JP2006089804A - Method for producing high strength electric resistance welded tube for instrument panel reinforcement having excellent tube shrinking property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength electric resistance welded tube, while maintaining strength in the class of ≥980 N, having the sufficient strength even when being thinned, having excellent tube shrinking workability, and suitable as an automobile instrument panel reinforcement requiring high collision safety. <P>SOLUTION: A C-Si-Mn steel whose carbon equivalent C<SB>eq</SB>(C+Si/24+Mn/6) is controlled to 0.55 to 0.75 mass% is subjected to rough rolling at a heating temperature of ≥1,000°C, and is thereafter hot-rolled at a finishing temperature of Ar<SB>3</SB>+50°C or higher and a coiling temperature of ≤700°C. After cold rolling, the steel is subjected to continuous annealing of soaking at ≥830°C×60 s, primary cooling so as to be cooled to 720 to 600°C for ≤10°C/s, second cooling so as to be cooled at ≥7°C/s to a second cooling temperature T: (-248×C<SB>eq</SB>+538)°C, and isothermal treatment at T+≥30°C or higher for ≥3 min. Then, the obtained cold rolled-annealed sheet is made into a tube, thus a high strength electric resistance welded tube having excellent tube shrinking properties while maintaining its high strength is produced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a high-strength ERW steel pipe suitable for instrument panel reinforcement, which has a tensile strength of 980 N class or higher, uses excellent tube shrinkage, and is a reinforcing part of an automobile.

In order to achieve both fuel economy and collision safety, steel materials used as automotive parts are required to maintain the required strength even if they are thinned. Uses high-strength ERW steel pipes with a strength of 750 N or higher. Being started. For example, reinforcing members such as instrument panel reinforcements, pillars, and members are incorporated so as to prevent deformation of the interior of the automobile at the time of a collision in order to secure the life of the passenger.
Instrument safety that is built into the dashboard is an important security component because crash safety is generally evaluated in full-lap crashes and offset crash tests. Instrument panel reinforcement is classified into a driver's seat side and a passenger seat side, and has different strength and thickness. On the driver's seat where importance is placed on rigidity and vibration control, a relatively large diameter and thick steel pipe member is required. On the passenger seat side, a relatively small diameter and thin steel pipe member is used to reduce the weight of the vehicle body. Has been.

  For example, in order to weld the small passenger-side steel pipe 1 to the driver-side steel pipe 2, the pipe end 2a of the driver-side steel pipe 2 is reduced in diameter to design a welded joint. The welder side steel pipe 1 and the driver side steel pipe 2 which are integrated by welding are fixed to the bracket 3 by laser welding w or the like, and are incorporated into the dashboard along the vehicle width direction via the bracket 3. -Fig. 1

Since the diameter and thickness of the driver's seat and passenger's seat are different, the instrument panel reinforcement is usually made by joining two steel pipes by arc welding, laser welding or the like. It is common to use ERW steel pipes with a strength of up to 440 N on the driver side, and the steel pipe member on the passenger side is prepared by reducing the steel pipe on the driver side to the required diameter on the passenger side. .
Even if the thickness is reduced to reduce the weight of the steel pipe member on the driver's side, it is necessary to maintain the absorbed energy at the time of collision and the maximum load until buckling at a predetermined level or higher, so the strength of the reduced thickness is required. Is done. Specifically, when reducing the weight of a steel pipe member, a tensile strength of at least 980 N class or more is required, and switching between increasing the strength and reducing the thickness from the conventional 440 N steel pipe to the 980 N class steel pipe is promoted by the processing manufacturer. ing.

Steel materials generally show a tendency for workability to decrease as the strength increases, and it is extremely difficult to contract a steel pipe member having a tensile strength of 980 N class or higher to a required diameter on the passenger seat side. Various methods for improving the workability of a high-strength steel sheet have been proposed in the past, but no means effective for reducing the tube-pipe of steel pipe members has been put into practical use so far.
For example, Patent Document 1 improves stretch flangeability by regulating the amount and size of martensite even in a high-strength steel sheet having a tensile strength of 590N. However, its use as a material for ERW steel pipes, and even the shrinkability of ERW steel pipes, has not been elucidated.
JP-A-3-264646

Patent Document 2 imparts necessary local ductility to a high-strength steel sheet by reducing the hardness difference between ferrite and the second phase. In this case as well, the applicability to the material for ERW steel pipe is not mentioned, and no analogy can be made about the relationship between the shrinkability of ERW steel pipe and the metal structure, particularly retained austenite.
JP-A 63-293121

In Patent Document 3, once a high-strength steel pipe is expanded, it is contracted at a contraction ratio of about 0.1 to 1.0, thereby suppressing uneven thickness and improving plate thickness accuracy. The tube contraction rate of about 0.1 to 1.0 is different from the tube contraction rate of about 30% planned for instrument panel reinforcement, and of course, the tube contractibility of the electric resistance welded steel tube for instrument panel reinforcement can be inferred. It is not a thing.
JP 2003-94113 A

  Even in thin and strong instrument panel reinforcements, the passenger side is straight tube type and workability is not particularly required, and the impact absorption energy and maximum buckling load are emphasized, and the driver side has much higher shock absorption. Since energy and maximum buckling load are required, a steel pipe with a larger outer diameter than the passenger side is used. Therefore, the method of reducing the diameter of the pipe end of the driver's side steel pipe and welding it to the passenger side's steel pipe is still the same as the conventional method, and excellent shrinkage even with ERW steel pipes with a tensile strength of 980 N class or higher. Pipelines are required.

  The inventors of the present invention have studied the types and manufacturing conditions of ERW steel pipes that affect the shape and required characteristics of instrument panel reinforcement from various viewpoints. As a result, C-Si-Mn steel was used and some martensite was used during continuous annealing. , And by microstructuring while securing 5 area% or more of retained austenite in the steel sheet due to the bainite transformation promotion effect by transformation strain, excellent tube shrinkage is obtained even when the tensile strength is 980 N class or more. It was clarified that An object of the present invention is to provide a high-strength electric resistance welded steel pipe for instrument panel reinforcement suitable for weight reduction and high strength based on such elucidation.

The present invention includes C: 0.15-0.20 mass%, Si: 1.2-1.8 mass%, Mn: 2.2-3.0 mass%, P: 0.030 mass% or less, S : 0.010% by mass or less, Total Al: 0.01 to 0.1% by mass, the balance is substantially Fe, and the carbon equivalent C _eq defined by the formula (1) is 0.55 to 0.75. Use a slab adjusted to a mass% range.
C _eq (%) = C + Si / 24 + Mn / 6 (1)

A cold-rolled annealed sheet is manufactured through rough rolling, hot rolling, cold rolling, and continuous annealing. The manufacturing conditions in each process are controlled as follows.
In the rough rolling process, the slab is heated to 1000 ° C. or higher and rough rolled.
In the hot rolling process, the finishing temperature is set to Ar ₃ + 50 ° C. or higher, and after finishing rolling, the steel is cooled and wound at 700 ° C. or lower to temper the ferrite + pearlite structure.
Next, after annealing and pickling, cold rolling is performed to a target plate thickness at a rolling rate of 30% or more.

By subjecting the obtained cold-rolled steel sheet to the following continuous annealing, a high-strength cold-rolled steel sheet having a tensile strength of 980 N class or more and excellent in tube shrinkability is obtained.
・ Soaking: 830 ° C. or more, 60 seconds or more ・ Primary cooling: After soaking, average cooling rate: 10 ° C./sec or less to 720 to 600 ° C. By this primary cooling, ferrite is generated at an area ratio of 50% or more. .
Secondary cooling: Average cooling rate: Cooling to secondary cooling temperature T at 7 ° C./s or more Secondary cooling temperature T is defined by equation (2), preferably 10-30 at secondary cooling temperature T ± 5 ° C. Hold for seconds.
T (° C.) = − 248 × C _eq +538 (2)
・ Constant temperature treatment: Hold at T + 30 ° C or higher for 3 minutes or more, then cool to room temperature

Effects and embodiments of the invention

The shrinkage required for an electric resistance steel pipe for instrument panel reinforcement is affected by metallurgical properties such as low yield strength, good elongation, and high homogeneity of the metal structure. In addition, a steel type that has a low yield ratio and good workability up to the steel plate stage and that is strengthened by work hardening during pipe making is preferable.
Usually, the tensile strength is about 980 N grade or higher, and the limit of elongation is about 15%. However, the high-strength steel plate manufactured under appropriate conditions according to the present invention has a retained austenite of 5 area% or more (specifically Specifically, it has a structure of about 8 area%), tensile strength: 980 N / mm ² or more, yield ratio: 70% or less, and total elongation: 18% or more.

The high-strength electric resistance welded steel pipe manufactured by passing the high-strength steel sheet through a pipe-making line and giving a pipe-forming strain of, for example, about 8% has a structure of retained austenite: 2 area% or more and has a tensile strength. : 980 N / mm ² or more, yield ratio: 80% or less, and total elongation: 20% or more. The reason why excellent tube shrinkage can be obtained is not necessarily clear, but the yield ratio of the steel sheet is as low as 80% or less, and the retained austenite remains in the steel tube after pipe forming, so that uniform elongation is improved, and uneven deformation occurs during tube contraction. It is considered that no meat, crunch, buckling, cracking, etc. occur.

Although martensite is generated by secondary cooling during continuous annealing, the present invention optimizes the component design and manufacturing conditions, and regulates the secondary cooling temperature T in relation to the carbon equivalent C _eq , so that fine residual A composite structure of ferrite + bainite + martensite + residual austenite in which austenite is uniformly dispersed is generated. Specifically, by properly managing the hot rolling conditions and annealing conditions of C—Si—Mn steel, some martensite is generated during continuous annealing, and the bainite transformation promoting effect using transformation strain is 5% by area. The above retained austenite is secured in the steel.

  When some martensite is generated, the prior austenite grain boundaries are divided, transformation strain is imparted, and crystal grains are refined. Further, since the lower bainite transformation due to transformation strain is promoted immediately above the martensite generation temperature, C enrichment also proceeds. As a result, stable retained austenite increases and remains at the steel pipe stage. In addition, as the lower bainite transformation is promoted, the untransformed austenite is further divided, and a finer dispersion of retained austenite is achieved.

Hereinafter, various conditions defined in the present invention will be described.
[Alloy components]
C: 0.15 to 0.20% by mass
It is a component that improves the strength of steel and affects the formation and stabilization of retained austenite necessary for excellent tube shrinkage. That is, C, which is an austenite stabilizing element, concentrates from ferrite to austenite during bainite transformation, and improves the chemical stability of austenite. Necessary amount of retained austenite: C content is 0.15% by mass or more in order to obtain 2% or more, but if excessive amount of C exceeding 0.20% by mass is contained, the weldability is deteriorated or excessive. The strength is increased, and on the contrary, the ductility deteriorates. Preferably, the C content is selected in the range of 0.16 to 0.18% by mass.

・ Si: 1.2 to 1.8% by mass
It is a component that increases strength while improving the strength-elongation balance by solid solution strengthening, and also has an effect of concentrating C in the austenite phase by promoting ferrite transformation. Therefore, the retained austenite is stabilized, and it becomes easy to secure the retained austenite amount that exhibits transformation-induced plasticity at room temperature. Furthermore, precipitation of cementite is suppressed in the bainite transformation temperature range, and C concentration to untransformed austenite is further promoted. Such an effect can be obtained by adding Si in an amount of 1.2% by mass or more. However, if an excessive amount of Si exceeding 1.8% by mass is added, a scale having poor descalability is likely to occur during hot rolling, and pickling properties. Detrimental to weldability and product surface properties. Preferably, the Si content is selected in the range of 1.4 to 1.6 mass%.

Mn: 2.2 to 3.0% by mass
It is an effective component for improving hardenability and stabilizing austenite. It also suppresses pearlite transformation during cooling, and also has the effect of securing the necessary amount of bainite effective for strength improvement and retained austenite effective for improvement of fatigue properties. Such an effect is observed when Mn is added in an amount of 2.2% by mass or more. However, an excessive addition of Mn exceeding 3.0% by mass increases the hardenability excessively, causes an excessive increase in strength and ductility, and causes deterioration in both weldability and ductility. Preferably, the Mn content is selected in the range of 2.2 to 2.6 mass%.

-P: 0.03 mass% or less Although it is a component that affects the formation of ferrite in the same manner as Si, excessive content deteriorates ductility and ductility, so an upper limit was set to 0.03 mass%.
-S: 0.010 mass% or less Although it does not have a bad influence on the production | generation of a retained austenite, A type | system | group inclusion harmful | toxic to fatigue strength produces | generates abundantly with the increase in S content. Since the increasing tendency of the A-based inclusions becomes remarkable when the content exceeds 0.010% by mass, the upper limit of the S content is regulated to 0.010% by mass (preferably 0.005% by mass).

-Total Al: 0.01-0.1 mass%
Like Si, it is an important component for securing retained austenite that is stable at room temperature. It does not dissolve in cementite and suppresses the precipitation of cementite during the isothermal treatment for bainite transformation. If Al, which has a stronger ferrite forming ability than Si, is added, the transformation starts earlier than Si addition, and C is concentrated in austenite even if kept for a very short time. As a result, the austenite phase is further stabilized, the C concentration of the generated austenite is increased, the amount of retained austenite is increased, and high work hardening characteristics are exhibited. The effect of addition of Al is seen at 0.01% by mass or more as the total Al, but when the total Al exceeds 0.1% by mass, the weldability tends to decrease. Preferably, the Al content is selected in the range of 0.05 to 0.08 mass%.

-Carbon equivalent C _eq : 0.55-0.75 mass%
In this component system, the carbon equivalent C _eq is calculated by the equation (1). In order to obtain the required strength, a carbon equivalent C _eq of 0.55% by mass or more is necessary. However, in the component design in which the carbon equivalent C _eq exceeds 0.75% by mass, the hardenability becomes excessive and the strength increases rapidly. It causes a significant decrease in bendability. Preferably, the carbon equivalent C _eq is selected in the range of 0.6 to 0.65% by mass.
T (° C.) = − 248 × C _eq +538 (2)

[Rough rolling]
Molten steel adjusted to predetermined components and composition is melted in a converter, and a slab obtained by continuous casting is roughly rolled after heating. In the heating prior to the rough rolling, the heating temperature is set to 1000 ° C. or more in order to prevent coarsening of crystal grains and promote ferrite transformation.

(Hot rolling)
The slab after rough rolling is hot-rolled to a predetermined plate thickness. In hot rolling, the finishing temperature is set to Ar ₃ + 50 ° C. or higher, and the hot rolled steel strip is wound at a winding temperature of 700 ° C. or lower to be tempered into a ferrite + pearlite structure. At a finishing temperature that does not reach Ar ₃ + 50 ° C., deformation resistance during hot rolling is high, and narrowing tends to occur. Conversely, when the finishing temperature exceeds 900 ° C., coarsening proceeds, so it is preferable to adjust the finishing temperature in the range of (Ar ₃ + 50 ° C.) to (Ar ₃ + 100 ° C.).
The hot-rolled steel strip is wound at a relatively high temperature in order to stabilize the material obtained after annealing. In low-temperature winding, bainite and martensite are generated and austenite becomes unstable during subsequent annealing, and partial hardening due to excessive cooling speed of the sheet width edge is likely to occur. On the other hand, in high temperature winding exceeding 700 ° C., ferrite crystal grains become coarse, and austenitization along the plate width direction becomes unstable during subsequent annealing in the subsequent process. Preferably, the winding temperature is selected in the temperature range of 550 to 680 ° C.

[Descaling]
If the steel strip surface is descaled one or more times during the hot rolling process, the fatigue properties of the high-strength cold-rolled steel sheet are improved. A typical method for descaling is to adjust the heating temperature of the slab to 1170 ° C or lower and make high-pressure water collide with the steel strip surface before finish rolling at a collision pressure of 2.0 kgf / cm ² or more. Descaling by rolling before pickling in which a similar rolling force is applied by a roll can also be employed.
Since it is based on Si steel that easily generates firelight that adversely affects descaling properties, it is preferable to lower the heating temperature of the slab than the melting point of firelight (1170 ° C.). When the heating temperature exceeds 1170 ° C., the firelite bites into the crystal grain boundaries and is difficult to peel off even by descaling. In addition, by performing descaling once or more at a collision pressure of 2.0 kgf / cm ² or more, the surface skin of the hot-rolled steel strip is dramatically improved, and fatigue failure due to scale due to grain boundary oxidation during annealing, etc. Reduces fatigue properties.

(Cold rolling)
The hot-rolled steel strip is cold-rolled to a predetermined thickness after annealing and pickling. In cold rolling, the rolling rate is set to 30% or more with the aim of lowering the recrystallization temperature and stabilizing austenite. In light reduction rolling where the rolling rate does not reach 30%, strain effective for lowering the recrystallization temperature hardly accumulates, and the stability of austenite tends to decrease.

[Continuous annealing]
By continuously annealing the cold-rolled steel strip, it is tempered into a composite structure of ferrite + bainite + martensite + residual austenite. Continuous annealing undergoes steps of soaking, primary cooling, secondary cooling, and isothermal treatment.
-Soaking process The austenitization is sufficiently advanced by soaking the cold-rolled steel strip at a holding temperature of 830 ° C or more and a holding time of 60 seconds or more. At a holding temperature that does not reach 830 ° C., austenitization becomes insufficient, residual austenite is reduced, and stable tube shrinkability is difficult to obtain. However, if it is kept at an excessively high temperature, the coarsening of the crystal grains that delay the ferrite transformation occurs, and the C concentration in the untransformed austenite tends to be insufficient. Therefore, the upper limit of the soaking temperature is preferably 900 ° C. .

-Primary cooling When the steel strip subjected to soaking is cooled to 720 to 600 ° C. at an average cooling rate of 10 ° C./second or less, ferrite is generated at an area ratio of 50% or more. The generation of ferrite is affected by the cooling rate, and ferrite with an area ratio of 50% or more is secured by setting the average cooling rate to 10 ° C./second or less. When the cooling rate exceeds 10 ° C./second, the necessary ferrite area ratio cannot be obtained, and C concentration to untransformed austenite becomes insufficient. Primary cooling temperature: 720 to 600 ° C. is also an important condition for producing ferrite at an area ratio of 50% or more. If the temperature is out of the temperature range, a sufficient amount of ferrite cannot be obtained.

-Secondary cooling After forming ferrite with an area ratio of 50% or more by primary cooling, secondary cooling is performed to the secondary cooling temperature T at an average cooling rate of 7 ° C / second or more. The secondary cooling temperature T is set to (−248 × C _eq +538) ° C. in order to generate an appropriate amount of martensite, preferably 10 seconds or more at the secondary cooling temperature T ± 5 ° C. (preferably 20 to 30). Seconds). When the secondary cooling is performed to an excessively low temperature, the amount of martensite produced becomes excessive, and the tube forming workability deteriorates.

  With slow cooling that does not reach 7 ° C / second, not only does pearlite form from untransformed austenite and the amount of retained austenite decreases, but cracks are likely to occur at the interface between hard and coarse pearlite and ferrite formed at high temperatures, Extensibility deteriorates. By once cooling to the secondary cooling temperature T, a part of martensite is generated from unstable austenite, and the bainite transformation is promoted by transformation strain generated during the martensite transformation. Further, in order to obtain transformation strain necessary for promoting bainite transformation, the secondary cooling temperature T is maintained for 10 seconds or more. If the holding time does not reach 10 seconds, the transformation strain that promotes bainite transformation tends to be insufficient in quantity.

-Constant temperature treatment By holding the steel strip after secondary cooling at a holding temperature: T + 30 ° C or higher for 3 minutes or longer, the bainite transformation proceeds while maintaining an appropriate amount of retained austenite. If the holding time is too short, the bainite transformation is insufficient, the amount of retained austenite is reduced, and the C concentration to the austenite phase is insufficient. As a result, martensite is generated from untransformed austenite in the process of cooling from the bainite transformation temperature to room temperature, the strength is increased, and the tube shrinkability is deteriorated.

  A steel material having the composition shown in Table 1 is melted in a converter, continuously cast into a slab, hot rolled to a thickness of 2.0 mm through rough rolling, and further cold rolled to a thickness of 1.2 mm. A cold-rolled steel strip was manufactured. In the hot rolling step, the slab heated to 1250 ° C. was hot-rolled at a finishing temperature of 880 ° C. and wound into a coil at 600 ° C. In the cold rolling process, the pickled hot rolled steel strip was cold rolled at a rolling rate of 40%.

Subsequently, the cold-rolled steel strip was manufactured by continuously annealing the cold-rolled steel strip. In the continuous annealing process, after soaking to 840 ° C. for 120 seconds, primary cooling to 680 ° C. at an average cooling rate of 8 ° C./second, followed by cooling to 380 ° C. at a secondary cooling rate of 12 ° C./second, holding temperature : Held at 410 ° C. for 3 minutes.
After the cold-rolled annealed plate was cut into a cut plate having a predetermined width, the cut plate was roll-formed into an open pipe shape, and then both ends in the width direction were subjected to high-frequency welding to produce an ERW steel pipe having a diameter of 54 mm. After pipe making, in order to prevent shrinkage deterioration due to hardening of the welded portion, the welded portion hardness was tempered to a base material hardness of ± 30 HV by seam annealing at 600 ° C. for 5 seconds.

The specimens cut from the cold rolled annealed plate before pipe making and the manufactured ERW steel pipe were subjected to a tensile test to investigate the mechanical properties. A JIS No. 5 tensile test piece in a direction perpendicular to the rolling direction was prepared as a cold-rolled annealed specimen, and a JIS No. 11 tensile test piece along the rolling direction was prepared as an ERW steel pipe test piece.
For electric resistance welded steel pipes, the test piece was applied to a die drawing test with the drawing ratio [(outer diameter before diameter reduction-outer diameter after diameter reduction processing) / (outer diameter before diameter reduction processing) × 100] changed. The thickness ratio (thickness after tube reduction / wall thickness before tube reduction) was set to 1.2, and the limit drawing ratio at which no wrinkle, buckling or cracking occurred was obtained as the tube reduction ratio.

The survey results are shown in Table 2 together with the amount of retained austenite quantified by the X-ray diffraction method.
As apparent from Table 2, the test Nos. 1 to 5 satisfying the requirements specified in the present invention for the component conditions are all tensile strengths of 980 N / mm ² or more in the steel pipe tensile test, yield ratio: 80%. In the following, the amount of retained austenite at the steel pipe stage: 2 area% or more and the tube shrinkage rate in the die drawing test: 25% or more were satisfied, and it was confirmed that this was an electric resistance steel tube excellent in tube shrinkability.
On the other hand, in test No. 6 in which the precipitate was dispersed by adding Ti, the yield ratio was increased due to the precipitate, and the retained austenite was small and the tube contraction rate was small. In Test No. 7 in which C was excessive and Nb was added, the required amount of retained austenite was secured, but the yield ratio was high and the tube contraction rate was also small. Test No. 8 in which C and Si are insufficient has a small amount of retained austenite and a small tube contraction rate.

  Furthermore, with regard to Nos 1 and 4 steel pipes that showed good tube shrinkage rates, the continuous annealing conditions were changed to conditions A to J (Table 3), and the effects of continuous annealing conditions on mechanical properties and tube shrinkage were investigated in detail. did.

Table 5 shows the results of a similar test conducted on the physical properties of each manufactured cold-rolled annealed plate and the ERW steel tube manufactured from the cold-rolled annealed plate.
The ERW steel pipes manufactured under the annealing conditions A, C, D, G, and J according to the present invention all have a low yield ratio and a residual austenite amount of 2 area% or more. Showed tubular. Excellent tuberculosis is stable due to the formation of some martensite, which breaks the former austenite grain boundaries, refines the crystal grains by imparting transformation strain, and C concentration due to the promotion of lower bainite transformation. This is because the retained austenite increases, the necessary amount of retained austenite is secured even at the steel pipe stage, the untransformed austenite is further divided by the lower bainite transformation, and finer retained austenite is uniformly dispersed.

On the other hand, in the secondary cooling temperature, the annealing condition B in which the holding temperature during the temperature lowering process is too high, some martensite is not generated, and although the bainite transformation occurs, the temperature is high, so the upper bainite becomes high and the yield ratio becomes high. Since there is no C concentration, the result of the decrease in retained austenite appears as a small contraction rate.
In the annealing condition E where the soaking time is too short, the amount of austenite is small and the bainite transformation time is also short, so that the reduced austenite amount is reduced, resulting in a reduction in the tube contraction rate.
Under the annealing condition F in which the primary cooling temperature is too high, the ferrite transformation is delayed and C enrichment to austenite does not occur, and as a result of the reduction in the amount of retained austenite, the tube contraction rate is reduced.

Under the annealing condition H in which the secondary cooling temperature is lower than the temperature T defined by the formula (2), the yield ratio becomes high because the amount of martensite generated increases, and the bainite transformation is promoted, but the structure becomes uneven. The amount of retained austenite decreased, and the ductility deteriorated.
In the annealing condition I where the soaking temperature is too low, the amount of austenite decreases due to soaking in the two-phase region, and the C concentration increases. In addition, since the primary cooling temperature was low, only the bainite transformation proceeded without the formation of ferrite, resulting in a high yield ratio, a decrease in the amount of retained austenite, and a reduction in tube shrinkage.

  As is clear from the above comparison, even when the same steel type is used, it can be tempered into a composite structure of ferrite + bainite + martensite + residual austenite by appropriately managing the continuous annealing conditions, and tensile strength: 980N class It is confirmed that a high-strength electric resistance welded steel pipe with the above strength and excellent tube shrinkability is obtained.

  As described above, a high-strength ERW steel pipe excellent in tube shrinkability can be obtained while maintaining a tensile strength of 980 N class or higher by appropriate management of composition and production conditions. This high-strength electric resistance welded steel pipe has sufficient strength even if it is thinned for weight reduction, and it is high on both the driver's side and passenger's side by utilizing the fact that it can be reduced without defects such as wrinkles, buckling, and cracks. It is used as a material suitable for instrument panel reinforcement that requires collision safety.

Schematic of instrument panel reinforcement built into the dashboard

Explanation of symbols

1: Passenger side steel pipe 2: Driver side steel pipe 2a: Reduced diameter pipe end of driver side steel pipe 3: Bracket

Claims (1)

C: 0.15-0.20 mass%, Si: 1.2-1.8 mass%, Mn: 2.2-3.0 mass%, P: 0.030 mass% or less, S: 0.010 Less than mass%, total Al: 0.01 to 0.1 mass%, the balance is substantially Fe, and the carbon equivalent C _eq defined by the formula (1) is in the range of 0.55 to 0.75 mass%. Prepare a slab,
The slab is heated to 1000 ° C. or higher and roughly rolled,
The hot rolled steel strip is tempered into a ferrite + pearlite structure by hot rolling by cooling from a finishing temperature of Ar ₃ + 50 ° C. or higher and winding at 700 ° C. or lower.
After pickling, the rolling rate is cold rolled at 30% or more,
The average cooling rate after heating and holding at 830 ° C. or higher for 60 seconds or longer is reduced to 720 to 600 ° C. at a rate of 10 ° C./second or lower. Cooling to the secondary cooling temperature T (° C.) defined by the formula (2) at a rate of at least / sec, further holding for 3 minutes or more at a temperature of T + 30 ° C.
The cold-rolled annealed plate cooled to room temperature is cut into a cut plate of a predetermined width,
A method of manufacturing a high-strength ERW steel pipe for instrument panel reinforcement, which is excellent in tube shrinkage, characterized by high-frequency welding at both ends in the width direction after forming a cut plate into an open pipe shape.
C _eq (%) = C + Si / 24 + Mn / 6 (1)
T (° C.) = − 248 × C _eq +538 (2)

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