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.

Description

TECHNICAL FIELD

The present invention relates to a field of manufacturing special round steel, and specially relates to a high-hardenability, medium-carbon, low-alloy round steel, which satisfies the mechanical performance requirements of Ø65 mm fasteners after quenching and tempering treatment, and the manufacturing method thereof.

BACKGROUND ART

At present market, the fasteners are manufactured mostly according to the requirements of standard ISO898-1: Mechanical properties of fasteners—Part 1: Bolts, screws and studs. The table 3 of this standard puts forward very strict requirements of mechanical and physical performance for fasteners, wherein the difficulties are: {circle around (1)} after quenching, the martensite structure in core of the steel should be 90% or more; {circle around (2)} on the basis of satisfying the strength and the hardness in each strength grades, the difference of HV 0.3 hardness between surface and core should be less than 30 HV; {circle around (3)} taking a whole tensile sample whose tensioned part has a diameter of 75% to the diameter of fastener for testing the mechanical performance of strength, elongation, reduction of fracture and etc.; {circle around (4)} on the basis of satisfying a high-grade strength and plastic properties, the impact energy Akv 2 measured at −20° C. is equal to or more than 27 J. It requires that the steel for fasteners has good hardenability, and sufficient depth of quenched zone can be obtained in quenching, so as to ensure martensite percentage, thereby to ensure the uniformity of structure and hardness of the finished product to match the strength and toughness. Bigger is the size of the fastener, requirements of hardenability of the steel is higher and the difficulty of manufacturing is greater. At present, the steels such as 32CrB4 or 42CrMo etc. are used to manufacture fasteners of diameter less than 45 mm, the steels such as 40CrNiMo, 4140MOD etc. with higher contents of noble metals like Mo and Ni are used to manufacture fasteners of diameter equal to or more than 45 mm. Otherwise, the core quenched structure, the hardness distribution in transversal cross-section, and the strength and toughness would not satisfy the matching requirements.

DISCLOSURE OF INVENTION

Aiming at above technical requirements, the technical problem to be solved in the present invention is to provide a round steel of CrB class with medium carbon and low alloy, to replace the steels such as 40CrNiMo, 34CrNiMo6 etc. having high content of Mo and high content of Ni. It can manufacture fasteners with size up to 65 mm and with mechanical performance satisfying standard of ISO 898-1, and achieve the purpose of reducing the cost.

The solution in the present invention is as follows: 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 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 940 MPa, tensile strength Rm is 1040˜1140 MPa, elongation is z,28 9% , reduction of area at fracture is z,28 48% Charpy impact energy measured at −20° C. is z,28 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.

The chemical constituents of round steel in the present invention are determined by this way:

C is used for improving hardenability, strength and hardness, but it decreases plasticity and toughness, and increases ductile-brittle transition temperature. Medium carbon content is applied in the present invention. The C content is controlled to be 0.36˜0.44%.

Si is a deoxidizing element in steel, and is used for increasing the strength by solid solution strengthening. When Si content is lower than 0.1%, the deoxidation effect is poor, when Si content is too high, the toughness is reduced. The Si content is controlled to be 0.15˜0.40%.

Mn is an element for improving hardenability of steel, and has effect of solid solution strengthening to increase the strength. But excessive Mn content is easy to reduce the plasticity and toughness of steel. In order to match the strength, plasticity and toughness, the Mn content is controlled to be 0.80˜1.00%

Cr and Mo are used for improving hardenability and improving the strength and toughness of material, Mo also has effects of reducing ductile-brittle transition temperature, inhibiting tempering brittleness, improving precipitation strengthening of carbon and niobium nitride, inhibiting massive ferrite, and hindering P segregation. However, Mo is a noble metal element, excessive addition increases the manufacture cost. So the Cr content is controlled to be 1.00˜1.15%, Mo content is controlled to be 0.05˜0.25%.

Ni is an element for improving hardenability of steel and obviously improving the toughness under low temperature, it has good influence on impact toughness and ductile-brittle transition temperature. In addition, Ni is also a noble metal element, excessive addition increases the manufacture cost. Considering all the above, the Ni content is controlled to be 0.05˜0.25%, helping to achieve the best cost performance.

Cu can improve hardenability of steel. However, excessive addition of Cu is easy to cause copper brittleness, and worsens the surface property. The Cu content of the present invention is 0.05˜0.25%.

B is an element which most obviously improves the hardenability of steel, and plays an important role in the present invention in replacing noble metals Mo and Ni, thereby to reduce cost. The B content of the present invention is controlled to be 0.0010˜0.0050%.

Ti can mainly fixate nitrogen. Ti can combine with N in the early stage of solidifying continuous casting billet, and forms TiN particles in the grains, thereby to reduce the reactions between B and N, to fully exert the effect of B on improving hardenability. The Ti content is controlled to be 0.020˜0.050%.

Al can mainly fixate nitrogen and deoxidize. The AlN formed by the combination of Al and N can effectively refine grains. However, excessive addition can destroy the toughness of steel, and worsens the casting performance of molten steel. The Al content of the present invention is controlled to be 0.015˜0.050%.

A further object of present invention is to provide the method of manufacturing the high-hardenability, medium-carbon, low-alloy round steel for fasteners. Special steps are as follows: 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.

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.

Compared with prior arts, the advantages of present invention focus on: taking a very small amount of alloy elements B and Ti to replace parts of noble metal elements Mo and Ni so as to improve hardenability of steel bar, manufacturing the fasteners with maximum diameter up to 65 mm, and on the basis of satisfying the mechanical and physical requirements of quenched martensite content in steel core being more than 90%, difference of the hardness of the whole cross-section being within 30 HV, impact energy measured at −20° C. being z,28 27 J in standard ISO 898-1, obviously reducing the manufacture cost of alloy raw material.

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

FIG. 1 is quenched and tempered core structure (100×) of embodiment 1;

FIG. 2 is quenched and tempered core structure (100×) of embodiment 2;

FIG. 3 is quenched and tempered core structure (100×) of embodiment 3.

MODE(S) FOR CARRYING OUT THE INVENTION

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

Embodiment 1

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.

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 15˜30° 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 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 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

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.

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 15˜30° 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 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 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

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.

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 15˜30° 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 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 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.

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.

Claims

1. A high-hardenability, medium-carbon, low-alloy round steel for fasteners, wherein its chemical constituents by mass percentage are as follows: C: 0.36˜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.