High-strength cold-rolled steel having excellent bake hardenability and method for manufacturing the same

Abstract

This invention discloses a high-strength cold-rolled steel with excellent baking performance, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages: C: 0.06-0.10%, 0 < Si ≤ 0.05%, Mn: 0.40-0.7%, Al: 0.03-0.05%; the microstructure of the high-strength cold-rolled steel has recrystallized equiaxed ferrite with a volume fraction of more than 95%, and slender ferrite extending along the rolling direction with a volume fraction of 3-5%. This invention also discloses a method for manufacturing high-strength cold-rolled steel, comprising the following steps: smelting and continuous casting; hot rolling: the furnace exit temperature is 1150-1250℃, the hot rolling final rolling temperature is 830-890℃, the coiling temperature is 520-650℃, laminar flow cooling is adopted after rolling, and the rapid cooling rate of the laminar flow front section is 80-120℃ / s; pickling and cold rolling; continuous annealing: controlling the annealing homogenization temperature to be 680-750℃; leveling.

Description

Technical Field

[0001] This invention relates to a steel plate and a method for manufacturing the same, and more particularly to a cold-rolled steel and a method for manufacturing the same. Background Technology

[0002] Currently, the yield strength of cold-rolled steel sheets used in home appliances is generally around 150-280 MPa, the tensile strength is around 250-300 MPa, and the elongation after fracture is around 20-30%. Thin-gauge high-strength cold-rolled steel sheets for home appliances are generally thicker than 0.45 mm. However, due to their relatively low strength, cold-rolled steel sheets for home appliances are currently only used for non-critical components such as the back panel of appliances.

[0003] In addition, the following problems exist in the current use of cold-rolled steel sheets for home appliances: For large and thin cold-rolled steel sheets for home appliances, there is insufficient material rigidity, and bending is prone to occur during production and use, which will have an adverse effect on the appearance and cannot be eliminated; In addition, after foaming, cold-rolled steel sheets for home appliances are prone to condenser tube marks, which will have an adverse effect on the product appearance; After assembly, cold-rolled steel sheets for home appliances are prone to deformation and cracking during subsequent drop tests.

[0004] In the prior art, existing patent documents have addressed the above-mentioned areas:

[0005] For example, Chinese patent document CN110714165A, published on January 21, 2020, entitled "A 320MPa grade cold-rolled sheet for home appliance panels and its production method", discloses a 320MPa grade cold-rolled sheet for home appliance panels and its production method. It strengthens the structure by adding the high-cost alloying element Ti to achieve the goal of high strength, and controls the final product shape through annealing and leveling.

[0006] For example, Chinese patent document CN103194670A, published on July 10, 2013, entitled "A Steel Strip for Home Appliance Panels and a Manufacturing Method Thereof," discloses a steel strip for home appliance panels and a manufacturing method thereof.

[0007] For example, Chinese patent document CN101008067A, published on August 1, 2007, entitled "Method for producing cold-rolled appliance sheets using a bell-type furnace," discloses a method for producing cold-rolled appliance sheets using a bell-type furnace. The cold-rolled appliance sheets produced by this method have a yield strength of 304 MPa, a tensile strength of 379 MPa, and an elongation of 30%, which meets the performance requirements of high-strength appliance sheets. However, this bell-type furnace has low production efficiency, poor sheet shape, and low surface quality. In addition, it increases the strength by adding Nb, which results in high costs. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high-strength cold-rolled steel with excellent baking hardening properties, high strength and high elongation.

[0009] To achieve the above objectives, the present invention provides a high-strength cold-rolled steel with excellent baking performance, which contains Fe and unavoidable impurities; it also contains the following chemical elements in the following mass percentages:

[0010] C: 0.06~0.10%, 0<Si≤0.05%, Mn: 0.40~0.7%, Al: 0.03~0.05%;

[0011] The microstructure of the high-strength cold-rolled steel has recrystallized equiaxed ferrite with a volume fraction of more than 95%, and slender ferrite extending along the rolling direction with a volume fraction of 3-5%.

[0012] Furthermore, in the high-strength cold-rolled steel described in this invention, the mass percentage of each chemical element is as follows:

[0013] C: 0.06-0.10%, 0 < Si ≤ 0.05%, Mn: 0.40-0.7%, Al: 0.03-0.05%, balance being Fe and unavoidable impurities.

[0014] In the technical solution described in this invention, the design principles of each chemical element are as follows:

[0015] C: In the high-strength cold-rolled steel described in this invention, carbon (C) is the most common strengthening element in steel. Carbon content has a significant impact on the mechanical properties of steel. As the carbon content increases, the amount of pearlite increases, leading to a substantial increase in the strength and hardness of the steel, but a significant decrease in its plasticity and toughness. Furthermore, while carbon increases the strength of steel, it decreases its plasticity; therefore, the carbon content should not be too high. If the mass percentage of carbon is too high, obvious network carbides will appear in the steel, and the presence of these network carbides will significantly reduce its strength, plasticity, and toughness. Simultaneously, the strengthening effect of increasing the carbon content in the steel will also be significantly reduced, resulting in poorer welding and forming process performance. Therefore, in the high-strength cold-rolled steel described in this invention, the mass percentage of carbon is controlled between 0.06% and 0.10%.

[0016] Si: In the high-strength cold-rolled steel described in this invention, silicon has a strong deoxidizing ability and is a commonly used deoxidizer in steelmaking. Silicon can also increase the fluidity of molten steel. However, when the mass percentage of silicon in the steel is too high, its plasticity and toughness will decrease significantly. In addition, an excessively high mass percentage of silicon will form iron oxide scale defects on the surface. Therefore, in the high-strength cold-rolled steel described in this invention, the mass percentage of Si is controlled to be 0 < Si ≤ 0.05%.

[0017] Mn: In the high-strength cold-rolled steel described in this invention, manganese is an inexpensive strengthening alloying element. Mn is infinitely miscible in steel and also provides solid solution strengthening. Therefore, in the high-strength cold-rolled steel described in this invention, the mass percentage of Mn is controlled between 0.40% and 0.7%.

[0018] Al: In the high-strength cold-rolled steel described in this invention, aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to steel can refine the grains and improve impact toughness. Aluminum can reduce the steel's sensitivity to notches, reduce or eliminate aging phenomena, and in particular lower the ductile-brittle transition temperature, improving the steel's toughness at low temperatures. Aluminum also has a significant solid solution strengthening effect, and high-aluminum steel has the advantage of higher specific strength. When the mass percentage of aluminum is too high, it will affect the hot working performance, weldability, and machinability of the steel. In the high-strength cold-rolled steel with excellent baking performance described in this invention, the mass percentage of Al is controlled to be 0.03 < Al ≤ 0.05%.

[0019] Furthermore, the high-strength cold-rolled steel described in this invention also contains at least one of Nb, Ti, and V, and the mass percentage content satisfies (Nb+Ti+V) / C≤0.5.

[0020] In this invention, the high-strength cold-rolled steel may also contain Nb, Ti, and V elements, which can further improve the steel's properties. Wherein:

[0021] Ti, Nb, V: In the high-strength cold-rolled steel described in this invention, the addition of trace amounts of microalloying elements Nb, V, and Ti ensures that, even with a low carbon equivalent, the steel's strength and toughness are greatly improved, particularly its low-temperature toughness, through the dispersed precipitation of carbide and nitride particles (smaller than 5 nm) and the solid solution of Nb, V, and Ti. This results in refined grains and significantly enhanced weldability and usability. Furthermore, Nb, V, and Ti are carbide and nitride forming elements, and their concentrations are sufficient to meet requirements. Therefore, in the high-strength cold-rolled steel described in this invention, microalloying elements are beneficial but not essential. Considering factors such as increased cost, the addition amount should not be excessive; the mass percentage of Nb, Ti, and V should be satisfied as (Nb+Ti+V) / C≤0.5.

[0022] Furthermore, in the high-strength cold-rolled steel described in this invention, the mass percentage of carbon element is 0.06–0.08%.

[0023] Furthermore, in the high-strength cold-rolled steel described in this invention, the mass percentage content of Mn element is 0.5% to 0.7%.

[0024] Furthermore, in the unavoidable impurities of the high-strength cold-rolled steel described in this invention: P≤0.018%, S≤0.0045%, N≤0.006%.

[0025] It should be noted that in the technical solution described in this invention, P, S, and N are all unavoidable impurity elements in steel. Wherein:

[0026] P: In the high-strength cold-rolled steel described in this invention, phosphorus is considered a harmful element that easily causes segregation, reducing plasticity and toughness. Therefore, in the high-strength cold-rolled steel described in this invention, the mass percentage content of phosphorus is controlled to P≤0.018%.

[0027] S: In the high-strength cold-rolled steel described in this invention, sulfur is considered a harmful element. Higher sulfur content easily forms large-sized manganese sulfides, reducing the final plasticity and toughness of the steel plate. Therefore, in the high-strength cold-rolled steel described in this invention, the mass percentage of sulfur is controlled to S≤0.0045%.

[0028] Furthermore, in the high-strength cold-rolled steel described in this invention, it also satisfies Mn / S≥150.

[0029] Since a certain amount of sulfur (S) remains in molten steel, S has the negative effect of increasing the hot brittleness of the slab and deteriorating the mechanical properties of the steel. To reduce the negative effects of S, this invention increases the Mn / S ratio in the steel plate; Mn / S ≥ 150 can effectively reduce the negative impact of S.

[0030] Furthermore, the microstructure of the high-strength cold-rolled steel described in this invention also has finely dispersed carbides.

[0031] Furthermore, the high-strength cold-rolled steel described in this invention has the following properties: yield strength ≥ 300 MPa, tensile strength ≥ 400 MPa, elongation ≥ 30%, and bake hardening value > 30 MPa.

[0032] One of the objectives of this invention is to provide a method for manufacturing high-strength cold-rolled steel, which can produce a high-strength cold-rolled steel with good bake hardening properties, high strength and high elongation.

[0033] To achieve the above objectives, the present invention provides a method for manufacturing high-strength cold-rolled steel, comprising the following steps:

[0034] Smelting and continuous casting;

[0035] Hot rolling: The furnace exit temperature is 1150~1250℃, the hot rolling finishing temperature is 830~890℃, the coiling temperature is 520~650℃, and laminar flow cooling is adopted after rolling. The rapid cooling rate of the laminar flow front section is 80~120℃ / s.

[0036] Pickling and cold rolling;

[0037] Continuous annealing: Control the annealing temperature to be 680-750℃;

[0038] smooth.

[0039] Furthermore, in the cold rolling step of the manufacturing method described in this invention, the cumulative cold rolling reduction rate is controlled to be 60-80%.

[0040] Furthermore, in the leveling step of the manufacturing method described in this invention, the leveling rate is controlled to be 0.8% to 1.2%.

[0041] The high-strength cold-rolled steel with excellent baking performance and its manufacturing method described in this invention have the following advantages and beneficial effects compared with the prior art:

[0042] The high-strength cold-rolled steel with excellent baking performance and its manufacturing method described in this invention improves material strength by using low-cost elements such as C and Mn, thereby avoiding the addition of excessively low-cost, high-cost alloying elements such as Ti and Nb, thus reducing alloy costs and manufacturing difficulty. Simultaneously, adjustments to hot rolling and continuous annealing process parameters ensure a good solid solution strengthening effect of Mn.

[0043] In some specific embodiments, the high-strength cold-rolled steel with excellent baking performance described in this invention has a yield strength ≥300MPa, tensile strength ≥400MPa, elongation ≥30%, and bake hardening value >30MPa, exhibiting good bake hardening performance, high strength, and high elongation. Attached Figure Description

[0044] Figure 1 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 1 of the present invention is shown.

[0045] Figure 2 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Embodiment 2 of the present invention is shown.

[0046] Figure 3 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 3 of the present invention is shown.

[0047] Figure 4 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 10 of the present invention is shown.

[0048] Figure 5 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 11 of the present invention is shown.

[0049] Figure 6 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 12 of the present invention is shown.

[0050] Figure 7 The microstructure of the comparative steel of Comparative Example 1 of the present invention is shown.

[0051] Figure 8 The microstructure of the comparative steel of Comparative Example 2 of the present invention is shown.

[0052] Figure 9 The microstructure of the comparative steel of Comparative Example 3 of the present invention is shown. Detailed Implementation

[0053] The high-strength cold-rolled steel with excellent baking performance and its manufacturing method described in this invention will be further explained and illustrated below with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0054] Examples 1-12 and Comparative Examples 1-6

[0055] The high-strength cold-rolled steels with excellent baking performance described in Examples 1-12 of this invention and the comparative steels of Comparative Examples 1-6 were all prepared using the following steps:

[0056] (1) Smelting and continuous casting: Smelting and casting into slabs through continuous casting.

[0057] (2) Hot rolling: The furnace exit temperature is controlled at 1150~1250℃, the hot rolling final rolling temperature is 830~890℃, the coiling temperature is 520~650℃, and laminar flow cooling is adopted after rolling. The rapid cooling rate of the laminar flow front section is 80~120℃ / s.

[0058] In some specific implementations, the hot rolling furnace exit temperature can be further controlled to 1180–1220°C, the hot rolling final rolling temperature can be further controlled to 850–870°C, the coiling temperature can be further controlled to 550–620°C, and the rapid cooling rate of the laminar flow front section can be further controlled to 100–120°C / s.

[0059] (3) Pickling and cold rolling: Pickling is performed to clean the iron oxide scale on the surface; then cold rolling is performed, and the cumulative reduction rate of cold rolling is controlled to be 60-80%.

[0060] In some specific implementations, the cold rolling reduction rate can be further controlled to 70-80%.

[0061] (4) Continuous annealing: Continuous annealing is carried out in a non-oxidizing continuous annealing furnace. The annealing process includes a heating section, a soaking section, and a cooling section in sequence. The soaking temperature is controlled at 680-750℃.

[0062] In some specific implementations, the annealing homogenization temperature can be further controlled to 680–720°C.

[0063] (5) Leveling: The leveling rate should be controlled at 0.8% to 1.2%.

[0064] Table 1 lists the mass percentage of each chemical element in the high-strength cold-rolled steel with excellent baking performance in Examples 1-12 of the present invention and the comparative steels in Comparative Examples 1-6.

[0065] Table 1. (The balance is Fe and other unavoidable impurities besides P, S, and N)

[0066]

[0067] Table 2 lists the specific process parameters of the high-strength cold-rolled steel with excellent baking performance in Examples 1-12 of the present invention and the comparative steel in Comparative Examples 1-6.

[0068] Table 2.

[0069]

[0070] To verify the implementation effect of the present invention, samples were taken from the high-strength cold-rolled steel with excellent baking performance in Examples 1-12 and the control steel in Comparative Examples 1-6, and their microstructure was observed using a Leica metallographic microscope. The observation results are listed in Table 3.

[0071] Table 3 lists the microstructure observation results of the high-strength cold-rolled steel with excellent baking performance in Examples 1-12 of the present invention and the comparative steels in Comparative Examples 1-6.

[0072] Table 3.

[0073]

[0074]

[0075] As can be seen from Table 3 above, the volume fraction of recrystallized equiaxed ferrite in the high-strength cold-rolled steels with excellent baking performance in Examples 1-12 is greater than or equal to 95%, and the volume fraction of slender ferrite extending along the rolling direction is between 3-5%.

[0076] Figure 1 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 1 of the present invention is shown.

[0077] Figure 2 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Embodiment 2 of the present invention is shown.

[0078] Figure 3The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 3 of the present invention is shown.

[0079] Figure 4 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 10 of the present invention is shown.

[0080] Figure 5 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 11 of the present invention is shown.

[0081] Figure 6 The microstructure of the high-strength cold-rolled steel with excellent baking performance of Example 12 of the present invention is shown.

[0082] like Figure 1-6 As shown, the microstructures of Examples 1-3 and Examples 10-12 of the present invention all have recrystallized equiaxed ferrite with a volume fraction of 95-97% or more, 3-5% elongated ferrite along the rolling direction, and less than 1% of fine carbides in a dispersed portion.

[0083] Figure 7 The microstructure of Comparative Example 1 of the present invention is shown.

[0084] Figure 8 The microstructure of Comparative Example 2 of the present invention is shown.

[0085] Figure 9 The microstructure of Comparative Example 3 of the present invention is shown.

[0086] like Figure 7-9 As shown, the microstructures of Comparative Examples 1-3 are all coarse equiaxed ferrite grains with very little carbide content.

[0087] The high-strength cold-rolled steels with excellent baking performance from Examples 1-12 of this invention and the control steels from Comparative Examples 1-6 were sampled again, and various performance tests were performed on them. The test results are listed in Table 4. The various performance tests included:

[0088] Mechanical properties: The mechanical property testing methods were conducted in accordance with the national standard GB / T 228.1-2021 Metallic materials, Tests – Part 1: Tests at room temperature. Tensile specimens were P17 specimens with a transverse gauge length of 50 mm.

[0089] Baking hardening value: The baking hardening value was determined according to the national standard GB / T 24174-2022 "Determination of baking hardening value (BH) of steel". The sample used was a transverse P17 sample with a gauge length of 50 mm.

[0090] Table 4 lists the performance test results of the high-strength cold-rolled steels with excellent baking performance in Examples 1-12 of the present invention and the comparative steels in Comparative Examples 1-6.

[0091] Table 4.

[0092]

[0093] As can be seen from Table 4 above, the high-strength cold-rolled steels with excellent baking performance in Examples 1-12 all have a yield strength greater than 300 MPa, a tensile strength greater than 400 MPa, and an elongation greater than 30%, exhibiting good bake hardening performance, high strength, and high elongation.

[0094] It should be noted that the scope of protection of this invention is not limited to the embodiments given in this application. All prior art that does not contradict the solution of this invention, including but not limited to prior patent documents, prior publications, prior uses, etc., can be included in the scope of protection of this invention. Furthermore, the combination of technical features in this application is not limited to the combinations described in the claims or the specific embodiments. All technical features described in this application can be freely combined or combined in any way, unless they contradict each other.

[0095] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A high-strength cold-rolled steel with excellent baking performance, containing Fe and unavoidable impurities; characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.06~0.10%, 0<Si≤0.05%, Mn: 0.40~0.7%, Al: 0.03~0.05%; The microstructure of the high-strength cold-rolled steel has recrystallized equiaxed ferrite with a volume fraction of more than 95%, and slender ferrite extending along the rolling direction with a volume fraction of 3-5%.

2. The high-strength cold-rolled steel as described in claim 1, characterized in that, The mass percentage of each chemical element is as follows: C: 0.06-0.10%, 0 < Si ≤ 0.05%, Mn: 0.40-0.7%, Al: 0.03-0.05%, balance being Fe and unavoidable impurities.

3. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, It also contains at least one of Nb, Ti, and V, and the mass percentage content satisfies (Nb+Ti+V) / C≤0.

5.

4. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, The carbon content is 0.06% to 0.08% by mass.

5. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, The mass percentage of Mn is 0.5% to 0.7%.

6. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, In unavoidable impurities: P≤0.018%, S≤0.0045%, N≤0.006%.

7. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, It also satisfies Mn / S≥150.

8. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, Its microstructure also contains finely dispersed carbides.

9. The high-strength cold-rolled steel as described in claim 1 or 2, characterized in that, Its performance meets the following requirements: yield strength ≥ 300 MPa, tensile strength ≥ 400 MPa, elongation ≥ 30%, and bake hardening value > 30 MPa.

10. The method for manufacturing high-strength cold-rolled steel according to any one of claims 1-9, characterized in that, Including the following steps: Smelting and continuous casting; Hot rolling: The furnace exit temperature is 1150~1250℃, the hot rolling finishing temperature is 830~890℃, the coiling temperature is 520~650℃, and laminar flow cooling is adopted after rolling. The rapid cooling rate of the laminar flow front section is 80~120℃ / s. Pickling and cold rolling; Continuous annealing: Control the annealing temperature to be 680-750℃; smooth.

11. The manufacturing method as described in claim 10, characterized in that, In the cold rolling process, the cumulative reduction rate is controlled to be 60-80%.

12. The manufacturing method as described in claim 10, characterized in that, During the leveling process, the leveling rate is controlled to be between 0.8% and 1.2%.

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