JP4008378B2 - Low yield ratio high strength steel with excellent toughness and weldability - Google Patents

Description

【００３３】
得られた熱延鋼板に対し、下記要領にて組織観察を行い、ポリゴナルフェライト量（面積％）、ＭＡ量（面積％）およびＭＡの平均サイズを測定した。なお、ポリゴナルフェライト、ＭＡ以外の組織はベイナイトであった。
ポリゴナルフェライト量は、熱延鋼板の板厚１／４部位について、光学顕微鏡を用いて倍率２００倍で３００μm ×３００μm の領域を５視野観察し、画像解析ソフトを用いてポリゴナルフェライトの面積率の平均値を求め、その値をポリゴナルフェライト量とした。また、ＭＡ量は、熱延鋼板の板厚１／４部位について、レペラー腐食をした後、光学顕微鏡を用いて倍率１０００倍で５０μm ×５０μm の領域を１０視野観察し、画像解析ソフトを用いてＭＡの面積率の平均値を求め、その値をＭＡ量とした。また、ＭＡの平均サイズは、前記観察視野について画像解析ソフトを用いてＭＡ粒の面積と同面積の円（相当円）を想定し、その相当円の直径の平均値を求め、その値を平均サイズとした。
【００３４】
また下記要領にて引張試験、衝撃試験を行い、母材の機械的性質を調べた。
・引張試験
各鋼板の板厚１／４部位からＪＩＳ４号試験片を得て、引張試験を行い、０．２％耐力、引張強さを測定した。
・衝撃試験
各鋼板の板厚１／４部位からＪＩＳ４号試験片を採取し、シャルピー衝撃試験（試験温度−２０℃）を行い、吸収エネルギー（ｖＥ_-20 ）を求めた。
【００３５】
さらに、以下の要領にて溶接を模擬した熱サイクル試験を行い、ＨＡＺ靭性を調べた。溶接入熱量が１０００ｋＪ／ｃｍに相当する熱サイクル（１３５０℃に加熱した後、８００℃から５００℃までの冷却時間が７２０sec である熱履歴）を与えた後、ＪＩＳ４号試験片を採取し、シャルピー衝撃試験（試験温度−２０℃）を行い、ボンド部の吸収工ネルギ（ｖＥ_-20 ）の平均値を求めた。なお、サンプル数は３とした。
【００３６】
【表１】

【００３７】
上記調査結果を表２に併せて示す。表２より、発明例（試料No. １〜１４）は母材の降伏比（ＹＲ）が７０〜７７％と低降伏比でありながら、吸収エネルギーが２５０Ｊ以上と靭性に優れる。さらに大入熱溶接後のＨＡＺにおいても、その吸収エネルギーが２００Ｊ以上であり、優れたＨＡＺ靭性が得られた。一方、成分が本発明範囲外の比較例（試料No. １５〜２０）は、引張強さ、降伏比は本発明レベルを満足するものの、母材靭性あるいはＨＡＺ靭性のいずれか一方が著しく劣化した。また、本発明の成分条件を満足するものの、製造条件が不適切であるため、本発明の組織条件を満足するに至らなかった比較例（試料No. ２１〜２５）は、強度およびＨＡＺ靭性は本発明レベルを満足するものの、降伏比が高くなったり、降伏比が低い場合には母材靭性が劣化した。
【００３８】
【発明の効果】
本発明鋼によれば、ＨＡＺ靭性を確保すべく合金成分を調整しつつ、組織をポリゴナルフェライト量を５〜３０％に抑制しつつ、平均サイズが５μm 以下の微細なＭＡ量を３〜１５％および残部ベイナイトとしたので、４９０〜５９０ＭＰａの引張強さを確保しながら、ＨＡＺ靭性に優れ、しかも優れた母材靭性を有する。[0001]
[Technical field to which the invention belongs]
The present invention relates to a high-strength steel having a tensile strength of 490 to 590 MPa, which is mainly used for large structures such as building structures and bridges.
[0002]
[Prior art]
Building steels used for large structures such as building structures and bridges are required to have earthquake resistance while ensuring a certain level of strength, and also have good weldability for large heat input welding. The earthquake resistance is ensured by lowering the yield ratio of the steel material.
[0003]
In order to achieve a low yield ratio and improve earthquake resistance while ensuring the strength of the steel, it is known that the form of the low-temperature transformation product forming ferrite and the second phase structure is important. In general, a low yield ratio is realized by dispersing a hard structure such as martensite in a soft ferrite structure, but the base material toughness tends to deteriorate. In recent years, demands for improving the safety of structures, such as seismic performance, have increased, and improvement in the base metal toughness has been demanded even in high strength steel sheets having a tensile strength of 490 to 590 MPa.
[0004]
As a technique for improving the toughness of the base metal while having a low yield ratio, for example, Japanese Patent Laid-Open No. 2002-266022 (Patent Document 1) discloses that the toughness of the entire two-phase structure is improved by increasing the toughness of ferrite, which is a soft phase. A method for producing a high toughness and high strength steel to ensure the strength is proposed. Japanese Patent Laid-Open No. 2001-342538 (Patent Document 2) describes that the fraction of martensite or MA (Martensite-Austenite Constituents) is 1 to 10% and the size is 3 μm or less. Limited low-yield ratio high-strength steels and methods for their production are described.
[0005]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2002-266022 (Claims)
[Patent Document 2]
JP 2001-342538 A (Claims)
[0006]
[Problems to be solved by the invention]
The technique described in Patent Document 1 requires ultra-fine graining of the structure before the heat treatment in the two-phase region, and therefore requires special hot rolling in a low temperature region, resulting in a large rolling load and productivity. bad. Moreover, since the technique of patent document 2 rolls in a non-recrystallized region, the rolling load is still large, and anisotropy may occur in the mechanical properties of the steel sheet, and the uniformity of the material is impaired. Furthermore, although only the martensite fraction and size are specified, the rolling end temperature is specified so as to avoid the formation of ferrite (paragraph number 0045), but in the end, the structure structure other than martensite or MA Is unknown and the technical content is unclear.
[0007]
The present invention has been made in view of such problems, and has a tensile strength of 490 to 590 MPa, which can be produced without performing hot rolling in a low temperature range, and has excellent base material toughness and weldability. An object is to provide high strength steel with low yield ratio.
[0008]
[Means for Solving the Problems]
The high strength steel of the present invention is mass%,
C: 0.01 to 0.15%,
Si: 0.8% or less,
Mn: 0.88 to 2.0%,
Al: 0.10% or less,
Ti: 0.004 to 0.050%,
B: 0.0003 to 0.0050%,
And the balance is Fe and impurities, and the average value of the area ratio of polygonal ferrite when observing five fields of 300 μm × 300 μm at 200 × magnification using an optical microscope at a 1/4 thickness portion of the steel sheet Is an area ratio of MA (a mixture of martensite and austenite) when observing 10 fields of 50 μm × 50 μm at a magnification of 1000 times in an area of 1/4 to 5% by area using an optical microscope . The average value is 5 to 15 area%, the balance is a bainite structure, and the average size of the MA is 5 μm or less.
[0009]
In the steel of the present invention, in addition to the above basic components, (1) one or more of Cu 1.5% or less, Ni 1.5% or less, Cr 1.5% or less, Mo 1.5% or less, (2 Nb: 0.05% or less, V: Any one or two of 0.20%, (3) Zr: 0.05% or less, Mg: 0.005% or less, 4) Ca: An element selected from each group of 0.005% or less can be further added alone or in combination.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The inventor, as a steel having a low yield ratio with excellent ductility characteristics, forms a soft phase with polygonal ferrite rich in ductility and a hard phase with MA and bainite, which are a mixture of martensite and austenite (residual austenite). As a result of research on methods for improving the toughness of high-strength steels with steel, the results of cracks propagating along the grain boundaries produced by these polygonal ferrites in steels produced by polygonal ferrites along the former austenite grain boundaries It has been found that the toughness of the base material decreases. In addition, it has been found that as the amount of MA increases, the particle size also increases, and since MA becomes a starting point of fracture, the toughness of the base metal is deteriorated. Based on this knowledge, the present invention secures HAZ (welding heat affected zone) toughness during high heat input welding by optimizing the alloy elements, and appropriately controls the structure of steel, thereby reducing ductility (low A high-strength steel excellent in yield ratio), base metal toughness, and weldability (HAZ toughness) during high heat input welding was completed. That is, the high-strength steel of the present invention is an average value of the area ratio of polygonal ferrite when observing five fields of 300 μm × 300 μm at 200 × magnification using an optical microscope at a ¼ portion of the thickness of the steel sheet ( Hereinafter, it is simply referred to as “polygonal ferrite amount”)) of 5 to 30% by area, and the MA when the 50 μm × 50 μm region is observed at 10 × magnification using an optical microscope at a 1/4 thickness portion of the steel sheet. The average area ratio (hereinafter simply referred to as “MA amount”) of (a mixture of martensite and austenite) is 5 to 15 area%, the balance is a bainite structure, and the average size of the MA is 5 μm or less. It is said that.
[0011]
Polygonal ferrite is a structure excellent in ductility and effective in reducing the yield ratio. On the other hand, polygonal ferrite is produced along the prior austenite grain boundaries, so that when the amount increases, the structure becomes non-uniform and the toughness deteriorates. For this reason, in the present invention, the lower limit of the amount of polygonal ferrite is 5%, preferably 7%, while the upper limit is 30%, preferably 20%.
[0012]
MA is a structure effective for securing strength and reducing the yield ratio. However, if the average size exceeds 5 μm, the massive MA becomes a crack generation source and the toughness is lowered. Therefore, in the present invention, it is set to 5 μm or less. If the average size is too small, the effect is too small. If the average amount of MA is less than 5%, the effect of improving the strength and reducing the yield ratio is too small, whereas if it exceeds 15%, the toughness is lowered. Therefore, the lower limit of the MA amount is 5%, preferably 5%, and the upper limit is 15%, preferably 10%.
[0013]
The remaining structure is composed of bainite. In order to disperse and produce the fine MA in the structure, it is important that the structure before the two-phase heat treatment after hot rolling is a bainite structure in which the formation of polygonal ferrite is reduced. By forming austenite at the lath interface of bainite and transforming a part thereof into martensite, a fine MA structure with the above average size can be obtained. Further, by using bainite as the remaining structure, it is possible to increase the strength to 490 MPa or more in the presence of 5 to 30% polygonal ferrite and 3 to 15% MA. In addition, the measuring method of the average size of MA is demonstrated in the below-mentioned Example.
[0014]
Next, a suitable steel component (unit mass%) in forming the three-phase structure will be described.
[0015]
C: 0.01 to 0.15%
C is an element necessary for ensuring the strength of the base material. If it is less than 0.01%, the intended strength level of the present invention cannot be obtained. On the other hand, if it exceeds 0.15%, the formation of MA in HAZ is promoted during welding, and the toughness deteriorates. Therefore, in the present invention, the lower limit of the C amount is 0.01%, preferably 0.03%, and the upper limit is 0.15%, preferably 0.09%.
[0016]
Si: 0.8% or less Si is an element having a deoxidizing action, and since it has an effect of improving strength while suppressing deterioration of ductility, 0.1% or more is preferably added. However, if it exceeds 0.8%, the production of HAZ MA is promoted and the toughness deteriorates. For this reason, the upper limit of the amount of Si is made into 0.8%, preferably 0.6%.
[0017]
Mn: 0.88 to 2.0%
Mn is effective for improving the hardenability and securing the strength and toughness, and if it is less than 0.88 %, such action is too small, while if it exceeds 2.0%, the strength becomes excessive and the toughness deteriorates. For this reason, the lower limit of the Mn content is 0.88 %, preferably 1.2%, and the upper limit is 2.0%, preferably 1.8%.
[0018]
Al: 0.10% or less Since Al has an effect of improving the base metal toughness by deoxidation and refinement of the microstructure, it is preferable to add 0.01% or more. However, if added in excess, like Si, the formation of HAZ MA is promoted and the toughness is reduced. For this reason, the upper limit is made 0.10%, preferably 0.05%.
[0019]
Ti: 0.004 to 0.050%
Ti is an element effective in improving HAZ toughness by combining with N to reduce solid solution N and to suppress the coarsening of HAZ austenite grains. If the amount of Ti is less than 0.004 %, the effect of refining is too small. On the other hand, if it exceeds 0.050%, TiN becomes coarse and the number of TiNs decreases, so that the HAZ toughness returns and deteriorates. . For this reason, the lower limit of the Ti amount is 0.004 %, and the upper limit is 0.050%, preferably 0.030%.
[0020]
B: 0.0003 to 0.0050%
B has the effect of suppressing the formation of polygonal ferrite from the austenite grain boundaries and improving the HAZ toughness, so 0.0003% or more is added. However, since it has an effect of improving the hardenability, when it is added excessively, the strength becomes excessive, and the HAZ toughness decreases. Therefore, the content is limited to 0.0050% or less, preferably 0.0040% or less.
[0021]
The steel of the present invention is formed by the remaining Fe and impurities in addition to the above basic components. Impurities P and S are preferably as small as possible because they deteriorate the base metal toughness and the HAZ toughness. However, it does not impair the effect of the basic component as a steel component or prevent the addition of an element that further improves the mechanical properties. For example, (1) any one or more of Cu 1.5% or less, Ni 1.5% or less, Cr 1.5% or less, Mo 1.5% or less, (2) Nb = 0. 05% or less, V: Any one or two of 0.20%, (3) Zr: 0.05% or less, Mg: One or two of 0.005% or less, (4) Ca: 0 An element selected from each group of 0.005% or less can be further added alone or in combination. Hereinafter, the reason for adding these elements will be described.
[0022]
Cu: 1.5% or less, Ni: 1.5% or less, Cr: 1.5% or less, Mo: 1.5% or less These elements have the effect of improving the hardenability and impair the HAZ toughness. Improve the strength and toughness of the base material without any problems. In order to effectively exhibit such an action, it is preferable to add 0.10% or more of each of these elements. However, when it is added excessively, the strength becomes excessive, and the HAZ toughness returns and decreases. For this reason, each is 1.5% or less, preferably 1.0% or less.
[0023]
Nb: 0.05% or less, V: 0.20% or less These elements can refine the bainite structure after rolling, finely disperse carbides, and bainite before two-phase region heat treatment. The tissue can be in a preferred form. Furthermore, the precipitation of carbides can suppress non-uniformization due to the coarsening of the structure during the two-phase heat treatment, and the strength and toughness can be prevented from being lowered. In order to effectively exhibit such an action, it is preferable to add 0.01% or more of Nb and 0.05% or more of V. However, excessive addition causes the carbides to become coarse and deteriorates the HAZ toughness, so Nb is limited to 0.05% or less and V is limited to 0.20% or less.
[0024]
Zr: 0.05% or less, Mg: 0.005% or less These elements have an action of suppressing coarsening of HAZ austenite grains and an action of improving HAZ toughness. Zr, like Ti, forms nitrides, and Mg finely disperses oxides that form nuclei of nitrides such as TiN, thereby achieving the above-described action. In order to effectively express such an action, it is preferable to add 0.005% or more of Zr and 0.001% or more of Mg. However, if it is added excessively, the inclusions become coarse, and the HAZ toughness returns and decreases. For this reason, Zr is 0.05% or less, preferably 0.03%, while Mg is 0.005% or less, preferably 0.004% or less.
[0025]
Ca: 0.005% or less
Ca has the effect | action which reduces the anisotropy of an inclusion, and contributes to the improvement of HAZ toughness. In order to effectively exhibit such an action, it is preferable to add 0.0005 % or more. However, if it is added excessively, the inclusions become coarse and return to deteriorate the HAZ toughness. For this reason, the upper limit of the addition amount is 0.005%, preferably 0.004%.
[0026]
The steel of the present invention is used as a steel material in an appropriate form such as a plate, an H shape, and a pipe, but is mainly used as a steel plate as a construction steel material. In particular, a 490 to 590 MPa grade steel plate is often used as a thick steel plate having a thickness of about 30 to 80 mm. Then, the case where the manufacturing method of this invention steel is manufactured as a board | plate material is demonstrated as an example. In addition, not only a board | plate material but when manufacturing steel materials, such as H-shaped steel materials, by rolling, it can manufacture on the same conditions.
[0027]
The steel plate according to the present invention is manufactured by melting steel having the above components, hot rolling the steel slab, and then heat-treating it in a two-phase region. Hot rolling may be performed by a conventional method, and the rolling end temperature does not need to be performed in a recrystallization temperature range or lower temperature range as in the past, and may be about 850 to 900 ° C.
[0028]
As for cooling after the end of rolling, it is typically preferable to cool up to 500 ° C. at 2 ° C./s to 7 ° C./s so as to refine the bainite structure while suppressing the formation of polygonal ferrite. The cooling rate of 500 ° C. or lower is not particularly limited. By refining the bainite structure, it is possible to refine the MA structure during the subsequent two-phase heat treatment.
[0029]
As described above, a steel sheet having an appropriate amount of polygonal ferrite and a fine bainite structure is heated to a two-phase temperature of about 700 to 760 ° C. for about 1.5 to 3.0 hours, depending on the plate thickness. By this heating, austenite is generated between the laths of bainite. Thereafter, the austenite is transformed and finely dispersed MA is obtained by cooling at a cooling rate of 1 to 3 ° C./s. MA is controlled to a predetermined structure fraction by the temperature and holding time of the two-phase region heat treatment.
[0030]
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limitedly interpreted by this Example.
[0031]
【Example】
The steel shown in Table 1 was melted and the slab (thickness 250 mm) obtained by casting the molten metal was heated at 1100 ° C., then hot-rolled to a thickness of 50 mm, and the rolling shown in Table 2 was completed. The hot rolling was finished at the temperature, and cooled at the cooling rate shown in the same table. Thereafter, the mixture was heated to the two-phase region temperature shown in the same table, maintained for about 2.0 hours, and then cooled at the cooling rate shown in the same table. In Table 1, the content underlined means outside the scope of the invention.
[0032]
[Table 1]

[0033]
The obtained hot-rolled steel sheet was observed for structure in the following manner, and the amount of polygonal ferrite (area%) , the amount of MA (area%), and the average size of MA were measured. The structure other than polygonal ferrite and MA was bainite.
The amount of polygonal ferrite is the area ratio of polygonal ferrite by using an optical microscope to observe five fields of 300 μm × 300 μm at a magnification of 200 times for a 1/4 thickness portion of a hot-rolled steel sheet, and using image analysis software. The average value was obtained, and the value was defined as the amount of polygonal ferrite . In addition, the amount of MA was determined by observing 10 fields of 50 μm × 50 μm area at 1000 × magnification using an optical microscope after repeller corrosion at a thickness 1/4 part of the hot-rolled steel sheet, and using image analysis software. The average value of the area ratio of MA was determined, and the value was defined as the MA amount . The average size of MA is assumed to be a circle (equivalent circle) having the same area as that of the MA grain by using image analysis software for the observation field of view, and an average value of the diameters of the equivalent circles is obtained. Size.
[0034]
In addition, a tensile test and an impact test were performed in the following manner to examine the mechanical properties of the base material.
-Tensile test A JIS No. 4 test piece was obtained from a 1/4 thickness portion of each steel plate, a tensile test was performed, and 0.2% yield strength and tensile strength were measured.
-Impact test A JIS No. 4 test piece was collected from a 1/4 thickness portion of each steel plate, subjected to a Charpy impact test (test temperature-20 ° C), and an absorbed energy (vE- ₂₀ ) was obtained.
[0035]
Furthermore, a thermal cycle test simulating welding was performed in the following manner to examine the HAZ toughness. After giving a heat cycle corresponding to 1000 kJ / cm of the heat input of welding (heat history with a cooling time from 800 ° C. to 500 ° C. of 720 sec after heating to 1350 ° C.), a JIS No. 4 test piece was collected and Charpy An impact test (test temperature −20 ° C.) was performed, and an average value of absorption energy (vE ₋₂₀ ) of the bond portion was obtained. The number of samples was 3.
[0036]
[Table 1]

[0037]
The survey results are also shown in Table 2. From Table 2, the invention examples (Sample Nos. 1 to 14) are excellent in toughness with absorbed energy of 250 J or more, while the yield ratio (YR) of the base material is 70 to 77% and a low yield ratio. Furthermore, even in HAZ after high heat input welding, the absorbed energy was 200 J or more, and excellent HAZ toughness was obtained. On the other hand, in Comparative Examples (Sample Nos. 15 to 20) whose components are outside the scope of the present invention, the tensile strength and the yield ratio satisfy the present invention level, but either the base material toughness or the HAZ toughness is remarkably deteriorated. . Moreover, although the component conditions of the present invention were satisfied, the manufacturing conditions were inappropriate, so the comparative examples (samples Nos. 21 to 25) that did not satisfy the structural conditions of the present invention had the strength and HAZ toughness Although the present invention level was satisfied, the base material toughness deteriorated when the yield ratio was high or the yield ratio was low.
[0038]
【The invention's effect】
According to the steel of the present invention, while adjusting the alloy components to ensure the HAZ toughness, the amount of polygonal ferrite is suppressed to 5 to 30%, and the fine MA amount with an average size of 5 μm or less is 3 to 15 % . % And the balance bainite, it has excellent HAZ toughness and excellent base material toughness while ensuring a tensile strength of 490 to 590 MPa.

Claims (5)

mass%
C: 0.01 to 0.15%,
Si: 0.8% or less,
Mn: 0.88 to 2.0%,
Al: 0.10% or less,
Ti: 0.004 to 0.050%,
B: 0.0003 to 0.0050%,
And the balance is Fe and impurities, and the average value of the area ratio of polygonal ferrite when observing five fields of 300 μm × 300 μm at 200 × magnification using an optical microscope at a 1/4 thickness portion of the steel sheet Is an area ratio of MA (a mixture of martensite and austenite) when observing 10 fields of 50 μm × 50 μm at a magnification of 1000 times in an area of 1/4 to 5% by area using an optical microscope . A low-yield ratio high-strength steel excellent in toughness and weldability, having an average value of 5 to 15 area%, the balance being a bainite structure, and an average size of the MA of 5 μm or less. further,
Cu: 1.5% or less,
Ni: 1.5% or less,
Cr: 1.5% or less,
The low yield ratio high-strength steel according to claim 1, which contains any one or more of Mo: 1.5% or less. further,
Nb: 0.05% or less,
The low yield ratio high-strength steel according to claim 1 or 2, which contains any one or two of V: 0.20% or less. further,
Zr: 0.05% or less,
The low yield ratio high-strength steel according to any one of claims 1 to 3, containing any one or two of Mg: 0.005% or less. further,
The low yield ratio high strength steel according to any one of claims 1 to 4, containing Ca: 0.005% or less.