JPH05148576A - Steel for bolt excellent in delayed breakdown characteristic - Google Patents

Abstract

PURPOSE:To attain excellent delayed breakdown characteristics even after heat-treating by decreasing content of Si, Mn, S, Cr, Ti and increasing Mo in a steel composition. CONSTITUTION:The composition of steel for bolt consists of 0.30-0.50% C, <=0.15% Mn, <=0.010% S, <=0.70% Cr, 0.8-2.0% Mo, <=0.003% Ti, 0.005-0.070% Al, 0.010-0.025% N and balance Fe and inevitable impurities. The steel has lack of quenching property even if adding Mo in some case because of low content of Cr. In this case, lack of quenching property is made up by adding 0.2-3.0% Ni.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high strength bolt steel having excellent delayed fracture characteristics.

[0002]

2. Description of the Related Art In construction, bridges, automobiles, etc., many bolts are used for fastening parts. Bolt
It is suitable for use in places where welding is not possible or inappropriate, but it is disadvantageous in terms of weight compared to welding, the weight of the above parts is reduced, and there is a strong demand for fuel consumption reduction in automobiles etc. However, it is desired to provide bolts with high strength. Currently, high-strength bolts are stipulated in ^JIS B1051 ^-1991 by strength rank, and are specified as 9.8, 10.9, and 12.9, and the material is generally Cr-M such as SCM435 and SCM440.
o Hardened and tempered steel is the most widely used material.

[0003]

[Problems to be Solved by the Invention] According to the aforementioned JIS standard,
The ones with the highest strength are bolts in the strength category of 12.9 (tensile strength 124 kgf / mm ² or more, hardness Hv 435 or less). However, if the strength is equal to or higher than this, that is, if the tensile strength is 130 kgf / mm ² or more, it is said to be delayed fracture, which causes almost no plastic deformation when used for a long time. The phenomenon that it is likely to occur. Therefore,
It is easy to heat-treat so that the tensile strength exceeds 130 kgf / mm ² using JIS-standard Cr-Mo steel such as SCM440, which is conventionally used, but the delayed fracture phenomenon described above is ignored. Since it becomes so large that it cannot be achieved, it was difficult to reduce the tensile strength by setting it to 130 kgf / mm ² or more.

In order to deal with the above problems, many steel grades have been developed for use at 130 kgf / mm ² or more. However, the heat treatment in actual operation is often performed in an atmosphere using RX gas, etc., and the delayed fracture characteristics in the high-strength region are markedly reduced compared to the heat treatment in a vacuum performed in the research / trial stage. Therefore, an appropriate countermeasure method has not been found, which has been an obstacle to practical use.

The present invention is applied to a conventional atmosphere heat treatment furnace.
An object of the present invention is to provide a steel for high strength bolts which can obtain excellent delayed fracture characteristics even when heat-treated to have a high tensile strength of 130 kgf / mm ² or more.

[0005]

Means for Solving the Problems The present inventor has obtained the following findings as a result of research and investigation into the cause of deterioration of delayed fracture characteristics in a high strength region when a bolt material is heat-treated in an atmosphere heat treatment furnace. Among bolts that were heat-treated in an atmosphere quenching furnace using RX gas, etc., a comparative examination was conducted with those with poor delayed fracture characteristics as good ones. The formation of oxides in
We have found that the delayed fracture characteristics are degraded by this effect. Therefore, if the present inventors can prevent carbon infiltration into the surface layer (hereinafter referred to as light carburization) and grain boundary oxidation,
We thought that delayed fracture characteristics would improve.

In order to prevent light carburization and grain boundary oxidation, a vacuum quenching furnace may be used, but the productivity is poor, the equipment is expensive, the cost is high, and it is not practical. It is difficult to use. Therefore, the present inventors have proceeded with the development of a steel for bolts in which the delayed fracture characteristics are not deteriorated even by the treatment in a normal atmosphere heat treatment furnace.

In order to prevent light carburization, it is considered effective to lower the carbon potential (hereinafter referred to as CP) of the atmosphere gas in the heat treatment furnace, but conversely decarburization and grain boundary oxidation are likely to occur. Therefore, it is necessary to be careful to change the CP value. Normally, the CP value is controlled for each steel type based on the amount of carbon contained in the steel. However, the optimum CP value varies depending on the alloy composition between steel types or lots and the distribution state of carbides.Therefore, in actual operation, control the CP value so that both light carburization and decarburization do not occur. Is difficult Therefore, the present inventors have conducted an investigation on the conditions under which the light carburization can be suppressed without changing the CP value, and have found that the lighter carburization is more likely to occur as the Cr content is higher.

As a result of further study on the relationship between grain boundary oxidation and steel components, Si, Mn, which are oxidizable elements,
It was found that reducing Cr is effective. However, if these elements are reduced, the required hardenability cannot be obtained, and unless the tempering temperature is set low, the tensile strength of 130 kgf / mm ² or more cannot be obtained, the toughness decreases, and rather delayed fracture occurs. The characteristics deteriorate. The steel of the present invention is intended to be used with higher strength than conventional steels, and it is necessary to set a high tempering temperature to obtain the same strength, and it is necessary to consider that the toughness does not decrease even with high strength.

Therefore, the problem was overcome by increasing the amount of Mo to improve the tempering softening resistance, and by securing the necessary hardenability by increasing the amount of Mo and adding an appropriate amount of C. It was also found that the hardenability can be controlled by adding an appropriate amount of Ni as needed without deteriorating the delayed fracture property.

On the other hand, the adverse effects of applying Ti and Nb, which are elements effective for grain refinement, to bolt steels have not been taken into consideration until now.
The presence of carbonitrides of i and Nb decreases the strength at the starting point of fracture during delayed fracture, and Ti and Nb that exist as impurities combine with N to inhibit the formation of AlN and It has been found that the grain refining effect of is adversely affected. Therefore, in the present invention, for Ti that may be contained as an impurity, the upper limit of the content rate is strictly regulated, and Nb is not positively added.

The present invention completed on the basis of the above findings is that C: 0.30 to 0.50% by weight, Si: 0.15% or less, Mn: 0.40% or less, S: 0.010% or less, Cr: 0.70% or less. , Mo: 0.8-2.0%, Ti:
It is a steel for bolts with excellent delayed fracture characteristics, characterized by containing 0.003% or less, Al: 0.005 to 0.070%, N: 0.010 to 0.025%, and the balance Fe and unavoidable impurity elements. In the invention, the first invention steel contains 0.2 to 3.0% of Ni in order to improve the hardenability and enable the production of a large-diameter bolt.

The reasons for limiting the components of the steel for bolts of the present invention will be described below. C: 0.30 to 0.50% C is an essential element for ensuring the required strength. In order to achieve the object of the present invention, it is necessary to obtain a tensile strength of 140 kgf / mm ² or more even at a higher tempering temperature than the conventional steel, and the lower limit was made 0.30%. However, when contained in a large amount, the toughness decreases, grain boundary carbides increase, and conversely the delayed fracture property deteriorates. Therefore, the upper limit was made 0.50%.

Si: 0.15% or less Si is an element effective as a deoxidizing agent, but promotes grain boundary oxidation that occurs during high temperature heating during quenching, and reduces delayed fracture strength. Therefore, the upper limit is set to 0.15% because it is necessary to reduce it as much as possible.

Mn: 0.40% or less Mn is an element added to improve the hardenability in conventional steels, but like Si, promotes grain boundary oxidation that occurs during high-temperature heating during quenching and The solid solution in cementite promotes the growth of the precipitated cementite and reduces the delayed fracture strength, so it is necessary to reduce it as much as possible, and the upper limit was made 0.40%.

S: 0.010% or less S is usually contained as an impurity and present in steel as a non-metallic inclusion such as MnS. Since this non-metallic inclusion deteriorates the delayed fracture property, it is necessary to reduce S as much as possible.
The upper limit was 0.010%.

Cr: 0.70% or less Cr is an element effective in improving hardenability, but as described above, it is also an element that promotes light carburization and grain boundary oxidation and deteriorates delayed fracture characteristics. Therefore, in the present invention in which the improvement of delayed fracture characteristics is of the utmost importance, it is necessary to reduce it as much as possible. However, hardenability is insufficient when Cr is reduced, and in the present invention in which Mn is reduced, it is necessary to secure necessary hardenability by increasing Mo and Ni described later, but these elements are expensive. Therefore, it was decided that the content could be added up to 0.70%. However, it is important not to add Cr more than necessary.

Mo: 0.80 to 2.00% In the present invention in which Mn and Cr are reduced, Mo is an essential element for ensuring the required hardenability. Mo also has the effect of imparting temper softening resistance. In order to fully obtain such effects, the amount should be increased compared to conventional steel,
It is necessary to contain 0.80% or more. However, even if contained in a large amount, the effect will be saturated, and there is a possibility that coarse carbides will be formed and the delayed fracture property will be deteriorated.
It was set to 00%.

Ti: 0.003% or less Ti is an element contained as an impurity in steel, but is not particularly restricted. However, when Ti is present, TiN, which has a large stress concentration effect, precipitates and embrittles the grain boundaries, degrading delayed fracture characteristics. Therefore, it is necessary to reduce it as much as possible, and the upper limit was made 0.003%.

Al: 0.005 to 0.070% Al is effective as a deoxidizing agent, and at the same time nitrides (A
lN) has the effect of refining the crystal grains. Therefore, it is necessary to contain 0.005% or more. However, if a large amount of AlN 3 is contained, the formed AlN becomes coarse and the grain refining effect is rather reduced, so the upper limit was made 0.070%.

N: 0.010 to 0.025% As described above, N is an element that forms a nitride with Al and is effective in refining crystal grains, and is 0.010% or more, preferably 0.
It is necessary to contain 012% or more. However, if a large amount is contained, blowholes are generated during steelmaking and manufacturing becomes difficult, so the upper limit was made 0.025%.

Ni: 0.2 to 3.0% Since the present invention reduces Mn and Cr, the hardenability may be insufficient even if Mo is added. In that case, it is necessary to supplement the hardenability by adding Ni, and the lower limit was made 0.2%. However, even if a large amount is contained, the effect is saturated and the cost increases, so the upper limit was made 3.0%.

[0022]

EXAMPLES Next, the characteristics of the steel of the present invention will be clarified by examples in comparison with comparative steel and conventional steel. Table 1 shows the chemical components of the test materials used in the examples. In Table 1, 1 to
Steel No. 9 is the steel of the present invention, Steel Nos. 1 to 5 are the first inventions, and Steel Nos. 6 to 9 are the steels corresponding to the second invention. Further, 10 to 15 steels are comparative steels which are partially outside the composition range of the present invention, and 16 steels are JIS-SCM440 which is a conventional steel.

[0023]

[Table 1]

The test materials having the components shown in Table 1 were melted in an electric furnace, and slab rolling was used to produce a steel slab. A part of the steel slab was sampled before rolling into a wire rod, and the quenching temperature and The relationship between tempering temperature and tensile strength was investigated. And the tensile strength
The heat treatment conditions of about 145 kgf / mm ² were obtained, and the evaluation described below was performed.

Table 2 shows heat treatment conditions obtained as a result of the investigation. It can be seen that the steel of the present invention has a larger resistance to temper softening and a higher tempering temperature than the conventional steel.

Next, a wire rod having a diameter of 15 mm is manufactured by rolling,
The materials heat-treated under the conditions shown in Table 2 were subjected to a tensile test and a delayed fracture test by the methods described below to evaluate the performance of the steel of the present invention. The test results are also shown in Table 2 above.

For the tensile test, a JIS No. 14 A test piece was prepared from the above wire, heat treated under the above conditions, and then the tensile speed was 1 m.
It was performed under the condition of m / min.

In the delayed fracture test, bolts were manufactured from wire rods, heat-treated, immersed in a 5% HCl solution for 30 minutes to absorb hydrogen, tightened under various stresses, and allowed to stand within 200 hours. The method was carried out by obtaining the ratio of the maximum stress that does not break to the tensile strength (delayed fracture strength ratio). In addition, the heat treatment of the bolt is performed by the widely used RX.
Performed using both an atmosphere heat treatment furnace using gas (CP value was the same as the carbon content) and a vacuum heat treatment furnace.
The effect on the delayed fracture characteristics due to the difference in atmosphere was investigated.

[0029]

[Table 2]

As is clear from Table 2, the comparison steel is 10
When steel Nos. 15 to 15 are compared with steels 1 to 9 which are examples of the steel of the present invention, steel 10 has a low C content, so unless the tempering temperature is lowered, the required tensile strength cannot be obtained and the toughness decreases. 11 to 13 steels have a high content of Si, Mn, and Cr, which are elements that reduce delayed fracture characteristics due to light carburization or grain boundary oxidation, However, the 14th steel has a high S content and the 15th steel has a high Ti content.Therefore, due to the influence of MnS or TiN, Delayed fracture characteristics are inferior.

Further, SCM440, which is a conventional steel, has a smaller resistance to temper softening than the steel of the present invention, and the elements that deteriorate delayed fracture properties due to grain boundary oxidation, non-metallic inclusions, etc. are not reduced, so the steel of the present invention is Compared with, the delayed fracture characteristics are significantly inferior.

On the other hand, the steels 1 to 9 of the present invention are
Elements such as Si, Mn, S, Cr and Ti, which are elements that deteriorate delayed fracture characteristics due to grain boundary oxidation, non-metallic inclusions, etc., are reduced as much as possible, and the resulting decrease in hardenability is delayed fracture. The amount of Mo, which has a small effect on the properties, is compensated by increasing the amount of Mo or adding Ni as necessary, and adding an appropriate amount of C and
By making it possible to set the tempering temperature high by increasing the amount of Mo, excellent delayed fracture characteristics are obtained even when heat treatment is performed in a normal atmosphere heat treatment furnace and a high tensile strength of about 145 kgf / mm ² is obtained. I was able to do it.

[0033]

As described above, unlike the conventional steel, the bolt steel of the present invention can be used under high stress because delayed fracture phenomenon does not easily occur even if the tensile strength is increased. Further, even if heat treatment is performed in a normal atmosphere heat treatment furnace, excellent delayed fracture characteristics can be stably obtained. Therefore, by effectively utilizing this characteristic, many places that cannot be fixed by welding, such as automobiles, can be fixed with smaller bolts, which can greatly contribute to weight saving and energy saving.

Claims (2)

[Claims] 1. C: 0.30 to 0.50%, Si: 0.15% by weight
Below, Mn: 0.40% or less, S: 0.010% or less, Cr: 0.70% or less, M
o: 0.8-2.0%, Ti: 0.003% or less, Al: 0.005-0.070%, N:
A steel for bolts having excellent delayed fracture characteristics, characterized by containing 0.010 to 0.025% and the balance being Fe and inevitable impurity elements. 2. C: 0.30 to 0.50%, Si: 0.15% by weight
Below, Mn: 0.40% or less, S: 0.010% or less, Cr: 0.70% or less, M
o: 0.8-2.0%, Ti: 0.003% or less, Al: 0.005-0.070%, N:
A steel for bolts having excellent delayed fracture characteristics, characterized by containing 0.010 to 0.025% and Ni: 0.2 to 3.0%, and the balance being Fe and inevitable impurity elements.