HIGH-HARDENABILITY, MEDIUM-CARBON, LOW-ALLOY ROUND STEEL FOR FASTENERS AND THE MANUFACTURING METHOD THEREOF

Abstract

The present invention relates to a high-hardenability, medium-carbon, low-alloy round steel for fasteners, the chemical constituents by mass percentage are as follows: C: 0.360.44%, Si: 0.150.40%, Mn: 0.801.00%, Cr: 1.001.15%, Mo: 0.050.25%, Ni: 0.050.25%, Cu: 0.050.25%, Al: 0.0150.050%, B: 0.00100.0050%, Ti: 0.0200.050%, the balance is Fe; the maximum diameter of the round steel is 65 mm. The manufacturing process are as follows: the raw materials are processed, in sequence, by converter smelting, LF refining, RH/VD degassing to obtain molten steel, feeding Ti wires and ferroboron, continuous casting, rolling into the bar, obtaining the quenched and tempered round steel after quenching and tempering treatment; the quenched and tempered round steel is able to be directly used in processing fasteners which meet ISO 898-1 standard for grade 10.9, such as bolts and the like.

Claims

1. A high-hardenability, medium-carbon, low-alloy round steel for fasteners, wherein its chemical constituents by mass percentage are as follows: C: 0.360.44%, Si: 0.150.40%, Mn: 0.801.00%, Cr: 1.001.15%, Mo: 0.050.25%, Ni: 0.050.25%, Cu: 0.050.25%, Al: 0.0150.050%, B: 0.00100.0050%, Ti: 0.0200.050%, the balance is Fe and unavoidable impurity elements; the maximum diameter of the round steel is 65 mm, after quenching and tempering treatment, its mechanical structure and performance meet the requirements of ISO 898-1 standard or other equivalent standards for grade 10.9 fasteners, specific requirements are as follows: quenched martensite content is z,27 90%, after quenching and tempering treatment, when a tensile sample is tested, the diameter of a tensioned part of the sample is 75% of nominal size, yield strength of the round steel Rp0.2 is z,27 3940 MPa, tensile strength Rm is 10401140 MPa, elongation is z,27 9% , reduction of area at fracture is z,27 48%, Charpy impact energy measured at 20 C. is 27 J when a Akv2 impact sample is taken at radius, HV0.3 hardness of the whole cross-section of round steel is 320380 HV, difference of the hardness of the whole cross-section is within 30 HV.

2. A method of manufacturing the high-hardenability, medium-carbon, low-alloy round steel for fasteners according to claim 1, wherein the raw materials are processed, in sequence, by converter smelting or electric furnace smelting, LF refining, RH/VD degassing to obtain molten steel, feeding Ti wires and ferroboron after LF refining; the molten steel is casted into a continuous casting billet by adopting low superheat degree of 1540 C. and under a full argon protection, the continuous casting billet is fed warmly at a temperature of 300600 C. or delivered to a slow-cooling pit and is slowly cooled for 32 hours or more, and then is taken out of the pit; the continuous casting billet is heated to 12001250 C. and kept warm for 310 hours in the furnace, and then is taken out of the furnace; after high-pressure water descaling, the continuous casting billet is rolled in to round steel bar at a temperature of 11001150 C.; the bar is processed by means of quenching and tempering treatment in a continuous furnace thereby to obtain quenched and tempered round steel; the quenched and tempered round steel is able to be directly used in processing fasteners which meet ISO 898-1 standard for grade 10.9, such as bolts and the like; the above quenching and tempering treatment includes quenching and tempering, the treatment can be performed in a roller hearth continuous furnace or in induction lines, when in a roller hearth continuous furnace, the quenching heating temperature is 840880 C., the time in furnace is 60360 min, the bar is water quenched by quenching ring; the tempering heating temperature is 530620 C., the time in furnace is 300600 min, the bar is air-cooled outside the furnace to room temperature or is water-cooled to room temperature; when in induction lines, the quenching heating temperature is 880950 C., the time in furnace is 310 min, the bar is water quenched by quenching ring; the tempering heating temperature is 600700 C., the time in furnace is 310 min, the bar is air-cooled outside the furnace to room temperature or is water-cooled to room temperature.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0018] FIG. 1 is quenched and tempered core structure (100) of embodiment 1;

[0019] FIG. 2 is quenched and tempered core structure (100) of embodiment 2;

[0020] FIG. 3 is quenched and tempered core structure (100) of embodiment 3.

MODE(S) FOR CARRYING OUT THE INVENTION

[0021] The present invention is further described in details with reference to embodiments.

Embodiment 1

[0022] The diameter of the steel for fastener relating to the present embodiment is 64 mm, and the steel is processed with heat treatment according to the requirements of 10.9 grade fastener, the chemical constituents by mass percentage are as follows: C: 0.42%, Si: 0.25%, Mn: 0.95%, Cr: 1.10%, Mo: 0.10%, Ni: 0.12%, Cu: 0.12%, Al: 0.018%, B: 0.0020%, Ti: 0.025%, the balance is Fe and unavoidable impurity elements.

[0023] The manufacturing processes are as follows: the main raw materials are processed, in sequence, by electric furnace smelting, LF refining, VD degassing to obtain molten steel, feeding Ti wires and ferroboron after LF refining; the molten steel is casted into a continuous casting billet by adopting low superheat degree of 1530 C. and under a full argon protection, the continuous casting billet is slowly cooled for 32 hours; the continuous casting billet is heated to 1250 C. and kept warm for 4 hours in the furnace, and then is taken out of the furnace; after high-pressure water descaling, the continuous casting billet is rolled in to round steel bar at a temperature of 1150 C.; in a roller hearth continuous furnace, the bar is heated to 840 C. for 3 hours for austenization, after austenization, the bar is high-press water quenched by quenching ring, and the bar is further heated to 620 C. for 6 hours for tempering, the bar is air-cooled outside the furnace to room temperature. Comparison with the chemical constituents, the mechanical performance and the metallographic structure of 40CrNiMo steel which is usually used for manufacturing the fasteners with large size, is shown in table 1, table 2 and FIG. 1. It can be seen that Mo content and Ni content in the present embodiment are greatly reduced, the noble metal addition is less and at the same time the mechanical performance, the distribution of hardness in transversal cross-section and the metallographic structure at the core all satisfy the requirements of standard ISO 898-1.

TABLE-US-00001 TABLE 1 Comparison of chemical constituents between CrB middle-carbon steel in Embodiment 1 and round steel in Comparative Example 1 (wt %) C Si Mn Cr Mo Ni Cu Al Ti B Comparative 0.40 0.25 0.70 0.78 0.18 1.30 0.03 0.022 0.003 0.0001 Example 1 Embodiment 1 0.42 0.25 0.95 1.10 0.10 0.12 0.12 0.018 0.025 0.0020

TABLE-US-00002 TABLE 2 Comparison of performance after quenching and tempering treatment between bar in Embodiment 1 and round steel in Comparative Example 1 Hardness Yield Tensile of Vickers hardness of Strength Strength Reduction 20 C. tempered transversal cross section Rp0.2 Rm Elongation of Area Akv core HV0.3 Steel Grade (MPa) (MPa) A % Z % J HRC Surface 1/2R Core Requirement 930 1040 9 48 27 30 + 1/2*C % 320-380 Comparative 1000 1100 13 52 62/56/60 55 354 348 342 Example 1 Embodiment 1 970 1050 11 49 42/43/38 53 335 332 328 take a whole sample for measuring tensile properties; Vickers hardness of transversal cross section: max-min 30HV.

Embodiment 2

[0024] The diameter of the steel for fastener relating to the present embodiment is 54 mm, and the steel is processed with heat treatment according to the requirements of 10.9 grade fastener, the chemical constituents by mass percentage are as follows: C: 0.37%, Si: 0.22%, Mn: 0.95%, Cr: 1.10%, Mo: 0.08%, Ni: 0.10%, Cu: 0.10%, Al: 0.022%, B: 0.0023%, Ti: 0.028%, the balance is Fe and unavoidable impurity elements.

[0025] Above round steel is processed, in sequence, by electric furnace smelting, LF refining, VD degassing to obtain molten steel, feeding Ti wires and ferroboron after LF refining; the molten steel is casted into a continuous casting billet by adopting low superheat degree of 1530 C. and under a full argon protection, the continuous casting billet is fed warmly at a temperature of 400 C.; the continuous casting billet is heated to 1200 C. and kept warm for 4 hours in the furnace, and then is taken out of the furnace; after high-pressure water descaling, the continuous casting billet is rolled in to round steel bar at a temperature of 1100 C.; in induction lines, the bar is heated to 880 C. for 5 minutes for austenization, after austenization, the bar is quenched by quenching ring, and the bar is further heated to 550 C. for 5 hours for tempering in a roller hearth continuous furnace, the bar is air-cooled outside the furnace to room temperature. Comparison with the chemical constituents, the mechanical performance and the metallographic structure of the conventional 40CrNiMo steel, is shown in table 3, table 4 and FIG. 2. It can be seen that Mo content and Ni content in the present embodiment are greatly reduced, and at the same time the mechanical performance, the distribution of hardness in transversal cross-section and the metallographic structure at the core all satisfy the requirements of standard ISO 898-1.

TABLE-US-00003 TABLE 3 Comparison of chemical constituents between CrB middle-carbon steel in Embodiment 2 and round steel in Comparative Example 2 (wt %) C Si Mn Cr Mo Ni Cu Al Ti B Comparative 0.41 0.23 0.72 0.80 0.18 1.31 0.05 0.028 0.003 0.0001 Example 2 Embodiment 2 0.37 0.22 0.95 1.10 0.08 0.10 0.10 0.022 0.028 0.0023

TABLE-US-00004 TABLE 4 Comparison of performance after quenching and tempering treatment between bar in Embodiment 2 and round steel in Comparative Example 2 Hardness of Vickers hardness of Yield Tensile Reduction 20 C. tempered transversal cross Strength Strength Elongation of Area Akv core section HV0.3 Steel Grade Rp0.2(MPa) Rm(MPa) A % Z % J HRC Surface 1/2R Core Requirements 930 1040 9 48 27 30 + 1/2*C % 320-380 Comparative 1020 1120 14 55 65/60/68 55 350 348 345 Example 2 Embodiment 980 1060 11 51 47/47/45 52 342 337 335 2 take a whole sample for measuring tensile properties; Vickers hardness of transversal cross section: max-min 30HV.

Embodiment 3

[0026] The diameter of the steel for fastener relating to present embodiment is 48 mm, and the steel is processed with heat treatment according to the requirements of 8.8 grade fastener, the chemical constituents by mass percentage are as follows: C: 0.36%, Si: 0.21%, Mn: 0.92%, Cr: 1.08%, Mo: 0.08%, Ni: 0.06%, Cu: 0.08%, Al: 0.023%, B: 0.0025%, Ti: 0.026%, the balance is Fe and unavoidable impurity elements.

[0027] Above round steel is processed, in sequence, by KR pre-treatment, converter smelting, LF refining, RH degassing to obtain molten steel, feeding Ti wires and ferroboron after LF refining; the molten steel is casted into a continuous casting billet by adopting low superheat degree of 1530 C. and under a full argon protection, the continuous casting billet is fed warmly at a temperature of 400 C.; the continuous casting billet is heated to 1200 C. and kept warm for 4 hours in the furnace, and then is taken out of the furnace; after high-pressure water descaling, the continuous casting billet is rolled in to round steel bar at a temperature of 1100 C.; in continuous induction lines, the bar is heated to 950 C. for 4 minutes for austenization, after austenization, the bar is quenched by quenching ring, and the bar is further heated to 700 C. for 4 minutes for tempering, the bar is air-cooled outside the furnace. Comparison with the chemical constituents, the mechanical performance and the metallographic structure of the conventional 4140MOD steel, is shown in table 5, table 6 and FIG. 3. It can be seen that Mo content and Ni content in the present embodiment are greatly reduced, and at the same time the mechanical performance, the distribution of hardness in transversal cross-section and the metallographic structure at the core all satisfy the requirements of standard ISO 898-1.

TABLE-US-00005 TABLE 5 Comparison of chemical constituents between CrB middle-carbon steel in Embodiment 3 and 40CrNiMo round steel in Comparative Example 3 (wt %) C Si Mn Cr Mo Ni Cu Al Ti B Comparative 0.41 0.25 0.95 1.05 0.23 1.31 0.05 0.028 0.003 0.0001 Example 3 Embodiment 3 0.36 0.21 0.92 1.08 0.08 0.06 0.08 0.023 0.026 0.0025

TABLE-US-00006 TABLE 6 Comparison of performance after quenching and tempering treatment between Embodiment 3 and 40CrNiMo round steel Yield Tensile Hardness of Vickers hardness of Strength Strength Reduction 20 C. tempered transversal cross Rp0.2 Rm Elongation of Area Akv core section HV0.3 Steel Grade (MPa) (MPa) A % Z % J HRC Surface 1/2R Core Requirements 660 830 12 52 27 30 + 1/2*C % 255-335 Comparative 790 900 18 60 100/98/90 56 288 283 279 Example 3 Embodiment 3 760 880 15 56 78/72/65 52 280 280 270 take a whole sample for measuring tensile properties; Vickers hardness of transversal cross section: max-min 30HV.

[0028] Besides above embodiments, the present invention further includes other embodiments, and any technical solution formed by equivalent transformation or equivalent substitution shall fall within the protection scope of claims of the present invention.