JP2000328191A - Steel for high strength bolt and production of high strength bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength bolt having high strength, excellent in delayed fracture resistance characteristics and suitable for joining in a plastic region. SOLUTION: Steel for a high strength bolt contg., by weight, 0.15 to 0.45% C, >0.40 to 1.50% Mn, 0.30 to 2.00% Cr, 0.10 to 0.35o/o Mo, >0.20 to 0.40% V and 0.010 to 0.100% Al, also balanced in Cr, Mo and V so as to satisfy the relation of 1.20<0.125×(Cr% +Mo%+5.22×V%<2.30, in which the content of Si is limited to <=0.10% (including 0%), P to <=0.015% (including 0%) and S to <=0.015% (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 500 to 650 deg.C.

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 and a method of manufacturing a high-strength bolt excellent in delayed fracture resistance and suitable for fastening in a plastic region.

[0002]

2. Description of the Related Art High performance and light weight of automobiles and industrial machines,
In addition, as the building structure becomes larger, the tensile strength becomes 1200M.
Development of high-strength steel for bolts of Pa or higher has been demanded.

[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] Recently, adoption of a plastic zone fastening method in which a bolt is tightened above a yield point has been expanding. The plastic region fastening is characterized in that a high axial force can be obtained as compared with the conventional elastic region fastening, and that the variation in the axial force is small, and high-accuracy, stable fastening is possible. The characteristics required for bolts for fastening in the plastic zone are as follows: (1) Variations in yield strength are directly linked to variations in the fastening axial force of bolts.
There is little variation in yield strength, and (2) from the viewpoint of re-tightening, the elongation from the yield point to the maximum axial force is large.

[0005] Variations in yield strength are largely affected by variations in tempering temperature in the case of ordinary steel grades. For example,
In a normal industrial production furnace, there is a possibility that a temperature variation of about ± 20 ° C. may occur, but in the case of a normal steel type, this variation may cause a variation in yield strength of 100 MPa or more. In order to reduce this variation, it is necessary to control the temperature in a narrower range than the normal operation range, and this still leaves production problems in actual industrial production furnaces. Absent.

[0006] Further, in the case of ordinary steel types, the elongation inevitably decreases as the strength increases, so that it is necessary to cope with such changes by changing the shape of the bolts. Gender issues remain. Of course, when a high-strength bolt is subjected to fastening in a plastic region, a high axial force is applied, so that the material must be superior in delayed fracture characteristics to the conventional one. However, there is no example that proposes a high-strength bolt steel suitable for fastening in a plastic region, and there is hardly any study on the material.

[0007] High-strength bolt steel intended to improve delayed fracture resistance is disclosed in, for example, JP-A-58-117856, JP-A-62-199751, and JP-A-61-2.
No. 23168, JP-A-3-243745, and Japanese Patent No. 2670937. But,
Since these steel types do not take into account plastic zone fastening, the above-described characteristics cannot be obtained. JP-A-58-1
No. 17856 describes a method for improving the ductility (elongation, drawing) and delayed fracture resistance by reducing impurities in steel, but the amounts of C, Cr, Mo and V are optimized. Therefore, variation in yield strength cannot be reduced, and improvement in elongation and delayed fracture resistance is just one step away. Japanese Patent No. 2670937 describes a method for improving delayed fracture resistance and fatigue characteristics by limiting the component system and heat treatment conditions, but this method also has the same problem as described above.

[0008] As described above, excellent delayed fracture resistance,
At the present time, there is no steel for high strength bolts suitable for fastening in the plastic zone.

[0009]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and provides a method for producing high-strength bolt steel and a high-strength bolt excellent in delayed fracture resistance and suitable for fastening in a plastic region. With the goal. In detail, even with high strength, it has better delayed fracture resistance than SCM435, which is currently widely used as a 1000 MPa class bolt, and has a small change in yield strength even if the tempering temperature fluctuates slightly, and has good elongation. It is an object of the present invention to provide a method for manufacturing a high-strength bolt steel having a tensile strength of 1200 MPa or more and a high-strength bolt described below.

[0010]

The gist of the present invention is as follows.

(1) In weight%, C: 0.15 to 0.4
5%, Mn: more than 0.40 to 1.50%, Cr: 0.30
-2.00%, Mo: 0.10-0.35%, V: 0.
More than 20 to 0.40%, Al: 0.010 to 0.100%
And Cr, Mo, V are set to 1.20 <0.125.
× (Cr% + Mo%) + 5.22 × V% <2.30 The balance satisfying the relationship of: Si: 0.10% or less (0
%), P: 0.015% or less (including 0%),
S: for high-strength bolts, each of which is limited to 0.015% or less (including 0%), with the balance being Fe and unavoidable impurities, having excellent delayed fracture resistance and being suitable for fastening in a plastic region. steel.

(2) In weight%, C: 0.15 to 0.4
5%, Mn: more than 0.40 to 1.50%, Cr: 0.30
-2.00%, Mo: 0.10-0.35%, V: 0.
More than 20 to 0.40%, Al: 0.010 to 0.100%
And Nb: 0.005 to 0.100%,
Ti: one or two of 0.005 to 0.100%
Containing seeds and Cr, Mo, V 1.20 <0.12
5 × (Cr% + Mo%) + 5.22 × V% <2.30 The balance satisfying the relationship: Si: 0.10% or less (including 0%), P: 0.015% or less (0% ), S: limited to 0.015% or less (including 0%), the balance being Fe and unavoidable impurities, excellent in delayed fracture resistance, suitable for fastening in a plastic region High strength bolt steel.

(3) In weight%, C: 0.15 to 0.4
5%, Mn: more than 0.40 to 1.50%, Cr: 0.30
-2.00%, Mo: 0.10-0.35%, V: 0.
More than 20 to 0.40%, Al: 0.010 to 0.100%
And B: 0.0005 to 0.0050%
And Cr, Mo, V are set to 1.20 <0.125.
× (Cr% + Mo%) + 5.22 × V% <2.30 The balance satisfying the relationship of: Si: 0.10% or less (0
%), P: 0.015% or less (including 0%),
S: for high-strength bolts, each of which is limited to 0.015% or less (including 0%), with the balance being Fe and unavoidable impurities, having excellent delayed fracture resistance and being suitable for fastening in a plastic region. steel.

(4) In weight%, C: 0.15 to 0.4
5%, Mn: more than 0.40 to 1.50%, Cr: 0.30
-2.00%, Mo: 0.10-0.35%, V: 0.
More than 20 to 0.40%, Al: 0.010 to 0.100%
And Nb: 0.005 to 0.100%,
Ti: one or two of 0.005 to 0.100%
Seeds, and B: 0.0005 to 0.0050
% And Cr, Mo, V 1.20 <0.12
5 × (Cr% + Mo%) + 5.22 × V% <2.30 The balance satisfying the relationship: Si: 0.10% or less (including 0%), P: 0.015% or less (0% ), S: limited to 0.015% or less (including 0%), the balance being Fe and unavoidable impurities, excellent in delayed fracture resistance, suitable for fastening in a plastic region High strength bolt steel.

(5) After forming the high-strength bolt steel according to any of the above (1) to (4) into a desired shape, the steel is heated to a temperature of A _C3 or higher, and then subjected to a quenching treatment, and then 500 to 650 ° C
A method for producing a high-strength bolt excellent in delayed fracture resistance and suitable for fastening in a plastic region, characterized in that it is tempered in the above temperature range.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have obtained the following knowledge on various factors affecting delayed fracture characteristics. That is,
(1) Cr, Mo, and V are added together in a certain component range and 5
By performing tempering in a temperature range of 00 ° C. or more, a brittle range in low-temperature tempering can be avoided. In addition, the form of cementite precipitated at the grain boundaries during tempering can be spheroidized to increase the bonding strength of the grain boundaries and prevent grain boundary cracking.
(2) By adding a specific amount of V, V carbonitrides, which are finely precipitated in the matrix during tempering, become intragranular trap sites for hydrogen, reduce the amount of hydrogen accumulated at the grain boundaries, and delay resistance. Breakage characteristics are greatly improved. (3) The former austenite grain boundary is strengthened by regulating the amounts of impurities P and S segregating at the grain boundary to a certain amount or less,
The delayed fracture resistance is improved.

The present inventors have also obtained the following knowledge in terms of materials regarding various characteristics necessary for fastening in a plastic region. That is,
(1) The addition amounts of C, Cr, Mo, and V are limited to a certain component range, and Cr, Mo, and V are set to 1.20 <0.125.
× (Cr% + Mo%) + 5.22 × V% <2.30
In the tempering temperature range of 620 ° C, tempering softening and tempering softening resistance, tempering secondary hardening are offset, tensile strength,
The fluctuation of 0.2% proof stress (yield strength) is very small, and a flat tempering performance can be obtained. This allows
Even if the tempering temperature fluctuates somewhat, the variation in the yield strength of the bolt can be kept small. (2) Cr, Mo,
By adding V in the above range and performing tempering at a high temperature of 500 ° C. or higher, both high strength and good elongation can be achieved.

[0018] Further, by reducing Si, which is a solid solution strengthening element of ferrite, as much as possible, a decrease in cold forgeability due to the addition of Cr, Mo, and V is compensated for, and a high strength can be obtained without impairing the cold forgeability of the bolt. Can be planned.

Hereinafter, the present invention will be described in detail.

C: Since C is an element effective for obtaining strength, it is added in an amount of 0.15% or more. However, if added in excess of 0.45%, cold forgeability and toughness are reduced.
It must be in the range of ~ 0.45%. The preferred range is 0.
20 to 0.40%.

Mn: Mn is an element effective for improving the hardenability, so Mn is added in an amount exceeding 0.40%.
If it is added in excess of 50%, the delayed fracture resistance and cold forgeability deteriorate. Therefore, it is necessary that the content be in the range of more than 0.40 to 1.50%. The preferred range is 0.50 to 0.80%.

Cr: Cr is an element effective for improving hardenability and has an effect of imparting temper softening resistance to steel. Therefore, Cr is added in an amount of 0.30% or more. When added, the cold forgeability is reduced.
It is necessary to be in the range of 2.00%. The preferred range is 1.
It is 00 to 1.40%.

Mo: Mo is an element that causes remarkable secondary hardening during tempering and improves the delayed fracture resistance by enabling high-temperature tempering. Further, the strength-ductility balance is improved by high-temperature tempering, and high elongation can be obtained.
If it is added in excess of 5%, the cold forgeability decreases, and the amount of secondary hardening becomes excessive, so that flat tempering properties cannot be obtained. Further, at the time of quenching and heating, it is difficult for the alloy carbide to form a solid solution in the matrix, and the amount of coarse undissolved carbide is increased, whereby the elongation is reduced and the alloy carbide is unsuitable for fastening in a plastic region. Must be in the range. The preferred range is 0.20 to 0.35%.

V: V has the effect of remarkably refining the prior austenite crystal grains, and also causes remarkable secondary hardening during tempering, thereby improving the delayed fracture resistance by enabling high temperature tempering. Element. Also,
The high-temperature tempering improves the strength-ductility balance and enables high elongation to be obtained. Furthermore, V carbonitride, which is finely precipitated in the matrix during tempering, becomes an intragranular trap site for hydrogen, and is an element that reduces the amount of hydrogen accumulated at the grain boundaries and greatly improves delayed fracture resistance. When added in excess of 0.20%, addition in excess of 0.40% not only saturates the effect but also lowers the cold forgeability, increases the amount of secondary hardening and increases the flat tempering property. I can't get it. Furthermore, at the time of quenching and heating, the alloy carbide is hardly dissolved in the matrix, and the amount of coarse undissolved carbide is increased, whereby the elongation is reduced and the alloy carbide is unsuitable for fastening in the plastic region. It must be in the range of%. The preferred range is 0.25 to 0.35
%.

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%.

Nb: Like Al, Ti and V, Nb has the effect of refining the crystal grains and the effect of improving the delayed fracture resistance, so it is preferable to add 0.005% or more. If it is added in excess of 0.100%, not only does the effect become saturated, but also the cold forgeability deteriorates. A preferred range is 0.010 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, it is preferable to add 0.005% or more,
If it is added in excess of 0.100%, not only does the effect become saturated, but also the cold forgeability deteriorates.
It is necessary to be in the range of 0.05 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%. It is preferable to add
If the addition exceeds 050%, the effect is saturated. Therefore, the addition needs to be in the range of 0.0005 to 0.0050%. The preferred range is 0.0010 to 0.0030%
It is.

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 former austenite grain boundary, embrittles the grain boundary, and has the effect of remarkably reducing the delayed fracture resistance. Therefore, P must be limited to 0.015% or less.
It should be reduced as much as possible. The preferred range is 0.010% 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 0.015% or less.
It should be reduced as much as possible. The preferred range is 0.010% or less.

Cr, Mo, V addition balance: In the present invention, the addition amount of Cr, Mo, V is 1.20 <0.125 × (C
r% + Mo%) + 5.22 × V% <2.30 By making the above balance, tempering softening and tempering softening resistance and tempering secondary hardening are offset in the tempering temperature range of 500 ° C to 650 ° C, and the tensile strength, 0.2% proof stress (yield strength) ) Is very small and a flat tempering performance can be obtained.
As a result, even if the tempering temperature fluctuates to some extent, it is possible to reduce the variation in the yield strength of the bolt. However, when the value of the above formula is lower than the lower limit, the tempering softening resistance, the tempering secondary curing becomes insufficient, and flat tempering performance cannot be obtained, and when the value is higher than the upper limit, the tempering secondary curing is excessive. Therefore, not only flat tempering performance cannot be obtained, but also an increase in material strength and a decrease in cold forgeability, so that the above range is required.
The preferred range is 1.50 <0.125 × (Cr% + Mo%)
+ 5.22 × V% <1.90.

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-described components is most effective in the bolt manufacturing method 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 not _lower than the _{AC 3} point to impart strength to the steel, and then quenched by water cooling or oil cooling. Perform processing. If the heating temperature is too high, the crystal grains will be coarsened, and the toughness and delayed fracture resistance will be deteriorated. Because it is not preferable to heat it to a very high temperature,
It is preferable to set the quenching heating temperature to 850 to 950 ° 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 the temperature range of 150 ° C. to A _C1 point, but in the present invention, it is necessary to limit the temperature range to 500 to 650 ° C. The reason is that at temperatures of 500 ° C. or lower, the form of cementite precipitated at the grain boundaries cannot be spheroidized to increase the bonding strength of the grain boundaries, and the precipitation of V carbonitride, which serves as a hydrogen trap site, is insufficient. That the delayed fracture resistance cannot be improved, and that the elongation, which is a necessary property of the plastic region fastening bolt, cannot be improved below 500 ° C.,
Above 50 ° C, softening is remarkable and tensile strength is 1200 MPa
The above high strength cannot be obtained.
This is because flat tempering performance cannot be obtained outside the temperature range of 0 ° C., and variation in yield strength due to variation in tempering temperature cannot be reduced. The preferred range is 525-625 ° 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, it was heated under predetermined conditions, quenched in an oil bath, and tempered under the conditions shown in Tables 2 and 3. 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 a notch with a circular notch (diameter of a parallel portion 8 mm, diameter of a notch portion 6 mm) were manufactured by machining to obtain mechanical properties. And delayed fracture properties 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 often used as a bolt having a tensile strength of 1000 MPa class, is about 0.5, it is judged that those having a delayed fracture strength ratio of less than 0.5 are inferior in delayed fracture resistance. did. Tables 2 and 3 also show the results of these various tests. FIG. 1 shows the relationship between the delayed fracture strength ratio and the tensile strength. It can be seen that the examples of the present invention exhibit good delayed fracture characteristics in spite of high strength as compared with the comparative examples.

In the tensile test, 0.2% proof stress (yield strength),
Tensile strength and elongation were measured. Growth is 30m between the grade points
It was measured as m. 15% elongation of SCM435 which is often used as a bolt with tensile strength of 1000MPa class
Therefore, those having an elongation of less than 15% were judged to be unsuitable for fastening in the plastic region. FIG. 2 shows the relationship between elongation and tensile strength. It can be seen that the example of the present invention shows good elongation in spite of high strength as compared with the comparative example, and is suitable for fastening in a plastic region. FIG. 3 shows the relationship between the tempering temperature and the 0.2% proof stress (yield strength). It can be seen that the steel of the present invention has a very small 0.2% proof stress (yield strength) variation as compared with the comparative steel, and can obtain flat tempering performance. This allows
Even if the tempering temperature fluctuates somewhat, variation in the yield strength of the bolt can be suppressed to a small value, and the accuracy of the fastening axial force of the bolt is improved.

[0043]

[Table 2]

[0044]

[Table 3]

As is apparent from the above, the present invention example has higher strength than the comparative example, excellent delayed fracture resistance and elongation, small variation in yield strength due to variation in tempering temperature, and low plasticity. Suitable for area conclusion.

[0046]

According to the present invention, the tensile strength is 1200MP.
It is possible to provide inexpensively a bolt suitable for fastening in a plastic region, which has high strength of a or more, has excellent delayed fracture characteristics,
Bolt fastening axial force increase, weight reduction by size reduction,
The effect, such as further application expansion of the plastic zone fastening, is extremely large.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a relationship between elongation and tensile strength.

FIG. 3 is a diagram showing the relationship between tempering temperature and 0.2% proof stress (yield strength).

Claims (5)

[Claims] C .: 0.15 to 0.45%, Mn: more than 0.40 to 1.50%, Cr: 0.30 to 2.00%, Mo: 0.10 to 0% by weight% 0.35%, V: more than 0.20 to 0.40%, Al: 0.010 to 0.100%, and Cr, Mo, V are 1.20 <0.125.
× (Cr% + Mo%) + 5.22 × V% <2.30 Si: 0.10% or less (including 0%), P: 0.015% or less (0% S: limited to 0.015% or less (including 0%), with the balance being Fe and unavoidable impurities, characterized by excellent delayed fracture resistance, suitable for fastening in a plastic region. Steel for strength bolts. 2. In% by weight, C: 0.15 to 0.45%, Mn: more than 0.40 to 1.50%, Cr: 0.30 to 2.00%, Mo: 0.10 to 0% 0.35%, V: more than 0.20 to 0.40%, Al: 0.010 to 0.100%, Nb: 0.005 to 0.100%, Ti: 0.005 to 0 100% or more, and containing Cr, Mo, V
1.20 <0.125 × (Cr% + Mo%) + 5.2
Si: 0.10% or less (including 0%), P: 0.015% or less (including 0%), S: 0.015% A high-strength bolt steel excellent in delayed fracture resistance and suitable for fastening in a plastic region, characterized by being limited to the following (including 0%), with the balance being Fe and inevitable impurities. 3. In% by weight, C: 0.15 to 0.45%, Mn: more than 0.40 to 1.50%, Cr: 0.30 to 2.00%, Mo: 0.10 to 0% 0.35%, V: more than 0.20 to 0.40%, Al: 0.010 to 0.100%, B: 0.0005 to 0.0050%, and Cr, Mo , V is 1.20 <0.125
× (Cr% + Mo%) + 5.22 × V% <2.30 Si: 0.10% or less (including 0%), P: 0.015% or less (0% ), S: limited to 0.015% or less (including 0%), with the balance being Fe and unavoidable impurities, and having excellent delayed fracture resistance and high suitable for fastening in a plastic region. Steel for strength bolts. 4. In% by weight, C: 0.15 to 0.45%, Mn: more than 0.40 to 1.50%, Cr: 0.30 to 2.00%, Mo: 0.10 to 0% 0.35%, V: more than 0.20 to 0.40%, Al: 0.010 to 0.100%, Nb: 0.005 to 0.100%, Ti: 0.005 to 0 .100%, B: 0.0005-0.0050%, and Cr, Mo, V 1.20 <0.125
× (Cr% + Mo%) + 5.22 × V% <2.30 Si: 0.10% or less (including 0%), P: 0.015% or less (0% ), S: limited to 0.015% or less (including 0%), with the balance being Fe and unavoidable impurities, and having excellent delayed fracture resistance and high suitable for fastening in a plastic region. Steel for strength bolts. 5. The high-strength bolt steel according to any one of claims 1 to 4, which is formed into a desired shape, heated to a temperature of A _C3 or more, and then subjected to a quenching treatment at a temperature of 500 to 650 ° C. A method for manufacturing a high-strength bolt having excellent delayed fracture resistance, characterized by tempering, and suitable for fastening in a plastic region.