JPH02267243A - High strength bolt steel having excellent delayed fracture characteristics - Google Patents

Abstract

PURPOSE:To increase limiting diffusible hydrogen in the steel and to improve its delayed fracture resistance by increasing the amounts of Si and Cr in a steel compared with those in a conventional steel. CONSTITUTION:The compsn. of the high strength bolt steel is constituted of, by weight, 0.18 to 0.35% C, >0.50 to 1.50% Si, 0.20 to 0.60% Mn, >1.50 to 3.50% Cr, 0.10 to 0.50% Mo, 0.008 to 0.070% Al and the balance Fe with impurities. The steel is applicable to the manufacture of bolts having excellent delayed fracture characteristics even if its strength is regulated to about 140 to 160kg/mm<2> by heat treatment. When about 0.005 to 0.030% Nb or Ti is furthermore added to the compsn., the steel is provided with high strength and can be fined.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention is a steel applicable to high-strength bolts having a tensile strength of 140 to 160 kgf/mJ and excellent delayed fracture resistance.

(Conventional technology) High-strength bolts (hereinafter referred to as bolts) are used in machines, automatic tunnels, bridges, etc.
It is often used in buildings. Bolt strength is 125kg
It is well known that exceeding f/mA increases the risk of delayed fracture, and the strength of bolts currently used is
The main types are 0kg f/ij or 110kg f/m4.

(Problem to be solved by the invention) However, in recent years, structures have tended to become larger and larger, and in order to increase joint efficiency or reduce weight,
There is a strong demand for higher strength bolts. That's where strength comes in! 25kgf/ml! It is necessary to solve the problem of delayed fracture of bolts exceeding

The delayed fracture of bolts is said to be caused by hydrogen in the bolts. Hydrogen involved in delayed fracture is so-called diffusible hydrogen (hereinafter referred to as diffusible hydrogen), which exists in lattice gaps, dislocations, grain boundaries, etc., and can easily move between structures in steel at room temperature. If higher strength bolts are used, the steel must be resistant to hydrogen, especially diffusible hydrogen.

(Means and actions for solving problems) The inventors have focused on adjusting the chemical composition of steel, particularly Sl.

We were able to solve the above problem by obtaining the knowledge that it is possible to increase the limit diffusible hydrogen that does not lead to delayed fracture (hereinafter referred to as limit diffusible hydrogen) by increasing C. be.

The present invention has been made based on the above findings, and by applying heat treatment, it is possible to reduce the
This relates to a steel for high-strength bolts with excellent delayed fracture resistance, which is characterized by exhibiting a higher limit diffusivity of hydrogen than conventional steels at a high strength of G Okg f / mJ.

The present inventors investigated the influence of alloying elements on delayed fracture resistance and found that an increase in the amount of Sl, especially an increase in Cr, is effective compared to those used in conventional steel for bolts. I discovered that.

That is, the range of alloy components of the steel for bolts of the present invention was determined for the following reasons.

C should be 0 in order to obtain high strength through quenching and tempering.
．． On the other hand, if it exceeds 0.35%, the toughness and delayed fracture resistance will deteriorate and the characteristics of a high-strength bolt cannot be satisfied, so it is set at 0.18 to 0.35%.

Sl is originally necessary for deoxidation, but here it particularly improves the delayed fracture resistance of the steel. However, in order to fully exhibit its effect, it is necessary to exceed 0.50%, but on the other hand, based on the decrease in the toughness of steel due to this increase, the upper limit was set at 1.50%.

Mn is necessary for deoxidation and desulfurization, and 0.20% or more is also necessary to ensure hardenability, but if it exceeds 0.60%, delayed fracture properties will deteriorate, so ~0.80
%.

As mentioned above, Cr is 1,
More than 50% was required. It also has the function of increasing the tempering temperature to obtain a strength of 140 kgf/II+4 or higher, and generally the higher the tempering temperature, the better the delayed fracture properties. However, even if added in a large amount, the effect is saturated, so the upper limit was set at 3.50%.

Mo is a very effective element for increasing the toughness of steel, and the required amount in this case is 0.10% or more. However, since it is an expensive element, the upper limit was set at 0.50% in consideration of its effectiveness.

The lower limit of Ag was set as o, oog% as an effective amount to refine the structure of steel and improve the strength and toughness of steel, but if it is too large, nonmetallic inclusions will increase and the effect will be reduced. The upper limit was set at 0.070%.

To the above-mentioned basic ingredient feed, one or more types of Nb or TI can be added in an amount of 0.005 to 0.030% in order to increase the strength and refine the steel.

The upper limit was determined by the amount at which the added effect was saturated.

(Example) Table 1 shows the chemical composition of the test steel.

A, B, C, D, E, and F are steels for bolts according to the present invention, and G, H, and I are existing steels prepared for comparison. The tensile strength is 140) cg f / o + j ~
Heat treatment (quenching and tempering) was performed with a target of 160 kgf/-. Table 2 shows the heat treatment conditions and tensile properties at this time.

We will investigate how much diffusible hydrogen these steels can tolerate against delayed fracture. That is, the limit amount of hydrogen for 6 steels is determined.

Next, a method for determining the limit amount of hydrogen will be described.

A test piece with a circumferential notch of 2+a+sV on the shaft part was made using the MIO bolt shown in Figure 1, and the two pieces were put together and immersed in a 20% HCj2 solution for 20 minutes to enrich hydrogen. The amount of hydrogen in the specimen is varied by leaving it in the atmosphere and varying the length of time it is left in the atmosphere. thus,
One of them measures hydrogen using a thermal analysis method, and the other one performs delayed fracture using the testing machine shown in Figure 2. In the figure, 1 indicates a test piece, 2 indicates a balance weight, and 3 indicates a fulcrum.

Note that if the amount of hydrogen penetrating is small even after immersing the test piece in a 20% MCI solution for 20 minutes, the amount of hydrogen is adjusted by the immersion time and the concentration of the HCg solution.

Further, the test load in the delayed fracture test was kept constant at 70% of the fracture load of each test piece before being immersed in the HCI solution.

Table 3 shows the relationship between the amount of diffusible hydrogen and the rupture time in the delayed fracture test when the immersion time and standing time were varied.

From the same table, the upper limit diffusible hydrogen amount that does not cause delayed fracture of the 6th steel, ie, the limit diffusible hydrogen amount, is estimated as shown in Table 4.

From this, A, B, C, and D are within the scope of the present invention.

E and F have a higher limit hydrogen content than the comparative materials G, H, and I, indicating that they are less likely to undergo delayed fracture.

Table Steel type symbol Estimated limit diffusible hydrogen amount A 0.62ppm B O, 58ppm CO, 44ppm D 0.50ppm E O, 46ppm F O, 38ppm G (80M440) 0.19ppmH (
SN0M240) 0.10ppm1 (S
N0M439) O, 14ppm

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the shape of the test piece; It is an explanatory view of a destructive test device. Figure 2 is delayed

Claims (1)

[Claims] 1. C: 0.18 to 0.35% by weight, Si: more than 0.50 1.50%, Mn: 0.20 to 0.60%, Cr: more than 1.50 3.50%, Mo: 0.10~0.50%, Al: 0.008~0.070%, residual Fe and impurities. High strength bolt steel with excellent delayed fracture resistance. . 2. High strength with excellent delayed fracture resistance according to claim 1, which contains one or more types of Nb or Ti in a weight ratio of 0.005 to 0.030%, and is composed of residual Fe and impurities. Steel for bolts.