JPH07278672A - Manufacture of high strength bolt excellent in delayed crack resistance - Google Patents

Abstract

PURPOSE:To manufacture a high strength bolt excellent in the delayed crack resistance property by forming the bolt of the steel having the composition consisting of C, Si, Mn, Cr, P, S, Al and Fe, and performing a specific heat treatment to increase the strength. CONSTITUTION:A bolt is formed of the steel having the composition consisting of, by weight, 0.15-0.50% C, 0.1-2.0% Si, 0.05-2.0% Mn, 0.1-3.0% Cr, <=0.015% P, <=0.02% S and 0.005-0.1% Al, and one or two or more kinds of Mo, Ni, V, Ti, Nb and B as necessary, and the balance Fe and inevitable impurities. This formed body is heat treated of hardening and tempering. The high speed heating is performed at the heating speed of >=5 deg.C/sec, preferably >=10 deg.C/sec as the tempering condition, and kept at the temperature of >=350 deg.C to obtain the strength of >=120kgf/mm<2>, preferably 130kgf/mm<2>.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving delayed fracture resistance of high strength bolts used in civil engineering / building structures, automobiles, industrial machines and the like.

[0002]

2. Description of the Related Art Higher strength of bolts is required for the purpose of simplifying construction work and reducing the weight of automobiles. However, the strength of the bolt is 120 kgf / mm ²
It is well known that if it exceeds, it will be destroyed later.

Various techniques have been developed in the past for improving the delayed fracture characteristics of bolts. JP-A-5-17135
No. 6 discloses a fine austenite grain size, and JP-A-5-117811 proposes a delayed fracture property by increasing the tempering temperature in the quenching and tempering process. Furthermore, JP-A-2-2402
36, JP-A-2-240237, JP-A-2
In JP-A-240244, delayed fracture characteristics are improved by improving corrosion resistance.

However, both methods have a strength of 12
It has not been possible to completely suppress the delayed fracture of bolts exceeding 0 kgf / mm ² .

[0005]

SUMMARY OF THE INVENTION In view of the above situation, the present invention has an object to drastically improve the delayed fracture characteristics of a bolt having a strength of 120 kgf / mm ² or more.

[0006]

The inventors of the present invention have studied the effect of heat treatment conditions on the delayed fracture characteristics of bolts, and as a result, at the time of quenching and tempering treatment after molding of bolts, the condition for tempering is 5 ° C./sec. Heating at the above heating rate, 3
The present invention has been completed based on the finding that excellent delayed fracture characteristics can be realized when a strength of 120 kgf / mm ² or more is obtained by holding at a temperature of 50 ° C. or more.

[0007] The gist of the matter is C:
0.15-0.50%, Si: 1.0-2.0%, M
n: 0.05 to 2.0%, Cr: 0.1 to 3.0%,
P: 0.015% or less, S: 0.02% or less, Al:
Steel containing 0.01 to 0.1%, or further to the above steel components, Mo: 0.05 to 1.2%, Ni: 0.05 to
2.0%, V: 0.10 to 0.50%, Ti: 0.00
5 to 0.10%, Nb: 0.005 to 0.10%, B:
After performing bolt forming using steel containing one or more of 0.0005 to 0.01%, at the time of heat treatment for quenching and tempering, as a tempering condition, at a heating rate of 5 ° C / sec or more. It is a method for producing a high-strength bolt excellent in delayed fracture resistance, which is characterized by heating and holding at a temperature of 350 ° C. or higher to give a strength of 120 kgf / mm ² or higher.

[0008]

[Function] 120kg manufactured by changing various heat treatment conditions
As a result of conducting an exposure test of bolts having a strength of f / mm ² or more and analyzing delayed fracture occurrence characteristics, the following was found. That is, in the heat treatment of quenching and tempering performed to obtain the required strength after forming the bolt, the tempering condition is 5
It was found that the delayed fracture property can be remarkably improved by heating at a heating rate of not less than ° C / sec, preferably at a heating rate of not less than 10 ° C / sec and holding at 350 ° C or more.

It was revealed by the metallographic analysis of bolts that this is because the number of carbides on the grain boundaries, which causes deterioration of delayed fracture characteristics, is significantly reduced as the heating rate of tempering is increased. The number of carbides on the grain boundaries is about 1 × 10 ⁸ grains / mm ^{2 in} a normal heat treatment, but can be reduced to 4 × 10 ⁶ grains / mm ² or less by the heat treatment of this patent. Although the present invention has been completed based on these findings, the reason why the alloy components of the bolt and the tempering conditions are determined in the present invention will be described in detail below.

C is required to be 0.15% or more in order to obtain high strength by quenching and tempering, but if it exceeds 0.50%, it is an element that deteriorates toughness and delayed fracture resistance. And 0.50% or less.

Si is an element necessary for deoxidizing steel and enhancing strength, and is added in an amount of 0.1% or more. If it exceeds 2.0%, it causes embrittlement, so the content was made 0.1% to 2.0%.

Mn is required to be 0.05% or more in order to secure the deoxidation and hardenability of steel. If it exceeds 2.0%, it segregates to the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and delays resistance Since it is an element that deteriorates the fracture characteristics, it is set to 0.05% or more and 2.0% or less.

Cr is 0.1 in order to obtain hardenability of steel.
% Or more, but if it is too much, it is an element that causes deterioration of toughness and cold workability, so the content was made 3.0% or less.

Although P is effective as a hardenable element,
Since it is an element that causes micro-segregation during solidification and segregates to grain boundaries during heating in the austenite region to arm the grain boundaries and deteriorate delayed fracture resistance, the content was made 0.015% or less.

Although S is an unavoidable impurity, it is 0.02 because it is an element that segregates to the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates the delayed fracture resistance.
% Or less.

Since Al is an element effective in deoxidizing steel, 0.005% or more is required, but if it exceeds 1.0%, toughness is deteriorated, so 0.005% or more and 1.0% or more. Below.

Further, alloying elements which affect delayed fracture resistance,
When the influence of the tempering temperature was investigated, Mo, Ni, A1,
It has been found that the addition of V, Ti, Nb and B is effective. Therefore, if necessary, one or more of these elements are contained.

Mo is added in order to obtain hardenability of steel and to obtain a tempering softening resistance and a tensile strength of 120 kgf / mm ² or more stably at a tempering temperature of 350 ° C. or more.
It is an element that is necessary in an amount of 05% or more, and if it exceeds 1.2%, its effect is saturated and the cost increases, so the content was made 0.05% or more and 1.2% or less.

Ni is required to be 0.05% or more in order to improve the toughness and the delayed fracture resistance, but if it exceeds 2.0%, the effect is saturated and the cost is rather increased. It was set to 05% or more and 2.0% or less.

V, Ti and Nb are elements effective for improving the delayed fracture resistance by contributing to the refinement of crystal grains and having a high affinity for hydrogen, and suppressing the diffusion and accumulation of hydrogen in steel. Therefore, V: 0.10% or more and Ti:
0.005% or more and Nb: 0.005% or more are required. However, if it is too large, the effect is saturated and rather the toughness is deteriorated, so V: 0.50% or less, Ti: 0.10% or less, and Nb: 0.10% or less, respectively.

B contributes to the improvement of hardenability, but 0.00
If it is less than 05%, the effect is not exerted. On the other hand, if it exceeds 0.01%, carbonitrides of boron are formed to rather deteriorate the delayed fracture property.
%.

The heating rate for tempering is 130 kgf / mm.
^For bolts with a strength of ² or more, the delayed fracture characteristics are remarkably improved at a heating rate of 10 ° C / sec or more, but 120 kgf / mm
^For bolts having a strength of ² or more, the delayed fracture characteristics are improved at a heating rate of 5 ° C / sec or more, so 5 ° C / sec was set.

Regarding the tempering temperature, even a bolt manufactured by using a steel having the composition according to the present invention and tempering at 5 ° C./sec or more can improve delayed fracture characteristics when the tempering temperature is less than 350 ° C. Since it cannot be done, the temperature was set to 350 ° C or higher.

A method for performing such rapid tempering is not particularly specified, but a method of heating in a high-temperature tank such as an induction heating furnace, a salt, a fluidized bed or a furnace set at a temperature of 900 ° C. or higher can be mentioned. .

[0025]

[Examples] Table 1 shows the composition of bolts used in the exposure test, tempering conditions, mechanical properties, and the frequency of delayed fracture in the exposure test for two years in Okinawa. (1) to (19) are in accordance with the steel composition and tempering conditions of the present invention, and (2)
0) to (43) are comparative examples. While (1) to (19) show extremely excellent delayed fracture characteristics, (20) to
In (43), the tempering conditions deviate from the conditions of the present invention even when the steel composition is within the range of the present invention, or conversely, tempering was performed according to the conditions of the present invention, but the steel composition was outside the present invention. Therefore, it is shown that the delayed fracture characteristics are inferior.

[0026]

[Table 1A]

[0027]

[Table 1B]

[0028]

[Table 1C]

[0029]

[Table 1D]

[0030]

According to the present invention, 12 has excellent delayed fracture characteristics.
It is possible to manufacture a bolt having a strength of 0 kgf / mm ² or more.

Claims (2)

[Claims] 1. By weight%, C: 0.15 to 0.50% Si: 0.1 to 2.0% Mn: 0.05 to 2.0% Cr: 0.1 to 3.0% P : 0.015% or less S: 0.02% or less Al: 0.005 to 0.1% is contained, and the balance is Fe and inevitable impurities. After the bolt forming is performed using a steel, the heat treatment of quenching and tempering is performed. In performing the heat treatment, heating is performed at a heating rate of 5 ° C./sec or more as a tempering condition, and 350
A method for producing a high-strength bolt having excellent delayed fracture resistance, which is characterized by giving a strength of 120 kgf / mm ² or more by holding at a temperature of ℃ or more. 2. By weight%, C: 0.15 to 0.50% Si: 0.1 to 2.0% Mn: 0.05 to 2.0% Cr: 0.1 to 3.0% P : 0.015% or less S: 0.02% or less Al: 0.005 to 0.1% in addition to the steel composition, weight%
Mo: 0.05 to 1.2% Ni: 0.05 to 2.0% V: 0.10 to 0.50% Ti: 0.005 to 0.10% Nb: 0.005 to 0. 10% B: 0.0005-0.01% out of 0.0005 to 0.01%, and when performing heat treatment for quenching and tempering after bolt forming using steel with the balance being Fe and unavoidable impurities As a tempering condition, heating is performed at a heating rate of 5 ° C./sec or more, and the temperature is kept at 350 ° C. or more, and 1
A method for producing a high-strength bolt having excellent delayed fracture resistance, which is characterized by giving a strength of 20 kgf / mm ² or more.