Cold-drawn steel wire for 8.8-grade cold heading non-quenched and tempered bolt and manufacturing method of cold-drawn steel wire

Abstract

The invention relates to a cold-drawn steel wire for a 8.8-grade cold heading non-quenched and tempered bolt and a manufacturing method of the cold-drawn steel wire, and belongs to the field of production of steel wires for fasteners. The steel wire comprises the following chemical components in percentage by mass: 0.18-0.26% of C, less than or equal to 0.35% of Si, 0.60-1.00% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, less than or equal to 0.30% of Cr, less than or equal to 0.30% of Ni, less than or equal to 0.30% of Cu and the balance of Fe and inevitable impurity elements. And the diameter range of the produced wire rod is phi 5.5-25mm. The production process of the steel wire comprises the steps of converter or electric furnace smelting, LF refining, vacuum degassing, continuous casting, heating, wire rod temperature control rolling, EDC water bath + stelmor capping cooling and wire drawing. The cold-drawn steel wire produced by the method has a uniform and fine ferrite and bainite structure, is good in plasticity, and can be directly formed into a bolt in a cold heading manner after being drawn, and spheroidizing annealing treatment of a wire rod and quenching and tempering heat treatment of bolt forming are omitted.

Description

Technical Field

[0001] The invention relates to the technical field of steel wires for fasteners, and in particular to a cold-drawn steel wire for 8.8-grade non-quenched and tempered bolts and a manufacturing method thereof. Background Art

[0002] The conventional bolt manufacturing process is generally as follows: alloy steel wire rods are annealed and then drawn to the corresponding specifications. The wire rods are cold-forged on cold heading equipment to form the corresponding shapes and then subjected to quenching and tempering heat treatment. After quenching and tempering, the corresponding mechanical property grades are obtained. In the context of dual carbon emission reduction and green manufacturing, it is now necessary to reduce carbon emissions in the manufacturing process of 8.8 grade U-bolts, achieve green production, and consider canceling the quenching and tempering heat treatment.

[0003] Non-quenched and tempered 8.8-grade bolts specifically refer to the cancellation of the quenched and tempered heat treatment after bolt forming, and the cancellation of the wire annealing treatment before the 8.8-grade bolt forming. The manufacturing process of non-quenched and tempered 8.8-grade bolts is roughly as follows: hot-rolled wire rod-cold-drawn steel wire-cold heading-bolt thread processing-stabilization treatment. Non-quenched and tempered 8.8-grade 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 after processing meets or approaches the mechanical properties of quenched and tempered 8.8-grade bolts. Summary of the invention

[0004] The purpose of the present invention is to provide a cold-drawn steel wire for cold-forging 8.8-grade non-quenched and tempered bolts and a manufacturing method thereof. By rationally configuring the alloy components, coordinating the rolling process and the EDC water bath cooling process, the wire rod obtains a uniform and fine ferrite bainite structure, and the wire rod has good performance. After drawing, the 8.8-grade bolts are directly cold-forged, and the 8.8-grade high-strength U-shaped bolts can be met through a simple stabilization heat treatment. The whole process and all-round low energy consumption of the steel wire production and bolt manufacturing for 8.8-grade bolts is achieved.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problem is: a cold-drawn steel wire and a manufacturing method for cold heading to make 8.8 grade ferrite + bainite non-adjustable bolts, the chemical composition of the steel is designed as follows by mass percentage: C: 0.18~0.26%, Si: ≤0.35%, Mn: 0.60~1.00%, P: ≤0.025%, S: ≤0.025%, Cr: ≤0.30%, Ni≤0.30%, Cu: ≤0.30%, and the balance is Fe and unavoidable impurity elements.

[0006] Furthermore, the chemical composition of the cold-drawn steel wire for the 8.8-grade non-adjustable cold-heading bolts of this patent is based on medium carbon steel, and through the use of organizational strengthening, the ferrite + bainite structure is achieved through cooling, thereby improving the steel's yield strength ratio and the bolt's ability to resist relaxation. Through the EDC water bath cooling technology + Stelmor cooling process, the fine austenite grains are transformed into a uniform ferrite + bainite structure, which improves the strength while providing good plasticity. The content of impurity elements such as P and S is strictly controlled to improve the purity of the steel's grain boundaries and increase the grain boundary strength. The hydrogen content of the steel in the present invention is limited to less than 2ppm to reduce the hydrogen embrittlement sensitivity of the bolts. The chemical composition of the steel for the 8.8-grade non-adjustable cold-heading bolts of the present invention is designed as follows: 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 0.18~0.26%.

[0007] 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 likely to cause segregation of the continuous casting billet. In order to match the strength and plasticity of the non-tempered cold-forged 8.8-grade bolts, the present invention controls the manganese content in the steel to 0.60-1.00%.

[0008] Cr: Chromium is an element that enhances the hardenability of steel. The addition of an appropriate content can effectively increase the hardenability of medium carbon steel and improve the strength of the steel. However, if the chromium content is added too much, the hot rolling strength will be too high. Therefore, the present invention controls the chromium content in the steel to 0-0.30%.

[0009] In addition, Ni, Cu and Al are non-essential added elements, and their contents are controlled within the range of 0 to 0.30%.

[0010] 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 quality permits, the present invention controls the phosphorus and sulfur content to below 0.025%.

[0011] 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.

[0012] Another object of the present invention is to provide a method for manufacturing the above-mentioned 8.8 grade non-adjusted cold-headed bolt cold-drawn steel wire: Molten steel smelting: The process steps are to smelt raw materials in turn through converter or electric furnace smelting, LF refining, RH or VD degassing to produce molten steel. The converter or electric furnace process controls the steel tapping P≤0.015%. When tapping, slag and alloy are added along the steel flow. After refining, the molten steel is allowed to stand for 20 minutes and then sent to the continuous casting machine for casting. The superheat of the molten steel with qualified chemical composition is controlled at 15-40℃, and the continuous casting tundish is cast into steel billets under argon protection throughout the process. After fire cutting, the continuous casting billets are cooled in piles or in slow cooling pits; Blank preparation: Remove the surface defects of the cooled blank; Billet rolling: Billet rolling is divided into heating process, rolling process and cooling process. Heating is carried out by a walking beam heating furnace. The temperature of the preheating section of the heating furnace is controlled at ≤650℃, the temperature of the heating equalization section of the heating furnace is controlled at 1100~1200℃, and the total heating time is controlled at 90~160min. After heating, the billet is descaled by high-pressure water, and the descaling pressure is ≥180bar. After descaling, it enters the rolling mill for rolling. Rolling 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℃. Due to the high rolling speed of wire rod, the corresponding cooling water tank is arranged after pre-finishing rolling and before finishing rolling to reduce the rolling temperature. The finishing rolling adopts temperature-controlled rolling, and the rolling temperature is controlled at 800-850℃. The purpose is to roll in the non-recrystallization zone of austenite. The total deformation of temperature-controlled rolling is controlled at 30-50%, and the wire drawing temperature is controlled at 800-850℃; after rolling, it enters EDC for cooling, and the water temperature in EDC is kept above 96℃, and the wire rod is evenly cooled in EDC. The temperature of the wire rod out of EDC is controlled at 500-550℃, and the temperature difference between the overlap point and the middle point of the wire rod does not exceed 50℃. After leaving EDC, the wire rod is cooled in the Stelmor cooling line, and the cooling line insulation cover is fully covered. The roller speed is 0.2-0.5m / s, the total cooling time is required to be greater than 100 seconds, and the temperature out of the cover should be 200-400℃. During the cooling process, austenite is transformed into ferrite + bainite. After the wire rod is out of the hood, the coil is collected and cooled naturally. The above process can obtain the hot rolled wire rod for 8.8 grade non-adjustable U-bolts.

[0013] Steel wire processing: The hot-rolled wire rods are pickled and saponified before being drawn, and the wire drawing reduction rate is controlled at 20%-30%.

[0014] Compared with the prior art, the advantages of the present invention are: through the EDC water bath cooling technology + the cooling process inside the cover of the Stelmor cooling line, the fine austenite grains are transformed into ferrite + bainite structure, which improves the strength while having good plasticity. The non-quenched and tempered cold-drawn steel wire of the present invention replaces the quenched and tempered steel wire to manufacture 8.8 grade cold heading bolts, which greatly reduces the production cost and realizes the green manufacturing of 8.8 grade cold heading bolts. The mechanical properties of the cold-drawn steel wire for 8.8 grade non-quenched and tempered cold heading bolts are: specifications φ5.5~25mm, yield strength 650~700 MPa, yield strength 850~900 MPa, elongation ≥10%, and cross-sectional shrinkage ≥50%. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a typical microstructure of the cold-drawn steel wire for non-quenched and tempered bolts of the present invention, with an observation magnification of ×100. DETAILED DESCRIPTION

[0016] 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. Example

[0017] This embodiment relates to the production of 8.8 grade non-quenched and tempered cold-headed bolts. The elements and weight percentages contained in the steel wire are: C: 0.22%, Si: 0.21%, Mn: 0.85%, P: ≤0.024%, S: ≤0.001%, Cr: 0.25%, Al: 0.023%, Cu: 0.02%, Ni: 0.03%, H: 0.00015%, and the balance is Fe and unavoidable impurity elements. The wire diameter is 18mm, and its process flow is as follows: the main smelting raw materials are successively smelted in a converter, LF+RH furnace number refining, and alloys are added in the middle of LF refining to complete the element regulation. After the RH refining is completed, the molten steel is allowed to stand for 20 minutes and sent to the continuous casting machine for casting. The molten steel is cast into a continuous casting billet under argon protection at a low superheat of 15~30℃, and the billet is slowly cooled and surface treated to remove surface defects. After treatment, the billet is heated to 1180℃ in a heating furnace, kept warm for 120min and then 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, it first enters the EDC water bath for cooling. The EDC water temperature is 98℃, the roller speed is 0.45m / s, and the EDC material temperature is 510℃. The wire rod is slowly cooled on the Stelmor cooling line, the insulation cover is fully covered, the roller speed is 0.25m / s, and the temperature out of the cover is 240℃. After leaving the cover, it is naturally cooled in the air. The yield strength of the hot-rolled wire rod obtained is 413MPa, the tensile strength is 670MPa, the elongation is 13%, and the cross-sectional shrinkage is 70%. After drawing with a 25% reduction rate, the yield strength of the steel wire is 672MPa, the tensile strength is 890MPa, the elongation is 12.5%, and the cross-sectional shrinkage is 68%. Example

[0018] The cold drawn steel wire of this embodiment contains the following weight percentages: C: 0.23%, Si: 0.15%, Mn: 0.90%, P: ≤0.015%, S: ≤0.003%, Cr: 0.20%, Al: 0.032%, Cu: 0.02%, Ni: 0.03%, H: 0.00015%, and the remainder is Fe and unavoidable impurity elements. The wire diameter is 24mm, and the process flow is as follows: the main raw materials are successively smelted in a converter and refined by LF+RH furnace number. After the RH refining is completed, the molten steel is allowed to stand for 20 minutes and then sent to the continuous casting machine for casting. The molten steel is cast into a continuous casting billet under argon protection at a low superheat of 15~30℃, and the billet is slowly cooled and surface treated to remove surface defects. 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 the EDC water bath for cooling. The EDC water temperature is 99℃, the roller speed is 0.40m / s, and the EDC wire rod temperature is 500℃. After EDC, it is slowly cooled in the Stelmor cooling line. The roller speed is 0.4m / s, and the hood temperature should be 320℃. The yield strength of the hot-rolled wire rod obtained is 415Pa, the tensile strength is 666MPa, the elongation is 15%, and the cross-sectional shrinkage is 70%. After drawing with a 30% reduction rate, the steel wire has a yield strength of 689MPa, a tensile strength of 883MPa, an elongation of 13%, and a cross-sectional shrinkage of 67%.

[0019] 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 8.8 grade cold heading non-quenched and tempered bolts, characterized in that: include, Step 1, molten steel smelting: the smelting composition is C: 0.18-0.26%, Si: ≤0.35%, Mn: 0.60-1.00%, P: ≤0.025%, S: ≤0.025%, Cr: ≤0.30%, Ni≤0.30%, Cu: ≤0.30%, and the balance is Fe and unavoidable impurity elements; the molten steel with qualified composition is cast into a steel billet; Step 2: Billet preparation: Surface treatment of the billet in step 1 to remove surface defects; Step 3, wire rod forming: divided into heating process, rolling process and cooling process, the steel billet is heated to make the structure completely austenitized, and the steel billet is descaled to remove the surface oxide layer after heating. After descaling, rolling begins. The rolling 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 cooling water tanks are arranged after pre-finishing rolling and before finishing rolling to reduce the temperature of the rolled material. The finishing rolling adopts temperature-controlled rolling, and the finishing rolling temperature is controlled at 800-850℃, so that the finishing rolling is completed in the austenite non-recrystallization zone. The total deformation of the temperature-controlled rolling is controlled at 30-50%, and the wire-spinning temperature is controlled at 1000-1100℃. The temperature is controlled at 800-850℃; after rolling, the wire rod enters the EDC water bath for cooling, the water temperature in the EDC water bath is maintained above 96℃, 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℃, the temperature difference between the overlap point and the middle point of the wire rod is no more than 50℃, and the wire rod is cooled in the Stelmor cooling line after leaving the EDC, the cooling line is fully covered with an insulation cover, the roller speed is controlled at 0.2-0.5m / s, the total cooling time is greater than 100 seconds, and the temperature out of the cover is controlled at 200-400℃; during the cooling process, austenite is transformed into ferrite + bainite, and the coil is collected and cooled naturally after the wire rod leaves the cover; Step 4, steel wire processing: the non-quenched and tempered wire rod in step 3 is subjected to room temperature drawing after pickling and phosphorus saponification, and the wire drawing reduction rate is controlled at 20%-30% to obtain a non-quenched and tempered cold-drawn steel wire.

2. The method according to claim 1, characterized in that: Step 1, the molten steel smelting steps are converter or electric furnace smelting, LF refining, RH or VD vacuum degassing, the converter or electric furnace smelting controls the steel tapping P≤0.015%, when tapping, slag and part of the alloy block are added along the steel flow; the alloy block is continued to be added during the LF refining process to adjust the composition, after the vacuum degassing refining is completed, the molten steel is allowed to stand for more than 20 minutes before casting, a continuous casting process is adopted, the superheat of the molten steel is controlled at 15-40°C, and the molten steel with qualified composition is cast into a billet under the protection of argon throughout the process, and the continuous casting billet is pile-cooled or cooled in a slow cooling pit after fire cutting.

3. The method according to claim 1, characterized in that: Step three, use a walking-beam heating furnace for heating, the temperature of the preheating section of the heating furnace is controlled at ≤650°C, the temperature of the heating furnace heating equalization section is controlled at 1100~1200°C, and the total heating time is controlled at 90-160min; the descaling pressure is ≥180bar, and after descaling, the steel is introduced into the rolling mill for rolling.

4. The method according to claim 1, characterized in that: The specification φ of the non-quenched and tempered cold-drawn steel wire is 5.5~25mm, the yield strength is: 650-700MPa, the tensile strength is: 850-900MPa, the elongation is ≥10%, and the cross-sectional shrinkage is ≥50%.

5. The method according to claim 1, characterized in that: In step 1, the composition also includes 0-0.3% Al.

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