JPH0468374B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention has a high strength of 140 to 160 Kgf/mm, grade ² , 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 also 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 bolts that fasten various parts. In other words, for example, if the parts are made smaller, the bolts must also be made smaller, and in order to secure the specified tightening force, it is possible to increase the number of bolts used or increase the strength of the bolts, but miniaturization Increasing the number of bolts to fasten these 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 it is known that delayed fracture resistance deteriorates when the tensile strength exceeds 120 Kgf/ ^mm2 . Therefore, the weight is 140~160Kg
There are few examples of f/mm ^2nd class use. (Purpose of the Invention) This invention has been made by focusing on the above-mentioned conventional problems, and has developed a steel for bolts that has a high strength of 140 to 160 Kgf/mm class ² and has excellent delayed fracture resistance. is intended to provide. (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 origin and propagation path, and this invention was completed as a result of research based on this. That is, the steel for high-strength bolts according to the present invention has C: 0.30% or more in weight%.
0.50% or less, Si: less than 0.15%, Mn: 0.10% or more
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 0.70
% 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
One or more of 0.15% or less, and Si
(%) + Mn (%) + 10 (P (%) + S (%)): 0.45%
The remainder is essentially made of Fe, with a strength of 140 to 160 kg.
It is characterized by outstanding delayed fracture resistance even when tempered to a high strength of f/mm ² class. Next, the limiting theorem for the composition range (weight %) of the steel for high-strength bolts according to the present invention will be explained. C (carbon): 0.30% or more and 0.50% or less C has a strength of 140 to 160 Kgf/mm by heat refining.It is necessary to contain 0.30% or more to ensure high strength of class ² , but if it is too much, it will deteriorate the toughness. Since it deteriorates ductility and delayed fracture resistance, it is 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 accelerates grain boundary oxidation, which becomes the starting point of delayed fracture. Therefore, in order to prevent such P segregation and grain boundary oxidation,
It was set to less than 0.15%. Mn (manganese): 0.10% 0.40% or less Mn acts as a deoxidizing and desulfurizing agent during melting and is an element that contributes to improving hardenability. to
0.10% to obtain 100% martensitic structure
It is necessary to contain the above amount. However, Mn
Like Si, P is an element that promotes segregation of P and promotes oxidation at grain boundaries, so it is set to 0.40% or less to prevent segregation of P and oxidation at grain boundaries. P (phosphorus): 0.015% or less P segregates at austenite grain boundaries during austenitization, embrittles the grain boundaries, reduces grain boundary strength, and deteriorates delayed fracture resistance, so it is set to 0.015% or less. S (sulfur): 0.010% or less S segregates at austenite grain boundaries during austenitization, and also exists as MnS, which deteriorates delayed fracture resistance, so the content was set to 0.010% or less. Cr (Chromium): 0.50% to 4.50% Cr not only ensures hardenability but also increases strength during high-temperature tempering (approximately 500°C or higher) that ensures toughness and ductility.
140~160Kgf/mm 0.50% to obtain class ² high strength
It is necessary to contain the above amount. However, Cr
If the amount increases, the tempering hardness in the region exceeding about 550°C will decrease rapidly, making it difficult to obtain stable strength, so it was set to 4.50% or less. Mo (molybdenum): 0.10% to 0.70% Mo depends on the balance with C and Cr, but about
Strength 140~160Kgf/at tempering temperature of 500℃ or higher
It is necessary to contain P at least 0.10% to obtain high strength of mm ² class, and it also prevents grain boundary segregation of P.
Since it has the effect of increasing grain boundary strength and improving delayed fracture resistance, from these points of view it is set to 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 set at 0.70% or less. Si (%) + Mn (%) + 10 (P (%) + S (%)) 0.5%
Below, Si, Mn, P, and S are regulated within the ranges mentioned above.
In the above component range, Si (%) + Mn (%) + 10 (P
If the value calculated by (%) + S (%)) is 0.45% or less, sufficient delayed fracture resistance can be obtained even with a high strength of 140 to 160 Kgf/mm class ² , and the current 130 Kgf/mm
It was confirmed through various experiments that delayed fracture resistance equivalent to mm ² class JIS SCM440 can be obtained. V (vanadium): 0.05% to 0.15%, Nb (niobium): 0.05% to 0.15%, Ti (titanium): 0.05
% to 0.15% V, Nb, and Ti all form carbonitrides, are effective in refining crystal grains, and are effective elements in improving yield strength and toughness and ductility. Therefore, if necessary, add one or more of these elements to 0.05 for each element.
% or more may be added. 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,
Naturally, the addition of Ni within the range of 0.25% or less is also included in the high-strength bolt steel of the present invention. (Example) Steel having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace with a capacity of 50 kg, then 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, and a delayed fracture test piece was used as shown in Figure 1: L=20 mm, D=6 mm, d=4 mm, R=0.1.
A material with dimensions of mm was used. Next, each test piece was heated at 950°C for 30 minutes and then quenched with oil cooling, and then heated to the temperature shown in Table 2 to obtain a tensile strength of 150Kgf/mm ² ±5Kgf/mm ² for each steel type. Tempering was performed by heating for 1 hour and then air cooling, and then a tensile test and a delayed fracture test were performed on each test piece. The delayed fracture test was performed by an accelerated bending test, and 0.1N-HCl was applied to the small diameter part of the test piece shown in Figure 1.
Applying bending stress while dropping 30 minutes, we investigated the relationship between bending stress and rupture time to create a delayed fracture curve.
Evaluation was made using the value of time strength (σ _30hr )/static bending stress (σSB), that is, the 30 hour strength ratio. The results are also shown in Table 2 and FIG.

【table】

[Table] As shown in Table 2 and Figure 2 above, strength 140
In terms of high strength of ~160Kgf/mm ^2nd grade, the steels of the present invention (,) are significantly superior in delayed fracture resistance compared to comparison steel and conventional steel (SCM440), and are tempered to a strength of 130Kgf/mm ^2nd grade. SCM440
This is almost equivalent to the delayed fracture resistance of It was also confirmed that ductility and delayed fracture resistance can be further improved by adding V, Nb, and Ti. Furthermore, when we investigated the grain boundary oxidation status in inventive steel A and conventional steel L, we found that in inventive steel A, the
As shown in the figure, there was significantly less grain boundary oxidation, whereas in ordinary steel L, it was recognized that grain boundary oxidation occurred considerably. (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, 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 0.70% or less, and as required 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
0.15% or less of one or more of the following, and Si
(%) + Mn (%) + 10 (P (%) + S (%)): 0.45%
Hereinafter, since the remainder essentially consists of Fe,
Strength: 140-160Kgf/mm Even when tempered to a high strength of class ² , 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, it has a very excellent effect that it can be suitably used as a material for high-strength bolts, which are required as automobile parts become stronger and smaller.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of a delayed fracture test piece used in an example of this invention, and Fig. 2 is an explanatory diagram of a delayed fracture test piece used in an example of this invention.
A graph showing the results of investigating the influence of S content on delayed fracture properties. Figure 3 is a metallographic micrograph (500x) showing the grain boundary oxidation status of invention steel A. Figure 4 shows grain boundaries of conventional steel L. This is a metallographic micrograph (500x) showing the state of field oxidation.

Claims (1)

[Claims] 1% by weight, C: 0.30% 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
4.50% or less, Mo: 0.10% or more and 0.70% or less, big
Si (%) + Mn (%) + 10 (P (%) + S (%)): 0.45
%
The following is a high-strength steel for bolts having a strength of 140 to 160 Kgf/mm ² and having excellent delayed fracture resistance, the remainder being substantially composed of Fe. 2 In weight%, C: 0.30% 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
4.50% or less, Mo: 0.10% or more and 0.70% or less, and V: 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 Si (%) + Mn (%) + 10 (P
(%) + S (%)): 0.45% or less, the balance being substantially Fe, and a 140-160 Kgf/mm ² class high-strength bolt steel with excellent delayed fracture resistance.