JPS61117248A - Steel for high-strength bolt - Google Patents

Abstract

PURPOSE:To obtain the steel for the high-strength bolt of 120-140kgf/mm<2> grade having superior resistance to delayed fracture, by lowering the quantity of Si and Mn each having a strong affinity for oxygen in order to inhibit e oxidation caused by heating at the high temp. at austenitizing, and by reducing intergranular oxidation. CONSTITUTION:The steel or a bolt consists of, by weight, 0.3-0.5% C, <0.1% Si, 0.1-0.5% Mn, <0.015% P, <0.01% S, 0.3-1% Cr. 0.1-0.5% Mo, and the balance Fe with inevitable impurities, with which, if necessary, 1 or >=2 kinds among 0.05-0.15% V, 0.05-0.15% Nb, and 0.05-0.15% Ti may further be combined.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a steel for high-strength bolts having a strength of 120 to 140 kgF/smz class and having a relatively low alloy and excellent delayed fracture resistance.

(Prior Art) In recent years, as bridges, buildings, mechanical structures, etc. have become larger, high-strength bolts have come to be used in large quantities. Bolts are generally required to have excellent yield ratio, impact resistance, delayed fracture strength, etc., but in particular, steel for high-strength bolts has a strength of 120 kgF/1.
It is known that when the value exceeds a+m2, delayed fracture resistance deteriorates significantly.

It is essential to have stable delayed fracture resistance.

Delayed fracture is a hydrogen embrittlement phenomenon that occurs due to the intrusion and diffusion of hydrogen, mainly when used in a humid environment, and is a phenomenon in which a member subjected to a static load suddenly breaks brittle. It is known that in tempered martensitic steel having a tensile strength of 120l20-14O/weight2, cracks occur and propagate along prior austenite grain boundaries. This is said to occur when one grain boundary becomes embrittled due to segregation of impurities such as P and S or precipitation of carbides, and embrittlement due to hydrogen penetrating from the usage environment is superimposed.

Therefore, from this point of view, attempts have been made to add various alloying elements or improve the manufacturing method in order to improve the delayed fracture resistance.

(Object of the invention) This invention was made against this background,
Although it is a low alloy, it has a strength of 120 to 140 kgf/m
With the aim of obtaining a steel for high-strength bolts that has M2 class high strength and excellent delayed fracture resistance, we have carried out various studies, and as a result of a more detailed investigation of practical bolts, we have found that ordinary quenching and heating ( It was found that a small amount of grain boundary oxidation occurred when the temperature was normally 830 to 880°C (atmospheric atmosphere with only the carbon potential adjusted). Although this is slight compared to the grain boundary oxidation that has been reported in the past during carburization of case hardened steel, delayed fracture occurs from the surface to the prior austenite grain boundaries, which deteriorates delayed fracture resistance. We found that this is one of the important factors.

(Structure of the Invention) This invention was made based on the above-mentioned knowledge, and in order to prevent oxidation due to high temperature heating during austenitization, Si and Mn, which have a strong affinity with oxygen, are kept particularly low, and grain boundary oxidation is prevented. This is intended to improve delayed fracture resistance.

That is, the steel for high-strength bolts according to the present invention contains 1% by weight.
So, C: 0.3 to 0.5%, Si: 0.1% or less, Mn
: 0.1 to 0.5%, P: 0.015% or less, S: 0.
010% or less, Cr: 0.3-1.5%, Mo: 0.1
~0.5%, and if necessary, V:0.05~0.
15%, Nb: 0.05-0.15%.

Made of one or more of Ti: 0.05-0.15%, balance Fe and impurities, 120-140 kgf/m class 2 high-strength bolt steel with excellent delayed fracture resistance. It is characterized by certain things.

The reason for limiting the composition range (weight %) of the steel for high-strength bolts according to the present invention will be explained below.

C: 0.3 to 0.5% C must be contained in an amount of 0.3% or more in order to obtain the required strength through heat treatment, but if it is contained in an amount exceeding 0.5%, toughness and ductility may deteriorate. However, since the delayed fracture properties may deteriorate, the content was set in the range of 0.3 to 0.5%.

Si: 0.1% or less Si is an element that promotes grain boundary oxidation due to high temperature heating during austenitization, and can become a starting point for delayed fracture, so it deteriorates delayed fracture resistance. Therefore, it is desirable that the amount of Si be as low as possible, particularly preferably 0.05% or less, but here the upper limit is set to 0.1%.

Mn: 0.1 to 0.5% Mn is an element that is effective as a deoxidizing agent during melting and also contributes to improving hardenability, and for this purpose, it is added in an amount of 0.1% or more. However, since Mn promotes grain boundary oxidation together with Si and deteriorates delayed fracture resistance, from this knowledge, the lower the content, the more preferable it is, and in particular, it is more preferable to keep it at 0.3% or less. The upper limit was set at 0.5%.

P: O, 0.015% or less P causes segregation at austenite grain boundaries due to high temperature heating during austenitization, embrittles one grain boundary, and deteriorates delayed fracture properties, so P is set to 0.015% or less.

S: 0.010% or less Similar to P, S causes segregation at austenite grain boundaries due to high 7μ heating during austenitization, embrittles grain boundaries, deteriorates delayed fracture resistance, and forms M n S. Since this causes deterioration of delayed fracture properties, the content was set to 0.0% or less.

Cr: 0.3-1.5% Cr is an element that contributes to improving hardenability, so it is best to adjust the amount added depending on the dimensions of the bolt, etc. This will improve the hardenability of the bolt. secure. Further, by adding Cr, it is possible to ensure a tempering temperature of 400° C. or higher and avoid grain boundary precipitation regions of carbides. Therefore, from the above point of view, the Cr content was set to 0.3% or more. However, the addition of Cr increases the strength from 120 to 140 kgf/w.
Dimensions for mz class high strength bolts (for example, up to 1
2) Adding up to 1.5% of S) can sufficiently improve hardenability, but adding too much promotes grain boundary oxidation and deteriorates delayed fracture resistance, similar to Si and Mn. Therefore, the content was set in the range of 0.3 to 1.5%.

Mo: 0.1 to 0.5% MO is an element that contributes to improving hardenability, as well as refining crystal grains and improving the strength of austenite grain boundaries. The content was set to 0.1% or more. However, even if a large amount is added, the effect is saturated, so the upper limit was set at 0.5%.

V: 0.05-0.15%, Nb: 0.05-0.15
%, Ti: type of work from 0.05 to 0.15% or 2
V, Nb, and Ti are all elements that form carbides, are effective in refining crystal grains, and contribute to improving delayed fracture resistance. It is also good to add two or more types at 0.05% or more each.
However, even if it is added in an amount exceeding 0.15%, the effect is not significantly improved, so each content is set at 0.15% or less.

(Example) Steels A-F and comparative steel (SCM
440), then ingots, forged into a round bar with a diameter of 20 x 1φ, and then heated at 900℃
After hr holding, air-cooled normalization was performed.Next, after processing test pieces from each normalized material, each test piece was
After holding at 0°C for 30 minutes, quenching was performed under oil cooling conditions, and then the second
After holding at the specified temperature Xi time shown in the table, each was tempered under air cooling conditions, and the tensile strength of each was 120 to 140K.
It was tempered to become gf/■zen 2.

Thereafter, the tensile properties, delayed fracture strength, and depth of the intergranular oxidation layer of each specimen were investigated. At this time, a JIS No. 4 test piece was used for the tensile test.

The results are shown in Table 2. Also, the delayed fracture test was performed on the test piece shown in Figure 1 (vertical t = 20 + u, d, = 6 m layer, d
2 = 4as, R = 0, 1mm), and a cantilever bending load was applied. Also, the test environment is 0,
The sample was prepared as IN-80 and was dropped into the notch of the test piece.

The delayed fracture characteristics of each test material were expressed as the ratio of the strength after 30 hours of the delayed fracture test to the static bending stress, and the delayed fracture strength ratio σ30hr/σSR. The results are shown in Table 2 and FIG.

2,/7,-'' As shown in Table 2, the steel of the present invention has a tensile strength of 120 to 1
When tempered to 40 kgf/mm2, the steel exhibits good elongation and reduction of area, and in particular, the 30-hour strength ratio is higher than that of the comparative steel. Furthermore, it is clear that the depth of one grain boundary oxidation layer is also quite small.

(Effects of the Invention) As explained above, the steel for high-strength bolts according to the present invention has C: 0.3% to 0.5% and Si: 0% by weight.
．． 1% or less, Mn: 0.1-0.5%, P: 0.015
% or less, S: 0.010% or less, Cr: 0.3 to 1.5
%.

Mo:O, 1-0.5%, and as needed.

V: 0.05-0.15%, Nb: 0.05-0.15
%, Ti: 1 or 2 of 0.05 to 0.15%
Since it is made of at least one species, the balance is Fe and impurities, it has a strength of 120 to 140 kg4/m even though it is a low alloy.
This is a steel for high-strength bolts that has a high strength of m2, has good toughness and ductility, and has extremely good delayed fracture resistance.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the test piece used in the delayed fracture test, Figure 2
The figure is a graph showing test results of delayed fracture strength ratio.

Claims (1)

[Claims] (1) In weight %, C: 0.3% to 0.5%, Si: 0.
1% or less, Mn: 0.1-0.5%, P: 0.015%
Below, S: 0.010% or less, Cr: 0.3 to 1.5%
, Mo: 0.1-0.5%, balance Fe and impurities. 120-1 with excellent delayed fracture resistance.
40kgf/mm^2 class high strength steel for bolts. (2) In weight%, C: 0.3-0.5%, Si: 0.1
% or less, Mn: 0.1 to 0.5%, P: 0.015% or less, S: 0.010% or less, Cr: 0.3 to 1.5%,
Mo: 0.1-0.5%, and V: 0.05-0.1
5%, Nb: 0.05 to 0.15%, Ti: 0.05 to 0.15%, and the balance is Fe and impurities. Excellent 120-140kgf/mm^2 class high strength steel for bolts.