JPH05195064A - Production of high strength resistance welded tube - Google Patents

Abstract

PURPOSE:To inexpensively produce a high strength resistance welded tube without using particularly expensive alloying elements. CONSTITUTION:The tube can be produced by subjecting a low carbon steel which has a composition containing 0.1-2.0% Mo, 0.01-0.30% C, 0.10-2.0% Cu, and 0.10-9.0% Ni and containing, if necessary, 0.01-0.15% Nb and/or 0.010-0.30% V to hot rolling and tube making by ordinary stages, applying, if necessary, cold strain, regulating cumulative strain in a longitudinal direction to 10-30%, and then performing low temp. heat treatment at 200-300 deg.C.

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, a bill concerning low fuel consumption of automobiles has been developed. 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 of 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 of increasing the strength by cold rolling a hot rolled sheet 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 during 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 provides the following two methods for the purpose of avoiding troubles during pipe making and assembling into an automobile when the strength of the electric resistance welded steel pipe for automobiles is increased. Is provided. (1) Relatively low strength during pipe making, and a predetermined strength is obtained by subsequent low temperature heat treatment. (2) The electric resistance welded steel pipe for an automobile has a relatively low strength when attached to the automobile, and a predetermined strength is obtained by a treatment after the attachment (for example, low temperature heat treatment).

[0007]

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 a relatively low strength during processing, and a bake coating treatment after processing (for example, 17
It becomes possible to harden the steel sheet at 0 ° C. × 20 min) and reduce the sheet thickness corresponding to the increase in strength, and the industrial value thereof is extremely high as an ideal steel sheet for automobiles and the like.

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 using the bake hardening characteristics in cold rolled steel sheets for automobiles, the strength increase 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,
For that purpose, it is considered 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 was found that the material has a curing property in a wider temperature range, the expression "hardening 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 S), which was a previously unthinkable amount of cure, was obtained. 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 was found that the effect is increased when Nb and V are added in combination. Furthermore, C
It has been found that the effect is further increased by simultaneously adding u and Ni together. Specifically, Mo
Curing amount 10k by low temperature heat treatment by combined addition of Nb and V
What was gf / mm ² or more (ΔTS) was C
It has been found that the combined addition of u and Ni results in a curing amount of 15 kgf / mm ² or more (ΔTS) due to the low temperature heat treatment.

It has been found that it is not necessary to specify 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 almost no effect on hardening by low temperature heat treatment. I found that I did not give.

Further, regarding the winding condition, when low temperature winding is performed, martensite and bainite are mixed in the second phase, so that a so-called two-phase steel is formed and the yield ratio is lowered. Therefore, the yield ratio increases due to work hardening at the time of manufacturing a steel pipe, that is, at the time of pipe making. Further, when the coiling temperature is high, carbides of Mo are precipitated and the tensile strength and the yield ratio are increased. Therefore, there is almost no increase in the yield ratio during steel pipe manufacturing. That is, it is considered that the difference due to the winding temperature has a level of tensile strength and a yield ratio of the material, and slightly affects the ease of manufacturing the steel pipe.

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 hardening amount in the low temperature heat treatment is (1) at most 10 kgf / in the Mo + Cu + Ni addition system.
mm ² (2) Only 10 to 12 kgf / mm ² can be expected in the composite additive system of Mo + Cu + Ni + (Nb or V), whereas the curing amount in low temperature heat treatment is (1) 13 kgf / mm ² or more in the Mo + Cu + Ni additive system (2) In order to obtain 15 kgf / mm ² or more in the Mo + Cu + Ni + (Nb or V) composite addition system, it was clarified that the total strain amount in the longitudinal direction needs to be 10% or more. At the same time, the upper limit of the total strain in the longitudinal direction was also examined, and it was clarified that if the strain was too large, the bake-hardening amount was decreased. More specifically, in order to ensure 15 kgf / mm ² or more in the cured amount in low-temperature heat treatment (1) 13kgf / mm ² or more Mo + Cu + Ni addition system (2) Mo + Cu + Ni + (Nb or V) combined addition system, longitudinal It was found that the total strain in the direction should be 30% or less.

Based on the above findings, the present invention has made it possible to enhance the strength of the electric resistance welded steel pipe by low temperature heat treatment. The gist of the invention is (1) wt% Mo at 0%. 1 to 2.0% and C: 0.01 to 0.30% and Cu: 0.10 to 2.0
%, And low carbon steel containing Ni: 0.10 to 9.0% or (2) wt%, containing 0.1 to 2.0% of Mo, and Nb: 0.010 to 0. 15%, V: 0.01
0 to 0.30% of one or two, and C: 0.
01-0.30% and Cu: 0.10-2.0%, and N
i: A low carbon steel containing 0.10 to 9.0% is hot-rolled and pipe-formed in a usual process, cold strain is given as necessary, and a cumulative strain in the longitudinal direction is 10%. -30%, and then low-temperature heat treatment in a temperature range of 200 ° C to 300 ° C, which is a method for producing a high strength electric resistance welded steel pipe.

[0017]

In the present invention, after the material having the defined components is made into the electric resistance welded steel pipe, the secondary working is carried out if necessary, and then the strength is remarkably increased by the low temperature heat treatment.

Next, the components of the present invention, hot rolling conditions, and steel pipe manufacturing conditions will be described. The method of the present invention comprises (1) C:
0.01 to 0.30% and Mo: 0.10 to 2.0% and C
u: 0.10 to 2.0%, and Ni: 0.10 to 9.0
% Low carbon steel or (2) C: 0.01 to 0.3
0% and Mo: 0.10 to 2.0% and Cu: 0.10
2.0% and Ni: 0.10 to 9.0%, and Nb: 0.01 to 0.15% and V: 0.010 to 0.
It can be applied to low carbon steel containing 30% of one or two with good results. Other preferable component compositions include: Si: 0.02 to 0.50% Mn: 0.10 to 1.00% Al: 0.001 to 0.100% N: 0.005 to 0.0100% Depending on required properties of low carbon steel or strength steel to be used, Cr: 5.5% or less Ti: 0.15% or less B: 0.003 to 0.0030% Ca: 0.0080% or less One or more types You may add.

C is indispensable for age hardening as solid solution C, and may be C> 0. However, since C <0.01% leads to high manufacturing cost, the lower limit of the C amount is 0.01
%. C is also useful for increasing the strength level of the material, but if the addition amount increases, the workability deteriorates, so the upper limit was made 0.30%.

Mo is known as an element having a very large interaction with dislocations. That is, since Mo exists as a cluster in the steel in the vicinity of dislocations, C attracted to Mo is used for fixing dislocations, resulting in a remarkable increase in age hardening as compared with the case where Mo is not added. It is considered to improve the bake hardenability. For the above reasons, addition of Mo is indispensable to the present invention, but if the addition amount is too large, weldability is impaired, so the upper limit of the content was made 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 promote the formation of carbide of Mo, but if it increases, it impairs the 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%.

Regarding the contribution of Cu to the hardening characteristics in the low temperature heat treatment, its mechanism has not been completely clarified, but it is considered to promote the carbide formation of Mo in some way. In order to bring out the effect, it is necessary to add 0.10% or more, but even if it is added over 2.0%, further improvement of the curing characteristics cannot be expected, so the range is set to 0.
It was set to 10 to 2.0%.

Regarding the contribution of Ni to the hardening characteristics in the low temperature heat treatment, as in the case of Cu, its mechanism has not been completely elucidated, but it is considered that Mo carbide formation is promoted in some form. In order to bring out the effect, 0.10% or more needs to be added, but since it is an expensive element, the range was made 0.10 to 9.0%.

Further, Si, Mn, and
With respect to Al and N, the range of commonly used components may be used, and Si has the action of improving the strength and ductility of the steel material by the solid solution strengthening action. 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%.

If 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

The components added for the purpose of increasing the strength will be described. 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 specified. However, it is necessary to determine the manufacturing conditions in consideration of the required characteristics of the final product, in consideration of the grain size under the finish rolling conditions, and the tensile strength level and the yield ratio under the winding 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%.

[0035]

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

[0036]

[Table 1]

[0037]

[Table 2]

Steel pipes NoA1, B1, C1, shown in Table 2
D1, J1, N1, O1, P1, Q1, R1, S1, T
1, U1 and V1 are steels that have been subjected to this treatment, and the target of the present invention is 13 kgf / mm ² or more (Mo + Cu + Ni added system) or 15 kgf / mm ² or more (Mo + Cu + Ni +).
(Nb or V) composite addition system) shows the curing characteristics by low temperature heat treatment.

Further, since E1 and F1 do not contain Mo, bake hardening hardly occurs.

Further, in G1, Mo was added, but Cu and Ni were not added, so that the hardening amount by the low temperature heat treatment is less than 13 kgf / mm ² .

Further, H1 contains Mo but does not contain Cu but only Ni, so that the amount of hardening by the low temperature heat treatment is less than 13 kgf / mm ² .

Regarding I1, the amount of hardening by low temperature heat treatment is less than 13 kgf / mm ² because Mo is added but Ni is not added but Cu is added.

Further, J2 has Mo, Cu and Ni added thereto, but the total amount of strain in the longitudinal direction is less than 10%, so that the amount of hardening by low temperature heat treatment is less than 13 kgf / mm ² .

Further, J3 has a total strain amount in the longitudinal direction of more than 30%, so that the hardening amount by the low temperature heat treatment is 13 kgf / mm ²
It is less than.

Further, J4 has a curing amount of less than 13 kgf / mm ² due to the low temperature heat treatment because the heat treatment temperature is too low.

Further, J5 has a curing amount of less than 13 kgf / mm ² due to the low temperature heat treatment because the heat treatment temperature is too high.

Further, K1 has Nb or V added in combination to Mo, but since neither Cu nor Ni is added, the amount of hardening by the low temperature heat treatment is 15 kgf / mm.
It is less than ² .

Further, L1 contains Nb or V added in combination to Mo, but since Ni is not added and Cu is not added, the hardening amount by low temperature heat treatment is 15 kgf / mm.
It is less than ² .

Further, M1 contains Nb or V added in combination to Mo, but since Ni is not added but only Cu, the amount of hardening by the low temperature heat treatment is 15 kgf / mm.
It is less than ² .

Although N2 is added with Mo, Cu, Ni and Nb or V, the total strain amount in the longitudinal direction is 1
Since it is less than 0%, the amount of hardening by low temperature heat treatment is 15 kgf
/ Mm ^{2 or} less.

Since N3 is the total strain amount in the longitudinal direction or more than 30%, the curing amount by the low temperature heat treatment is 15 kgf / mm ²
It is less than.

Further, N4 has a curing amount of less than 15 kgf / mm ² due to the low temperature heat treatment because the heat treatment temperature is too low.

Further, N5 has a curing amount of less than 15 kgf / mm ² due to the low temperature heat treatment because the heat treatment temperature is too high.

[0054]

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

Claims (2)

[Claims] 1. A Mo content of 0.1 to 2.0% by weight, and C: 0.01 to 0.30% and Cu: 0.10.
-2.0% and low carbon steel containing Ni: 0.10 to 9.0% are hot-rolled and pipe-formed in a usual process, and given a cold strain as necessary, a long length Direction cumulative strain is 10% ~
A method for producing a high-strength electric resistance welded steel pipe, which comprises 30%, and then low-temperature heat treatment in a temperature range of 200 ° C to 300 ° C. 2. The content of Mo is 0.1 to 2.0% by weight, and Nb is 0.010 to 0.15% and V is 0.0.
10 to 0.30% of one or two, and C:
A low carbon steel containing 0.01 to 0.30%, Cu: 0.10 to 2.0%, and Ni: 0.10 to 9.0% is hot-rolled in a normal process to form a pipe. High-strength electric resistance welded steel pipe, characterized in that cold strain is applied as necessary, the 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. Manufacturing method.