WO2011161927A1 - Manufacturing method for steel pipe - Google Patents

Abstract

In the disclosed manufacturing method for a steel pipe, when quenching that is rapidly cooled after being heated is applied to a steel pipe, the pipe is heated to a quenching temperature defined by formula (1) by means of high frequency induction heating, and the time required from when the temperature of the pipe heated by high frequency induction heating has reached the temperature which is the transformation point of Ac₃ to when rapid cooling is initiated is not more than 6 seconds if the quenching temperature is at least 975˚C, not more than 8 seconds if the quenching temperature is at least 950˚C but less than 975˚C, and not more than 9 seconds if the quenching temperature is less than 950˚C. Thus, the structure of the steel pipe undergoes complete Martensite transformation, and a highly strong and tough steel pipe can be obtained because the crystal grains are miniaturised. In formula (1) T1 is the quenching temperature (˚C), TAc3 is the Ac₃ transformation point temperature (˚C). (1)TAc3+40˚C≤T1≤1000˚C…

Description

Steel pipe manufacturing method

The present invention relates to a method of manufacturing a steel pipe by quenching by high-frequency induction heating, and more particularly to a method of manufacturing a steel pipe capable of obtaining a high-strength and high-toughness steel pipe.

In automobile airbag systems, accumulators made using steel pipes are often used. The accumulator using the steel pipe is filled with high-pressure gas, and the high-pressure gas sealed in the accumulator is ejected into the airbag at a time when the airbag is activated. Therefore, steel pipes used in accumulators are subjected to stress at a large strain rate in a very short time, so high strength, high toughness and excellent burst resistance are required in addition to high dimensional accuracy, workability and weldability. Is done.

The steel pipe used for such an air bag is a steel pipe made of steel by hot pipe making, quenched and tempered after being heated and then tempered on the steel pipe, and then cold worked to finish the steel pipe to a predetermined dimension, It may be manufactured by removing the residual stress by annealing.

Quenching and tempering are applied to the steel pipe to ensure the strength and toughness required for the steel pipe for airbags. When applying quenching and tempering to a steel pipe after hot pipe making and before cold working, the toughness of the steel pipe, which is reduced by cold working, should be adjusted by adding alloy components to the steel material. Need to compensate. When alloy components are added to steel, it is difficult to thin the steel pipe by hot pipe making, and high-strength steel pipes that have been quenched and tempered are cold-worked. It becomes difficult to reduce the thickness. In addition, an increase in raw material cost due to the addition of alloy components is also a problem.

In order to solve the problems of thinning in hot pipe and cold working and the problem of increased raw material costs due to the addition of alloy components, the steel is made into steel pipe by hot pipe making and steel pipe by cold working. After finishing to a predetermined dimension, a method of manufacturing a steel pipe for an air bag is studied in which the steel pipe is quenched by induction induction heating and then tempered. In this case, the toughness of the steel pipe can be ensured by quenching and tempering performed after the cold working, and it is not necessary to add an alloy component to the steel material. For this reason, while thinning by hot pipe making and cold work becomes easy, the raise of the raw material cost by addition of an alloy component can be suppressed.

Various proposals have heretofore been made with respect to a method for manufacturing a steel pipe for an airbag. For example, Patent Document 1 is available. In the method for manufacturing a steel pipe for an air bag described in Patent Document 1, a steel material having a predetermined steel composition is made into a steel pipe by hot pipe making, quenching after heating to the steel pipe and quenching, and tempering at a temperature below the Ac ₁ transformation point. After that, the steel pipe is finished to a predetermined dimension by cold working. By reducing the workability (area reduction rate) during cold working, the X-ray integrated intensity ratio of the {110} plane measured in a cross section perpendicular to the axial direction L of the steel pipe is perpendicular to the circumferential direction T of the steel pipe. The ratio of the {110} plane to the X-ray integral intensity ratio measured with a simple cross section is set to 50 or less, and excellent burst resistance can be ensured.

According to Patent Document 1, it is preferable that quenching performed on a steel pipe is rapidly heated to a quenching temperature and then held for a short time and rapidly cooled. The quenching temperature is 900 to 1000 ° C., and the heating means is preferably high frequency induction heating.

Further, in the method of manufacturing a steel pipe for an air bag described in Patent Document 2, a steel material having a predetermined steel composition is made into a steel pipe by hot pipe making, and the steel pipe is finished to a predetermined size by cold working, and then 900 to 960 ° C. The steel pipe is subjected to quenching and tempering to make the austenite grain size 11.0 or more. In patent document 1, it is supposed that the strength and toughness required for a steel pipe for an airbag can be secured by setting the austenite grain size to 11.0 or more. Further, it is preferable to use high-frequency induction heating at the time of quenching and to keep the holding time at 900 to 1000 ° C. for 10 seconds or less because the crystal grains of the obtained steel pipe become finer.

In the method of manufacturing a steel pipe for an air bag described in Patent Document 3, a steel material having a predetermined steel composition is made into a steel pipe by hot pipe making, and after finishing the steel pipe to a predetermined dimension by cold working, the temperature of the Ac ₃ transformation point The steel pipe is subjected to quenching as described above and tempering at a temperature equal to or lower than the Ac ₁ transformation point. According to Patent Document 3, when the contents of Mn and Ti added as alloy components satisfy the relational expression, it is possible to ensure a tensile strength of 1000 MPa or more and high toughness. The quenching applied to the steel pipe is preferably rapidly heated to the quenching temperature, then held for a short time and rapidly cooled. The quenching temperature is 900 to 1000 ° C., and the heating means is preferably high frequency induction heating.

In the method for manufacturing a steel pipe for an air bag which is quenched by high frequency induction heating described in Patent Documents 1 to 3, there is a description of the time for maintaining the temperature of the steel pipe heated by high frequency induction. It is difficult to keep the temperature constant from the principle of high frequency induction heating. For this reason, when hardening a steel pipe, it cannot be said that it has fully examined about the temperature and time which hold | maintain a steel pipe at high temperature after high frequency induction heating.

Further, in the method for manufacturing a steel pipe for an air bag that is hardened by high-frequency induction heating described in Patent Documents 1 to 3, the steel pipe for an air bag can be used even if the steel pipe is quenched and tempered while satisfying the conditions described therein. In some cases, the strength and toughness required by the method cannot be secured.

WO2006 / 046503 JP 2002-194501 A WO2004 / 104255

As described above, in the conventional method for manufacturing a steel pipe for an airbag that is quenched by high-frequency induction heating, the temperature and time for holding the steel pipe at a high temperature after high-frequency induction heating have not been sufficiently studied. Moreover, even if the steel pipe is subjected to quenching and tempering by conventional high-frequency induction heating, the strength and toughness required for the steel pipe for airbags may not be ensured.

The present invention has been made in view of such a situation, and the steel pipe having high strength and high toughness is obtained by completely martensifying the structure of the steel pipe by quenching and tempering and by refining the crystal grains. It aims at providing the manufacturing method of the steel pipe which can obtain.

As a result of conducting extensive studies and performing tests shown in Examples described later, the present inventor has found that the time required for rapid cooling to start after the temperature of the high-frequency induction heated steel pipe reaches the temperature of the Ac ₃ transformation point. And the temperature at which induction heating is performed during quenching. As a result, it has been found that a steel pipe having high strength and high toughness can be obtained by making the structure of the steel pipe subjected to quenching and tempering completely martensite and making the crystal grains finer.

The present invention has been completed on the basis of the above findings, and the gist of the method of manufacturing a steel pipe is as follows (1) to (3):

(1) When the steel pipe is quenched after being heated, the steel pipe is heated to a quenching temperature defined by the following formula (1) by high-frequency induction heating, and the temperature of the high-frequency induction heated steel pipe reaches the temperature of the Ac ₃ transformation point. When the quenching temperature is 975 ° C. or higher, the time required to start the rapid cooling after reaching is 6 seconds or less, and when the quenching temperature is lower than 975 ° C. and 950 ° C. or higher, the quenching temperature is 8 seconds or less. When the temperature is lower than 950 ° C., the manufacturing method of the steel pipe is characterized in that it is 9 seconds or less.
Tac3 + 40 ° C. ≦ T1 ≦ 1000 ° C. (1)
However, T1 is the quenching temperature (° C.), and TAc3 is the temperature of the Ac ₃ transformation point (° C.).

(2) In the method for manufacturing a steel pipe described in (1) above, the raw pipe obtained by hot pipe making is finished to a predetermined size by cold working to obtain a steel pipe, and the steel pipe is subjected to the quenching, and then Ac _A method for producing a steel pipe, comprising tempering the steel pipe at a temperature of ₁ transformation point or less.

(3) The method for manufacturing a steel pipe according to (1) or (2) above, wherein the obtained steel pipe is a steel pipe for an airbag.

In the present invention, “temperature of the steel pipe” means a temperature determined based on an outer surface temperature measured by a radiation thermometer on the outlet side (downstream side) of the high-frequency induction heating device (heating coil). Specifically, the quenching temperature T1 is a temperature measured by a radiation thermometer on the exit side of the high-frequency induction heating device, and the temperature of the steel pipe in the process of high-frequency induction heating is the quenching temperature T1 (that is, the radiation thermometer on the exit side). ) And the heating rate.

The steel pipe manufacturing method of the present invention has the following remarkable effects.
(1) By defining the time required for the quenching to start after the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point and the quenching temperature, the structure of the obtained steel pipe is completely martensitic and crystallized. By refining the grains, a steel pipe with high strength and high toughness can be obtained.
(2) By performing quenching and tempering after cold working, it is easy to reduce the thickness in hot pipe making and cold working, and it is possible to suppress an increase in raw material cost due to the addition of alloy components.

FIG. 1 is a diagram for explaining the time required from the time when the temperature of a steel pipe specified in the present invention reaches the temperature of the Ac ₃ transformation point until the rapid cooling starts, and the steel pipe is quenched by the method of manufacturing a steel pipe according to the present invention. It is a figure which shows the relationship between the time at the time of giving, and the temperature of a steel pipe. FIG. 2 shows that the time required for quenching to start after the temperature of the steel pipe at the time of quenching reaches the temperature of the Ac ₃ transformation point and the quenching temperature depend on the hardness and austenite grain size of the heat-treated specimen. It is a figure which shows the influence which acts.

Below, the manufacturing method of the steel pipe of this invention is demonstrated.

In the method of manufacturing a steel pipe according to the present invention, when the steel pipe is quenched after being heated, the steel pipe is heated to a quenching temperature defined by the following formula (1) by high-frequency induction heating, and the temperature of the steel pipe heated by high-frequency induction is Ac. _The time required to start rapid cooling after reaching the temperature of the _third transformation point is 6 seconds or less when the quenching temperature is 975 ° C. or more, and 8 seconds or less when the quenching temperature is less than 975 ° C. and 950 ° C. or more. When the quenching temperature is less than 950 ° C., it is set to 9 seconds or less.
Tac3 + 40 ° C. ≦ T1 ≦ 1000 ° C. (1)
However, T1 is the quenching temperature (° C.), and TAc3 is the temperature of the Ac ₃ transformation point (° C.).

FIG. 1 is a diagram for explaining the time required from the time when the temperature of the steel pipe specified in the present invention reaches the temperature of the Ac ₃ transformation point until the rapid cooling starts, and the steel pipe is quenched by the method of manufacturing the steel pipe of the present invention. It is a figure which shows the relationship between the time at the time of giving, and the temperature of a steel pipe. As shown in the figure, in the method for manufacturing a steel pipe according to the present invention, the time required from when the temperature of the high-frequency induction heated steel pipe reaches the temperature of the Ac ₃ transformation point to start quenching is managed.

As described above, it is difficult to keep the temperature of the high-frequency induction-heated steel pipe constant from the principle of high-frequency induction heating, so the temperature of the steel pipe decreases between the time of heating to the quenching temperature and the time of rapid cooling. Alternatively, there may be a case where sufficient time cannot be ensured for fully austenitizing the heated steel pipe structure. In the present invention, the time required from when the structure of the heated steel pipe reaches the temperature of the Ac ₃ transformation point at which austenitization starts to start quenching is managed. By performing the heat treatment on the steel pipe within the range specified in the present invention after the temperature of the steel pipe reaches the Ac ₃ transformation point temperature, the structure of the heated steel pipe is completely austenite. The structure of the obtained steel pipe can be completely martensitic.

When the quenching temperature is outside the range defined by the above formula (1) and becomes less than the temperature obtained by adding 40 ° C. to the temperature of the Ac ₃ transformation point, as shown in FIG. May not be fully martensitic and may lack strength and toughness. On the other hand, when the quenching temperature exceeds 1000 ° C. outside the range defined by the above formula (1), the crystal grain size of the steel pipe is coarsened by heating, the austenite crystal grain size of the resulting steel pipe is lowered, and the toughness is insufficient.

The time required from when the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point to start quenching is 6 seconds or less when the quenching temperature is 975 ° C. or more, and the quenching temperature is less than 975 ° C. and 950 ° C. or more. If the quenching temperature is less than 950 ° C., it is 9 seconds or less. This is to prevent the austenite crystal grain size of the obtained steel pipe from being lowered and the toughness being insufficient as shown in FIG.

Quenching using high-frequency induction heating is generally performed by sequentially arranging a coil for high-frequency induction heating of a steel pipe and a cooling zone for cooling the heated steel pipe, and feeding the steel pipe by a conveying device such as a roller. The steel pipe is subjected to high-frequency induction heating to the quenching temperature by passing through the water, and when passing through the cooling zone, cooling water or the like is injected to quench the steel pipe. In addition, the temperature of a steel pipe is measured using the radiation thermometer arrange | positioned at the outgoing side (downstream side) of a high frequency induction heating apparatus (heating coil).

The temperature measured by this radiation thermometer is the temperature of the steel pipe immediately after coming out of the high frequency induction heating device, and is the highest temperature in quenching, and corresponds to the quenching temperature T1. When the steel pipe is passed through the heating device and induction-heated at high frequency, the steel pipe reaches the temperature of the Ac ₃ transformation point in the heating device, but it is difficult to measure the temperature of the steel pipe in the heating device. Here, in the case where the steel pipe is subjected to high-frequency induction heating under the same conditions, generally, the heating rate is the same pattern. Therefore, if the heating rate is confirmed in advance, the temperature of the steel pipe in the process of induction heating can be estimated from the quenching temperature T1 (that is, the temperature measured by the radiation thermometer on the exit side) and the heating rate. The time when the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point can be grasped.

Since the heating rate (° C./s) is difficult to confirm directly, for example, assuming that the heating rate during high-frequency induction heating is constant, the temperature (° C.) of the steel pipe measured by the radiation thermometer on the outlet side ) And the temperature (° C.) of the steel pipe before heating (incoming side) can be derived by dividing by the heating time (s). Further, the heating time can be calculated from the conveying speed of the steel pipe and the length of the heating device.

In addition, when using equipment in which a heating device and a cooling zone are arranged in order for quenching, the time required for the quenching to start after the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point includes the heating device and the cooling zone. It can be adjusted by changing the interval at the time of placement or the feed speed of the transport device.

In the method for producing a steel pipe of the present invention, the raw pipe obtained by hot pipe making is finished to a predetermined dimension by cold working to form a steel pipe, and the steel pipe is subjected to the above-mentioned quenching, and then the temperature of the Ac ₁ transformation point or lower. It is preferable to temper the steel pipe. As mentioned above, the required toughness can be ensured by quenching and tempering after cold working, so there is no need to add alloy components to the steel material, and thinning by hot pipe making and cold working is eliminated. This is because it becomes easy and an increase in raw material cost due to the addition of alloy components can be suppressed.

As described above, in the method of manufacturing a steel pipe according to the present invention, when quenching is performed on the steel pipe using high-frequency induction heating, it is necessary to start quenching after the quenching temperature and the temperature of the steel pipe reach the temperature of the Ac ₃ transformation point. Specify time. Thereby, the structure of the obtained steel pipe is completely martensitic and crystal grains are refined. For this reason, since the obtained steel pipe is high strength and toughness, it is suitable for the steel pipe for airbags used for an accumulator by the airbag system of a motor vehicle.

The test which heat-processes to a test piece was done, and the effect by the manufacturing method of the steel pipe of this invention was verified.

[Test method]
The test was performed by subjecting the solid round bar test piece to heat treatment by high-frequency induction heating to the quenching temperature, followed by quenching and quenching, and then tempering at a temperature below the Ac ₁ transformation point. The time required for rapid cooling to start after the temperature of the high-frequency induction heated steel pipe reaches the temperature of the Ac ₃ transformation point during heat treatment varies in the range of 0.2 to 9 seconds in each test. I let you. The quenching temperature was varied in the range of 830 to 1050 ° C. in each test.

The test conditions in this test were as follows.
Test pieces:
Outer diameter 3mm, length 6mm
Material C: 0.16% by mass, Mo: 0.01% by mass, Cr: 0.30% by mass, Ni: 0.26% by mass and Cu: 0.25% by mass of carbon steel Ac ₃ transformation point Temperature 832 ° C

[Evaluation index]
As evaluation indexes, the hardness and austenite grain size of the heat-treated specimen were measured. Hardness measured the value of HV10 by the test force 98.07N according to the method prescribed | regulated to JISZ2244. The austenite crystal grain size was evaluated by corroding the test piece subjected to the above-mentioned heat treatment with a saturated aqueous solution of viric acid by the Bechet-Beaujard method described in JIS G 0551, thereby evaluating the austenite crystal grain size. .

In each test piece, pass / fail was judged based on a hardness of 380 HV or more and an austenite grain size of 11 or more. The criterion for the hardness was 380 HV, which is the hardness when the structure of a steel material containing 0.16% by mass of C is 95% by mass or more. That is, when the hardness is equal to or higher than the criterion, it is considered that the structure of the test piece is completely martensitic.

[Test results]
FIG. 2 shows that the time required for quenching to start after the temperature of the steel pipe at the time of quenching reaches the temperature of the Ac ₃ transformation point and the quenching temperature depend on the hardness and austenite grain size of the heat-treated specimen. It is a figure which shows the influence which acts. In this figure, the time (seconds) required to start quenching after the temperature of the steel pipe at the time of quenching reaches the temperature of the Ac ₃ transformation point is plotted on the horizontal axis using a logarithmic scale, and the quenching temperature (° C) is plotted on the vertical axis. Show. Also, heat-treated specimens with hardness and austenite crystal grain size that are both above the standard value are black circles, and those with hardness above the standard value and austenite crystal grain size below the standard value are outlined. In the triangle marks, those whose hardness and austenite grain size are both less than the standard value are indicated by x marks.

Since the Ac ₃ transformation point of the test piece is 832 ° C., when the quenching temperature is less than 872 ° C. and falls outside the range defined by the above equation (1), as shown in FIG. Both austenite grain sizes were below the standard value. On the other hand, when the quenching temperature exceeded 1000 ° C. and was outside the range defined by the above formula (1), in all cases, the austenite grain size was less than the standard value, and in part, the hardness was also less than the standard value.

The case where the quenching temperature is 1000 ° C. or less and 975 ° C. or more, the time required for the quenching to start after the steel pipe temperature reaches the Ac ₃ transformation point exceeds 6 seconds, and the quenching temperature is less than 975 ° C. 950 The austenite grain size was less than the reference value in all cases where the temperature required was 8 ° C or more after the temperature of the steel pipe reached the temperature of the Ac ₃ transformation point and the rapid cooling started.

On the other hand, the quenching temperature is within the range defined by the above formula (1), and the time required for the quenching to start after the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point is 975 ° C. or higher. 6 seconds or less, 8 seconds or less when the quenching temperature is less than 975 ° C. and 950 ° C. or more, 9 seconds or less when the quenching temperature is less than 950 ° C., and heat treatment was performed according to the conditions specified in the present invention. All of the test pieces had hardness and austenite grain size equal to or larger than the standard values.

From these, the time required for the steel pipe to be heated to the quenching temperature defined by the above formula (1) and the steel pipe temperature to reach the temperature of the Ac ₃ transformation point to start quenching is defined by the present invention. It was confirmed that by performing quenching in the range, the heat-treated test piece had a hardness and austenite grain size equal to or higher than the reference value. Therefore, it has been clarified that the structure of the obtained steel pipe can be completely martensite and the crystal grains can be refined by the steel pipe manufacturing method of the present invention.

The steel pipe manufacturing method of the present invention has the following remarkable effects.
(1) By defining the time required for the quenching to start after the temperature of the steel pipe reaches the temperature of the Ac ₃ transformation point and the quenching temperature, the structure of the obtained steel pipe is completely martensitic and crystallized. By refining the grains, a steel pipe with high strength and high toughness can be obtained.
(2) By performing quenching and tempering after cold working, it is easy to reduce the thickness in hot pipe making and cold working, and it is possible to suppress an increase in raw material cost due to the addition of alloy components.

Since the steel pipe manufacturing method of the present invention can provide a steel pipe with high strength and high toughness, it is useful for manufacturing a steel pipe for an airbag used for an accumulator in an automobile airbag system.

Claims (3)

When the steel pipe is quenched after being heated, it is heated to a quenching temperature defined by the following equation (1) by high-frequency induction heating, and the temperature of the steel pipe heated by high-frequency induction reaches the temperature of the Ac ₃ transformation point. The time required from the start to rapid cooling is 6 seconds or less when the quenching temperature is 975 ° C. or more, 8 seconds or less when the quenching temperature is less than 975 ° C. and 950 ° C. or more, and the quenching temperature is 950 ° C. The method of manufacturing a steel pipe, characterized in that if it is less than 9 seconds, it is set to 9 seconds or less.
Tac3 + 40 ° C. ≦ T1 ≦ 1000 ° C. (1)
However, T1 is the quenching temperature (° C.), and TAc3 is the temperature of the Ac ₃ transformation point (° C.). 2. The method of manufacturing a steel pipe according to claim 1, wherein the raw pipe obtained by hot pipe making is finished to a predetermined size by cold working to form a steel pipe, and the steel pipe is subjected to the quenching, and then the Ac ₁ transformation point or less. A method of manufacturing a steel pipe, characterized by subjecting the steel pipe to tempering at a temperature of 3. The method for manufacturing a steel pipe according to claim 1, wherein the obtained steel pipe is a steel pipe for an airbag.

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