JPH11106825A - Production of extra-low carbon nonheat treated type high workability electric resistance welded tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an extra-low carbon steel tube having the hardness in the weld zone equal to that of a base metal part which the steel tube is produced. SOLUTION: A steel slab contg. <=0.01% C, <=0.05% Si, <=0.3% Mn, <=0.03% P, <=0.02% S and <=0.08% sol. Al, contg. one or two kinds of 0.002 to 0.10% Ti and 0.002 to 0.10% Nb, and the balance Fe with inevitable impurities is heated to the temp. range of 1100 to 1280 deg.C and is thereafter subjected to hot rolling so as to regulate the finishing temp. to >= the Ar3 transformation point and the coiling temp. to >=500 deg.C, and the obtd. hot rolled coil or a cold rolle coil obtd. by subjecting the hot rolled coil to cold rolling is used as a stock, the 2 to 4 mm widths of both edge parts of an opening pipe cylindrical subjected to continuous forming by a forming roll group are preheated at 700 to 1000 deg.C, are subjected to normal heating and are subject to electric resistance welding, and the weld zone is allowed to cool to obtain the hardness of the weld zone approximately equal to that of the base metal part while electric resistance-weld is performed, by which the heat treating cost can be reduced, and, moreover, the problem of scale exerting a bad influence on a die in bulging can be solved as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic forming method (hydraulic forming method) in which a liquid pressure is applied to the inner surface of a pipe to form an overhang.
The present invention relates to a method for producing a non-heat-treated electric resistance welded steel pipe having excellent workability under severe processing such as c forming (hereinafter referred to as bulging).

[0002]

2. Description of the Related Art In the automotive industry, as global environmental problems are increasingly attracting attention, efforts to reduce fuel weight to reduce fuel consumption have been given top priority. For this reason,
Recently, as a method of realizing a drastic weight reduction of an automobile, development of severe processing technology such as bulging using a steel pipe as a material has been advanced.

Conventional methods for producing a steel pipe having excellent workability include: C: 0.01% or less; Si: 0.05% or less;
n: 0.30% or less, P: 0.025% or less, S: 0.
015% or less, sol. Al: 0.080% or less, Ti
Or one or two types of Nb: 0.002 to 0.10.
%, The balance consisting of Fe and unavoidable impurities, was hot-rolled to a predetermined thickness at a hot strip finishing temperature of Ar ₃ + 40 ° C. or higher and a winding temperature of 500 ° C. or higher, and the obtained hot-rolled steel sheet was cooled. Then, a method of heat-treating a steel pipe formed into a tubular shape and subjected to electric resistance welding at 700 to 900 ° C. (Japanese Patent Laid-Open No. 4-365815), C: 0.01% or less, S
i: 0.05% or less, Mn: 0.40% or less, P: 0.
025% or less, S: 0.025% or less, sol. Al:
0.050% or less, B: 0.0005 to 0.0030
%, The balance consisting of Fe and unavoidable impurities, is formed into a tubular body, and then heat-treated at a temperature of 850 to 900 ° C. (JP-A-56-119756).
Etc. have been proposed.

[0004]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Laid-Open No. 4-365 is disclosed.
In the method disclosed in Japanese Patent Publication No. 815, the electric resistance welded portion is hardened by the effect of improving the hardenability of Ti and Nb added to improve the workability of the material, and the hardness of the base material portion and the welded portion becomes uneven. Therefore, heat treatment of the entire steel pipe after pipe production is indispensable, which increases the production costs such as heat treatment, and causes heat treatment scale to be generated on the surface, which is accumulated in the mold during bulging and causes the mold to wear. Becomes

[0005] Further, in the ultra-mild steel pipe disclosed in JP-A-56-119756, the electric resistance welded portion is hardened due to the effect of improving the hardenability of B added to improve the workability of the material.
Since the hardness of the base material and the welded part becomes uneven, heat treatment of the entire steel pipe after pipe production is indispensable, which increases production costs such as heat treatment, and also causes heat treatment scale to be generated on the surface. And accumulates in the mold, causing wear of the mold. The extremely mild steel pipe is used for tap water and gas pipes, not for automobiles or other mechanical structures.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned disadvantages of the prior art and to obtain an ultra-low carbon, non-heat-treated type in which the hardness of a welded portion equivalent to that of a base material can be obtained in a pipe-made state (without heat treatment). An object of the present invention is to provide a method for manufacturing a high-workability electric resistance welded steel pipe.

[0007]

According to the first aspect of the present invention, there is provided a method for producing an ultra-low carbon, non-heat-treated, high-workability ERW steel pipe, comprising:
0.01% or less, Si: 0.05% or less, Mn: 0.3
%, P: 0.03% or less, S: 0.02% or less, s
ol. Al: 0.08% or less, Ti: 0.002
０．１0.10%, Nb: A steel slab containing one or two of 0.002 to 0.10%, the balance being Fe and inevitable impurities is heated to a temperature range of 1100 to 1280 ° C. After that, hot rolling at a finishing temperature of not less than the transformation point of Ar ₃ and a winding temperature of not less than 500 ° C., and using the obtained hot-rolled coil as a raw material, both edges of an open pipe continuously formed into a cylindrical shape by forming rolls. Part 2-4mm width 70
After preheating to 0 to 1000 ° C., main heating is performed, electric resistance welding is performed, and the weld is allowed to cool.

Thus, the steel slab having the above chemical composition is
After heating to a temperature range of 1100 to 1280 ° C., hot rolling is performed at a finishing temperature of Ar ₃ transformation point or higher, and a winding temperature of 500 ° C. or higher, and the obtained hot-rolled coil is used as a material. After preheating 2 to 4 mm width of both edges of the open pipe continuously formed to 700 to 1000 ° C, main heating and ERW welding, and letting the welded portion cool down, the base metal remains as ERW welded. It is possible to obtain a weld hardness substantially equal to that of the welded portion, and it is possible to eliminate the need for heat treatment after pipe production.

The method for producing an ultra-low carbon, non-heat-treated, high-workability ERW steel pipe according to claim 2 of the present invention is characterized in that: C: 0.01%
Hereinafter, Si: 0.05% or less, Mn: 0.3% or less,
P: 0.03% or less, S: 0.02% or less, sol. A
l: 0.08% or less, Ti: 0.002 to 0.1
After heating a steel slab containing 0%, Nb: one or two of 0.002 to 0.10%, and the balance being Fe and unavoidable impurities, to a temperature range of 1100 to 1280 ° C, Finishing temperature Ar ₃ transformation point or higher, winding temperature 5
The hot-rolled coil obtained by hot rolling at 00 ° C. or higher was pickled and then cold-rolled, and the cold-rolled coil subjected to annealing and heat treatment was used as a raw material. Edge part 2-4mm width 700-1000
After preheating to ℃, the main heating and ERW welding, and the weld is allowed to cool.

As described above, the steel slab having the above chemical composition is
After heating to a temperature range of 1100 to 1280 ° C., hot rolling was performed at a finishing temperature of Ar ₃ transformation point or higher and a winding temperature of 500 ° C. or higher, and the obtained hot-rolled coil was cold-rolled after being pickled and subjected to annealing heat treatment. After using a cold-rolled coil as a raw material and preheating 2 to 4 mm width on both edges of an open pipe continuously formed into a cylindrical shape by a forming roll group to 700 to 1000 ° C,
Main heating, ERW welding, and letting the ERW weld cool down allow it to meet the requirements for thin wall thickness and high wall thickness accuracy. Approximately equivalent weld hardness can be obtained, and heat treatment after pipe production can be eliminated.

[0011]

DETAILED DESCRIPTION OF THE INVENTION In the method for producing an ultra-low carbon, non-heat-treated, high-workability ERW steel pipe according to claim 1 of the present invention, a hot-rolled coil is used as a material, and both edges 2 to 2 of an open pipe are used. After preheating 4mm width to 700-1000 ° C,
Main heating and ERW welding, and the ERW weld is allowed to cool.By increasing the amount of heat in the weld, the cooling rate is reduced, and the formation of martensite and bainite hardened structures in the weld is reduced. Thus, the hardness of the welded portion can be substantially the same as that of the base material as it is in the electric resistance welding, and the heat treatment after the pipe production can be eliminated. For this reason, the heat treatment cost can be reduced, and good processing can be performed without a scale problem that adversely affects the mold during bulging.

Further, in the method for manufacturing an ultra-low carbon, non-heat-treated, high-workability ERW steel pipe according to the second aspect of the present invention, since a cold-rolled coil is used as a material, a case where the wall thickness is small or a high wall thickness is used. In addition to meeting the demand for precision, the width of both edges of the open pipe 2 to 4 mm can be adjusted to 700 to 1000.
After preheating to ℃, the main heating and ERW welding, and the ERW weld is allowed to cool, so by increasing the amount of heat in the weld,
The cooling rate is reduced, and the formation of martensite and bainite quenched structure is prevented in the welded portion, and the welded portion hardness can be substantially equal to that of the base material portion as it is in ERW welding. Heat treatment can be eliminated. For this reason, the heat treatment cost can be reduced, and good processing can be performed without a scale problem that adversely affects the mold during bulging.

In the present invention, the chemical components of the steel slab are limited for the following reasons. C increases the mechanical strength of the steel due to the increase and decreases the workability,
Ultra-low carbon steel of 0.01% or less. In practice, 0.0
If it exceeds 1%, the desired workability of the steel of the present invention cannot be obtained.

[0014] Si is an element effective as a deoxidizing element in steel, but if it exceeds 0.05%, the strength is increased and the workability is reduced.

Mn is an indispensable element for securing the toughness of steel, but if it exceeds 0.3%, the strength increases and the workability decreases, so Mn is set to 0.3% or less.

If P exceeds 0.03%, it precipitates at the grain boundaries and impairs the workability. Therefore, P is set to 0.03% or less.

If S exceeds 0.02%, S bonds with Mn to form inclusions as MnS, impairing workability.
0.02% or less.

Sol. Al is an element necessary for preventing strain aging by fixing N in the steel, but if it exceeds 0.08%, the steel is hardened. Therefore, Al is set to 0.08% or less.

Ti and Nb are elements necessary to prevent excessive coarsening of crystal grains and to improve toughness and workability, but if less than 0.002%, the effect cannot be obtained.
On the other hand, if it exceeds 0.10%, the strength will be increased,
In order to reduce the workability, the content was made 0.002 to 0.10%.

In the present invention, the heating temperature of the steel slab, the hot rolling condition, the cold rolling condition, and the heating condition of both edges of the open pipe are limited for the following reasons.

If the heating temperature of the steel slab is less than 1100 ° C., the rolling temperature is too low, so that the finishing temperature in hot rolling cannot be maintained at the Ar ₃ transformation point or higher, and the finish rolling is performed at a low temperature. , Causes an increase in the load on the rolling equipment, and, when the temperature exceeds 1280 ° C., the surface of the slab such as roughened surface and indentation of the scale is reduced.
1100-1280 degreeC.

If the finishing temperature in the hot rolling is equal to or higher than the Ar ₃ transformation point, sufficient workability can be ensured by the growth of crystal grains in the steel strip, but further processing is performed by securing the grain growth from the Ar ₃ transformation point. In order to improve the properties, it is preferable to finish at as high a temperature as possible.

If the winding temperature in the hot rolling is lower than 500 ° C., the workability cannot be improved by the self-tempering effect.

The cold rolling of the hot-rolled coil is performed when the thickness of the ERW steel pipe to be manufactured is small or when high wall thickness accuracy is required. However, since workability decreases due to work hardening, it is necessary to perform annealing after cold rolling.

As for the heating of both edges of the open pipe, generally, when the frequency f is reduced, the current penetration depth δ (δ = 5.03√ (ρ / f · μ) c at the edges)
m, μ: relative magnetic permeability, ρ: specific resistance μΩ-cm, f: frequency Hz), the heating width is widened, and the cooling characteristics of the welded portion tend to be improved. Accordingly, in the present invention, the preheating width of both edges of the open pipe continuously formed into a cylindrical shape by the forming roll group is, as shown in FIG. 1, the frequency f of the high-frequency preheating device so that the preheating width W is 2 mm to 4 mm. What is necessary is just to select appropriately. The preheating width W is 2 mm
If it is less than 1, the heat input is insufficient and the amount of heat itself is small, so that the temperature rapidly drops and the rapid heating and quenching of the ERW weld is not improved, and if it exceeds 4 mm, the preheating effect does not change. , The amount of heat input is so large that power is wasted,
mm to 4 mm.

In the present invention, the preheating temperature of both edges of the open pipe continuously formed into a cylindrical shape by the forming roll group is as follows:
If the temperature is less than 700 ° C., the preheating is insufficient to prevent rapid cooling of the ERW weld. If the temperature exceeds 1000 ° C., burn-through occurs during ERW, or the metal flow is poor and molten steel discharge is insufficient, resulting in poor welding quality. 700 ° C ~ 100
0 ° C.

In the present invention, the main heating at the time of welding is 1200 to 1400 ° C., which is the heating temperature at the time of ordinary electric resistance welding. If the temperature is less than 1200 ° C., the joining surface temperature is low, welding is incomplete, and cold welding and the like are likely to occur.
If the temperature exceeds ℃, the pressure at the bonding surface decreases, so that burn-through occurs, penetrators are generated, spatter increases, and beads are increased in size, resulting in a decrease in workability.

In the present invention, the reason why the welded portion is allowed to cool after the electric resistance welding is to prevent the formation of a quenched structure of martensite and bainite in the welded portion due to rapid cooling after the electric resistance welding.

In the present invention, before the electric resistance welding, the width of both edges of the open pipe at 2 to 4 mm is increased to 700 ° C. to 1000 ° C.
Since the temperature is preheated to ° C., the amount of heat input during the electric resistance welding can be controlled by the components and the wall thickness of the pipe, so that excessive power consumption can be avoided.

[0030]

EXAMPLE Steel No. 1 shown in Table 1 was used. After melting steels having the chemical compositions of A to D, a coil having a thickness of 1.80 mm to 3.00 mm manufactured under hot rolling conditions and the like shown in Table 2 was used as a material, and was continuously formed into a cylindrical shape by a forming roll. 280 KH for each of the cases where both edges of the open pipe were preheated under the preheating conditions shown in Table 3 and when they were not preheated.
ERF welding using high-frequency current of z, outer diameter 38.1 mm
An electric resistance welded steel pipe having a thickness of ６60.3 mm and a wall thickness of 1.80 mm to 3.00 mm was manufactured. A test piece was collected from each of the obtained electric resistance welded steel pipes, and the tensile strength and the total elongation were measured according to the metal material tensile test method specified in JIS Z2241.
Vickers hardness (HV) at a test load of 49.03 N according to the Vickers hardness test method specified in Z2244.
5) was measured. Table 4 shows the results.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

As shown in Tables 2 to 4, Test Nos. Of Comparative Examples in which the winding temperature using steel type A was out of the range of the present invention. For the steel pipe of No. 5, the self-tempering effect was not obtained, and test No. 5 of the example using the same steel type A was used. Compared with the steel pipe No. 1, the tensile strength increases by 50 N / mm ² , the elongation decreases, and the hardness of the base material and the welded portion increases significantly, resulting in inferior workability. Furthermore, the test No. of the comparative example in which the preheating condition using steel type B is out of the range of the present invention. In the steel pipe No. 6, the difference between the hardness of the weld portion and the hardness of the base metal portion is as large as 58. Furthermore, the test No. of the comparative example which did not preheat both edges of the open pipe using steel type D. In the steel pipe No. 7, the difference between the weld hardness and the base metal hardness is as large as 52. On the other hand, in the test Nos. Of the examples using steel types A to D. Each steel pipe of 1-4 is
The difference between the hardness of the welded part and the hardness of the base material shows almost the same Vickers strength of 1 to 4 with no increase in tensile strength or decrease in elongation as it is in a pipe-made tube. A sewn steel pipe was obtained.

[0036]

According to the method for producing an ultra-low carbon, non-heat-treated, high-workability electric resistance welded steel pipe of the present invention, it is possible to obtain a welded part hardness substantially equal to that of the base material part in the electric resistance welded state. Since heat treatment after pipe production is not required, heat treatment costs can be reduced, and good processing can be performed without a scale problem that adversely affects a mold during bulging.

Further, the method for manufacturing an ultra-low carbon, non-heat-treated, high-workability ERW steel pipe according to the second aspect of the present invention can cope with a case where the wall thickness is thin or a demand for high wall thickness accuracy. In addition, it is possible to obtain almost the same hardness of the welded portion as the base material portion in the electric resistance welding as it is, and it becomes unnecessary to perform the heat treatment after pipe making. Therefore, the heat treatment cost can be reduced, and good processing can be performed without a scale problem that adversely affects the mold during bulging.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a heating width of both edge portions of a coil.

Claims (2)

[Claims] 1. C: 0.01% or less, Si: 0.05%
Hereinafter, Mn: 0.3% or less, P: 0.03% or less, S:
0.02% or less, sol. Al: 0.08% or less, Ti: 0.002 to 0.10%, Nb: 0.002
A steel slab containing one or two of 0.1% to 0.10%, with the balance being Fe and unavoidable impurities,
After heating to a temperature range of 0 to 1280 ° C., the finishing temperature A
r ₃ transformation point or more, then hot rolled at the coiling temperature 500 ° C. or higher,
Using the obtained hot-rolled coil as a material, after preheating both edges 2 to 4 mm width of an open pipe continuously formed into a cylindrical shape by a forming roll group to 700 to 1000 ° C.
A method for producing an ultra-low carbon, non-heat-treated, high-workability electric resistance welded steel pipe, characterized by main heating, electric resistance welding, and allowing the welded part to cool. 2. C: 0.01% or less, Si: 0.05%
Hereinafter, Mn: 0.3% or less, P: 0.03% or less, S:
0.02% or less, sol. Al: 0.08% or less, Ti: 0.002 to 0.10%, Nb: 0.002
A steel slab containing one or two of 0.1% to 0.10%, with the balance being Fe and unavoidable impurities,
After heating to a temperature range of 0 to 1280 ° C., the finishing temperature A
r ₃ transformation point or more, then hot rolled at the coiling temperature 500 ° C. or higher,
The obtained hot-rolled coil was cold-rolled after pickling, using a cold-rolled coil subjected to annealing heat treatment as a material, and both edges 2 to 2 of an open pipe continuously formed into a cylindrical shape by a group of forming rolls.
A method for producing a non-heat-treated high-workability ERW steel pipe, comprising preheating a 4 mm width to 700 to 1000 ° C., performing main heating, performing ERW welding, and allowing the ERW weld to cool.