JPH03122219A - Production of high strength steel tube for machine structural use having >=120kgf/mm2 tensile strength - Google Patents

Abstract

PURPOSE:To produce a steel tube having superior elongation and excellent in tensile strength by subjecting a steel tube formed by using, as a stock, a carbon steel or low alloy steel containing specific trace amounts of B to hardening treatment. CONSTITUTION:A steel tube 5 is produced by using a carbon steel or a low alloy steel having a composition which contains, by weight, 0.15-0.23% C, <=1.5% Mn, <=0.5% Si, <=0.04% Ti, 0.0003-0.0035% B as a hardenability-improving element, and <=0.0080% N for reducing the hardening effect or further contains one or >=2 kinds among <=0.5% Ni, %h0.5% Cr, and <0.5% Mo and in which trace amounts of sol. Al are allowed to remain by using Al for the deoxidation of the steel. Both ends of the above steel tube 5 are held by means of thrust P by jigs 6, and, while turning the steel tube 5, a high frequency induction heating coil 7 is moved in the direction of an arrow to heat the steel tube 5 up to a temp. in an austenite region, followed by hardening by means of water quenching using a water quenching ring 8. By this method, the steel tube excellent in ductility and having >=120kgf/mm<2> tensile strength can easily be produced.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is particularly useful for improving the tensile strength of mechanical structural steel pipes that require high strength, such as door impact bars and core materials for bumpers, which are reinforcing steel pipes for automobiles. The present invention relates to a method for manufacturing high-strength steel pipes for mechanical structures with a strength of 120 kgf/mm2 or more.

(Prior art) Tensile strength of 75 achieved by heat treating steel pipes
As a method for manufacturing high strength steel pipes of kgf/mm2 or more,
A method for manufacturing high-tensile resistance welded steel pipes is known, which is described in Japanese Patent Application Laid-Open No. 56-46538, but it requires two heat treatment steps and is slow due to the tempering treatment being carried out after the quenching treatment. It was complicated. In addition, quenching and tempering treatments were necessary to restore the toughness and ductility of steel pipes, but it was difficult to simultaneously obtain high strength and good elongation. As an example, FIG. 1 shows the tempering temperature conditions and tensile properties.

The test piece size is outer diameter 38.1 mm x plate thickness 2.0 mm.
This is a JIS II test piece. After induction hardening, the tensile strength when treated at the tempering temperature on the horizontal axis is indicated by Q, the yield strength is indicated by -, and the elongation is indicated by Δ. When the tempering temperature is raised, a slight recovery in elongation is observed, but the strength drops significantly before sufficient ductility is recovered. 120kgf
/mm2 or more while maintaining a tensile strength of, for example, 10
It was extremely difficult to secure an increase of more than 1%.

(Problems to be Solved by the Invention) For example, in the case of a steel pipe for an impact beam, as shown in FIG. 2, in an actual use environment, when a load 1 is applied due to a collision, the impact beam 3 along with the body 2 undergoes plastic deformation. However, it is important to determine how much energy can be absorbed before the inside of the door comes into contact with the passenger 4, so the impact beam has extremely high strength, a certain amount of plastic deformation, and
It is important to have a so-called elongation amount.

An object of the present invention is to obtain a high-strength steel pipe that can simultaneously ensure such high strength and elongation at a low cost.

(Means for Solving the Problems) The present invention provides C: 0.15 to 0.23% (weight%, the same applies hereinafter) Mn≦
0.5% Si≦0.5% Ti≦0.04% B: 0.0003 to 0.0035% N≦0.008
0%, or in addition, SiSO0.
5% Cr≦0.5% Mo≦0.5% Contains one or more of the following, and the remainder consists of remaining sol, AI, Fe, and inevitable impurities by adjusting the degree of deoxidation.The steel pipe is then quenched. The gist of the present invention is to provide a method for manufacturing high-strength steel pipes for mechanical structures having a tensile strength of 120 kgf/mm2 or more, which is characterized by carrying out the following steps.

(Function) The present invention has been made to solve the above problems, and
By selecting the ingredients and performing a quenching treatment after forming the steel pipe, a high-strength steel pipe that exhibits good elongation can be obtained.

The reason for limiting the manufacturing conditions for the electric resistance welded steel pipe in the present invention will be described below.

First, regarding the component system, the present invention aims at strengthening by a quenched martensitic structure, and the strength of the as-quenched martensitic structure is determined by the C content.

It is thought that the amount of solid solute C introduced into supersaturation through the use of transformation is a controlling factor. Therefore, in order to secure a strength of 120 kgf/mm2 or more on the premise of obtaining a martensitic structure of 90% or more, as shown in Figure 3, as shown in Figure 3, a carbon content of 0.15% or more is required. was confirmed. That is, when the C content is 0.15% or more, a tensile strength of 120 kgf/mm2 or more can be obtained, and C
Tensile strength increases with increasing amount. However, it is necessary to consider ductility as well as tensile strength, and as the amount of C increases, elongation decreases.
The amount was set to 0.23% or less (other components other than the amount of C shown in Figure 3 are S: 0.20%, Mn: 2.20%,
P: 0.016%, Si: 0.003%.

N: 0.025%, Cr: 0.20%, Ti:
Using a sample with B: 0.001%,
As the hardening treatment, induction hardening was performed). As seen in JP-A No. 56-46538, the conventional tempering treatment after quenching is performed to ensure ductility, but solid solution C is It aggregates and transitions to carbide. Therefore, the strengthening mechanism of steel when tempering is performed changes from solid solution strengthening to precipitation strengthening, and unlike precipitation strengthening, the present invention performs solid solution strengthening by not performing tempering. However, the strengthening mechanism is very different.

Mn is an element that has the effect of lowering the martensitic transformation temperature of steel and improving hardenability, as well as avoiding self-tempering after transformation during the hardening process and maintaining high strength. However, Mn tends to cause welding defects when manufacturing steel pipes by electric resistance welding, for example, and Mn
The upper limit is 1.50%.

Compared to Mn, Ni, Cr, and Mo are very expensive, but adding these Ni, Cr, and Mo in addition to Mn lowers the martensitic transformation temperature, avoids self-tempering, and is effective in increasing strength. It has the following.

From the viewpoint of weldability, the upper limit is 0.5% for each.

Si, together with Mn, is a very important element in maintaining the integrity of the weld when manufacturing steel pipes by electric resistance welding. The upper limit of St is 0.5% to prevent the formation of oxides called penetrators in welds.
At the same time, the balance of Mn/Si ratio is set to 3 to 10.
It is desirable to do so.

B is an element that dramatically improves hardenability, and the steel of the present invention is characterized by the addition of B in order to obtain a martensite fraction of 90% or more at a relatively low C.
If it is less than 0.00%, no improvement in hardenability can be expected;
If the amount is more than 35%, not only will the cost be high,
It can easily cause surface flaws and initial deterioration. Therefore, the B content was set to 0.0003 to 0.0035%.

This hardenability improving effect of B is lost if N is present in an amount of 0.003% or more, so Ti is added for the purpose of fixing this N. If the amount of Ti added exceeds 0.04%, quality problems such as occurrence of scratches and machinability are likely to occur, so the amount of Ti added is limited to 0.04% or less.

Note that N inevitably exists in steel, forms BN, and reduces the effect of B. Therefore, in order to reduce N as much as possible,
The upper limit is set to 0.0080%.

By subjecting a steel pipe made of the above-mentioned components to a tensile strength of 120 kgf/mm2 or more, the steel pipe for use in impact beams, etc., targeted by the present invention, has an outer diameter of 40 mm. Below, the wall thickness is 2.0
Sensible heat is very small, on the order of mm, and the temperature drop is significant.

Figure 4 shows a comparison between the conventional furnace-heated water quenching method used for boat fishing and the quenching conditions required for the present invention. generation of,
The temperature drops rapidly after extraction from the furnace, and temperature unevenness is likely to occur. Even if such materials are subsequently rapidly cooled, uneven heating, structure variations, strength variations, and insufficient strength occur. Therefore, in the quenching heat treatment of the present invention, it is important to try to reduce the structure variation by shortening the time during which the material is allowed to cool at a high temperature and by forcibly controlling the temperature by rapid heating and cooling. . As a method for this purpose, it is effective to use high frequency induction heating, and for example, a heat treatment method as shown in FIG. 5 can be considered. That is,
The dimensional accuracy (bending,
The steel pipe is fixed by sandwiching it from both sides with a constant thrust P to ensure proper direction restraint during heat treatment, and by transmitting rotation from this jig, while rotating the steel pipe, From one end, the high frequency induction heating coil 7
In this method, the tube is heated sequentially from the tube end to the temperature of the austenite region by moving the tube, and then rapidly cooled to the water temperature by the water cooling ring 8 that follows immediately behind. As mentioned above, the induction hardening method is effective, but there is no particular restriction on the method as long as it can provide instant heating and cooling.

In this way, it is possible to obtain a high-strength steel pipe for mechanical structures that exhibits good ductility and has a tensile strength of 120 kgf/mm 2 or more just by quenching heat treatment.

(Example) Table 1 shows examples and comparative examples of the present invention.

Outer diameter 38.1 mmφX 2. JIS 1 when heat treatment is performed on a steel pipe of Omm t using the method shown in Figure 5.
The results of the No. 1 tensile test are shown. In Examples A to G, the tensile strength was 120 kgf/mm2 or more and the elongation was 10 in all cases.
% or more can be obtained.

Comparative Example H is a case in which the C content is lower than the range of the components of the present invention,
Targeted strength is not achieved. Comparative example! This is a case where the amount of C is higher than the range of the components of the present invention, and the elongation is very low.

Comparative Example J is a case of furnace heating and water quenching, and the quenched structure is insufficient, and compared to Example C, the strength is lower and the elongation is less. When evaluating the hardness, there were very large variations in the longitudinal, thickness, and circumferential directions.

In addition, the comparative example -N is an example in which the target strength/elongation balance was achieved through annealing, but it is extremely difficult to stably satisfy the strength/ductility requirements.

(Effects of the Invention) As explained above, according to the present invention, a steel pipe with good elongation and a tensile strength of 120 kgf/mm 2 or more can be obtained by only one heat treatment called quenching treatment.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the tensile properties of steel pipes obtained by tempering treatment, Fig. 2 is a diagram showing an example of the use of the steel pipe of the present invention, and Fig. 3 is a diagram showing the carbon content when the heat treatment of the present invention is performed. FIG. 4 is a diagram showing the characteristics of the quenching heat cycle required to be carried out in the present invention, and FIG. 5 is a diagram showing the method of carrying out the quenching heat treatment of the present invention. Figure 2 Figure 3 C content (utt phantom Figure 1 Return temperature (°C) Figure 4 Natural history comparison

Claims (1)

[Claims] C: 0.15 to 0.23% (weight %, same hereinafter) Mn≦
1.5% Si≦0.5% Ti≦0.04% B: 0.0003 to 0.0035% N≦0.0080%, or in addition to these, Ni≦0.
5% Cr≦0.5% Mo≦0.5% Contains one or more of the following, and the remainder is sol. A method for manufacturing a high-strength steel pipe for mechanical structures having a tensile strength of 120 kgf/mm^2 or more, which comprises subjecting a steel pipe made of Al, Fe and inevitable impurities to a quenching treatment.