JP4084581B2 - Manufacturing method of high impact steel pipe and high impact steel pipe manufactured by the method - Google Patents

Description

【００２４】
【表２】

【００２５】
【発明の効果】
以上に説明したように、本発明の高耐衝撃性鋼管は従来品よりも高強度、低降伏比の材料特性を備えたものであるうえ、０.１％耐力を採用したことにより衝撃吸収特性を正しく反映したものとなるから、限界付近までの軽量化を図った超軽量、高衝突安全性のドアインパクトビーム、バンパー用材料、バンパー補強用材料等の衝撃吸収エネルギーを必要とされる部材に適したものとなる。また本発明の製造方法によれば、このような高耐衝撃性鋼管を安定して製造することが可能となる。
【図面の簡単な説明】
【図１】鋼の応力-歪み曲線を示す模式図である。
【図２】冷却速度とＹＲとの関係を示すグラフである。
【図３】冷却水温とＹＲとの関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a high impact steel pipe used as a member that requires impact absorption energy, such as an automobile door impact beam, a bumper material, and a bumper reinforcing material, and a high impact resistance manufactured by the method. This is related to the steel pipe .
[0002]
[Prior art]
Steel pipes for door impact beams intended to absorb shocks during side impacts of automobiles are required to have high tensile strength and low yield strength (yield stress). This shock absorption characteristic is expressed by a yield ratio defined as a ratio of proof stress and tensile strength. However, in general, when the tensile strength of the steel pipe is increased, the proof stress is increased, and in the conventional product, the tensile strength is 1500 to 1600 MPa and the yield ratio is 70 to 80%. For this reason, in order to further reduce the weight and increase the collision safety, a steel pipe with higher strength and lower yield ratio has been demanded.
[0003]
In general, the yield strength is generally determined by a method of measuring a stress that causes a permanent strain of 0.2% on a test piece as defined in JIS, and the yield ratio is calculated based on the 0.2% yield strength. . FIG. 1 is a diagram schematically showing a stress-strain curve of steel, and the impact energy that can be absorbed by the deformation of the impact absorbing member at the time of collision is represented by the area of the hatched portion. As described above, since the evaluation was conventionally performed with the 0.2% proof stress, the design was made with the area of the hatched portion that descends to the right as the ability to absorb impact energy. However, according to the study of the present inventor, in the case of a steel material having a stress-strain curve of the shoulder where the yield point does not appear clearly, the steel pipe has a permanent strain of 0.2% at the time of collision as shown in FIG. It was found that the yield ratio using the 0.2% proof stress specified in JIS underestimated the impact absorption characteristics in order to absorb considerable impact energy until it occurred.
[0004]
[Problems to be solved by the invention]
The present invention solves the above-mentioned conventional problems, and has a new high impact resistant steel pipe having material properties of higher strength and lower yield ratio than conventional products, and having both ultralight weight and high collision safety, and its manufacture It was made to provide a method.
[0005]
[Means for Solving the Problems]
Process for producing a high impact resistance steel of the invention of claim 1 has been made in order to solve the aforementioned problem, the mass ratio, C:. 0.19~0 35%, Si: 0.1~0.3 %, Mn: 1.0 to 1.6%, P: 0.025% or less, S: 0.02% or less, Al: 0.010 to 0.050%, B: 2 to 35 ppm, Ti: 0.0. . 005-0 0.05% was contained as essential components, further Nb:.. 0 005~0 050% , V:.. 0 005~0 070%, Cu:.. 0 005~0 5%, Cr: 0 .. 005~0 5%, Mo: .. 0 1~0 5%, Ni:.. 0 1~0 5%, Ca:. 0 01% or less, rare earth element (REM):. 0 1% or less of An ERW steel pipe containing at least one selected component selected from the group and having the balance consisting of inevitable impurities and Fe, at 900 ° C. or higher After having heated, cooling rate 100 ° C. / sec or higher, and is characterized in that the water cooling quenching under the following conditions the cooling water temperature 35 ° C..
[0006]
The high impact resistant steel pipe of the present invention is manufactured by the method of claim 1 , and the high impact resistant steel pipe of claim 2 has a tensile strength TS of 1700 MPa or more and 0.1%. The yield ratio YR, which is the ratio between the proof stress YS and the tensile strength TS (YS / TS), is 72% or less. The high impact steel pipe according to claim 3 has a tensile strength TS of 1800 MPa or more, and a yield ratio YR which is a ratio (YS / TS) of 0.1% proof stress YS to tensile strength TS is 70% or less. It is characterized by that. Further, the high impact steel pipe of claim 4 has a tensile strength TS of 1900 MPa or more, and a yield ratio YR which is a ratio (YS / TS) of 0.1% proof stress YS to tensile strength TS is 68% or less. It is characterized by that. The high impact steel pipe according to claim 5 has a tensile strength TS of 2000 MPa or more, and a yield ratio YR which is a ratio of 0.1% proof stress YS to tensile strength TS (YS / TS) is 66% or less. It is characterized by that.
[0007]
Furthermore, a high impact resistant steel pipe according to claim 6 is the high impact resistant steel pipe according to any one of claims 2 to 5 , wherein the dislocation density is 10 ¹⁰ to ¹⁴ mm ^-2. It is.
[0008]
As described above, the present invention has been developed with the aim of further improving the material properties of high TS and low YR compared to conventional products. In general, such a high TS material can be obtained by subjecting the structure to martensite by water-cooling quenching after heating. Conventionally, a part of soft austenite or ferrite remains in a hard martensite structure, thereby reducing the yield strength and obtaining material characteristics of a low yield ratio. However, in such a conventional method, as described above, the strength is 1500 to 1600 MPa and the yield ratio is 70 to 80%.
[0009]
In contrast, the present invention eliminates residual austenite and residual ferrite in the martensite structure and achieves a higher TS compared to the conventional product, while the dislocation density in the hard martensite structure is much higher than that of the conventional product. YS and YR are reduced by facilitating deformation under stress. The dislocation density of the product of the present invention is 10 ¹⁰ to ¹⁴ mm ^−2, which is extremely high compared to the dislocation density of the conventional product of 10 ⁸ to ⁹ mm ⁻² . By realizing such a high dislocation density, the high impact resistant steel pipe of the present invention has a lower YR while having a higher TS than the conventional product. In addition, the YS value used to calculate YR is 0.1% proof stress, which has not been used in the past, so the material characteristics more accurately reflect the impact absorption characteristics, and the door impact beam is designed to reach the limit. The weight can be reduced.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The contents of the present invention will be described in detail below.
As described in claim 1 , the high impact steel pipe of the present invention has a mass ratio of C: 0.19 to 0.35%, Si: 0.1 to 0.3%, Mn: 1.0 to 1.6%, P: 0.025% or less, S: 0.02% or less, Al: 0.010 to 0.050%, B: 2 to 35 ppm, Ti: 0.005 to 0.5. Containing 05% as an essential component, Nb: 0.005 to 0.050%, V: 0.005 to 0.070%, Cu: 0.005 to 0.5%, Cr: 0.005 to 0.00. 5%, Mo: 0.1-0.5%, Ni: 0.1-0.5%, Ca: 0.01% or less, rare earth element (REM): selected from the group of 0.1% or less It is manufactured by induction heating an ERW steel pipe containing the selected components and then water-cooling and quenching. The reason for the numerical limitation of each component is as follows.
[0011]
C is an essential component for strengthening martensite itself and improving hardness, and at least 0.19% is necessary to obtain TS of 1700 MPa or more. However, if C is excessive, the martensite structure becomes brittle and causes cracking that breaks during quenching, so the content is made 0.35% or less. In order to obtain a steel pipe with TS of 1700 MPa or more and YR of 72% or less according to claim 2 , C is set to about 0.21%, and a steel pipe with TS of 1800 MPa or more and YR of 70% or less is claimed in claim 3. In order to obtain a steel pipe in which C is about 0.24%, TS of claim 4 is 1900 MPa or more, and YR is 68% or less, C is about 0.28%, and TS of claim 5 is In order to obtain a steel pipe having 2000 MPa or more and YR of 66% or less, C is preferably about 0.30%.
[0012]
Si, Mn, and Ti are all components that promote martensitic transformation from austenite during quenching, and Si: 0.1 to 0.3%, Mn: 1.0 to 1.6%, Ti: 0.00. If it is less than each numerical limit range of 005 to 0.05%, the hardenability is lowered to produce retained austenite and retained ferrite, and the desired material properties cannot be obtained. On the other hand, exceeding the numerical limit range is not preferable because it causes burning cracking and segregation. In addition, Ti has the effect | action which improves hardenability by fixing N.
[0013]
B is a component that suppresses the precipitation of ferrite, but when it is combined with N contained as a gas component in the steel and becomes BN, its effect is lost, so the content is made 2 ppm or more. However, if it exceeds 35 ppm, segregated inclusions are formed. P and S become segregated inclusions and make the martensite structure brittle, so P: 0.025% or less and S: 0.02% or less are required. Al is a deoxidizing agent, and if it is less than 0.010%, the deoxidizing effect is insufficient, and if it exceeds 0.050%, the oxide becomes an intercrystalline inclusion, which is not preferable.
[0014]
Nb and V are precipitation strengthening components that improve the strength by forming precipitates in the martensite structure and preventing the passage of dislocations. Cu, Cr, Mo, and Ni are solid solution strengthening components that improve the strength by being dissolved in the martensite crystal and preventing the passage of dislocations. Cr and Mo also act as precipitation strengthening components. These components contribute to an increase in strength, but increase the cost and add excessive segregation inclusions. Therefore, Nb: 0.005 to 0.050%, V: 0.005 to 0.070%, Cu: 0.005-0.5%, Cr: 0.005-0.5%, Mo: 0.1-0.5%, Ni: 0.1-0.5% are suitable.
[0015]
Ca and rare earth elements (REM) are components that contribute to the control of the morphology of inclusions. However, excessive addition causes harmful segregation leading to destruction of the martensite structure, so Ca: 0.01% or less, REM: 0 Less than 0.1% is appropriate. These Nb, V, Cu, Cr, Mo, Ni, Ca, and rare earth elements (REM) are not essential components, but are optional components that are added as necessary. For example, Y, La, Ce, or Sm can be used as the rare earth element (REM).
[0016]
In this invention, after manufacturing the steel pipe which consists of steel of the said composition by electric-welding welding, it is induction-heated to 900 degreeC or more through the work coil for high frequency heating, and is water-cooled and hardened from an austenite state. At this time, even if the steel pipe is passed through the work coil and the water-cooled quenching apparatus fixed while being continuously conveyed on the conveyor, a method of moving the work coil and the water-cooled quenching apparatus while fixing the steel pipe may be adopted.
[0017]
This water cooling quenching instantaneously causes transformation from austenite to martensite, and at the same time, expansion of about 7 to 8% occurs due to transformation strain, and the dislocation density in the martensite structure increases rapidly. The dislocation density is measured by observing the specimen after a tensile test according to JIS with a transmission electron microscope, measuring the number of dislocations per field of 1 μm × 1 μm in 10 fields, and taking the average value. It is. Since the dislocation density is represented by the dislocation length per unit volume, the unit is mm ⁻² .
[0018]
As described above, the dislocation density of the product of the present invention is 10 ¹⁰ to ¹⁴ mm ^-2, which is extremely high compared to the dislocation density of the conventional product of 10 ⁸ to ⁹ mm ^-2 . By realizing such a high dislocation density, the yield point is lowered, and the high impact-resistant steel pipe of the present invention has a low YR while having a higher TS than the conventional product.
[0019]
Thus, in order to obtain a high-impact steel pipe having high TS and low YR, it is preferable to set the cooling rate of water-cooling quenching to 100 ° C./sec or more. FIG. 2 is a graph showing the relationship between the cooling rate and YR, and when Y is 100 ° C./sec or more, a rapid decrease in YR is observed. This is presumably because transformation strain is suddenly generated by rapid cooling and the dislocation density is increased.
[0020]
In order to obtain a high TS and low YR high impact resistant steel pipe, it is preferable that the cooling water temperature of water cooling and quenching is 35 ° C. or lower. As shown in FIG. 3, YR rises when the cooling water temperature rises. This is thought to be because quenching becomes insufficient as the cooling water temperature rises and ideal martensitic transformation cannot be realized.
[0021]
The high impact-resistant steel pipe of the present invention thus obtained has a much lower strength and a lower YR than the conventional product, so it can be used for automobile door impact beams, bumper materials, and bumper reinforcement materials. If it is used as a member that requires shock absorbing energy such as the above, excellent shock absorbing performance can be exhibited. Also, unlike conventional products, 0.1% proof stress was adopted, so the impact absorption capacity increased by the area shown by horizontal hatching in Fig. 1 compared to the shock absorption capacity calculated based on 0.2% proof stress. Therefore, the correlation with the shock absorbing performance at the time of actual collision becomes more accurate. For this reason, combined with the high strength, the weight can be reduced to near the limit. For this reason, it is possible to provide an impact absorbing member having both ultralight weight and high collision safety.
Examples of the present invention are shown below.
[0022]
【Example】
ERW steel pipes made of steels having the respective compositions shown in Table 1 were produced, moved on a conveyor at a constant speed, passed through a work coil, induction-heated, and rapidly cooled to room temperature with an adjacent water-cooled quenching apparatus. The cooling rate and cooling water temperature are shown in Table 2. The cut specimen was subjected to a tensile tester to measure 0.1% yield strength and breaking strength. Table 2 also shows the results of observing the specimen after the tensile test with a transmission electron microscope and measuring the dislocation density.
[0023]
[Table 1]

[0024]
[Table 2]

[0025]
【The invention's effect】
As explained above, the high impact resistant steel pipe of the present invention has material properties of higher strength and lower yield ratio than conventional products, and also has a shock absorbing property by adopting 0.1% proof stress. Therefore, it is suitable for members that require shock absorption energy such as ultra-lightweight, high impact safety door impact beams, bumper materials, bumper reinforcement materials, etc. It will be suitable. Moreover, according to the manufacturing method of this invention, it becomes possible to manufacture such a high impact-resistant steel pipe stably.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a stress-strain curve of steel.
FIG. 2 is a graph showing a relationship between a cooling rate and YR.
FIG. 3 is a graph showing the relationship between cooling water temperature and YR.

Claims (6)

  By mass ratio, C: 0.19 to 0.35%, Si: 0.1 to 0.3%, Mn: 1.0 to 1.6%, P: 0.025% or less, S: 0.02 %: Al: 0.010 to 0.050%, B: 2 to 35 ppm, Ti: 0.005 to 0.05% as essential components, Nb: 0.005 to 0.050%, V : 0.005 to 0.070%, Cu: 0.005 to 0.5%, Cr: 0.005 to 0.5%, Mo: 0.1 to 0.5%, Ni: 0.1 to 0 A composition comprising at least one selected component selected from the group of 0.5%, Ca: 0.01% or less, and rare earth element (REM): 0.1% or less, with the balance being inevitable impurities and Fe A water-welded steel tube having a cooling rate of 100 ° C./sec or higher and a cooling water temperature of 35 ° C. or lower is heated after heating to 900 ° C. or higher. Manufacturing method of high impact steel pipe.   It is manufactured by the method of claim 1 and has a tensile strength TS of 1700 MPa or more, and a yield ratio YR which is a ratio (YS / TS) of 0.1% proof stress YS to tensile strength TS is 72% or less. High impact resistant steel pipe.   It is manufactured by the method according to claim 1, wherein the tensile strength TS is 1800 MPa or more, and the yield ratio YR, which is the ratio of the 0.1% proof stress YS to the tensile strength TS (YS / TS), is 70% or less. High impact resistant steel pipe.   It is manufactured by the method of claim 1 and has a tensile strength TS of 1900 MPa or more and a yield ratio YR which is a ratio of the 0.1% proof stress YS to the tensile strength TS (YS / TS) is 68% or less. High impact resistant steel pipe.   It is manufactured by the method of claim 1, wherein the tensile strength TS is 2000 MPa or more, and the yield ratio YR, which is the ratio of the 0.1% proof stress YS to the tensile strength TS (YS / TS), is 66% or less. High impact resistant steel pipe. High impact steel pipe according to any one of claims 2-5 dislocation density of ¹⁰ 10 ~ ¹⁴ mm ^-2.

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