KR20040059294A - Method for manufacturing linepipe steel strips to minimize the deviation of yield strength through the longitudinal direction - Google Patents

Method for manufacturing linepipe steel strips to minimize the deviation of yield strength through the longitudinal direction Download PDF

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KR20040059294A
KR20040059294A KR1020020085889A KR20020085889A KR20040059294A KR 20040059294 A KR20040059294 A KR 20040059294A KR 1020020085889 A KR1020020085889 A KR 1020020085889A KR 20020085889 A KR20020085889 A KR 20020085889A KR 20040059294 A KR20040059294 A KR 20040059294A
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steel
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rolling
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KR100946052B1 (en
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강기봉
배진호
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주식회사 포스코
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

PURPOSE: A manufacturing method of hot rolled strip is provided which is capable of reducing deviation of yield strength in a longitudinal direction of coil by controlling micro-structure by formation of precipitate and controlling distribution and size of the precipitate itself. CONSTITUTION: The method comprises the steps of reheating a steel slab comprising 0.03 to 0.10 wt.% of C, 0.01 to 0.50 wt.% of Si, 1.2 to 2.0 wt.% of Mn, 0.03 wt.% or less of P, 0.010 wt.% or less of S, 0.01 to 0.10 wt.% of Ti, 0.01 wt.% or less of N, 0.02 to 0.07 wt.% of Nb, 0.10 wt.% or less of V, 0.01 to 0.3 wt.% of Mo, 0.01 to 0.3 wt.% of Cr, 0.1 to 0.5 wt.% of Ni and a balance of Fe and other inevitable impure elements in the temperature range of 1,250 to 1,350 deg.C; hot-rolling the reheated steel slab to a finish delivery temperature of 780 to 820 deg.C; and coiling the cooled steel sheet at a temperature of 500 to 600 deg.C by controlling injection quantity of cooling water in such a way that temperature of top end part and tail end part of the hot rolled steel sheet are higher than a central part of the hot rolled steel sheet by at least 30 deg.C.

Description

길이방향의 항복강도 편차가 적은 라인파이프용 열연강판의 제조방법{Method for manufacturing linepipe steel strips to minimize the deviation of yield strength through the longitudinal direction}Method for manufacturing linepipe steel strips to minimize the deviation of yield strength through the longitudinal direction}

본 발명은 건축, 파이프라인 및 해양구조물 등의 용도로 사용되는 항복강도 540~570MPa의 라인파이프 강의 제조방법에 관한 것으로, 보다 상세하게는 석출물 형성에 의한 미세조직의 관리와 석출물 자체의 분포 및 크기의 관리에 의해 코일 길이 방향 항복강도 편차를 줄일 수 있는 열연강판의 제조방법에 관한 것이다.The present invention relates to a method for producing a line pipe steel having a yield strength of 540 to 570 MPa for use in construction, pipelines and offshore structures. More specifically, the microstructure management and the distribution and size of the precipitate itself The present invention relates to a method for manufacturing a hot rolled steel sheet which can reduce the variation in the yield strength of the coil in the longitudinal direction.

우수한 저온 인성을 지닌 고강도 열연강판을 제조하기 위해서 전 세계에 걸쳐 많은 시도가 이루어 지고 있으며, 이중 열연강판 선후단부의 재질편차를 최소화하기 위한 것으로 한국 공개특허공보 2000-0038781호가 있다. 이 선행기술은 C:0.04-0.06%, Si:0.025%이하, Mn:0.2-1.0%, S:0.008%이하, Cu:0.4%이하, Sn:0.001-0.04%, Pb:0.001~0.012%, Ni:0.2%이하, Cr:0.1%이하 및 Ca:0.0002~0.008%를 함유한 1차 압연된 열연 바를 가열대에서 중앙부보다 선후단부가 40~60℃ 높게 가열하고, 코일박스의 분위기 온도를 1020℃이상으로 유지하며, 후압연시에는 압연온도를 850℃이상에서 압연을 실시하여 열연강판을 제조후, 선후단부가 중앙부보다 20℃이상 높은 상태로 코일링하는 것이다. 그러나 이러한 조업조건은 바 히터와 코일박스가 설치된 미니밀 공정에는 적용될 수 있지만, 일반 열간압연 조업공정에는 적용할 수 없는 단점이 있다.Many attempts have been made all over the world to manufacture high strength hot rolled steel sheets having excellent low temperature toughness, and Korean Laid-Open Patent Publication No. 2000-0038781 is for minimizing the material deviation of the front and rear ends of the double hot rolled steel sheet. This prior art is C: 0.04-0.06%, Si: 0.025% or less, Mn: 0.2-1.0%, S: 0.008% or less, Cu: 0.4% or less, Sn: 0.001-0.04%, Pb: 0.001 to 0.012%, The first rolled hot-rolled bar containing Ni: 0.2% or less, Cr: 0.1% or less, and Ca: 0.0002 to 0.008% is heated at the front and rear ends 40-60 ° C higher than the center part in the heating table, and the ambient temperature of the coil box is 1020 ° C. In this case, after rolling, the rolling temperature is rolled at 850 ° C. or higher to manufacture the hot rolled steel sheet. However, these operating conditions can be applied to the mini-mill process in which the bar heater and the coil box are installed, but are not applicable to the general hot rolling operation process.

따라서, 본 발명에서는 일반 열간압연 조업공정에서 열연강판의 길이방향 재질 편차를 최소화할 수 있는 열연강판의 제조방법을 제공하는데, 그 목적이 있다.Accordingly, the present invention provides a method for manufacturing a hot rolled steel sheet which can minimize the longitudinal material variation of the hot rolled steel sheet in a general hot rolling operation process.

상기 목적을 달성하기 위한 본 발명의 열연강판 제조방법은, 중량 %로, C:0.03-0.10%, Si:0.01%-0.50%, Mn:1.2-2.0%, P:0.03%이하, S:0.010%이하, Ti:0.01-0.10%, N:0.01%이하, 여기에 Nb:0.02%-0.07%, V:0.10%이하, Mo:0.01~0.3%, Cr:0.01~0.3%, Ni:0.1~0.5%의 그룹 중 선택된 1종 이상 및 나머지 Fe와 기타 불가피한 불순원소들로 구성된 강슬라브를 1250-1350℃의 온도에서 재가열하고 780-820℃의 마무리 압연온도로 열간압연한 다음, 열연판의 길이방향으로 선단부와 후단부의 온도가 열연판의 중앙부 보다 30℃ 이상 높도록 주수량을 조절하여 500~600℃의 온도에서 권취하는 것을 포함한다.Hot-rolled steel sheet manufacturing method of the present invention for achieving the above object, in weight%, C: 0.03-0.10%, Si: 0.01% -0.50%, Mn: 1.2-2.0%, P: 0.03% or less, S: 0.010 % Or less, Ti: 0.01-0.10%, N: 0.01% or less, here Nb: 0.02% -0.07%, V: 0.10% or less, Mo: 0.01 to 0.3%, Cr: 0.01 to 0.3%, Ni: 0.1 to The steel slab consisting of at least one selected from the group of 0.5% and the remaining Fe and other unavoidable impurities is reheated at a temperature of 1250-1350 ° C and hot rolled to a finish rolling temperature of 780-820 ° C, and then the length of the hot rolled sheet It includes winding at a temperature of 500 ~ 600 ℃ by adjusting the amount of water so that the temperature of the front end and the rear end in the direction is 30 ℃ or more higher than the center of the hot rolled sheet.

이하, 본 발명을 상세히 설명한다.Hereinafter, the present invention will be described in detail.

본 발명자들은 소량의 합금원소 첨가와 열간압연 제어만으로도 길이 방향 항복강도편차가 적은 강관용 고장력 열연강판을 제조하는 방안을 모색하던 중 강도를 증가시키기 위해 석출물을 석출시키는 경우 석출물 형성에 의한 미세조직의 변화와 석출물 자체의 분포 및 크기 등을 적절히 관리하면 길이 방향 재질이 크게 변화될 수 있다는 것을 실험을 통해 확인하고, 그 실험결과에 기초하여 본 발명을 제안하게 되었다. 본 발명에서는 Mo, Cr, V, Ti을 복합첨가하여 항복강도 540~570MPa급으로 길이방향 재질편차가 적은 열연판을 제공한다. 먼저, 본 발명강의 성분한정이유를 설명한다.The inventors of the present invention have found that when a precipitate is precipitated to increase its strength while seeking to manufacture a high tensile hot rolled steel sheet having a low longitudinal yield strength deviation with only a small amount of alloying element and hot rolling control, It was confirmed through experiments that the longitudinal material can be greatly changed by appropriately managing the change and the distribution and size of the precipitates themselves, and the present invention was proposed based on the experimental results. In the present invention, Mo, Cr, V, Ti is added to provide a hot rolled sheet with a low longitudinal material deviation in the yield strength of 540 ~ 570MPa class. First, the reason for component limitation of the present invention steel will be described.

C의 함량은 0.03~0.10%가 바람직하다.The content of C is preferably 0.03 to 0.10%.

C는 강을 강화시키는데 가장 경제적이며 효과적인 원소로서, 그 함량이 0.03%미만이면 동일한 강도를 발휘시키기 위하여 다른 합금원소를 상대적으로 다량 첨가하여야 하므로 경제적이지 못하며, 0.10% 초과하면 용접성, 성형성 및 인성이 저하하므로 바람직하지 않다.C is the most economical and effective element for reinforcing steel, and if its content is less than 0.03%, it is not economical because other alloy elements must be added in order to exert the same strength, and if it exceeds 0.10%, weldability, formability and toughness It is unpreferable since it falls.

Si의 함량은 0.01~0.50%가 바람직하다.The content of Si is preferably 0.01 to 0.50%.

Si는 용강을 탈산시키고 고용강화원소로서, 그 함량이 0.01%미만이면 용강의 탈산역할을 충분히 하지 않아 청정한 강을 얻기 어려우며, 0.5% 초과하면 열간압연시 Si에 의한 붉은형 스케일이 형성되어 강판표면 형상이 매우 나쁘게 되며 연성도 저하되므로 바람직하지 않다.Si deoxidizes molten steel and is a solid solution element. If the content is less than 0.01%, it is difficult to obtain a clean steel because it does not sufficiently deoxidize the molten steel.If it exceeds 0.5%, a red scale formed by Si is formed during hot rolling. It is not preferable because the shape becomes very bad and the ductility is lowered.

Mn의 함량은 1.2~2.0%가 바람직하다.The content of Mn is preferably 1.2 to 2.0%.

Mn은 강을 고용강화시키는데 효과적인 원소로서 1.2%이상 첨가되어야 소입성 증가효과와 더불어 고강도를 발휘할 수 있다. 그러나, Mn의 함량이 2.0%초과하면 제강공정에서 슬라브를 주조시 두께 중심부에서 편석부가 크게 발달되고 최종제품의 용접성을 해치므로 바람직하지 않다.Mn is an effective element to solidify the steel to be added more than 1.2% can exhibit high strength with the effect of increasing the hardenability. However, if the Mn content exceeds 2.0%, the segregation part is greatly developed at the center of the thickness when casting the slab in the steelmaking process, which is not preferable because it impairs the weldability of the final product.

P의 함량은 0.03%이하가 바람직하지 않다.The content of P is preferably 0.03% or less.

P는 강 중에 존재하는 불순물 원소로서 Mn 등과 결합하여 비금속개재물을 형성하며 이에 따라 강의 인성 및 강도를 크게 손상시키므로 가능한 0.03%이하로 제한하는 것이 바람직하다.P is an impurity element present in the steel and forms a non-metallic inclusion by combining with Mn and the like, and thus, it is preferable to limit it to 0.03% or less as it greatly impairs the toughness and strength of the steel.

S의 함량은 0.01%이하가 바람직하다.The content of S is preferably 0.01% or less.

S도 역시 강 중에 존재하는 불순물 원소로서 Mn 등과 결합하여 비금속개재물을 형성하며 이에 따라 강의 인성 및 강도를 크게 손상시키므로 가능한 0.01%이하로 제한하는 것이 바람직하다.S is also an impurity element present in the steel and forms a non-metallic inclusion by combining with Mn and the like, and thus, it is preferable to limit it to 0.01% or less as it greatly impairs the toughness and strength of the steel.

Ti의 함량은 0.01~0.10%가 바람직하다.The content of Ti is preferably 0.01 to 0.10%.

Ti은 결정립을 미세화시키는데 아주 유용한 원소로서 강 중에 TiN으로 존재하여 열간압연을 위한 가열 과정에서 결정립의 성장을 억제하는 효과가 있으며 또한 질소와 반응하고 남은 Ti이 강 중에 고용되어 탄소와 결합하여 TiC의 석출물이 형성되고 TiC의 형성은 매우 미세하여 강의 강도를 대폭적으로 향상시킨다. 따라서, TiN 석출에 의한 오스테나이트 결정립 성장 억제 효과 및 TiC 형성에 의한 강도 증가를 얻기 위해서는 적어도 0.01%이상의 Ti이 첨가되어야 하나, 0.10% 초과이면 강판을 용접하여 강관으로 제조시 용융점까지 급열됨에 의해서 TiN이 재고용됨에 따라 용접 열영향부의 인성이 열화된다.Ti is a very useful element for refining grains. It exists as TiN in steel and has the effect of inhibiting the growth of grains during heating for hot rolling. Also, Ti remaining after being reacted with nitrogen is dissolved in carbon to bond with carbon. Precipitates are formed and the formation of TiC is very fine, greatly improving the strength of the steel. Therefore, in order to obtain the effect of inhibiting austenite grain growth due to TiN precipitation and increasing the strength due to TiC formation, at least 0.01% of Ti should be added. However, if it exceeds 0.10%, the steel sheet is welded to a melting point when the steel sheet is manufactured to be melted. As this is reused, the toughness of the weld heat affected zone deteriorates.

N의 함량은 0.01%이하가 바람직하다.The content of N is preferably 0.01% or less.

N의 성분 한정 사유는 상기의 Ti 첨가에 기인한 것이다. 일반적으로 N은 강중에 고용되었다가 석출되어 강의 강도를 증가시키는 역할을 하며 이러한 능력은 탄소보다도 훨씬 크다. 그러나, 한편으로 강 중에 질소가 존재하면 할수록 인성은 크게 저하하는 것으로 알려져 있어 가능한 한 질소 함유량을 감소시키려는 것이 일반적인 추세이다. 그러나, 본 발명에서는 적정량의 질소를 존재케하여 Ti과 반응시켜 TiN를 형성, 재가열 과정에서의 결정립 성장을 억제시키는 역할을 부여한다. 그런데, Ti의 일부는 N와 반응하지 않고 남아 이후의 공정에서 탄소와 반응하므로 그 상한을 0.01%이하로 하는 것이 바람직하다.The reason for component limitation of N is attributable to the above Ti addition. In general, N is employed in steel and precipitates to increase the strength of the steel, which is much greater than carbon. On the other hand, however, it is known that toughness decreases as more nitrogen exists in steel, and it is a general trend to reduce nitrogen content as much as possible. However, in the present invention, a proper amount of nitrogen is present to react with Ti to form TiN, thereby imparting a role of suppressing grain growth during reheating. By the way, since part of Ti does not react with N and reacts with carbon in a subsequent process, it is preferable to make the upper limit into 0.01% or less.

상기와 같이 조성되는 강에 Nb, V, Mo, Cr, Ni의 1종 또는 2종 이상을 첨가한다. 바람직하게는 이들 모두를 첨가하는 것이다.One or two or more of Nb, V, Mo, Cr, and Ni are added to the steel formed as described above. Preferably all of them are added.

Nb의 함량은 0.02~0.07%가 바람직하다.The content of Nb is preferably 0.02 to 0.07%.

Nb은 결정립을 미세화시키는데 아주 유용한 원소이며 동시에 강의 강도도 크게 향상시키는 역학을 하므로 0.02% 이상을 첨가하여야 하나, 0.07%를 초가하면 과도한 Nb 탄질화물의 석출에 기인되어 오스테나이트 미재결정온도를 지나치게 높여서 재질이방성을 증가시킨다.Nb is a very useful element for refining grains and at the same time, it has a dynamic effect that greatly improves the strength of steel. Therefore, more than 0.02% should be added. Increase material anisotropy

V의 함량은 0.10%이하가 바람직하다.The content of V is preferably 0.10% or less.

V은 Nb과 같이 탄질화물로 석출하여 강의 강도를 높이는데 유효한 원소로서 첨가하는데, 그 첨가량이 너무 많은 경우 석출량이 포화되어 강도 증가는 크지 않고 제조비용만 높아지므로 0.10%이하로 제한하는 것이 바람직하다.V is added as an effective element to increase the strength of the steel by precipitation as carbonitrides, such as Nb, but if the amount is too large, it is preferable to limit it to 0.10% or less since the precipitation amount is saturated, the strength increase is not large and the manufacturing cost is high. .

Mo의 함량은 0.1~0.3%가 바람직하다.The content of Mo is preferably 0.1 to 0.3%.

Mo은 소재의 강도를 상승시키는데 매우 유효하며, 저온변태 조직인 침상형(acicular) 페라이트 생성을 조장함에 의해 항복비를 낮추는 역할을 한다. 또한 시멘타이트와 탄화물이 집적되어 있어 열화한 충격특성을 보이는 펄라이트 조직의 생성을 억제하여 양호한 충격인성도 확보할 수 있다. 이를 위해 Mo은 0.1%이상을 첨가하여야 하나, 고가의 원소이므로 0.3%이하로 제한하는 것이 바람직하다.Mo is very effective in increasing the strength of the material, and serves to lower the yield ratio by promoting the formation of acicular ferrite, which is a low temperature metamorphic tissue. In addition, cementite and carbides are integrated to suppress the formation of pearlite structures exhibiting deteriorated impact characteristics, thereby ensuring good impact toughness. To this end, Mo should be added more than 0.1%, but is an expensive element is preferably limited to less than 0.3%.

Cr의 함량은 0.01~0.03%가 바람직하다.The content of Cr is preferably 0.01 to 0.03%.

Cr은 Mo과 동일한 작용을 하며, Mo과 복합첨가하면 상호 보완작용에 의해 저항복비와 우수한 저온인성을 동시에 얻는 것이 가능하므로 0.01-0.03%를 첨가하는 것이바람직하다.Cr has the same effect as Mo, and when added in combination with Mo, it is preferable to add 0.01-0.03% since it is possible to simultaneously obtain a resistance ratio and excellent low-temperature toughness by complementary action.

Ni의 함량은 0.1~0.5%가 바람직하다.The content of Ni is preferably 0.1 to 0.5%.

Ni은 오스테나이트 안정화 원소로서 펄라이트 형성을 억제하며, 저온변태 조직인 침상형(acicular) 페라이트 형성을 용이하게 하는 원소로, 이를 위해 0.1%이상 첨가하나 고가의 원소이므로 0.5%이하로 제한하는 것이 바람직하다.Ni is an austenite stabilizing element that inhibits the formation of pearlite and facilitates the formation of acicular ferrite, which is a low-temperature metamorphic structure. For this purpose, Ni is added at least 0.1%, but is preferably an elemental element, and is preferably limited to 0.5% or less. .

본 발명의 강은 상기 성분외에 나머지 기타 불가피한 불순물과 Fe로 조성된다.The steel of the present invention is composed of Fe and other remaining unavoidable impurities in addition to the above components.

다음으로 본 발명의 제조방법에 대해서 설명한다.Next, the manufacturing method of this invention is demonstrated.

상기와 같이 조성되는 슬라브를 재가열하여 열간압연하고, 권취한다. 본 발명에서는 열연 코일의 길이 방향 항복강도 편차는 열간압연 중에 생성된 미세조직 및 권취 전후에 형성된 석출물에 의해 결정된다는 점을 고려하여, 슬라브 재가열조건, 열간압연조건, 권취조건을 제어하는데 특징이 있다.The slab formed as above is reheated, hot rolled and wound up. In the present invention, in consideration of the fact that the longitudinal yield strength variation of the hot rolled coil is determined by the microstructure generated during hot rolling and the precipitate formed before and after winding, the slab reheating condition, the hot rolling condition, and the winding condition are characterized. .

먼저, 슬라브를 열간압연하기전에 재가열하는데, 재가열온도는 1250~1350℃가 바람직하다. 재가열온도가 1250℃미만이면 석출물이 충분히 재고용되지 않아 열간압연 이후의 공정에서 NbC 등의 석출물이 감소하게 된다. 또한, 재가열 온도가 너무 높으면 오스테나이트 결정립의 이상입성장에 의하여 강도가 저하되므로 재가열대 온도 상한은 1350℃로 하는 것이 좋다. 상기와 같이 재가열하고 슬라브를 추출하는데, 바람직하게는 추출 온도를 1170℃이상으로 한다. 추출온도는 추출물의 재고용을 조장하고 적당한 크기의 오스테나이트 결정립도를 유지하므로써 소재의 강도수준도 향상시키면서 코일의 길이 방향으로 균일한 미세조직을 얻기 위한 것이다.First, the slab is reheated before hot rolling, and the reheating temperature is preferably 1250 to 1350 ° C. If the reheating temperature is less than 1250 ° C, the precipitates are not sufficiently reused, and thus precipitates such as NbC are reduced in the process after hot rolling. If the reheating temperature is too high, the strength decreases due to abnormal grain growth of the austenite grains, so the upper limit of the reheating temperature is preferably 1350 ° C. Reheating as above and extracting the slab, Preferably the extraction temperature is 1170 ℃ or more. The extraction temperature is intended to obtain a uniform microstructure in the longitudinal direction of the coil while improving the strength level of the material by promoting the re-use of the extract and maintaining the austenite grain size of the appropriate size.

다음으로 가열된 슬라브를 열간압연하는데, 마무리압연온도(FDT;finish delivery temperature)는 780~820℃에서 하는 것이 바람직하다. 본 발명에서는 Mo, Cr, V, Ti이 복합첨가 되어 있으며, 이와 같은 석출원소의 석출강화에 의한 강도 증대 효과를 위해서 열간압연 마무리온도를 780-820℃의 범위로 정하는 것이다. 열간압연 마무리 온도가 780℃보다 낮으면, 열간압연 도중에 오스테나이트에 강한 변형집합조직이 형성되어 상변태에 의해 생성된 페라이트 및 침상형(acicular) 페라이트에도 강한 변태집합조직이 형성되게 된다. 한편, 열간압연 마무리 온도를 820℃보다 높게 설정하면 마무리 압연개시온도가 높게 되어 변태에 의해 생성되는 페라이트의 결정립이 미세하지 않게 되며, 그 결과 원하는 강도 수준을 나타내지 못하기 때문에 경제적이지 못하다.Next, the heated slab is hot rolled, and the finish delivery temperature (FDT) is preferably set at 780 to 820 ° C. In the present invention, Mo, Cr, V, Ti is a composite addition, the hot rolling finish temperature is set in the range of 780-820 ℃ for the strength increase effect by the precipitation strengthening of the precipitation element. When the hot rolling finish temperature is lower than 780 ° C, a strong deformation set structure is formed in austenite during hot rolling, so that a strong transformation set structure is formed even in the ferrite and acicular ferrite produced by phase transformation. On the other hand, when the hot rolling finish temperature is set higher than 820 ℃, the finish rolling start temperature is high and the grains of the ferrite produced by the transformation is not fine, as a result it is not economical because it does not exhibit the desired strength level.

다음으로 열연판을 권취하는데, 권취온도는 500-600℃가 바람직하다. 권취온도가 600℃보다 높으면 미세조직이 조대한 페라이트와 펄라이트로 형성되고 석출물이 증가하여 인성이 나빠지게 되며, 500℃보다 낮으면 베이나이트 변태가 일어나 강도는 증가하나 소재의 인성이 크게 저하하고 재질이방성도 증가하기 때문이다. 권취온도는 강판의 중앙부온도를 기준으로 한다.Next, the hot rolled sheet is wound, but the winding temperature is preferably 500-600 ° C. If the coiling temperature is higher than 600 ℃, the microstructure is formed of coarse ferrite and pearlite, and the precipitate increases, resulting in poor toughness. If it is lower than 500 ℃, the bainite transformation occurs to increase the strength, but the toughness of the material is greatly reduced and the material Anisotropy also increases. The coiling temperature is based on the central temperature of the steel sheet.

한편, 열간압연 후 공냉시 강판의 길이방향으로 선단부, 후단부, 중심부의 냉각이력이 다른 것은 여러 문헌을 통하여 알려져 있다. 대기와 직접 접촉하는 선후 단부의 냉각 속도가 대기와 접촉하지 않는 중심부 보다 빠르며, 그 결과 열연 코일 권취후 미세 Nb, V 석출물의 양이 코일 부위별로 달라지게 된다. 따라서, 본 발명에서는 권취할 때, 주수량을 조정하여 열연강판의 길이방향으로 선단부와 후단부가 중앙부 보다 30℃이상 높은 상태가 되도록 하는데, 이는 권취 후 코일 부위별 냉각 이력의 차이를 고려하여 설정한 것이다.On the other hand, it is known from the various documents that the cooling history of the front end, the rear end, and the center in the longitudinal direction of the steel sheet during hot-rolling after hot rolling is different. The cooling rate of the trailing end in direct contact with the atmosphere is faster than that of the center not in contact with the atmosphere. As a result, the amount of fine Nb and V precipitates varies after coiling after coiling. Therefore, in the present invention, when the winding, adjust the amount of water so that the front end and the rear end in the longitudinal direction of the hot-rolled steel sheet 30 ° C or more higher than the center portion, which is set in consideration of the difference in the cooling history of each coil site after winding will be.

본 발명에서는 선단부는 코일 최선단으로부터 10M이내, 후단부는 최후단으로부터 10M이내이며, 선단부와 후단부의 사이가 중앙부가 된다.In the present invention, the leading end is less than 10M from the coil leading end, the rear end is within 10M from the last end, and the center is between the leading end and the rear end.

이하, 본 발명을 실시 예를 통하여 보다 구체적으로 설명한다.Hereinafter, the present invention will be described in more detail with reference to Examples.

[실시예]EXAMPLE

표 1과 같은 화학성분을 갖는 발명강을 용해하여 슬라브로 제조한 후, 표 2의 조건으로 슬라브 재가열, 열간압연, 권취하여 판재를 제조하였다. 이 판재를 인장시험하여 그 결과를 표 3에 나타내었다. 표 2에서 강종 A와 B는 최종 코일 두께가 14.6mm이며, C와 D는 최종 코일 두께가 17.5mm로 압연하였다.After dissolving the invention steel having the chemical composition as shown in Table 1 to produce a slab, and slab reheating, hot rolling, and winding under the conditions of Table 2 to prepare a plate. Tensile tests were carried out on the sheet and the results are shown in Table 3. In Table 2, steel grades A and B had a final coil thickness of 14.6 mm, and C and D had a final coil thickness of 17.5 mm.

강종Steel grade CC SiSi MnMn PP SS NbNb VV NiNi CrCr MoMo TiTi NN AA 0.0610.061 0.210.21 1.61.6 0.0110.011 0.0010.001 0.0530.053 0.0510.051 0.240.24 0.020.02 0.220.22 0.0180.018 0.00690.0069 BB 0.0520.052 0.230.23 1.581.58 0.0110.011 0.0010.001 0.0570.057 0.0540.054 0.240.24 0.020.02 0.220.22 0.020.02 0.00570.0057 CC 0.0540.054 0.20.2 1.551.55 0.0090.009 0.0020.002 0.0530.053 0.0550.055 0.230.23 0.020.02 0.230.23 0.0230.023 0.00710.0071 DD 0.0540.054 0.20.2 1.551.55 0.0090.009 0.0020.002 0.0530.053 0.0550.055 0.230.23 0.020.02 0.230.23 0.0230.023 0.00710.0071

강종Steel grade 가열온도(℃)Heating temperature (℃) 추출온도(℃)Extraction temperature (℃) 마무리압연온도(℃)Finish rolling temperature (℃) 권취온도(℃)Winding temperature (℃) 두께(mm)Thickness (mm) 비고Remarks 선단부Tip 중앙부Center 후단부Rear end AA 12501250 11701170 801801 610610 600600 600600 14.614.6 발명강-비교압연Inventive Steel-Comparative Rolling BB 12511251 11701170 797797 590590 550550 580580 14.614.6 발명강-발명압연Inventive Steel-Invention Rolling CC 12561256 11701170 781781 620620 590590 620620 17.517.5 발명강-비교압연Inventive Steel-Comparative Rolling DD 12551255 11701170 788788 600600 520520 550550 17.517.5 발명강-발명압연Inventive Steel-Invention Rolling

강종Steel grade YP(MPa)YP (MPa) 비고Remarks 코일의 선단부Tip of coil 코일의 중앙부Central part of coil 코일의 후단부Rear end of coil AA -23-23 590590 -48-48 발명강-비교압연Inventive Steel-Comparative Rolling BB -28-28 580580 -20-20 발명강-발명압연Inventive Steel-Invention Rolling CC +34+34 590590 -11-11 발명강-비교압연Inventive Steel-Comparative Rolling DD -13-13 590590 -15.5-15.5 발명강-발명압연Inventive Steel-Invention Rolling

표 3에는 발명강의 비교압연재와 발명강의 발명압연재의 30도 방향 코일 중심부(MID)의 인장강도와 이 중심부의 인장강도에 대한 선단부(TOP) 및 후단부(BOT)의 인장강도 편차를 나타낸 것이다.Table 3 shows the tensile strength variation of the coil center (MID) in the 30-degree direction of the comparative rolled material of the inventive steel and the inventive rolled material, and the tensile strength deviations of the top and rear ends (BOT) with respect to the tensile strength of the core. will be.

발명강을 이용하여 비교압연한 강종 A와 C의 경우 코일 길이 방향 항복강도 편차가 11~34MPa로 매우 크다. 이에 반해, 발명강을 이용하여 발명압연 조건으로 제조한 강종 B와 D의 경우에는 코일 길이 방향 항복강도 편차가 15.5~28MPa로 비교재에 비하여 항복강도 편차가 감소하였음을 알 수 있다.For steel grades A and C, which were rolled using the inventive steel, the variation in yield strength of the coil in the longitudinal direction was 11 ~ 34MPa. On the contrary, in the case of steel grades B and D manufactured using the invention steel under the invention rolling condition, the yield strength variation of the coil in the longitudinal direction of the coil was 15.5 to 28 MPa, indicating that the yield strength variation decreased compared to the comparative material.

이와 같이, 본 발명에 따르면 코일 길이방향 재질편차가 강판두께 14.6mm의 경우 28MPa이하, 17.5mm의 경우 16MPa이하로 길이 방향 항복강도 편차가 적은 우수한 라인파이프용 열연강판을 얻을 수 있는 것이다.As described above, according to the present invention, an excellent line pipe hot rolled steel sheet having a low longitudinal yield strength variation of less than 28 MPa for a steel sheet thickness of 14.6 mm and 16 MPa for a sheet thickness of 17.5 mm can be obtained.

상술한 바와 같이, 본 발명에 따르면 페라이트 및 침상형(acicular) 페라이트 조직 형성과 동시에 미세한 석출물을 형성하여 길이 방향 항복강도 편차를 감소시킬 수 있는 것이다. 또한, 비교적 정확한 열간압연 조건을 사용함으로써 압연 생산성을 향상시킬 수 있으며, 코일 길이 방향 재질편차가 적으므로 부품 제조시의 실수율 및 생산성을 향상시킬 수 있어 제조원가의 저하를 도모할 수 있는 유용한 효과가 있는 것이다.As described above, according to the present invention it is possible to form a fine precipitate at the same time as the formation of ferrite and acicular ferrite tissue to reduce the longitudinal yield strength variation. In addition, it is possible to improve the rolling productivity by using a relatively accurate hot rolling condition, and because the coil longitudinal material deviation is small, it is possible to improve the error rate and productivity in manufacturing parts, which has a useful effect of reducing the manufacturing cost. will be.

Claims (1)

중량 %로, C:0.03-0.10%, Si:0.01%-0.50%, Mn:1.2-2.0%, P:0.03%이하, S:0.010%이하, Ti:0.01-0.10%, N:0.01%이하, Nb:0.02%-0.07%, V::.10%이하, Mo:0.01~0.3%, Cr:0.01~0.3%, Ni:0.1~0.5% 및 나머지 Fe와 기타 불가피한 불순원소들로 구성된 강슬라브를 1250-1350℃의 온도에서 재가열하고 780-820℃의 마무리 압연온도로 열간압연한 다음, 열연판의 길이방향으로 선단부와 후단부의 온도가 열연판의 중앙부 보다 30℃ 이상 높도록 주수량을 조절하여 500~600℃의 온도에서 권취하는 것을 포함하여 이루어지는 길이방향의 항복강도 편차가 적은 라인파이프용 열연강판의 제조방법.By weight%, C: 0.03-0.10%, Si: 0.01% -0.50%, Mn: 1.2-2.0%, P: 0.03% or less, S: 0.0110% or less, Ti: 0.01-1.10%, N: 0.01% or less , Steel slab consisting of Nb: 0.02% -0.07%, V: .10% or less, Mo: 0.01 ~ 0.3%, Cr: 0.01 ~ 0.3%, Ni: 0.1 ~ 0.5% and the remaining Fe and other unavoidable impurities After reheating at 1250-1350 ℃ and hot rolling to finish-rolling temperature of 780-820 ℃, adjust the amount of water so that the temperature of the front end and the rear end in the longitudinal direction of the hot rolled sheet is 30 ℃ higher than the center of the hot rolled sheet. Method for producing a hot-rolled steel sheet for line pipe with less variation in yield strength in the longitudinal direction, including winding at a temperature of 500 ~ 600 ℃.
KR1020020085889A 2002-12-28 2002-12-28 Method for manufacturing linepipe steel strips to minimize the deviation of yield strength through the longitudinal direction KR100946052B1 (en)

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KR100711464B1 (en) * 2005-12-15 2007-04-24 주식회사 포스코 A method for manufacturing hot rolled steel sheet for linepipe having low yield ratio and excellent yield strength anisotropic properties
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KR101091368B1 (en) * 2004-09-30 2011-12-07 주식회사 포스코 Method for manufacturing a high-strength hot rolled steel sheet for linepipe with superior DWTT characteristics at low temperature
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KR101091368B1 (en) * 2004-09-30 2011-12-07 주식회사 포스코 Method for manufacturing a high-strength hot rolled steel sheet for linepipe with superior DWTT characteristics at low temperature
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KR100711464B1 (en) * 2005-12-15 2007-04-24 주식회사 포스코 A method for manufacturing hot rolled steel sheet for linepipe having low yield ratio and excellent yield strength anisotropic properties
KR100957907B1 (en) * 2007-12-24 2010-05-13 주식회사 포스코 High Strength Ferritic Steel Sheet having Excellent Yield Strength and Low Temperature Toughness and Manufacturing Method Thereof
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JP2015175004A (en) * 2014-03-13 2015-10-05 Ｊｆｅスチール株式会社 Production method of high strength steel sheet excellent in formability

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