JP2008095156A - Method for manufacturing hollow stabilizer with excellent delayed fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a hollow stabilizer from an electric resistance welded tube having metallic structure mainly of bainite thereby providing high strength. <P>SOLUTION: A steel material containing 0.10-0.30% C, ≤0.5% Si, 0.25-2.50% Mn, ≤0.03% P, ≤0.01% S, 0.5-1.5% Cr, 0.1-0.5% Mo, 0.0005-0.0100% B, 0.01-0.10% Ti, ≤0.01% N, 0.02-0.08% Al, is hot-rolled at a finish-temperature of 800-950°C and a coiling temperature of 400-600°C, consequently the metallic structure thereof is adjusted to ≤20 area% ferrite, ≤20 area% pearlite, ≤5 area% retained austenite and the balance bainite. After hot-rolling, an acid-pickling is applied to the steel material to make it into the electric resistance welded tube, desirably in order to control the maximum hardness difference between an electric resistance welded part and a basic material part to ≤50 HV, a tempering is applied to the electric resistance welded tube at the temperature range of (Ac<SB>1</SB>transforming point) to (Ac<SB>1</SB>transforming point-100°C) after cooling the electric resistance welded tube to a temperature of Ms point or below. The obtained electric resistance welded tube is annealed at 450-710°C for 1hr or shorter, and after cooling, the bending-work into the targeted stabilizer shape is performed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a hollow stabilizer that is mounted on an automobile and exhibits excellent delayed fracture resistance.

Stabilizers are necessary parts to alleviate rolling of the car body during cornering of the car and ensure the running stability of the car body at high speeds, but they have excellent fatigue because they are used for a long time while attached to the car. Durability is required. In order to satisfy the required fatigue durability, a material having a strength of 900 N / mm ² or more is required, but a hollow steel pipe is frequently used as a stabilizer material with an emphasis on weight reduction.

  The hollow stabilizer is hardened and tempered to improve the strength after bending the ERW steel pipe with the pipe end processed into a stabilizer shape. Furthermore, shot peening, painting, or the like may be applied to improve the fatigue characteristics of the stabilizer surface. In order to give the required strength to the hollow stabilizer that has undergone such a manufacturing process, carbon steel having a high quenching effect of C: about 0.20% by mass is used as the material.

The shape of the stabilizer is often U-shaped, and the U-shaped stabilizer is manufactured by bending a plurality of portions of the electric resistance welded steel pipe. In the bending process, a strict bending workability of a bending angle of 90 degrees is required, and the bending process is performed at room temperature. The stress / strain introduced during processing causes delayed fracture that impairs the function of the stabilizer.
However, the delayed fracture susceptibility increases as the strength of the electric resistance welded steel pipe used for the stabilizer increases. For example, a strength of 900 N / mm ² or more is necessary to satisfy fatigue durability, but it is a problem for delayed fracture resistance. Among them, in the tempered martensitic steel having a metal structure of tempered martensite by quenching and tempering and imparted a strength of 900 N / mm ² or more, delayed fracture susceptibility tends to increase rapidly.

The origin of delayed fracture is thought to be in the former austenite grain boundary. In tempered martensitic steel, carbides precipitated at the prior austenite grain boundaries promote the occurrence of delayed fracture cracks, resulting in a rapid increase in delayed fracture susceptibility. Therefore, various methods for improving the delayed fracture resistance of the electric resistance welded steel pipe used for the stabilizer have been proposed.
For example, in Patent Document 1, a hot-rolled steel strip having a strength of 900 N / mm ² or more and excellent in delayed fracture resistance is obtained by finishing and rolling in a temperature range of Ar ₃ or higher, and then rapidly cooling and winding. Although manufactured, it does not teach or suggest the manufacture of ERW steel pipes with improved delayed fracture resistance. In Patent Document 2, an ultra-high-strength ERW steel pipe is manufactured by performing aging treatment and cold-drawing and annealing as necessary after pipe making without quenching. Since it is wound up in the following, it is a disadvantage that it becomes martensite with a large amount of precipitated carbides as the starting point of delayed fracture.
JP-A-7-197184 Japanese Patent Laid-Open No. 5-9579

Quenching / tempering is an effective heat treatment for increasing the strength, but the formation of tempered martensite, which is undesirable for delayed fracture resistance, is a drawback. Therefore, the present inventors examined various heat treatments for obtaining a metal structure effective for delayed fracture resistance, which is not different from quenching and tempering in terms of strength. As a result, it was clarified that, after annealing an ERW steel pipe having a metal structure mainly composed of bainite and bending it into a stabilizer shape, quenching and tempering can be omitted, and a stabilizer having excellent delayed fracture resistance can be obtained.
Based on such knowledge, the present invention is made of an electric-welded steel pipe having a bainite-based metal structure whose strength does not substantially decrease even if annealed, and is quenched and tempered after being bent into a stabilizer shape. The object is to produce a hollow stabilizer exhibiting excellent delayed fracture resistance at low cost.

The present invention manufactures an electric resistance steel pipe for a hollow stabilizer through the following steps.
C: 0.10 to 0.30 mass%, Si: 0.5 mass% or less, Mn: 0.25 to 2.50 mass%, P: 0.03 mass% or less, S: 0.01 mass% or less , Cr: 0.5-1.5 mass%, Mo: 0.1-0.5 mass%, B: 0.0005-0.0100 mass%, Ti: 0.01-0.10 mass%, N : Hot rolled a steel material having a composition containing 0.01% by mass or less, Al: 0.02 to 0.08% by mass, the balance of Fe and impurities. The steel material may contain one or more of Ni: 0.1 to 1.0% by mass, V: 0.5% by mass or less, and Nb: 0.01 to 0.10% by mass as necessary.

In the hot rolling process, ferrite: 20 area% or less, pearlite: 20 area% or less, retained austenite: 5 area% or less, finishing temperature: 800 to 950 ° C., winding so that a bainite metal structure is obtained. Temperature: Select in the range of 400-600 ° C.
After hot rolling, the steel strip is pickled and sent to a pipe making line to make an electric resistance welded steel pipe. Preferably, an ERW steel pipe having a thickness of 2.0 to 4.5 mm and an outer diameter of 15 to 40 mm is manufactured.

In order to reduce the maximum hardness difference between the ERW weld and the base metal part to 50 HV or less, after cooling to a temperature below the M _s point, (Ac ₁ transformation point) to (Ac ₁ transformation point − A heat treatment for tempering to a temperature range of 100 ° C. may be applied to the ERW weld.
When manufacturing hollow stabilizers from ERW steel pipes, anneal them for 1 hour or less in a temperature range of 450 to 710 ° C, then cool to room temperature and bend the annealed ERW steel pipes to the target stabilizer shape. To do.

Effects and embodiments of the invention

  In the present invention, an ERW steel pipe having a bainite-based metal structure is used as a stabilizer material, and the annealing conditions (heat treatment conditions) are regulated, so that the ERW steel pipe (element pipe) before bending is high in strength and workability. And heat treatment after bending into a stabilizer shape is not required. In this regard, in the conventional method, strength is imparted by quenching and tempering after bending, but as described above, the delayed fracture susceptibility increases when tempered martensite is generated during quenching and tempering. That is, the order of bending and heat treatment differs between the present invention and the conventional method as follows.

In this way, by adopting the component design and manufacturing conditions that express high strength in the steel pipe stage, the quenching and tempering process can be omitted, and the strength design is difficult to reduce even by annealing, so the ductility is improved by annealing. it can. The annealing may be performed before or after the pipe end processing before the bending processing.
Since ductility is improved without causing a decrease in strength by annealing, the ERW steel pipe can be cold-worked, and the strength level required for the stabilizer is ensured. Moreover, since it is not a tempered martensite, which is disadvantageous for delayed fracture resistance, but a bainite-based metal structure, a hollow stabilizer excellent in both delayed fracture resistance and fatigue durability can be obtained.

Hereinafter, the alloy components, contents, production conditions and the like defined in the present invention will be described.
(Ingredient design)
The hot-rolled steel strip, finish-rolled at 800-950 ° C. and wound at 400-600 ° C., has a bainite-based metal structure so that a strength of 900 N / mm ² or more can be obtained without requiring quenching and tempering. Further, each alloy component is defined as follows.
C: 0.10 to 0.30% by mass
It is an alloy element necessary for obtaining the strength required as a hollow stabilizer, and if it is less than 0.10% by mass, a strength of 900 N / mm ² or more cannot be obtained. However, when the C content exceeds 0.30% by mass, bending workability, toughness, and delayed fracture resistance deteriorate.

Si: 0.5 mass% or less Although it is an alloy component that contributes to high strength without deteriorating delayed fracture resistance, excessive addition may cause frequent scale defects, quality deterioration of hot-rolled steel strip, and toughness reduction. Therefore, the upper limit was made 0.5 mass%.

Mn: 0.25 to 2.50% by mass
It has the effect of suppressing the formation of ferrite during the cooling process of the hot-rolled steel strip, and has the effect of forming a tough bainite-based structure even at a slow cooling rate. The action of Mn is observed at 0.25% by mass or more, but excessive addition promotes grain boundary segregation and lowers the melt strength, so 2.50% by mass was made the upper limit.
P: 0.03 mass% or less An element harmful to ductility, toughness, and bending workability, and excessive addition tends to induce cracks in the weld during ERW welding. Moreover, since it segregates at a grain boundary and increases delayed fracture susceptibility, the upper limit was set to 0.03 mass%.

S: 0.01% by mass or less An element that easily binds to Mn, and forms MnS, which is an inclusion in steel, to deteriorate bending workability. MnS becomes a stress concentration site and causes delayed fracture resistance deterioration. Therefore, the upper limit of the S content is set to 0.01% by mass.
Cr: 0.5 to 1.5% by mass
It is an element that increases the softening resistance during warming and the tempering softening resistance during reheating, and contributes to higher strength. The effect of Cr is seen at 0.5% by mass or more, but excess Cr exceeding 1.5% by mass brings about a decrease in bending workability.

Mo: 0.1-0.5% by mass
It is an alloy component that is effective in improving strength and hardenability, and has the effect of generating a tough bainite-based metal structure even at a low cooling rate. It is also effective in improving softening resistance. These effects are observed with 0.1% by mass or more of Mo, but even if excess Mo exceeding 0.5% by mass is included, further improvement in strength and hardenability cannot be expected, and expensive Mo is It is economically disadvantageous because it consumes a large amount.

B: 0.0005-0.0100 mass%
It is an effective alloy component for improving strength and toughness, and also exhibits the effect of delaying ferrite transformation during the cooling process. Moreover, the strain energy of a grain boundary is reduced, the effect | action which strengthens a grain boundary is exhibited, and it contributes to the improvement of fatigue durability. Such an effect is observed at 0.0005 mass% or more, but is saturated at 0.0100 mass%, and excessive addition causes deterioration of toughness.
Ti: 0.01-0.10 mass%
It is an alloy component that fixes solute N as nitride, contributes to preventing coarsening, improving strength and toughness, and is effective in improving delayed fracture resistance. Such an effect is observed at Ti: 0.01% by mass or more, but excess Ti exceeding 0.10% by mass generates coarse nitrides and causes toughness deterioration.

N: 0.01% by mass or less Combined with Ti to produce TiN, exhibiting the effect of increasing the strength of the steel material and refining the crystal grains. Although refinement of crystal grains is effective for delayed fracture resistance, excess N exceeding 0.01 mass% degrades toughness.
Al: 0.02 to 0.08 mass%
It is an element used as a deoxidizer in the steelmaking stage, and 0.02% by mass or more is necessary. However, excessive addition exceeding 0.08 mass% reduces the cleanliness of steel and promotes the generation of surface defects.

Ni: 0.1-1.0 mass%
It is an alloy component added as necessary, and is effective in improving corrosion resistance, toughness, and suppressing hydrogen intrusion. The addition effect of Ni is seen at 0.1% by mass or more, but is saturated at 1.0% by mass, and adding more than that causes an increase in steel material cost.
V: 0.5% by mass or less V is an alloy component added as necessary, and is effective in increasing strength and improving delayed fracture resistance, but is an expensive element and excessive addition leads to an increase in steel material cost. The upper limit was 0.5% by mass.
Nb: 0.01 to 0.10% by mass
It is an alloy component added as necessary, and forms carbonitrides to suppress coarsening of crystal grains and exhibits an effect of improving toughness. These effects are observed when Nb is added in an amount of 0.01% by mass or more. However, excessive addition causes toughness deterioration, so 0.10% by mass was made the upper limit.

(Hot rolling)
In hot rolling, the finishing temperature is adjusted to 800 to 950 ° C. and the winding temperature is adjusted to 400 to 600 ° C. so that a bainite-based structure is formed.
When the finishing temperature is less than 800 ° C., the deformation resistance increases, which impairs the sheet passing property. Furthermore, the two-phase rolling is easy to produce processed ferrite. Conversely, if it exceeds 950 ° C, the hot-rolled structure becomes coarse, the workability deteriorates, and the steel strip shape deteriorates due to increased cooling strain. It is.

The control of the coiling temperature is important in obtaining a bainite-based metal structure, and is adjusted to a range of 400 to 600 ° C. When it is wound at a temperature that is too low, the strength is significantly increased and the stability of the mechanical properties is impaired due to the influence of fluctuations in hot rolling conditions. The influence of fluctuations in hot rolling conditions can be suppressed by setting the coiling temperature to 450 ° C. or higher. On the other hand, at a high temperature winding exceeding 600 ° C., a strength of 900 N / mm ² or more may not be obtained depending on fluctuations in hot rolling conditions, and grain boundary oxidation tends to occur, resulting in poor fatigue characteristics.

[Metal structure of hot-rolled steel strip]
Tensile strength: A tempered martensitic steel having a strength of 900 N / mm ² or more is more susceptible to delayed fracture due to precipitation of carbides at the prior austenite grain boundaries than bainitic steel, and also tends to rapidly increase delayed fracture susceptibility. It is in. On the other hand, when the metal structure is bainite: 55 area% or more, a tensile strength of 900 N / mm ² or more is obtained, and delayed fracture resistance is also improved.

The metal structure of the hot-rolled steel strip is controlled by appropriately controlling the finishing temperature and the coiling temperature in addition to the alloy design. Ferrite: 20 area% or less, pearlite: 20 area% or less, retained austenite: 5 area% or less, balance Adjust to the metal structure of bainite. The area ratio is calculated as an average value obtained by measuring each tissue with ten fields of view of a 1000 times SEM image. However, in a metal structure containing a small amount of retained austenite, it is difficult to quantify the retained austenite by the area ratio, so the result quantified by X-ray diffraction is converted into an area ratio and displayed.
Ferrite is rich in ductility and advantageous for pipe forming properties, but it is more preferable as it is less strengthened. In order to secure the tensile strength necessary for the steel pipe for stabilizer: 900 N / mm ² or more, the ferrite is regulated to 20 area% or less.

Although pearlite contributes to high strength but adversely affects pipe forming properties, it is restricted to 20 area% or less in order to ensure good bending workability and toughness. Moreover, if the pearlite is 20% by area or less, cracks that occur during bending are greatly reduced.
Residual austenite has a high hydrogen solubility, and when it is subjected to processing-induced transformation to martensite, which has a low hydrogen solubility during bending, etc., hydrogen dissolved in the residual austenite is expelled, triggering delayed fracture resistance. Become a source of working hydrogen. In order to suppress the influence of hydrogen on the deterioration of delayed fracture resistance, the retained austenite is made 5 area% or less.

  The hot-rolled steel strip is pickled according to a conventional method and conveyed to a pipe making line. In the pipe making line, a hot-rolled steel strip cut into a predetermined width is formed into an open pipe by roll forming, rollless forming, etc., electric resistance welding (high frequency welding), arc welding (TIG welding, etc.), laser welding For example, a pipe is formed by welding both ends of the steel strip in the width direction by any of the welding methods. In addition, the steel strip cut to a predetermined width is cut in the longitudinal direction to form a cut plate, and both end portions in the width direction by any of welding methods such as plate winding, arc welding (MIG welding, TIG welding, etc.), laser welding, etc. A single pipe having a predetermined length may be manufactured by welding. Normally, a welded steel pipe manufactured by high-frequency welding is referred to as an electric resistance welded pipe, but in this specification, the term “electric resistance welding” is used to include arc welding, laser welding, and the like.

[East-welded welded parts]
Next, the inner surface bead and the outer surface bead of the ERW steel pipe are removed as necessary, and the ERW welded portion is tempered by tempering. In general, a welded portion that is in a hardened state is harder and less stretched than the base metal portion, and therefore, it is a place that governs the quality of workability when bending an ERW steel pipe. Therefore, workability can be improved by tempering the hardened and hardened ERW weld so that the maximum hardness difference from the base metal is 50HV or less.

In tempering, the ERW weld is rapidly cooled to below the M _s point and then tempered to a temperature range of (Ac ₁ transformation point) to (Ac ₁ transformation point−100 ° C.). For the tempering, a local heating method represented by high-frequency induction heating is adopted. When the tempering temperature exceeds the Ac ₁ point, it becomes austenite, and since it is quenched by subsequent cooling, the hardness does not decrease. However, at a heating temperature that does not reach (Ac ₁ transformation point −100 ° C.), the time required for softening becomes longer and the productivity is lowered. The lower limit of the tempering temperature is preferably (Ac ₁ transformation point -70 ° C.).

[Annealing before bending]
Bending angle: When bending at 90 degrees, a large amount of strain is introduced into the workpiece, and it is not suitable for machining high-strength materials as well as ERW steel pipes. It is conceivable to reduce strain and improve elongation by annealing prior to bending, but generally the strength tends to decrease due to annealing. In this respect, in the present component system, Cr, Mo, Ti, etc., which increase the softening resistance, are used as alloy components, so that a decrease in strength due to annealing is suppressed.

In annealing, the ERW steel pipe (base pipe) is heated to 450 to 710 ° C. within 1 hour and cooled to room temperature. A heating temperature of 450 ° C. or higher is necessary for removing strain and improving elongation, but at a high temperature exceeding 710 ° C., the softening proceeds too much, and the target strength: 900 N / mm ² or higher cannot be obtained. Even in the temperature range of 450 to 710 ° C, softening proceeds and the strength decreases in soaking for a long period of time, so the soaking time is limited to 1 hour.

  A steel material having the composition shown in Table 1 is melted and cast, and then hot-rolled at a finishing temperature: 830 ° C., a coiling temperature: 350-650 ° C., and a sheet thickness: 2.0-4.5 mm. Manufactured.

After subjecting the hot-rolled steel strip to skin pass rolling and pickling, pipes were formed by a roll forming method and further subjected to electro-welding to produce ERW steel pipes having an outer diameter of 19.1 to 38.1 mm. The ERW weld was water cooled to 150 ° C. below the M _s point immediately after welding and tempered to 600-700 ° C. by high frequency heating. Next, the ERW steel pipe was induction-heated by high frequency induction and annealed at 350-730 ° C.
In the example of the present invention, the electric resistance welded steel pipe after annealing was subjected to various tests, and in the comparative example, the electric resistance welded steel pipe subjected to quenching, water cooling, and tempering immediately after pipe forming was subjected to various tests.

[Metal structure]
The L section of the hot-rolled steel strip was mirror-polished and etched to prepare a test piece. The specimen was observed by SEM at a magnification of 1000 times, the area of each phase was measured for ten visual fields, the area for each visual field was averaged, and the area ratio of each phase was calculated.

[Bending test]
The ERW steel pipe was cut into a 1 m long test piece, mounted on a pipe bender, and bent by a bending radius: twice the diameter of the steel pipe, a bending angle: 90 degrees, and a drawing method. The occurrence of cracks on the outside of the bent part was investigated, and cracks that did not lead to breakage were evaluated as cracks.
[Tensile test]
The ERW steel pipe was cut into a tensile piece having a predetermined length, and the test piece was chucked in a state where a rod-like jig having a diameter substantially equal to the inner diameter was inserted into the pipe, and a tensile test was performed.

(Delayed fracture test)
A strip-shaped test piece is taken in the longitudinal direction of the ERW steel pipe from the position of 180 degrees with respect to the ERW welded part, and is subjected to surface grinding to form a flat plate having a length of 100 mm, a width of 8.0 mm, and a thickness of 1.0 mm. processed. A test piece 1 having a V notch (FIG. 1) at the edge in the width direction at the center position in the longitudinal direction of the flat plate was produced and immersed in a corrosion tank 3 (FIG. 2) filled with a corrosion solution 2. A 5% aqueous HCl solution was used as the corrosive solution 2, a bending stress was applied to the test piece 1 with the weight 4, and the time to break was measured while measuring the deformation of the test piece 1 with the displacement meter 5. The highest bending stress at which the specimen did not break even after 100 hours had passed was regarded as the delayed fracture limit, and the delayed fracture limit: 1300 N / mm ² or more was evaluated as “good delayed fracture resistance”.

As can be seen from the investigation results in Table 2, in the example in which the ERW welded part was tempered under the conditions according to the present invention, the difference between the maximum hardness of the ERW welded part and the hardness of the steel pipe base part was 50 HV or less. It was settled.
Test Nos. 1, 5, 11, 15, 17, 18, and 20 had a tensile strength of 900 N / mm ² or more, but the fraction exceeding 20 area% of the metal structure was martensite, tempered martensite, Since it was occupied by pearlite, cracks occurred in the outer periphery of the bent part in the bending test. In addition, carbides were precipitated at the prior austenite grain boundaries, cracks occurred frequently, and the delayed fracture limit was inferior.

In test No. 3 where the annealing temperature was low, cracks occurred in the outer periphery of the bent portion, and conversely, in test No. 8 where the annealing temperature was too high, the tensile strength was insufficient.
Test Nos. 14 and 16 were soft because of ferrite of 20 area% or more, and no crack was detected on the outer periphery of the bent portion, but a tensile strength of 900 N / mm ² or more could not be obtained. Test No. 7 satisfied the bending workability and tensile strength, but the delayed austenite resistance deteriorated due to the processing-induced transformation of retained austenite to martensite.
On the other hand, test Nos. 2, 4, 6, 9, 10, 12, and 13 in which the steel components, the coiling temperature, the area ratio of the metal structure, and the annealing conditions before bending satisfy the conditions specified in the present invention are as follows: All of them exhibited a tensile strength of 900 N / mm ² or more, could be bent 90 degrees without cracking, and were excellent in delayed fracture resistance.

As described above, by combining the specified component design and hot rolling conditions, the hot rolled steel strip is made into a bainite-based metal structure and does not require quenching / tempering after bending into a stabilizer shape. Is manufacturing. Therefore, unlike the tempered martensitic steel, there is no precipitation of carbide or the like that increases the delayed fracture susceptibility, and a hollow stabilizer excellent in delayed fracture resistance with a tensile strength of 900 N / mm ² or more can be stably obtained.

Diagram showing the size of the specimen used for the delayed fracture test Diagram showing test equipment used for delayed fracture test

Explanation of symbols

1: Test piece 2: Corrosion solution 3: Corrosion tank 4: Weight 5: Displacement meter

Claims (3)

C: 0.10 to 0.30 mass%, Si: 0.5 mass% or less, Mn: 0.25 to 2.50 mass%, P: 0.03 mass% or less, S: 0.01 mass% or less , Cr: 0.5-1.5 mass%, Mo: 0.1-0.5 mass%, B: 0.0005-0.0100 mass%, Ti: 0.01-0.10 mass%, N : Steel containing 0.01% by mass or less, Al: 0.02 to 0.08% by mass, the balance being composed of Fe and impurities, is sent to the hot rolling process,
Ferrite: 20 area% or less, pearlite: 20 area% or less, retained austenite: 5 area% or less, finishing temperature: 800 to 950 ° C., winding temperature: 400 to 600 ° C. so that the balance is bainite. The steel material is hot-rolled under the conditions selected in the range of
After pickling and making ERW steel pipe,
A method for producing a hollow stabilizer excellent in delayed fracture resistance, characterized by annealing in a temperature range of 450 to 710 ° C. for 1 hour or less, cooling to room temperature, and bending into a target shape.   Furthermore, the steel material containing 1 type or 2 types or more of Ni: 0.1-1.0 mass%, V: 0.5 mass% or less, Nb: 0.01-0.10 mass% is used. Production method. After cooling to a temperature below the M _s point, the ERW weld is heat treated continuously to a temperature range of (Ac ₁ transformation point) to (Ac ₁ transformation point −100 ° C.). The manufacturing method of Claim 1 or 2 which makes the maximum hardness difference between base material parts 50HV or less.

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