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: 0.36˜0.44%, Si: 0.15˜0.40%, Mn: 0.80˜1.00%, Cr: 1.00˜1.15%, Mo: 0.05˜0.25%, Ni: 0.05˜0.25%, Cu: 0.05˜0.25%, Al: 0.015˜0.050%, B: 0.0010˜0.0050%, Ti: 0.020˜0.050%, 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.

1 . A high-hardenability, medium-carbon, low-alloy round steel for fasteners, wherein its chemical constituents by mass percentage are as follows: C: 0.36˜0.44%, Si: 0.15˜0.40%, Mn: 0.80˜1.00%, Cr: 1.00˜1.15%, Mo: 0.05˜0.25%, Ni: 0.05˜0.25%, Cu: 0.05˜0.25%, Al: 0.015˜0.050%, B: 0.0010˜0.0050%, Ti: 0.020˜0.050%, 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 1040˜1140 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 ½ radius, 
 HV0.3 hardness of the whole cross-section of round steel is 320˜380 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 15˜40° C. and under a full argon protection, the continuous casting billet is fed warmly at a temperature of 300˜600° 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 1200˜1250° C. and kept warm for 3˜10 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˜1150° 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 840˜880° C., the time in furnace is 60˜360 min, the bar is water quenched by quenching ring; the tempering heating temperature is 530˜620° C., the time in furnace is 300˜600 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 880˜950° C., the time in furnace is 3˜10 min, the bar is water quenched by quenching ring; the tempering heating temperature is 600˜700° C., the time in furnace is 3˜10 min, the bar is air-cooled outside the furnace to room temperature or is water-cooled to room temperature.