A production method for low-yield strength low-carbon cold heading steel

Abstract

A method for producing low-yield strength, low-carbon, cold-heading steel, wherein the chemical composition of the steel is, by weight, C = 0.10% to 0.20%, Si = 0.10% to 0.30%, Mn = 0.60% to 0.90%, P ≤ 0.030%, S ≤ 0.030%, and the remainder is Fe and unavoidable impurities; the process steps include heating, rolling, spinning, and Stelmor cooling. The present invention mainly controls the hot rolling process, spinning temperature, and Stelmor controlled cooling process to control the reasonable combination of the room-temperature grain size, dislocation density, and grain boundaries of the wire rod, thereby achieving the purpose of achieving low yield strength in the hot-rolled low-carbon, cold-heading steel wire rod, which is conducive to deep drawing, special-section drawing, and easy cold-heading deformation, thereby reducing the yield strength of the low-carbon, cold-heading steel wire rod, improving the quality of downstream processed parts, reducing the drawing force, reducing the power consumption of the drawing machine during the drawing process, and reducing the cost of product parts.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production method of low-yield strength low-carbon cold heading steel. Background Art

[0002] Low-carbon, cold-heading steel wire rod is the primary material for processing and manufacturing small, unusually shaped electrical components and small structural parts. Typically, 5.5-16.0mm wire rod is drawn into 0.5-3.5mm diameters, mechanically descaled or pickled, then drawn in multiple passes. Finally, parts are cold-headed or bent. Finished parts must have a smooth surface, be free of internal microscopic drawing defects, and have irregular cross-sections. The wire rod's cold drawing properties and resistance to cold-heading deformation play a crucial role in the quality of finished parts, placing stringent requirements on its yield strength, which influences these properties. A high yield strength in hot-rolled, low-carbon, cold-heading steel wire rod will prevent deformation during the drawing and cold-heading processes, potentially leading to internal and external cracks in the steel, resulting in material rejection.

[0003] Therefore, reducing the yield strength of low-carbon cold heading steel wire rod can improve the quality of downstream processed parts and has practical research significance. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for producing low-yield strength low-carbon cold heading steel, which can control the room temperature grain size, dislocation density and reasonable combination of grain boundaries of the hot-rolled wire rod of the low-carbon cold heading steel, and develop a low-yield strength low-carbon cold heading steel that is conducive to deep drawing, special-section drawing and easy cold heading deformation.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for producing low-yield strength low-carbon cold heading steel, wherein the chemical composition of the steel is C=0.10%-0.20%, Si=0.10%-0.30%, Mn=0.60%-0.90%, P≤0.030%, S≤0.030%, and the remainder is Fe and unavoidable impurities; the method comprises the following process steps:

[0007] 1) Heating: Using a walking beam heating furnace, the furnace time is 90 to 150 minutes, and the temperature of the billet out of the furnace is 1000 to 1100 ° C;

[0008] 2) Rolling: The wire rod is rolled to the finished wire rod size through roughing and intermediate rolling, pre-finishing rolling and finishing rolling mills. The cooling water tank of the rolling line is opened to control the final rolling temperature at 1000-1050℃;

[0009] 3) Wire drawing: Adjust the water volume of the water cooling line after the final rolling to control the wire drawing temperature at 880~950℃, to avoid the wire drawing temperature being too low or too high, which will make the surface oxide scale too thin and not conducive to mechanical removal of the oxide scale, and the local coarsening of the grains causing the drawn steel wire to be delaminated;

[0010] 4) Stelmor cooling: All cooling fans are turned off and a slow cooling process is adopted; the speed of the Stelmor insulation roller is adjusted to ensure that the cooling rate of the wire rod in the temperature range of 700-900℃ is 1-2℃ / s; in the temperature range of 550-700℃, the delayed cooling method is used to control the cooling rate of the wire rod to 0.3-1.0℃, reducing the phase change cooling rate of the wire rod; in the temperature range of 350-550℃, the delayed cooling method is used to control the cooling rate of the wire rod to 0.5-2.0℃, facilitating the elimination of hot rolling stress and phase change stress and reducing dislocation density;

[0011] 5) Through the above process, the hot-rolled structure is obtained as a wire rod with uniform equiaxed ferrite, a small amount of dispersed pearlite and low yield strength.

[0012] Principle of the invention:

[0013] The Hall-Petch equation (Os = σ0 + kd~1 / 2) indicates that reducing the yield strength of a material requires increasing its grain size. In step 1), the billet exit temperature is controlled between 1000°C and 1100°C to ensure uniform austenite grain growth, facilitating subsequent hot rolling and increasing the original austenite grain size. In step 2), the cooling water tanks are activated to control the final rolling temperature between 1000°C and 1050°C. This maintains high temperatures for dynamic and static recrystallization recovery after rolling deformation, accelerating recrystallization recovery and increasing austenite grain size. In step 3), the water flow rate in the cooling water line after final rolling is adjusted between 600 and 1000 L / min, maintaining the spinning temperature between 880°C and 950°C. Too low a spinning temperature will result in insufficient driving force for austenite grain growth, while too high a spinning temperature can lead to the formation of giant grains, impairing the cold workability of the wire rod. The spinning temperature should be controlled so that the austenite grains of the wire rod have sufficient driving force to grow uniformly before the solid-state phase transformation on the Stelmor roller.

[0014] Control of the reasonable matching of wire rod's room temperature grain size, dislocation density and grain boundaries. How to control the room temperature grain size, dislocation density and grain boundary matching of wire rod. Stelmor air-cooled roller adopts the method of appropriately increasing the cooling rate at high temperature of wire rod + slow cooling + appropriately increasing the cooling rate of wire rod. Appropriately increase the cooling rate at high temperature of wire rod to control the grains from growing too much, and dynamic recovery and recrystallization are fully carried out to reduce the dislocation density. Appropriately reduce the cooling rate during the phase transformation process to form uniform and dispersed small-block pearlite and ferrite structures. After the phase transformation is completed, continue to use a slow cooling rate, which is appropriately higher than the phase transformation cooling rate, to facilitate the elimination of hot rolling stress and phase transformation stress, reduce dislocation density and iron oxide scale weight and collection temperature. Finally, control the reasonable matching of wire rod's room temperature grain size, dislocation density and grain boundaries.

[0015] This invention achieves the goal of achieving a low yield strength in hot-rolled low-carbon cold-heading steel wire rod by controlling the hot-rolling process, spinning temperature, and Stelmor-controlled cooling to optimize the balance of room-temperature grain size, dislocation density, and grain boundaries. This facilitates deep drawing, special-section drawing, and easy cold-heading deformation. Beneficial effects include lowering the yield strength of low-carbon cold-heading steel wire rod, improving the quality of downstream processed parts. Lowering the drawing force reduces power consumption of the drawing machine during the drawing process, lowering the cost of finished parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a metallographic structure photo of the wire rod in Example 1.

[0017] Figure 2 This is a metallographic structure photo of the wire rod in Example 2.

[0018] Figure 3 This is a metallographic photograph of the comparative wire rod. DETAILED DESCRIPTION Example 1

[0019] A method for producing low-yield strength low-carbon cold heading steel, wherein the chemical composition of the steel is C=0.15%, Si=0.20%, Mn=0.76%, P=0.015%, S=0.006%, and the remainder is Fe and unavoidable impurities; the method comprises the following steps:

[0020] 1) Heating: The furnace time is 120 minutes and the billet temperature out of the furnace is 1020℃;

[0021] 2) Rolling: Through roughing and intermediate rolling, pre-finishing rolling and finishing rolling mills, the finished wire rod is rolled to the finished wire rod size. The cooling water tank of the rolling line is opened to control the final rolling temperature to 1005℃;

[0022] 3) Wire drawing: Adjust the water volume of the water cooling line after final rolling to ensure the wire drawing temperature is 890℃;

[0023] 4) Stelmor cooling process: The cooling fan is completely turned off and a slow cooling process is adopted; the speed of the Stelmor insulation roller is adjusted to ensure that the cooling rate of the wire rod in the range of 700-900℃ is 1.5℃ / s; the delayed cooling method is used to control the cooling rate of the wire rod in the temperature range of 550-700℃ to 0.8℃, reducing the phase transformation cooling rate of the wire rod; the delayed cooling method is used to control the cooling rate of the wire rod in the temperature range of 350-550℃ to 1.5℃, and the hot-rolled structure is uniform equiaxed ferrite and a small amount of dispersed pearlite, and the wire rod has a low yield strength. The hot rolling properties are shown in Table 1, and the metallographic structure pictures are shown in Figure 1 . Example 2

[0024] A method for producing low-yield strength low-carbon cold heading steel, wherein the chemical composition of the steel is C=0.15%, Si=0.20%, Mn=0.76%, P=0.015%, S=0.006%, and the remainder is Fe and unavoidable impurities; the method comprises the following steps:

[0025] 1) Heating: The furnace time is 120 minutes and the billet temperature out of the furnace is 1080℃;

[0026] 2) Rolling: Through roughing and intermediate rolling, pre-finishing rolling and finishing rolling mills, the finished wire rod is rolled to the finished wire rod size. The cooling water tank of the rolling line is opened to control the final rolling temperature at 1045℃.

[0027] 3) Wire drawing: Adjust the water volume of the water cooling line after final rolling to ensure the wire drawing temperature is 940℃;

[0028] 4) Stelmor cooling process: All cooling fans are turned off and a slow cooling process is adopted; the speed of the Stelmor insulation roller is adjusted to ensure that the cooling rate of the wire rod in the temperature range of 700-900℃ is 1.5℃ / s; in the temperature range of 550-700℃, the delayed cooling method is used to control the cooling rate of the wire rod to 0.8℃, reducing the phase change cooling rate of the wire rod; in the temperature range of 350-550℃, the delayed cooling method is used to control the cooling rate of the wire rod to 1.5℃, facilitating the elimination of hot rolling stress and phase change stress and reducing dislocation density;

[0029] 5) Through the above process, the hot rolled structure is uniform equiaxed ferrite and a small amount of dispersed pearlite, and the wire rod has low yield strength. The hot rolled properties are shown in Table 1, and the metallographic pictures are shown in Figure 2 .

[0030] Comparative Example

[0031] A comparative example was produced using a steel billet raw material having the same chemical composition. The chemical composition of the steel was as follows: C = 0.15%, Si = 0.20%, Mn = 0.76%, P = 0.015%, S = 0.006%, and the remainder was Fe and unavoidable impurities. The process comprised the following steps:

[0032] 1) Heating: The furnace time is 120 minutes and the billet temperature out of the furnace is 950℃;

[0033] 2) Rolling: The wire rod is rolled to the finished wire rod size through roughing and intermediate rolling, pre-finishing rolling and finishing rolling mills, and the cooling water tank of the rolling line is opened to control the final rolling temperature to 950℃;

[0034] 3) Wire drawing: Adjust the water volume of the water cooling line after final rolling to ensure the wire drawing temperature is 800℃;

[0035] 4) Stelmor cooling process: Adjust the Stelmor insulation roller speed to ensure that the cooling rate of the wire rod in the range of 700-900℃ is 2.5℃ / s; the wire rod temperature range of 550-700℃ adopts the delayed cooling method to control the wire rod cooling rate to 1.5℃; the wire rod temperature range of 350-550℃ adopts the delayed cooling method to control the wire rod cooling rate to 2.5℃, and the hot-rolled structure is equiaxed ferrite and a small amount of dispersed pearlite with high yield strength. The hot rolling properties are shown in Table 1, and the metallographic structure pictures are shown in Figure 3 .

[0036] Table 1 Wire rod hot rolling performance test data

[0037] .

Claims

1. A method for producing low-yield strength low-carbon cold heading steel, characterized by: The chemical composition of steel is C=0.10%~0.20%, Si=0.10%~0.30%, Mn=0.60%~0.90%, P≤0.030%, S≤0.030%, and the rest is Fe and unavoidable impurities; The process steps include: 1) Heating: Using a walking beam heating furnace, the furnace time is 90 to 150 minutes, and the temperature of the billet out of the furnace is 1000 to 1100 ° C; 2) Rolling: The wire rod is rolled to the finished wire rod size through roughing and intermediate rolling, pre-finishing rolling and finishing rolling mills. The cooling water tank of the rolling line is opened to control the final rolling temperature at 1000-1050℃; 3) Wire drawing: Adjust the water volume of the water cooling line after final rolling to control the wire drawing temperature at 880~950℃; 4) Stelmor cooling: The cooling fans are all turned off and a slow cooling process is adopted; the speed of the Stelmor insulation roller is adjusted to ensure that the cooling rate of the wire rod in the range of 700-900℃ is 1-2℃ / s; the delayed cooling method is used in the temperature range of 550-700℃ to control the cooling rate of the wire rod to 0.3-1.0℃, thereby reducing the phase transformation cooling rate of the wire rod; the delayed cooling method is used in the temperature range of 350-550℃ to control the cooling rate of the wire rod to 0.5-2.0℃, thereby obtaining a wire rod with a hot-rolled structure of uniform equiaxed ferrite, a small amount of dispersed pearlite, and low yield strength.

Images