Seamless Steel Pipe

The present invention relates to a seamless steel pipe.

The automotive industry has been actively introducing safety-oriented equipment. In particular, airbag systems have been installed, which inflate an airbag with gas or the like between an occupant and a steering wheel, an instrument panel, or the like at the time of a collision before the occupant impacts these objects, so as to absorb the kinetic energy of the occupant, thus reducing injuries of the occupant. Although airbag systems of a type that uses an explosive chemical have been adopted to date, a system that uses high-pressure fill gas has been developed from the viewpoint of environmental recyclability, and the system is increasingly applied.

In the system, gas or the like to blow into an airbag at the time of a collision is always kept at high pressure, and at the time of a collision, the gas blows all at once. Accordingly, a stress is to be loaded to a pipe used for a high-pressure gas accumulator at a high strain rate in an extremely short time. Therefore, a pipe to be used for the accumulator is required to have excellent strength and resistance to burst.

Recently, there are increasing demands for weight reduction of automobiles. From this viewpoint, there is also a demand for a decreased wall thickness and weight of a pipe for an onboard airbag. To keep a high bursting pressure even in a thin-wall airbag, accumulators produced from high-strength seamless steel pipes having a tensile strength of 900 MPa or more or even 1000 MPa or more have become used in airbag systems.

Further, an accumulator is required to have excellent low-temperature toughness so as not to cause brittle fracture of the accumulator at the time of a collision, leading to a secondary accident, for example in cold regions.

In view of these circumstances, for example, Patent Document 1 discloses a seamless steel pipe for an airbag accumulator that has a tensile strength of 850 MPa or more and resistance to burst at −20° C. and can be produced only by normalizing heat treatment, without quenching and tempering.

Patent Document 2 discloses a seamless steel pipe for an airbag system having a tensile strength of 1000 MPa or more that is subjected to cold working followed by quenching+tempering and has excellent low-temperature resistance to burst when used as an airbag accumulator component with a shrunk portion.

Patent Document 3 discloses a process for producing a pipe for a high-strength, high-toughness airbag that enables simplification of a cold draw step and reduction in alloy cost.

With the techniques described in Patent Documents 1 to 3, a pipe for an airbag having high strength and excellent low-temperature toughness can be provided. However, because of a further request for further weight reduction in recent years, there is a demand for a seamless steel pipe for an airbag having a tensile strength of 1200 MPa or more.

The present inventors thus conducted studies about a method for increasing strength while maintaining low-temperature toughness and found that simply increasing strength of a pipe may result in significant decrease in hydrogen embrittlement resistance properties of the pipe. To maintain higher reliability of a pipe for an airbag, restraint of embrittlement by hydrogen entering a pipe during a production step and in a usage environment is required even when high strength is given to the pipe.

An objective of the present invention is to provide a seamless steel pipe that has high strength and excellent low-temperature toughness and further has excellent hydrogen embrittlement resistance properties.

The present invention is made to solve the above problems and has a gist of the following seamless steel pipe.

(1) A seamless steel pipe having a chemical composition consisting of, in mass %:

(2) The seamless steel pipe according to (1), wherein the chemical composition contains one or more elements selected from, in mass %:

According to the present invention, a seamless steel pipe that has high strength and excellent low-temperature toughness and further has excellent hydrogen embrittlement resistance properties can be provided.

The present inventors conducted diligent studies about a method for increasing strength of a seamless steel pipe with low-temperature toughness of the seamless steel pipe being maintained and further keeping hydrogen embrittlement resistance properties. As a result, the present inventors obtained the following findings.

(a) To achieve increase in strength of a seamless steel pipe, contents of elements that enhance hardenability need to be increased. In particular, keeping sufficient contents of C, Mo, and Cris effective. From such a viewpoint, Formula (i) shown below has to be satisfied.

(b) Mn is also an element that enhances hardenability. However, Mn excessively contained segregates in grain boundaries to degrade low-temperature toughness. In addition to Mn, P is also an element that segregates in grain boundaries to degrade low-temperature toughness. In contrast, N precipitates in the form of nitrides, and if a content of N is excessive, the number of nitrides is increased to degrade low-temperature toughness.

(c) Here, a degree of decrease in low-temperature toughness due to grain-boundary segregation varies based on a prior-austenite grain size number. For this reason, the present inventors evaluated an influence of contents of Mn, P, and N and the prior-austenite grain size number GN on low-temperature toughness and consequently found that excellent low-temperature toughness can be maintained by adjusting the content of each element within the specified range and satisfying Formula (ii) shown below.

(d) If the content of Mn is excessive, a diffusion velocity of hydrogen is decreased, which causes not only localized concentration of hydrogen but also production of MnS, leading to degradation in hydrogen embrittlement resistance properties. In addition, P segregates in grain boundaries to degrade hydrogen embrittlement resistance properties. In contrast, Ca has the effect of restraining the production of MnS and thus enhances hydrogen embrittlement resistance properties.

(e) Studies by the present inventors revealed that a degree of degradation in hydrogen embrittlement resistance properties varies based on a prior-austenite grain size number as well. The present inventors evaluated an influence of contents of Mn, P, and Ca and the prior-austenite grain size number GN on hydrogen embrittlement resistance properties and consequently found that excellent hydrogen embrittlement resistance properties can be obtained by adjusting the content of each element within the specified range and satisfying Formula (iii) shown below.

(f) Furthermore, in order to improve hydrogen embrittlement resistance, it is necessary to perform preheating in a tempering process. The mechanism by which the hydrogen embrittlement resistance is improved by preheating has not been clarified, but it is considered that this is because the temperature distribution in the thickness direction is eliminated and the metal structure becomes uniform.

(g) Cu, Ni, Cr and Mo are elements that enhance hardenability, as with Mn. On the other hand, Ti and Nb are elements that have the effect of strongly pinning grain boundaries. In the present invention, in order to achieve both strength and low-temperature toughness, it is necessary to utilize the effects of all these elements, and it is necessary to contain all elements in a well-balanced manner at a predetermined content or more.

The present invention has been made based on the above findings. Requirements of the present invention will be described below in detail.

Reasons for limiting a chemical composition of a seamless steel pipe according to an embodiment of the present invention are as follows. In the following description, the symbol “%” for a content of each element means “mass %”.

C (carbon) is an element that is effective in increasing strength of steel inexpensively. If a content of C is less than 0.05%, it is difficult to provide a desired tensile strength, and if the content of C is more than 0.20%, workability and weldability are decreased. Therefore, the content of C is set to 0.05 to 0.20%. A range of the content of C is preferably 0.07% or more to 0.18% or less, and more preferably 0.09% or more to 0.17% or less. It should be noted that when forming the seamless steel pipe into the shape of an airbag, it is necessary to perform a diameter reduction process or the like. Therefore, when the workability is particularly important, the C content is more preferably less than 0.17%.

Si (silicon) is an element that has a deoxidation action and increases hardenability of steel to enhance strength of steel. For this purpose, a content of Si is set to 0.05% or more. However, if the content of Si is more than 0.50%, toughness is decreased. Therefore, the content of Si is set to 0.50% or less. A range of the content of Si is preferably 0.10% or more to 0.40% or less, and more preferably 0.15% or more to 0.30% or less.

Mn (manganese) is an element that has a deoxidation action and is effective in increasing hardenability of steel to enhance strength and toughness of steel. However, if a content of Mn is less than 0.40%, sufficient strength and toughness cannot be provided. On the other hand, if the content of Mn is more than 1.50%, coarsening of MnS occurs, and coarsened MnS elongates and expands at the time of hot rolling, resulting in decrease in toughness and hydrogen embrittlement resistance properties. Further, excessive Mn decreases a diffusion velocity of hydrogen, which causes localized concentration of hydrogen, leading to decrease in hydrogen embrittlement resistance properties. For this reason, the content of Mn is set to 0.40 to 1.50%. A range of the content of Mn is preferably 0.45% or more to 1.20% or less, and more preferably 0.50% or more to 1.00% or less.

P: 0.025% or less

P (phosphorus) is contained in steel as an impurity and leads to decrease in toughness and hydrogen embrittlement resistance properties due to grain-boundary segregation. In particular, if a content of P is more than 0.025%, the decrease in toughness and hydrogen embrittlement resistance properties becomes significant. Therefore, the content of P is set to 0.025% or less. The content of P is preferably 0.020% or less, and more preferably 0.015% or less.

S: 0.020% or less

S (sulfur) is contained in steel as an impurity and decreases toughness particularly in a T direction of a pipe (a direction perpendicular to a pipe axis direction of the pipe). If a content of S is more than 0.020%, the decrease in toughness in the T direction of a pipe becomes significant. Therefore, the content of S is set to 0.020% or less. The content of S is preferably 0.010% or less.

Cu (copper) increases hardenability of steel to enhance strength and toughness of the steel. The effect appears when 0.10% or more of Cu is contained. However, a content of Cu more than 0.50% leads to increase in alloy cost. Therefore, the content of Cu is set to 0.10 to 0.50%. The content of Cu is preferably 0.15% or more, and more preferably 0.20% or more. The content of Cu is preferably 0.40% or less, and more preferably 0.35% or less.

Ni (nickel) increases hardenability of steel, thereby enhancing strength and toughness of the steel. The effect appears when 0.10% or more of Ni is contained. However, a content of Ni more than 0.50% leads to increase in alloy cost. Therefore, the content of Ni is set to 0.10 to 0.50%. The content of Ni is preferably 0.15% or more, and more preferably 0.20% or more. The content of Ni is preferably 0.45% or less, and more preferably 0.40% or less.

Cr (chromium) increases hardenability of steel and increases temper softening resistance to enhance strength and toughness. The effect appears when 0.10% or more of Cr is contained. However, a content of Cr more than 1.20% leads to increase in alloy cost. Therefore, the content of Cr is set to 0.10 to 1.20%. The content of Cr is preferably 0.15% or more, and more preferably 0.20% or more. The content of Cr is preferably 1.00% or less, and more preferably 0.90% or less.

Mo (molybdenum) increases hardenability of steel and increases temper softening resistance to enhance strength and toughness. The effect appears when 0.10% or more of Mo is contained. However, a content of Mo more than 0.50% leads to increase in alloy cost. If the content of Mo is excessively high, a resultant seamless steel pipe tends to increase in strength even in air cooling after hot rolling, which requires softening heat treatment before cold drawing work, leading to increase in production cost. Therefore, the content of Mo is set to 0.10 to 0.50%. The content of Mo is preferably 0.15% or more, and more preferably 0.20% or more. The content of Mo is preferably 0.45% or less, and more preferably 0.40% or less.

Ti (titanium) fixes N in steel, enhancing toughness. In addition, Ti nitrides finely dispersed strongly pin grain boundaries to subject grains to grain refinement, enhancing toughness of steel. To provide the effect, 0.005% or more of Ti needs to be contained. However, if more than 0.050% of Ti is contained, its nitrides are coarsened, rather decreasing toughness. Therefore, a content of Ti is set to 0.005 to 0.050%. The content of Ti is preferably 0.040% or less, and more preferably 0.030% or less.

Nb (niobium) is finely dispersed in steel in the form of its carbides, strongly pinning crystal grain boundaries. Nb has the effect of subjecting grains to grain refinement, enhancing toughness of steel. To provide the effect, 0.005% or more of Nb needs to be contained. However, if more than 0.100% of Nb is contained, its carbides are coarsened, rather decreasing toughness. Therefore, a content of Nb is set to 0.005 to 0.100%. The content of Nb is preferably 0.010% or more, and more preferably 0.015% or more. The content of Nb is preferably 0.050% or less, and more preferably 0.030% or less.

Ca (calcium) fixes S that is present in steel as an unavoidable impurity in the form of its sulfide and improves anisotropy of toughness to increase toughness in a T direction of a pipe, thereby increasing resistance to burst. In addition, Ca restrains production of MnS, thus contributing to enhancement in hydrogen embrittlement resistance properties. The effect appears when 0.0005% or more of Ca is contained. However, if more than 0.0025% of Ca is contained, inclusions increase, rather decreasing toughness. Therefore, a content of Ca is set to 0.0005 to 0.0025%. In order to reliably obtain the effect of improving hydrogen embrittlement resistance, the Ca content is preferably 0.0010% or more, more preferably more than 0.0010%, further preferably 0.0012% or more, and further preferably 0.0015% or more.

Al: 0.080% or less

Al (aluminum) is an element that has a deoxidation action and is effective in increasing toughness and workability. However, if more than 0.080% of Al is contained, occurrence of macro-streak-flaw becomes significant. Therefore, a content of Al is set to 0.080% or less. The content of Al is preferably 0.060% or less, and more preferably 0.040% or less. The content of Al may be on the level of impurity. Thus, a lower limit of the content of Al is not limited to a particular content. However, the content of Al is preferably set to 0.005% or more. The content of Al in the present invention refers to a content of acid-soluble Al (what is called sol. Al).

N: 0.0100% or less

N (nitrogen) forms fine nitrides, thereby strongly pinning grain boundaries to subject grains to grain refinement, thus enhancing toughness of steel. However, if more than 0.0100% of N is contained, nitrides are coarsened, rather decreasing toughness. Therefore, a content of N is set to 0.0100% or less. The content of N is preferably 0.0080% or less, and more preferably 0.0050% or less. The content of N may be on the level of impurity. Thus, a lower limit of the content of N is not limited to a particular content. However, the content of N is preferably set to 0.0005% or more, and more preferably 0.0010% or more.

V (vanadium) is an element that keeps toughness and increases strength through precipitation strengthening. Thus, V may be contained as necessary. However, more than 0.100% of V contained leads to decrease in toughness. Therefore, in a case where V is contained, a content of V is set to 0.100% or less. The content of V is preferably 0.050% or less, and more preferably 0.010% or less. Even a trace quantity of V enables the action of V to be recognized. However, to provide the effect sufficiently, 0.001% or more of V is preferably contained.

B (boron) is an element that segregates in grain boundaries in steel to enhance hardenability of steel significantly. Therefore, B may be contained as necessary. However, if more than 0.0050% of B is contained, there is a tendency for borides to precipitate coarsely in crystal grain boundaries, decreasing toughness. Therefore, in a case where B is contained, a content of B is set to 0.0050% or less. The content of B is preferably 0.0030% or less, and more preferably 0.0020% or less. Even a trace quantity of B enables the action of B to be recognized. However, to keep the effect sufficiently, 0.0001% or more of B is preferably contained, and 0.0005% or more of B is more preferably contained.