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

Abstract

The invention discloses a cold-rolled IF steel and hot-dip galvanized steel plate. The cold-rolled IF steel and hot-dip galvanized steel plate further comprises the following chemical elements in percentage by mass: 0.004 to 0.007 percent of C, 0.50 to 0.80 percent of Mn, 0.025 to 0.040 percent of P, 0.04 to 0.08 percent of Nb, 0.05 to 0.2 percent of Cr, 0.01 to 0.1 percent of Al and the balance of Fe. (Nb * 12) / (C * 93) is greater than or equal to 1.2 and less than or equal to 2.0, 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 manufacturing method of the steel plate. The manufacturing method comprises the following steps: preparing a plate blank; hot rolling; cold rolling; and annealing. The cold-rolled IF steel and the hot-dip galvanized steel plate have good stamping performance stability, excellent surface appearance and good deformation resistance.

Description

Technical Field

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

[0002] At present, the painting process of automotive outer panels can effectively reduce energy consumption because it reduces one coating layer and one baking process, and the VOC emissions are also significantly reduced. However, due to the reduction of coating and baking passes, the covering ability of the new painting process for the surface topography of the substrate has decreased, and higher requirements are put forward for the surface topography of the material, especially the surface waviness. In addition, automotive lightweighting is also an inevitable path in the automotive industry. By increasing the strength of steel plates, the thickness of automotive panels can be reduced, thereby effectively reducing the vehicle weight, reducing fuel consumption, and lowering emissions.

[0003] As a common raw material for automotive panels, IF steel mainly uses Ti / Nb elements to fix C and N atoms in crystal interstitial sites, and solid solution strengthening is carried out by adding Mn, Si, and P elements. There are no interstitial atoms dissolved in the matrix phase of this steel, and it has a low yield strength. The addition of solid solution elements also effectively increases the tensile strength of the material. Therefore, it has the characteristics of a low yield ratio, good deep drawing performance, and no aging phenomenon, and is widely used in the production of automotive outer panels.

[0004] However, in the actual application process, the stamped automotive outer panels made of thinner IF steel are prone to deformation during handling. However, when using high-strength grade steel to manufacture automotive stampings, it is found that their yield ratios are often greater than 0.65, the uniform plastic deformation range is significantly shortened, the material is not easy to deform, which is not conducive to the stamping forming of automotive panels. Therefore, it is necessary to improve the deformation resistance after stamping on the basis of maintaining the excellent stamping forming performance of IF steel.

[0005] Therefore, in order to make the product meet the requirements of users for stamping, handling, weight reduction, and painting, the steel plate should not only ensure that the mechanical properties of the material meet the standards, but also control its yield ratio, waviness after stamping, and deformation resistance after stamping.

[0006] For example, a Chinese patent document with the publication number CN109023050B, publication date December 18, 2018, and title "A 390MPa Grade High-Strength IF Steel and Its Production Method" discloses a high-strength IF steel and its production method. Its composition and mass fraction are as follows: 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%, and the balance is iron and unavoidable impurities. In this invention, the diameter of the precipitated phase is greater than 50 nm, and the main precipitated phases are FeNbP and FeTiP. It changes the material texture by changing the content of the precipitated phase, improves the plastic strain ratio, and has a certain improvement on the stamping performance of the material, but does not consider the coating quality of the material.

[0007] For another example, a Chinese patent document with the publication number CN114196882B, publication date October 28, 2022, and title "A Steel Strip Coil for High-Surface-Quality High-Strength Automotive Panels and Its Preparation Method" discloses a production method for automotive outer panels. Its composition and mass fraction 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%, and the rest is Fe and unavoidable impurities. In this invention, the composition and process of Nb-IF steel are mainly controlled to prepare materials with higher strength and excellent formability, but do not consider the influence of the material surface morphology and deformation resistance on the coating and transportation of automotive outer panels. Summary of the Invention

[0008] One of the purposes of the present invention is to provide a cold-rolled IF steel, which has good stamping performance stability, excellent surface morphology, and good deformation resistance, can ensure that during the use by users, the stamping parts are avoided from being deformed during handling, ensure that the surface waviness of the material after electrophoresis is low, and there is no obvious orange peel phenomenon. This cold-rolled IF steel can effectively improve the deformation resistance and production stability of stamping parts, improve the surface quality of the coating of automotive outer panels, is suitable for the production of automotive outer coverings, and has broad application prospects and value.

[0009] To achieve the above purpose, the present invention provides a cold-rolled IF steel, which contains Fe and unavoidable impurities, and in addition, it also contains the following chemical elements with the following mass percentages:

[0010] C: 0.004 - 0.007%, Mn: 0.50 - 0.80%, P: 0.025 - 0.040%, Nb: 0.04 - 0.08%, Cr: 0.05 - 0.2%, Al: 0.01 - 0.1%;

[0011] It also satisfies: 1.2 ≤ (Nb × 12) / (C × 93) ≤ 2.0, where each chemical element is substituted with 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 contents of its various chemical elements are as follows:

[0013] C: 0.004 - 0.007%, Mn: 0.50 - 0.80%, P: 0.025 - 0.040%, Nb: 0.04 - 0.08%, Cr: 0.05 - 0.2%, Al: 0.01 - 0.1%; the balance is Fe and unavoidable impurities.

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

[0015] C: In the cold-rolled IF steel of the present invention, the C element in the steel will affect the content of NbC precipitation phase, and NbC particles can play the role of fine grain strengthening and second phase strengthening. When the content of C element is greater than 0.007%, it is easy to form solid solution C atoms, which will lead to the formation of {111} texture, and this texture is not conducive to the stamping performance of the material, especially the r value of the material. When the content of C is less than 0.004%, the precipitation quantity of NbC particles is small, which will lead to the reduction of the material grain size, and at the same time will weaken the effect of second phase strengthening and reduce the material strength. Therefore, in the cold-rolled IF steel of the present invention, the mass percentage of C element can be controlled between 0.004 - 0.007%.

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

[0017] P: In the cold-rolled IF steel described in the present invention, the P element, as a solid-solution strengthening element, is also one of the elements with the strongest effect on strengthening ferrite, and can effectively improve the strength of the material. When the content of the P element is too low, the strengthening effect of the P element is not significant. When the content of the P element is too high, segregation is likely to occur, increasing the brittleness of the steel, affecting the welding performance, and at the same time having an adverse effect on the alloying treatment of the zinc layer. Therefore, in the cold-rolled IF steel described in the present invention, the mass percentage of the P element can be controlled between 0.025 - 0.040%.

[0018] Nb: In the cold-rolled IF steel described in the present invention, the Nb element mainly forms carbides with the C element and precipitates, and the surplus part exists as a solid-solution element. The fine and dispersed carbides precipitated can effectively refine the grains, and a precipitate-free zone (PFZ) can be formed during the nucleation and growth process, which can effectively improve the strength of the material and reduce the yield ratio. If the content of the Nb element is too low, the free C atoms cannot be completely fixed, reducing the yield strength. Therefore, in the cold-rolled IF steel described in the present invention, the mass percentage of the Nb element can be controlled between 0.04 - 0.08%.

[0019] Cr: In the cold-rolled IF steel described in the present invention, the Cr element is used as a solid-solution strengthening element in the present invention. Its characteristics are as follows: the carbide-forming ability of the Cr element is lower than that of the Nb element; the Cr element has a strong affinity for carbon, which can effectively reduce the diffusion coefficient of the C element in the iron matrix; the atomic radius of the Cr element is larger than that of the Mn element, resulting in a higher degree of lattice distortion; the solubility of the Cr element in the iron matrix is greater than that of the Nb element, and the degree of lattice distortion caused is less than that of the Nb element. When the mass percentage content of Cr ≤ 0.05%, it is not sufficient to significantly increase the lattice resistance of dislocation slip during deformation, refine the NbC precipitation phase, and increase the work-hardening value; when its mass percentage content ≥ 0.2%, due to the opposite effect of lattice distortion on the Fe matrix caused by the P element, it will instead reduce the solid-solution strengthening effect of the P and Cr elements. Adding Cr elements with a mass percentage content between 0.05 - 0.2% can reduce the diffusion rate of carbon atoms, promote the precipitation of the NbC phase, slow down the growth of the NbC phase, and at the same time refine the grains, thereby increasing the work-hardening value and deformation resistance of the material. Based on this, in the cold-rolled IF steel described in the present invention, the mass percentage of the Cr element can be controlled between 0.05 - 0.2%.

[0020] Al: In the cold-rolled IF steel described in the present invention, the Al element, as an impurity element of the deoxidizer, can fix free N atoms. When the content of the Al element is low, the nitrogen-fixing and deoxidizing effects cannot be guaranteed; when the content of the Al element is too high, it is easily dissolved in the matrix, reducing the plasticity of the material and generating more inclusions. Therefore, in the cold-rolled IF steel described in the present invention, the mass percentage of the Al element can be controlled between 0.01 - 0.1%.

[0021] It should be noted that while controlling the mass percentage of a single element in the present invention, the range of (Nb×12) / (C×93) also needs to be controlled between 1.2 and 2. This is the key factor to ensure that the steel coil described in the present invention has a low yield ratio and high deformation resistance, and is also an important factor to ensure that the steel coil has low waviness. When the mass percentages of Nb and C elements are lower than the lower limit of the range of (Nb×12) / (C×93), the effects of fine grain strengthening and second phase strengthening will be weakened; when the mass percentages of Nb and C elements are higher than the upper limit of the range of (Nb×12) / (C×93), it will cause the annealing recrystallization temperature to rise, increase the yield strength of the material, and is not conducive to the formation of favorable textures and cost control.

[0022] Furthermore, in the cold-rolled IF steel described in the present invention, among the inevitable impurities, S≤0.01%, B≤0.0006%, N≤0.004%, Si≤0.001%.

[0023] In the cold-rolled IF steel described in the present invention, the elements S, B, N, and Si are all inevitable impurities. In order to ensure that the cold-rolled IF steel described in the present invention has good performance, when conditions permit, it is desirable that their contents are as low as possible. Among them:

[0024] S: In the cold-rolled IF steel described in the present invention, the S element is controlled as a harmful impurity element in the steel. It forms a low-melting-point precipitation phase in the steel, causing hot brittleness of the material and being unfavorable to the welding performance of the material. Based on this, in the cold-rolled IF steel described in the present invention, the mass percentage of the S element can be controlled below 0.01%.

[0025] B: In the cold-rolled IF steel described in the present invention, the B element will reduce the plasticity of the material. When its content is too high, it is not conducive to the progress of recrystallization and inhibits the formation of the PFZ band. Based on this, in the cold-rolled IF steel described in the present invention, the mass percentage of the B element can be controlled below 0.0006%.

[0026] N: In the cold-rolled IF steel described in the present invention, the dissolved N atoms will cause the r value of the material to decrease and cause aging phenomena, which is not conducive to the anti-aging performance and stamping performance of the material. Therefore, in the cold-rolled IF steel described in the present invention, the mass percentage of the N element can be controlled below 0.004%.

[0027] Si: In the cold-rolled IF steel described in the present invention, the Si element is an impurity element in the steelmaking process and is prone to segregation at the grain boundaries, forming a hot brittle phase. When the content of the Si element is relatively high, it will also have an adverse effect on the anisotropy and plasticity of the material. When the technical conditions permit, its content in the steel should be reduced as much as possible. In the present invention, in the cold-rolled IF steel described in the present invention, the mass percentage of the Si element can be controlled below 0.001%.

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

[0029] In the present invention, controlling the grain size grade to at least 10 can ensure that the material has sufficient strength, while reducing the waviness after stamping of the stamped parts, enabling the material to have high painting quality.

[0030] Furthermore, in the cold-rolled IF steel of the present invention, it has a precipitate-free zone (PFZ), and the width of the precipitate-free zone is 0.3 μm to 0.8 μm.

[0031] In the present invention, the PFZ band refers to the precipitate-free zone (PFZ) formed around the grain boundaries of the material, and the width of this PFZ band has a significant impact on the yield ratio of the material. When the width of the precipitate-free zone ≤ 0.3 μm, the accumulated dislocations are not conducive to the formation of a dislocation pile-up group with a large size, resulting in a high local stress, and at the same time, it is not conducive to the occurrence of cross-slip, and cannot effectively reduce the yield strength of the material, increasing the stamping difficulty. When the width of the precipitate-free zone ≥ 0.8 μm, it is necessary to significantly increase the annealing holding time, which is not conducive to cost control. Therefore, in the cold-rolled IF steel of the present invention, the average width of the precipitate-free zone can be controlled between 0.3 μm and 0.8 μm.

[0032] Furthermore, in the cold-rolled IF steel of the present invention, its microstructure has NbC precipitated in the grains and NbC precipitated at the grain boundaries, wherein the size of the NbC precipitated in the grains ≤ 30 nm, and the number is 600 - 1000 per square micron.

[0033] In the present invention, the main precipitated phases in 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 sufficient to effectively refine the grains and hinder the dislocation slip, enabling the material to have high 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 content of Nb element, inevitably increasing the economic cost and increasing the yield strength, which is not conducive to stamping. At the same time, when the diameter of the precipitated phases at the grain boundaries of the cold-rolled IF steel ≥ 60 nm, it is conducive to the formation of the PFZ band, and when the diameter of the precipitated phases is less than 60 nm, it is not conducive to grain growth and the formation of the PFZ band. Based on this, in the cold-rolled IF steel of the present invention, it can be controlled that its microstructure has NbC precipitated in the grains and NbC precipitated at the grain boundaries, wherein the size of the NbC precipitated in the grains ≤ 30 nm, and the number is 600 - 1000 per square micron; furthermore, in the cold-rolled IF steel of the present invention, the performance meets at least one of the following items:

[0034] The yield strength is 235 - 270 MPa, the tensile strength is 385 - 430 MPa, and the yield ratio ≤ 0.63;

[0035] The work-hardening value at 2% strain ≥ 45 MPa;

[0036] The waviness Wsa before deformation ≤ 0.18 μm, and the waviness Wsa after deformation ≤ 0.22 μm.

[0037] In the present invention, the reason for limiting the yield strength within the range of 235 - 270 MPa is as follows: too high a yield strength is not conducive to the stamping forming of the material, while too low a yield strength is not conducive to the dent resistance and deformation resistance of the material. At the same time, limiting the tensile strength within the range of 385 - 430 MPa is mainly restricted by the process and material composition. When the tensile strength of the material is relatively high, its alloy composition or microstructure will change accordingly, and the yield strength of the material will also increase, which is not conducive to the stamping forming of the material. While too low a tensile strength is likely to lead to insufficient material strength and reduced deformation resistance.

[0038] In addition, in the present invention, the work-hardening value is used to characterize the ability of the part to resist deformation after forming, that is, the increase in the yield strength of the material after tensile deformation by a certain amount of strain. The traditional IF steel has a relatively low yield strength, and the increase in yield strength after stamping is small, so the parts made are prone to deformation during handling. For automotive steels, in the past, the strain hardening rate (n) was usually used to measure the ability of the material to resist deformation during the stamping deformation process. The selected measurement range of this value is often the uniform plastic deformation section, that is, the strain hardening rate in the large deformation range (usually in the range of 10% - 20% strain), which cannot measure the deformation resistance of low yield strength materials after deformation. The deformation amount of automotive outer panels is usually between 3% and 7%. Increasing the work-hardening value in this strain range can effectively increase the deformation resistance of the stamped parts and avoid the deformation of the stamped parts during handling. Therefore, limiting the work-hardening value (WH) ≥ 45 MPa at a small strain amount (for example, the strain amount corresponding to the work-hardening value in the present invention can be 2%) is beneficial to maintaining the shape stability of the stamped parts during transportation.

[0039] In the present invention, the 5% strain cupping can be used to simulate the stamping forming process of automotive outer panels, and the waviness value after deformation can be controlled below 0.22 μm, which can ensure that the orange peel R value of the material is at a relatively low level after subsequent electrophoresis and painting, and improve the painting quality.

[0040] Another object of the present invention is to provide a hot-dip galvanized steel sheet, which has good stamping, painting and deformation resistance properties, and can meet the requirements of users for the stamping forming of materials, the application of the process without intermediate coating, and the maintenance of the shape and size stability of automotive stamped parts during handling.

[0041] To achieve the above object, the present invention also provides a hot-dip galvanized steel sheet, the base plate of which is the cold-rolled IF steel of the present invention as described above, and a hot-dip galvanized layer is plated on the base plate.

[0042] Another object of the present invention is to provide a manufacturing method of cold-rolled IF steel. By controlling the production process and the element composition of the steel plate, this method can increase the work hardening value of the steel, enable the stamping parts to have a higher deformation resistance, and at the same time enable the material to obtain a lower yield ratio and waviness, so as to ensure that the steel has good stamping performance and painting effect, and at the same time maintain the shape stability of the stamping parts during handling.

[0043] To achieve the above object, the present invention also provides a manufacturing method of cold-rolled IF steel, which includes the steps of:

[0044] Manufacturing a slab;

[0045] Hot rolling: The tapping temperature of the hot-rolled billet is 1150°C to 1200°C, the finish rolling temperature is 890°C to 920°C, and the coiling temperature is 600°C to 650°C;

[0046] Cold rolling;

[0047] Annealing: Control the annealing temperature between 800°C and 840°C, the holding time is 80s to 110s, and the cooling rate is 180°C / min to 230°C / min.

[0048] In the above hot rolling step of the present invention, when the tapping temperature of the hot-rolled billet is too low, it is not conducive to the uniform heating of the billet and the complete austenitization process; when the tapping temperature of the hot-rolled billet is too high, it is easy to cause problems such as an increase in scale and severe coarsening of the original austenite grains, which have an adverse impact on grain refinement, the surface quality of the steel plate, and the descaling process. Based on this, the tapping temperature of the hot-rolled billet can be controlled between 1150°C and 1200°C.

[0049] In the above hot rolling step of the present invention, when the finish rolling temperature is too high, it is easy to cause an increase in grain size; when the finish rolling temperature is too low, it is easy to cause relatively severe work hardening, increase the rolling difficulty, and cause an increase in the yield strength of the finished product. Based on this, the finish rolling temperature of the hot-rolled billet can be controlled between 890°C and 920°C.

[0050] In the above hot rolling step of the present invention, the coiling temperature of the steel coil should be controlled between 600°C and 650°C. The reason is that when the coiling temperature is too low, it is easy to cause deterioration of the processing performance and is not conducive to coiling; when the coiling temperature is too high, it is easy to cause grain growth and the growth of NbC precipitation phase, which is not conducive to the control of its grain size in the subsequent steps.

[0051] In the annealing step of the present invention, the annealing temperature is controlled between 800°C and 840°C, the holding time is between 80 s and 110 s, and the cooling rate is controlled between 180°C / min and 230°C / min. The reason is as follows: When the annealing temperature is too low, recrystallization is insufficient, the width of the PFZ band formed is small, which is not conducive to reducing the yield ratio of the material; when the annealing temperature is too high, it is easy to cause grain coarsening and the disappearance of lath ferrite, which has an adverse effect on the material strength and the surface after stamping. If the holding time is too short, annealing is incomplete, which is not conducive to deep drawing performance; if the holding time is too long, grain coarsening will occur, resulting in a decrease in material strength. If the cooling rate is too slow, production efficiency will be reduced; if the cooling rate is too fast, it will be unfavorable to form a relatively wide low vacancy concentration region, and at the same time, Ostwald ripening is insufficient, reducing the formation width of the PFZ band.

[0052] Further, in the cold rolling step of the manufacturing method of the cold-rolled IF steel of the present invention, the cold rolling reduction is controlled to be 75% - 85%.

[0053] In the cold rolling step of the manufacturing method of the cold-rolled IF steel of the present invention, when the cold rolling reduction is too low, the grains obtained by rolling are coarser, the stored energy of deformation is lower, which is not conducive to annealing recrystallization, and at the same time, it is not sufficient to form lath ferrite grains, which is not conducive to the stamping performance of the material; when the cold rolling reduction is too high, abnormally grown grains are likely to appear, resulting in uneven grain size, which is not conducive to the elongation of the material and the surface morphology after stamping. Based on this, the cold rolling reduction is controlled between 75% and 85%.

[0054] Another object of the present invention is to provide a manufacturing method of a hot-dip galvanized steel sheet. By controlling the element composition and production process of the steel sheet, the work hardening value of the steel is increased, so that the stamped parts have a relatively high deformation resistance, and at the same time, the material obtains a relatively low yield ratio and waviness, so as to ensure that the steel has good stamping performance and painting effect, and at the same time, the shape of the stamped parts can be maintained stable during handling.

[0055] In order to achieve the above object, the present invention also provides a manufacturing method of a hot-dip galvanized steel sheet, which includes the steps of:

[0056] Producing a slab;

[0057] Hot rolling: The slab discharging temperature of the hot-rolled steel billet is 1150°C - 1200°C, the finish rolling temperature is 890°C - 920°C, and the coiling temperature is 600°C - 650°C;

[0058] Cold rolling;

[0059] Annealing: Controlling the annealing temperature between 800°C and 840°C, the holding time is 80 s - 110 s, and the cooling rate is 180°C / min - 230°C / min to obtain the cold-rolled IF steel;

[0060] Hot dip galvanizing;

[0061] Skin pass.

[0062] Furthermore, in the cold rolling step of the manufacturing method of the hot dip galvanized steel sheet of the present invention, the cold rolling reduction is controlled to be 75% - 85%.

[0063] Furthermore, in the skin pass step of the manufacturing method of the hot dip galvanized steel sheet of the present invention, the rolling force per unit width is controlled to be 2.7 kN / mm - 4.0 kN / mm, and the unit rolling tension ≤ 40 MPa.

[0064] In the skin pass process of the manufacturing method of the hot dip galvanized steel sheet of the present invention, the rolling force per unit width and the unit rolling tension have a strong improving effect on the sheet shape quality, surface topography and mechanical properties of the material. By controlling the skin pass 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 topography of the material after skin pass, and the drawn cup of the obtained material also has a lower waviness. However, if the rolling force is too large, roll marks are likely to occur. If the tension is too small, it is easy to cause poor sheet shape control and a large increase in the rolling force, which is not conducive to production; if the tension is too large, significant differences will appear in the deformation degree and dislocation density of various textures in the material along the rolling direction, which is not conducive to the surface quality of the material after forming. Based on this, in the present invention, the rolling force per unit width can be controlled to be 2.7 kN / mm - 4.0 kN / mm, and the unit rolling tension ≤ 40 MPa.

[0065] A cold rolled IF steel, a hot dip galvanized steel sheet and a manufacturing method thereof according to the present invention have the following advantages and beneficial effects:

[0066] A cold rolled IF steel and a hot dip galvanized steel sheet according to the present invention have good stamping, painting and anti-deformation properties, and can meet the requirements of users for stamping and forming of materials, application of the primerless process, and maintaining the shape and size stability during the handling of automotive stamped parts. It can be used to produce automotive outer panels, such as engine hoods and outer door panels.

[0067] The manufacturing method of a cold rolled IF steel and a hot dip galvanized steel sheet according to the present invention controls the elemental composition and production process of the steel sheet, improves the work hardening value of the steel to make the stamped parts have a higher deformation resistance, and at the same time enables the material to obtain a lower yield ratio and waviness, so as to ensure that the steel has good stamping performance and painting effect, and can maintain the shape stability of the stamped parts during handling. Description of the Drawings

[0068] Figure 1 Shows a transmission electron microscope photograph of the PFZ band of Example 1 of the present invention.

[0069] Figure 2 Shows a transmission electron microscope photograph of the intragranular precipitation phase of Example 1 of the present invention. Detailed implementation manners

[0070] The following will further explain and illustrate the cold-rolled IF steel, hot-dip galvanized steel sheet and their manufacturing methods according to the present invention with specific embodiments. However, such explanations and illustrations shall not unduly limit the technical solutions of the present invention.

[0071] Examples 1-9 and Comparative Examples 1-9

[0072] The cold-rolled IF steel and hot-dip galvanized steel sheet of Examples 1-9 of the present invention are all obtained by the following steps:

[0073] (1) Prepare a slab according to the chemical composition ratios described in Table 1-1 and Table 1-2;

[0074] (2) Hot rolling: The tapping temperature of the hot-rolled billet is 1150°C to 1200°C, the finish rolling temperature is 890°C to 920°C, and the coiling temperature is 600°C to 650°C;

[0075] (3) Cold rolling; control the cold rolling reduction to be 75% to 85%;

[0076] (4) Annealing: Control the annealing temperature to be between 800°C and 840°C, the holding time to be 80 s to 110 s, and the cooling rate to be 180°C / min to 230°C / min to obtain the cold-rolled IF steel;

[0077] (5) Hot-dip galvanizing;

[0078] (6) Temper rolling. Control the rolling force per unit width to be 2.7 kN / mm to 4.0 kN / mm, and the unit rolling tension ≤ 40 MPa. Control the unit rolling tension ≤ 40 MPa to obtain the hot-dip galvanized steel sheet.

[0079] The comparative steel and steel sheet of Comparative Examples 1-9 are also obtained by the above steps, but their specific composition ratios and process parameters do not meet the design requirements of the present invention.

[0080] Table 1-1. (The balance is Fe and other inevitable impurities except S, B, N, and Si)

[0081] Number C Mn P Nb Cr Al Example 1 0.004 0.65 0.035 0.04 0.15 0.07 Example 2 0.0052 0.7 0.04 0.08 0.2 0.012 Example 3 0.0051 0.6 0.028 0.048 0.05 0.03 Example 4 0.0055 0.65 0.035 0.064 0.15 0.025 Example 5 0.0061 0.6 0.03 0.064 0.1 0.049 Example 6 0.005 0.5 0.035 0.064 0.16 0.05 Example 7 0.0065 0.5 0.025 0.07 0.09 0.024 Example 8 0.007 0.8 0.034 0.08 0.14 0.06 Example 9 0.0051 0.6 0.035 0.064 0.15 0.1 Comparative Example 1 0.005 0.6 0.034 0.063 <![CDATA 0.008 > 0.009 Comparative Example 2 0.0051 0.6 0.034 0.063 <![CDATA 0.3 > 0.014 Comparative Example 3 0.0051 0.6 0.034 <![CDATA 0.039 > 0.15 0.03 Comparative Example 4 0.0035 <![CDATA 0.47 > 0.02 0.055 0.14 0.12 Comparative Example 5 0.0051 0.6 0.035 0.064 0.14 0.09 Comparative Example 6 <![CDATA 0.008 > <![CDATA 0.9 > 0.043 <![CDATA 0.09 > 0.15 0.026 Comparative Example 7 0.0051 0.6 0.035 0.064 0.16 0.03 Comparative Example 8 <![CDATA 0.0073 > 0.65 0.034 0.064 0.14 0.07 Comparative Example 9 0.0051 0.65 0.035 0.064 0.14 0.04

[0082] Table 1-2. (The balance is Fe and other inevitable impurities except S, B, N, and Si)

[0083]

[0084]

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

[0086] Table 2.

[0087]

[0088] The cold-rolled IF steel of Examples 1-5, the hot-dip galvanized steel sheets of Examples 6-9, and the comparative steel materials of Comparative Examples 1-9 obtained were respectively sampled and subjected to 4% nitric acid corrosion to prepare metallographic specimens and electrolytic twin-jet to prepare transmission specimens. The width of the PFZ band, the size of the precipitated phases, and the number of the precipitated phases were statistically observed and recorded in Table 3. Among them:

[0089] The number and diameter of the precipitates are the results of statistical analysis of the area size per square micron on the cross-section of the IF steel of the examples and comparative examples under a JEM-2100F transmission electron microscope.

[0090] The width value of the PFZ band is the average value statistically obtained by selecting three observation fields of view for the IF steel of the examples and comparative examples.

[0091] In addition, the grain sizes of the cold-rolled IF steel of Examples 1-5, the hot-dip galvanized steel sheets of Examples 6-9, and the comparative steel materials of Comparative Examples 1-9 obtained were observed and recorded in Table 3. Among them:

[0092] The grain size level was obtained by referring to GB / T 6394-2002 Determination Method for Average Grain Size of Metals, preparing metallographs for each example and comparative example, and measuring by selecting three observation fields of view.

[0093] It should be noted that the microstructures of the hot-dip galvanized steels obtained in Examples 1-9 and the cold-rolled IF steels obtained before the hot-dip galvanizing step are the same. Therefore, the microstructure of the hot-dip galvanized steel can also be used to represent the microstructure of the corresponding cold-rolled IF steel of the examples or comparative examples.

[0094] Table 3.

[0095]

[0096] As can be seen from Table 3 above, the cold-rolled IF steel of Examples 1-5 and the hot-dip galvanized steel sheets of Examples 6-9 of the present invention have obtained ideal tissue characteristics through reasonable chemical element composition design and combined with optimized process parameters. Their microstructure grain size levels are all greater than or equal to 10, the width of the no-atom precipitation zone is between 0.3 μm and 0.8 μm, the intragranular precipitates are all NbC and the size is less than 30 nm, and the number per square micron is between 600 and 1000.

[0097] In addition,Figure 1 The transmission electron microscope photograph of the PFZ band of Example 1 of the present invention is shown. Figure 2 The transmission electron microscope photograph of the intragranular precipitation phase of Example 1 of the present invention is shown.

[0098] As Figure 1 and Figure 2 shown, through reasonable chemical element composition design and combined with optimized process parameters in Example 1 of the present invention, ideal tissue characteristics are obtained.

[0099] In addition, 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 are respectively sampled, and the mechanical properties and work hardening values of the specimens are measured with reference to GB / T 228.1-2021 Metallic materials - Tensile testing, and the test results are recorded in Table 4. The pre-deformation waviness and the waviness after 5% deformation cup drawing of the specimens are detected with reference to the PV1054 standard, and the results are listed in Table 4.

[0100] It should be further noted that the mechanical properties and anti-deformation properties of the hot-dip galvanized steels obtained in Examples 1-9 and the cold-rolled IF steels obtained before the hot-dip galvanizing step are the same. Therefore, the relevant properties of the hot-dip galvanized steels can also be used to represent the properties of the cold-rolled IF steels of the corresponding examples or comparative examples.

[0101] Table 4.

[0102]

[0103] As can be seen from Table 4 above, through reasonable chemical element composition design and combined with optimized process parameters, the cold-rolled IF steels of Examples 1-5 and the hot-dip galvanized steel sheets of Examples 6-9 of the present invention obtain cold-rolled IF steels and hot-dip galvanized steel sheets with excellent properties. Their yield strengths are all between 235 MPa and 270 MPa, their tensile strengths are all between 385 MPa and 430 MPa, their yield ratio is less than 0.63, and their work hardening values at 2% deformation are all greater than 45 MPa. Their waviness Wsa is less than 0.18 μm, and the Wsa after 5% deformation cup drawing is less than 0.22 μm.

[0104] At the same time, it can be seen from Tables 1, 2, 3 and 4 above that the chemical element composition designs of Comparative Examples 1-3 do not meet the design specification requirements of the present invention, resulting in that their work hardening values (the deformation resistance after the steel is processed and deformed) and yield ratio cannot meet the indexes of the present invention at the same time.

[0105] The tapping temperature of Comparative Example 4 exceeds the upper limit of the index requirements of the present invention, resulting in the formation of coarse grains in the rough rolling stage, ultimately reducing the effect of fine grain strengthening in the present invention, reducing the tensile strength of the steel, and making the final finished product grains relatively coarse;

[0106] The finishing temperature of comparative example 5 exceeds the upper limit of the index requirement of the present invention, resulting in the grain level of the finished steel material not meeting the standard, and the waviness Wsa after stamping deformation exceeds the upper limit of the index requirement;

[0107] 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;

[0108] 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;

[0109] 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, and ultimately causing the PFZ band width of the steel to be lower than the lower limit of the index, and the yield ratio and the waviness Wsa value before and after stamping to be higher than the upper limit of the index;

[0110] 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 requirements and the yield strength ratio higher than the index requirements.

[0111] The hot-dip galvanized steel sheets corresponding to the cold-rolled IF steels of Examples 1-5 described in the present invention and the cold-rolled IF steels corresponding to the hot-dip galvanized steel sheets of Examples 6-9 described in the present invention both have the properties described in the present invention.

[0112] From the above, it can be seen that the cold-rolled IF steel and hot-dip galvanized steel plates 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 outer panels, such as engine hoods and door outer panels, and have broad application prospects and use value.

[0113] 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.

[0114] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A cold-rolled IF steel, which contains Fe and inevitable impurities, characterized in that, it further contains the following chemical elements with the mass percentage contents as follows: C: 0.004 - 0.007%, Mn: 0.50 - 0.80%, P: 0.025 - 0.040%, Nb: 0.04 - 0.08%, Cr: 0.05 - 0.2%, Al: 0.01 - 0.1%; it further satisfies: 1.2 ≤ (Nb × 12) / (C × 93) ≤ 2.0, where each chemical element in the formula is substituted with 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 contents of its chemical elements are: C: 0.004 - 0.007%, Mn: 0.50 - 0.80%, P: 0.025 - 0.040%, Nb: 0.04 - 0.08%, Cr: 0.05 - 0.2%, Al: 0.01 - 0.1%; 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%, Si ≤ 0.001%.

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

5. The cold-rolled IF steel according to claim 1 or 2, characterized in that, it has a no-atom-precipitation zone, and the width of the no-atom-precipitation zone is 0.3 μm to 0.8 μm.

6. The cold-rolled IF steel according to claim 1 or 2, characterized in that, its microstructure has NbC precipitated in grains and NbC precipitated at grain boundaries, where the size of the NbC precipitated in grains ≤ 30 nm and the number is 600 - 1000 per square micron.

7. The cold-rolled IF steel according to claim 1 or 2, characterized in that, its properties satisfy at least one of the following items: yield strength is 235 - 270 MPa, tensile strength is 385 - 430 MPa, yield ratio ≤ 0.63; work hardening value at 2% deformation ≥ 45 MPa; waviness Wsa before deformation ≤ 0.18 μm, waviness Wsa after deformation ≤ 0.22 μm.

8. A hot-dip galvanized steel sheet, characterized in that, its substrate is the cold-rolled IF steel according to any one of claims 1 - 7, and a hot-dip galvanized layer is plated on the substrate.

9. The manufacturing method of the cold-rolled IF steel according to any one of claims 1 - 7, characterized in that, it includes the steps: producing a slab; hot rolling: the tapping temperature of the hot-rolled steel billet is 1150°C - 1200°C, the finish rolling temperature is 890°C - 920°C, and the coiling temperature is 600°C - 650°C; cold rolling; annealing: controlling the annealing temperature to be between 800°C - 840°C, the holding time to be 80 s - 110 s, and the cooling rate to be 180°C / min - 230°C / min.

10. The manufacturing method according to claim 9, characterized in that, in the cold rolling step, controlling the cold rolling reduction to be 75% - 85%.

11. The manufacturing method of the hot-dip galvanized steel sheet according to claim 8, characterized in that, it comprises the steps of: producing a slab; hot rolling: the tapping temperature of the hot-rolled steel billet is 1150°C to 1200°C, the finish rolling temperature is 890°C to 920°C, and the coiling temperature is 600°C to 650°C; cold rolling; annealing: controlling the annealing temperature to be between 800°C and 840°C, the holding time to be 80 s to 110 s, and the cooling rate to be 180°C / min to 230°C / min to obtain the cold-rolled IF steel; hot-dip galvanizing; skin pass rolling.

12. The manufacturing method according to claim 11, characterized in that, in the cold rolling step, the cold rolling reduction is controlled to be 75% to 85%.

13. The manufacturing method according to claim 11, characterized in that, in the skin pass rolling step, the rolling force per unit width is controlled to be 2.7 kN / mm to 4.0 kN / mm, and the unit rolling tension ≤ 40 MPa.

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