A cold drawn steel wire for 10.9 grade non-quenched and tempered U-bolt and a manufacturing method thereof

Abstract

The present invention relates to a cold-drawn steel wire for 10.9-grade non-quenched and tempered U-bolts and a manufacturing method thereof, and belongs to the field of steel wire production for fasteners. The chemical composition of the steel wire is designed to be: C: 0.45-0.65%, Si: 1.0-2.0%, Mn: 0.70-1.00%, P: ≤0.025%, S: ≤0.025%, Cr: 0.10-0.70%, Nb: 0.01-0.06%, H: ≤0.0002%, the remainder is Fe and unavoidable impurity elements, and the structure is ferrite and troostite. Strengthening is performed by increasing the content of Si element, thereby increasing the yield strength of the wire rod, and the yield strength ratio reaches more than 0.85, and the yield strength ratio reaches 0.88-0.93 after wire drawing. Cold-drawn steel wire has a ferrite + troostite structure. After the steel wire is used to make 10.9 grade U-bolts, the U-bolts do not need to be tempered and heat treated after forming, which greatly reduces the production cost of the U-bolts and realizes the green and energy-saving production of U-bolts.

Description

Technical Field

[0001] The invention belongs to the field of wire rod steel, and in particular relates to a cold-drawn steel wire for bolts and a manufacturing method thereof. Background Art

[0002] U-bolts, also known as riding bolts, are often used to fix tubular or sheet objects such as leaf springs in cars. The manufacturing process of U-bolts is generally as follows: alloy steel wire rods are annealed and heat treated, and then wired to the corresponding specifications. The wire rods after drawing are made into the corresponding shapes on the U-bolt manufacturing equipment and then subjected to quenching and tempering heat treatment, and the corresponding mechanical property grades are obtained after quenching and tempering. In the context of dual carbon emission reduction and green manufacturing, it is now necessary to reduce carbon emissions in the manufacturing process of grade 10.9 U-bolts, achieve green manufacturing, consider canceling the quenching and tempering heat treatment, and use non-quenched and tempered steel to make U-bolts.

[0003] Non-tempered U-bolts specifically refer to the elimination of the tempering heat treatment after the U-bolt is formed, and the elimination of the annealing treatment before the U-bolt is made. The manufacturing process of non-tempered U-bolts is roughly as follows: hot-rolled wire rod-cold-drawn steel wire-U-bolt forming-bolt thread processing-stabilization treatment. Non-tempered U-bolts are completely achieved by controlling the mechanical properties of hot-rolled wire rods and matching the wire rod drawing reduction rate. The steel wire produced meets or approaches the mechanical properties of the quenched and tempered U-bolts after processing. Summary of the invention

[0004] The purpose of the present invention is to provide a cold-drawn steel wire for making 10.9-grade non-quenched and tempered U-bolts and a manufacturing method thereof. By rationally designing the alloy composition, coordinating the rolling process and the EDC water bath cooling process, the wire rod obtains a uniform and fine ferrite + troostite structure, and the wire rod has good performance. After being cold-drawn into steel wire, it is directly processed into a U-bolt, and finally a 10.9-grade high-strength U-bolt can be obtained after a simple stabilization heat treatment. The whole process and all-round process can achieve low-cost production of steel wire for 10.9-grade U-bolts and bolt manufacturing, so as to achieve the purpose of energy saving and consumption reduction.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problem is: a cold-drawn steel wire for 10.9-grade non-quenched and tempered U-shaped bolts, characterized in that the chemical composition of the steel wire is designed as follows by mass percentage: C: 0.45-0.65%, Si: 1.0-2.0%, Mn: 0.70-1.00%, P: ≤0.025%, S: ≤0.025%, Cr: 0.10-0.70%, Nb: 0.01-0.06%, H: ≤0.0002%, the remainder is Fe and unavoidable impurity elements, and the structure is ferrite and troostite.

[0006] Furthermore, the chemical composition of the steel wire is Ni≤0.30%, and Cu≤0.30%.

[0007] The chemical composition of the cold-drawn steel wire for non-adjustable U-bolts of the present application is based on medium carbon steel. By utilizing the solid solution strengthening of Si, Mn, and Cr alloy elements, Si can solid solution strengthen ferrite, improve the yield strength ratio of steel, and improve the anti-relaxation ability of the bolts. Adding an appropriate amount of Nb element to the steel increases the recrystallization temperature of austenite and combines it with the rolling process to obtain fine austenite grains in the steel. Then, the EDC water bath cooling + Stelmor cooling process is used to transform the fine austenite grains into a uniform ferrite + troostite structure. Strictly control the content of impurity elements such as P and S in the product, improve the purity of the grain boundaries of the steel, and increase the grain boundary strength.

[0008] The hydrogen content of the steel in this application is limited to less than 2ppm, in order to reduce the hydrogen embrittlement sensitivity of the U-bolts of the bolts. The chemical composition of the steel wire for the 10.9 grade non-quenched and tempered U-bolts in this application is designed as follows:

[0009] C: Carbon is one of the main strengthening elements of steel. If the carbon content is too low, the strength and hardness of the non-adjustable U-bolt will be too low. If the carbon content is too high, while the strength and hardness increase, the plasticity will decrease, affecting the cold bending performance of the U-bolt. Therefore, the present invention controls the carbon content to 0.45-0.65%.

[0010] Si: Silicon is a ferrite solid solution strengthening element and also a deoxidizing element. Silicon can effectively strengthen ferrite. A high silicon content will increase the wire drawing strengthening coefficient, especially the yield strength of steel and the yield strength ratio. At the same time, silicon can improve the bolt's ability to resist stress relaxation. Therefore, the present invention controls the silicon content in the steel to 1.00-2.00%.

[0011] Mn: Manganese improves the strength of steel through solid solution strengthening. Too low manganese will reduce the steel's plastic toughness and also reduce the Mn / S ratio in the steel, which is easy to cause segregation of the continuous casting billet. In order to match the strength and plasticity of the non-adjustable U-shaped bolt, the present invention controls the manganese content in the steel to 0.70-1.00%.

[0012] Cr: Chromium is an element that enhances the hardenability of steel. The addition of a suitable content can effectively increase the hardenability of medium carbon steel and improve the strength of the steel. However, if the chromium content is too much, the hot rolling strength will be too high. Therefore, the present invention controls the chromium content in the steel to 0.10-0.70%.

[0013] Nb: Niobium is the most significant element for refining the actual austenite grain size during rolling. In the present invention, the actual austenite grain size during rolling is refined by appropriately adding niobium element. The niobium content in the present invention is controlled at 0.01-0.06%.

[0014] P, S: In principle, the lower the phosphorus and sulfur content, the better, but too low a content will undoubtedly increase the smelting cost. Under the condition that the quality allows, the present invention controls the phosphorus and sulfur content to be below 0.025%.

[0015] H: The present invention strictly controls the hydrogen content in the steel to below 2 ppm to reduce the risk of hydrogen-induced delayed fracture.

[0016] Another object of the present invention is to provide a method for manufacturing the above-mentioned 10.9 grade non-adjustable U-bolt cold-drawn steel wire, the steps comprising:

[0017] Molten steel smelting: the smelting raw materials are sequentially subjected to primary refining, refining and vacuum degassing. During the primary refining, P is ≤0.015%. During the primary refining, slag is added along the steel flow. The slag is used to remove inclusions, element alloy blocks and deoxidized alloys during refining. Nb alloy is added after the reduction slag is formed in the refining process. After the vacuum degassing, the molten steel is left to stand for more than 20 minutes.

[0018] Billet preparation: Molten steel that meets the element design is cast into billets;

[0019] Billet heating: reheat the structure to make it fully austenitized;

[0020] Wire rod rolling: The rolling process is divided into rough rolling, intermediate rolling, pre-finishing rolling and finishing rolling. The rough rolling temperature is controlled at 1000-1100℃, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 850-950℃, and the finishing rolling adopts temperature-controlled rolling, the rolling temperature is controlled at 800-850℃, and the rolling is carried out in the austenite non-recrystallization zone. The total deformation of temperature-controlled rolling is controlled at 30-50%, and the wire laying temperature is controlled at 800-850℃;

[0021] Wire rod cooling: After spinning, the wire rod enters the EDC water bath for cooling. The water temperature of the EDC is maintained above 96°C, and the wire rod is evenly cooled in the EDC. The temperature of the wire rod out of the EDC is controlled at 500-550°C. After exiting the water bath, the wire rod is coiled. The temperature difference between the two overlapping points and the middle point of adjacent wire rod coils does not exceed 50°C. The middle point refers to the midpoint of the two overlapping points. The coiled wire rod is cooled in the Stelmor cooling line. The cooling line is fully covered with an insulation cover. The total cooling time of the wire rod in the insulation cover is ≥100 seconds. The temperature when leaving the insulation cover should be 200-400°C. During the cooling process, austenite is transformed into ferrite + troostite. After the wire rod exits the cover, the coil is collected and cooled naturally.

[0022] The hot rolled wire rod for 10.9 grade non-quenched and tempered U-bolts can be obtained from the above.

[0023] Steel wire drawing: The wire rod is acid-washed and saponified before being drawn to obtain the steel wire.

[0024] Preferably, the molten steel smelting includes converter or electric furnace smelting, LF refining, RH or VD degassing, and the alloy added along the steel flow during the initial smelting is silicon-manganese alloy or low-calcium ferrosilicon to adjust the content of Si and Mn in the molten steel. When adding Cr-containing alloys, low-manganese alloys are also preferred.

[0025] Preferably, in the billet preparation stage, molten steel is sent to a continuous casting machine to be cast into billets, the superheat of the molten steel is controlled at 15-40°C, the continuous casting tundish is cast into billets under argon protection throughout the process, and the continuous casting billets are cooled by pile cooling or slow cooling pit cooling after fire cutting.

[0026] Preferably, the cooled continuous casting billet is treated to remove surface defects of the billet to prevent surface defects from being introduced into the final wire rod.

[0027] Preferably, in the billet heating stage, a walking-beam heating furnace is used for heating, the temperature of the preheating section of the heating furnace is controlled at ≤650°C, the temperature of the heating equalization section of the heating furnace is controlled at 1100-1200°C, and the total heating time is controlled at 90-160min; after heating, the billet is descaled by high-pressure water, the descaling pressure is ≥180bar, and rolling is carried out after descaling.

[0028] Preferably, in the wire rod rolling stage, since the wire rod rolling speed is relatively fast, a cooling water tank is arranged after pre-finishing rolling and before finishing rolling to reduce the temperature of the billet to reach the control temperature of rolling.

[0029] Preferably, during the wire rod cooling stage, the roller speed of the Stelmor cooling line is 0.2-0.5 m / s.

[0030] Preferably, in the steel wire drawing stage, the wire drawing area reduction ratio is controlled at 6%-15%.

[0031] Compared with the prior art, the advantages of the present invention are:

[0032] The yield strength of the wire rod is increased by increasing the Si content, and the yield strength ratio reaches above 0.85. After drawing, the yield strength ratio reaches 0.88-0.93. The cold-drawn steel wire has a ferrite + troostite structure. After the steel wire is used to make 10.9 grade U-bolts, the U-bolts do not need to be tempered after forming, which greatly reduces the production cost of the U-bolts and realizes the green and energy-saving production of U-bolts. The mechanical properties of the cold-drawn steel wire for non-quenched and tempered U-bolts are shown in Table 1 below:

[0033] Table 1 Mechanical properties of cold drawn steel wire for non-adjustable U-bolts of grade 10.9

[0034] Specification / mm Yield strength / MPa Tensile strength / MPa Elongation % Sectional shrinkage / % Room temperature impact value / J 16～25 900～1000 1040～1150 ≥10 ≥50 20～30

[0035] The yield strength of the hot-rolled wire rod of the present application is 820-880MPa, tensile strength is 1000-1100MPa, elongation is 13-15%, cross-sectional shrinkage is 55-60%, and impact energy at room temperature is 25-40J. The yield strength of the steel wire after drawing with a surface reduction rate of 6-15% is 900-1000MPa, tensile strength is 1040-1150MPa, elongation is ≥11%, cross-sectional shrinkage is ≥50%, and impact energy at room temperature is 20-30J.

[0036] The reason why the wire rod of the present application does not need to be annealed before drawing is that the wire rod of the present application has a structure of ferrite and troostite, and there is no other abnormal structure that is not conducive to drawing. The present invention does not need to be annealed to eliminate it. The purpose of annealing is to eliminate the abnormal structure. The strength and other properties of the steel wire after the wire rod of the present application have met the use requirements of the U-bolt, so the quenching and tempering heat treatment can be cancelled after being formed into a U-bolt. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 : This is the ferrite+troostite microstructure of the steel wire in the embodiment of the present invention. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0039] Example 1

[0040] This embodiment relates to a cold-drawn steel wire for producing 10.9-grade non-quenched and tempered U-bolts and a manufacturing method thereof. The steel wire contains the following elements and weight percentages: C: 0.50%, Si: 1.45%, Mn: 0.78%, P: ≤0.018%, S: ≤0.003%, Cr: 0.28%, Nb: 0.04%, Al: 0.03%, Cu: 0.02%, Ni: 0.02%, H: 0.00015%, and the remainder is Fe and unavoidable impurity elements. The wire diameter is 18 mm, and the process flow is as follows: the main raw materials are successively smelted in a converter and refined by LF+RH furnace number, wherein niobium iron alloy is added in the middle of LF refining. After RH refining, the molten steel is allowed to stand for 20 minutes and then sent to a continuous casting machine for casting. The molten steel is cast into a continuous casting billet under argon protection at a low superheat of 15 to 30°C, and the billet is surface treated after slow cooling. After treatment, the billet is heated to 1180℃ in a heating furnace, kept warm for 120min and taken out of the furnace. The starting rolling temperature is 1050℃, the intermediate rolling temperature is 920℃, the pre-finishing rolling temperature is 880℃, the finishing rolling temperature is 830℃, and the wire drawing temperature is 810℃. After wire drawing, the wire rod first enters the EDC for cooling. The EDC water temperature is 98℃, the roller speed is 0.45m / s, and the temperature of the wire rod out of the EDC is 510℃. After the EDC is coiled, it is slowly cooled in the insulation cover of the Stelmor cooling line, and the cooling process is a phase change of the organization. The yield strength of the obtained hot-rolled wire rod is 880MPa, the tensile strength is 1030MPa, the elongation is 13%, the cross-sectional shrinkage is 55%, and the room temperature impact energy is 30J. After drawing with a 10% reduction rate, the yield strength of the steel wire is 960MPa, the tensile strength is 1130MPa, the elongation is 11%, the cross-sectional shrinkage is 51%, and the room temperature impact energy is 28J.

[0041] Example 2

[0042] This embodiment relates to a cold-drawn steel wire for producing 10.9-grade non-adjustable U-bolts and a manufacturing method thereof. The steel wire contains the following elements and weight percentages: C: 0.57%, Si: 1.65%, Mn: 0.70%, P: ≤0.018%, S: ≤0.003%, Cr: 0.40%, Nb: 0.03%, Al: 0.03%, Cu: 0.02%, Ni: 0.02%, H: 0.00010%, and the remainder is Fe and unavoidable impurity elements. The wire diameter is 24 mm, and the process flow is as follows: the main raw materials are successively smelted in a converter, refined by LF+RH furnace number, and niobium iron alloy is added in the middle of LF refining. After RH refining, the molten steel is allowed to stand for 20 minutes and then sent to a continuous casting machine for casting. The molten steel is cast into a continuous casting billet under argon protection at a low superheat of 15 to 30°C, and the billet is surface treated after slow cooling. After treatment, the billet is heated to 1120℃ in a heating furnace, kept warm for 130min and then taken out of the furnace. The starting rolling temperature is 1010℃, the intermediate rolling temperature is 900℃, the pre-finishing rolling temperature is 870℃, the finishing rolling temperature is 840℃, and the wire drawing temperature is 820℃. After wire drawing, it first enters EDC cooling, the EDC water temperature is 99℃, the roller speed is 0.40m / s, the EDC temperature is 500℃, and it is slowly cooled in Stelmor after EDC. The yield strength of the obtained hot-rolled wire rod is 900MPa, the tensile strength is 1060MPa, the elongation is 12%, the cross-sectional shrinkage is 54%, and the room temperature impact energy is 29J. After drawing with a 10% reduction rate, the yield strength of the steel wire is 990MPa, the tensile strength is 1150MPa, the elongation is 10%, the cross-sectional shrinkage is 50%, and the room temperature impact energy is 27J.

[0043] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A method for manufacturing a cold drawn steel wire for a 10.9 grade non-quenched and tempered U-bolt, characterized in that: The chemical composition of the steel wire is designed to be: C: 0.45-0.65%, Si: 1.0-2.0%, Mn: 0.70-1.00%, P: ≤0.025%, S: ≤0.025%, Cr: 0.10-0.70%, Nb: 0.01-0.06%, H: ≤0.0002%, the remainder is Fe and unavoidable impurity elements, and the structure is ferrite and troostite; The method comprises: Molten steel smelting: the smelting raw materials are sequentially subjected to primary refining, refining and vacuum degassing. During primary refining, P is ≤0.015%. During primary refining, slag is added along the steel flow. The slag is used to remove inclusions, element alloy blocks and deoxidized alloys during refining. Nb alloy is added after the reduction slag is formed in the refining process. After the vacuum degassing is completed, the molten steel is left to stand for more than 20 minutes; Billet preparation: Molten steel that meets the element design is cast into billets; Billet heating: reheat the structure to make it fully austenitized; Wire rod rolling: The rolling process is divided into rough rolling, intermediate rolling, pre-finishing rolling and finishing rolling. The rough rolling temperature is controlled at 1000-1100℃, the intermediate rolling temperature and pre-finishing rolling temperature are controlled at 850-950℃, and the finishing rolling adopts temperature-controlled rolling, the rolling temperature is controlled at 800-850℃, and the rolling is carried out in the austenite non-recrystallization zone. The total deformation of temperature-controlled rolling is controlled at 30-50%, and the wire laying temperature is controlled at 800-850℃; Wire rod cooling: After spinning, the wire rod enters the EDC water bath for cooling. The water temperature of the EDC is maintained above 96°C. The temperature of the wire rod out of the EDC is controlled at 500-550°C. After exiting the water bath, the wire rod is coiled. The temperature difference between the two overlapping points and the middle point of the adjacent wire rod coils does not exceed 50°C. The middle point refers to the midpoint of the two overlapping points. The coiled wire rod is cooled in the Stelmor cooling line. The cooling line is fully covered with an insulation cover. The total cooling time of the wire rod in the insulation cover is ≥100 seconds. The temperature when leaving the insulation cover should be 200-400°C. During the cooling process, austenite is transformed into ferrite + troostite. After the wire rod exits the cover, the coil is collected and cooled naturally. Steel wire drawing: The wire rod is acid-washed and saponified before being drawn to obtain the steel wire.

2. The method according to claim 1, characterized in that: The chemical composition of the steel wire is Ni≤0.30%, Cu≤0.30%.

3. The method according to claim 1, characterized in that: The molten steel smelting includes converter or electric furnace smelting, LF refining, RH or VD degassing. The alloys added along the steel flow during the initial steelmaking are silicon-manganese alloy and low-calcium ferrosilicon to adjust the content of Si and Mn in the molten steel.

4. The method according to claim 1, characterized in that: In the billet preparation stage, molten steel is sent to the continuous casting machine to be cast into billets, the superheat of the molten steel is controlled at 15-40°C, and the continuous casting tundish is cast into billets under argon protection throughout the process. After fire cutting, the continuous casting billets are cooled by pile cooling or slow cooling pit cooling.

5. The method according to claim 4, characterized in that: After cooling, the continuous casting billet is treated to remove surface defects.

6. The method according to claim 1, characterized in that: In the billet heating stage, a walking beam heating furnace is used for heating, the temperature of the preheating section of the heating furnace is controlled at ≤650°C, the temperature of the heating equalization section of the heating furnace is controlled at 1100-1200°C, and the total heating time is controlled at 90-160min; after the billet is heated, it is descaled by high-pressure water.

7. The method according to claim 1, characterized in that: In the wire rod rolling stage, a cooling water tank is arranged after pre-finishing rolling and before finishing rolling to reduce the temperature of the billet to reach the control temperature of rolling.

8. The method according to claim 1, characterized in that: During the wire rod cooling stage, the roller speed of the Stelmor cooling line is 0.2-0.5 m / s.

9. The method according to claim 1, characterized in that: During the steel wire drawing stage, the wire drawing area reduction rate is controlled at 6%-15%.

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