JPH08120408A - High strength bolt steel excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE: To produce a high strength bolt steel having >=140kgf/mm<2> tensile strength and excellent in delayed fracture resistance. CONSTITUTION: This steel has a basic composition containing 0.20-0.50% C, >0.30-1.50% Si, 0.10-<O.50% Mn, <=0.020% P, <=0.020% S, >1.50-3.5O% Cr, 0.20-0.50% Mo, >0.20-0.5O% V, 0.010-0.050% Al, and <0.0080% N. If necessary, 0.020-0.050% Ti and/or 0.020-0.050% Nb and further <0.0005% B are incorporated into this basic composition. By this method, even in the case of steels having tensile strength exceeding 140kgf/mm<2> , delayed fracture can be inhibited and the strength of a structure can be increased, and joint efficiency can be improved.

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 having a tensile strength of 140 kgf / mm ² or more, which is excellent in delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength bolts are widely used in automobiles, civil engineering, and construction. Conventionally, there is a strong demand for higher strength bolts for the purpose of simplifying construction and reducing weight by improving joint efficiency.

However, it is well known that when the tensile strength exceeds 125 kgf / mm ² , the risk of delayed fracture increases, and for example, the tensile strength of high-strength bolts for friction welding currently recognized by JIS standard is: At present, the upper limit is 110 kgf / mm ² class.

Various constituent systems have been proposed for improving delayed fracture of high strength steel for bolts. For example, JP-A-1-9
Japanese Patent No. 6354 discloses a steel containing increased amounts of Si and Cr. This is intended to suppress the intergranular embrittlement associated with tempering and improve delayed fracture by designing a component that ensures high strength even at high temperature tempering of 450 ° C. or higher. In addition, Japanese Patent Laid-Open No. 5-
In 171356, in addition to increasing the amounts of Si and Cr, V
A steel in which delayed fracture is improved by strengthening the grain boundary by increasing the amount of B and adding a small amount of B is disclosed.

[0005]

However, in the steel having a tensile strength of more than 140 kgf / mm ² , the delayed fracture has not been sufficiently suppressed, which is an obstacle to increasing the strength of the bolt.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-strength bolt steel having a tensile strength of 140 kgf / mm ² or more and excellent delayed fracture resistance. And

[0007]

With respect to the above problems, the present inventors have studied the chemical composition of steel and found that Ti, Al,
The amount of solid solution N is reduced by adjusting the amount of N, and in addition, grain boundary segregation of N, which causes grain boundary embrittlement due to grain refinement due to an increase in V, can be suppressed. The grain boundary precipitation of B nitride, which causes the grain boundary embrittlement, can be suppressed by utilizing the strengthening of the grain boundary of B, and the increase of V causes V carbide and V nitride to precipitate in the grain during the tempering process. The ability to trap hydrogen to suppress the accumulation of diffusible hydrogen at the grain boundaries, and increase Si, Cr and V
The tempering temperature for obtaining a strength of more than kgf / mm ² is 4
It is possible to perform the treatment in a high temperature range of 50 ° C. or higher, strengthen the grain boundaries, and by the synergistic effect of suppressing grain boundary embrittlement and detoxifying hydrogen, it is possible to dramatically improve delayed fracture characteristics. I got the knowledge.

The present invention is constructed on the basis of these new findings, and its gist is as follows.

The first aspect of the present invention is to provide C: 0.2 by weight%.
0 to 0.50%, Si: more than 0.30 to 1.50%, M
n: 0.10 to less than 0.50%, P: 0.020% or less, S: 0.020% or less, Cr: more than 1.50 to 3.
50%, Mo: 0.20 to 0.50%, V: 0.20
Super ~ 0.50%, Al: 0.010 to 0.050%, N
: High strength bolt steel having excellent delayed fracture resistance, characterized by containing less than 0.0080% and the balance being Fe and inevitable impurities.

The second aspect of the present invention is, in% by weight, C: 0.2.
0 to 0.50%, Si: more than 0.30 to 1.50%, M
n: 0.10 to less than 0.50%, P: 0.020% or less, S: 0.020% or less, Cr: more than 1.50 to 3.
50%, Mo: 0.20 to 0.50%, V: 0.20
Super ~ 0.50%, Al: 0.010 to 0.050%, N
: 0.0080% or less, and as a steel component in weight%, Ti: 0.020 to 0.050%, Nb: 0.
A high-strength bolt steel having excellent delayed fracture resistance, characterized by containing 020 to 0.050% of one or two kinds, and the balance being Fe and unavoidable impurities.

A third aspect of the present invention is, in% by weight, C: 0.2.
0 to 0.50%, Si: more than 0.30 to 1.50%, M
n: 0.10 to less than 0.50%, P: 0.020% or less, S: 0.020% or less, Cr: more than 1.50 to 3.
50%, Mo: 0.20 to 0.50%, V: 0.20
Super ~ 0.50%, Al: 0.010 to 0.050%, B
: Less than 0.0005%, N: less than 0.0080%, the balance being Fe and unavoidable impurities, and a steel for high strength bolts excellent in delayed fracture resistance.

In a fourth aspect of the invention, the weight percentage is C: 0.2.
0 to 0.50%, Si: more than 0.30 to 1.50%, M
n: 0.10 to less than 0.50%, P: 0.020% or less, S: 0.020% or less, Cr: more than 1.50 to 3.
50%, Mo: 0.20 to 0.50%, V: 0.20
Super ~ 0.50%, Al: 0.010 to 0.050%, B
: 0.0005% or less, N: less than 0.0080%, and further, as a steel component, by weight%, Ti: 0.020 to.
0.055%, Nb: 0.020 to 0.050% of 1 type or 2 types, the balance consisting of Fe and unavoidable impurities, for high strength bolts excellent in delayed fracture resistance. It is steel.

[0013]

In the following, the reasons for limiting the alloy components and heat treatment conditions in the present invention will be described in detail.

C: 0.20% or more is necessary to obtain high strength by quenching and tempering, but if too much, toughness is deteriorated and delayed fracture property is also deteriorated.
It was set to 0.50% or less.

Si: Necessary for deoxidation of steel, has resistance to softening by tempering, and 140 at tempering temperature of 450 ° C. or higher.
It is an element effective for obtaining a tensile strength of kgf / mm ² or more, and needs to be more than 0.30%. However, if too much,
In order to cause deterioration of cold workability, it was set to 1.50% or less.

Mn: 0.10% or more is necessary to secure deoxidation and tempering properties of steel, but in order to segregate to the grain boundaries during heating in the austenite region to embrittle the grain boundaries and deteriorate delayed fracture properties. , Less than 0.50%.

P: Effective as a tempering element,
Since it is an element that causes microsegregation during solidification and further segregates at grain boundaries during heating in the austenite region, embrittles the grain boundaries and deteriorates delayed fracture characteristics, the content was made 0.020% or less.

S: An unavoidable impurity, but like P, it is an element that segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates the delayed fracture characteristics.
It was set to 20% or less.

Cr: An element which is necessary for obtaining the hardenability of steel and has temper softening resistance, and is effective for obtaining a tensile strength of 140 kgf / mm ² or more at a tempering temperature of 450 ° C. or more. > 50% is required. However, if it is too large, the toughness and the cold workability are deteriorated, so the content is set to 3.50% or less.

Mo: an element which is necessary for obtaining the hardenability of steel and has temper softening resistance, and is effective for obtaining a tensile strength of 140 kgf / mm ² or more at a tempering temperature of 450 ° C. or higher, and 0 20% or more is required. However, if the amount is too large, the effect is saturated and the cost is increased, so the content was made 1.20% or less.

V is an important element in the present invention,
Ultra-fine crystal grains (preferably Japanese Industrial Standard JIS-G
According to the austenite grain size test method according to No. 0551, grain boundary segregation of N is reduced by grain size 11 or more fine grain) to strengthen the grain boundary, and further V precipitated in the grain.
It is necessary to exceed 0.20% in order to trap hydrogen around carbides and V-nitrides, suppress the accumulation of diffusible hydrogen at grain boundaries, and dramatically improve delayed fracture characteristics. However, if too much,
The content is set to 0.50% or less because coarse precipitates are formed during solidification to deteriorate the toughness and delayed fracture characteristics.

Al: an element effective in deoxidizing steel, and an element that enables precipitation of nitrides to make crystal grains fine and to reduce the amount of solute N, and is required to be 0.010% or more. However, if it is too large, the toughness is deteriorated.
It was set to 050% or less.

N: an element essential for the formation of a nitride, but if it is too much, it segregates at the grain boundaries during austenite heating, or combines with B to precipitate a boron compound (BN) at the grain boundaries,
Since the grain boundary is embrittled and the delayed fracture property is deteriorated, the content is set to less than 0.0080%.

Ti: an element which is added as required to precipitate nitrides or carbides to make crystal grains finer and to reduce solid solution N by forming nitrides, and 0.020% or more Although it is necessary, if it is too much, coarse precipitates are formed during solidification, and the toughness and delayed fracture properties are deteriorated, so the content was made 0.050% or less.

Nb: added as required to precipitate nitrides or carbides to refine the crystal grains, and
It is an element that enables the reduction of solute N by forming a nitride, and is required to be 0.020% or more. However, if it is too much, coarse precipitates are formed during solidification and the toughness and delayed fracture properties are deteriorated. It was set to 0.050% or less.

B: It is added as necessary from the viewpoint of improving the grain boundary strength and the hardenability, but if it is too much, it bonds with N in the steel to precipitate a boron compound (BN) at the grain boundaries. ,
Since the grain boundary is embrittled and the delayed fracture property is deteriorated, the content is set to less than 0.0005%.

[0027]

[Examples] Table 1 shows the chemical composition of the test steel. Symbols 1-7
Is a steel of the present invention, and symbols 8 to 14 are steels of comparative examples. The symbols 8, 9 and 10 are Si, Cr and V, respectively.
Is below the range of the present invention. Symbol 1
Nos. 1 and 12 are examples in which the amount of N exceeds the range of the present invention, and symbols 13 and 14 are examples in which the amount of B exceeds the range of the present invention.

Using these 20 mmφ steel bars, quenching and tempering were carried out with the goal of a tensile strength of 140 kgf / mm ^{2 to} 160 kgf / mm ² . Table 2 shows the heat treatment conditions and the tensile strength at this time.

The bolt steels of the examples of the present invention are all 450 ° C.
140 kgf / mm ² even if tempered in the above high temperature range
The above high strength is obtained. The extent of diffusible hydrogen that these steels can tolerate for delayed fracture, that is, the critical hydrogen content of each steel type, was investigated.

First, a method for obtaining the limit hydrogen amount will be described. Using a M10 bolt 1 shown in FIG. 1, a test piece having a V-shaped annular notch with a notch radius of 0.25 mmR was made on the shaft portion, and two pieces were made into a set, and a maximum of 180 was put in a 25% or 36% HCl solution. The amount of hydrogen in the test piece is changed by immersing it for a minute.

One of these was taken out from the HCl soak,
After leaving it in the air for 30 minutes, measure the amount of hydrogen by thermal analysis method, and for the other one, leave it in the air for 30 minutes after soaking, and then delay it for a maximum of 100 hours with the delayed fracture tester shown in Fig. 2. Perform a destructive test. The test load in the delayed fracture test is 90% of the tensile strength of each material.

When the concentration of HCl and the immersion time were variously changed, the relationship between the diffusible hydrogen amount and the fracture time in the delayed fracture test was obtained, and the upper limit of the diffused hydrogen that did not cause delayed fracture within 100 hours was obtained. Calculate the amount, that is, the limit hydrogen amount.

Table 3 shows the limiting amount of hydrogen obtained according to the above procedure. The steels of the invention examples of the symbols 1 to 7 which were in the composition range of the invention and which were tempered in a high temperature range of 450 ° C. or higher were 2.
While the limit hydrogen amount as high as 00 ppm or more is shown, the steels of symbols 8 to 14 which are comparative examples show only the limit hydrogen amount of 1.00 ppm or less, and it is difficult for the steel of the present invention example to undergo delayed fracture. it is obvious.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

As described above, the high-strength steel for bolts according to the present invention can sufficiently suppress delayed fracture even when the tensile strength of the steel exceeds 140 kgf / mm ² and delays the high-strength steel. The destruction problem can be solved and the strength of the bolt can be increased. Therefore, the efficiency of the joint of the bolt, which is often used for automobiles, civil engineering, construction, etc., can be improved to simplify the construction and reduce the weight. There is a big contribution to improving productivity.

[Brief description of drawings]

FIG. 1 is a side view showing an example of the shape of a test piece used for a delayed fracture test.

FIG. 2 is an explanatory diagram showing an example of a test by a delayed fracture test device.

[Explanation of symbols]

 1 Test piece 2 Balance weight 3 Support point

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hideo Kanizawa 12 Nakamachi, Muroran-shi, Hokkaido Nippon Steel Corporation Muroran Works

Claims (4)

[Claims] 1. By weight%, C: 0.20 to 0.50%, Si: more than 0.30 to 1.
50%, Mn: 0.10 to less than 0.50%, P: 0.
020% or less, S: 0.020% or less, Cr: 1.5
More than 0 to 3.50%, Mo: 0.20 to 0.50%, V
: Over 0.20 to 0.50%, Al: 0.010 to 0.
050%, N: less than 0.0080%, the balance being Fe and unavoidable impurities, and high strength bolt steel having excellent delayed fracture resistance. 2. In weight%, C: 0.20 to 0.50%, Si: more than 0.30 to 1.
50%, Mn: 0.10 to less than 0.50%, P: 0.
020% or less, S: 0.020% or less, Cr: 1.5
More than 0 to 3.50%, Mo: 0.20 to 0.50%, V
: Over 0.20 to 0.50%, Al: 0.010 to 0.
050%, N: less than 0.0080%, and by weight% as a steel component, Ti: 0.020 to 0.050%, Nb: 0.020 to
A steel for high-strength bolts having excellent delayed fracture resistance, characterized by containing 0.050% of one or two kinds, and the balance being Fe and inevitable impurities. 3. In weight%, C: 0.20 to 0.50%, Si: more than 0.30 to 1.
50%, Mn: 0.10 to less than 0.50%, P: 0.
020% or less, S: 0.020% or less, Cr: 1.5
More than 0 to 3.50%, Mo: 0.20 to 0.50%, V
: Over 0.20 to 0.50%, Al: 0.010 to 0.
050%, B: less than 0.0005%, N: 0.00
A steel for high-strength bolts containing less than 80%, the balance being Fe and unavoidable impurities and having excellent delayed fracture resistance. 4. In weight%, C: 0.20 to 0.50%, Si: more than 0.30 to 1.
50%, Mn: 0.10 to less than 0.50%, P: 0.
020% or less, S: 0.020% or less, Cr: 1.5
More than 0 to 3.50%, Mo: 0.20 to 0.50%, V
: Over 0.20 to 0.50%, Al: 0.010 to 0.
050%, B: less than 0.0005%, N: 0.00
It contains less than 80%, and Ti:
0.020 to 0.050%, Nb: 0.020 to 0.0
A high-strength bolt steel having excellent delayed fracture resistance, which is characterized by containing 50% of one or two kinds, and the balance being Fe and inevitable impurities.