Method for improving surface quality of cold heading steel wire rod

Abstract

The application discloses a method for improving the surface quality of cold upsetting steel wire rod and belongs to the technical field of metallurgy. The average thickness of the oxide skin of the continuous casting blank is controlled to be 0.25-0.3 mm. The formation of the oxide skin is specifically controlled by controlling the holding temperature of the continuous casting blank to be greater than or equal to 900 DEG C and the holding time to be not less than 40 min. The application realizes the thickening of the original blank industrialized oxide skin through the pouring residual heat of the continuous casting blank, so that the defects on the surface of the blank form the oxide skin to fall off, the process flow is shortened, the production cost is reduced, and the low-carbon environmental protection requirement is met.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a method for improving the surface quality of cold heading steel wire rod. Background Technology

[0002] Cold heading steel wire rod is mainly used to produce cold heading workpieces and is widely used in aerospace, aviation, automotive and equipment manufacturing fields. Its surface quality requirements are extremely stringent. Even very slight defects can cause the material to crack and fail during cold heading. The main reason is that during the cold heading deformation process, the surface tension forms a stress concentration phenomenon at the defect location, which in turn leads to cracking failure.

[0003] CN116140502A discloses a method for producing cold heading steel wire rod and the resulting cold heading steel wire rod, which solves the problem of surface scribing quality defects in the wire rod by controlling rolling parameters. CN102873293B discloses a continuous casting process for cold heading steel for low-carbon boron-containing standard parts, which improves surface quality by controlling the composition of molten steel and protective slag, and by improving the continuous casting process.

[0004] Tracing back to the source revealed that over 80% of the defects were inherited from the surface defects (vibration marks, inclusions, bubbles) of the base material (continuously cast billet). Therefore, the former patent only considered the surface quality problems that are prone to occur during the rolling stage and did not solve the surface defect problem from the source; the latter directly changed the original continuous casting process, requiring a significant change to the existing mature continuous casting production method, and may not be suitable for cold heading steel grades with different requirements.

[0005] Therefore, the industry currently mainly solves the problem of inherited surface defects in the base material through the following methods: ① Small-sized billet → grinding → rolling; ② Large-sized billet → grinding and rolling; ③ Large-sized billet → billet preparation → rolling; ④ Large-sized billet → billet preparation → grinding → rolling. In other words, surface defects in the billet are removed through billet preparation or grinding, thereby enabling the rolled product to meet the required surface quality requirements.

[0006] However, the above production processes all require additional steps, resulting in the depletion of manpower and resources. Therefore, a method is needed that can shorten the process and solve the problem of inherited surface defects in the continuous casting billets of cold heading steel wire rod. Summary of the Invention

[0007] Technical problem solved: To address the above-mentioned technical problems, this invention provides a method for improving the surface quality of cold heading steel wire rod. By utilizing the residual heat from the continuous casting of the billet, the industrial-scale oxide scale of the original billet is thickened, thereby causing the oxide scale on its surface defects to fall off. This shortens the process flow, reduces production costs, and meets the requirements of low carbon and environmental protection.

[0008] Technical solution: A method for improving the surface quality of cold heading steel wire rod, controlling the average thickness of the oxide scale in the continuously cast billet to be 0.25~0.3mm.

[0009] Preferably, the oxide scale is generated by controlling the holding temperature of the continuously cast billet to be ≥900℃ and the holding time to be 40min.

[0010] Preferably, the heat preservation temperature is 950~1000℃.

[0011] Preferably, the heat for heat preservation is provided by the residual heat after the continuous casting billet is poured.

[0012] Preferably, the heat preservation temperature is monitored by a temperature sensor installed below the continuous casting cooling bed.

[0013] Furthermore, a steel insulation cover is provided above the continuous casting cooling bed, and the insulation cover is embedded with asbestos.

[0014] Furthermore, a blower and a heating device are also provided below the continuous casting cooling bed. When the temperature sensor detects a temperature >1000℃, the blower is turned on to cool the continuous casting billet and increase the oxygen content; when the temperature sensor detects a temperature of 950~1000℃, the blower is not turned on; when the temperature sensor detects a temperature <950℃, the heating device is turned on.

[0015] Beneficial effects: This invention utilizes the residual heat from the casting of continuously cast billets to achieve industrial-scale thickening of the original billet oxide scale, thereby causing the oxide scale to fall off the surface defects. This not only shortens the process flow but also reduces production costs and meets the requirements of low carbon and environmental protection.

[0016] The principle of this invention is as follows: According to field tests, 95% of the high-risk thickness of surface defects in continuously cast billets is concentrated within 3mm. This portion of heritable surface defects is the source of over 80% of defects in cold-heading steel wire rods, and therefore generally needs to be removed by grinding. Iron oxides (Fe2O3, Fe3O4) possess good hardness and brittleness, low deformation performance, and easy peeling properties. Furthermore, according to the ferro-oxygen phase diagram (see appendix...) Figure 1 It can be seen that its formation is closely related to temperature, surrounding oxygen concentration and time.

[0017] Cold heading steel primarily uses aluminum for deoxidation, so the relationship between the free oxygen content and carbon content in the steel itself is no longer significant and can be ignored. The effect of different temperatures on iron oxides (see appendix) is also relevant. Figure 2 As can be seen, in order to form a large amount of oxygen-rich oxides, the temperature must be ≥900℃. In fact, the temperature that the continuously cast billet can effectively maintain after being cut from the continuous casting machine is around 1000℃, which fully meets the temperature conditions for the formation of oxygen-rich oxides of iron.

[0018] The continuously cast billet is in direct contact with air, and oxygen in the air is economical, inexpensive and readily available, with a concentration of about 21%, which can meet the oxygen concentration requirements of iron oxides.

[0019] Based on experimental data (see appendix) Figure 3 It is known that approximately 3 μm thick iron oxide can be formed in about 0.25 minutes, therefore, it takes about 25 minutes for the billet to form a thickness of about 0.3 mm. However, in reality, constant temperature and oxygen concentration conditions cannot be achieved in industrial environments, so the holding time usually needs to be appropriately extended. Depending on different field tests, it generally requires about 40 minutes. Attached Figure Description

[0020] Figure 1 It is a ferrite phase diagram;

[0021] Figure 2 This is a graph showing the effect of different temperatures on iron oxides;

[0022] Figure 3 This is a graph showing the relationship between the thickness of the oxide layer and time.

[0023] Figure 4 This is a schematic diagram of an embodiment of the present invention. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Example 1

[0025] like Figure 4 As shown, a steel insulation cover is installed above the continuous casting cooling bed. The insulation cover is embedded with asbestos and can divide the continuous casting cooling bed into several zones according to different conditions, thus providing insulation for the continuously cast billet on the cooling bed. Temperature sensors, blowers, and heating devices are respectively installed in different zones below the continuous casting cooling bed. The temperature sensors monitor the zone temperature. When the temperature sensor detects a temperature >1000℃, the blower turns on to cool the continuously cast billet and increase its oxygen content; when the temperature sensor detects a temperature between 950~1000℃, the blower does not turn on; when the temperature sensor detects a temperature <950℃, the heating device is activated.

[0026] The above-mentioned device can control the holding temperature of the continuous casting billet to ≥900℃ and the holding time to ≥40min, thereby controlling the oxide scale thickness of the continuous casting billet to be close to 0.3mm. The oxide scale is an oxygen-rich oxide of iron (a microstructure of iron(III) oxide and iron(II) oxide with an oxygen content of ≥28%).

[0027] Taking the Comcast 9-meter arc 140 cubic meter continuous casting machine as an example, without the process of increasing the thickness of the iron oxide scale, the average thickness of the iron oxide scale at the center of the billet surface is about 0.18 mm. When cold heading steel with a rolling diameter of 30 mm is rolled, the linear defect rate on the wire rod surface is about 35%. After adopting the process of increasing the thickness of the iron oxide scale, the thickness of the iron oxide scale on the billet surface is about 0.25 mm, and the thickness of the iron oxide scale in some local locations can reach 0.3 mm. The linear defect rate of wire rod with a rolling diameter of 30 mm is reduced to about 1.5%.

[0028] Therefore, this invention utilizes the residual heat from the casting of the continuous casting billet and the heat provided by the heating device to achieve industrial thickening of the original billet oxide scale, thereby causing the oxide scale to fall off the defects on its surface. This shortens the process flow, reduces production costs, and meets the requirements of low carbon and environmental protection.

Claims

1. A method for improving the surface quality of cold-heading steel wire rod, characterized in that, The continuously cast billet is in direct contact with air, and the residual heat after casting provides the heat for insulation. The insulation temperature of the continuously cast billet is controlled at 950~1000℃, and the insulation time is ≥40min, thereby controlling the average thickness of the oxide scale of the continuously cast billet to be 0.25~0.3mm. The insulation temperature is monitored by a temperature sensor installed below the continuous casting cooling bed. A blower and a heating device are also installed below the continuous casting cooling bed. When the temperature sensor detects a temperature >1000℃, the blower is turned on to cool the continuously cast billet and increase the oxygen content; when the temperature sensor detects a temperature of 950~1000℃, the blower is not turned on; when the temperature sensor detects a temperature <950℃, the heating device is turned on.

2. The method for improving the surface quality of cold heading steel wire rod according to claim 1, characterized in that, A steel insulation cover is provided above the continuous casting cooling bed, and the insulation cover is embedded with asbestos.

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