JP2007056283A - High-strength thick-wall electric resistance welded steel tube having excellent hardenability and decarburization resistance, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength thick-wall electric resistance welded steel tube having excellent hardenability and decarburization resistance and also to provide its manufacturing method. <P>SOLUTION: The thick-wall electric resistance welded steel tube has a composition consisting of, by mass, 0.15 to 0.5% C, 0.05 to 0.5% Si, 0.3 to 2% Mn, ≤0.05% P, ≤0.05% S, ≤0.05% Al, 0.005 to 0.05% Ti, 0.0005 to 0.01% B, 0.001 to 0.01% N, further either or both of 0.01 to 0.2% Sb and 0.2 to 0.5% Cu and the balance Fe with inevitable impurities. Moreover, critical cooling velocity Vc represented by equality logVc=2.94-0.75β (where β=2.7+0.4Si+Mn) is <30°C/s, and t/D as a ratio between wall thickness (t) and outside diameter D ranges from >0.15 to 0.30. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength thick-walled electric-welded welded steel pipe that is suitable for a hollow stabilizer for ensuring the running stability of an automobile, and is excellent in hardenability and decarburization resistance, and a method for manufacturing the same.

As one of the measures to improve the fuel efficiency of automobiles, weight reduction of the car body is being promoted. Stabilizers that reduce the rolling of the car body when cornering an automobile and ensure the stability of the car body when driving at high speed are also cited as targets. Conventionally, stabilizers were manufactured by processing solid materials such as steel bars into the required shape, but they must be manufactured using hollow materials such as seamless steel pipes and ERW welded steel pipes to reduce weight. Has increased.
As an electric resistance welded steel pipe for stabilizers, in Patent Document 1, by defining the composition, the metal structure of the electric resistance welded part and the base metal part is uniform, and the hardness difference between the electric resistance welded part and the base metal part is small. In addition, an electric resistance welded steel pipe for a hollow stabilizer having excellent workability is disclosed, and Patent Document 2 discloses electric resistance welding for a hollow stabilizer that ensures hardenability by defining the contents of Ti and N. A steel pipe is disclosed.
Japanese Patent Laid-Open Publication No. 2003-28259 proposes an electric resistance welded steel pipe for a hollow stabilizer in which the ratio t / D between the wall thickness and the outer diameter of the steel pipe is 20% or more and the tensile strength is 400 to 755 N / mm ² It is disclosed to increase the wall thickness by radial rolling.
Patent Document 4 proposes a high-strength steel pipe for automobile structural members that is excellent in workability to withstand hydroforming, in which a raw pipe is reduced in diameter, has a tensile strength of over 580 MPa, and a yield ratio is 70% or less. Furthermore, Patent Document 5 discloses a high-tensile steel pipe excellent in composite secondary workability such as bending, diameter reduction, and tube end flattening by specifying the heating temperature, diameter reduction ratio, etc. in diameter reduction rolling. The manufacturing method of this is proposed.
Further, in Patent Document 6, in the forming step of forming the raw material pipe into a stabilizer shape by cold bending, and the manufacturing method of the hollow stabilizer for quenching and tempering the formed steel pipe, the raw material pipe is A method for producing a hollow stabilizer excellent in fatigue resistance is disclosed in which a base steel pipe is subjected to heat treatment and then drawn at a rolling temperature of 600 to 850 ° C. and a cumulative diameter reduction ratio of 40% or more.
Patent Document 7 includes, in mass%, C: 0.15 to 0.3%, Mn: 0.5 to 2.0%, Cu: 0.05 to 0.30%, and Si ≦ 0.41%, P ≦ 0.02%, Al ≦ 0.03%, Nb ≦ 0.020%, B ≦ 0.001%, Ti ≦ 0.01%, Cr ≦ 0.42% Containing at least one selected, limited to 0 ≦ Ni + Mo <0.15% and S ≦ 0.003% as unavoidable impurities, the balance being excellent in workability made of Fe, after hydrogen intrusion due to corrosion A hardened ERW steel pipe with a high residual strength rate is disclosed.

By the way, for example, a stabilizer further reduces the diameter of an electric resistance welded steel pipe to obtain a required shape by cold forming such as bending 1). Or after tempering by heating, cooling with water and then tempering, or 2) heating the thick ERW welded steel pipe and forming it into the required shape by hot forming such as pressing. It is manufactured by tempering after cooling with water and quenching. The latter method by hot forming is advantageous as a manufacturing process because it is easier to process and can handle complex shapes than the former method by cold forming.
However, in this method, since the molding is performed after heating, the time until quenching becomes long, the temperature of the molded member decreases, the temperature decreases due to the contact between the press die and the material steel pipe (electrically welded steel pipe), Alternatively, due to generation of non-uniform temperature due to the generation of a heating scale, it is difficult to ensure a sufficient quenching state as a whole, and there is a concern that insufficient quenching may occur, and a material with excellent quenchability is required.

Further, in the case of hot forming, heat treatment increases as compared with cold forming, and therefore decarburization is unavoidable in the formed steel pipe member. That is, in addition to heating for quenching and tempering, heating for molding is added, and thus it is inevitable that a ferrite decarburized layer is easily formed. When such a ferrite decarburized layer is present, in the case of an automobile member, mechanical materials such as the strength and hardness of the surface layer portion are lowered, and not only the required surface hardness and surface strength can be obtained, but also bending processing is performed. In the case of secondary processing such as cutting, threading, and joining with other members, it may cause processing defects such as cracks, cutting defects, and bonding defects. Accordingly, there is a need for an electric resistance welded steel pipe having excellent decarburization resistance.
Further, the weight reduction of the vehicle body is in a trend of further acceleration, and there is a demand for a high strength and toughness as an electric resistance welded steel pipe for a stabilizer.

WO2002 / 070767 Japanese Patent Laid-Open No. 2004-011009 Japanese Patent Laid-Open No. 2004-009126 JP 2003-201543 A JP 2004-292922 A JP-A-2005-076047 Japanese Patent No. 3655381

The electric resistance welded steel pipe for stabilizers and the high strength steel pipes described in Patent Documents 1 to 7 are useful as steel pipes for automobile structural members, but as described above, due to process changes in the manufacturing process of automobile structural members It is not enough to deal with the problems that occur.
In view of the above-described problems, an object of the present invention is to provide a high-strength thick-walled electric-welded steel pipe having sufficient hardenability and excellent decarburization resistance and a method for producing the same.

The thick-walled ERW welded steel pipe of the present invention is made to solve the above-mentioned problems, and increases the strength (hardness) by increasing C as much as possible without impairing the weldability and toughness. The decarburization resistance is improved by adding Sb, and the hardenability is ensured by adjusting the composition of the steel material so that the critical cooling rate Vc is in a specific range. And in manufacture of the thick-walled electric resistance welded steel pipe of this invention, diameter reduction rolling is given to an electric resistance welded steel pipe by making heating temperature and a cross-sectional reduction rate into a specific range.
The gist is as follows.
(1) In mass%,
C: 0.15-0.5%, Si: 0.05-0.5%, Mn: 0.3-2%, P: 0.05% or less, S: 0.05% or less, Al: 0 0.05% or less, Ti: 0.005-0.05%, B: 0.0005-0.01%, N: 0.001-0.01%,
Cu: 0.2 to 0.5%, Sb: 0.01 to 0.2% of one or two kinds, the balance consists of Fe and inevitable impurities, the criticality represented by the formula <1> Hardening and decarburization resistance, characterized in that the cooling rate Vc is less than 30 ° C./s, and the ratio of the wall thickness t to the outer diameter D is in the range of more than 0.15 to 0.30. High strength thick wall ERW welded steel pipe with excellent properties
logVc = 2.94−0.75β <1>
However, β = 2.7C + 0.4Si + Mn.
(2) In mass%,
Cr: 0.1 to 1%, Mo: 0.05 to 1%, Nb: 0.01 to 0.1%, V: 0.01 to 0.5%, Ni: 0.1 to 1% The high-strength thick-walled electric-welded steel pipe excellent in hardenability and decarburization resistance according to (1), characterized by containing seeds or two or more kinds.
However, in formula <1>
β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo.
(3) The hardenability and decarburization resistance according to (1) or (2), wherein the thickness of the ferrite decarburized layer after holding at 800 ° C. for 1 hour in the air is less than 0.15 mm Excellent high-strength thick-walled ERW welded steel pipe.
(4) The ERW welded steel pipe having the component described in (1) or (2) is heated to 800 to 1200 ° C. and reduced in diameter in the range of 40 to 80% in cross-section reduction rate. A method for producing a high-strength thick-walled ERW welded steel pipe with excellent hardenability and decarburization resistance.

Since the thick-walled ERW welded steel pipe of the present invention is extremely excellent in hardenability, a sufficient quenching effect can be obtained even when a quenching treatment is performed immediately after hot forming in the manufacture of a stabilizer or the like. Further, the quenching means is not limited to water cooling, and a sufficient quenching effect can be obtained by oil quenching with a cooling rate lower than that of water cooling.
In addition, since it has excellent decarburization resistance, the formation of a ferrite decarburized layer is much smaller than before even when hot forming, quenching, and tempering when manufacturing an automobile member, and the surface of the member is hard. The strength can be maintained in the same manner as the inside, and the occurrence of processing defects in the secondary processing can be reduced.
Furthermore, it is excellent in strength and toughness, and is excellent in durability against repeated loads, and the stabilizer can be further reduced in weight.

Inventors examined the method of improving the hardenability of the thick-wall ERW welded steel pipe for stabilizers, and further strengthening strength and toughness.
First, with regard to strength, water-cooled quenching is performed on steel materials (test materials A, B, C) for ERW welded steel pipes having compositions as shown in Table 1 with varying amounts of C, and comparative steel materials (conventional materials A, B). The hardness after tempering was investigated. FIG. 1 shows the relationship between the change in hardness after water-cooled quenching and tempering and the C content.
As can be seen from FIG. 1, the hardness increases as the amount of C increases. For example, the hardness increases by about 10% or more from the hardness level of the conventional material, and it can be seen that the strength can be improved. In addition, it turns out that the effect is more remarkable when C: 0.25% or more.

Next, the hardenability was examined.
FIG. 1 also shows the relationship between the hardness and C content of the 100% and 90% martensite structures, but the test materials A, B, and C are at least 10% of the conventional materials if they are quenched to at least 90% martensite structure. It can be seen that the hardness of can be secured. Therefore, as an index of hardenability, for example, iron and steel 74 (1988) p. A critical cooling rate Vc (° C./sec), which is conventionally known from 1073 and can obtain a 90% martensite structure, may be used. This is usually expressed by the following formula <1>.
logVc = 2.94−0.75β <1>
However, β = 2.7C + 0.4Si + Mn or β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo.
From FIG. 1, it is clear that a martensite structure of 90% or more is obtained by water quenching, but in the case of oil quenching, the cooling rate is greatly reduced compared to water quenching. Usually, the cooling rate achieved when oil-quenching a steel pipe of a size used for a stabilizer is 30 ° C./s. Therefore, in order to ensure a 90% martensite structure even in oil quenching, the critical cooling rate Vc is set to less than 30 ° C./sec in the present invention.
As described above, in order to improve the strength of the material steel pipe, the amount of C is increased, and the components are selected so that the critical cooling rate Vc represented by <1> is less than 30 ° C./sec.

Next, the present inventors examined a method for improving the decarburization resistance.
Conventionally, in a high C steel having a C content exceeding 0.5%, such as spring steel, decarburization can be suppressed by adding Cu, Sb, Sn, etc. when heat treatment is performed at a temperature in a two-phase region. It is known to be possible.
However, in the medium C steel having a C content of around 0.3%, and when the temperature in the two-phase region is rapidly passed, that is, in the case of quenching by cooling from the γ-phase temperature region to water, It is not clear whether or not there is an effect of suppressing decarburization when heating is applied in the inter-forming.
Therefore, the inventors manufactured a test material in which Sb was added to medium C steel with a C content of around 0.3%, and suppressed decarburization when the medium C steel was heated and quenched and tempered. The effect of Sb addition on the was confirmed.

That is, a steel material for thick-walled electric-welded welded steel pipe containing 0.05% Sb and test material A ′ prepared in test material A (C: 0.28%) shown in Table 1 was further compared. Therefore, steel materials for thick-walled ERW welded steel pipes shown in Table 1, test material A (C: 0.28%), conventional steel materials for ERW welded steel pipes for hollow stabilizers, conventional material A (C: 0.26%) ) Were prepared, and the specimens taken from these steel materials were subjected to decarburization heat treatment in the atmosphere at 800 ° C. for 1 hour, and then the cross-sectional structure was observed.
Since this decarburization heat treatment was intended to clarify the difference in decarburization properties, the thick-walled ERW welded steel pipe of the present invention was used in the process of forming and tempering treatment for manufacturing automobile structural members. It is a condition that is much easier to decarburize than the heating condition considered to be received.

FIG. 2 is a photograph showing a cross-sectional structure, where (a) shows a conventional material A, (b) shows a test material A, and (c) shows a test material A ′ containing Sb.
As can be seen from FIG. 2, in the conventional material A and test material A to which Sb is not added, the surface layer portion from the surface to 180 to 200 μm becomes a decarburized ferrite phase, and the crystal grains are coarsened. In the test material A ′ to which is added, no ferrite decarburized layer is observed.
When the decarburization suppression effect was confirmed also about Cu and Sn using the same method, the decarburization suppression effect was able to be confirmed about Cu similarly to Sb. However, it has been found that Sn is not sufficient.
It is most desirable that the ferrite decarburized layer is not formed, but if the thickness is less than 0.15 mm from the surface under heating conditions of 800 ° C. × 1 hour, the thickness of the ferrite decarburized layer in the actual manufacturing process is In the present invention, the thickness of the ferrite decarburized layer formed when heated at 800 ° C. for 1 hour in the atmosphere is less than 0.15 mm. Is.
In order to suppress the ferrite decarburization layer in this way, Sb: 0.01 to 0.2%, preferably 0.01 to 0.10%, or Cu: 0.2 to 0.5% alone or It is necessary to contain in combination.

Conventionally, in medium C steel with a C content of around 0.3%, the effect of addition of Sb and / or Cu on the decarburization suppressing effect was not clear, but as described above, in medium C steel, Sb and / or Alternatively, it was confirmed that the decarburization resistance can be improved by adding Cu.
Thus, the thick ERW welded steel pipe of the present invention has high strength by increasing the C content and improves decarburization resistance by adding Sb and / or Cu, and other components. By appropriately controlling the temperature, the critical cooling rate is lowered and the hardenability is improved.

Below, the chemical component of the thick-walled electric resistance welded steel pipe in this invention is demonstrated.
C is an element that increases the strength of steel by being precipitated as solid solution or carbide in the matrix.
A general automobile structural member needs to have a strength of at least 100 kg / mm ² , and a corresponding hardness of around Hv320 is obtained with a 90% martensite structure when the C content is 0.15%. Therefore, it is necessary that C is contained in an amount of 0.15% or more. However, if it exceeds 0.5%, the workability and weldability deteriorate, so the content is 0.15 to 0.5. % Range. In addition, Preferably, it is 0.25%-0.40%.

Si is an alloy element that contributes to solid solution strengthening, and in order to obtain its effect, it is necessary to contain 0.05% or more. Si-Mn-based inclusions that become welding defects are easily generated, which adversely affects the soundness of the ERW weld.
For this reason, content is made into the range of 0.05 to 0.5%. In addition, Preferably, it is 0.10 to 0.3%.

  Mn is an element that improves the hardenability. If the content is less than 0.3%, the effect of improving the hardenability cannot be sufficiently ensured. In order to adversely affect the soundness of the film, the content is made 0.3 to 2%. In addition, Preferably, it is 0.8 to 1.5%.

  Since Al is an element necessary as a deoxidizer for molten steel and also an element for fixing N, its amount has a great influence on the crystal grain size and mechanical properties. If the content exceeds 0.05%, the crystal grain size becomes coarse and the toughness is reduced, or non-metallic inclusions increase and surface defects are likely to occur in the product. % Or less. In addition, Preferably, it is 0.03% or less.

  B is an element that greatly improves the hardenability of the steel material by adding a small amount, and also has an effect of strengthening the grain boundary. If the content is less than 0.0005%, the effect of improving the hardenability cannot be expected. On the other hand, if the content exceeds 0.01%, a coarse B-containing phase tends to be formed, and embrittlement tends to occur. For this reason, the content is made 0.0005% to 0.01%. In addition, Preferably, it is more than 0.0010 to 0.0020%.

  Ti acts to stably and effectively improve the hardenability by adding B by fixing N in steel and suppressing precipitation of BN, but if the content is less than 0.005%, The effect cannot be expected, but if it exceeds 0.05%, the toughness tends to deteriorate, so the content is made 0.005 to 0.05%. In addition, Preferably, it is 0.01 to 0.02%.

  N is an important element for precipitating nitride or carbonitride and increasing the strength. The effect is exerted by adding 0.001% or more, but if the content exceeds 0.01%, the toughness deteriorates due to a decrease in hardenability due to precipitation of BN, coarsening of nitride, and age hardening. The tendency to do is seen. For this reason, the content is made 0.001 to 0.01% of range. In addition, Preferably, it is 0.002 to 0.005%.

  P is an element that adversely affects weld crack resistance and toughness, so is limited to 0.05% or less. In addition, Preferably, it is 0.03% or less.

  S affects the formation of non-metallic inclusions in the steel material, and deteriorates workability such as bendability and flatness of the steel pipe, and causes toughness deterioration and anisotropy and increased reheat cracking sensitivity. Also, the soundness of the welded part has an adverse effect. For this reason, the content is limited to 0.05% or less. In addition, Preferably, it is 0.01% or less.

  Cu, like Sb, is an element that has an effect of suppressing decarburization. However, if the content is less than 0.2%, the effect cannot be sufficiently obtained, while it exceeds 0.5%. Even if contained, the effect is saturated, while the hot workability is lowered and the surface properties are easily impaired. For this reason, content is made into 0.2 to 0.5%. In addition, Preferably, it is 0.2 to 0.4%.

As described above, Sb is an element having an effect of suppressing decarburization. However, if the content is less than 0.01%, the effect cannot be sufficiently obtained, while it exceeds 0.2%. Even if contained, the effect is saturated, while the hot workability is lowered and the surface properties are easily impaired. For this reason, content is made into 0.01 to 0.2%. In addition, Preferably, it is 0.01 to 0.10%.
In the present invention, as described above, Cu and Sb can be contained alone or in combination within the range of the respective contents.

  The thick-walled electric resistance welded steel pipe of the present invention may contain one or more of Cr, Mo, Nb, V, and Ni as required.

Cr is an element that improves the hardenability and has the effect of precipitating M ₂₃ C ₆ type carbide in the matrix, and has the effect of increasing the strength and miniaturizing the carbide. If the content is less than 0.1%, these actions and effects cannot be sufficiently expected, and if it exceeds 1%, defects are likely to occur during ERW welding. For this reason, the content shall be 0.1 to 1% of range. In addition, Preferably, it is 0.1 to 0.6%.

  Mo is an element having an effect of improving the hardenability and is an element having an effect of causing solid solution strengthening. If the content is less than 0.05%, these effects cannot be sufficiently expected. On the other hand, if the content exceeds 1%, coarse carbides are likely to be precipitated and the toughness is deteriorated. % Range. In addition, Preferably, it is 0.1 to 0.5%.

In addition to having the effect of precipitation strengthening by Nb carbonitride, Nb has an effect of improving the toughness by refining the crystal grain size of the steel material in a medium carbon level steel material having a C content of around 0.3%. have.
If the content is less than 0.01%, the effect of improving the strength and toughness is not sufficient. If the content exceeds 0.1%, carbides increase and the toughness decreases. For this reason, the content is made 0.01 to 0.1% of range. In addition, Preferably, it is 0.02 to 0.04%.

  V is an element having an effect of improving hardenability, and an element having an effect of precipitation strengthening by V carbonitride. If the content is less than 0.01%, these effects cannot be sufficiently expected. On the other hand, if the content exceeds 0.5%, coarse carbides are liable to precipitate and the toughness is deteriorated. It is made into the range of -0.5%. In addition, Preferably, it is 0.02 to 0.05%.

  Ni is an element having an effect of improving hardenability and toughness. If the content is less than 0.1%, the effect cannot be expected. On the other hand, if it exceeds 1%, residual γ may exist even after quenching, and fatigue durability is deteriorated. For this reason, the content shall be 0.1 to 1% of range. In addition, Preferably, it is 0.015-0.5%.

Next, the reason why the range of t / D, which is the ratio between the thickness t (mm) of the steel pipe and the outer diameter D (mm) of the steel pipe, is set to be more than 0.15 to 0.30 will be described.
A smaller t / D is desirable for reducing the weight of the stabilizer. However, the smaller the t / D, the greater the main stress applied during use, so the fatigue characteristics deteriorate. On the other hand, when t / D is increased, the effect of weight reduction is reduced, and in addition, it is difficult to manufacture an electric resistance welded steel pipe. In order to ensure the minimum fatigue strength, the lower limit of t / D was set to more than 0.15, and the upper limit was set to 0.30 from the viewpoints of manufacturability and weight reduction.

The manufacturing method of the thick-walled ERW welded steel pipe of the present invention will be described.
Molten steel melted to have the required chemical composition is cast into a slab, or once made into a steel ingot, then hot rolled into a steel slab, and this slab or steel slab is hot rolled Thus, a hot rolled steel sheet is obtained.
This hot-rolled steel sheet is made into an ERW welded steel pipe by a conventional method for producing ERW welded pipe, for example, hot or cold electric resistance welding.
The thickness electric resistance welded steel pipe of the present invention has a thickness / outer diameter ratio, t / D, of more than 0.15 to 0.30 as described above. When the capacity of the machine has the ability to make an electric resistance welded steel pipe having a wall thickness / outer diameter ratio in such a range, the electric resistance welded steel pipe of the present invention is directly manufactured using the hot-rolled steel sheet. be able to.

However, an ERW welded steel pipe becomes more difficult to manufacture as the wall thickness increases, the outer diameter of the pipe decreases, and the strength of the steel material for steel pipe increases. Generally, an electric resistance welded steel pipe having a wall thickness / outer diameter ratio t / D of 0.15 or less can be manufactured by an ordinary electric resistance welded steel pipe making machine, but if t / D exceeds 0.15, the manufacturing capability Therefore, it is often difficult to directly manufacture the thick-walled electric-welded steel pipe of the present invention having a t / D of more than 0.15 to 0.30 in an ordinary electric-welded steel pipe-making machine.
Accordingly, an electric resistance welded steel pipe having a wall thickness / outer diameter ratio of 0.15 or less (also referred to as a mother pipe) is manufactured by an ordinary electric resistance welded steel pipe making machine, and is further hot-compressed. Thick-walled ERW welded steel pipe having a thickness / outer diameter ratio of more than 0.15 to 0.30 is manufactured by diameter rolling.

Reduction rolling can be performed using a stretch reducer or the like.
The Strech Reducer is a rolling device equipped with a plurality of rolling stands having three or four rolls around the rolling axis and in series with the rolling shaft, and the roll rotation speed and rolling force of each rolling stand of this rolling device are adjusted. By doing so, it is possible to perform reduction rolling that controls the tension in the tube axis direction (rolling direction) and the compressive force in the circumferential direction of the steel pipe, thereby increasing the thickness / outer diameter ratio.
That is, in the diameter reduction rolling, the outer diameter is reduced by the rolling force of the outer diameter of the steel pipe while the wall thickness is increased, but on the other hand, the wall thickness is decreased by the tension acting in the tube axis direction of the steel pipe. The final wall thickness is determined by the balance. Since the thickness of the steel pipe reduced in diameter in this way is mainly determined by the tension between the rolling stands, the tension between the rolling stands to obtain the target thickness is obtained from the rolling theory and the tension works. Thus, it is necessary to set the number of roll rotations of each rolling stand.

As described above, the present invention heats the above-mentioned ERW welded steel pipe (mother pipe) to 800 to 1200 ° C., and performs hot diameter reduction rolling with a cross-section reduction rate of 40 to 80%, so that the thickness / outside This is a thick ERW welded steel pipe having a diameter ratio of more than 0.15 to 0.30.
Here, the cross-sectional reduction rate is (outer diameter of steel pipe before diameter reduction-outer diameter of steel pipe after diameter reduction) / outer diameter of steel pipe before diameter reduction × 100 (%).

  When the heating temperature of the ERW welded steel pipe at the time of diameter reduction rolling is less than 800 ° C., the deformation resistance is large. For this reason, heating temperature shall be the range of 800-1200 degreeC.

  Also, if the cross-section reduction rate during reduction rolling is less than 40%, the compressive force is insufficient, and the wall thickness / outer diameter ratio from an electric resistance welded steel pipe (master tube) with a wall thickness / outer diameter ratio of 0.15 or less. Is more than 0.15 to 0.30, it is difficult to make a thick ERW welded steel pipe. On the other hand, if the cross-section reduction rate exceeds 80%, the generation of surface flaws on the steel pipe due to reduced diameter rolling becomes significant, and it becomes difficult to ensure a uniform shape. For this reason, the cross-sectional reduction rate in the reduced diameter rolling is 40 to 80%.

Whether or not the thick-walled ERW welded steel pipe of the present invention is manufactured by reduced diameter rolling is determined by observing the angled state of the inner surface of the cross section (C cross section) perpendicular to the pipe axis direction, It can be judged by measurement.
For example, as described above, a stretch reducer used for diameter reduction rolling is a rolling device provided with a plurality of rolling stands having three or four rolls around a rolling axis in series with the rolling axis, and is usually adjacent to each other. The rolls of the rolling stands (for example, N and N + 1 rolling stands) are out of phase, and are arranged out of phase by 60 ° for a 3 roll rolling stand and 45 ° for a 4 roll rolling stand. .
Therefore, the inner surface shape of the cross section (C cross section) perpendicular to the tube axis direction of the thick-walled ERW welded steel pipe manufactured by reduced diameter rolling is hexagonal, four rolls when the stretch reducer has a three roll rolling stand. When a rolling stand is provided, an octagon is formed.
Also, in the four rolling stands (for example, N, N + 1, N + 2, N + 3 rolling stands) of the stretch reducer, the phase of the roll is shifted to 30 °, 60 °, 90 ° in the case of the 3-roll rolling stand, When the rolling stand is shifted to 22.5 °, 45 ° or 67.5 °, the inner surface shape of the cross section (C cross section) perpendicular to the tube axis direction of the thick-walled ERW welded steel pipe after diameter reduction rolling Are respectively a dodecagonal shape when equipped with a three-roll rolling stand and a hexagonal shape when equipped with a four-roll rolling stand.
Thus, when the inner surface shape of the cross section perpendicular to the tube axis direction of the thick-walled ERW welded steel pipe forms the polygonal shape as described above, this thick-walled ERW welded steel pipe is manufactured by reduction rolling. It can be seen that

Hereinafter, the present invention will be described more specifically with reference to examples.
Various steels having the compositions shown in Table 2 were melted and cast into slabs. This slab was heated to 1150 ° C. and hot-rolled at a rolling finish temperature of 890 ° C. and a winding temperature of 630 ° C. to obtain a steel plate having a thickness of 6 mm. This hot-rolled steel sheet was slit into a predetermined width, and an ERW welded steel pipe (master pipe) having an outer diameter of 90 mm was formed by high-frequency ERW welding. Subsequently, the steel pipe was heated to 980 ° C. by high-frequency induction heating, and then subjected to reduction rolling to obtain a thick ERW welded steel pipe having a thickness of 7 mm and an outer diameter of 35 mm.
In Table 2, No. About the ERW welded steel pipe manufactured with 8 steel, the cross-section reduction rate in diameter reduction rolling was changed, and the thickness ERW welded steel pipe of thickness 5-7.5mm and outer diameter 30-35mm was manufactured.
The obtained thick-walled electric resistance welded steel pipe was heated to 960 ° C., cooled with water, quenched, and tempered at 300 ° C. × 1 hr. Specimens were collected from these steel pipes and subjected to various tests to confirm the characteristics of the thick-walled ERW welded steel pipe of the present invention.
The hardness was measured at five points at the center of the wall thickness at Hv 9.8N, and the average value was obtained. Elasticity was tested using a 5 mm sub-size 2 mm V notch Charpy test piece to determine vTrs. Fatigue properties are described in Spring Journal, 28 (1983) p. 49, the bending radius was 80 mm, and the first principal stress amplitude corresponding to the solid material was calculated to be 600 MPa. The thickness of the ferrite decarburized layer was measured from a photograph taken with an optical microscope after heating in air at 800 ° C. for 1 hour. The results of these characteristics are shown in Tables 2 and 3.

No. 2 having the chemical components of the present invention shown in Table 2. It turns out that the steel of 1-10 has the characteristic excellent in hardness, toughness, and decarburization resistance.
In contrast, no. Steel No. 11 is an example in which the Vc is large, so that the steel does not sufficiently baked and sufficient hardness cannot be obtained at 0.22% C. No. Steel No. 12 is an example of intense decarburization because Cu and Sb are not added. No. Steel No. 13 was an example in which the amount of C was insufficient, so that the minimum required hardness for an automobile structural member could not be obtained, and in addition, Cu and Sb were not added, so that the steel was vigorously decarburized.
As shown in Table 3, the present steel pipe No. It can be seen that a to e have sufficient fatigue strength at which the number of repeated fractures exceeds 50 × 10 ³ times.
In contrast, no. The steel pipe of f is an example where t / D is too small to obtain sufficient fatigue strength.

It is a figure which shows the relationship between the hardness after hardening and tempering of the steel material for thick-walled electric-welded steel pipes, and C amount. It is a photograph which shows the cross-sectional structure of the steel material after decarburization heat processing, (a) is the case of a conventional material, (b) is the case of the test material A containing 0.28% of C, (c) is the present invention. The case of test material A ′ containing C: 0.28% and Sb: 0.05% is shown.

Claims (4)

% By mass
C: 0.15-0.5%
Si: 0.05 to 0.5%,
Mn: 0.3-2%,
P: 0.05% or less,
S: 0.05% or less,
Al: 0.05% or less,
Ti: 0.005 to 0.05%,
B: 0.0005 to 0.01%
N: 0.001 to 0.01% is contained,
Further, it contains one or two of Cu: 0.2 to 0.5% and Sb: 0.01 to 0.2%, and the balance is composed of Fe and inevitable impurities, and is represented by the formula <1>. The critical cooling rate Vc is less than 30 ° C./s, and the ratio of the thickness t to the outer diameter D is t / D in the range of more than 0.15 to 0.30. High-strength thick-walled ERW welded steel pipe with excellent decarburization.
logVc = 2.94−0.75β <1>
However, β = 2.7C + 0.4Si + Mn In mass%,
Cr: 0.1 to 1%,
Mo: 0.05 to 1%
Nb: 0.01 to 0.1%,
V: 0.01-0.5%
The high-strength thick-walled electric-welded steel pipe excellent in hardenability and decarburization resistance according to claim 1, comprising Ni: 0.1 to 1% or two or more.
However, in formula <1>
β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo   The high strength excellent in hardenability and decarburization resistance according to claim 1 or 2, wherein the thickness of the ferrite decarburized layer after holding at 800 ° C for 1 hour in the air is less than 0.15 mm Thick ERW welded steel pipe.   An electric resistance welded steel pipe having the component according to claim 1 or 2 is heated to 800 to 1200 ° C. and reduced in diameter in a range of 40 to 80% in cross-section reduction ratio and hardenability and resistance to desorption. Manufacturing method of high strength thick wall ERW welded steel pipe with excellent charcoal properties.