JP5157257B2 - Low yield ratio steel sheet - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 49
- 239000010959 steel Substances 0.000 title claims description 49
- 239000002184 metal Substances 0.000 claims description 8
- 229910052751 metal Inorganic materials 0.000 claims description 8
- 239000000203 mixture Substances 0.000 claims description 8
- 229910052799 carbon Inorganic materials 0.000 claims description 7
- 229910001562 pearlite Inorganic materials 0.000 claims description 7
- 229910052804 chromium Inorganic materials 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 6
- 229910000859 α-Fe Inorganic materials 0.000 claims description 6
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 229910052710 silicon Inorganic materials 0.000 claims description 5
- 229910052720 vanadium Inorganic materials 0.000 claims description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 4
- 229910052717 sulfur Inorganic materials 0.000 claims description 3
- 229910052782 aluminium Inorganic materials 0.000 claims description 2
- 229910052757 nitrogen Inorganic materials 0.000 claims description 2
- 229910052698 phosphorus Inorganic materials 0.000 claims 1
- 238000001816 cooling Methods 0.000 description 14
- 238000005098 hot rolling Methods 0.000 description 14
- 238000000034 method Methods 0.000 description 11
- 239000000463 material Substances 0.000 description 6
- 238000009864 tensile test Methods 0.000 description 6
- 230000000694 effects Effects 0.000 description 5
- 238000010438 heat treatment Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 239000010953 base metal Substances 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- 229910001035 Soft ferrite Inorganic materials 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 239000000498 cooling water Substances 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000001186 cumulative effect Effects 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052745 lead Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 238000005496 tempering Methods 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
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Description
本発明は、耐震性を必要とする建築構造物用として好適な、降伏比が７５％以下、引張り強さ４９０ＭＰａ以上の低降伏鋼板に関する。 The present invention relates to a low yield steel sheet having a yield ratio of 75% or less and a tensile strength of 490 MPa or more, which is suitable for a building structure requiring earthquake resistance.
近年、建築構造物などでは、地震時の安全性確保の観点から、優れた耐震性を有する鋼板が要求されている。また、降伏比の低い鋼板ほど耐震性に優れることが、従来の研究結果から明らかにされており、建築構造物には、降伏比が８０％以下の鋼材を使用することが義務付けられている。 In recent years, steel sheets having excellent earthquake resistance have been required for building structures and the like from the viewpoint of ensuring safety during an earthquake. Moreover, it has been clarified from the conventional research results that the steel plate having a lower yield ratio is superior in earthquake resistance, and it is obliged to use a steel material having a yield ratio of 80% or less for the building structure.
しかし、耐震性をより改善するためには、降伏強度レベルを一定としつつ、降伏比をより低下させる必要がある。これまでにも降伏比７５％以下の鋼を製造する方法がいくつか提案されている。 However, in order to further improve earthquake resistance, it is necessary to lower the yield ratio while keeping the yield strength level constant. Several methods for producing steel with a yield ratio of 75% or less have been proposed so far.
特許文献１では、特定成分の鋼について熱間圧延仕上温度を規定し、鋼の成分と所望するＴＳから、板厚に応じた冷却水量密度を求めるなど冷却条件を厳密に管理した加速冷却を行いＹＲ７５％以下の加速冷却型５０キロ級低降伏比鋼を製造する方法が提案されている。 In Patent Document 1, the hot rolling finishing temperature is specified for steel of a specific component, and accelerated cooling is performed with strictly controlled cooling conditions such as obtaining the cooling water density according to the plate thickness from the steel component and the desired TS. A method of producing an accelerated cooling type 50 kg class low yield ratio steel with YR of 75% or less has been proposed.
特許文献２では、Ｃ量と炭素当量を規定した鋼について９００℃以下での累積圧下率５０％以上熱間圧延を行い、板厚中心部のオーステナイト分率が９０％以下になるまで５℃／ｓ以上で冷却後、二相域まで再加熱する引張強度６０キロ以下の低降伏比鋼の製造方法が提案されている。 In Patent Document 2, hot rolling is performed at a cumulative reduction of 50% or more at 900 ° C. or less on a steel in which the C content and carbon equivalent are defined, and 5 ° C./until the austenite fraction at the center of the plate thickness becomes 90% or less A method of producing a low yield ratio steel having a tensile strength of 60 kg or less, which is reheated to a two-phase region after cooling at s or higher is proposed.
特許文献３では、Ｎｂ，Ｖなどの析出硬化元素を含有せず、Ｃｕ，Ｍｏなども析出硬化しない量に制限した特定成分の鋼を熱間圧延終了後、二相域でフェライトを生成した後に水冷し、その後、焼戻す際、予め高温に保持した加熱炉内に搬入して焼戻し処理を行う方法が提案されている。 In Patent Document 3, after hot rolling is completed on a steel having a specific component that does not contain precipitation hardening elements such as Nb and V, and Cu and Mo are not precipitated and hardened, ferrite is generated in a two-phase region. There has been proposed a method of carrying out tempering by carrying it into a heating furnace which has been previously kept at a high temperature when water-cooled and then tempered.
また、特許文献４、５では、特定成分の鋼を熱間圧延終了後、予備冷却を行い、一旦冷却を停止し、ある一定割合のフェライトが生成するまで待機させ、その後、再度冷却するプロセスにより降伏強度の変動幅が小さい、引張強度４９０Ｎ／ｍｍ２以上、降伏比７５％以下の低降伏比鋼を製造することが提案されている。
しかしながら、特許文献１〜５記載の低降伏比鋼は、いずれも熱間圧延後に水冷を施すことによって、主相がフェライト−ベイナイト組織で、軟質なフェライトと硬質なベイナイト相とすることにより、組織間の強度差をつけて、降伏比を低下させる方法である。このような方法では、熱間圧延後の水冷時に厳密な温度管理が必要で、容易なプロセスではない。 However, the low yield ratio steels described in Patent Documents 1 to 5 are all made by subjecting the main phase to a ferrite-bainite structure and a soft ferrite and a hard bainite phase by performing water cooling after hot rolling. This is a method of reducing the yield ratio by giving a difference in strength between the two. Such a method is not an easy process because it requires strict temperature control during water cooling after hot rolling.
本発明は、上記したような従来技術の問題点を解決し、降伏比が７５％以下、ＴＳ４９０ＭＰａ以上の低降伏比鋼板を提供することを目的とする。 An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a low yield ratio steel sheet having a yield ratio of 75% or less and TS490 MPa or more.
本発明者らは、上記した目的を達成するために、低降伏比化に影響する要因について鋭意研究した。その結果、鋼材組成に、微量のＣｒを添加することにより、降伏比が低下することを見出した。 In order to achieve the above-mentioned object, the present inventors diligently studied factors that influence the reduction in yield ratio. As a result, it has been found that the yield ratio is lowered by adding a small amount of Cr to the steel composition.
まず、本発明で基礎となった実験結果について説明する。Ｍａｓｓ％で０．１６％Ｃ−０．３５％Ｓｉ−０．０１５％Ｐ−０．００３％Ｓを基本成分として、Ｍｎ：０．６０〜１．５０％、Ｃｒ：０〜０．７０％含み、炭素当量Ｃｅｑ（＝Ｃ＋Ｍｎ／６＋Ｓｉ／２４＋Ｎｉ／４０＋Ｃｒ／５＋Ｍｏ／４＋Ｖ／１４）を０．４２％と一定としたスラブに、熱間圧延を施して２０ｍｍ厚の厚鋼板とし、その後空冷した。 First, experimental results based on the present invention will be described. Mass% 0.16% C-0.35% Si-0.015% P-0.003% S as a basic component, Mn: 0.60 to 1.50%, Cr: 0 to 0.70% In addition, a slab containing carbon equivalent Ceq (= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14) constant at 0.42% was hot-rolled into a 20 mm thick steel plate and then air-cooled.
得られた鋼板について、ＪＩＳ Ｚ２２０１の規定に準拠して、ＪＩＳ５号試験片を採取して引張試験を実施し、引張特性（引張強さ（以下、ＴＳ）、降伏強さ（以下、ＹＳ）、降伏比（以下、ＹＲ））を求め、Ｃｒ添加量との関係で整理した。 For the obtained steel sheet, in accordance with the provisions of JIS Z2201, a JIS No. 5 test piece was collected and subjected to a tensile test, and tensile properties (tensile strength (hereinafter referred to as TS), yield strength (hereinafter referred to as YS), Yield ratio (hereinafter referred to as YR)) was determined and arranged in relation to the Cr addition amount.
図１に結果を示す。Ｃｒ添加量が０．１％を超えると、ＴＳレベルが同等でありながら、降伏比が低下し、７５％以下となっている。Ｃｒを添加することにより、フェライト組織が軟質化し、その結果、同等強度レベルでありながら、ＹＳが低下したためと推察される。 The results are shown in FIG. When the amount of Cr added exceeds 0.1%, the yield ratio is reduced to 75% or less while the TS level is the same. It is presumed that the addition of Cr softened the ferrite structure, and as a result, YS was reduced while maintaining the same strength level.
本発明は、上記した知見に基づき、さらに検討を加えて、完成されたものである。すなわち、本発明の要旨は次のとおりである。
（１）ｍａｓｓ％で、Ｃ：０．１０〜０．１８％、Ｓｉ：０．０５〜０．５０％，Ｍｎ：０．６〜１．３％，Ｃｒ：０．１〜１．０％、Ｐ：０．０２０％以下、Ｓ：０．００５％以下、Ａｌ：０．１％以下、Ｎ：０．００６０％以下、０．３８≦Ｃｅｑ≦０．４３、残部Ｆｅおよび不可避的不純物の組成とフェライト−パーライトを主相とする金属組織を有することを特徴とする低降伏比鋼板。
Ｃｅｑ＝Ｃ＋Ｓｉ／２４＋Ｍｎ／６＋Ｎｉ／４０＋Ｃｒ／５＋Ｍｏ／４＋Ｖ／１４
但し、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ：各元素の含有量（ｍａｓｓ％）
（２）更に、Ｎｂ：０．００５〜０．０５％を含有することを特徴とする（１）に記載の低降伏比鋼板。
（３）更に、Ｔｉ：０．００５〜０．０５％を含有することを特徴とする（１）または（２）に記載の低降伏比鋼板。
The present invention has been completed based on the above findings and further investigations. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.6 to 1.3%, Cr: 0.1 to 1.0% , P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% or less, 0.38 ≦ Ceq ≦ 0.43, balance Fe and inevitable impurities A low yield ratio steel sheet having a composition and a metal structure having ferrite-pearlite as a main phase.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
(2) The low yield ratio steel sheet according to (1), further containing Nb: 0.005 to 0.05%.
(3) The low yield ratio steel sheet according to (1) or (2), further containing Ti: 0.005 to 0.05%.
本発明によれば、高価な合金元素の多量添加や、熱間圧延後の水冷など製造条件の厳密な管理を必要とせずに、降伏比が７５％以下、引張り強さ４９０ＭＰａ以上の低降伏鋼板を安価に製造でき、産業上格段の効果を奏でる。 According to the present invention, a low yield steel sheet having a yield ratio of 75% or less and a tensile strength of 490 MPa or more without requiring a strict control of production conditions such as a large amount of expensive alloy elements or water cooling after hot rolling. Can be manufactured at a low cost, and has a remarkable industrial effect.
本発明に係る低降伏比鋼板は、所定の組成と金属組織を有し、ＴＳ：４９０ＭＰａ以上、ＹＲ：７５％以下の低降伏比を有する。
［成分組成］以下、特にことわらない限り、％はｍａｓｓ％を意味する。
The low yield ratio steel sheet according to the present invention has a predetermined composition and metal structure, and has a low yield ratio of TS: 490 MPa or more and YR: 75% or less.
[Component Composition] Hereinafter, unless otherwise specified,% means mass%.
Ｃ：０．１０〜０．１８％
Ｃは、鋼の強度を増加させる元素であり、本発明でＴＳ：４９０ＭＰａ以上を確保するため、０．１０％以上とする。一方、０．１８％を超えてＣを含有すると母材靭性が劣化し、さらに、低温溶接割れ感受性を増大させる。このため、本発明ではＣは０．１０〜０．１８％の範囲に限定した。
C: 0.10 to 0.18%
C is an element that increases the strength of the steel, and in the present invention, TS is 0.10% or more in order to ensure TS: 490 MPa or more. On the other hand, if the C content exceeds 0.18%, the base metal toughness deteriorates, and the low-temperature weld cracking sensitivity is further increased. For this reason, in this invention, C was limited to 0.10 to 0.18% of range.
Ｓｉ：０．０５〜０．５０％，
Ｓｉは、製鋼において脱酸剤として作用させる場合、０．０５％以上の含有を必要とする。一方、０．５０％を超えて含有すると母材靭性が低下する。このため，Ｓｉは０．０５〜０．５０％の範囲に限定した。なお、好ましくは０．１０〜０．４０％である。
Si: 0.05 to 0.50%,
When Si acts as a deoxidizer in steelmaking, it needs to contain 0.05% or more. On the other hand, if the content exceeds 0.50%, the base material toughness decreases. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10 to 0.40%.
Ｍｎ：０．６〜１．３％，
Ｍｎは固溶強化により、強度を向上させる元素である。このような効果を確保するためには、０．６％以上の含有を必要とする。一方、１．３％を超える含有は、溶接性を著しく低下させる。このため、本発明では、Ｍｎは０．６〜１．３％の範囲に限定した。
Mn: 0.6 to 1.3%,
Mn is an element that improves the strength by solid solution strengthening. In order to ensure such an effect, the content of 0.6% or more is required. On the other hand, if the content exceeds 1.3%, weldability is significantly reduced. For this reason, in this invention, Mn was limited to 0.6 to 1.3% of range.
Ｃｒ：０．１〜１．０％
Ｃｒは、本発明では重要な元素であり、０．１％以上含有することにより、フェライト相を軟化させ、降伏比を低下させる。一方、１．０％を超える含有は、溶接性を著しく劣化させる。このため、本発明では、Ｃｒは０．１〜１．０％の範囲に限定した。
Cr: 0.1 to 1.0%
Cr is an important element in the present invention. By containing 0.1% or more, Cr softens the ferrite phase and lowers the yield ratio. On the other hand, the content exceeding 1.0% significantly deteriorates the weldability. For this reason, in this invention, Cr was limited to 0.1 to 1.0% of range.
Ｐ：０．０２０％以下
Ｐは、不純物として鋼中に不可避的に含有される元素であり，鋼の靭性を劣化させるためにできるだけ低減することが望ましい．特に、０．０２０％を超えての含有は、著しく靭性を低下させるため、本発明ではＰは０．０２０％以下に限定した。
P: 0.020% or less P is an element inevitably contained in steel as an impurity, and is desirably reduced as much as possible in order to deteriorate the toughness of the steel. In particular, if the content exceeds 0.020%, the toughness is remarkably lowered. Therefore, in the present invention, P is limited to 0.020% or less.
Ｓ：０．００５％以下
Ｓは、不純物として鋼中に不可避的に含有される元素であり、鋼の靭性や板厚方向引張試験における絞りを劣化させるため、できるだけ低減することが望ましい。特に、０．００５％を超えて含有すると、上記特性の劣化傾向が著しくなる。そのため、Ｓは０．００５％以下に限定した。
S: 0.005% or less S is an element inevitably contained in steel as an impurity, and it is desirable to reduce it as much as possible in order to deteriorate the toughness of steel and the drawing in the thickness direction tensile test. In particular, when the content exceeds 0.005%, the deterioration tendency of the above characteristics becomes remarkable. Therefore, S is limited to 0.005% or less.
Ａｌ：０．１％以下
Ａｌは，溶鋼の脱酸プロセスにおいて、脱酸剤としてもっとも汎用的に使用される元素である。０．１％を超える含有は、粗大な酸化物を形成して、鋼母材の延性を著しく劣化させる。このため、Ａｌは０．１％以下に限定した。好ましくは、０．０２０〜０．０８０％である。
Al: 0.1% or less Al is an element most widely used as a deoxidizer in a deoxidation process of molten steel. If the content exceeds 0.1%, a coarse oxide is formed, and the ductility of the steel base material is significantly deteriorated. For this reason, Al was limited to 0.1% or less. Preferably, it is 0.020 to 0.080%.
Ｎ：０．００６０％以下
Ｎは、固溶Ｎとして存在すると、歪時効後の母材靭性や溶接熱影響部靭性を低下させるため、０．００６０％以下に限定した。
N: 0.0060% or less When N is present as solid solution N, the base metal toughness after strain aging and the weld heat affected zone toughness are lowered, so the content is limited to 0.0060% or less.
０．３８≦Ｃｅｑ≦０．４３
本発明でＣｅｑは、Ｃｅｑ＝Ｃ＋Ｓｉ／２４＋Ｍｎ／６＋Ｎｉ／４０＋Ｃｒ／５＋Ｍｏ／４＋Ｖ／１４（但し、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ：各元素の含有量（ｍａｓｓ％））で求め、建築用鋼としての機械的特性と溶接性を満たすため、０．３８≦Ｃｅｑ≦０．４３とする。
0.38 ≦ Ceq ≦ 0.43
In the present invention, Ceq is determined by Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (where C, Si, Mn, Ni, Cr, Mo, V: content of each element (mass%)). In order to satisfy the mechanical characteristics and weldability as steel for construction, 0.38 ≦ Ceq ≦ 0.43.
本発明に係る低降伏比鋼板は、上記した成分を基本成分とするが、更に特性を向上させる場合、Ｎｂ、Ｔｉを添加する。 The low yield ratio steel sheet according to the present invention has the above-described components as basic components, but Nb and Ti are added to further improve the characteristics.
Ｎｂ：０．００５〜０．０５％
Ｎｂは、析出強化により鋼板の強度を向上させる有効な元素であり、このような効果を得るためには、０．００５％以上の含有を必要とする。一方、０．０５％を超える含有は、溶接性を低下させる。このため、Ｎｂは添加する場合は、０．００５〜０．０５％の範囲に限定することが好ましい。
Nb: 0.005 to 0.05%
Nb is an effective element that improves the strength of the steel sheet by precipitation strengthening, and in order to obtain such an effect, the Nb content needs to be 0.005% or more. On the other hand, the content exceeding 0.05% lowers the weldability. For this reason, when adding Nb, it is preferable to limit to 0.005 to 0.05% of range.
Ｔｉ：０．００５〜０．０５％
Ｔｉは、Ｎを固定し、溶接熱影響部靭性を改善するために有効な元素であり、このような効果を得るためには、０．００５％以上の含有を必要とする。一方、０．０５％を超えて含有すると、溶接熱影響部靭性が低下する。このため、Ｔｉは０．００５〜０．０５％の範囲に限定することが好ましい。上記した成分以外の残部は、Ｆｅおよび不可避的不純物である。尚、不可避的不純物としては、Ｃｕ：０．１％以下、Ｎｉ：０．１％以下、Ｖ:０．１％以下、Ｂ：０．０００５％以下、Ｃａ：０．００５０％以下、ＲＥＭ：０．００５０％以下、Ｏ：０．０１％以下、Ｚｒ：０．０１％以下、Ｃｏ，Ｓｎ，Ｐｂ，Ｓｂは各０．０１％以下、も許容できる。
Ti: 0.005 to 0.05%
Ti is an element effective for fixing N and improving the weld heat affected zone toughness, and in order to obtain such an effect, it needs to contain 0.005% or more. On the other hand, if it exceeds 0.05%, the weld heat-affected zone toughness decreases. For this reason, it is preferable to limit Ti to 0.005 to 0.05% of range. The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Cu: 0.1% or less, Ni: 0.1% or less, V: 0.1% or less, B: 0.0005% or less, Ca: 0.0050% or less, REM: 0.0050% or less, O: 0.01% or less, Zr: 0.01% or less, and Co, Sn, Pb, and Sb are each acceptable to 0.01% or less.
［金属組織］
本発明に係る低降伏比鋼板は、上記した組成を有し、板厚全域にわたって、フェライト−パーライトを主相とする金属組織を有する。主相とは、体積率で９０％以上有することを意味する。すなわち、フェライトとパーライトの両者の和が体積率で全体の９０％以上を占めている。
[Metal structure]
The low yield ratio steel sheet according to the present invention has the above-described composition, and has a metal structure having ferrite-pearlite as a main phase over the entire plate thickness. The main phase means having a volume ratio of 90% or more. That is, the sum of both ferrite and pearlite accounts for 90% or more of the total volume.
ＹＲ７５％以下の低降伏比鋼とするため、フェライト−パーライトを主相とする金属組織とする。 In order to obtain a low yield ratio steel with a YR of 75% or less, a metal structure having ferrite-pearlite as a main phase is used.
本発明に係る低降伏比鋼板は以下に述べる製造方法で製造可能である。上記した組成の溶鋼を、転炉などの常用の溶解炉で溶製し、連続鋳造法や、造塊−分塊法などの常用の方法で、スラブ（鋼素材）とする。次いで、熱間圧延し、所望の板厚とした後に、空冷する。 The low yield ratio steel sheet according to the present invention can be manufactured by the manufacturing method described below. The molten steel having the above composition is melted in a conventional melting furnace such as a converter and is made into a slab (steel material) by a conventional method such as a continuous casting method or an ingot-bundling method. Next, after hot rolling to a desired plate thickness, air cooling is performed.
熱間圧延の加熱温度は、特に限定する必要はないが、１０５０〜１２５０℃の範囲の温度に加熱することが好ましい。加熱温度が１０５０℃未満では、変形抵抗が大きくなり、圧延機負荷が増大する。 The heating temperature for hot rolling is not particularly limited, but it is preferable to heat to a temperature in the range of 1050 to 1250 ° C. If heating temperature is less than 1050 degreeC, a deformation resistance becomes large and a rolling mill load will increase.
一方、１２５０℃を超えると、熱間圧延時に表面疵が発生しやすくなるため、加熱温度は１０５０〜１２５０℃の範囲とするのが望ましい。 On the other hand, if it exceeds 1250 ° C, surface flaws are likely to occur during hot rolling, so the heating temperature is preferably in the range of 1050 to 1250 ° C.
熱間圧延の終了温度は、鋼板の表面温度で８００〜９５０℃の範囲とする。圧延終了温度が８００℃未満では、所望の強度が確保できず、一方、９５０℃を超える温度では、厚鋼板の母材靭性が低下する。このため、熱間圧延の圧延終了温度は、鋼板の表面温度で８００〜９５０℃の範囲に限定した。好ましくは、８００〜９００℃とする。 The end temperature of the hot rolling is in the range of 800 to 950 ° C. as the surface temperature of the steel sheet. If the rolling end temperature is less than 800 ° C., the desired strength cannot be ensured. On the other hand, if the temperature exceeds 950 ° C., the base material toughness of the thick steel plate decreases. For this reason, the rolling end temperature of hot rolling was limited to the range of 800 to 950 ° C. as the surface temperature of the steel sheet. Preferably, it is set to 800-900 degreeC.
熱間圧延後、空冷する。板厚が増加した場合などに限り、２．０℃／ｓ以下の冷却速度で加速冷却しても、本発明の目的組織であるフェライト−パーライト組織が得られさえすれば、本発明の効果を損なうものではない。ここで冷却速度とは、板厚１／４ｔ位置での８００〜５００℃の平均冷却速度とする。 Air-cool after hot rolling. As long as the plate thickness is increased, the effect of the present invention can be obtained as long as the ferrite-pearlite structure, which is the target structure of the present invention, can be obtained even when accelerated cooling is performed at a cooling rate of 2.0 ° C./s or less. There is no loss. Here, the cooling rate is defined as an average cooling rate of 800 to 500 ° C. at the position of 1/4 t thickness.
表１に示した組成を有する鋼素材に熱間圧延を施して、種々の板厚の厚鋼板を製造し、引張試験を実施した。表２に熱間圧延条件と引張試験の結果を合わせて示す。 The steel material having the composition shown in Table 1 was hot-rolled to produce thick steel plates having various thicknesses, and a tensile test was performed. Table 2 shows the hot rolling conditions and the results of the tensile test.
引張試験は、得られた厚鋼板から、ＪＩＳＺ２２０１の規定に準拠して、ＪＩＳ５号試験片を採取し、ＪＩＳＺ２２４１の規定に準拠して引張試験を実施し、引張特性（ＹＳ、ＴＳ、ＹＲ）を求めた。また、金属組織の観察を行った。 In the tensile test, a JIS No. 5 test piece is collected from the obtained thick steel plate in accordance with the provisions of JISZ2201, the tensile test is performed in accordance with the provisions of JISZ2241, and tensile properties (YS, TS, YR) are obtained. Asked. Moreover, the metal structure was observed.
本発明例（Ｎｏ．１〜５）はいずれも、フェライト−パーライトを主相とする金属組織であり、ＴＳが４９０ＭＰａ以上、ＹＲが７５％以下の、低降伏比を有する非調質高張力厚鋼板で、一方、本発明の範囲を外れる比較例は、強度が不足しているか、降伏比が７５％を上回っている。 Each of the inventive examples (Nos. 1 to 5) is a metal structure having ferrite-pearlite as a main phase, TS is 490 MPa or more, YR is 75% or less, and has a low yield ratio. On the other hand, the comparative example which is out of the scope of the present invention is a steel plate, and the strength is insufficient or the yield ratio exceeds 75%.
Claims (3)
Ｃｅｑ＝Ｃ＋Ｓｉ／２４＋Ｍｎ／６＋Ｎｉ／４０＋Ｃｒ／５＋Ｍｏ／４＋Ｖ／１４
但し、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ：各元素の含有量（ｍａｓｓ％） In mass%, C: 0.10 to 0.18%, Si: 0.05 to 0.50%, Mn: 0.6 to 1.3%, Cr: 0.1 to 1.0%, P: 0.020% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.0060% or less, 0.38 ≦ Ceq ≦ 0.43, composition of remaining Fe and inevitable impurities and ferrite -A low yield ratio steel sheet characterized by having a metal structure having pearlite as a main phase.
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
However, C, Si, Mn, Ni, Cr, Mo, V: Content of each element (mass%)
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