Boron microalloyed Q235 steel wire rod for 8.8-grade fastener and manufacturing method of boron microalloyed Q235 steel wire rod

Abstract

The invention discloses a boron microalloyed Q235 steel wire rod for a 8.8-grade fastener and a manufacturing method of the boron microalloyed Q235 steel wire rod for the 8.8-grade fastener, and the boron microalloyed Q235 steel wire rod for the 8.8-grade fastener comprises the following components in percentage by mass: 0.16%-0.22% of C, 0.31%-0.59% of Mn, 0.10%-0.35% of Si, less than or equal to 0.032% of P, less than or equal to 0.031% of S, 0.0006%-0.0030% of B and the balance of Fe. The wire is prepared through molten iron pretreatment, converter smelting, LF refining, continuous casting, rolling and controlled cooling. According to the invention, the extremely low content of B is added on the basis of Q235, so that the hardenability is improved by more than 30%, the total cross-section martensization of low-carbon steel after hardening and tempering is realized, the 8.8-grade performance is realized at extremely low cost, the cost barrier of the traditional high alloy steel is broken through, and the low-carbon steel is particularly suitable for the fields of buildings and general machinery sensitive to price, and promotes the popularization of high-strength fasteners.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of fastener production, in particular to a 8.8-grade fastener wire made of boron micro-alloyed Q235 steel and a manufacturing method thereof. BACKGROUND

[0002] The 8.8-grade fastener is widely used in the fields of automobiles and machines, and is traditionally made of medium-carbon steel (such as ML35 and SWRCH35K) or medium-carbon alloy steel (such as 35CrMo) and needs to be quenched and tempered to meet the strength requirement. For example, the production process disclosed in patent CN200710021168.0 has the problems of complex process, high energy consumption and high cost. The non-quenched and tempered steel can omit the heat treatment, but depends on expensive micro-alloying elements such as V, Nb and Ti and has limited adaptability to the cold upsetting process.

[0003] The Q235 carbon steel is low in cost, but the conventional composition cannot meet the performance requirement (tensile strength >= 800 MPa, yield strength >= 640 MPa) of the 8.8-grade fastener. How to make the Q235 carbon steel treated to achieve the balance between high strength and toughness is a difficulty in the research and development of enterprises. SUMMARY

[0004] The application aims at overcoming the shortcoming that the Q235 carbon steel in the prior art cannot meet the performance of the 8.8-grade fastener and provides a 8.8-grade fastener wire made of boron micro-alloyed Q235 steel and a manufacturing method thereof.

[0005] The application is realized by the following technical scheme: a 8.8-grade fastener wire made of boron micro-alloyed Q235 steel, which is composed of the following components in percentage by mass: C 0.16%-0.22%, Mn 0.31%-0.59%, Si 0.10%-0.35%, P <= 0.032%, S <= 0.031%, B 0.0006%-0.0030% and the balance of Fe.

[0006] Preferably, in percentage by mass, C is 0.18%, Mn is 0.33%, Si is 0.13%, P is 0.032%, S is 0.031%, B is 0.0009% and the balance is Fe.

[0007] A manufacturing method of the 8.8-grade fastener wire made of boron micro-alloyed Q235 steel, which comprises the following steps, S1. molten iron pretreatment; S2. converter smelting; S3. LF refining, S / P control; S4. continuous casting, overheat degree control 15-25 DEG C; S5. rolling, finish rolling temperature 820-850 DEG C; S6. Controlled cooling, spinning temperature 800-830 DEG C, Stelmor air cooling.

[0008] A manufacturing method of a 8.8-grade fastener of boron micro-alloyed Q235 steel, comprising the following steps, S7. Drawing, reduction 6-23%; S8. Cold upsetting forming; S9. Wire rolling; 10. Quenching and tempering treatment, 860 DEG C oil quenching and 480 DEG C tempering; S11. Surface treatment.

[0009] A 8.8-grade fastener of boron micro-alloyed Q235 steel, the hardness difference from the surface to the core is less than or equal to 0.3 HRC, the tensile strength is greater than or equal to 800 MPa, and the yield strength is greater than or equal to 640 MPa.

[0010] Component C, basic strength guarantee, and cold upsetting plasticity.

[0011] Component Mn, solid solution strengthening, and improving hardenability.

[0012] Component Si, deoxidation, and inhibiting grain boundary segregation.

[0013] Component P, impurity control (too high reduces toughness).

[0014] Component S, improves machinability, and too high causes hot brittleness.

[0015] The present application has the following advantages: 1. By adding very low content of B (0.0006-0.0030%) on the basis of Q235, the hardenability is increased by more than 30%, and full-section martensitization after quenching and tempering of low-carbon steel is realized; 2. Abandoning Cr, Mo, V and other alloys, reducing quenching time and alloy use, the raw material cost is reduced by 20% compared with ML35, by 30% compared with non-quenching steel (containing V / Ti), ton steel carbon emission is reduced by 15%, ton steel cost is reduced by about 25%, and the manufactured 8.8-grade high-strength fastener (such as bolt, screw) also meets the requirements of GB / T 3098.1-2010 and GB 5783-2010 standards; 3. The traditional medium-carbon steel needs to be annealed multiple times, and the wire rod of the present application can be directly drawn and cold upset, simplifying the process. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all the embodiments.

[0017] Example 1 M12 bolt (composition: 0.16% C, 0.31% Mn, 0.10% Si, 0.032% P, 0.031% S, 0.0006% B) was prepared by the following steps, S1. Hot metal pretreatment; S2. Converter smelting; S3. LF refining, control S / P; S4. Continuous casting, continuous casting billet heated to 1150℃, control overheat 15℃-25℃; S5. Rolling, finish rolling temperature 820℃-850℃, rolled into Φ12mm wire rod, S6. Controlled cooling, wire laying temperature 800℃-830℃, Stelmor air cooling, control cooling rate 15℃ / s; S7. Drawing, drawn to Φ11.5mm, control reduction 8.16%; S8. Cold upsetting forming; S9. Wire twisting; 10. Quenching and tempering treatment, 860℃ oil quenching and 480℃ tempering; S11. Surface treatment.

[0018] The performance results are: after quenching and tempering, uniform tempering sorbite, tensile strength 840 MPa, yield strength 731 MPa, core hardness HRC 25.8, elongation 18%, surface to core hardness difference ≤2 HRC, indicating excellent hardenability.

[0019] Example 2 M10 bolt (composition: 0.19% C, 0.36% Mn, 0.10% Si, 0.032% P, 0.031% S, 0.0012% B), process same as Example 1, the difference is drawn to Φ9.5mm.

[0020] Results: tensile strength 890 MPa, yield strength 778 MPa, elongation 19%, full cross-section hardness uniform (HRC 25.5).

[0021] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to part of the technical features, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A boron-microalloyed Q235 steel wire for 8.8 grade fasteners, characterized in that, It consists of the following components by mass percentage: C 0.16%-0.22%, Mn 0.31%-0.59%, Si 0.10%-0.35%, P≤0.032%, S≤0.031%, B 0.0006%-0.0030%, with the balance being Fe.

2. The boron microalloyed Q235 steel wire for 8.8 grade fasteners according to claim 1, characterized in that: By mass percentage, it contains 0.18% C, 0.33% Mn, 0.13% Si, 0.032% P, 0.031% S, 0.0009% B, with the balance being Fe.

3. The method for manufacturing boron microalloyed Q235 steel wire for 8.8 grade fasteners according to claim 1, characterized in that, Includes the following steps, S1. Hot metal pretreatment; S2. Converter smelting; S3.LF refining, controlling S / P; S4. Continuous casting, with superheat controlled at 15℃-25℃; S5. Rolling, finishing temperature 820℃-850℃; S6. Controlled cooling, spinning temperature 800℃-830℃, Stellmore air cooling.

4. A method for manufacturing boron microalloyed Q235 steel grade 8.8 fasteners based on the wire obtained in claim 3, characterized in that, Includes the following steps, S7. Stringy texture, reducing dough size by 6%-23%; S8. Cold heading; S9. Twisting the threads; S10. Quenching and tempering treatment, oil quenching at 860℃ and tempering at 480℃; S11. Surface treatment.

5. A grade 8.8 fastener based on the boron microalloyed Q235 steel obtained in claim 4, characterized in that, Its surface-to-core hardness difference is ≤3 HRC, tensile strength is ≥800 MPa, and yield strength is ≥640 MPa.