JP2009235499A - Method for manufacturing hollow stabilizer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hollow stabilizer superior in fatigue properties, with high productivity and processability while suppressing the manufacturing cost. <P>SOLUTION: The method for manufacturing the hollow stabilizer comprises sequentially: a step of preparing a slab having a composition comprising, by mass%, 0.15 to 0.40% C, 0.30% or less Si, 0.3 to 2.0% Mn, 0.030% or less P, 0.010% or less S, 0.2 to 0.8% Cr, 0.005 to 0.1% Ti, 0.005 to 0.10% sol. Al, 0.010% or less N, 0.0010 to 0.0070% B, and the balance Fe with unavoidable impurities; a hot rolling step of finish-rolling the slab at a finishing temperature of 800 to 900&deg;C, and then winding the plate at a winding temperature of 550 to 680&deg;C to obtain a hot-rolled steel plate; a step of electric-resistance-welding the wound hot-rolled steel plate and tempering only a weld bead part at a transformation point Ac<SB>1</SB>or lower to obtain an electric-resistance-welded steel pipe; and steps of forming the electric-resistance-welded steel pipe into the hollow stabilizer, then hardening the inside of the hollow stabilizer with an average cooling rate of 50&deg;C/second or higher, and subsequently tempering the hollow stabilizer at 400&deg;C or lower. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for manufacturing a hollow stabilizer, and more particularly, to a method for manufacturing a hollow stabilizer that ensures running stability of an automobile.

  The stabilizer is an important part that ensures the running stability of the vehicle body at high speeds. Conventional stabilizers are solid materials made of steel bars into product shapes, but are hollow materials such as seamless steel pipes and ERW welded steel pipes to reduce the weight of the vehicle body as one of the measures to improve the fuel efficiency of automobiles. Steel pipes are increasingly used. In recent years, the design stress required for a stabilizer has been increased due to an increase in the output of an automobile, so that it is also desired to improve the fatigue life of the stabilizer under a high stress.

  As a method for improving the fatigue life of a hollow stabilizer (hereinafter referred to as “hollow stabilizer”), a method of imparting compressive residual stress to the hollow stabilizer by shot peening is known. However, although the method by shot peening can be easily applied to the outer surface of the hollow stabilizer (steel pipe), there is a problem that it is industrially difficult to apply to the inner surface of the hollow stabilizer. Therefore, the thin hollow stabilizer cannot sufficiently cope with high cyclic stress. Therefore, by producing a thick hollow stabilizer using a thick ERW steel pipe having a large ratio (t / D) between the steel pipe wall thickness t and the steel pipe outer diameter D, the stress applied to the inner surface of the hollow stabilizer is reduced. It is reduced. Here, when increasing the thickness of the electric resistance welded steel pipe, a large strain is applied to the material surface during pipe forming or forming into the stabilizer, and therefore, the electric resistance welded steel pipe is required to have high workability. In particular, since the portion to be bent, the bending angle, the bending radius, and the like vary depending on the shape of the target hollow stabilizer, the electric resistance welded steel pipe is often processed with a small bending radius. Therefore, the ERW steel pipe needs to have good workability enough to withstand severe processing.

ERW steel pipes used for hollow stabilizers are manufactured by forming steel sheets, but during the solidification of pipe welding, element partitioning occurs between the solid phase and the liquid phase, resulting in solidification in the solid phase. The amount of carbon in the liquid phase is significantly lower than the amount of carbon in the liquid phase. Therefore, in the weld bead portion of the electric resistance welded steel pipe, the hardenability is lowered when the hollow stabilizer is manufactured, and a portion having a lower quenching hardness than the base material portion is generated. As a result, the soft weld bead portion becomes the starting point of fatigue failure, and the fatigue characteristics of the hollow stabilizer are reduced.
Also, in the manufacture of hollow stabilizers, when quenching after forming an ERW steel pipe into a hollow stabilizer, the hollow stabilizer heated to a high temperature is cooled with a coolant such as water, but only from the outer surface of the hollow stabilizer. For this reason, the cooling rate on the inner surface side becomes lower than the cooling rate on the outer surface side, and a quenching failure is likely to occur. Therefore, the quenching hardness is lowest at the weld bead portion on the inner surface side of the hollow stabilizer.

As described above, since fatigue characteristics are regarded as important in the hollow stabilizer, it is conceivable to use a structural alloy steel pipe excellent in fatigue characteristics as a steel pipe used in the hollow stabilizer. However, the structural alloy steel pipe has a problem that the addition of the alloy leads to an increase in manufacturing cost.
Therefore, a hollow steel with improved fatigue strength by using an ERW steel pipe heated to an austenite region having a predetermined chemical composition, reduced in diameter with a hot drawing mill, and then subjected to quenching treatment. A method for manufacturing a stabilizer has been proposed (see, for example, Patent Document 1). In this method, it is considered that by reducing the diameter, carbon diffuses into the portion that has been reduced in carbon by welding, and the hardness of the weld bead portion can be prevented from decreasing.
On the other hand, a method for manufacturing a hollow stabilizer in which the inner surface of a steel pipe is reinforced by forming a carburized hardened layer on the inner surface of the steel pipe has also been proposed (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2004-11009 JP 2000-118224 A

However, in the method of Patent Document 1, it is necessary to heat the steel to the austenite region after pipe making and to reduce the diameter, and in order to remove the thermal strain and make a straight pipe, correction by a pipe drawing process is required. Therefore, the method of Patent Document 1 has a problem that the manufacturing cost increases and the productivity is also poor. Furthermore, in the method of Patent Document 1, since a predetermined amount of Mn is blended as a steel component in order to improve the hardenability of the electric resistance welded steel pipe, there is a concern that the workability of the hot rolled steel sheet and the electric resistance welded steel pipe is lowered. Nevertheless, no means for ensuring the workability is shown.
Further, in the method of Patent Document 2, it is difficult to stably strengthen the weld bead portion of the electric resistance welded pipe in the thickness direction, and after forming the pipe, the coating type carburizing composition is applied to the inner surface of the steel pipe and dried. Therefore, there is a problem in that the manufacturing cost increases. Furthermore, in the method of Patent Document 2, it is necessary to strictly control the quenching temperature and the heating time.

  The present invention has been made to solve the above-described problems, and provides a method for manufacturing a hollow stabilizer excellent in fatigue characteristics that can be manufactured with high productivity and workability while suppressing manufacturing cost. The purpose is to provide.

The manufacturing method of the hollow stabilizer of this invention is C: 0.15-0.40 mass%, Si: 0.30 mass% or less, Mn: 0.3-2.0 mass%, P: 0.030 mass% Hereafter, S: 0.010 mass% or less, Cr: 0.2-0.8 mass%, Ti: 0.005-0.1 mass%, sol.Al:0.005-0.10 mass%, N : After finishing and rolling a slab containing 0.010% by mass or less, B: 0.0010 to 0.0070% by mass and the balance of Fe and inevitable impurities at a finishing temperature of 800 to 900 ° C., 550 A hot rolling step of obtaining a hot-rolled steel sheet by winding at a coiling temperature of ˜680 ° C., and after electric welding of the wound hot-rolled steel sheet, only the weld bead portion is tempered at a temperature below the Ac ₁ transformation point. A process of obtaining an electric resistance steel pipe, and the electric resistance steel pipe was formed into a hollow stabilizer , Quenching the average cooling rate of the inner surface of the hollow stabilizer as above 50 ° C. / sec, and then characterized by sequentially performing the step of tempering at 400 ° C. or lower.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the hollow stabilizer excellent in the fatigue characteristic which can be manufactured with high productivity and workability, suppressing manufacturing cost can be provided.

  As described above, the decrease in fatigue characteristics of the hollow stabilizer is due to fatigue failure starting from the weld bead where the carbon content has decreased significantly during solidification of pipe-forming welds. It is important to prevent fatigue failure at the bead portion (particularly, the weld bead portion on the inner surface side of the ERW steel pipe). In order to prevent fatigue failure starting from this weld bead, the ERW pipe is formed into a hollow stabilizer and then hardened and tempered to transform the metal structure of the weld bead to martensite and generate residual stress. It is necessary to ensure hardness by making it. Based on such a viewpoint, the composition of the steel component was designed to achieve both the workability of steel (hot-rolled steel sheet and ERW steel pipe) that is the material of the hollow stabilizer and the fatigue characteristics of the hollow stabilizer.

Hereinafter, the composition of steel used as the material for the hollow stabilizer of the present invention will be described in detail.
Steel used as a material for the hollow stabilizer is C, Si, Mn, P, S, Cr, Ti, sol. Including Al, N, and B, the balance is made of Fe and inevitable impurities. Among these components, the upper limit of C, Ti, Mn, Cr, and S is regulated from the viewpoint of workability, and the lower limit of C, Mn, and Cr is regulated from the viewpoint of heat treatment. B and Ti are effective components for improving hardenability. Hereinafter, the composition of steel will be described in detail.

C: 0.15-0.40 mass%
C is the most basic component in carbon steel, and the quenching hardness and the amount of carbide vary greatly depending on the content thereof. Steel with a C content of less than 0.15% by mass cannot provide a quenching hardness sufficient to be applied to a hollow stabilizer. Further, when element distribution occurs between the solid phase and the liquid phase during welding solidification, C of 0.15% by mass or more is necessary in order to secure a C content of 0.05% by mass or more. On the other hand, if the C content exceeds 0.40% by mass, the workability of the steel is deteriorated so that it cannot be formed, or there is a concern about burning cracks. From the viewpoint of obtaining a steel having better quenching hardness and workability, the C content is preferably 0.20 to 0.25% by mass.

Si: 0.30 mass% or less Si is one of the components having a great influence on ductility. If Si is contained excessively, the ferrite is hardened by the solid solution strengthening action, which causes cracking during the molding process. Moreover, when Si content increases, there exists a tendency for a scale flaw to generate | occur | produce on the steel plate surface in the manufacturing process of a hot-rolled steel plate, and causes the fall of surface quality. In particular, if the Si content exceeds 0.3% by mass, a penetrator is generated during pipe-forming welding, resulting in a weld defect, and the strength of the steel becomes too high and workability decreases, so the upper limit of the Si content Needs to be 0.3 mass%.

Mn: 0.3 to 2.0% by mass
Mn is an effective component for enhancing the hardenability and toughening of the ERW steel pipe. In order to obtain sufficient hardenability, the Mn content needs to be 0.3% by mass or more. However, if the Mn content exceeds 2.0 mass%, defects are likely to occur in the weld bead portion. From the viewpoint of further ensuring the hardenability and workability of the ERW steel pipe, the content of Mn is preferably 1.0 to 1.8% by mass.

P: 0.030% by mass or less P is a component that deteriorates ductility and toughness. If P exceeding 0.030% by mass is included, the toughness of prior austenite grain boundaries deteriorates after quenching, and the fatigue characteristics decrease. To do. Therefore, the upper limit of the P content needs to be 0.030% by mass.

S: 0.010 mass% or less S is a component that forms MnS inclusions. Since ductility deteriorates as the amount of inclusions increases, the S content in the steel needs to be reduced as much as possible. Therefore, the upper limit of the S content needs to be 0.010% by mass. From the viewpoint of further improving the workability of the hot-rolled steel sheet and the ERW steel pipe and the fatigue strength of the hollow stabilizer, the upper limit of the S content is preferably 0.005% by mass.

Cr: 0.2-0.8 mass%
Cr is one of the effective components for improving hardenability and miniaturizing austenite crystal grains during quenching heating. If the austenite crystal grains can be reduced, the fatigue strength of the hollow stabilizer can be improved. In order to obtain this effect, it is necessary to add at least 0.2% by mass of Cr. However, if a large amount of Cr exceeding 0.8% by mass is blended, the workability before quenching deteriorates. In order to further improve the workability, the upper limit of the Cr content is preferably 0.5% by mass, and more preferably 0.30% by mass.

Ti: 0.005 to 0.1% by mass
Ti is a component blended for adjusting the deoxidation of molten steel, and also has a denitrification action. Further, Ti fixes N dissolved in the steel sheet as a nitride, so that the effective B amount for improving the hardenability can be increased. Furthermore, Ti has the effect of forming carbonitrides and preventing crystal grain coarsening during quenching heating. In order to stably obtain these effects, it is necessary to contain 0.005 mass% or more of Ti. However, when a large amount of Ti exceeding 0.1% by mass is contained, not only is it economically disadvantageous, but it also causes deterioration in ductility. In order to obtain the above effect more stably, the Ti content is preferably 0.01 to 0.05% by mass.

sol. Al (acid-soluble Al): 0.005 to 0.10% by mass
sol. Al is a component used as a deoxidizer for molten steel. Also, sol. Al has the effect of suppressing the abnormal growth of austenite grains during heat treatment by generating AlN and suppressing the generation of BN by this N fixing effect and promoting the improvement of hardenability by B. In order to stably obtain this effect, sol. It is necessary to contain Al. On the other hand, sol. When Al is contained, it is not only economically disadvantageous, but also the workability of hot-rolled steel sheets and ERW steel pipes is lowered. In order to obtain the above effect more stably, sol. The content of Al is preferably 0.010 to 0.050% by mass.

N: 0.010% by mass or less N is a component that improves the hardness and tensile strength of steel. By forming AlN and TiN, the austenite grains are refined, and the impact resistance and fatigue characteristics are improved. There is. Such an effect is obtained when the N content is 0.001% by mass or more. However, when the N content exceeds 0.010% by mass, it leads to an increase in material strength at the time of molding and deteriorates moldability. In addition, if N remains beyond the fixing ability of AlN and TiN, BN is generated and inhibits the effect of improving hardenability, so the upper limit of the N content needs to be 0.010% by mass. In order to obtain the above effect stably, the N content is preferably 0.001 to 0.005 mass%.

B: 0.0010 to 0.0070% by mass
B is a component that greatly improves the hardenability of the ERW steel pipe by adding a very small amount. Further, B has an effect of strengthening the grain boundary by reducing the strain energy of the grain boundary. In order to obtain such an effect stably, the lower limit of the B content needs to be 0.0010% by mass. However, even if B is added in an amount exceeding 0.0070% by mass, the effect is saturated, and conversely, the toughness is deteriorated. In order to obtain the above effect more stably, the B content is preferably 0.0020 to 0.0050 mass%.

  In the method for producing a hollow stabilizer of the present invention, a hot-rolled steel sheet having the above composition is used as a material, but the final pass temperature (finishing temperature) in finish rolling in the production process of the hot-rolled steel sheet is a hot-rolled steel sheet or It has a great influence on the characteristics of ERW steel pipe and hollow stabilizer. Therefore, in order to obtain the effect of the present invention, it is necessary to finish-roll a slab having the above composition at a finishing temperature of 800 to 900 ° C. to obtain a hot-rolled steel sheet. When the finishing temperature is less than 800 ° C., the hot deformation resistance increases, the load applied to the rolling mill increases, and the productivity decreases. In addition, when the finishing temperature is too low, the strength of the hot-rolled steel sheet increases due to the refinement of the metal structure and the accumulation of processing strain, making it difficult to form a pipe. On the other hand, when the finishing temperature exceeds 900 ° C., the ferrite decarburization of the plate thickness surface layer becomes deep. As a result, a hollow stabilizer having a desired surface hardness cannot be obtained after quenching, or ferrite is present on the inner and outer surfaces of the electric resistance welded steel pipe, and the fatigue characteristics of the hollow stabilizer are also lowered. In order to obtain the effect of the present invention more stably, the upper limit of the finishing temperature is preferably 880 ° C.

  Next, the hot rolled steel sheet is wound into a coil shape. The coiling temperature in the coiling process also has a great influence on the properties of the hot-rolled steel sheet, the electric resistance welded steel pipe and the hollow stabilizer made of the same. In particular, from the viewpoint of minimizing surface decarburization while ensuring the workability of hot-rolled steel sheets and ERW steel pipes, the hot-rolled steel sheets are wound at a winding temperature of 550 to 680 ° C, preferably 600 to 650 ° C. There is a need. If the coiling temperature is less than 550 ° C., the workability of the hot-rolled steel sheet is lowered, making pipe making difficult, or even if pipe making is possible, the electric resistance welded steel pipe cannot be formed into a hollow stabilizer. On the other hand, when the coiling temperature exceeds 680 ° C., surface decarburization becomes deep during cooling after coiling, so that the surface hardness after quenching is insufficient, and a hollow stabilizer excellent in fatigue characteristics cannot be obtained.

Next, the wound hot-rolled steel sheet is electro-welded using a known means. Specifically, after pickling the hot-rolled steel sheet, the hot-rolled steel sheet is slit to a predetermined width, and the end faces are welded and water-cooled by roll forming and high-frequency heating. Since the molten metal (this is referred to as “weld bead”) is raised at the welded portion of the outer surface and the inner surface of the electric resistance welded steel pipe thus obtained, the raised bead is removed as necessary. Normally, only the weld bead having a raised outer surface that can be easily removed is removed by cutting or the like. Next, since the weld bead part is a rapidly solidified structure and has a hard and brittle nature, only the weld bead part is tempered at a temperature equal to or lower than the Ac ₁ transformation point to recover the toughness of the weld bead part. When the tempering temperature exceeds the Ac ₁ transformation point, it becomes austenite, so that it is quenched by subsequent cooling, and the desired toughness cannot be obtained. On the other hand, the lower limit of the tempering temperature may be appropriately set in accordance with the kind of hot-rolled steel sheet, but if considering the productivity and the like, preferably in the (Ac ₁ transformation point -100 ℃), (Ac ₁ transformation It is more preferable that it is a point -70 degreeC.
The method for heating the weld bead portion is not particularly limited, and a known method such as high-frequency heating can be used.
The electric resistance welded steel pipe manufactured in this way is modified in shape as necessary to increase the straight pipe degree or cut to a predetermined length.

Next, in order to obtain a hollow stabilizer having a desired shape, the electric resistance welded steel pipe is cut into a required length and formed into a hollow stabilizer by bending the required portion. The processing method is not particularly limited, and a known method such as a rotational pulling bending method can be used.
After the forming process, the hollow stabilizer is sequentially quenched and tempered in order to improve the fatigue strength. In quenching and tempering, the metal structure is transformed into martensite to impart compressive stress to the hollow stabilizer and improve fatigue characteristics. In particular, by applying compressive stress accompanying martensitic transformation to the weld bead portion on the inner surface side, it becomes possible to improve the fatigue characteristics from the inner surface side, which is difficult by shot peening or the like. In order to achieve the improvement of the fatigue characteristics, it is necessary that the martensite has a metal structure of 80% or more and the balance is bainite.

The heating method in quenching is not particularly limited, and a known method such as an electric heating method or a high-frequency heating method can be used.
In the quenching, the metal structure of the hollow stabilizer needs to be austenitized, and in order to prevent the ferrite structure and carbide from remaining in the metal structure, it is necessary to heat to a temperature equal to or higher than the Ac ₃ transformation point. However, it should be noted that when soaking for a long time in this temperature range, austenite grains grow abnormally and the fatigue characteristics may be remarkably deteriorated. Therefore, it is preferable that the quenching temperature is (Ac ₃ transformation point + 50 ° C.), and soaking is performed for at least 10 seconds in order to decompose cementite and form uniform austenite. Also, from the viewpoint of reducing the heat treatment cost, it is desirable that the soaking time is relatively short, so the soaking time is preferably within 1 minute.
In quenching, in order to obtain a metal structure with martensite of 80% or more and the balance of bainite, the average cooling rate of the inner surface of the hollow stabilizer must be 50 ° C./second or more, preferably 100 ° C./second or more. Here, the average cooling rate means an average cooling rate from the quenching temperature to the Ms point (martensitic transformation start point). On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 200 ° C./second, more preferably 160 ° C./second, from the viewpoint of preventing the occurrence of distortion. The average cooling rate can be easily controlled by appropriately managing the temperature of the coolant.

The heating method in tempering is not particularly limited as long as it is a heating method capable of maintaining the whole at a uniform temperature, and can be performed using a known method.
Tempering needs to be performed at a temperature of 400 ° C. or lower, and a metal structure with tempered martensite of 80% or more is obtained by this tempering. When the tempering temperature exceeds 400 ° C., the residual compressive stress applied to the inner surface side of the hollow stabilizer is released, and the fatigue characteristics from the inner surface side of the hollow stabilizer cannot be improved. On the other hand, the lower limit of the tempering temperature is not particularly limited, but is preferably 200 ° C. The tempering time may be appropriately set according to the tempering temperature, the steel composition, and the like, and is not particularly limited, but is generally 30 to 120 minutes. In addition, it does not specifically limit about the cooling in tempering.

The hollow stabilizer thus quenched and tempered exhibits excellent fatigue strength.
In order to further improve the fatigue characteristics of the hollow stabilizer, compressive residual stress by shot peening or the like may be applied to the outer surface of the hollow stabilizer after quenching and tempering. Then, a coating process can also be performed as needed.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by these.
[Example 1]
Each steel having the chemical composition shown in Table 1 (with the balance consisting of Fe and inevitable impurities) was produced in a converter, and a slab for hot rolling was obtained by a continuous casting method. Steels A to L are steels whose Ac ₁ transformation point temperature is in the range of 735 to 760 ° C.

Next, each hot rolling slab was finish-rolled at each finishing temperature shown in Table 2, and then wound up at each winding temperature shown in Table 2 to obtain a hot-rolled steel sheet. The wound hot-rolled steel sheet is pickled and slit, and electro-welded by high frequency welding. Then, bead cutting is performed only on the weld bead portion on the outer surface side of the steel pipe, followed by water cooling, and then only the weld bead portion. By tempering at 680 ° C. or 750 ° C. within 1 minute, an electric resistance welded steel pipe was obtained. Here, a is the 680 ° C. tempering temperature of the weld bead portion is in the range of relative steel A to L (Ac ₁ transformation point -55 ℃) ~ (Ac ₁ transformation point -80 ° C.). Meanwhile, 750 ° C. is a tempering temperature of the weld bead portion, since Ac ₁ transformation point of steel B and steel D are each 735 ° C. and 740 ° C., either Ac ₁ transformation point or above for these steels It is. The electric resistance welded steel pipe thus obtained had a steel pipe outer diameter of 30 mm and a steel pipe thickness of 6 mm.
The workability of this electric resistance welded steel pipe was evaluated. The workability was evaluated in accordance with the flattening test specified in JIS G 3472. At this time, the bead portion was perpendicular to the compression direction. In this evaluation, when the amount of processing at the weld bead portion is 2/3 of the outer diameter D of the steel pipe, the case where there was no crack is indicated as ◯, and the case where cracks occurred or the case where the pipe could not be formed is indicated as x. To express.

Next, in the evaluation of the workability, a short pipe having a length of 600 mm was cut out from an ERW steel pipe having an evaluation of ◯, and quenched and tempered. Here, quenching was performed by energization heating in the atmosphere, the quenching temperature was 900 ° C., and the quenching time was 10 minutes. Moreover, cooling in quenching was performed by putting it in a coolant, and the average cooling rate on the inner surface of the ERW steel pipe was about 80 ° C./second. In addition, since this cooling is cooling by charging into the coolant, it is cooling only from the outer surface. In addition, the average cooling rate on the inner surface of the ERW steel pipe was calculated from the measured value of the temperature change by the thermocouple with a thermocouple with a protective tube welded at a position of 100 mm from the pipe end of the inner surface of the ERW steel pipe before quenching. . Tempering was performed at a tempering temperature of 300 ° C. and a tempering time of 30 minutes.
With respect to the ERW steel pipe after quenching and tempering, the surface hardness of the weld bead portion was evaluated using a Vickers hardness tester with a test load of 196N. In this evaluation, a sample having a Vickers hardness of 380 Hv or more is represented by a circle, and a sample having a Vickers hardness of less than 380 Hv is represented by x.
Table 2 shows the results of the workability of the above-mentioned ERW steel pipe and the surface hardness after quenching and tempering.

As shown in Table 2, Example No. of the present invention. 1, 3, 4, 6-8, 11, 12, 14, and 18-21 had high workability of the electric resistance welded steel pipe and high surface hardness after quenching and tempering.
On the other hand, comparative example No. whose finishing temperature and winding temperature are lower than the range specified by this invention. In Nos. 2, 10 and 17, the obtained hot-rolled steel sheet had high strength, so that the workability of the electric resistance welded pipe could not be sufficiently ensured. Moreover, comparative example No. whose finishing temperature and winding temperature are higher than the range specified by this invention. 5, 15 and 16 show that the obtained hot-rolled steel sheet has a comparative example No. Although it was softer than 2, 10 and 17, and the workability of the ERW steel pipe was good, the surface decarburization was deep and the surface hardness after quenching and tempering was low. Furthermore, comparative example No. whose tempering temperature of a weld bead part is higher than the range specified by this invention. Even in 9 and 13, the workability of the ERW steel pipe could not be secured sufficiently.
Of the comparative examples where the chemical composition of the steel is outside the range specified in the present invention, No. 22 (those with low C content), 23 (those with low Mn content), and 24 (those with low Cr content and no addition of Ti and B) have the workability of ERW steel pipes Although it was good, the surface hardness after quenching and tempering was low. No. 25 (those with a high content of C), 26 (those with a high content of Mn), and 27 (those with a high content of Si) have high strength and workability is too low. As a result, plate breakage occurred during manufacture of the ERW steel pipe, and the ERW steel pipe was not obtained. As can be seen from the above results, the ERW steel pipe has good workability and surface hardness after quenching and tempering. In order to obtain the above, it is necessary that the finishing temperature, the coiling temperature, the tempering temperature of the weld bead and the chemical composition of the steel are within the scope of the present invention.

[Example 2]
After obtaining the hot-rolling slabs A to I in the same manner as in Example 1, each hot-rolling slab was finish-rolled at each finishing temperature shown in Table 3, and wound at each winding temperature shown in Table 3 to hot-roll. A steel plate was obtained. The wound hot-rolled steel sheet is pickled and slit, electro-welded by high frequency welding, and then bead-cut at the weld bead portion on the outer surface side of the steel pipe, and only the weld bead portion is tempered as shown in Table 3. An electric resistance welded steel pipe was produced by tempering at a temperature. The electric resistance welded steel pipe thus obtained had a steel pipe outer diameter of 30 mm and a steel pipe thickness of 6 mm.
Next, a short pipe having a length of 600 mm was cut out from each obtained ERW steel pipe, and quenched and tempered. Here, quenching was performed by energization heating in the atmosphere, the quenching temperature was 900 ° C., and the quenching heating time was 1 minute. Moreover, cooling in quenching was performed by putting it in a coolant so that the average cooling rate on the inner surface of the electric resistance welded steel tube became the average cooling rate in Table 3, and cooling was performed only from the outer surface of the electric resistance welded steel tube. Here, the average cooling rate was changed by using water, oil, or polymer as the coolant or changing the degree of stirring of the coolant. In addition, the measuring method of each average cooling rate is the same as that of Example 1. Tempering was performed at each tempering temperature shown in Table 3 with a tempering time of 30 minutes.

About the ERW steel pipe after quenching and tempering obtained as described above, the surface hardness of the weld bead part, the metal structure, and the fatigue test were evaluated. In this example, a straight tubular hollow stabilizer was assumed, and bending was omitted.
The surface hardness of the weld bead portion was evaluated using a Vickers hardness tester with a test load of 196N.
The metallographic structure was evaluated by polishing the weld bead part to give a mirror finish, etching with a 5% concentration of nital liquid, and then observing the weld bead part with an optical microscope. In this observation, the area ratio of martensite on the surface of the weld bead portion was determined. In addition, although this evaluation is a result in a weld bead part, it estimated with the ratio of the metal structure and martensite in the whole ERW steel pipe.
In the fatigue test, a stress of 700 MPa was applied to the surface of the ERW steel pipe, and the repetition rate was 10 Hz. In this evaluation, fatigue tests were performed up to 50,000 repetitions, and the presence or absence of fracture at the welded portion was observed.
The evaluation results are shown in Table 4.

As shown in Table 4, Example No. of the present invention. Nos. 31, 33, 34, 36 to 40, 42, 48 to 51, 53 and 54 had high surface hardness of the weld bead portion and a metal structure having martensite of 80% or more. Further, these examples of the present invention were not damaged even in a fatigue test with 50,000 repetitions, and had sufficient fatigue characteristics to be used as a hollow stabilizer.
On the other hand, Comparative Example No. in which the average cooling rate on the inner surface of the ERW steel pipe during quenching is lower than the value specified in the present invention (50 ° C./second). 32, 45 to 47 and 52, since ferrite and pearlite, a mixed structure of martensite and bainite and ferrite having a metal structure of less than 80%, or ferrite, pearlite and bainite, the surface hardness on the inner surface of the steel pipe is low, Fatigue properties sufficient for use as a hollow stabilizer were not obtained.
Moreover, comparative example No. whose tempering temperature of an electric resistance steel pipe is higher than the value (400 degreeC) specified by this invention. Also in 35, 41, 43 and 44, the surface hardness of the weld bead part was low, and sufficient fatigue characteristics for use as a hollow stabilizer were not obtained.
Furthermore, Comparative Example No. in which the chemical composition of the steel is outside the range specified in the present invention. In 55-58, since hardenability was low, the metal structure with few martensites was formed. For this reason, the surface hardness on the inner surface of the steel pipe is low, and sufficient fatigue properties for use as a hollow stabilizer cannot be obtained.

  As can be seen from the above results, the hollow stabilizer manufacturing method of the present invention can be manufactured with high productivity and workability while suppressing the manufacturing cost, and can provide a hollow stabilizer excellent in fatigue characteristics.

Claims (1)

C: 0.15-0.40 mass%, Si: 0.30 mass% or less, Mn: 0.3-2.0 mass%, P: 0.030 mass% or less, S: 0.010 mass% or less , Cr: 0.2-0.8 mass%, Ti: 0.005-0.1 mass%, sol.Al: 0.005-0.10 mass%, N: 0.010 mass% or less, B: A slab containing 0.0010 to 0.0070% by mass and having the balance consisting of Fe and inevitable impurities is finish-rolled at a finishing temperature of 800 to 900 ° C., and then wound at a winding temperature of 550 to 680 ° C. A hot rolling process to obtain a hot rolled steel sheet,
A step of electro-welding the wound hot-rolled steel sheet and then tempering only the weld bead portion at a temperature below the Ac ₁ transformation point to obtain an ERW steel pipe;
After forming the ERW steel pipe into a hollow stabilizer, quenching is performed at an average cooling rate of the inner surface of the hollow stabilizer of 50 ° C./second or more, and then tempering at a temperature of 400 ° C. or less. A method for manufacturing a hollow stabilizer.