Differentially surface treated cold rolled fine blanking steel and method of manufacturing a steel sheet

Abstract

This invention discloses a method for manufacturing cold-rolled precision stamping steel and steel plates with differentiated surface treatment. The method includes the following processing steps: 1. Smelting and casting into billets; 2. Hot rolling; 3. Cold rolling; 4. Annealing; 5. Leveling; 6. Degreasing treatment using organic solvents; 7. Upper surface surface conditioning and activation: the surface conditioning agent is a solution containing colloidal titanium phosphate, sprayed onto one side, with a concentration of 0.1% to 0.5% and a pH value of 8.5 to 9.5 in the activation solution; 8. Lower surface passivation and barrier treatment: the lower surface is passivated and barrier treated with a passivation solution of a mixture of NaNO2 and Na2CO2, with a pH of 9 to 10; the treatment is carried out until the tensile strength of the steel plate is 440 to 480 MPa, the yield strength is 280 to 350 MPa, the elongation at break is >30%, and the spheroidization rate in the microstructure is ≥95%.

Description

Technical Field

[0001] This invention relates to a cold-rolled precision stamping steel sheet and its manufacturing method, and more particularly to a cold-rolled precision stamping steel sheet and its strip steel sheet manufactured and processed with differential treatment of the upper and lower surfaces. Background Technology

[0002] Fine-stamped steel typically has a microstructure primarily consisting of ferrite and spheroidal cementite, with spheroidal cementite accounting for over 90% of the total cementite. It lacks banded structures and a decarburized layer. Fine-stamped steel is commonly used for precision-stamped components in adjusting mechanisms of precision machinery. Currently, in actual processing, many components require phosphating treatment for fine-stamped steel products. Phosphating is a process where the metal workpiece is brought into contact with a phosphate solution containing metal ions, forming a water-insoluble crystalline or amorphous phosphate film on the metal surface. In particular, sometimes differentiated treatments are required on the upper and lower surfaces of the fine-stamped strip to allow for single-sided phosphating in subsequent processing. This differentiated surface treatment of cold-rolled fine-stamped steel products is increasingly used in key automotive components.

[0003] Chinese patent document with application number 201810048364.5 discloses a cold-rolled precision stamping steel strip for a car child seat locking device and its preparation method. The chemical composition is C: 0.50-0.55, Si: 0.15-0.35, Mn: 0.80-1.10, Cr: 0.90-1.20, V: 0.10-0.20. The addition of more C and Si improves the strength.

[0004] Chinese patent document application number 200910196007.4 discloses a hot-rolled steel plate for precision stamping and its manufacturing method. Its chemical composition percentages are: C: 0.05–0.30; Si: ≤0.50; Mn: 0.50–1.50; Cr: 0.5–1.5; Ni: 0.02–0.50; Al: ≤0.04, and also includes Ti: ≤0.05. This patent adds a large number of expensive alloying elements, which increases the product manufacturing cost; moreover, the composition design is too broad, failing to disclose key indicators related to production control and testing, and precision stamping; and the strength is relatively low, failing to meet the requirements for final heat treatment of the parts.

[0005] Chinese patent document with application number 202010744672.9 discloses a method for manufacturing a cold-rolled steel strip for a low-alloy, low-yield-strength ratio automotive shift fork. The method includes readjusting the chemical composition, microstructure, and mechanical properties of 16MnCr5 steel so that the yield strength of the cold-rolled fine-stamping material 16MnCr5 is 450 MPa, the elongation is A5>30%, and the spheroidization rate is not less than 95%. During processing, Ni and Cr trace elements are added during the annealing process.

[0006] Chinese patent document application number 201110429986.0 discloses a MnCr carburized gear steel and its manufacturing method for producing large gears. Its chemical composition percentages are: C: 0.25–0.30; Si: ≤0.12; Mn: 0.60–0.80; Cr: 0.80–1.10; β: ≤0.035; S: 0.020–0.050, with the remainder being Fe and unavoidable impurities. This invention employs electric furnace smelting, continuous casting to obtain continuously cast billets, and hot rolling into round steel. The elemental composition design incorporates a high C content, while a certain amount of S is added to ensure machinability.

[0007] The cold-rolled precision stamping steel products mentioned in these documents can only undergo simultaneous phosphating on both sides during the subsequent phosphating process, which cannot meet the requirements of specific application scenarios. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the above-mentioned deficiencies of the prior art and provide a cold-rolled precision stamping steel with differentiated surface treatment, having a tensile strength of 440-480 MPa, a yield strength of 280-350 MPa, a fracture elongation of >30%, and a spheroidization rate of ≥95% in the microstructure.

[0009] The technical problem it aims to solve can be addressed through the following technical solutions.

[0010] A cold-rolled precision stamping steel, wherein the chemical composition of the steel grade, by mass percentage, includes 0.15–0.25% C, 0.8–1.5% Mn, 0.8–1.5% Cr, 0.02–0.03% Ti, less than 0.05% Si, less than 0.008% N, less than 0.02% P, and less than 0.05% S.

[0011] Furthermore, by mass percentage, this steel grade consists of 0.15–0.25% C, 0.8–1.5% Mn, 0.8–1.5% Cr, 0.02–0.03% Ti, less than 0.05% Si, less than 0.008% N, less than 0.02% P, less than 0.05% S, and the balance Fe and unavoidable impurities.

[0012] Preferably, the composition is as follows: C: 0.18–0.23%, Mn: 0.8–1.5%, Cr: 0.9–1.3%, Ti: 0.02–0.03%, Si<0.03%, N<0.005%, P<0.015%, S<0.03%. The preferred C content allows for better control of the final spheroidizing annealing effect. The preferred Cr element range facilitates the passivation process. The preferred mass percentage of Si is below 0.03%, and the preferred mass percentage of P is below 0.015%. Compounds such as MnS generated from S can negatively impact forming performance, therefore the mass percentage of S is preferably below 0.003%. N is prone to causing surface cracks or bubbles in the slab, so it is preferably below 0.005%.

[0013] In the composition design of the steel plate described in this invention:

[0014] C: In the steel plate described in this invention, carbon (C) is an important solid solution element. Appropriate C addition can improve the strength of the steel and ensure sufficient strength and hardness after quenching. Therefore, the amount of C added is selected to be 0.15-0.25%. This is because if the mass percentage of C is less than 0.15%, the strength after annealing cannot be guaranteed to meet the requirements; if the mass percentage of C is greater than 0.25%, the strength of the steel plate will be too high, and subsequent quenching processing will result in excessively high hardness.

[0015] Mn: Under appropriate conditions, increasing the amount of manganese can increase the strength and hardness of steel. Manganese has deoxidizing and desulfurizing effects (forming MnS), preventing hot brittleness. Therefore, manganese can improve the forgeability and plasticity of steel and eliminate the effects of sulfur and oxygen on the hot brittleness of steel. Therefore, the added content is 0.8% to 1.5%. Too little Mn will lead to insufficient strength after final heat treatment, while too much Mn will lead to quenching cracks.

[0016] Cr: The addition of Cr can improve strength and hardness. Cr is also an element with a passivating tendency; therefore, adding a certain amount of Cr to steel will give it a certain degree of corrosion resistance during subsequent processing and facilitate the formation of an oxide film on the steel surface. Therefore, the amount added should be between 0.8% and 1.5%. Too little Cr will result in insufficient strength after final heat treatment, while too much Cr can increase the steel's temper brittleness, leading to cracking during subsequent heat treatment.

[0017] Ti: Ti has a certain grain-refining effect. Adding an appropriate amount to this product can refine the grains, resulting in a more uniform microstructure. However, Ti can react with other elements in the steel, such as N and C, to form hard precipitates like TiN and TiC, which are detrimental to plastic deformation during fine blanking. Therefore, the amount added is between 0.02% and 0.03%.

[0018] Another technical problem to be solved by the present invention is to provide a method for manufacturing cold-rolled fine-stamped steel sheets with differentiated surface treatments as described above, comprising the following steps:

[0019] 1) Smelting and casting: Smelting and casting billets according to the composition described in the previous steel grades;

[0020] 2) Hot rolling: First, heat to 1150-1250℃ and hold for more than 0.5 hours. After rolling, cool rapidly at 30-100℃ / s and coil at 400-550℃. Coiling within this temperature range can make the microstructure more uniform, which is conducive to the dispersion and distribution of carbides in the later stage and improves the spheroidization rate.

[0021] 3) Cold rolling: The reduction rate of cold rolling is controlled between 30% and 50%. Suitable deformation is conducive to the refinement of microstructure and the formation of more uniformly dispersed carbides after annealing, thereby improving the spheroidization rate.

[0022] 4) Annealing: Use a full hydrogen bell furnace for 25-35 hours of heat preservation annealing, with an annealing temperature range of 660-730℃.

[0023] 5) Leveling: The leveling reduction rate is 1-4%. A suitable leveling reduction rate can eliminate the yield strength plateau, which is beneficial for subsequent fine blanking.

[0024] 6) Degreasing: The present invention uses organic solvent degreasing treatment, the main component of which is anhydrous ethanol.

[0025] 7) Surface conditioning and activation: The concentration of the surface conditioner in the activation solution is 0.1% to 0.5%. The components of the activation solution can be a solution containing colloidal titanium phosphate, a surface conditioner for zinc salt phosphating, or a surface conditioner for manganese phosphating. A solution containing colloidal titanium phosphate is preferred.

[0026] 8) Lower surface passivation and barrier treatment: The lower surface is passivated and barrier treated. The passivation solution used in this invention is a mixture of NaNO2 (purity 5%–20%) and Na2CO2 (purity 0.8%), with a pH of 9–10. The preferred mixing ratio is 1:1.

[0027] The fine-stamping steel involved in this invention mainly comprises C, Mn, Cr, and a small amount of Ti, with a strength of approximately 440-480 MPa and an elongation greater than 30%. More importantly, the upper and lower surfaces of the strip undergo different final treatments: one surface is pre-phosphated for subsequent phosphate treatment, while the other undergoes surface passivation. This satisfies both the corrosion resistance requirements of the two surfaces and the phosphate treatment requirements of one surface.

[0028] The cold-rolled fine-stamped steel included in this patent undergoes special process treatment to obtain differentiated surface states, so that one of the surface treatments becomes a pre-phosphating state with good phosphating effect. After subsequent spray zinc phosphating, cold-rolled fine-stamped strip steel with single-sided phosphating treatment can be obtained. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the differentiated surface steel plate product manufactured according to the present invention;

[0030] Figure 2 Metallographic diagram of the steel plate matrix; Detailed Implementation

[0031] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0032] The chemical composition (by mass percentage) of the precision stamping steel product of this invention is as follows: C: 0.15–0.25%; Mn: 0.8–1.5%; Cr: 0.8–1.5%; Ti: 0.02–0.03%; Si < 0.05%; with the remainder being Fe and unavoidable impurities. The tensile strength of the steel plate is 440–480 MPa, and the elongation at break is ≥ 30%.

[0033] The steel of this invention is mainly composed of C, Mn, and Cr. After annealing, the carbide precipitates in the steel should be uniformly dispersed with a spheroidization rate greater than 95% to facilitate fine blanking. Therefore, the composition needs to be rationally designed to ensure both good uniform plastic deformation capacity and good hardenability and quenching hardness during subsequent heat treatment. The addition of C is mainly to ensure a certain strength and elongation. The content of Mn and Cr must be reasonable to ensure both hardenability and quenching hardness during subsequent heat treatment, while avoiding problems such as cracking due to quenching brittleness caused by excessive amounts.

[0034] After spheroidizing annealing, the final precipitated carbides are uniformly dispersed spherical, with a size of about 1 micrometer, and the proportion of spherical carbides is over 95%. The precipitation of lamellar carbides or network-like carbides should be avoided.

[0035] The manufacturing method of the precision stamping steel plate with differentiated surface treatment in this invention, based on product specifications of 2.0mm to 3.0mm, includes the following steps:

[0036] 1) Smelting and casting: smelting and casting billets according to the set composition;

[0037] 2) Hot rolling: First, heat to 1150-1250℃ and hold for at least 0.5 hours. After rolling, cool rapidly at 30-100℃ / s and coil at 400-550℃. Coiling within this temperature range results in a more uniform microstructure, which is beneficial for the subsequent dispersion and distribution of carbides and improves the spheroidization rate.

[0038] 3) Cold rolling: The reduction rate of cold rolling is controlled between 30% and 50%. Suitable deformation is conducive to the refinement of microstructure and the formation of more uniformly dispersed carbides after annealing, thereby improving the spheroidization rate.

[0039] 4) Annealing: Annealing is performed in a full-hydrogen bell-type furnace with a holding temperature range of 660-730℃, preferably 690-730℃. This process serves three purposes:

[0040] ① Reduces hardness and improves machinability. After spheroidizing annealing, the spherical carbides formed in steel have lower hardness than lamellar carbides, which is beneficial for improving machinability.

[0041] ② Refine the microstructure to prepare for quenching. During the quenching heating process, spheroidal carbides are less soluble in austenite than lamellar carbides, thus preventing grain growth and reducing or preventing overheating of the steel. The uniform microstructure obtained after spheroidizing annealing helps reduce quenching distortion and cracking tendency.

[0042] ③ Improve the wear resistance of quenched workpieces. Since the spherical carbides are completely retained after quenching and are evenly distributed on the martensitic matrix, these fine and hard particles can effectively improve the wear resistance of the workpiece.

[0043] 5) Leveling: The leveling reduction rate is 1-4%. A suitable leveling reduction rate can eliminate the yield strength plateau, which is beneficial for subsequent fine blanking.

[0044] 6) Degreasing: The present invention uses organic solvent degreasing treatment, the main component of which is anhydrous ethanol.

[0045] 7) Surface Conditioning and Activation: To improve the quality of phosphating film formation and reduce film thickness, especially for zinc phosphate films used as coating underlayers, it is essential to achieve technical requirements such as fine crystallization, low film weight, high phosphorus ratio, and good corrosion resistance. Activation treatment before phosphating plays a crucial role. The surface conditioner used in this invention is a solution containing colloidal titanium phosphate, applied via single-sided spraying. The concentration of the surface conditioner in the activation solution is 0.1%–0.5%, and the pH value is 8.5–9.5. When the pH value is below the lower limit, an inactive blue film easily forms on the steel surface, inhibiting phosphating film formation; when the pH value is above the upper limit, the activation effect decreases, and the service life of the solution is shortened. The activation solution can be a solution containing colloidal titanium phosphate, a surface conditioner for zinc salt phosphating, or a surface conditioner for manganese-based phosphating, with a solution containing colloidal titanium phosphate being preferred.

[0046] 8) Lower surface passivation and barrier treatment: The lower surface is passivated and barrier treated. The passivation solution used in this invention is a mixture of NaNO2 (purity 5%~20%) and Na2CO2 (purity 0.8%), with a pH of 9~10. The mixing ratio is 1:1.

[0047] After the above treatment, the product has good corrosion resistance, and one side has been coated with a paint-friendly pre-phosphating treatment.

[0048] The present invention will be further described below with reference to examples. Table 1 shows the chemical composition of the steel examples and comparative examples of the present invention, and Table 2 shows the manufacturing processes of the steel examples and comparative examples of the present invention. Figure 1 This is a schematic diagram of the structure of the differentiated surface steel plate product manufactured according to the present invention. Figure 2 Table 3 shows the metallographic structure of the steel plate substrate; Table 4 shows the mechanical properties and surface condition of the steel plate substrate of the present invention; Table 5 shows the microstructure and composition analysis of the steel plate surface in Example 2; and Table 6 shows the test results of the microscopic physical profile and roughness of the steel plate surface in Example 2. It can be seen that the present invention differs from the prior art, achieving a cold-rolled precision-stamped steel with a differentiated surface treatment, while maintaining a microstructure spheroidization rate ≥95%.

[0049] Table 1: Chemical composition (wt%) of the steel of this invention

[0050] C Mn Cr Ti Si P S N Example 1 0.15 1.5 1.5 0.02 0.03 0.0084 0.002 0.0004 Example 2 0.20 0.8 1.2 0.03 0.02 0.007 0.001 0.0001 Example 3 0.25 1.0 0.8 0.02 0.02 0.009 0.003 0.0002 Example 4 0.18 1.4 1.0 0.02 0.04 0.008 0.002 0.0003 Example 5 0.22 1.1 1.1 0.03 0.02 0.008 0.002 0.0003 Example 6 0.23 1.3 1.2 0.02 0.03 0.008 0.002 0.0003 Comparative Example B1 0.08 1.1 1.4 0.03 0.05 0.007 0.003 0.0004 Comparative Example B2 0.12 0.05 1.4 0.03 0.04 0.006 0.001 0.0001 Comparative Example B3 0.16 1.2 0.03 0.02 0.03 0.009 0.003 0.0002 Comparative Example B4 0.16 1.2 1.3 0.005 0.04 0.009 0.003 0.0002 Comparative Example B5 0.25 1.0 1.0 0.04 0.03 0.009 0.003 0.0002 Comparative Example B6 0.22 1.1 1.0 0.04 0.02 0.009 0.003 0.0002

[0051] Table 2: Steel Manufacturing Process of the Invention

[0052]

[0053] Table 3: Mechanical properties and surface condition of the steel plate substrate of the present invention

[0054]

[0055] Table 4: Microstructure and composition of steel plate surface

[0056]

[0057] Table 5: Surface Microscopic Physical Profile and Roughness Test

[0058]

[0059] This invention relates to a cold-rolled precision-stamped steel with differentiated surface treatment, exhibiting a tensile strength of 440–480 MPa, a yield strength of 280–350 MPa, an elongation at break >30%, and a spheroidization rate ≥95% in its microstructure. The manufacturing process involves a method for processing cold-rolled precision-stamped steel with differentiated treatment of its upper and lower surfaces. This product possesses excellent corrosion resistance, with one surface undergoing surface activation treatment to facilitate subsequent phosphating, and the other surface undergoing passivation treatment.

Claims

1. A cold-rolled precision stamping steel, characterized in that, By mass percentage, the chemical composition of the steel grade includes 0.15–0.25% C, 0.8–1.5% Mn, 0.8–1.5% Cr, 0.02–0.03% Ti, less than 0.05% Si, less than 0.008% N, less than 0.02% P, and less than 0.05% S; The cold-rolled precision stamping steel undergoes differentiated treatment on its upper and lower surfaces during processing, with one surface undergoing pre-phosphating treatment and the other surface undergoing passivation treatment. The steel plate is treated to achieve a tensile strength of 440–480 MPa, a yield strength of 280–350 MPa, a fracture elongation of >30%, and a spheroidization rate of ≥95% in the microstructure.

2. The cold-rolled precision stamping steel according to claim 1, characterized in that, By mass percentage, this steel grade consists of 0.15–0.25% C, 0.8–1.5% Mn, 0.8–1.5% Cr, 0.02–0.03% Ti, less than 0.05% Si, less than 0.008% N, less than 0.02% P, less than 0.05% S, and the balance Fe and unavoidable impurities.

3. The cold-rolled precision stamping steel according to claim 1 or 2, characterized in that, By mass percentage, this steel grade contains the following chemical composition: C: 0.18–0.23%, Mn: 0.8–1.5%, Cr: 0.9–1.3%, Ti: 0.02–0.03%, Si<0.03%, N<0.005%, P<0.015%, S<0.03%.

4. A method for manufacturing cold-rolled fine-stamped steel sheets with differentiated surface treatment as described in claim 1, 2, or 3, characterized in that, The processing steps include the following: 1) Smelting and casting into billets; 2) Hot rolling; 3) Cold rolling: The reduction rate of cold rolling is controlled at 30%-50%; 4) Annealing; 5) Flat; 6) Degreasing treatment using organic solvents; 7) Surface conditioning and activation of the upper surface: The surface conditioning agent used is a solution containing colloidal titanium phosphate, which is sprayed onto one side. The concentration of the surface conditioning agent in the activation solution is 0.1% to 0.5%, and the pH value is 8.5 to 9.

5. 8) Passivation barrier on the lower surface: The lower surface is passivated and blocked by a mixture of NaNO2 and Na2CO2 with a pH of 9-10. The steel plate is treated until the tensile strength is 440-480 MPa, the yield strength is 280-350 MPa, the elongation at break is >30%, and the spheroidization rate in the microstructure is ≥95%.

5. The method for manufacturing cold-rolled precision stamped steel sheet according to claim 4, characterized in that, The hot rolling in step 2) is performed using the following method: First, heat to 1150-1250℃ and hold for more than 0.5 hours. After rolling, cool rapidly at 30-100℃ / s and wind up at 400-550℃. In step 4), a full hydrogen bell furnace is used for heat preservation annealing for 25-35 hours, with the annealing temperature range being 660-730℃.

6. The method for manufacturing cold-rolled precision stamped steel sheet according to claim 4, characterized in that, In step 5), the flattening reduction rate is 1-4%.

7. The method for manufacturing cold-rolled precision stamping steel sheet according to claim 4, characterized in that, The main component of the organic solvent in step 6) is anhydrous ethanol.

8. The method for manufacturing cold-rolled precision stamped steel sheet according to claim 4, characterized in that, The activation solution in step 7) is selected from a solution containing colloidal titanium phosphate, a surface conditioner for zinc salt phosphating, or a surface conditioner for manganese phosphating.

9. The method for manufacturing cold-rolled precision stamped steel sheet according to claim 4, characterized in that, In step 8), the passivation solution is a mixture of NaNO2 with a purity of 5% to 20% and Na2CO2 with a purity of 0.8%.

10. The method for manufacturing cold-rolled precision stamped steel sheet according to claim 9, characterized in that, The mixing ratio of NaNO2 and Na2CO2 is 1:1.

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