Production method of long-fatigue-life fastener SCM435 cold forging steel

Abstract

The invention discloses a production method of long-fatigue-life fastener SCM435 cold forging steel, which comprises the following steps of: eliminating surface defects of a steel billet by adopting a method of combining full grinding and double flaw detection; the surface of the steel billet is coated with anti-decarburization paint, and it is guaranteed that the steel billet is not decarburized during heating; after the blank is heated, high-pressure water is adopted for descaling, and it is ensured that anti-decarburization paint and oxide scale are removed thoroughly; the product is produced under the optimal tool condition, and the optimal product quality is ensured through the optimal tool quality; and a multi-dimensional inspection method is adopted, so that the microscopic structure and the surface quality of the outgoing wire rod are ensured to be qualified, and the fatigue life of the cold heading fastener is further prolonged. The SCM435 cold forging steel produced by the method is used for manufacturing outer hexagonal flange bolts, the fatigue life of the SCM435 cold forging steel can reach 8 million times, and the fatigue life of bolts of the same model manufactured by a common method can only reach 5 million times; and economic loss and social harm caused by fatigue fracture failure can be effectively avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of high fatigue life steel production, and in particular relates to a production method of SCM435 cold heading steel for high fatigue life fasteners. Background Art

[0002] Fatigue fracture failure has no obvious macroscopic plastic deformation before fracture, and there is no precursor to fracture, which often leads to catastrophic consequences, causing huge economic losses and social harm. Improving the fatigue life of bolts, finding ways to improve the fatigue resistance of materials and various measures to prevent fatigue fracture accidents are of great significance to the national economy and science and technology. The surface quality of cold heading steel products directly affects the manufacturing quality of fasteners, so it is of great significance to develop a production method of SCM435 cold heading steel for fasteners with high fatigue life. Summary of the invention

[0003] Technical problem to be solved: In view of the above technical problem, the present invention provides a production method of SCM435 cold heading steel for fasteners with high fatigue life, which can effectively avoid the economic losses and social harm caused by fatigue fracture failure.

[0004] Technical solution: A method for producing SCM435 cold heading steel for high fatigue life fasteners, comprising the following steps: S1. Use the method of combining full grinding and double flaw detection to eliminate the surface defects of the steel billet; S2. Coating anti-decarburization coating on the surface of the steel billet to ensure that the steel billet does not decarburize during heating; S3. After the blank is heated, high-pressure water is used to descale it to ensure that the anti-decarburization coating and iron oxide scale are completely removed; S4. Use the best tooling conditions to produce SCM435 cold heading steel for fasteners with high fatigue life, and ensure the best product quality with the best tooling quality; S5. Use multi-dimensional inspection methods to ensure that the microstructure and surface quality of the wire rods leaving the factory are qualified, and further improve the fatigue life of cold-headed fasteners.

[0005] Preferably, the specific process of step S1 is: adopting a full grinding method combining rough grinding and fine grinding to eliminate defects on the surface of the steel billet, and adopting a dual flaw detection method of magnetic particle flaw detection and ultrasonic flaw detection to check the grinding effect to ensure that the surface of the steel billet is smooth and defect-free.

[0006] Furthermore, a 16-mesh grinding wheel is used for rough grinding, and the grinding depth is ≥0.5mm. A 24-mesh grinding wheel is used for fine grinding, and the grinding depth is ≥1.0mm, until the surface of the steel billet after grinding is free of burrs and the surface roughness is Ra≤80.

[0007] Preferably, the total decarburization layer depth of SCM435 after coating the anti-decarburization coating in step S2 is ≤0.5%D, and the full decarburization layer depth is 0, wherein D is the nominal diameter of the cold heading steel wire rod.

[0008] Preferably, the pressure of high-pressure water descaling in step S3 is ≥25 Mpa.

[0009] Preferably, the best tooling conditions in step S4 refer to using other cold heading steel products to roll 230-280 tons of products before production, and achieving the best tooling conditions after the surface of the cold heading products is qualified by online pickling; specifically: through data analysis and experimental verification, the surface state of the rolling groove and the opening degree of the guide have a strong correlation with the surface quality of the cold heading steel finished product. When rolling 16 specifications, 230-280 tons of other products are rolled first after the rolling groove is replaced. Only after the surface of other products is checked by online pickling and qualified can the high fatigue life cold heading steel SCM435 be arranged to enter the furnace for rolling.

[0010] Preferably, the multi-dimensional inspection method in step S5 includes a method combining online pickling and cold heading and a method combining offline pickling, metallographic and magnetic particle inspection.

[0011] Furthermore, hydrochloric acid needs to be heated to 60°C in advance during online pickling, and the method of direct 1 / 3 cold forging without heat treatment is adopted during online cold forging; because SCM435 is a medium-carbon alloy cold forging steel, it is a product with a large amount of CrMo alloy elements added in the cold forging steel series, and the carbon content exceeds 0.30%, so in general, SCM435 must be heat treated before 1 / 3 cold forging to release stress and improve the structure to ensure that the 1 / 3 cold forging is qualified. Direct 1 / 3 cold forging without heat treatment has higher requirements for the original structure of SCM435 wire rod.

[0012] Preferably, when online stretching is performed in step S5, the upper limit of the tensile strength is 750 MPa.

[0013] Beneficial effects: The SCM435 cold heading steel produced by the method of the present invention is used to manufacture hexagonal flange bolts, and its fatigue life can reach 8 million times, while the fatigue life of the same type of bolts manufactured by ordinary methods can only reach 5 million times; it can effectively avoid the economic losses and social harm caused by fatigue fracture failure. DETAILED DESCRIPTION

[0014] The present invention is described in detail below in conjunction with specific embodiments: Example 1

[0015] A method for producing SCM435 cold heading steel for fasteners with high fatigue life comprises the following steps: S1. Use a method combining full grinding and double flaw detection to eliminate surface defects of the steel billet. The specific process is: use a full grinding method combining rough grinding and fine grinding to eliminate defects on the surface of the steel billet, and use a double flaw detection method of magnetic particle flaw detection and ultrasonic flaw detection to check the grinding effect to ensure that the surface of the steel billet is smooth and free of defects; specifically, use a 16-mesh grinding wheel for rough grinding with a grinding depth of ≥0.5mm, and use a 24-mesh grinding wheel for fine grinding twice with a grinding depth of ≥1.0mm to ensure that there are no burrs on the surface after grinding, and the surface roughness Ra≤80. Perform magnetic particle flaw detection after grinding. If there are defects on the surface that have not been ground clean, repeat grinding and magnetic particle flaw detection on the defective part until the defects of the billet are ground clean.

[0016] S2. Anti-decarburization coating is applied on the surface of the steel billet to ensure that the steel billet does not decarburize during heating. When the anti-decarburization coating is not applied, the total decarburization layer depth of 16-size SCM435 is ≤1.0% D (D is the nominal diameter of the cold heading steel wire rod), and the full decarburization layer depth is ≤0.02 mm; after using the anti-decarburization coating, the total decarburization layer depth of SCM435 is ≤0.5% D, and the full decarburization layer depth is 0.

[0017] S3. After the billet is heated, high-pressure water is used to descale it to ensure that the anti-decarburization coating and iron oxide scale are completely removed. When high-pressure water is used to dephosphorize the billet out of the furnace, the descaling pressure used is ≥25Mpa to ensure that the anti-decarburization coating and iron oxide scale are completely removed.

[0018] S4. Use the best tooling conditions to produce SCM435 cold heading steel for fasteners with high fatigue life, and ensure the best product quality with the best tooling quality. The best tooling conditions refer to using other cold heading steel products to roll 230~280 tons of products before production, and achieving the best tooling conditions after the surface of the cold heading products is qualified through online pickling. Specifically: through data analysis and experimental verification, the surface state of the rolling groove and the opening degree of the guide have a strong correlation with the surface quality of the cold heading steel products. When rolling 16 specifications, after replacing the rolling groove, roll 230~280 tons of other products first. After the surface of other products is checked by online pickling and qualified, the high fatigue life cold heading steel SCM435 can be arranged to enter the furnace for rolling.

[0019] S5. Use multi-dimensional inspection methods such as online pickling + cold heading, offline pickling + metallographic + magnetic particle inspection to ensure that the microstructure and surface quality of the wire rod are qualified, and further improve the fatigue life of cold heading fasteners; when online pickling SCM435, it is necessary to heat the hydrochloric acid to 60°C in advance. At this temperature, the time for pickling the surface oxide scale is shortened from 1 hour required for cold pickling to 20 minutes, which improves the efficiency of online pickling, reduces the waiting time for pickling, and greatly improves the efficiency of surface defect detection. When online stretching SCM435, the upper limit of tensile strength is constrained. The tensile strength of ordinary SCM435 is only tested without an upper limit. The upper limit of tensile strength of high fatigue life cold heading steel SCM435 is set to 750MPa. Once it exceeds 750MPa, it is judged as an ordinary SCM435 product.

[0020] The SCM435 cold heading steel produced by the method of the present invention is used to manufacture hexagonal flange bolts, and its fatigue life can reach 8 million times, while the fatigue life of the same type of bolts manufactured by ordinary methods can only reach 5 million times; the economic losses and social harm caused by fatigue fracture failure can be effectively avoided.

[0021] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for producing SCM435 cold heading steel for fasteners with high fatigue life, characterized in that: The following steps are involved: S1. Use the method of combining full grinding and double flaw detection to eliminate the surface defects of the steel billet; S2. Coating anti-decarburization coating on the surface of the steel billet to ensure that the steel billet does not decarburize during heating; S3. After the blank is heated, high-pressure water is used to descale it to ensure that the anti-decarburization coating and iron oxide scale are completely removed; S4. Use the best tooling conditions to produce SCM435 cold heading steel for fasteners with high fatigue life, and ensure the best product quality with the best tooling quality; S5. Use multi-dimensional inspection methods to ensure that the microstructure and surface quality of the wire rods leaving the factory are qualified, and further improve the fatigue life of cold-headed fasteners.

2. The method for producing SCM435 cold heading steel for high fatigue life fasteners according to claim 1, characterized in that: The specific process of step S1 is: adopting a full grinding method combining rough grinding and fine grinding to eliminate defects on the surface of the steel billet, and adopting a double flaw detection method of magnetic particle flaw detection and ultrasonic flaw detection to check the grinding effect to ensure that the surface of the steel billet is smooth and defect-free.

3. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 2, characterized in that: A 16-mesh grinding wheel is used for rough grinding, with a grinding depth of ≥0.5mm, and a 24-mesh grinding wheel is used for fine grinding, with a grinding depth of ≥1.0mm, until the surface of the steel billet after grinding is free of burrs and the surface roughness Ra≤80.

4. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 1, characterized in that: The total decarburization layer depth of SCM435 after coating the anti-decarburization coating in step S2 is ≤0.5%D, and the full decarburization layer depth is 0, where D is the nominal diameter of the cold heading steel wire rod.

5. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 1, characterized in that: The pressure of high-pressure water descaling in step S3 is ≥25 MPa.

6. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 1, characterized in that: The best tooling condition in step S4 refers to using other cold heading steel products to roll 230-280 tons of products before production, and achieving the best tooling condition after the surface of the online pickling cold heading products is qualified.

7. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 1, characterized in that: The multi-dimensional inspection method in step S5 includes a method combining online pickling and cold heading and a method combining offline pickling, metallographic and magnetic particle inspection.

8. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 7, characterized in that: During online pickling, the hydrochloric acid needs to be heated to 60°C in advance, and during online cold forging, the method of direct 1 / 3 cold forging without heat treatment is adopted.

9. The method for producing SCM435 cold heading steel for fasteners with high fatigue life according to claim 1, characterized in that: When online stretching is performed in step S5, the upper limit of the tensile strength is 750 MPa.