JP3235168B2 - Manufacturing method of high strength electric resistance welded steel pipe for automobile - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

This invention relates to a tensile strength of 70 to 12
The present invention relates to a method for producing a high-strength ERW steel pipe for automobiles having high strength of 0 kgf / mm ² and excellent in fatigue characteristics.

[0002]

2. Description of the Related Art In recent years, the field of application of ERW steel pipes has been extended to structural materials for automobiles, such as propeller shafts and impact bars, with the progress of the technology for manufacturing ERW pipes. However, the demand for thinner walls and smaller diameters required for structural materials for automobiles from the viewpoint of weight reduction is becoming increasingly severe. Therefore, measures to further increase the strength of ERW steel pipes for automobiles are desired. It has come to be. However, considering the characteristics required of ERW steel pipes as structural members for automobiles, simply increasing the static strength is not enough, and at the same time improving the durability (fatigue properties) against repeated loading. Had to be done.

However, the following problems have been pointed out from the viewpoint of fatigue characteristics for increasing the strength of ERW steel pipes used as automobile structural members. a) ERW steel pipes as automobile structural members are welded to other members when they are mounted on an actual vehicle. At that time, softening tends to occur at the welded joints. It is difficult to ensure the improvement of fatigue properties corresponding to "increase in tensile strength".

B) Further, in ERW during the production of steel pipes, the toughness of an ERW weld or its heat-affected zone is liable to decrease as the strength of the steel increases. Therefore, when a high strength ERW steel pipe is used as an automobile structural member, The occurrence of fatigue cracks near the ERW weld
It becomes a path of propagation and tends to cause a decrease in fatigue life.

However, as a technique for coping with the problem of softening of the welded joint, for example, as described in Japanese Patent Application Laid-Open No. 2-197525, "Nb, Cr, Mo is added as a steel component in combination with heat, A technique of suppressing the reduction in hardness of the heat-affected zone of the joint by devising the rolling conditions "is known. However, in the case of steel materials containing such alloying elements, the reduction in toughness of the electric resistance welded portion and the heat-affected zone during the production of steel pipes cannot be suppressed, and the reduction of fatigue life cannot be avoided.

[0006] The present invention has been made in view of the above-described circumstances to solve the above-mentioned problems pointed out when increasing the strength of an electric resistance welded steel pipe for an automobile, and to provide both an electric resistance welded joint and a joint welded part. The purpose was to establish means for stably mass-producing a highly reliable high-strength ERW steel pipe exhibiting sufficient fatigue properties on an industrial scale.

[0007]

Means for Solving the Problems The present inventor repeatedly conducted a number of experiments to achieve the above-mentioned object, and as a result, said, "We adjusted the total composition by adding appropriate amounts of Nb and Ti. After hot-rolled steel sheet is made from steel and the hot-rolling conditions and the cooling conditions after rolling are properly controlled to obtain a hot-rolled steel sheet, this pipe is made by electric resistance welding. High-strength ERW steel pipe is realized. "

The present invention has been made based on the above findings and the like. "C: 0.06 to 0.30% (hereinafter,% representing the component ratio is% by weight.
), Mn: 2.0% or less, Si: 1.0% or less,
Nb: 0.005 to 0.08%, Ti: 0.005 to 0.04%, sol.A
l: 0.005 to 0.05%, N: 0.0080% or less, or Cr: 1.5% or less, Mo: 0.05 to 0.8%, Cu:
1.0% or less, Ni: 3.0% or less, V: 0.10% or less,
B: A steel containing at least one of 0.0020% or less, and the balance consisting of Fe and unavoidable impurities is subjected to hot rolling at a cumulative draft of at least 40% or more at 950 ° C or less, and hot rolling is completed. 20 ° C after
/ s is forcibly cooled at a cooling rate of not less than 600-200 ° C. and then rolled in a temperature range of 600-200 ° C. Then, the obtained hot-rolled steel sheet is formed into a tube by electric resistance welding, so that the tube is formed as it is or the strain is removed. the weld toughness tensile strength after exhibit 70~120kgf / mm ²
R> The stable production of high strength ERW steel pipes for automobiles.

[0009]

[Effects] That is, the present invention provides a method for adding a trace amount of Nb,
A) An extremely fine and thermally stable microstructure can be obtained by adding Ti, and a decrease in the hardness of the welded joint can be suppressed. And the improvement of the toughness of ERW welds, and the fatigue characteristics of ERW welds obtained by rolling the above-mentioned steel in a temperature range of 950 ° C or less. (Especially, the addition of a small amount of Ti to the material steel will cause a fatigue crack in the ERW weld and reduce the fatigue life of the ERW steel pipe.) This is an extremely effective countermeasure for the problem that could not be solved conventionally). Hereinafter, the reason why the composition and the manufacturing conditions of the ERW steel pipe material steel in the present invention are limited as described above will be described.

(A) Ingredient composition of base steel a) C C is an important element for securing high strength to a steel pipe.
If the content is less than 0.06%, a tensile strength of 70 kg or more cannot be obtained. On the other hand, if the content exceeds 0.30%, the toughness of the joint weld and the ERW weld will be reduced. , C content is set to 0.06 to 0.30%.

B) Si Si is not only important as a deoxidizing element of steel, but also a necessary component for securing a desired tensile strength. The Si content was determined to be 1.0% or less because it has a bad effect on the toughness of the welded portion and the ERW weld, and at the same time, easily causes welding defects in the ERW weld.

C) Mn Mn is also an element necessary for ensuring high strength in the steel pipe, and is effective in refining the structure and has an effect of improving fatigue properties. If contained, Mn content is set to 2.0% or less, since defects are likely to occur in the ERW weld and fatigue strength is rather lowered.

D) Nb Nb not only brings about an increase in the strength of the steel material mainly due to the formation of precipitates, but also an effect of improving the toughness of the base metal through the action of refining the structure.
HAZ of the welded joint, as described in JP (HAZ portion) is prevented from softening and improved toughness of the electric resistance welding unit and the HAZ, the effect of increasing the fatigue strength However, if the content is less than 0.005%, the desired effect due to the above-mentioned effect cannot be obtained. On the other hand, if the content exceeds 0.08%, on the contrary, the toughness of the ERW weld will be reduced. Therefore, the Nb content was determined to be 0.005 to 0.08%.

E) Ti Ti has the effect of promoting the microstructural refinement of the base metal, the ERW weld and the joint weld, and also has the effect of improving the toughness of the ERW weld. Although it is an essential component for improving the strength properties, if the content is less than 0.005%, the desired effect due to the above-mentioned effects cannot be expected. On the other hand, if the content exceeds 0.04%, the fatigue strength is reduced. Therefore, the Ti content was determined to be 0.005 to 0.08%.

F) sol.Al Al is an element effective for deoxidizing steel and refining the structure of steel.
When the Al content is less than 0.005% in sol.Al content, the above effect cannot be sufficiently ensured. On the other hand, when the sol.Al content exceeds 0.05%, the toughness and fatigue properties of the ERW are adversely affected. Therefore, the sol.Al content was determined to be 0.005 to 0.05%.

G) NN N is an impurity element inevitably mixed into steel, but if the N content in the steel exceeds 0.0080%, the toughness of the base metal and the welded portion is significantly deteriorated, and the fatigue properties are also reduced. From lowering,
The N content was limited to 0.008% or less.

H) Cr, Mo, Cu, Ni, V and B These components each have an action of further improving the strength, toughness or fatigue properties of the ERW pipe, so that one or two of them may be used as necessary. More than one species can be contained, but the reasons for limiting the content of each component to a specific value are as follows.

[0018] Cr Cr has the effect of improving the strength and corrosion resistance of the ERW steel pipe and suppressing the softening of the welded joint.
If Cr is contained in excess of this, the toughness of the base metal and the ERW weld will be reduced, and at the same time, welding defects will easily occur in the ERW weld. Therefore, the Cr content is set to 1.5% or less.

Mo Mo is also a component that contributes to the strengthening of the ERW steel pipe by the solid-solution strengthening action, and is also an effective component for suppressing the softening of the joint welded HAZ and improving the fatigue characteristics as a structural member. ,
If the content is less than 0.05%, the above effect is not sufficient. On the other hand, if the content exceeds 0.8%, the toughness of both the base material and the welded parts (electrically welded parts, joint parts) decreases. Therefore, the Mo content is
0.05 to 0.8%.

Cu Cu has the effect of improving the strength and corrosion resistance of ERW steel pipes. However, if it is contained in excess of 1.0%, the hot workability is reduced and the toughness of the ERW weld is also reduced. Therefore, the Cu content was determined to be 1.0% or less.

Ni Ni has the effect of improving the strength, toughness, and corrosion resistance of the steel pipe, but is an expensive element. If it is contained in excess of 3.0%, there is a sign that the toughness of the electric resistance welded part is deteriorated. Therefore, the Ni content was determined to be 3.0% or less.

V V has the effect of improving the strength of the ERW pipe through the formation of precipitates and increasing the softening resistance of the welded joint.
%, The toughness of the base metal and the electric resistance welded portion is reduced. Therefore, the V content is set to 0.10% or less.

[0023] Although B B is an effective ingredient the intensity increase of electric resistance welded steel pipe, 0.0020
%, The toughness of the base metal and the welded parts (electrically welded parts and joints) is significantly reduced.
% Or less.

The inevitable impurities such as P and S are preferably as low as possible with respect to the toughness and fatigue properties of the base metal and the welded portion of the ERW steel pipe. However, since the reduction of these elements involves an increase in cost, for the purpose of the present invention, the P content is set to 0.02% or less (preferably 0.007% or less).
S content is 0.005% or less (preferably 0.001%
It is enough to keep it below.

(B) Manufacturing conditions of ERW steel pipes The hot rolling and cooling conditions for manufacturing ERW steel pipes are to obtain a fine grain ferrite structure and to make Nb and Ti carbonitrides fine and stable. It has been determined with the aim of improving the toughness of the base metal by dispersing it and at the same time significantly improving the toughness of the heat-affected zone during electric resistance welding and joint welding with other members. The reasons for setting the conditions in each step are as follows.

A) Cumulative rolling reduction at 950 ° C. or less During hot rolling, working in a temperature range of 950 ° C. or less improves the toughness of the base material by making the ferrite of the rolled material finer, and reduces the carbonitride. The effect of suppressing the coarsening and softening of the heat-affected zone at the time of welding is obtained by performing strain-induced precipitation. At this time, if the cumulative rolling reduction is less than 40%, the above effect cannot be expected. Therefore, the cumulative rolling reduction in the temperature range of 950 ° C. or less is 4%.
The requirement is to perform hot rolling of 0% or more.

B) Cooling rate during forced cooling after hot rolling Accelerated cooling after hot rolling is effective for increasing the strength of a steel sheet, and at the same time, reducing ferrite to prevent coarse agglomeration of carbonitrides. However, the effect of improving the toughness of the base metal, the electric resistance welded portion, and the joint welded portion is brought about, but a cooling rate of 20 ° C./s or more is required to ensure the effect.

C) Winding temperature Even when rolling in a low temperature range or accelerated cooling after rolling, winding at a temperature exceeding 600 ° C. promotes coarsening of ferrite and coarsening of carbonitride. As a result, high strength cannot be obtained, and at the same time, the toughness of the base material and the welded portion decreases. The lower the winding temperature is, the more effective the base material is in strength. However, if the winding temperature is lower than 200 ° C., a martensite structure is formed, and the base material strength is significantly increased to deteriorate toughness. The taking temperature was set in the range of 600 ° C to 200 ° C.

The hot-rolled steel sheet obtained through the above-described treatment is subjected to ERW welding in the usual manner to produce a pipe. The ERW steel pipe is kept as it is or has a tensile strength even after ordinary strain relief annealing. 7
Together exhibits high strength 0~120kgf / mm ^2, the base metal, but toughness of the electric resistance welding portion and the joint weld part is to have been significantly improved in comparison with conventional materials, the present invention, including its effect Examples will be described more specifically.

[0030]

EXAMPLE A slab having the composition shown in Table 1 was used.
After hot rolling and forced cooling / winding under the conditions shown in (1) to obtain a hot rolled sheet, the pipe is further made in the usual manner to form an outer diameter of 58 mm, a wall thickness of 1.6 mm, an outer diameter of 105 mm, and a wall thickness. Two types of ERW steel pipes having a thickness of 4.2 mm were used. After pipe production, a part of the tube was subjected to strain relief annealing.

[0031]

[Table 1]

[0032]

[Table 2]

Next, a test piece was taken from each of the obtained ERW steel pipes, and the tensile strength of the base material and the toughness of the ERW weld (−10 ° C.)
Was measured. These results are also shown in Table 2.

As is clear from the results shown in Table 2, all the steel pipes manufactured according to the present invention have sufficiently high strength as ERW steel pipes for automobiles, and are significantly improved as compared with the steel pipes according to the conventional method. It can be seen that they have improved ERW weld toughness and therefore exhibit improved fatigue properties .

[0035]

[Summary of Effects] As described above, according to the present invention, the welded portion has sufficiently excellent toughness for automobiles (toughness of a joint welded portion and an electric resistance welded portion), and also has satisfactory fatigue characteristics. Industrially extremely useful effects are obtained, for example, it is possible to stably provide a high-strength ERW steel pipe at a relatively low cost.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/10 C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) C: 0.06 to 0.30%, Si: 1.0% or less, Mn:
2.0% or less, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.04
%, Sol.Al: 0.005 to 0.05%, N: 0.0080% or less, with the balance being Fe and unavoidable impurities.
At least 950 ° C. or less, the hot rolling is performed at a cumulative draft of 40% or more. After the completion of the hot rolling, the material is forcibly cooled at a cooling rate of 20 ° C./s or more and wound in a temperature range of 600 to 200 ° C. after the obtained characterized by pipe-hot-rolled steel sheet at electric resistance welding was excellent weld toughness showing a still or strength of tensile strength 70~120kgf / mm ² after stress relief annealing of the pipe formation Of manufacturing high-strength ERW steel tubes for automobiles. 2. C: 0.06 to 0.30% by weight, Si: 1.0% or less, Mn:
2.0% or less, Nb: 0.005 to 0.08%, Ti: 0.005 to 0.04
%, Sol. Al: 0.005 to 0.05%, N: 0.0080% or less, Cr: 1.5% or less, Mo: 0.05 to 0.8%, Cu:
1.0% or less, Ni: 3.0% or less, V: 0.10% or less,
B: A steel containing at least one of 0.0020% or less, with the balance being Fe and unavoidable impurities, is subjected to hot rolling at a cumulative draft of at least 950 ° C or less and at least 40% and hot rolling is completed. 20 ° C after
/ s after forcibly cooling at a cooling rate of not less than 600 to 200 ° C., winding the obtained hot-rolled steel sheet by electric resistance welding, method of manufacturing an automobile high-strength electric resistance welded steel pipes with excellent weld toughness representing the intensity of the tensile strength 70~120kgf / mm ² after stress relief annealing.