Production method of 12.9-grade cold forging steel for automobiles

Abstract

The invention discloses a production method of cold forging steel for a 12.9-grade automobile. The steel comprises the following chemical components in percentage by weight: 0.34-0.37% of C, 0.15-0.35% of Si, 0.60-0.90% of Mn, less than or equal to 0.015% of P, less than or equal to 0.008% of S, 0.90-1.20% of Cr, 0.15-0.30% of Mo, more than or equal to 0.020% of Al and the balance of Fe and inevitable impurities. The method comprises the key process steps of converter treatment, refining, bloom continuous casting, bloom cogging, full coping, flaw detection, rolling hot eye monitoring, protection and bundling. After blooming of the blooms, complete repair and surface magnetic powder fluorescent flaw detection are performed, and hot eyes monitor the surfaces of the wire rods online during rolling, so that the internal quality of the billets and zero defects on the surfaces of the billets are guaranteed. The rolled wire rod is subjected to end face protection and bundling full packaging, and damage caused by follow-up hoisting and transportation of the wire rod is effectively prevented.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a production method of 12.9 grade cold heading steel for automobiles. Background Technology

[0002] With the development of metallurgical technology, higher requirements have been placed on the quality of cold heading steel, especially for 12.9 grade automotive cold heading steel, which requires both internal and surface quality.

[0003] Existing document CN114752857A discloses a 10.9 grade cold-heading steel wire rod for automotive fasteners and its production method. The composition of the cold-heading steel wire rod, by weight percentage, is: C: 0.32-0.40%, Si: 0.15-0.35%, Mn: 0.60-1.00%, P≤0.015%, S≤0.010%, Cr: 0.90-1.20%, Mo: 0.15-0.25%, Ni≤0.20%, Cu≤0.20%, N≤0.0060%, O≤0.0015%. The composition is H≤0.0002%, and also contains one or more of the following: Al: 0.015~0.050%, Ti: 0.02~0.05%, B: 0.0008-0.0035%, V: 0.02~0.05%, Nb: 0.01~0.04%, with the remainder being Fe and impurities. During the continuous casting of large round billets, low superheat and a three-stage combined electric stirring are employed. High-temperature diffusion is used during the billet opening process, and a high compression ratio is used during both the billet opening and rolling processes to improve the low-density porosity and segregation of the wire rod, meeting customer requirements for high fatigue and delayed fracture resistance. However, the performance only meets grade 10.9 and does not consider damage caused by the lifting and transportation of the wire rod after rolling. Summary of the Invention

[0004] To achieve the above objectives, this invention provides a production method for 12.9 grade automotive cold heading steel. Based on the chemical composition and the technological requirements for pure steel, a unique process is employed, including low-temperature dephosphorization in a converter and slag-blocking during tapping, LF (sulfurized oil recovery) for white slag deoxidation and desulfurization, and RH (hydrogenation) vacuum degassing, resulting in low phosphorus, low oxygen, low sulfur, low hydrogen, and minimal non-metallic inclusions. After billet preparation, the entire billet is repaired and subjected to surface magnetic particle fluorescent testing. During rolling, the surface of the wire rod is monitored online with thermal imaging, ensuring both the internal quality of the billet and zero defects on its surface. After rolling, the wire rod undergoes end-face protection and full packaging before bundling, effectively preventing damage during subsequent lifting and transportation.

[0005] The technical solution adopted in this invention is a method for producing 12.9 grade automotive cold heading steel, comprising the following main process steps:

[0006] (1) Converter: Low phosphorus slag blocking during steel tapping, P≤0.012%;

[0007] (2) LF furnace: to produce white slag for deoxidation and desulfurization, control the alkalinity to 8-10, and control S≤0.005%;

[0008] (3) RH furnace: vacuum degassing, maintain vacuum for ≥10 minutes, soft blowing for ≥15 minutes, control H≤0.0002%;

[0009] (4) Continuous casting: 350*430mm large cross section casting is used;

[0010] (5) Billet cutting: The 350*430mm steel billet is cut into 150*150mm billets;

[0011] (6) Grinding and flaw detection: The 150*150mm steel billet is fully ground with a 16-mesh grinding wheel, and then the surface magnetic particle fluorescent flaw detection is performed on all four sides to ensure zero defects on the surface of the steel billet.

[0012] (7) Rolling, protection, and bundling: Low-temperature rolling and air-cooled roller conveyor adopt delayed cooling, online thermal monitoring of the wire rod surface, and the wire rod adopts end face protection and bundling for full packaging after rolling;

[0013] The steel has the following chemical composition by weight percentage: C: 0.34-0.37%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P≤0.015%, S≤0.008%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, Al≥0.020%, with the remainder being Fe and unavoidable impurities.

[0014] Furthermore, in the grinding process, each side of the steel billet undergoes 12 grinding passes, and the corners undergo 5 passes.

[0015] Furthermore, in the rolling process, the final rolling temperature is 850°C and the wire drawing temperature is 750°C.

[0016] Furthermore, in the cooling process, the cooling rate is 0.5℃ / s.

[0017] The beneficial effects of this invention are:

[0018] (1) This invention employs low-temperature dephosphorization in a converter and slag-blocking during steel tapping, LF-based white slag deoxidation and desulfurization, and RH-based vacuum degassing to achieve low phosphorus, low oxygen, low sulfur, low hydrogen, and minimal non-metallic inclusions. Large billets undergo full repair after billet preparation, along with surface magnetic particle fluorescent testing. During rolling, the surface of the wire rod is monitored online with thermal imaging, ensuring both the internal quality of the billet and zero defects on its surface. After rolling, the wire rod is protected at the end face and fully packaged before bundling, effectively preventing damage caused by subsequent hoisting and transportation.

[0019] (2) Using the method of the present invention, the coarse non-metallic inclusion A+B+C+D series is ≤3.5, the fine non-metallic inclusion A+B+C+D series is ≤5.5, the inclusion DS is ≤1.5, the maximum depth of the total decarburized layer is allowed to be 0.8%D, and the tensile strength after heat treatment is 1350-1420MPa. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a picture of the product after it has been fully packaged and bundled for protection. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] A method for producing 12.9 grade automotive cold heading steel, wherein the chemical composition of the steel by weight percentage is: C: 0.34-0.37%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P≤0.015%, S≤0.008%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, Al≥0.020%, with the remainder being Fe and unavoidable impurities.

[0024] The production steps are as follows:

[0025] (1) Converter: Low phosphorus slag blocking during steel tapping, P≤0.012%;

[0026] (2) LF furnace: to produce white slag for deoxidation and desulfurization, control the alkalinity to 8-10, and control S≤0.005%;

[0027] (3) RH furnace: vacuum degassing, maintain vacuum for ≥10 minutes, soft blowing for ≥15 minutes, control H≤0.0002%.

[0028] (4) Continuous casting: 350*430mm large cross section casting is adopted.

[0029] (5) Billet cutting: The 350*430mm steel billet is cut into 150*150mm.

[0030] (6) Grinding and flaw detection: 12 grinding marks on each side and 5 grinding marks on the corners of the 150*150mm steel billet with 16-mesh grinding wheel, and then surface magnetic particle fluorescent flaw detection is carried out on all four sides to ensure zero defects on the surface of the steel billet.

[0031] (7) Rolling, protection, and bundling: Low-temperature rolling, with a final rolling temperature of 850℃ and a wire drawing temperature of 750℃; delayed cooling at a rate of 0.5℃ / s; online monitoring of the wire rod surface during rolling; after rolling, the wire rod is fully packaged with end-face protection and bundling, such as... Figure 1 As shown.

[0032] Effect testing: Samples of the products obtained by the above method are tested.

[0033] Table 1. Non-metallic inclusions

[0034]

[0035] Decarburization layer condition: Complete decarburization (free ferrite) is not allowed in wire rods, and the maximum depth of the total decarburization layer is allowed to be 0.8%D.

[0036] The following examples provide further details.

[0037] Example 1:

[0038] A method for producing 12.9 grade automotive cold heading steel, with a billet size of 350*430mm. The chemical composition of the steel by weight percentage is: C: 0.35%, Si: 0.20%, Mn: 0.73%, P: 0.012%, S: 0.002%, Al: 0.033%, H: 1.09ppm, Cr: 0.98ppm, Mo: 0.20ppm, with the remainder being Fe and unavoidable impurities.

[0039] The production process steps are as follows:

[0040] (1) Converter: slag blocking and steel tapping, P = 0.010%;

[0041] (2) LF furnace: produces white slag for deoxidation and desulfurization, with an alkalinity of 9.2 and S = 0.004%;

[0042] (3) RH furnace: vacuum degassing, maintain vacuum for 10 minutes, soft blowing for 18 minutes, S = 0.002%, H = 1.09ppm;

[0043] (4) Continuous casting: 350*430mm large cross section casting is used;

[0044] (5) Billet cutting: The 350*430mm steel billet is cut into 150*150mm billets;

[0045] (6) Grinding and flaw detection: 12 grinding marks on each side and 5 grinding marks on the corners of the 150*150mm steel billet with 16-mesh grinding wheel. Surface magnetic particle fluorescent flaw detection is carried out on all four sides. Steel billets with surface defects are ground and tested again. Unqualified billets are rejected.

[0046] (7) Rolling, protection, and bundling: The final rolling temperature is 850℃ and the wire drawing temperature is 750℃. The air-cooled roller conveyor adopts delayed cooling with a cooling rate of 0.5℃ / s. During rolling, the surface of the wire rod is monitored online with thermal imaging. Wire rods with surface defects are rejected. After rolling, the end face of the wire rod is protected with a swimming ring and fully packaged by bundling.

[0047] Test results: Non-metallic inclusions D (fine) 1.0, DS 1.0; no complete decarburization, total decarburized layer 0.58%D.

[0048] Example 2:

[0049] A method for producing 12.9 grade automotive cold heading steel, with a billet size of 350*430mm. The chemical composition of the steel by weight percentage is: C: 0.34%, Si: 0.18%, Mn: 0.75%, P: 0.013%, S: 0.001%, Al: 0.042%, H: 0.97ppm, Cr: 1.00ppm, Mo: 0.19ppm, with the remainder being Fe and unavoidable impurities.

[0050] The production process steps are as follows:

[0051] (1) Converter: slag blocking and tapping, P = 0.011%;

[0052] (2) LF furnace: produces white slag for deoxidation and desulfurization, with an alkalinity of 8.9 and S = 0.002%;

[0053] (3) RH furnace: vacuum degassing, maintain vacuum for 10 minutes, soft blowing for 20 minutes, S = 0.001%, H = 0.97ppm;

[0054] (4) Continuous casting: 350*430mm large cross section casting is used;

[0055] (5) Billet cutting: The 350*430mm steel billet is cut into 150*150mm billets;

[0056] (6) Grinding and flaw detection: 12 grinding marks on each side and 5 grinding marks on the corners of the 150*150mm steel billet with 16-mesh grinding wheel. Surface magnetic particle fluorescent flaw detection is carried out on all four sides. Steel billets with surface defects are ground and tested again. Unqualified billets are rejected.

[0057] (7) Rolling, protection, and bundling: The final rolling temperature is 850℃ and the wire drawing temperature is 750℃. The air-cooled roller conveyor adopts delayed cooling with a cooling rate of 0.5℃ / s. During rolling, the surface of the wire rod is monitored online with thermal imaging. Wire rods with surface defects are rejected. After rolling, the wire rod is fully packaged with end-face protection and bundling.

[0058] Test results: Non-metallic inclusions D (fine) 1.0; no complete decarburization, total decarburized layer 0.63%D.

[0059] In summary, by adopting the technical solution of this invention, the large billet undergoes full repair after billet preparation, surface magnetic particle fluorescent testing, and online thermal monitoring of the wire rod surface during rolling, ensuring both the internal quality of the billet and zero defects on its surface. After rolling, the wire rod is protected at the end face and fully packaged in bundles, effectively preventing damage caused by subsequent hoisting and transportation.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method for producing 12.9 grade automotive cold heading steel, characterized in that, The main process steps include the following: (1) Converter: Low phosphorus slag blocking during steel tapping, P≤0.012%; (2) LF furnace: to produce white slag for deoxidation and desulfurization, control the alkalinity to 8-10, and control S≤0.005%; (3) RH furnace: vacuum degassing, maintain vacuum for ≥10 minutes, soft blowing for ≥15 minutes, control H≤0.0002%; (4) Continuous casting: 350*430mm large cross section casting is used; (5) Billet cutting: The 350*430mm steel billet is cut into 150*150mm billets; (6) Grinding and flaw detection: The 150*150mm steel billet is fully ground with a 16-mesh grinding wheel, and then the surface magnetic particle fluorescent flaw detection is performed on all four sides to ensure zero defects on the surface of the steel billet. (7) Rolling, protection, and bundling: Low-temperature rolling and air-cooled roller conveyor adopt delayed cooling, online thermal monitoring of the wire rod surface, and the wire rod adopts end face protection and bundling for full packaging after rolling; The steel has the following chemical composition by weight percentage: C: 0.34-0.37%, Si: 0.15-0.35%, Mn: 0.60-0.90%, P≤0.015%, S≤0.008%, Cr: 0.90-1.20%, Mo: 0.15-0.30%, Al≥0.020%, with the remainder being Fe and unavoidable impurities.

2. The production method of 12.9 grade automotive cold heading steel as described in claim 1, characterized in that, In the grinding process, each side of the steel billet undergoes 12 grinding passes, and the corners undergo 5 passes.

3. The production method of 12.9 grade automotive cold heading steel as described in claim 2, characterized in that, In the rolling process, the final rolling temperature is 850℃ and the wire drawing temperature is 750℃.

4. The production method of 12.9 grade automotive cold heading steel as described in claim 3, characterized in that, In the cooling process, the cooling rate is 0.5℃ / s.

Images