JPS60114551A - High strength bolt steel - Google Patents

Abstract

PURPOSE:To improve the delayed fracture resistance and strength by adding prescribed percentages of C, Si, Mn, P, S, Cr and Mo. CONSTITUTION:This high strength bolt steel consists of, by weight, 0.3-0.5% C, <0.15% Si, 0.1-0.4% Mn, <=0.015% P, <=0.01% S, 0.5-4.5% Cr, 0.1-0.7% Mo and the balance Fe and satisfies an equation Si(%)+Mn(%)+10[P(%)+S(%)] <=0.45%. The steel has superior delayed fracture resistance and 140-160kgf/mm.<2> high strength.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention provides high strength of 140 to 160 kgf and 7 mm2 class, and also has excellent delayed fracture resistance.
The present invention relates to a steel for high-strength bolts that can sufficiently respond to increases in strength and miniaturization of bolts.

(Prior art) In recent years, there has been a strong demand for lighter and smaller mechanical structures, etc., and automobile parts are increasingly being made lighter and smaller in order to reduce fuel consumption and improve driving performance. We are starting to be able to move forward. Therefore, there is an increasing demand for high-strength design specifications for ports that connect various parts. In other words, if parts are made smaller, for example, the bolts must also be made smaller, and in order to ensure the required tightening force, it may be possible to increase the number of bolts used or increase the strength of the bolts. Increasing the number of bolts used to fasten smaller parts tends to be unreasonable, and as a result, it becomes necessary to increase the strength of the bolts.

Conventionally, when increasing the strength of bolts, consideration has often been given to component adjustment and thermal refining, but the tensile strength is l 20
It is known that when the weight exceeds 7mm2, the delayed fracture resistance deteriorates, and therefore the weight of 140 to 160 kgf
/ There are few examples of it being used in the mm2 class.

(Object of the Invention) This invention was made by focusing on the above-mentioned conventional problems, and provides a steel for bolts that has excellent delayed fracture resistance even though it has a high strength of 140 to 160 kgf and 7 mm2 class. The purpose is to

(Structure of the Invention) In accordance with the above objectives, the mechanism of delayed fracture occurrence in high-strength bolts was studied in detail, and the effects of alloying elements and impurity elements were investigated in detail. As a result, it became clear that delayed fracture cracks generally occur with austenite grain boundaries as the starting point and propagation path, and as a result of research based on this, the present invention was completed. The steel for bolts has, in weight percent, C: 0.30% or more and 0.50% or less, St: less than 0.15%, M n
: 0, 10% or more and 0.40% or less, P: 0.015
% or less, S: 0.010% or less, Cr: 0.50% or more and 4.50% or less, MO2 0.10% or more and 0.70% or less, and if necessary V: 0.05% or more and 0. .15% or less, Nb: 0.05% or more and 0.15% or less, Ti: 0.
05% or more and 0.15% or less, and Si (%) + Mn (%) + 10 (P, (%) +
S (%)): 0.45% or less, the remainder substantially consisting of Fe, and it has outstanding delayed fracture resistance even when tempered to a high strength of 140 to 160 kgf/ll 1 m2 class. It is a feature.

Next, the reason for limiting the composition range (weight %) of the high-strength port steel according to the present invention will be explained.

C (carbon): 0.30% or more and 0.50% or less C must be contained in an amount of 0.30% or more in order to ensure high strength of 140 to 160 kgf/mm2 class by heat refining. If too much, the toughness and ductility deteriorate as well as the delayed fracture resistance, so the content was set at 0.50% or less.

Si (silicon): less than 0.15% Si is an element that acts as a deoxidizing agent during melting, but if it is too large, it promotes the segregation of P and promotes grain boundary oxidation, which leads to delayed fracture. Since P serves as a starting point, the content is set to less than 0.15% in order to prevent such segregation of P and grain boundary oxidation.

Mn (Manga): O, 105 or more and 0.40% or less Mn acts as a deoxidizing and desulfurizing agent during melting and is an element that contributes to improving hardenability. In order to obtain a 100% martensitic structure up to the center of the MIO bolt, it is necessary to contain 0.10% or more. However, like St, Mn is an element that promotes segregation of P and promotes oxidation at grain boundaries, so in order to prevent segregation of P and oxidation at grain boundaries, Mn was set at 0.40% or less.

P (phosphorus): 0.015% or less P segregates at the austenite grain boundaries during austenitization, embrittles the grain boundaries, reduces grain boundary strength, and deteriorates delayed fracture resistance, so it should be 0.015% or less. did.

S (sulfur): 0.010% or less S segregates at austenite grain boundaries during austenitization, and also exists as MnS, which deteriorates delayed fracture properties, so it is set to 0.010% or less.

Cr (Chromium): 0.50% or more and 4.50% or less Cr ensures hardenability and has a strength of 140 to 1 at high temperature tempering (approximately 500°C or higher) that ensures toughness and ductility.
0.50 to obtain high strength of 60kgf 7mm2 class
% or more is necessary. However, Cr1l
If the temperature increases, the tempering hardness in the region exceeding about 550°C will decrease rapidly, making it difficult to obtain stable strength.
．． It was set to 50% or less.

Mo (molybdenum): 0.10% or more 20, 70% or less MO depends on the balance with C and Cr, but at about 500°C
Strength 140~l 60 kgf at tempering temperature above
/ It is necessary to contain P at least 0.10% to obtain high strength of class 2, and it also has the effect of preventing grain boundary segregation of P, increasing grain boundary strength, and improving delayed fracture resistance. Therefore, from these points, the content was set at 0.10% or more. However, in this steel, even if the content exceeds 0.70%, the effect does not improve much, and since it is an expensive element, the content was limited to 0.70% or less.

St (%) + Mn (%) +, 10 (P (%) + S (
%)): 0.45% or less St, Mn, P, and S were regulated within the above ranges, but within the above component ranges St (%) + Mn (%) + 10 (P (%)
) + S (%)) If the calculated value is 0.45% or less, sufficient delayed fracture resistance can be obtained even with a high strength of 140 to 160 kgf 7 mm2 class, and the current 130 k
It has been confirmed through various experiments that delayed fracture properties equivalent to JIS 30M440 of gf/mm2 class can be obtained.

■(Vanadium) 20.05% or more - H0015% or less,
(Nb): 0.05% or less 20, 15% or less, T
i (titanium): 0.05% or more” 20. One or more of the following 15% or less: V, Nb, and Ti all form carbonitrides and are effective in refining crystal grains. Since these elements are effective in improving yield strength and toughness and ductility, one or more of these elements may be added in an amount of 0.05% or more as required. However, even if more than necessary is added, the above effects will be saturated, so it is preferable to limit each element to 0.15% or less. Note that it is also possible to partially replace Nb with Ta.

Furthermore, elements such as Cu (copper) and Ni (nickel) must be below JIS standards (for example, Cu: 0.30% or less, N
It is naturally included in the steel for high-strength bolts of the present invention that it is added within the range of (i: 0.25% or less).

(Example) Steel having the chemical composition shown in Table 1 was melted in a 50 kg capacity J' (air induction melting furnace, formed into an ingot, forged and normalized, and then processed into a test piece. At this time, The test piece used was a tensile test piece that complied with the provisions of JIS No. 4,
In addition, L = 20 mm shown in Figure 1 as a delayed fracture test piece,
The dimensions of D=6 mm, d=4 mm, and R=0.1 ml11 were used.

Next, each test piece was heated at 950'CX for 30 minutes and then oil-quenched, and then the tensile strength l was determined for each steel type.
Tempering was performed by heating for 1 hour at the temperature shown in Table 2 to obtain 50 kgf/urn2 soil and 5 kgf/mm2, followed by air cooling, and then a tensile test and a delayed fracture test were conducted on each test piece.

In addition, the delayed fracture test was performed using a bending type accelerated test.
Bending stress was applied while dropping 0.111-HCM to the small diameter part of the test piece shown in the figure, and the relationship between bending stress and rupture time was investigated to create a delayed fracture record.
, -)/static stress (σSB), that is, the 30-hour strength ratio. The results are also shown in Table 2 and FIG. 2. As shown in Table 2 and FIG.
At a high strength of 60 kgF/class 12, the steel of the present invention (I
, If) are comparative steel and normal steel (30M44
Compared to 0), delayed fracture resistance is significantly superior, and strength
The delayed fracture resistance is almost equivalent to that of 30M440 tempered to 130 kgf/mmz class. And V, Nb
It was confirmed that ductility and delayed fracture resistance could be further improved by adding Ti.

Furthermore, when we investigated the grain boundary oxidation status in Invention Steel A and conventional steel, we found that in Invention VT4A, there was significantly less grain boundary oxidation as shown in Figure 3, whereas in ordinary steel, grain boundary oxidation was considerably low. It was observed that oxidation occurred.

(Effects of the Invention) As explained above, the high-strength bolt steel of the present invention has, in weight percent, C': 0, 30% or more and 0.50% or less, St:O, less than 15%, Mn: 0.10% or more 0.
40% or less, P: 0.015% or less, S: 0.010%
Below, Cr: 0.50% or more and 4.50% or less, Mo
: 0, 10% or more and 0.70% or less, and as necessary V: 0.05% or more and 0.15% or less, Nb: 0.0
5% or more and 0.15% or less, Ti: 0.05% or more and 0.1
One or more of the following 5% or less, Dekatsu S: (%
)+Mn(%)+10CP(%)+S(%)): 0.4
5% or less, the remainder being substantially composed of Fe,
Even when tempered to a high strength of 140 to 160 kgf/ml, class 112, it has outstanding delayed fracture resistance, and is fully compatible with increasing the strength of bolts and downsizing them with the same strength. For example, increasing the strength of automobile parts,
It has an extremely excellent effect that it can be suitably used as a material for high-strength bolts that are required as miniaturization progresses.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a delayed fracture test piece used in an example of the present invention, and FIG. 2 is an illustration of a specimen of Si and Mn. A graph showing the results of investigating the influence of the amounts of P and S on delayed fracture characteristics. Figure 3 is a metallographic micrograph (500x) showing the grain boundary oxidation status of invention steel A. Figure 4 is a graph showing the state of grain boundary oxidation of inventive steel A. This is a metallographic micrograph (500x magnification) showing the state of grain boundary oxidation. Patent Applicant Daido Steel Co., Ltd. Representative Patent Attorney Yutaka Oshio 851 Ward Figure 2

Claims (1)

[Claims] 5(1) Weight%: C: 0.30% 1 or more and 0.50% or less, Si: less than 0.15%, Mn: 0.10% or more and 0
．． 40% or less, P: 0.015% or less, S: 0.010
% or less, Cr: 0.50% or more and 4.50% or less, Mo:
0.10% or more and 0.701O% or less, and Si (%)
+Mn (%) + 10 (P (%) + S (%)): 0.45
% or less, the balance being substantially made of Fe, and has excellent delayed fracture resistance of 140 to 160 kgf/am.
Grade 2 high strength bolt steel. 15 (2) In weight%, C: 0.30% or more 0.50%
Below, Si: O, less than 15%, Mn: 0.10% or more and 0.40% or less, P: 0.015% or less, S: 0.01
0% or less, Cr: 0.50% or more and 4.50% or less, Mo
: 0.10% or more and 0.7020% or less, and V: 0
．． 0.05% or more and 0.15% or less, Nb: 0.05% or more and 0
．． 15% or less, Ti: one or more of 0.05% to 0.15%, and St (%) + Mn
(%) + 10 (P (%) + S (%)): 0.45% or less, the balance is essentially Fe, and has excellent strength with delayed fracture properties of 140 to 160 kgf /++++
N2 class high strength bolt steel.