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

Abstract

The application relates to a cold-drawing steel wire for 8.8-grade cold-upsetting non-tempered bolts and a manufacturing method thereof, and belongs to the field of steel wire production for fasteners. The chemical components of the steel wire are as follows in percentage by mass: 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 of Fe and inevitable impurity elements. The diameter of the produced wire rod ranges from phi 5.5 to 25 mm. The production process of the steel wire is as follows: converter or electric furnace smelting, LF refining, vacuum degassing, continuous casting, heating, wire control temperature rolling, EDC water bath + Stelmor cover cooling, and wire drawing. The cold-drawing steel wire produced by the application has uniform and fine ferrite + bainite structure, good plasticity, and can be directly cold-upsetting formed into bolts after drawing, thereby avoiding spheroidizing annealing treatment of the wire rod and quenching and tempering heat treatment of bolt forming.

Description

TECHNICAL FIELD

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

[0002] A conventional bolt manufacturing process is generally as follows: alloy steel wire rods are subjected to annealing heat treatment, and then are drawn to corresponding specifications. After being drawn, the wire rods are subjected to cold heading on a cold heading device to form corresponding shapes, and then are subjected to quenching and tempering heat treatment, so that corresponding mechanical properties are obtained. In the context of double-carbon emission reduction and green manufacturing, it is necessary to reduce carbon emissions in the manufacturing process of 8.8-grade U-shaped bolts and realize green production by considering canceling quenching and tempering heat treatment.

[0003] The non-tempered 8.8-grade bolt specifically refers to canceling quenching and tempering heat treatment after bolt forming, and canceling annealing treatment of the steel wire before forming the 8.8-grade bolt. The manufacturing process of the non-tempered 8.8-grade bolt is roughly as follows: hot-rolled wire rod-cold-drawn steel wire-cold heading-processed bolt thread-stabilization treatment. The non-tempered 8.8-grade bolt is completely produced by controlling the mechanical properties of the hot-rolled wire rod and the wire drawing reduction rate of the wire rod. The steel wire produced by the wire rod meets or approaches the mechanical properties of the 8.8-grade bolt after quenching and tempering. SUMMARY

[0004] The application aims to provide a cold-drawn steel wire for cold heading forming 8.8-grade non-tempered bolts and a manufacturing method. By reasonably configuring alloy components, cooperating with rolling processes and EDC water bath cooling processes, the wire rod obtains uniform and fine ferrite bainite structure, and the wire rod has good through-bar properties. The 8.8-grade bolt is directly manufactured by cold heading after drawing, and the 8.8-grade high-strength U-shaped bolt can be satisfied by simple stabilization heat treatment. The steel wire production for 8.8-grade bolts and the low-energy consumption of bolt manufacturing are achieved in the whole process.

[0005] The application adopts the technical scheme that a cold-drawn steel wire for cold heading forming 8.8-grade ferrite + bainite non-tempered bolts and a manufacturing method are provided. The chemical components of the steel are designed as follows in percentage by mass: 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 inevitable impurity elements.

[0006] Further, the chemical composition of the 8.8-grade non-tempered cold-drawn steel wire for cold heading bolts is based on medium carbon steel, and the ferrite + bainite structure is obtained by using microstructure strengthening and cooling, so as to improve the yield ratio of the steel and the anti-relaxation ability of the bolt. The fine austenite grain is converted into uniform ferrite + bainite structure by the EDC water bath cooling technology + Stelmor cooling process, so as to improve the strength and plasticity. The content of impurity elements such as P and S is strictly controlled, the purity of the grain boundary of the steel is improved, and the grain boundary strength is improved. The hydrogen content of the steel in the application is limited to less than 2 ppm, so as to reduce the hydrogen embrittlement sensitivity of the bolt. The chemical composition of the 8.8-grade non-tempered cold-drawn steel wire for cold heading bolts is designed as follows:

[0007] 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-shaped bolt will be too low. If the carbon content is too high, the plasticity will be reduced while the strength and hardness are increased, which will affect the cold bending performance of the U-shaped bolt. Therefore, the carbon content is controlled to be 0.18-0.26% in the application.

[0008] Mn: Manganese is a solid solution strengthening element that improves the strength of steel. Too low manganese will reduce the plasticity and toughness of the steel, 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 8.8-grade non-tempered cold-drawn steel wire for cold heading bolts, the manganese content in the steel is controlled to be 0.60-1.00%.

[0009] Cr: Chromium is an element that enhances the hardenability of steel. Appropriate addition of chromium can effectively increase the hardenability of medium carbon steel and improve the strength of the steel. However, if the chromium content is too high, the hot rolling strength will be too high. Therefore, the chromium content in the steel is controlled to be 0-0.30% in the application.

[0010] In addition, Ni, Cu and Al are optional additive elements, and their content is controlled to be 0-0.30%.

[0011] P and S: In principle, the lower the content of phosphorus and sulfur, the better. However, too low content will undoubtedly increase the smelting cost. Under the condition of quality permission, the content of phosphorus and sulfur is controlled to be less than 0.025% in the application.

[0012] H: The hydrogen content in the steel is strictly controlled to be less than 2 ppm in the application, so as to reduce the risk of hydrogen-induced delayed fracture.

[0013] Another object of the application is to provide a manufacturing method of the above-mentioned 8.8-grade non-tempered cold-drawn steel wire for cold heading bolts.

[0014] Steel smelting: the process steps are that the raw materials are sequentially smelted by converter or electric furnace, LF refining, RH or VD degassing to produce molten steel, the converter or electric furnace process controls the tapping P≤0.015%, and the slag and alloy are added along the steel flow at the time of tapping. After the refining is completed, the molten steel is placed for 20 minutes before being sent to the continuous casting machine for casting. The overheat degree of the molten steel with qualified chemical composition is controlled to be 15-40℃, the continuous casting tundish is cast into a billet under the whole-process argon protection, and the continuous casting billet is cooled by stacking or slow cooling in a pit after being cut by fire;

[0015] Billet preparation: the cooled billet is subjected to a surface defect removal treatment;

[0016] Billet rolling: the billet rolling includes a heating process, a rolling process and a cooling process. The heating is performed by using a walking beam furnace, the temperature of the preheating section of the furnace is controlled to be ≤650℃, the temperature of the heating uniform section of the furnace is controlled to be 1100-1200℃, and the total heating time is controlled to be 90-160min. After the billet is heated, the billet is descaled by high-pressure water, the descaling pressure is ≥180bar, and the billet is rolled after being descaled. The rolling includes rough rolling, intermediate rolling, pre-precision rolling and precision rolling. The rough rolling temperature is controlled to be 1000-1100℃. The intermediate rolling temperature and the pre-precision rolling temperature are controlled to be 850-950℃. Because the wire rod rolling speed is relatively fast, a corresponding cooling water tank is arranged after the pre-precision rolling and before the precision rolling to reduce the rolling temperature. The precision rolling adopts temperature control rolling, and the rolling temperature is controlled to be 800-850℃, so as to roll in the austenite unrecrystallization zone. The total deformation amount of the temperature control rolling is controlled to be 30-50%, and the wire drawing temperature is controlled to be 800-850℃. After rolling, the wire rod enters the EDC for cooling, the water temperature in the EDC is maintained to be higher than 96℃, and the wire rod is uniformly cooled in the EDC. The temperature of the wire rod out of the EDC is controlled to be 500-550℃, and the temperature difference between the lap joint point and the middle point of the wire rod is not more than 50℃. After the wire rod is out of the EDC, the wire rod is cooled on the Stelmor cooling line, the cooling line is fully covered by the heat preservation cover, the roller speed is 0.2-0.5m / s, the total cooling time is required to be greater than 100 seconds, and the out-of-cover temperature should be 200-400℃. In the cooling process, the austenite is transformed into ferrite+ bainite. After the wire rod is out of the cover, the wire coil is collected for natural cooling. Thus, the hot-rolled wire rod for the 8.8-grade non-adjusted U-shaped bolt can be obtained.

[0017] Steel wire processing: the above hot-rolled wire rod is subjected to acid pickling and phosphosulphurization, and then is drawn, and the drawing reduction is controlled to be 20%-30%.

[0018] Compared with the prior art, the application has the advantages that: through the EDC water bath cooling technology + Stelmor cooling on-line cover cooling process, the fine austenite grain is transformed into ferrite + bainite structure, the strength is improved, and the plasticity is good. The non-tempered cold-drawn steel wire of the application replaces the quenched and tempered steel wire to manufacture the 8.8-grade cold heading bolt, greatly reduces the production cost, and realizes the green manufacturing of the 8.8-grade cold heading bolt. The mechanical properties of the 8.8-grade non-tempered cold heading bolt cold-drawn steel wire are as follows: the specification is φ5.5~25mm, the yield strength is 650~700 MPa, the yield strength is 850~900 MPa, the elongation is ≥10%, and the reduction of area is ≥50%. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The typical microstructure of the non-tempered bolt cold-drawn steel wire of the application is shown in the figure, and the observation magnification is ×100. DETAILED DESCRIPTION

[0020] The application will be further described in detail in combination with the embodiments below, and the embodiments are exemplary and are intended to explain the application, and cannot be understood as the limitation of the application. EMBODIMENT

[0021] The embodiment relates to production of cold-drawing steel wire for 8.8-grade non-quenched cold-upsetting bolt, the steel wire contains the following elements and percentage by weight: 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 inevitable impurities. The wire diameter is 18 mm, and the technological process is as follows: the main smelting raw material is sequentially subjected to converter smelting, LF+RH furnace refining, and alloy addition in the middle period of LF refining to complete element regulation. After the RH refining, the molten steel is rested for 20 minutes, and then is sent to a continuous casting machine for casting. The molten steel is cast into a continuous casting blank under the protection of argon gas with a low superheat degree of 15-30 DEG C, the blank is slowly cooled, and then is subjected to surface treatment to remove surface defects. After the treatment, the blank is heated to 1180 DEG C in a heating furnace, is kept for 120 minutes, and is discharged, the opening rolling temperature is 1050 DEG C, the medium rolling temperature is 920 DEG C, the pre-precision rolling temperature is 880 DEG C, the precision rolling temperature is 830 DEG C, and the wire drawing temperature is 810 DEG C. After the wire drawing, the wire is first subjected to EDC water bath cooling, the EDC water temperature is 98 DEG C, the roller speed is 0.45 m / s, the EDC outlet temperature is 510 DEG C, the wire is slowly cooled on a Stelmor cooling line, the whole cover is kept, the roller speed is 0.25 m / s, the cover outlet temperature is 240 DEG C, and after the cover outlet, the wire is naturally cooled in air, the obtained hot-rolled wire rod has a yield strength of 413 MPa, a tensile strength of 670 MPa, an elongation of 13%, and a reduction of area of 70%. After 25% area reduction wire drawing, the steel wire has a yield strength of 672 MPa, a tensile strength of 890 MPa, an elongation of 12.5%, and a reduction of area of 68%. Embodiment

[0022] The cold drawn steel wire of the embodiment contains and has the weight percentage of 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 balance of Fe and inevitable impurity elements. The wire diameter is 24 mm, and the process flow is that the main raw materials are sequentially subjected to converter smelting, LF+RH furnace refining. After the RH refining is completed, the molten steel is placed for 20 minutes, and then is sent to a continuous casting machine for casting. The molten steel is cast into a continuous casting blank by using 15-30 ℃ low superheat argon protection in the whole process, and the blank is slowly cooled after surface treatment to remove surface defects. After the treatment, the blank is heated to 1120 ℃ in a heating furnace, is kept for 130 min, is discharged, and is subjected to the following processes: the opening rolling temperature is 1010 ℃, the intermediate rolling temperature is 900 ℃, the pre-precise rolling temperature is 870 ℃, the precise rolling temperature is 840 ℃, and the wire drawing temperature is 820 ℃. After the wire drawing, the hot rolled wire rod is first subjected to EDC water bath cooling, the EDC water temperature is 99 ℃, the roller speed is 0.40 m / s, the wire rod temperature after the EDC is 500 ℃, and after the EDC, the wire rod is slowly cooled in a Stelmor cooling line, the roller speed is 0.4 m / s, and the cover temperature is 320 ℃. The obtained hot rolled wire rod has the yield strength of 415 Pa, the tensile strength of 666 MPa, the elongation of 15%, and the cross-sectional reduction of 70%. After the wire drawing at the reduction rate of 30%, the steel wire has the yield strength of 689 MPa, the tensile strength of 883 MPa, the elongation of 13%, and the cross-sectional reduction of 67%.

[0023] In addition to the above-mentioned embodiments, the present application also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement shall fall within the protection scope of the claims of the present application.

Claims

1. A method for manufacturing cold-drawn steel wire for grade 8.8 cold-heading non-quenched and tempered bolts, characterized in that: include, Step 1: Steel smelting: The smelting composition by mass percentage 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%, with the balance being Fe and unavoidable impurity elements; the qualified molten steel is then cast into steel billets. Step 2, Billet Preparation: Remove surface defects from the steel billet from Step 1; Step 3, Wire Rod Forming: This involves heating, rolling, and cooling processes. Heating the billet completely austenitizes the microstructure. After heating, the billet is descaled to remove the surface oxide layer. Rolling then begins, consisting of roughing, intermediate rolling, pre-finishing, and finishing. The roughing temperature is controlled at 1000-1100℃, while the intermediate and pre-finishing temperatures are controlled at 850-950℃. Cooling water tanks are placed after pre-finishing and before finishing to lower the temperature of the rolled material. Finishing uses temperature-controlled rolling, with the temperature controlled at 800-850℃ to ensure finishing within the non-recrystallized austenite region. The total deformation during temperature-controlled rolling is controlled at 30-50%. The wire drawing temperature... 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 achieves uniform cooling in the EDC. The temperature of the wire rod exiting the EDC is controlled at 500-550℃, and the temperature difference between the overlap point and the midpoint of the wire rod does not exceed 50℃. After exiting the EDC, the wire rod is cooled on the Stellmore cooling line, which is fully covered by 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 exiting the cover is controlled at 200-400℃. During the cooling process, austenite transforms into ferrite + bainite. After exiting the cover, the wire rod is collected and cooled naturally. Step 4, Wire Processing: After pickling and phosphating, the non-quenched and tempered wire rod from Step 3 is drawn at room temperature. The wire drawing reduction rate is controlled at 20%-30% to obtain non-quenched and tempered cold-drawn steel wire.

2. The method according to claim 1, characterized in that: Step 1: The steelmaking process consists of the following steps: converter or electric furnace smelting, LF refining, and RH or VD vacuum degassing. During converter or electric furnace smelting, the steel output P is controlled to be ≤0.015%. When tapping, slag and some alloy blocks are added along the steel stream. During the LF refining process, alloy blocks are added to adjust the composition. After vacuum degassing and refining, the molten steel is allowed to stand for more than 20 minutes before casting. Continuous casting is used, and the superheat of the molten steel is controlled between 15-40℃. Under the protection of argon gas throughout the process, the qualified molten steel is cast into billets. The continuously cast billets are then cooled by fire cutting or placed in a slow cooling pit.

3. The method according to claim 1, characterized in that: Step 3: Heating is carried out using a walking beam furnace. The temperature of the preheating section of the furnace is controlled at ≤650℃, the temperature of the heating uniform section is controlled at 1100~1200℃, and the total heating time is controlled at 90-160min. The descaling pressure is ≥180bar. After descaling, the furnace is rolled in a rolling mill.

4. The method according to claim 1, characterized in that: The non-quenched and tempered cold-drawn steel wire has a diameter of 5.5~25mm, yield strength of 650-700MPa, tensile strength of 850-900MPa, elongation ≥10%, and reduction of area ≥50%.

5. The method according to claim 1, characterized in that: Step one, the ingredients also contain 0~0.3% Al.

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