JPH0742499B2 - Method for manufacturing high strength electric resistance welded steel pipe for automobiles in which heat-affected zone is hard to soften - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention shows a high tensile strength of 60 to 100 kgf / mm ² as it is in pipe forming or after strain relief annealing at low temperature, for example arc welding. The present invention also relates to a method for manufacturing a high-strength electric resistance welded steel pipe for automobiles in which the strength of the heat-affected zone is less likely to decrease, for example, a high-strength electric resistance welded steel pipe for rear axle covers, cross members, propeller shafts, impact bars, and the like. .

(Prior Art) Generally, a method for obtaining a high-strength electric resistance welded steel pipe for automobiles having a tensile strength of more than 60 kgf / mm ² is as follows. , A method of increasing the strength by cold drawing after forming an electric resistance welded pipe, a method of forming a high strength steel pipe by heat treatment such as quenching after forming an electric resistance welded pipe, for example, JP-A-61-272318 At the time of hot rolling as described, after rapidly cooling and low temperature winding to make a high strength steel plate,
There is a method of using high strength ERW steel pipe, etc., but in the cases of to, the process is complicated, such as cold rolling, cold drawing, heat treatment of steel pipe, etc., which causes high cost. In particular, in the case of, since it is reinforced by processing strain, there is a problem that the arc-welding in the actual vehicle equipment causes a large decrease in the partial strength of the heat-affected zone. Also, in the case of, since the strength is secured by quenching of the steel pipe and quenching of the coil, there is a problem that tempering occurs in the heat-affected zone during arc welding and the strength decreases.

(Problems to be Solved by the Invention) As shown in FIG. 1, a propeller shaft will be described. When the steel pipe 1 for a propeller shaft and the yoke 3 are connected by arc welding, the hardness changes at the arc welded portion 2. As compared with the conventional electric resistance welded pipe having the strength level, when the strength is increased, the hardness of the heat-affected zone is more significantly reduced.

The present invention reduces the strength reduction of the assembled arc welded portion as described above, which is the biggest problem for strengthening the strength of the steel pipe for automobiles, and the strength of the arc welded heat affected zone is close to that of the base metal. It is an object to obtain a high strength electric resistance welded steel pipe.

(Means for Solving the Problems) The gist of the present invention is to contain C: 0.08 to 0.23% (weight%, the same hereinafter), Mn ≦ 2.0%, Si ≦ 0.8%, Nb ≦ 0.10%, and Cr. ≤1.0%, Mo ≤0.60%, and the balance is sol.Al, Fe remaining by adjusting the deoxidation degree and the raw material steel slab consisting of unavoidable impurities. After rolling in a temperature range of 650 ° C to make a hot rolled steel sheet, it is welded by electric resistance welding to produce a pipe as it is or strain relief annealing to obtain tensile strength.
A method for producing a high-strength electric resistance welded steel pipe for automobiles, which is resistant to softening in a heat-affected zone, characterized by using a steel pipe of 60 to 100 kgf / mm ² .

(Operation) The present invention has been made to solve the above problems,
By selecting the components and hot rolling conditions, it is possible to reduce the decrease in strength of the assembled arc welded part and obtain a high strength electric resistance welded steel pipe that exhibits strength close to that of the base metal even in the arc welded heat affected part.

The reasons for limiting the conditions for producing electric resistance welded steel pipe in the present invention are described below.

First, in order to improve the resistance of the arc-welded heat-affected zone to softening, strengthening by working as a mechanism to increase strength,
It is basically based on the use of strengthening by refining ferrite crystal grains, solid solution strengthening, and precipitation strengthening rather than strength by quenching structure. Therefore, the component system is one of the major characteristics of the present invention.

In the present invention, as a result of detailed investigation of a composition system having high resistance to the arc-welded softened portion by using elements effective for atomization, solid solution strengthening, and precipitation strengthening, FIG. ; C: 0.16%, Si: 0.20%, Mn: 1.20%, P: 0.02%, S:
0.004%, Al: 0.03%, N: 0.004%; heating temperature: 1230 ℃, rolling stop temperature: 810 ℃, cooling rate: 20 ℃ / sec) N
b, Cr, Mo added individually or in combination (addition amount (wt
%) Is Nb: 0.03%, Cr: 0.60%, Mo: 0.20%) is H
It became clear that it is effective against AZ softening. However, when trying to obtain higher resistance to softening,
Combined addition is more effective, and addition of Cr and Mo is effective after addition of Nb, which has a significant effect on grain refinement in a small amount. That is, it is effective to add one or more of Cr and Mo with Nb as a basic component to reduce the softening of the heat-affected zone. Here, Nb can obtain a fine-graining effect with a small amount,
Also, due to cost constraints, the upper limit is 0.10 wt%.

Further, Mo is an element which has a high solid solution strength, forms special carbides, and is expected to have precipitation strength, but it is a very expensive element and has a problem of cost and hardenability when added in a large amount. Therefore, the upper limit is set to 0.60 wt% because the hardness of the HAZ part becomes very high.

Also, Cr has effects such as solid solution strengthening and formation of special carbides, and when added together with Mo, even if expensive Mo is reduced, equivalent resistance to softening can be obtained, but oxides Easy to form,
Since the penetrator is easily generated during electric resistance welding, the upper limit is set.
It was set to 1.0 wt%.

The components other than Nb, Cr, and Mo are the component systems of ordinary electric resistance welded steel pipes, and C, Si, and Mn were used as the elements that stably increase the strength.

Here, C is 0.08 wt% or more because high strength is a target, and deterioration of toughness is feared for the purpose of avoiding deterioration of toughness.
The upper limit was 0.23 wt%.

Further, Si is an element effective as a deoxidizing element in steel,
If it is contained excessively, penetrator defects due to the SiO ₂ component are likely to occur during electric resistance welding, so the content was limited to 0.8 wt% or less.

Like C, Mn is an element that increases the strength of the steel pipe and is also effective in improving toughness, but if it is contained in excess, it easily causes penetrator defects due to the MnO component during electric resistance welding. % Or less.

Next, the rolling / cooling conditions for obtaining high strength in which arc welding is less likely to be softened by using the above-described component system will be described.

The biggest point here is to obtain a fine-grained ferrite structure, which is shown in Fig. 3 (base steel composition; C: 0.18%, Si: 0.20%,
Mn: 1.20%, P: 0.02%, S: 0.003%, Nb: 0.03%, Mo: 0.4%, A
l: 0.03%, N: 0.004% (wt%); heating temperature: 1230 ℃, rolling stop temperature: 810 ℃, cooling rate: 20 ℃ / sec),
The coiling temperature is in the range of 450 to 650 ° C. Even if controlled rolling is performed in the unrecrystallized region, coiling at a coiling temperature above 650 ° C promotes coarsening of the structure and agglomeration and coarsening of carbides, making it difficult to obtain high strength. Was 650 ° C. On the contrary, when the coiling temperature is lower than 450 ° C, the steel type with relatively good hardenability has a bainite structure,
The lower limit was set to 450 ° C because a martensite structure is formed depending on the cold speed, the strength of the base metal part is significantly increased, and conversely the difference between the hardness of the arc weld and the hardness of the base metal becomes large.

The hot-rolled steel sheet thus obtained was used as an electric resistance welded steel pipe to obtain a tensile strength of 60 to 100 kgf in a pipe-formed state as in the example.
A high strength electric resistance welded steel pipe for automobiles that exhibits a high strength of / mm ² and is unlikely to soften the HAZ can be obtained. For products that require partial residual stress reduction, such as propeller shafts, the strength decreases significantly at 450 ° C or higher, where recovery occurs after pipe forming.
Sometimes strain relief annealing is carried out in the temperature range of 50 ° C or less, but in such a case as well, the tensile strength after strain relief annealing is
High strength of 60 to 100 kgf / mm ² can be obtained, and high strength electric resistance welded steel pipes for automobiles which are hard to soften HAZ can be obtained.

(Examples) Table 1 shows Examples and Comparative Examples of the present invention. After finishing rolling, each sample was water-cooled at 20 ° C / sec to 500 ° C, and a hot-rolled sheet wound at 500 ° C was used to form an electric resistance welded pipe with an outer diameter of 38.1 mm and a wall thickness of 3.2 mm. did. A high strength of 60 kgf / mm ² or more was obtained for all steel types including the comparative material, but the hardness of the arc welded part was significantly softened with ΔHv = 44 for the comparative material of I. On the other hand, Nb-Mo steels A to D, and E
~ G Nb-Cr-Mo steel and H Nb-Cr steel have a low softening margin of about 25 or less. Also, when strain relief annealing is performed as in D and G, softening of the HAZ part is similarly reduced.

As a representative example in FIG. 4, HAZ of Example A and Comparative Example I
The hardness profile of a part is shown. The steel type to which the present invention is applied has high hardness even in the softest part.

(Effects of the Invention) As described above, according to the present invention, it is possible to reduce the decrease in strength of the arc welded portion, which was the biggest obstacle to the increase in strength of the electric resistance welded steel pipe for automobiles. Therefore, the weight and the diameter of the steel pipe for automobiles can be reduced.

[Brief description of drawings]

FIG. 1 (a) is an explanatory view showing an arc welded portion between a propeller shaft and a yoke, and FIG. 1 (b) shows a heat effect of arc welding performed on an electric resistance welded steel pipe for an automobile when the strength of the electric resistance welded steel pipe is increased. Fig. 2 shows that the hardness decrease in the part becomes remarkable, Fig. 2 shows the relationship between the resistance to softening reduction of the arc welding heat affected zone and the additive element, and Fig. 3 shows the strength of the winding temperature. FIG. 4 shows the influence of the above, FIG.
It is a figure which shows the hardness distribution of the arc welding part about the comparative example I.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Isao Anai 5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Co., Ltd. Inside the Nippon Steel Works (56) Reference JP-A-1-172520 (JP, A)

Claims (1)

[Claims] 1. C: 0.08 to 0.23% (weight%, the same below), Mn ≦ 2.0%, Si ≦ 0.8%, Nb ≦ 0.10%, and Cr ≦ 1.0%, Mo ≦ 0.60%, Containing one or more, the balance is sol.Al, which remains by adjusting the deoxidation degree, a steel slab consisting of inevitable impurities and slabs, after hot rolling, wound in the temperature range of 450 ~ 650 ℃, Tensile strength after pipe forming or strain relief annealing by electric resistance welding after hot rolled steel sheet
A method for producing a high-strength electric resistance welded steel pipe for an automobile, which is resistant to softening in a heat-affected zone, characterized in that the steel pipe is 60 to 100 kgf / mm ² .