JPH0565541A - Manufacture of high strength resistance welded steel tube for automotive use excellent in ductility and three-point bendability - Google Patents

Abstract

PURPOSE:To improve the strength and toughness of a steel tube by subjecting steel sheet stock contg. specified amounts of C, Si, Mn, P, S, Cr, Ti, B, Ca and N to resistance seam welding and executing high-frequency heat treatment of high-frequency heating and hardening. CONSTITUTION:Steel sheet stock contg., by weight, 0.15 to 0.27% C, 0.5% Si, 0.80 to 1.70% Mn, <=0.02% P, <=0.02% S, 0.1 to 0.7% Cr, 0.005 to 0.040% Ti, 0.0005 to O.0025% B, 0.0004 to 0.0100% Ca, <=0.0080% N and the balance Fe with inevitable impurities is subjected to resistance seam welding. The obtd. steel tube is subjected to high-frequency heat treatment of high-frequency heating and hardening under the conditions of 850 to 1050 deg.C heating temp. and >=100 deg.C/sec cooling rate. The above compsn. is incorporated with optimum amounts of one or more kinds among Nb, V and Mo according to necessary. By the addition of Ca, sound seam properties can be obtd., and by high-frequency heating treatment, the objective resistance welded steel tube having 150 to 190kgf/mm<2> class tensile strength and excellent in three-point bendability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to and used for a high-strength steel pipe for structural materials required for automobiles, two-wheeled vehicles, etc. from the viewpoints of weight reduction and safety improvement. The present invention relates to a method for producing an electric resistance welded steel pipe having high strength and excellent bending characteristics, which can be applied as a reinforcing material (also called an impact beam), a core material for a bumper, or a front fork of a motorcycle.

[0002]

2. Description of the Related Art In order to further reduce the weight of automobiles, inexpensive materials are required to have high tensile strength. Regarding the high tensile strength itself, see, for example, “Heat Treatment of Steel”, edited by The Iron and Steel Institute of Japan 411
As explained on the page, 150-200 kgf / mm ² class high-strength steels are known to be 1) low alloy steel, 2) medium alloy steel, and 3) high alloy steel (maraging steel series). .. However, all of these alloy systems contain Cr or Ni in an amount of 1% or more. In particular, maraging steel contains a large amount of Ni and is very expensive. Moreover, all of these steels undergo tempering heat treatment as well as tempering heat treatment to obtain a good balance of strength, ductility and toughness, and two heat treatments are required for each process, resulting in a large load. In order to obtain high strength steel of 150 to 200 kgf / mm ² grade by quenching heat treatment without containing expensive elements, it is necessary to use medium and high carbon steel. For example, Kazama Kaizo “Steel Material Studies” page 175 shows the mechanical properties of 0.2% C steel and 0.4% C steel when quenched and tempered. For 0.4% C steel, 180kgf / Although a tensile strength of about mm ² can be obtained, there is almost no elongation and it is several percent or less. As described above, it is not possible to secure high strength and sufficient ductility by quenching only by using high carbon steel.

As an example of achieving high tensile strength of a high strength electric resistance welded steel pipe for automobiles, as disclosed in Japanese Patent Laid-Open No. 1-205032, 140 to 160 kgf / mm ² grade is obtained. However, when the tensile strength is increased, the ductility, that is, the elongation at break is lowered, and as shown in the examples, it is lowered to about 8%. In many cases, in order to obtain a high-strength electric resistance welded steel pipe for automobiles having a tensile strength of 80 kgf / mm ² or more, after being formed into an electric resistance welded steel pipe, it is strengthened by quenching heat treatment. For example, Japanese Laid-Open Patent Publication No. 60-215719 discloses a method for manufacturing an electric resistance welded steel pipe for a front fork of a motorcycle, which has been conventionally made to have a high C content or a large amount of alloy elements such as Ni and Cr to improve hardenability. Was changed to Ti
And B are added to improve the hardenability and suppress the amount of alloying elements, thereby improving the ductility. However, as shown in Fig. 1, the tensile strength is 150 kgf /
When it exceeds mm ² , the average elongation is 10% or less and 160 kgf.
It is disclosed that the elongation is further reduced when it exceeds / mm ² .

[0004]

The impact beam is required to have a high tension in order to further reduce the weight of the automobile, but there are the following problems. In the present invention, it is intended to increase the tensile strength to 150 kgf / mm ² or more, but if the strength is increased, the ductility decreases, for example, simply C
There is a problem in that the ductility is lowered to several percent or less by simply increasing the amount to increase the tensile strength, and in the extension of the conventional technique, the ductility is further decreased due to the high tensile strength. The reason is that when a high-strength steel pipe is applied to the impact beam, the role of the impact beam is to receive bending deformation at the time of collision and to absorb the energy.
This is because it is important that the bending characteristics such as maximum load and absorbed energy in the point bending test are excellent.

FIG. 2 shows a three-point bending test method for a steel pipe, and FIG. 3 shows an outline of a load displacement curve after the test. The energy absorbed during bending deformation of the steel pipe is obtained as the area of the hatched portion shown in FIG. Tensile strength is required to increase maximum load
(TS) needs to be large, and in order for absorbed energy to become large, not only TS must be large, but also bending displacement needs to be large without decreasing the load with respect to bending. Various improvement factors can be considered in order to increase the bending displacement without lowering the load, but it is important to improve the ductility and suppress the occurrence of constrictions and cracks during bending deformation.
In the example shown in FIG. 3, the curve 2 has smaller absorbed energy than the curve 1. However, the cause is not always clear, and there is a problem that when the tensile strength is increased, the load may be decreased before the sufficient deformation in the three-point bending test, or cracking or fracture may occur and the absorbed energy may decrease. Regarding cracking and rupture, it is also related that the higher the strength, the higher the susceptibility to cracking at the seam position and the easier it becomes to break, and therefore more sound seam characteristics are required. Further, although it is not directly related to the increase in strength, there is a problem that the pipe is likely to bend due to the quenching treatment, and a measure for preventing the bend has also been required. From this point of view, it is of great significance if the electric resistance welded steel pipe for automobiles can be made to have a high tensile strength with an inexpensive component system, at the same time it can be prevented from lowering the ductility, and the three-point bending property of the steel pipe can be improved. The effect will be even greater if bending can be prevented.

[0006]

Means for Solving the Problems As a result of various examinations of components and heat treatment conditions in order to meet the above-mentioned demands, the inventors of the present invention compare normal furnace heating, quenching with induction heating, and quenching. We have found that quenching gives higher strength. Based on this finding, the high-frequency heating and quenching have higher strength than the furnace heating and quenching, so that even if the C content is reduced, the same strength as in the case of high-carbon steel furnace heating and quenching can be obtained. B and Ti to improve hardenability
It is well known that the addition of Cr, Mn in addition to B and Ti, and by optimizing the quenching conditions at high frequency, the quenching characteristics are further improved, It was found that the amount of C and the amount of alloying elements can be suppressed more than in the case of increasing the tension by heating and quenching, and at the same time, the ductility can be improved. Furthermore, by adding Ca, the soundness of the seam portion during pipe making can be enhanced, and as a result, cracks during bending deformation disappear during the three-point bending test after increasing the tensile strength, and high load is maintained. It was found that the absorbed energy was increased because of sufficient deformation.

That is, the C content is adjusted to a proper level, Cr and Mn are added in appropriate amounts, Ti and B are added in appropriate amounts to improve hardenability, and further Ca is added in an appropriate amount to ensure a healthy seam even after welding by electric resistance welding. The characteristics were obtained, and further, by quenching this electric resistance welded steel pipe by induction heat treatment under appropriate conditions, a steel pipe having high strength and excellent ductility, which was not obtained conventionally, was obtained. Regarding the three-point bending property, cracking did not occur during bending, and the material was sufficiently deformed and a high load and large absorbed energy were obtained. In this way, by studying the materials and manufacturing method, we were able to obtain properties that were not achieved previously.

In the present invention, C: 0.15 to 0.27%, Si: 0.5
% Or less, Mn: 0.80 to 1.70%, P: 0.02% or less, S: 0.02
%, Cr: 0.1 to 0.7%, Ti: 0.005 to 0.040%,
B: 0.0005 to 0.0025%, Ca: 0.0004 to 0.0100%, N: 0.
After a raw steel sheet containing less than 0080% and the balance Fe and unavoidable impurities was formed into a steel pipe by electric resistance welding, the steel pipe was heated at a temperature of 850 to 1050 ° C and a cooling rate of 100 ° C / s.
Manufacture of high-strength electric resistance welded steel pipes for automobiles with a tensile strength of 150 to 190 kgf / mm ² class and excellent ductility and three-point bending characteristics, characterized by performing induction heating and quenching under the above conditions Method, and C: 0.15 to 0.27
%, Si: 0.5% or less, Mn: 0.80 to 1.70%, P: 0.02% or less, S: 0.02% or less, Cr: 0.1 to 0.7%, Ti: 0.005 to
0.040%, B: 0.0005 to 0.0025%, Ca: 0.0004 to 0.0100
%, N: 0.0080% or less, Nb: 0.1% or less, V:
A material steel sheet containing 0.1% or less, Mo: 0.5% or less, one or more of which is Fe and unavoidable impurities in the balance is made into a steel pipe by electric resistance welding, and then the heating temperature of the steel pipe is 8
Under conditions of 50 to 1050 ℃ and cooling rate of 100 ℃ / s or more,
Intensity of 150 to 190 kgf / mm, characterized by high-frequency heat treatment such as induction heating and quenching.
A high-strength electric resistance welded steel pipe for automobiles having excellent point bending characteristics. In addition, in the high-frequency heat treatment in the preceding paragraph or in addition, the high-frequency heat treatment is applied to the steel pipe while rotating the steel pipe around the pipe axis at a rotation speed of 300 times / min or more. Is a method for producing a high-strength electric resistance welded steel pipe for automobiles, which is excellent in ductility and three-point bending property.

[0009]

The reasons for limiting the components and production conditions in the present invention will be described below. C: 0.15 to 0.27% C is an inexpensive element that is effective in increasing the strength after quenching, but 0.15% or more is required to obtain a tensile strength (TS) of 150 kgf / mm ² or more. Is. On the other hand, if the addition amount is increased, TS increases but the elongation in the tensile test after quenching decreases. As will be described later, it is a component system in which B and Ti are added in appropriate amounts, and by adding appropriate amounts of Mn and Cr, and in some cases Mo, etc., it is possible to obtain an elongation of 10% or more up to 0.27% of C. Was 0.27%. Mn: 0.80 to 1.70% Mn is an element effective in enhancing the hardenability and increasing the tensile strength, but it is necessary to add 0.8% or more to obtain this effect effectively. On the other hand, if a large amount is added, oxides are easily generated in the weld zone and weld defects easily occur, so the upper limit was made 1.70%. P and S: 0.02% or less P and S impair ductility and toughness, as is well known.
If it exceeds 02%, it will have an adverse effect, so the upper limit is 0.02 for both cases.
%. Cr: 0.1 to 0.7% Cr is an element effective for significantly increasing the hardenability and increasing the tensile strength, and it is necessary to add 0.10% or more in order to obtain the effect effectively. As a result of various studies on components and heat treatment conditions,
Add appropriate amounts of C and Mn, and further enhance B to improve hardenability.
It was found that the addition of Ti and Ti in an appropriate amount and quenching at high frequency not only increases the strength but also improves the ductility. On the other hand, the addition of 0.7% or more not only increases the cost but also tends to cause defects in the welded portion during pipe making, so the upper limit was made 0.70%.

B: 0.0005 to 0.0025% B has an extremely large effect on the improvement of hardenability, but in order to obtain the effect, 0.0005% or more must be added, while if it exceeds 0.0025%, the effect is saturated. The upper limit is set because it tends to cause surface defects and deterioration of toughness.
It was set to 0.0025%. Ti: 0.005 to 0.040% Ti is an element added to fix N and exert the hardenability effect of B. For this purpose, addition of 0.005% or more is necessary, while addition of more than 0.040% tends to cause defects, so the upper limit was made 0.040%. Ca: 0.0004 to 0.0100% Ca has the effect of improving the ductility and toughness of the material by the deoxidizing action and the morphology control of inclusions, and also has the effect of suppressing defects in the electric resistance welded portion. 0.0004% to obtain this effect
The above-mentioned additions are necessary, while additions exceeding 0.0100% lower the ductility and toughness, so the upper limit was made 0.0100%. N: 0.0080% or less N is present in the steel as a gas component and acts with B to form BN, which reduces the hardenability effect of B, so it is preferable that the N content be 0.0080% or less. Nb, V, and Mo all have the effect of enhancing the hardenability of steel, but if added in large amounts, they may reduce toughness, so 0.1%, 0.1%, and 0.5%, respectively.
% Or less.

The reason why the heating temperature at high frequency is set to 850 to 1050 ° C. is the result of examination of the heating temperature and the strength and ductility after quenching. If it is less than 850 ° C, quenching is insufficient and high strength cannot be obtained, while if it exceeds 1050 ° C, ductility after quenching,
Since the toughness is deteriorated, the heating temperature is set to 850 to 1050 ° C. The cooling rate of 100 ° C./s or more is based on the result of the quenching experiment at high frequency, and at a cooling rate of less than 100 ° C./s, quenching is insufficient and sufficient tensile strength cannot be obtained. A higher cooling rate is preferable because quenching will be sufficient, but if it is too fast, the cooling equipment will be costly. Therefore, the upper limit is preferably 500 ° C / S or less. The reason why the steel pipe is rotated around the tube axis of the steel pipe at a rotation speed of 300 rpm or more is to prevent bending of the steel pipe during quenching, and a rotation speed of less than 300 rpm gives a sufficient effect. Because it is not possible. A higher rotation speed is preferable for uniform heating and cooling, but if it is set too high, the rotating equipment system will be expensive, so the upper limit is 1200 times /
Minutes or less are preferred.

[0012]

【Example】

(Example 1) ERW steel pipe 3 having the chemical composition shown in Table 1
1.8mmφ × 2.3mmt is heat-treated by two types of furnace heating, quenching, high frequency heating, and quenching, and then JIS11
A tensile test using a No. test piece and a three-point bending test of a steel pipe were performed. Furnace heating, quenching to 1000 ℃ and quenching in a water tank, induction heating, quenching to 930 ℃, cooling rate
Quenching was performed at 250 ° C / s. The three-point bending test of the steel pipe was performed by the test method shown in FIG. 2, but the radius R of the bending die 3 was:
The test conditions were 6 inches (152.4 mm), span L: 700 mm, bending speed: 5 mm / s, and bending displacement: 155 mm. The results are shown in Table 2. High-frequency heating and quenching have higher tensile strength and greater ductility, and even in the three-point bending test, high-frequency heating and quenching are superior to both maximum load and absorbed energy. There is.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

[0016]

[Table 4]

(Embodiment 2) With respect to the electric resistance welded steel pipe 25.4 mmφ × 2.8 mmt having the chemical composition shown in Table 3, under the same conditions as in Embodiment 1, heat treatment was conducted in two types of furnace heating, quenching and induction heating, and quenching. After that, a tensile test using JIS No. 11 test pieces and a three-point bending test of a steel pipe were performed. The results are shown in Table 4. High-frequency heating and quenching have higher tensile strength and greater ductility, and even in the three-point bending test, high-frequency heating and quenching are superior in both maximum load and absorbed energy. .. Further, in the comparative example in which Cr is not added, not only the strength after quenching decreases but also the elongation after induction hardening is lower than that in the case where Cr is added.

(Embodiment 3) With respect to the electric resistance welded steel pipe 31.8 mmφ × 2.3 mmt having the chemical composition shown in Table 5, heat treatment was performed by high frequency heating and quenching under the same conditions as in Embodiment 1, and then the bending die radius R. , Span L, and bending speed in Example 1
Same as above, bending displacement: 305mm, seam position: 0 degree, 90
The steel pipe was subjected to a three-point bending test under the conditions of 180 degrees and 180 degrees. Table 6 shows the occurrence of cracks in the direction of the pipe axis of the electric resistance seam after the three-point bending test. In Examples containing Ca, no cracks occurred in the direction of the seam of the seam. In the comparative example in which Ca was not added, partial cracking occurred regardless of the seam. By adding Ca in this way, the soundness of the seam portion is enhanced.

[0019]

[Table 5]

[0020]

[Table 6]

(Embodiment 4) With respect to the electric resistance welded steel pipe 25.4 mmφ × 2.8 mmt having the chemical composition shown in Table 7, heat treatment was carried out by induction heating and quenching under the same conditions as in Embodiment 1, and then a tensile test was conducted. The results are shown in Table 8. Whereas the component which belongs to the scope of the invention also 150 kgf / mm ² or more tensile strength and 15% or more of elongation either can be obtained, both in the present invention outside the scope of component 150 kgf / mm ² or less of the tensile The strength and / or the elongation is 10% or less, and high strength and high ductility cannot be satisfied at the same time.

[0022]

[Table 7]

[0023]

[Table 8]

(Example 5) Material symbols F and O shown in Table 7
The electric resistance welded steel pipe having a chemical composition of 25.4 mmφ × 2.8 mmt was subjected to heat treatment under different conditions of high frequency heating and quenching, and then subjected to a tensile and three-point bending test. The results are shown in Table 9. As is clear from the table, by performing high-frequency heating and quenching within the conditions of the present invention, it is possible to manufacture an electric resistance welded steel pipe having high strength, good elongation, and excellent absorbed energy characteristics during three-point bending.

Example 6 High-frequency heating and quenching heat treatment were performed on a 31.8 mmφ × 2.3 mmt ERW steel pipe. At that time, heat treatment was performed while changing the rotation speed of the steel pipe, and the bending examination after the heat treatment was performed. The results are shown in FIG. 300 of the present invention conditions
When the number of revolutions per minute or more is small, the curve is about 1 mm / m, which is small, but when the rotational speed is 300 times per minute or less, which is outside the conditions of the present invention, the curve is large.

[0026]

As described above, according to the present invention, a high-strength steel pipe having a tensile strength of 150 kgf / mm ² or more and excellent ductility can be manufactured by induction heating and quenching, and the product is an automobile door impact beam. Can be applied as a structural member.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing the relationship between the number of revolutions and the bending amount of a steel pipe when high-frequency heating and quenching are performed while rotating the steel pipe.

FIG. 2 is an explanatory diagram of a three-point bending test method.

FIG. 3 is a graph showing the relationship between load and displacement after a 3-point bending test.

[Explanation of symbols]

 1 Test piece 2 Support tool 3 Bending die L span

[Table 9]

Claims (3)

[Claims] 1. C: 0.15 to 0.27% (weight%, the same hereinafter), Si: 0.5% or less, Mn: 0.80 to 1.70%, P: 0.02%
Below, S: 0.02% or below, Cr: 0.1 to 0.7%, Ti: 0.005
~ 0.040%, B: 0.0005 ~ 0.0025%, Ca: 0.0004 ~ 0.01
00%, N: 0.0080% or less, the balance consisting of Fe and unavoidable impurities, the raw material steel plate is made into a steel pipe by electric resistance welding, and the steel pipe is heated at a temperature of 850 to 1050 ° C and a cooling rate of 100 ° C. Tensile strength of 150-190k, characterized by high-frequency heat treatment and high-frequency heat treatment such as hardening under conditions of / s or more
A method for manufacturing high strength ERW steel pipes for automobiles with gf / mm ² grade and excellent in ductility and 3-point bending properties. 2. C: 0.15 to 0.27% (weight%, the same below), Si: 0.5% or less, Mn: 0.80 to 1.70%, P: 0.02%
Below, S: 0.02% or below, Cr: 0.1 to 0.7%, Ti: 0.005
~ 0.040%, B: 0.0005 ~ 0.0025%, Ca: 0.0004 ~ 0.01
00%, N: 0.0080% or less, Nb: 0.1% or less, V:
0.1% or less, Mo: 0.5% or less, 1 or 2 or more, and the balance of Fe and unavoidable impurities, the raw material steel plate is made into a steel pipe by electric resistance welding, and then the heating temperature is 8
Under conditions of 50 to 1050 ℃ and cooling rate of 100 ℃ / s or more,
Intensity of 150 to 190 kgf / mm ² grade, characterized by performing induction heating and induction heating.
A method for producing a high-strength electric resistance welded steel pipe for automobiles having excellent point bending properties. 3. Ductility and three-point bending property, characterized in that in the induction heat treatment according to claim 1 or 2, the induction heat treatment is performed while applying a rotation speed of 300 times / min or more to the steel pipe around the pipe axis of the steel pipe. A method for producing a high-strength electric resistance welded steel pipe for automobiles, which has excellent properties.