JPH05171278A - Production of high strength resistance welded tube - Google Patents

Abstract

PURPOSE:To produce a high strength resistance welded tube. CONSTITUTION:A low carbon steel having a composition which contains, by weight, 0.1-2.0% Mo and 0.010-0.15% Nb and/or 0.010-0.30% V and in which C* (solid-solution carbon) represented by C*= C-(12/92.9).Nb-(12/50.9).V-(12/95.9).Mo satisfies C*>0 is hot-rolled by the ordinary process, coiled at 550-750 deg.C, and subjected to tube making. Cold strain is applied, if necessary, and cumulative strain in a longitudinal direction is regulated to 10-30% and then low temp. heat treatment is done at 200-300 deg.C, by which the high strength resistance welded tube can be obtained. By this method, the high strength resistance welded tube having a strength as high as >=50kgf/mm<2> tensile strength can be inexpensively produced while dispensing with particularly expensive alloying elements.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high strength electric resistance welded steel pipe. The field of application is 50 for tensile strength for automobiles, machine structures, industrial machines, etc.
It is applicable to fields requiring a strength level of kgf / mm ² or higher.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of resource saving and energy saving, for example, a bill concerning low fuel consumption of automobiles has been enacted. On the other hand, automobile companies are aiming to reduce the weight of automobiles, and the performance required of steel materials is thinning and high strength.

Generally, as a method for increasing the strength of an electric resistance welded steel pipe, a method of increasing the Ceq of the material or a method of increasing the strength by adding a precipitation strengthening element, or a method in which a hot rolled sheet is cold-rolled to have high strength, A method of forming a high strength steel pipe, a method of increasing the strength by cold drawing after forming an electric resistance pipe, a method of forming a high strength steel pipe by heat treatment such as quenching after forming an electric resistance weld pipe, for example As described in JP-A No. 61-272318, there is a method of forming a high-strength steel plate by rapid cooling and low-temperature winding at the time of hot rolling, and then using a high-strength electric resistance welded steel pipe, and the like. There is a drawback that workability is significantly deteriorated. Also,~
In the case of, the process is complicated such as cold rolling, cold drawing, heat treatment of steel pipe, etc., which causes a high cost. Also, in the case of (1), the yield is reduced due to the operation trouble caused by the low-temperature winding, resulting in an increase in cost.

[0004] Further, since the material is made to have a high strength and is made into a pipe, it may cause a trouble at the time of making a pipe.

Further, the mounting of these high-strength steel pipes on automobiles is considerably difficult because of their high strength. For example, even if the strength is not so high at the time of mounting, it will be strengthened by some processing thereafter. Steel pipes are also desired, but of course such steel pipes have not been developed so far.

[0006]

DISCLOSURE OF THE INVENTION The present invention aims to avoid the deterioration of workability when, for example, an electric resistance welded steel pipe for automobiles is made to have high strength, and troubles during pipe making and assembling into automobiles. The above two methods are provided.

It has a relatively low strength during pipe making, and a predetermined strength is obtained by subsequent low temperature heat treatment.

For example, when an electric resistance welded steel pipe for an automobile is attached to an automobile, it has a relatively low strength and a predetermined strength is obtained by a treatment (for example, low temperature heat treatment) after the attachment.

[0009]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made a detailed study to improve the strength after pipe making, and as a result, have considered the use of bake hardening characteristics used for cold rolled steel sheets for automobiles. It was A steel sheet having bake hardening characteristics has excellent workability during processing, and a bake coating treatment after processing (for example, 170
It becomes possible to cause hardening of the steel sheet at (° C. × 20 min) and reduce the thickness corresponding to the increase in strength, and the industrial value thereof is extremely high as an ideal steel sheet such as a steel sheet for automobiles.

The phenomenon of bake hardening is so-called age hardening which occurs when dislocations introduced by working are trapped in solid solution C, and the necessary conditions therefor are dislocations introduced by working and solid solution C. .. Since electric resistance welded steel pipe is manufactured by forming a hot coil, it satisfies certain conditions from the viewpoint of introducing work dislocations, and we considered its application to automobile electric resistance welded steel pipe.

However, when the bake hardening characteristics are used in cold-rolled steel sheets for automobiles, the increase in strength is at most 3-5 kgf /
Since it is about mm ² , the strength increase is too small for use for the purpose of this development. Therefore, 10 kgf / mm
^It is necessary to have bake hardening characteristics of about ² or more,
To that end, it seems that considerable innovation is required.

Therefore, the present inventors have conducted numerous experiments and detailed studies in order to apply the bake hardening characteristics to electric resistance welded steel pipes. As a result, the addition of Mo as an alloying element and the combination of Mo with Nb or V have been investigated. We have found that the addition is effective.
In addition, the temperature range for baking coating (170 ° C to 180 ° C)
Not only that, but since it has been found that it has a curing property in a wider temperature range, the expression "curing by low temperature heat treatment" will be used hereinafter.

Further, by using this method,
The amount of hardening by low temperature heat treatment, which was 3-5 kgf / mm ² level (ΔYP) in the past, was 10 kgf / mm ² or more (ΔT).
It was found that a bake-curing amount, which was previously unthinkable, was obtained as S). Further, from the viewpoint of improving the tensile strength, it can be said that this is an improvement in properties that has not been considered so far.

Regarding the components, it is essential to add Mo as described above, but it has been found that the effect is increased when Nb and V are added in combination.

When Mo, Nb, or V is added, these elements and C combine with each other depending on the winding temperature to form a precipitate, and the solid solution C decreases.
It was found that a sufficient amount of C was required to ensure solid solution C even when bonded with.

Specifically, since the precipitate is formed in the winding temperature range of 550 to 750 ° C., the above C ^* > 0.
The applicable range of is in this temperature range. When the winding temperature range is less than 550 ° C., the above-mentioned precipitates are not formed, so that C ^* > 0 is not necessary, and only C> 0 is sufficient.

It was found that it is not necessary to prescribe the hot rolling conditions among the manufacturing conditions of the material. Regarding the finish rolling conditions, there is a difference in crystal grains between rolling just above the transformation point and rolling at high temperature, which slightly affects the material itself, but has almost no effect on the bake hardening characteristics. I found that I did not give.

By the way, since the electric resistance welded steel pipe is formed with the hot coil as described above, the condition is satisfied to some extent from the viewpoint of introducing dislocations. It was found that the total amount of strain in the longitudinal direction was required to be 10% or more regardless of whether or not the subsequent strain was applied. That is, if the total strain amount in the longitudinal direction is less than 10%, the bake hardening amount can be expected to be at most about 8 to 10 kgf / mm ² , whereas the bake hardening amount is 12 kgf / mm ^2.
It was clarified that the total strain amount in the longitudinal direction needs to be 10% or more in order to obtain mm ² or more. At the same time, the upper limit of the total strain in the longitudinal direction was also examined, and it was revealed that if the strain was too large, the bake-hardening amount decreased. Specifically, it was found that the total strain amount in the longitudinal direction needs to be less than 30% in order to secure a bake hardening amount of 120 kgf / mm ² or more.

Based on the above knowledge, the present invention makes it possible to apply bake hardening characteristics to electric resistance welded steel pipes for automobiles and the like.
0.1 to 2.0%, and Nb: 0.010
0.15%, V: 0.010 to 0.30% of 1 type or 2 types, and C ^* = C- (12 / 92.9) · Nb
-(12 / 50.9) ・ V- (12 / 95.9) ・ Mo
A low carbon steel having a C ^* (solid solution carbon) represented by C that satisfies C ^* > 0 is hot-rolled in a usual process, wound in a temperature range of 550 to 750 ° C., and then pipe-formed and cooled. In the method for producing a high-strength electric resistance welded steel pipe, which is characterized in that a cumulative strain in the longitudinal direction is set to 10 to 30%, and then low temperature heat treatment is performed in a temperature range of 200 ° C. to 300 ° C. is there.

[0020]

In the present invention, the raw material having the defined components is made into the electric resistance welded steel pipe, and then the secondary working is carried out if necessary, and then the remarkable hardening characteristic by the low temperature heat treatment is succeeded.

Next, the components of the present invention, hot rolling conditions, and steel pipe manufacturing conditions will be described.

The method of the present invention contains C: 0.01 to 0.30% and Mo: 0.10 to 2.0%, and Nb: 0.0.
It can be applied to a low carbon steel containing 10 to 0.15% and V: 0.010 to 0.30% of one or two kinds, and good results can be obtained. Other preferable component compositions include: Si: 0.02-0.50% Mn: 0.10-1.00% Al: 0.001-0.100% N: 0.005-0.0100% Depending on the required characteristics of the low carbon steel to be used, or the strength steel, Cr: 5.5% or less Ti: 0.15% or less B: 0.003 to 0.0030% Ca: 0.0080% or less 1 type or 2 types You may add above.

C is indispensable for age hardening as solid solution C, but in steels added with Mo, Nb, and V, those elements are combined with C to eliminate solid solution C, which makes the developed steel meaningless. Therefore, it is necessary to leave the solid solution C even if the C combined with the alloy element is subtracted from the C content of the steel. In particular,
C ^* = C− (12 / 92.9) · Nb− (12/50.
9) C- ^* represented by V- (12 / 95.9) -Mo needs to satisfy C ^* > 0. However, this is Mo or N
b and V form a complex precipitate, the winding temperature (550
(° C. to 750 ° C.) only. Mo is known as an element having a very large interaction with dislocations. That is,
Since Mo exists as a precipitate in steel or as a cluster in the middle of forming precipitates in the vicinity of dislocations, C attracted to Mo is used for fixation of dislocations, and compared with the case where Mo is not added. It is considered that this brings about a remarkable increase in age hardening and improves the bake hardenability. For the above reasons, addition of Mo is indispensable to the present invention, but since the weldability is impaired if the addition amount is too large, the upper limit of the content is
It was set to 2.0%. Further, since the effect is not exhibited when the content is 0.10% or less, the lower limit was made 0.10%.

Nb is added to promote the formation of carbide of Mo, that is, to increase the force of attracting C, and indirectly contributes to the increase of age hardening, but if it increases, it hinders the weldability. The upper limit of the content is 0.15%. Further, since the effect is not exhibited when the content is 0.010% or less, the lower limit was made 0.010%.

Similar to Nb, V is added to accelerate the formation of carbide of Mo, but if it increases, it impairs weldability, so the upper limit of its content is 0.3%. Further, since the effect is not exhibited when the content is 0.010% or less,
The lower limit was made 0.010%.

Further, Si, Mn, and
With respect to Al and N, the generally used component range may be used, and Si has the action of improving the strength and ductility of the steel material by the solid solution strengthening action, and is required to be 0.02% or more, but 0.50% is required. If the content is exceeded, the toughness of the steel material will deteriorate, so the content should be 0.02-0.50.
Defined as%.

Since Mn is an element necessary for strength, 0.10
%, But the upper limit was made 1.0% to secure weldability and toughness.

Al is necessary for deoxidation in the steel making stage, and the lower limit was made 0.001%. However, if the content is too large, the absolute value of the inclusion itself increases, so the upper limit was made 0.10%.

When the content of N is too large, the toughness of the steel material is deteriorated, so the upper limit is made 0.010%, but the lower limit is made 0.005% considering the cost increase from the steelmaking capacity.
And

Regarding the components added for the purpose of increasing the strength, first, Cr is added because it is useful for increasing the strength and improving the corrosion resistance, but if it increases, it impairs the low temperature toughness and weldability, so the content is 5.5. % Was the upper limit.

Ti is useful for grain refinement of austenite grains and is added, but if it increases, the weldability is impaired. Therefore, the upper limit of the content is 0.15%.

B has an effect of remarkably enhancing the hardenability of steel by adding a trace amount. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added excessively, a B compound is formed and the toughness is deteriorated, so the upper limit was made 0.0030%.

Ca is useful for controlling the morphology of sulfide-based inclusions, but if it increases, it forms inclusions in the steel and deteriorates the properties of the steel. Therefore, the upper limit of the content is 0.0080%. .

The hot rolling conditions are not particularly specified other than the winding conditions as described above. Considering the required characteristics of the final product, it is necessary to determine the manufacturing conditions by considering the grain size and strength level under the finish rolling conditions.

The steel pipe manufacturing conditions are also not specified. When the bake hardening characteristics are to be obtained, if the amount of strain when forming the steel pipe from the hot coil is insufficient, secondary processing such as drawing after the steel pipe is manufactured can be allowed without any problem. Finally, the total strain in the longitudinal direction may be controlled to 10 to 30%.

[0036]

EXAMPLES Table 1 shows the chemical composition of the test material, and Table 2 shows the heat treatment conditions and the mechanical properties obtained.

Steel pipes NoA1, B1, C1 shown in Table 2
D1, Q1, R1, S1, T1, U1, and V1 are steels that have been subjected to this treatment, and the target of the present invention is 12 kgf / mm.
^It exhibits a curing property of ² or more.

Further, since E1 and F1 do not contain Mo, the baking and hardening hardly occur even when the low temperature heat treatment is performed.

[0039] Further, G1 has a small bake hardening of less than 10 kgf / mm ² because it contains Mo, but neither Nb nor V is added.

Further, since H'and I1 have negative C ', solid solution C is not secured, and hardening hardly occurs. Further, J1, K1, and L1 do not show the curing characteristics of 12 kgf / mm ² or more because the total strain in the longitudinal direction is less than 10%. Further, M1 and N1 do not exhibit a hardening property of 12 kgf / mm ² or more because the total longitudinal strain amount exceeds 30%.

Further, since the heat treatment temperature of O1 is too low, the curing amount by the low temperature heat treatment is less than 12 kgf / mm ² .

Further, since the heat treatment temperature of P1 is too high, the amount of hardening by the low temperature heat treatment is less than 12 kgf / mm ² .

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described in detail above, the present invention has a tensile strength of 5% without using any expensive alloying elements.
An electric resistance welded steel pipe having a strength of 0 kgf / mm ² or more can be manufactured at low cost, and the effect is industrially great.

Claims (1)

[Claims] 1. The composition contains 0.1 to 2.0% by weight of Mo, and Nb: 0.010 to 0.15% and V: 0.0.
10 to 0.30% of one or two, and C ^* =
C- (12 / 92.9) / Nb- (12 / 50.9) /
V- (12 / 95.9) · Mo, C ^* (solid solution carbon) satisfying C ^* > 0, low carbon steel is hot-rolled in a normal process and subjected to a temperature range of 550 to 750 ° C. After being rolled up, pipe forming is performed, with or without cold strain, cumulative strain in the longitudinal direction of 10% to 30%, and then 200 ° C to 300 ° C.
A method for producing a high-strength electric resistance welded steel pipe, characterized by performing low-temperature heat treatment in the temperature range of.