JP2001032044A - Steel for high strength bolt and production of high strength bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength bolt having high strength and excellent in delayed fracture characteristics. SOLUTION: Steel for a high strength bolt having a compsn. contg., by weight, 0.25 to 0.50% C, >0.40 to 1.50% Mn and 1.50 to 3.00% Mo or Mo+1/2W and 0.010 to 0.100% Al, moreover contg., at need, one or >= two kinds among 0.10 to 1.50% Cr, 0.01 to 0.40% V, 0.005 to 0.100% Nb, 0.005 to 0.100% Ti and 0.0005 to 0.0050% B, in which the content of Si is limited to <=0.10% (including 0%), P to <=0.012% (including 0%) and S to <=0.012% (including 0%), and the balance Fe with inevitable impurities is formed into a desired shape, is thereafter heated to >=Ac3, is subsequently subjected to quenching treatment and is tempered in the temp. range of 550 deg.C to Ac1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength bolt steel excellent in delayed fracture resistance and a method of manufacturing the high-strength bolt.

[0002]

2. Description of the Related Art High performance and light weight of automobiles and industrial machines,
Also, with the enlargement of building structures, development of high-strength bolt steel has been required.

[0003] Steel grades currently generally used as high-strength bolt steel are SCM435 defined by JIS,
Low alloy structural steel such as SCR435, which is manufactured by quenching and tempering. But,
When the tensile strength of these steel types exceeds 1200 MPa, the resistance to delayed fracture is sharply reduced, and the risk of fracture due to delayed fracture during the use of the bolt is increased. It was impossible.

[0004] Various companies have proposed high-strength bolt steel for the purpose of improving delayed fracture resistance. For example, JP-A-5-148576 and JP-A-5-148580 disclose preventing light carburization and grain boundary oxidation occurring during heat treatment of bolts, and increasing the amount of Mo to improve tempering softening resistance. However, when applied to a high-strength bolt of 1500 MPa class or more, the delayed fracture resistance becomes insufficient and practical application is difficult. Further, the optimum range of the tempering temperature is not shown, and the maximum value of the tensile strength described in the examples of the publication is 147.0 kgf / mm ² .

[0005] For example, Japanese Patent No. 2737913 is disclosed.
Mo and V are added in combination to _ThreeMinimize C generation
By stopping, the bolt of 1400MPa class can withstand
A technique for improving delayed fracture characteristics is described.
When applied to high strength bolts of 00MPa class or higher,
Delayed fracture resistance is not sufficient, and practical application is difficult
It is. Also, the optimum range of tempering temperature is not shown.
The maximum tensile strength described in the examples of the publication
The value is also 158.7kgf / mm^TwoStop.

From the viewpoint of reducing the weight and improving the performance of building structures and mechanical parts, the higher the strength level of the bolt is, the more preferable it is. However, if it is necessary to further increase the strength to the 1500 MPa or 1600 MPa class, the above-described method is used. Such conventional techniques cannot cope with the above, and the delayed fracture resistance deteriorates.
High strength bolts having a strength level of 500 MPa or more have not been put to practical use.

As described above, no high-strength bolt having a strength level of 1500 MPa or more and excellent in delayed fracture resistance has been found at present.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a method for manufacturing a high-strength bolt steel and a high-strength bolt excellent in delayed fracture resistance. In detail, even at high strength, 1000 MPa
It is an object of the present invention to provide a high-strength bolt steel having a tensile strength of 1500 MPa or more and a method of manufacturing a high-strength bolt, which is more excellent in delayed fracture resistance than SCM435, which is often used as a grade bolt.

[0009]

The gist of the present invention is as follows (1) to (7).

(1) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
0.100%, Si: 0.10% or less (0%
), P: 0.012% or less (including 0%), S:
A high-strength bolt steel excellent in delayed fracture resistance, characterized by being limited to 0.012% or less (including 0%) and the balance being Fe and unavoidable impurities.

(2) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
-0.100%, and further Cr: 0.10-0.10%
1.50%, V: contains one or two of 0.01 to 0.40%, Si: 0.10% or less (including 0%), P: 0.012% or less (0% %, S: 0.
012% or less (including 0%), and the balance is Fe
And high-strength bolt steel with excellent delayed fracture resistance, characterized by being composed of unavoidable impurities.

(3) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
００0.100%, and Nb: 0.005 to
0.100%, one or two of Ti: 0.005 to 0.100%, and Si: 0.10% or less (0
%, P: 0.012% or less (including 0%),
S: High-strength bolt steel excellent in delayed fracture resistance, characterized by being limited to 0.012% or less (including 0%) and the balance being Fe and unavoidable impurities.

(4) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
-0.100%, and further, Cr: 0.10-0.10%
1.50%, V: one or two of 0.01 to 0.40%, and Nb: 0.005 to 0.1
00%, one or two of Ti: 0.005 to 0.100%, Si: 0.10% or less (including 0%), P: 0.012% or less (0% S: 0).
012% or less (including 0%), and the balance is Fe
And high-strength bolt steel excellent in delayed fracture resistance, characterized by being composed of unavoidable impurities.

(5) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
００0.100%, and Nb: 0.005 to
0.100%, one or two of Ti: 0.005 to 0.100%, and B: 0.0005
0.0050%, Si: 0.10% or less (0
%, P: 0.012% or less (including 0%),
S: High-strength bolt steel excellent in delayed fracture resistance, characterized by being limited to 0.012% or less (including 0%) and the balance being Fe and unavoidable impurities.

(6) In weight%, C: 0.25 to 0.5
0%, Mn: more than 0.40 to 1.50%, Mo or Mo
+ 1 / 2W: 1.50 to 3.00%, Al: 0.010
-0.100%, and further Cr: 0.10-0.10%
1.50%, V: one or two of 0.01 to 0.40%, and Nb: 0.005 to 0.1
00%, one or two of Ti: 0.005 to 0.100%, and B: 0.0005 to 0.5%.
0050%, Si: 0.10% or less (including 0%), P: 0.012% or less (including 0%), S: 0.
012% or less (including 0%), and the balance is Fe
And high-strength bolt steel with excellent delayed fracture resistance, characterized by being composed of unavoidable impurities.

(7) After forming the high-strength bolt steel according to any one of the above (1) to (6) into a desired shape, the steel is heated to a temperature of A _C3 or more, and then subjected to a quenching treatment to perform 550 ° C. to A _C1. A method for producing a high-strength bolt having excellent delayed fracture resistance, characterized by tempering in a temperature range of:

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies on various factors affecting delayed fracture characteristics and found the following findings. That is, (1) the effect of tempering temperature is great on the delayed fracture characteristics of high-strength steel. Comparing the delayed fracture resistance characteristics of steel materials having the same tensile strength, the higher the tempering temperature, the lower the delayed fracture resistance. The tendency to improve is great. This is because the higher the tempering temperature, the more the cementite morphology that precipitates at the old austenite grain boundaries becomes spheroidized, increasing the bonding force of the grain boundaries, and the density of defects such as matrix dislocations decreases, resulting in embrittlement against hydrogen This is because the sensitivity is reduced. (2) The delayed fracture resistance characteristics of high-strength steel of 1500 MPa class or more can be measured using SCM435, which is currently widely used.
The tempering temperature must be set to at least 550.degree. C. in order to make it equivalent to the 1000 MPa class delayed fracture resistance. (3) In order to set the tempering temperature in the above temperature range and obtain high strength of 1500 MPa class or more, Mo alone or Mo + W is added in a large amount in a certain range, and Mo carbide and W carbide precipitated during tempering are used. It is effective to use precipitation strengthening. (4)
By restricting the amounts of P and S, which are impurities segregated at the grain boundaries, to a certain amount or less, the austenite grain boundaries are strengthened, and the delayed fracture resistance is improved.

In addition, Si, which is a solid solution strengthening element for ferrite, is reduced as much as possible, and addition of alloying elements such as Mn and Cr is minimized to compensate for a decrease in cold forgeability due to a large amount of Mo added. It has been found that high strength can be achieved without impairing the cold forgeability of the bolt.

Hereinafter, the present invention will be described in detail.

C: Since C is an effective element for obtaining strength, it is added in an amount of 0.25% or more. However, if added in excess of 0.50%, cold forgeability and toughness are reduced.
Must be in the range of ~ 0.50%. The preferred range is 0.
30 to 0.45%.

Mn: Mn is an element effective for improving hardenability and has an effect of preventing hot brittleness by fixing S in steel as MnS.
Although it is added in excess of 0%, if it is added in excess of 1.50%, the delayed fracture resistance and cold forgeability deteriorate.
It needs to be in the range of more than 0 to 1.50%. The preferred range is 0.45 to 0.65%.

Mo or Mo + 1 / 2W: Mo, W causes remarkable secondary hardening due to precipitation of fine Mo carbides and W carbides at the time of tempering, and remarkably exhibits delayed fracture resistance by enabling high temperature tempering. It is an element to improve. Further, the strength-ductility balance can be improved by high-temperature tempering. W has the same effect as Mo, but the effect is half of Mo. In the tempering temperature range of 550 ° C. or more, in order to obtain a high strength of 1500 MPa class by precipitation hardening of Mo carbonitride and W carbide, it is necessary to add Mo or Mo + ／ W of 1.50% or more. If the addition exceeds 0.000%, the cold forgeability is reduced, and the alloy carbide is hardly dissolved in the matrix at the time of quenching and heating, and the ductility is reduced by increasing the amount of coarse undissolved carbide. .
It needs to be in the range of 50 to 3.00%. The preferred range is 1.80 to 2.20%.

Al: Al is an element necessary for the deoxidation of steel, and has an effect of forming nitrides and refining old austenite grains. Therefore, Al is added in an amount of 0.010% or more.
If added in excess of 0.100%, not only does the effect become saturated, but also alumina-based inclusions increase and toughness decreases.
It is necessary to be in the range of 0.010 to 0.100%. A preferred range is from 0.020 to 0.050%.

Cr: Cr is an element effective for improving hardenability, and has an effect of imparting tempering softening resistance to steel. Therefore, Cr is added in an amount of 0.10% or more. When added, the cold forgeability decreases, so 0.10
Must be in the range of １．５1.50%. The preferred range is 0.
15 to 0.50%.

V: V is an element that has the effect of refining the prior austenite crystal grains, causes remarkable secondary hardening during tempering, and improves the delayed fracture resistance by enabling high temperature tempering. . Further, V carbonitride, which is finely precipitated in the matrix at the time of tempering, becomes an intragranular trap site of hydrogen, and is an element that reduces the amount of hydrogen accumulated at the grain boundaries and improves delayed fracture resistance. When the content exceeds 0.4%, the cold forgeability is reduced, and the alloy carbide is hardly dissolved in the matrix during quenching and heating, and the amount of coarse undissolved carbide increases. Therefore, the ductility decreases, so that the content needs to be in the range of 0.01 to 0.40%. The preferred range is 0.05-0.15%.

Nb: Similar to Al, Ti and V, Nb has the effect of refining crystal grains and the effect of improving delayed fracture resistance. Therefore, Nb is added in an amount of 0.005% or more. %, The effect is not only saturated, but also the cold forgeability deteriorates.
It must be in the range of 100%. The preferred range is 0.01
0 to 0.050%.

Ti: Ti, like Al, Nb, and V, has the effect of refining crystal grains, and has the effect of fixing solute N in steel as nitride and improving delayed fracture resistance. Therefore, 0.005% or more is added, but if added in excess of 0.100%, not only the effect is saturated, but also the cold forgeability decreases, so it is necessary to be in the range of 0.005 to 0.100%. . A preferred range is 0.010 to 0.050%.

B: B has the effect of improving hardenability with a small amount of addition, and has the effect of segregating at the former austenite grain boundary to strengthen the grain boundary and improving delayed fracture resistance, so 0.0005%. The effect is saturated when added in excess of 0.0050%.
Must be in the range of 0.0050%. A preferred range is 0.0010 to 0.0030%.

Si: Si is an element necessary for deoxidation of steel, but if added in excess of 0.10%, the cold forgeability is significantly reduced, so it must be limited to 0.10% or less. The preferred range is 0.08% or less.

P: P segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and has the effect of significantly reducing delayed fracture resistance. Therefore, it is necessary to limit P to at least 0.012% or less. Should. The preferred range is 0.01
0% or less.

S: S has the effect of segregating at the former austenite grain boundary to embrittle the grain boundary and significantly reduce the delayed fracture resistance, so it is necessary to limit it to at least 0.012% or less. Should. The preferred range is 0.01
0% or less.

In the present invention, the N content is not particularly specified, but since solid solution N in steel is an element which lowers the delayed fracture resistance, it is preferable to reduce it as much as possible. A preferred range is 0.0080% or less.

Although the present invention does not particularly define the secondary working step, in the case where a cold forging step is included in the manufacturing step, in order to improve the cold forgeability, the material after hot rolling is annealed or A spheroidizing annealing treatment may be performed. In the case of a bolt that requires dimensional accuracy of the material, it is common to draw the wire before cold forging.

The steel for bolts having the above-mentioned components is most effective in the method for manufacturing bolts described below.

After the bolt steel having the above-mentioned components is formed into a desired bolt shape by forging, cutting, or the like, the steel is heated to a temperature of A _C3 or more to impart strength to the steel, and then water-cooled or oil-cooled. A quenching process is performed. If the heating temperature is too low, the solution of the carbides of Mo, W, and V becomes insufficient, and the desired properties cannot be obtained. On the other hand, when the heating temperature is too high, the crystal grains become coarse, and the toughness and the delayed fracture resistance deteriorate. Further, from the viewpoint of the operation, the furnace body of the heat treatment furnace and the attached parts become remarkably damaged, and the production cost increases. Therefore, it is not preferable to heat the furnace to an excessively high temperature. Within the component range of the present invention, the quenching heating temperature is 900 to 100
The temperature is preferably set to 0 ° C.

In order to impart predetermined strength, toughness and ductility to the steel, it is necessary to perform tempering after quenching. Tempering is generally performed in a temperature range of 150 ° C. to A _C1, but in the present invention, it is necessary to limit the temperature range to 550 ° C. to A _C1 . The reason is that at 550 ° C. or lower, the form of cementite precipitated at the grain boundary cannot be made spherical to increase the bonding strength of the grain boundary, and the delayed fracture resistance is one of SCM435's.
The reason is that it cannot be equal to or higher than the 000 MPa class, and the precipitation strengthening by Mo carbide and W carbide precipitated at the time of tempering is remarkably exhibited at 550 ° C. or more. On the other hand, when the tempering temperature exceeds A _C1 , it becomes difficult to obtain a desired strength. The preferred range is 575-675 ° C.

[0037]

The present invention will be further described below with reference to examples.

Converter steelmaking steel having the composition shown in Table 1 was continuously cast and, if necessary, was subjected to a soaking process and a slab rolling process to obtain a 162 mm square rolled material. Subsequently, a wire rod shape was obtained by hot rolling.

[0039]

[Table 1]

Next, bolts were manufactured in order to investigate the delayed fracture characteristics of these materials. The rolled material was subjected to annealing or spheroidizing annealing as necessary, and was formed into a bolt shape by cold forging. Thereafter, the mixture was heated under predetermined conditions, quenched in an oil bath, and tempered under the conditions shown in Table 2. From the bolts manufactured in the above process, a tensile test piece having a diameter of 8 mm and a delayed fracture test piece with an annular notch notch (diameter of the parallel portion 8 m)
m, the diameter of the notch 6 mm) was manufactured by machining, and the mechanical properties and the delayed fracture characteristics were investigated.

In the delayed fracture test, a test piece was charged with hydrogen at a current density of 1.0 mA / cm ² in dilute sulfuric acid (solution temperature: 30 ° C.) at pH 3.0, a constant load was applied, and the time until fracture was measured. did. The test time was a maximum of 200 hours, and the maximum applied stress that did not break for 200 hours was measured. The value obtained by dividing the maximum applied stress that does not break for 200 hours by the breaking stress in the atmosphere was defined as “delayed fracture strength ratio” and used as an index of delayed fracture characteristics. Since the delayed fracture strength ratio of SCM435, which is generally used as a bolt having a tensile strength of 1000 MPa class, is about 0.5, the one having a delayed fracture strength ratio of less than 0.5 is inferior in delayed fracture resistance. It was judged. Table 2 also summarizes the results of these various tests.

[0042]

[Table 2]

No. of Comparative Example In No. 12, although the component range was within the range of the present invention, the desired delayed fracture characteristics were not obtained because the tempering temperature was low. The relationship between the delayed fracture strength ratio and the tensile strength shown in Table 2 is shown in FIG. It can be seen that the inventive examples show better delayed fracture characteristics than the comparative examples.

As is apparent from these, the inventive examples have higher strength and are superior in delayed fracture resistance as compared with the comparative examples.

[0045]

According to the present invention, the tensile strength is 1500MP.
It is possible to provide a bolt having a high strength of a or more and excellent in delayed fracture characteristics, thereby increasing the fastening axial force of the bolt,
The effect is extremely large because it can greatly contribute to weight reduction and high performance of structures and mechanical parts through weight reduction by downsizing and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a delayed fracture strength ratio and a tensile strength.

Claims (7)

[Claims] C .: 0.25 to 0.50%, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0% by weight 0.10% or less (including 0%), P: 0.012% or less (including 0%), S: 0.012% or less (including 0% High-strength bolt steel with excellent delayed fracture resistance, characterized by being limited to Fe and inevitable impurities. 2. In% by weight, C: 0.25 to 0.50%, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0 0.10 to 0.100%, further contains one or two of Cr: 0.10 to 1.50%, V: 0.01 to 0.40%, and Si: 0.1 to 0.10%. 10% or less (including 0%), P: limited to 0.012% or less (including 0%), S: limited to 0.012% or less (including 0%), and the balance is less than Fe and inevitable impurities. High strength bolt steel with excellent delayed fracture resistance. C .: 0.25 to 0.50% by weight, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0 Nb: 0.005 to 0.100%, Ti: 0.005 to 0.100%, or one or two of Nb: 0.005 to 0.100%
Seeds, Si: 0.10% or less (including 0%), P: 0.012% or less (including 0%), S: 0.012% or less (including 0%) A high-strength bolt steel excellent in delayed fracture resistance, characterized in that the balance consists of Fe and unavoidable impurities. 4. In% by weight, C: 0.25 to 0.50%, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0 0.10 to 0.100%, Cr: 0.10 to 1.50%, V: 0.01 to 0.40%, Nb: 0.005 to 0.100%, one of Ti: 0.005 to 0.100% or 2
Seeds, Si: 0.10% or less (including 0%), P: 0.012% or less (including 0%), S: 0.012% or less (including 0%) A high-strength bolt steel excellent in delayed fracture resistance, characterized in that the balance consists of Fe and unavoidable impurities. 5. Weight%, C: 0.25 to 0.50%, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0 Nb: 0.005 to 0.100%, Ti: 0.005 to 0.100%, or one or two of Nb: 0.005 to 0.100%
B: 0.0005% to 0.0050%, Si: 0.10% or less (including 0%), P: 0.012% or less (including 0%), S : A high-strength bolt steel excellent in delayed fracture resistance, characterized by being limited to 0.012% or less (including 0%) and the balance being Fe and unavoidable impurities. 6. In% by weight, C: 0.25 to 0.50%, Mn: more than 0.40 to 1.50%, Mo or Mo + 1 / 2W: 1.50 to 3.00%, Al: 0 0.10 to 0.100%, Cr: 0.10 to 1.50%, V: 0.01 to 0.40%, Nb: 0.005 to 0.100%, one of Ti: 0.005 to 0.100% or 2
B: 0.0005% to 0.0050%, Si: 0.10% or less (including 0%), P: 0.012% or less (including 0%), S : A high-strength bolt steel excellent in delayed fracture resistance, characterized by being limited to 0.012% or less (including 0%) and the balance being Fe and unavoidable impurities. 7. The high-strength bolt steel according to any one of claims 1 to 6, which is formed into a desired shape, heated to a temperature of A _C3 or more, and then subjected to a quenching treatment to perform a temperature range of 550 ° C. to A _C1 . A method of manufacturing a high-strength bolt having excellent delayed fracture resistance, characterized by tempering with a bolt.