, the mass percentage of each chemical element is:

[0013] C: 0.004-0.0070%, Mn: 0.50-0.80%, P: 0.025-0.040%, Nb: 0.040-0.080%, Cr: 0.05-0.20%, Al: 0.010-0.100%; the balance is Fe and inevitable impurities.

[0014] In the cold-rolled IF steel of the present invention, the design principles of each chemical element are specifically described as follows:

[0015] C: In the cold-rolled IF steel described herein, the presence of C affects the content of NbC precipitates, and NbC particles can provide both grain refinement and secondary phase strengthening. When the C content exceeds 0.007%, solid-solution C atoms are more likely to form, leading to the formation of a {111} texture, which is detrimental to the material's stamping performance, particularly its r-value. When the C content is less than 0.004%, the number of NbC particles precipitating is small, resulting in a reduction in the material's grain size, weakening the secondary phase strengthening effect, and lowering the material's strength. Therefore, in the cold-rolled IF steel described herein, the mass percentage of C can be controlled between 0.004% and 0.0070%.

[0016] Mn: In the cold-rolled IF steel described in the present invention, the Mn element is a commonly used solid solution element in steel, which can effectively increase the strength of the material and reduce the yield strength ratio. When the Mn element content is greater than 0.50%, it can combine with the S element to form MnS precipitation, thereby avoiding hot cracking of the steel billet and improving the bonding performance of the coating. When the Mn element content is increased, although the yield strength of the material increases significantly, considering the difference in the effects of the Mn element and the P element on the Fe-based lattice distortion state, its content should not be too high. Therefore, in the cold-rolled IF steel described in the present invention, the mass percentage of the Mn element can be controlled between 0.50-0.80%.

[0017] P: In the cold-rolled IF steel described herein, P, as a solid solution strengthening element, is one of the elements with the strongest ferrite strengthening effect, effectively improving material strength. However, when the P content is too low, the strengthening effect is insignificant. When the P content is too high, segregation is likely to occur, increasing the steel's brittleness, affecting weldability, and adversely affecting the alloying treatment of the zinc layer. Therefore, in the cold-rolled IF steel described herein, the mass percentage of P can be controlled between 0.025% and 0.040%.

[0018] Nb: In the cold-rolled IF steel described herein, Nb primarily forms carbide precipitations with C, with the remainder remaining as a solid solution element. The resulting fine, dispersed carbides effectively refine the grains and form a precipitation-free zone (PFZ) during nucleation and growth, effectively increasing material strength and reducing the yield strength ratio. However, if the Nb content is too low, the free C atoms cannot be completely fixed, reducing the yield strength. Therefore, in the cold-rolled IF steel described herein, the mass percentage of Nb can be controlled between 0.040 and 0.080%.

[0019] Cr: In the cold-rolled IF steel described in the present invention, Cr is used as a solid solution strengthening element. Its characteristics are that Cr has a lower carbide-forming ability than Nb; Cr has a stronger affinity for carbon, effectively reducing the diffusion coefficient of C in the iron matrix; Cr has a larger atomic radius than Mn, causing a higher degree of lattice distortion; Cr has a higher solid solubility in the iron matrix than Nb, but causes less lattice distortion than Nb. When the Cr content is ≤0.05% by mass, it is insufficient to significantly increase the lattice resistance to dislocation slip during deformation, refine the NbC precipitation phase, and increase the work hardening value; when its content is ≥0.2% by mass, it will reduce the solid solution strengthening effect of P and Cr due to the opposite effect of P on the Fe matrix caused by the lattice distortion. Adding Cr at a mass percentage between 0.05% and 0.20% can reduce the diffusion rate of carbon atoms, promote the precipitation of NbC phases, slow NbC phase growth, and refine the grains, thereby improving the material's work hardening value and deformation resistance. Based on this, the mass percentage of Cr in the cold-rolled IF steel described herein can be controlled between 0.05% and 0.20%.

[0020] Al: In the cold-rolled IF steel described herein, Al acts as a deoxidizing impurity, fixing free nitrogen atoms. Low Al content fails to ensure nitrogen fixation and deoxidation. Excessive Al content tends to dissolve in the matrix, reducing material plasticity and producing a high number of inclusions. Therefore, the mass percentage of Al in the cold-rolled IF steel described herein can be controlled between 0.010% and 0.100%.

[0021] It should be noted that while controlling the mass percentage of individual elements, the present invention also requires controlling the ratio (Nb×12) / (C×93) within a range of 1.20-2.00. This is a key factor in ensuring the steel coils described herein have a low yield ratio and high deformation resistance, and is also an important factor in ensuring low waviness. When the mass percentage of Nb and C elements is below the lower limit of the (Nb×12) / (C×93) range, the effects of grain refinement and second-phase strengthening are weakened. When the mass percentage of Nb and C elements is above the upper limit of the (Nb×12) / (C×93) range, the annealing recrystallization temperature increases, increasing the yield strength of the material while hindering the formation of favorable textures and cost control.

[0022] Furthermore, in the cold-rolled IF steel of the present invention, among the inevitable impurities, S≤0.010%, B≤0.0006%, N≤0.004%, and Si≤0.0010%.

[0023] In the cold-rolled IF steel of the present invention, S, B, N, and Si are all unavoidable impurities. To ensure that the cold-rolled IF steel of the present invention has good performance, it is desirable that their contents be as low as possible, if conditions permit.

[0024] S: In the cold-rolled IF steel described herein, S is controlled as a harmful impurity element. It forms low-melting-point precipitates in the steel, causing hot brittleness and impairing weldability. Therefore, the mass percentage of S in the cold-rolled IF steel described herein can be controlled to below 0.010%.

[0025] B: In the cold-rolled IF steel described herein, element B reduces the material's plasticity. Excessive B content can hinder recrystallization and inhibit the formation of PFZ bands. Therefore, the mass percentage of B in the cold-rolled IF steel described herein can be controlled to below 0.0006%.

[0026] N: In the cold-rolled IF steel described herein, dissolved N atoms can cause a decrease in the material's r-value and induce aging, negatively impacting the material's aging resistance and stamping performance. Therefore, the mass percentage of N in the cold-rolled IF steel described herein can be controlled to below 0.004%.

[0027] Si: In the cold-rolled IF steel described herein, Si is an impurity element present during the steelmaking process and is prone to segregation at grain boundaries, forming hot brittle phases. High Si content can also adversely affect the material's anisotropy and plasticity. Therefore, its content in the steel should be minimized, if technical conditions permit. In the present invention, the mass percentage of Si in the cold-rolled IF steel described herein can be controlled to below 0.0010%.

[0028] Furthermore, in the cold-rolled IF steel of the present invention, the microstructure grain size grade is at least grade 10.

[0029] In the present invention, controlling the grain size level to at least level 10 can ensure that the material has sufficient strength, and at the same time can reduce the waviness of the stamped part after forming, so that the material has higher coating quality.

[0030] Furthermore, in the cold-rolled IF steel of the present invention, it has an atom-free precipitation zone, and the width of the atom-free precipitation zone is 0.30 μm to 0.80 μm.

[0031] In the present invention, PFZ band refers to the atom-free precipitation zone (PFZ) formed around the grain boundary of the material, and the width of the PFZ band has a significant effect on the yield strength ratio of the material. When the atom-free precipitation zone width is ≤0.30μm, the accumulated dislocations are not conducive to the formation of larger dislocation pile-up groups, forming higher local stresses, and are also not conducive to the occurrence of cross-slip, and are unable to effectively reduce the yield strength of the material and reduce the difficulty of stamping. When the atom-free precipitation zone width is ≥0.80μm, it is necessary to significantly increase the annealing holding time, which is not conducive to cost control. Therefore, in the cold-rolled IF steel described in the present invention, the average width of the atom-free precipitation zone can be controlled to be between 0.30μm and 0.80μm.

[0032] Furthermore, in the cold-rolled IF steel of the present invention, its microstructure comprises NbC precipitated intragranularly and at grain boundaries, wherein the size of the NbC precipitated intragranularly is ≤30.0 nm and the number is 600-1000 per square micron. In some embodiments, the size of the NbC precipitated intragranularly is 19.0 to 30.0 nm, such as 19.0 to 25.0 nm.

[0033] In the present invention, the main precipitated phases within the grains and at the grain boundaries are both NbC. When the density of the precipitated phases in the cold-rolled IF steel is low, it is not enough to effectively refine the grains, hinder dislocation slip, and enable the material to have higher tensile and yield strengths. When the density of the precipitated phases in the cold-rolled IF steel is high, it is inevitable to increase the Nb element content, which inevitably increases the economic cost, increases the yield strength, and is not conducive to stamping. At the same time, when the diameter of the precipitated phase at the grain boundary of the cold-rolled IF steel is ≥60nm, it is conducive to the formation of the PFZ band. When the diameter of the precipitated phase is less than 60nm, it is not conducive to grain growth and the formation of the PFZ band. In some embodiments, the diameter of the precipitated phase at the grain boundary is 60 to 85nm. When the size of the precipitated phase within the grain is ≥30nm, the NbC phase formed in the material during the heat treatment process undergoes excessive Ostwald ripening, and the density of the precipitated phase is reduced, which reduces the obstruction of dislocation movement and makes the grains easy to grow, which is not conducive to the improvement of the tensile strength of the material. Based on this, in the cold-rolled IF steel of the present invention, its microstructure can be controlled to have NbC precipitated intragranularly and NbC precipitated at grain boundaries, wherein the size of the NbC precipitated intragranularly is ≤30.0 nm and the number is 600-1000 per square micron.

[0034] Furthermore, in the cold-rolled IF steel of the present invention, the performance satisfies at least one of the following items:

[0035] Yield strength is 235-270MPa, tensile strength is 385-430MPa, and yield strength ratio is ≤0.630;

[0036] 2% deformation work hardening value ≥45.0MPa;

[0037] The waviness before deformation is Wsa≤0.180μm, and the waviness after deformation is Wsa≤0.220μm.

[0038] In the present invention, the reason for limiting the yield strength to the range of 235 to 270 MPa is that too high a yield strength is not conducive to the stamping forming of the material, and too low a yield strength is not conducive to the material's dent resistance and deformation resistance. At the same time, limiting the tensile strength to 385 to 430 MPa is mainly subject to the process and material composition. The higher the tensile strength of the material, the alloy composition or microstructure will also change, and the yield strength of the material will also increase, which is not conducive to the stamping forming of the material. A lower tensile strength will easily lead to insufficient material strength and reduced deformation resistance. In some embodiments, the yield strength of the IF cold-rolled steel described in the present invention is 250 to 270 MPa. In some embodiments, the tensile strength of the IF cold-rolled steel described in the present invention is 400 to 430 MPa. In some embodiments, the yield strength ratio of the IF cold-rolled steel described in the present invention is 0.600 to 0.630.

[0039] Furthermore, in the present invention, the work hardening value is used to characterize a part's ability to resist deformation after forming. This refers to the increase in the material's yield strength after being stretched to a certain degree. Traditional IF steel has a low yield strength, resulting in a minimal increase in yield strength after stamping, making the resulting parts susceptible to deformation during handling. For automotive steel, the strain hardening rate (n) has traditionally been used to measure a material's resistance to deformation during stamping. This value is often measured within the uniform plastic deformation range, i.e., the strain hardening rate within a large deformation range (typically a strain range of 10% to 20%), and does not measure the deformation resistance of low-yield-strength materials after deformation. The deformation of automotive exterior panels typically ranges from 3% to 7%. Increasing the work hardening value within this strain range can effectively increase the deformation resistance of stamped parts and prevent deformation during handling. Therefore, limiting the work hardening value (WH) to ≥ 45.0 MPa for small deformations (for example, the work hardening value in the present invention can correspond to a deformation of 2%) helps maintain the shape stability of stamped parts during transportation. In some embodiments, the IF cold-rolled steel of the present invention has a 2% deformation work hardening value of 45 to 66.0 MPa, such as 48.0 to 66.0 MPa.

[0040] In the present invention, a 5% deformation cup can be used to simulate the stamping process of automobile exterior panels and can control the post-deformation waviness to below 0.220 μm. This ensures a low orange peel R value during subsequent electrophoresis and coating, improving coating quality. In some embodiments, the IF cold-rolled steel described herein has a waviness Wsa of 0.155 to 0.180 μm before deformation and 0.180 to 0.220 μm after deformation.

[0041] Another object of the present invention is to provide a hot-dip galvanized steel plate, which has good stamping, coating and anti-deformation properties, and can meet the user's requirements for material stamping forming, application of mid-coating-free process and maintaining shape and size stability during the transportation of automobile stamping parts.

[0042] In order to achieve the above object, the present invention further provides a hot-dip galvanized steel sheet, the substrate of which is the above-mentioned cold-rolled IF steel of the present invention, and the substrate is coated with a hot-dip galvanized layer.

[0043] Another object of the present invention is to provide a method for manufacturing cold-rolled IF steel. This method can improve the work hardening value of the steel by controlling the production process and coordinating the elemental composition of the steel plate, so that the stamping parts have a higher deformation resistance, and at the same time, the material can obtain a lower yield strength ratio and corrugation, thereby ensuring that the steel has good stamping performance and coating effect, and at the same time maintaining the shape stability of the stamping parts during transportation.

[0044] In order to achieve the above object, the present invention also provides a method for manufacturing cold-rolled IF steel, which comprises the steps of:

[0045] obtaining a slab;

[0046] Hot rolling: The furnace temperature of hot rolled steel billet is 1150℃~1200℃, the finishing rolling temperature is 890℃~920℃, and the coiling temperature is 600℃~650℃;

[0047] cold rolling;

[0048] Annealing: control the annealing temperature to be between 800℃ and 840℃, the holding time to be 80s to 110s, and the cooling rate to be 180℃ / min to 230℃ / min.

[0049] In the hot rolling step described above, if the discharge temperature of the hot-rolled steel slab is too low, it is not conducive to uniform heating of the steel slab and complete austenitization. If the discharge temperature of the hot-rolled steel slab is too high, it is likely to cause the increase of iron oxide scale and serious coarsening of the original austenite grains, which will adversely affect grain refinement, steel plate surface quality, and descaling. Therefore, the discharge temperature of the hot-rolled steel slab can be controlled between 1150°C and 1200°C.

[0050] During the hot rolling process of the present invention, if the finishing temperature is too high, the grain size may increase; if the finishing temperature is too low, it may lead to severe work hardening, increasing the difficulty of rolling and the yield strength of the finished product. Therefore, the finishing temperature of the hot-rolled steel slab can be controlled between 890°C and 920°C.

[0051] During the hot rolling step of the present invention, the coiling temperature should be controlled between 600°C and 650°C. This is because: if the coiling temperature is too low, workability may deteriorate and coiling may be impaired; if the coiling temperature is too high, grain growth may occur, leading to the growth of NbC precipitation, which may hinder grain size control in subsequent steps.

[0052] In the above-mentioned annealing step of the present invention, the annealing temperature is controlled between 800°C and 840°C, the holding time is between 80s and 110s, and the cooling rate is controlled between 180°C / min and 230°C / min. This is because: when the annealing temperature is too low, recrystallization is insufficient, and the width of the formed PFZ band is small, which is not conducive to reducing the material's yield strength ratio; when the annealing temperature is too high, it is easy to cause grain coarsening and the disappearance of cake-shaped ferrite, which has an adverse effect on the material strength and the surface after stamping. If the holding time is too short, the annealing is incomplete, which is not conducive to deep drawing performance; if the holding time is too long, the grains will coarsen, resulting in a decrease in material strength. If the cooling rate is too slow, it will lead to reduced production efficiency; if the cooling rate is too fast, it will be unfavorable to form a wider low vacancy concentration zone, and at the same time, Oswald ripening will be insufficient, reducing the width of the PFZ band.

[0053] Furthermore, in the cold rolling step of the method for manufacturing cold-rolled IF steel of the present invention, the cold rolling deformation is controlled to be 75% to 85%.

[0054] During the cold rolling step of the cold-rolled IF steel manufacturing method of the present invention, if the cold rolling deformation is too low, the resulting grains are coarse and the deformation energy storage is low, which is not conducive to annealing recrystallization. At the same time, it is insufficient to form pancake-shaped ferrite grains, which is detrimental to the material's stamping performance. If the cold rolling deformation is too high, abnormally large grains are likely to occur, resulting in uneven grain size, which is detrimental to the material's elongation and surface morphology after stamping. Therefore, the cold rolling deformation is controlled between 75% and 85%.

[0055] In some embodiments, the method for manufacturing cold-rolled IF steel of the present invention further comprises a leveling step after the annealing step. In some embodiments, during the leveling step, the unit width rolling force is controlled to be between 2.7 kN / mm and 4.0 kN / mm, and the unit rolling tension is ≤ 40 MPa, such as between 30 and 40 MPa.

[0056] Another object of the present invention is to provide a method for manufacturing hot-dip galvanized steel plates. By controlling the elemental composition and production process of the steel plates, the work hardening value of the steel is improved so that the stamping parts have higher deformation resistance, and at the same time the material has a lower yield strength ratio and corrugation, thereby ensuring that the steel has good stamping performance and coating effects, and at the same time maintaining the shape stability of the stamping parts during transportation.

[0057] In order to achieve the above object, the present invention also provides a method for manufacturing a hot-dip galvanized steel sheet, which comprises the steps of:

[0058] obtaining a slab;

[0059] Hot rolling: The furnace temperature of hot rolled steel billet is 1150℃~1200℃, the finishing rolling temperature is 890℃~920℃, and the coiling temperature is 600℃~650℃;

[0060] cold rolling;

[0061] Annealing: controlling the annealing temperature to be between 800° C. and 840° C., the holding time to be between 80s and 110s, and the cooling rate to be between 180° C. and 230° C. to obtain the cold-rolled IF steel;

[0062] Hot dip galvanizing;

[0063] smooth.

[0064] Furthermore, in the cold rolling step of the method for manufacturing the hot-dip galvanized steel sheet of the present invention, the cold rolling deformation is controlled to be 75% to 85%.

[0065] Furthermore, in the leveling step of the method for manufacturing hot-dip galvanized steel sheets of the present invention, the unit width rolling force is controlled to be between 2.7 kN / mm and 4.0 kN / mm, and the unit rolling tension is ≤40 MPa, such as between 30 and 40 MPa.

[0066] In the leveling process of the manufacturing method of the hot-dip galvanized steel sheet described in the present invention, the unit width rolling force and unit rolling tension have a strong improvement effect on the plate quality, surface morphology and mechanical properties of the material. By controlling the leveling elongation, the mechanical properties of the strip steel can meet the use requirements. The combination of large rolling force and small tension can better optimize the surface morphology of the material after leveling, and the resulting material punch cup also has a low waviness. However, if the rolling force is too large, it is easy to cause roller marks. If the tension is too small, it is easy to cause poor plate shape control and a large increase in rolling force, which is not conducive to production; if the tension is too large, the deformation degree and dislocation density of various textures in the material along the rolling direction will show significant differences, which is not conducive to the surface quality of the material after forming. Based on this, in the present invention, the unit width rolling force can be controlled to be 2.7kN / mm~4.0kN / mm, and the unit rolling tension can be ≤40MPa.

[0067] The cold-rolled IF steel, hot-dip galvanized steel sheet and manufacturing method thereof described in the present invention have the following advantages and beneficial effects:

[0068] The cold-rolled IF steel and hot-dip galvanized steel sheet described in the present invention have good stamping, coating and deformation resistance properties, can meet users' requirements for material stamping forming, application of mid-coating-free process and maintaining shape and size stability during the transportation of automobile stamping parts, and can be used to produce automobile exterior panels, such as engine hoods and door exterior panels.

[0069] The manufacturing method of cold-rolled IF steel and hot-dip galvanized steel plate described in the present invention controls the elemental composition and production process of the steel plate, improves the work hardening value of the steel material, so that the stamping parts have higher deformation resistance, and at the same time enables the material to obtain a lower yield strength ratio and corrugation, thereby ensuring that the steel material has good stamping performance and coating effect, and at the same time can maintain the shape stability of the stamping parts during transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG1 shows a transmission electron microscope photograph of the PFZ tape of Example 1 of the present invention.

[0071] FIG2 shows a transmission electron microscope photograph of the intracrystalline precipitated phase of Example 1 of the present invention. DETAILED DESCRIPTION

[0072] The cold-rolled IF steel, hot-dip galvanized steel sheet and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0073] Examples 1-9 and Comparative Examples 1-9

[0074] The cold-rolled IF steels of Examples 1-5 and the hot-dip galvanized steel sheets of Examples 6-9 of the present invention were prepared by the following steps:

[0075] (1) Slabs were prepared according to the chemical composition ratios described in Table 1-1 and Table 1-2;

[0076] (2) Hot rolling: The furnace temperature of the hot rolled steel billet is 1150℃~1200℃, the finishing rolling temperature is 890℃~920℃, and the coiling temperature is 600℃~650℃;

[0077] (3) Cold rolling; control the cold rolling deformation to 75% to 85%;

[0078] (4) Annealing: controlling the annealing temperature to be between 800°C and 840°C, the holding time to be between 80s and 110s, and the cooling rate to be between 180°C / min and 230°C / min to obtain cold-rolled IF steel;

[0079] (5) Hot-dip galvanizing;

[0080] (6) Flatness. Control the unit width rolling force to 2.7kN / mm~4.0kN / mm and the unit rolling tension to ≤40MPa. Control the unit rolling tension to ≤40MPa to obtain hot-dip galvanized steel sheet.

[0081] Examples 1-5 do not include step (5), and Examples 6-9 include step (5).

[0082] The comparative steel materials of Comparative Examples 1-5 and the comparative steel plates of Comparative Examples 6-9 were also produced using the above-described steps, but their specific composition ratios and process parameters did not meet the design requirements of the present invention. Similarly, Comparative Examples 1-5 did not include a hot-dip galvanizing step, while Comparative Examples 6-9 did.

[0083] Table 1-1. (The balance is Fe and other unavoidable impurities except S, B, N and Si)

[0084] Table 1-2. (The balance is Fe and other unavoidable impurities except S, B, N and Si)

[0085] Table 2 lists the specific process parameters of the cold-rolled IF steels of Examples 1-5 of the present invention, the hot-dip galvanized steel sheets of Examples 6-9, the comparative steels of Comparative Examples 1-5, and the steel sheets of Comparative Examples 6-9 in the above process steps.

[0086] Table 2.

[0087] 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 and subjected to 4% nitric acid etching to prepare metallographic specimens and electrolytic double-spray etching to prepare transmissive specimens. The PFZ band width, precipitate size, and precipitate quantity were statistically observed and recorded in Table 3.

[0088] The number and diameter of the precipitates are the statistical results of the area size per square micrometer on the cross section of the embodiment and comparative example IF steel under a JEM-2100F transmission electron microscope.

[0089] The PFZ band width value is the average value of three observation fields selected for the embodiment and comparative example IF steel.

[0090] In addition, the grain sizes of 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 observed and recorded in Table 3.

[0091] The grain size was determined by making metallographic preparations for each embodiment and comparative example and selecting three observation fields for measurement with reference to GB / T 6394-2002, Method for Determination of Average Grain Size of Metals.

[0092] It should also be noted that the microstructure of the hot-dip galvanized steel obtained in Examples 6-9 is the same as the microstructure of the cold-rolled IF steel obtained before the hot-dip galvanizing step. Therefore, the microstructure of the hot-dip galvanized steel can also express the microstructure of the cold-rolled IF steel in the corresponding examples or comparative examples.

[0093] Table 3.

[0094] As can be seen from Table 3 above, the cold-rolled IF steels of Examples 1-5 and the hot-dip galvanized steel sheets of Examples 6-9 of the present invention have ideal organizational characteristics through reasonable chemical element composition design combined with optimized process parameters. Their microstructural grain size levels are all greater than or equal to 10, the width of the atom-free precipitation zone is between 0.3 μm and 0.8 μm, and the intragranular precipitates are all NbC with a size of less than 30 nm, and the number per square micron is between 600 and 1000.

[0095] In addition, Figure 1 shows a transmission electron micrograph of the PFZ tape of Example 1 of the present invention. Figure 2 shows a transmission electron micrograph of the intragranular precipitation phase of Example 1 of the present invention.

[0096] As shown in FIG1 and FIG2 , Example 1 of the present invention achieves ideal structural characteristics through reasonable chemical element composition design combined with optimized process parameters.

[0097] In addition, samples of 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 taken and tested for mechanical properties and work hardening values ​​according to GB / T 228.1-2021 Tensile Test for Metallic Materials. The test results are recorded in Table 4. The waviness of the samples before deformation and after 5% deformation cupping were tested according to the PV1054 standard, and the results are listed in Table 4.

[0098] It should also be noted that the mechanical properties and deformation resistance of the hot-dip galvanized steel obtained in Examples 6-9 are the same as the mechanical properties and deformation resistance of the cold-rolled IF steel obtained before the hot-dip galvanizing step. Therefore, the relevant properties of the hot-dip galvanized steel can also express the properties of the cold-rolled IF steel in the corresponding examples or comparative examples.

[0099] Table 4.

[0100] As can be seen from Table 4, the cold-rolled IF steels of Examples 1-5 and the hot-dip galvanized steel sheets of Examples 6-9 of the present invention, through rational chemical element composition design and optimized process parameters, yield excellent cold-rolled IF steels and hot-dip galvanized steel sheets. Their yield strengths ranged from 235 MPa to 270 MPa, their tensile strengths ranged from 385 MPa to 430 MPa, their yield strength ratios were less than 0.63, and their work hardening values ​​at 2% deformation were greater than 45.0 MPa. Their waviness (Wsa) was less than 0.18 μm, and their Wsa after 5% deformation and cupping was less than 0.22 μm.

[0101] At the same time, in combination with Tables 1, 2, 3 and 4 above, it can be seen that the chemical element composition designs of Comparative Examples 1-3 do not meet the design specification requirements of the present invention, resulting in their work hardening values ​​(corresponding to the deformation resistance of the steel after processing deformation) and yield strength ratios not being able to simultaneously meet the indicators of the present invention.

[0102] The tapping temperature of Comparative Example 4 exceeds the upper limit of the indicator requirements of the present invention, resulting in the formation of coarse grains during the rough rolling stage, which ultimately reduces the effect of grain refinement strengthening in the present invention, reduces the tensile strength of the steel, and makes the grains of the final product relatively coarse;

[0103] The finishing temperature of Comparative Example 5 exceeded the upper limit of the index requirement of the present invention, resulting in the finished steel grain level not meeting the standard and the waviness Wsa after stamping deformation exceeding the upper limit of the index requirement;

[0104] The annealing holding time of Comparative Example 6 is lower than the design specification requirement of the present invention, resulting in its yield ratio reaching 0.668, far exceeding the upper limit of the indicator requirement;

[0105] The coiling temperature of Comparative Example 7 is higher than the upper limit required by the design specification of the present invention, resulting in the waviness Wsa value of the finished product before and after stamping exceeding the upper limit required by the index;

[0106] The annealing temperature of Comparative Example 8 is lower than the design specification requirements of the present invention, resulting in insufficient thermal activation energy during the recrystallization process. Ultimately, the PFZ width of the steel is lower than the lower limit of the index, and the yield ratio and the waviness Wsa value before and after stamping are higher than the upper limit of the index.

[0107] The cooling rate of the annealing section of Comparative Example 9 is higher than the design specification requirements of the present invention, resulting in a narrowing of the low vacancy concentration width, and ultimately making the steel PFZ width lower than the index requirement and the yield strength ratio higher than the index requirement.

[0108] The hot-dip galvanized steel sheets corresponding to the cold-rolled IF steels of Examples 1-5 of the present invention and the cold-rolled IF steels corresponding to the hot-dip galvanized steel sheets of Examples 6-9 all have the properties described in the present invention.

[0109] From the above, it can be seen that the cold-rolled IF steel and hot-dip galvanized steel sheets described in the present invention have good stamping, coating and anti-deformation properties, which can meet the user's requirements for material stamping forming, application of mid-coating-free process and maintaining shape and size stability during the transportation of automobile stamping parts. They can be used to produce automobile exterior panels, such as engine hoods and door exterior panels, and have broad application prospects and use value.

[0110] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0111] 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 therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A cold-rolled IF steel containing Fe and inevitable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.0040-0.0070%, Mn: 0.50-0.80%, P: 0.025-0.040%, Nb: 0.040-0.080%, Cr: 0.05-0.20%, Al: 0.010-0.100%; It also satisfies: 1.20≤(Nb×12) / (C×93)≤2.00, where each chemical element is substituted into the value before the percentage sign of its mass percentage content.

2. The cold-rolled IF steel according to claim 1, characterized in that: The mass percentage of each chemical element is: C: 0.0040-0.0070%, Mn: 0.50-0.80%, P: 0.025-0.040%, Nb: 0.040-0.080%, Cr: 0.05-0.20%, Al: 0.010-0.100%; the balance is Fe and inevitable impurities.

3. The cold-rolled IF steel according to claim 1 or 2, characterized in that: Among the inevitable impurities, S≤0.01%, B≤0.0006%, N≤0.004%, and Si≤0.0010%.

4. The cold-rolled IF steel according to claim 1 or 2, characterized in that: The microstructure grain size grade is at least 10.

5. The cold-rolled IF steel according to claim 1 or 2, characterized in that: It has an atom-free precipitation zone, and the width of the atom-free precipitation zone is 0.30 μm to 0.80 μm.

6. The cold-rolled IF steel according to claim 1 or 2, characterized in that: Its microstructure comprises NbC precipitated in the crystal and NbC precipitated in the grain boundary, wherein: the diameter of NbC precipitated at the grain boundary is ≥60nm; and / or the size of NbC precipitated in the crystal is ≤30nm, and the number is 600-1000 per square micron.

7. The cold-rolled IF steel according to claim 6, characterized in that: The diameter of NbC precipitated at the grain boundary is 60-85 nm, and the size of NbC precipitated within the grain is 19.0-30.0 nm.

8. The cold-rolled IF steel according to claim 1 or 2, characterized in that: Its performance meets at least one of the following: Yield strength is 235-270MPa, tensile strength is 385-430MPa, yield strength ratio ≤0.630; 2% deformation work hardening value ≥45.0MPa; The corrugation before deformation is Wsa≤0.180μm, and the corrugation after deformation is Wsa≤0.220μm.

9. A hot-dip galvanized steel sheet, characterized in that: The base is the cold-rolled IF steel as claimed in any one of claims 1 to 8, and the base is coated with a hot-dip galvanized layer.

10. The method for manufacturing cold-rolled IF steel according to any one of claims 1 to 8, characterized in that: It includes the steps of: obtaining a slab; Hot rolling: The furnace temperature of hot rolled steel billet is 1150℃~1200℃, the finishing temperature is 890℃~920℃, and the coiling temperature is 600℃~650℃; Cold rolling; Annealing: control the annealing temperature to be between 800℃ and 840℃, the holding time to be between 80s and 110s, and the cooling rate to be between 180℃ / min and 230℃ / min.

11. The manufacturing method according to claim 9, characterized in that: In the cold rolling step, the cold rolling deformation is controlled to be 75% to 85%.

12. The manufacturing method according to claim 9, characterized in that: After the annealing step, the method further comprises a leveling step; preferably, in the leveling step, the unit width rolling force is controlled to be between 2.7 kN / mm and 4.0 kN / mm, and the unit rolling tension is ≤40 MPa.

13. The method for manufacturing a hot-dip galvanized steel sheet according to claim 9, characterized in that: It includes the steps of: obtaining a slab; Hot rolling: The furnace temperature of hot rolled steel billet is 1150℃~1200℃, the finishing temperature is 890℃~920℃, and the coiling temperature is 600℃~650℃; Cold rolling; Annealing: controlling the annealing temperature to be between 800° C. and 840° C., the holding time to be between 80s and 110s, and the cooling rate to be between 180° C. / min and 230° C. / min, to obtain the cold-rolled IF steel; Hot dip galvanizing; smooth.

14. The manufacturing method according to claim 13, characterized in that: In the cold rolling step, the cold rolling deformation is controlled to be 75% to 85%.

15. The manufacturing method according to claim 13, characterized in that: In the leveling step, the unit width rolling force is controlled to be between 2.7 kN / mm and 4.0 kN / mm, and the unit rolling tension is ≤40 MPa.

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