KR100306149B1 - Method for manufacturing hot rolled steel sheets with high tensile strength higher than 80kg/mm¬2 - Google Patents

Method for manufacturing hot rolled steel sheets with high tensile strength higher than 80kg/mm¬2 Download PDF

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KR100306149B1
KR100306149B1 KR1019970059209A KR19970059209A KR100306149B1 KR 100306149 B1 KR100306149 B1 KR 100306149B1 KR 1019970059209 A KR1019970059209 A KR 1019970059209A KR 19970059209 A KR19970059209 A KR 19970059209A KR 100306149 B1 KR100306149 B1 KR 100306149B1
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steel
temperature
rolling
toughness
strength
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KR1019970059209A
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KR19990039203A (en
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노광섭
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이구택
포항종합제철 주식회사
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B3/00Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02Rolling special iron alloys, e.g. stainless steel
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B1/00Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B37/00Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • B21B37/16Control of thickness, width, diameter or other transverse dimensions
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B37/00Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • B21B37/74Temperature control, e.g. by cooling or heating the rolls or the product
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B1/00Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B2001/225Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by hot-rolling
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B15/00Arrangements for performing additional metal-working operations specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B2015/0057Coiling the rolled product
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B2261/00Product parameters
    • B21B2261/20Temperature
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B2265/00Forming parameters
    • B21B2265/14Reduction rate

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

Abstract

PURPOSE: Provided is a method for manufacturing a hot rolled steel sheet with high tensile strength, which can obtain acicular ferritic structure by controlling steel components proper and regulating its reduction ratio, finish rolling temperature, and coiling temperature. CONSTITUTION: The method includes the steps of process rolling a slab comprising C 0.03- 0.10wt.%, Si 0.01-0.50wt.%, Mn 1.2-2.2wt.%, P 0.015wt.% or less, S 0.005wt.% or less, Al 0.01-0.10wt.%, Mo 0.2-0.7wt.%, Ni 0.1-0.5wt.%, Nb 0.02-0.10wt.%, Ti 0.02-0.10wt.%, N 0.01wt.% or less, Ca 0.001-0.005wt.%, balance Fe and other inevitable impurities in the temperature range of 750 to 1000°C at a reduction ratio of over 60% on the basis of total reduction ratio(TRR); finish rolling in the temperature range of 750 to 850°C where the finish rolling temperature and total reduction ratio satisfies the follow relation: TRR >= 0.13xFDT-32; quenching followed by coiling in the temperature range of 500 to 630°C.

Description

고장력 열연강판의 제조방법Manufacturing method of high tensile hot rolled steel sheet

본 발명은 건축, 라인파이프, 해양구조물 등의 용도로 사용하기에 적합한 고장력 열연강판의 제조방법에 관한 것이며, 보다 상세하게는 저온인성이 우수한 고장력 열연강판의 제조방법에 관한 것이다.The present invention relates to a method for producing a high tensile strength hot rolled steel sheet suitable for use in construction, line pipes, marine structures and the like, and more particularly, to a method for manufacturing a high tensile strength hot rolled steel sheet excellent in low temperature toughness.

북극해 또는 시베리아 등의 지역에서 사용되는 원유 또는 가스 수송용 강관이나 해양구조물에 사용되는 철강소재는 그 사용되는 환경의 가혹화로 말미암아 우수한 성질이 요구되고 있으며, 특히 저온에서의 파괴방지를 위하여 저온인성이 각별히 요구되고 있다. 이에 따라 우수한 저온인성을 지닌 고강도 강판을 제조하기 위해서 전세계에 걸쳐 많은 시도가 이루어져 왔고, 이에 대한 다수의 연구결과 및 특허가 제안되어 있다.Steel materials used for oil or gas transportation steel pipes and marine structures used in the Arctic Ocean or Siberia are required to have excellent properties due to the harsh environment, and especially low temperature toughness to prevent destruction at low temperatures. It is particularly required. Accordingly, many attempts have been made all over the world to manufacture high strength steel sheets having excellent low temperature toughness, and many research results and patents have been proposed.

그중에서 몇가지를 예시하면, 먼저 일본 특개소53-119219호에서는 저합금 고장력강을 퀸칭(quenching) 및 템퍼링(tempering)을 실시하여 미세한 페라이트와 베이나이트로 구성된 미세조직을 형성시키므로써 한랭지용 소재를 제조할 수 있다. 또한 일본 특개소52-14486호에서는 저온인성이 우수한 고장력 강판을 제조하기 위해 권취온도를 250℃까지 낮추는 방법을 제시하고 있으며, 일본 특개평04-325626호에서는 침상 페라이트 조직을 형성시킴으로써 저온인성을 향상시키는 방법을 제시하고 있다. 또한 일본 특개평1-15320호에서는 저온인성을 향상시키기 위한 칼슘, 산소, 황의 제어방법을 제시해 놓고 있다.To illustrate some of them, Japanese Patent Laid-Open No. 53-119219 first quenchs and tempers low-alloy high-strength steel to form a microstructure consisting of fine ferrite and bainite to produce a cold-cooled material. can do. In addition, Japanese Patent Application Laid-Open No. 52-14486 proposes a method of lowering the coiling temperature to 250 ° C in order to manufacture high tensile steel having excellent low temperature toughness, and Japanese Patent Laid-Open No. 04-325626 improves low temperature toughness by forming a needle-like ferrite structure. It shows how to make it. In addition, Japanese Patent Laid-Open No. 1-15320 proposes a control method of calcium, oxygen, and sulfur to improve low temperature toughness.

이와 같이 저온인성과 동시에 고강도를 얻기 위하여 다양한 방법을 주장하고 있으나, 상기의 방법 들은 그 어느 것도 열간압연 만으로, 인장강도 80kg/㎟이상을 나타내면서 극히 우수한 저온인성을 얻는 방법을 제시하고 있지는 않다. 특히 열처리에 의해 저온인성을 향상시키는 경우에는 열간압연만에 의한 경우보다 제조비용이 높은 단점이 있으며 권취온도를 낮추거나 냉각속도를 제어하는 경우에는 생산공정이 복잡한 문제점 들이 있다.As described above, various methods are insisted on obtaining low-temperature toughness and high strength, but none of the above methods suggest a method of obtaining extremely excellent low-temperature toughness while exhibiting a tensile strength of 80 kg / mm 2 or more by hot rolling alone. In particular, in the case of improving the low temperature toughness by heat treatment, there is a disadvantage in that the manufacturing cost is higher than in the case of only hot rolling, and when the winding temperature is lowered or the cooling rate is controlled, the production process is complicated.

따라서, 이러한 문제점을 야기시키지 않기 위해 열간압연만을 행하여 고장력 열연강판을 제조하고자 하는 연구가 행해졌으며, 그들중에서 예를들면, 일본 특허공개공보 소52-107225호, 소52-14486호 및 평04-325626호를 열거할 수 있다.Therefore, in order not to cause such a problem, studies have been conducted to manufacture high-strength hot rolled steel sheets by only performing hot rolling. Among them, Japanese Patent Laid-Open Publication Nos. 52-107225, 52-2486, and 04- 325626 may be listed.

상기 소52-107225호의 기술적 요지는, 탄소(C)량:0.03-0.08%, Si<0.6%, Mn:0.7-2.0%, P<0.010%, S<0.008%, Nb:0.01-0.10%, V:0.01-0.15%, Mo<0.50%, Ni<1.0%, Cr<1.0%, 및 Cu<1.0%를 함유한 강을 열간압연하여 850-600℃에서 마무리 압연을 하고 680-500℃에서 권취함을 특징으로 하는 것이다. 그러나 이와 같은 방법은 비교적 다량의 합금원소를 사용함과 동시에 마무리압연온도를 850-600℃정도로 낮추었음에도 불구하고 인장강도 60kg/㎟이상에 불과하여 합금원소 첨가의 최대효과 및 압연공정의 적절화가 다소 미흡하다.The technical gist of the above-mentioned SO 52-107225 is carbon (C) content: 0.03-0.08%, Si <0.6%, Mn: 0.7-2.0%, P <0.010%, S <0.008%, Nb: 0.01-1.10%, Hot rolled steel containing V: 0.01-0.15%, Mo <0.50%, Ni <1.0%, Cr <1.0%, and Cu <1.0% to finish-roll at 850-600 ° C and roll at 680-500 ° C. It is characterized by intoxication. However, this method uses a relatively large amount of alloying elements and at the same time lowers the finish rolling temperature to about 850-600 ° C, but the tensile strength is more than 60kg / mm2. Do.

또한, 상기 소52-14486호에서는 C:0.01-0.20%, Si:0.02-2.0%, Mn:0.5-2.5%, Mo:0.1-2.5% 정도를 첨가하여 열간압연을 하고 Ar3온도 이상에서부터 15℃/sec 이상의 속도로 냉각하여 250℃정도에서 권취를 하는 방법으로 합금성분계가 비교적 단순하나 권취온도가 250℃정도로 매우 낮기 때문에 매우 큰 용량의 수냉각 설비를 보유하고 있어야 하며 동시에 권취기의 부하 증가 때문에 열간압연 후 소재를 권취하여야 하는 열연공정에서는 그다지 적합한 방법이 아니다.In addition, in SO 52-14486, C: 0.01-0.20%, Si: 0.02-2.0%, Mn: 0.5-2.5%, and Mo: 0.1-2.5% are added to hot-roll and 15 to Ar 3 temperature or higher. It is a method of winding at about 250 ℃ by cooling at a rate of above ℃ / sec, but the alloy component system is relatively simple, but the coiling temperature is very low, such as around 250 ℃, so it must have a very large capacity water cooling system and at the same time increase the load of the winder Therefore, it is not a very suitable method in the hot rolling process where the material must be wound after hot rolling.

또한, 상기 소04-325626호에서는 C:0.03-0.06%, Si<0.5%, Mn:1.3-1.6%, Nb:0.02-0.06%, S<0.002%, Ca:0.0015-0.0050%를 함유한 강을 열간압연 함에 있어 Ar3-990℃정도의 온도 영역에서 적어도 60% 이상의 압하를 실시하고 10℃/sec이상의 냉각속도로 수냉각하여 500-600℃ 온도영역에서 권취함으로써 우수한 저온인성을 발휘시킬 수 있다고 하였다. 하지만, 인장강도 수준이 비교적 낮고, 저온인성 역시 낮으며, 또한 열간압연 조건을 명쾌히 제시하고 있지도 않다. 따라서, 합금원소 첨가와 압연조건을 최적화시킴으로써 효과를 극대화시되어 있지 않아 제조의 경제성이 다소 나쁜 것을 알 수 있다In addition, in the above-mentioned SO 04-325626, steel containing C: 0.03-0.06%, Si <0.5%, Mn: 1.3-1.6%, Nb: 0.02-0.06%, S <0.002%, Ca: 0.0015-0.0050% In hot rolling, at least 60% of pressure reduction can be performed in the temperature range of Ar 3 -990 ℃, and water-cooled at a cooling rate of 10 ℃ / sec or higher and wound in 500-600 ℃ temperature range to show excellent low temperature toughness. It was said. However, the tensile strength level is relatively low, the low temperature toughness is low, and the hot rolling conditions are not clearly presented. Therefore, the optimization of the alloying elements and the rolling conditions are not maximized by optimizing the effect.

..

이에, 본 발명은 강성분을 적절히 제어하고, 열간압연시 압하율 및 마무리온도를 제어하고 적정 권취온도를 설정함으로써, 석출물 입자가 미세하게 석출되어 있는 침상페라이트 조직을 얻어 인장강도 80kg/㎟이상의 저온인성이 우수한 고장력 열연강판을 제조하는 방법을 제공하고자 하는데, 그 목적이 있다.Therefore, according to the present invention, the steel component is properly controlled, the rolling reduction rate and the finishing temperature during hot rolling are set, and the appropriate winding temperature is set, thereby obtaining a needle-like ferrite structure in which precipitate particles are finely precipitated, and having a low tensile strength of 80 kg / mm 2 or more. It is an object of the present invention to provide a method for manufacturing a high tensile hot rolled steel sheet having excellent toughness.

도 1은 본 발명강의 열간압연조건 변화에 따른 충격인성을 나타낸 그래프1 is a graph showing the impact toughness according to the change of hot rolling conditions of the present invention steel

도 2는 본 발명강의 열간압연조건 변화에 따른 DWTT 인성의 변화를 나타낸 그래프2 is a graph showing a change in the DWTT toughness according to the change of hot rolling conditions of the present invention steel

일반적으로 강에 있어서 석출강화는 강도증가 효과는 탁월하지만 상대적으로 강의 인성(특히 저온인성)은 저하시키게 되는 것으로 알려져 있다. 그러나, 강도증가를 위해서 석출을 시키는 경우 저온인성은 일반적으로 감소하지만, 석출물 형성에 의한 미세조직의 변화와 석출물 자체의 분포 및 크기 등에 의해서 저온인성이 크게 변화될 수 있다. 이에 본 발명자는 동일한 성분으로도 압연 제조공정 변화에 따라 저온인성이 크게 변화되는 현상을 찾아내어, 석출 및 조직 미세화에 의한 강도 증가와 더불어 저온인성 역시 우수한 고장력 열연강판을 얻을 수 있는 방법을 알아내었다.In general, precipitation strengthening in steel is excellent in increasing strength but relatively low in toughness (particularly low temperature toughness). However, in the case of precipitation for increasing the strength, low-temperature toughness generally decreases, but low-temperature toughness may be greatly changed due to the change in the microstructure and the distribution and size of the precipitate itself. Accordingly, the present inventors found a phenomenon in which the low temperature toughness greatly changes according to the change of the rolling manufacturing process with the same component, and found a method of obtaining a high strength hot rolled steel sheet having excellent low temperature toughness as well as increasing strength due to precipitation and microstructure. .

상기한 바와같은 관점으로 부터 출발한 본 발명은 중량비로, C:0.03-0.10%, Si:0.01-0.50%, Mn:1.2-2.2%, P:0.015%이하, S:0.005%이하, Al:0.01-0.10%, Mo:0.2-0.7%, Ni:0.1-0.5%, Nb:0.02-0.10%, Ti:0.02-0.10%, N:0.01%이하, Ca:0.001-0.005%를 함유하고, 나머지 Fe 및 불가피한 불순물로 이루어진 강재를 750-1000℃의 온도에서 총압하율(TRR)에 대하여 60% 이상의 압연을 실시하고, 마무리 압연온도(FDT)를 750-850℃의 온도에서 실시하는 열간압연을 행하는데, 이때 상기 마무리압연온도와 총압하율은 "TRR ≥0.13 x FDT - 32"의 관계를 만족하여 열간압연을 행하고, 압연후 급냉하여 500-630℃온도에서 권취하는 고장력 열연강판의 제조방법에 관한 것이다.Starting from the above point of view, the present invention has a weight ratio of C: 0.03-0.10%, Si: 0.01-0.50%, Mn: 1.2-2.2%, P: 0.015% or less, S: 0.005% or less, Al: 0.01-0.10%, Mo: 0.2-0.7%, Ni: 0.1-0.5%, Nb: 0.02-0.10%, Ti: 0.02-0.10%, N: 0.01% or less, Ca: 0.001-0.005%, and the rest Steel rolling consisting of Fe and unavoidable impurities is subjected to hot rolling at a temperature of 750-1000 ° C. with respect to the total reduction ratio (TRR) of 60% or more and hot rolling at a finish rolling temperature (FDT) of 750-850 ° C. In this case, the finish rolling temperature and the total rolling reduction ratio is a hot rolling to satisfy the relationship of "TRR ≥ 0.13 x FDT-32", hot rolling, quenching after rolling and winding at a temperature of 500-630 ℃ It is about.

다음에서, 본 발명의 강 성분조성에 대한 수치한정이유를 설명한다.Next, the numerical limitation reason for the composition of the steel composition of the present invention will be described.

상기 C는 강을 강화시키는데 가정 경제적이며 효과적인 원소이나 너무 많이 첨가되면 용접성, 성형성 및 인성이 저하되기 때문에 본 발명에서는 0.03-0.10%로 한정하였다. 즉, 첨가량이 0.03% 미만이 되면 동일한 강도를 발휘시키기 위하여 다른 합금원소를 상대적으로 다량 첨가하여야 하기 때문에 경제적이지 못하며, 0.10%를 초과하여 첨가하면 용접성, 성형성 및 인성 저하 뿐만 아니라, 후술하는 본 발명과 같은 압연조건에서는 기지조직이 침상 페라이트로 조직으로부터 페라이트-펄라이트 조직이 형성되어 강도가 크게 저하되거나 상부 베이나이트 조직이 형성되어 인성이 저하하기 때문에 바람직하지 않다.The C is assumed to be 0.03-0.10% in the present invention because it is an economical and effective element for reinforcing steel, but when too much is added, weldability, formability and toughness are reduced. In other words, if the added amount is less than 0.03%, it is not economical to add a relatively large amount of other alloy elements in order to exhibit the same strength, and if it is added more than 0.10%, the weldability, formability and toughness decrease, as well as the following In the rolling condition as in the present invention, the base structure is not preferable because the ferrite-pearlite structure is formed from the needle-like ferrite structure and the strength is greatly reduced or the upper bainite structure is formed and the toughness is reduced.

상기 Si은 용강을 탈산시키기 위해서도 필요하고 고용강화원소로도 효과를 나타내므로 0.01-0.50%범위의 첨가가 필요하다. 즉, 첨가량이 0.01%미만에서는 용강의 탈산 역할을 충분히 하지 않기 때문에 청정한 강을 얻기 어려우며, 0.5%를 초과하여 첨가하면 열간압연시 Si에 의한 붉은형 스케일이 형성되어 강판표면 형상이 매우 나쁘게 되며 연성도 저하되기 때문에 바람직하지 않다.The Si is necessary to deoxidize molten steel and also has an effect as a solid solution strengthening element, so an addition in the range of 0.01-0.50% is required. In other words, if the added amount is less than 0.01%, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel.If it is added more than 0.5%, the red scale is formed by Si during hot rolling, and the surface of the steel sheet becomes very bad and ductile. It is also unpreferable because it also falls.

상기 Mn은 강을 고용강화시키는데 효과적인 원소로서 1.2%이상 첨가되어야 소입성 증가효과와 더불어 고강도를 발휘할 수 있다. 그러나 2.2%를 초과하여 첨가시키면 제강공정에서 슬라브로 주조시 두께 중신부에서 편석부가 크게 발달되고 최종제품의 용접성을 해치기 때문에 바람직하지 않다.The Mn is an element effective to strengthen the solid solution of the steel should be added more than 1.2% to exhibit high strength with the effect of increasing the hardenability. However, the addition of more than 2.2% is not preferable because the segregation part in the thickness middle body during the casting into slab in the steelmaking process greatly develops and damages the weldability of the final product.

상기 P는 강 중에 존재하는 불순물 원소로서 강 내부에 중심 편석 및 미소편석이 크게 일어나게 하여 강의 인성을 저하시키기 때문에 없으면 없을수록 바람직하지만 0.015% 이하로 제어하면 이러한 P의 악영향은 상쇄될 수 있기 때문에 그 상한을 0.015%로 한다.P is an impurity element present in the steel, which causes the center segregation and the micro segregation to occur largely in the steel, thereby lowering the toughness of the steel, so it is preferable that it is absent, but when controlled to 0.015% or less, the adverse effect of P may be offset The upper limit is made 0.015%.

상기 S도 역시 강중에 존재하는 불순물 원소로서 Mn 등과 결합하여 비금속개재물을 형성하며 이에 따라 강의 인성 및 강도를 크게 손상시키기 때문에 가능한한 감소시키는 것이 바람직하다. 특히 본 발명에서는 후술되는 Ca을 동시에 첨가하여 비금속 개재물을 감소시킴과 동시에 개재물의 형상도 구상화시켜 줌으로써 S의 악영향을 극소화시키고자 하였으며, S 첨가의 한계량을 0.002%로 정하였다.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 desirable to reduce the toughness and strength of the steel as much as possible. In particular, the present invention was intended to minimize the adverse effect of S by simultaneously adding Ca to be described later to reduce non-metallic inclusions and to shape the inclusions at the same time, and the limit of S addition was set at 0.002%.

상기 Al은 탈산원소로서 강의 불순물을 저감시키는 역할을 하고 오스테나이트 결정립의 조대화를 억제하는데 유효하기 때문에 0.01% 이상의 첨가가 필요하나, 0.1% 를 초과하여 과도하게 첨가하면 개재물을 다량으로 형성시켜 인성이 저하되기 때문에 상한을 0.1%로 제한하여 강의 청정성을 향상시키도록 하였다.Al is a deoxidation element and serves to reduce impurities in the steel and is effective in suppressing coarsening of austenite grains. Therefore, it is required to add more than 0.01%, but when excessively added in excess of 0.1%, toughness is formed by forming a large amount of inclusions. Since this falls, the upper limit is limited to 0.1% to improve the cleanliness of the steel.

상기 Mo는 소입성 향상효과가 매우 크며 동시에 고용강화 정도로 매우 크기 때문에 많이 첨가될수록 강도는 증가되나 0.5%를 초과하면 용접 열영향부의 인성이 저하되고 경제성이 상실되므로 상한을 0.5%로 정하였으며, 0.2%미만을 첨가하는 경우에는 저온인성이 우수한 미세조직인 침상페라이트 조직을 본 발명 조건하에서 형성시키기 어렵고, 강도도 낮기 때문에 적어도 0.2% 이상을 첨가한다.Since Mo is very hard to improve the hardenability and at the same time, it is very large enough to enhance the solid solution, the more the strength is increased, but if it is more than 0.5%, the toughness of the weld heat affected zone is lowered and the economic efficiency is lost. When less than% is added, acicular ferrite structure, which is a microstructure having excellent low temperature toughness, is difficult to form under the conditions of the present invention, and since the strength is low, at least 0.2% or more is added.

상기 Ni은 강의 인성을 향상시키는 역할을 하며 연주시의 균열 발생을 방지하는 작용이 있기 때문에 Ni 첨가량이 0.1% 보다 작으면 열간취성 억제가 미흡하고 0.5% 를 초과하여 첨가하면 고가의 원소첨가에 의한 경제성을 잃어버리기 때문에 Ni 첨가량의 범위를 0.1-0.5%로 하였다.Since Ni plays a role of improving the toughness of the steel and prevents the occurrence of cracks during playing, when the Ni content is less than 0.1%, the hot brittleness is insufficient and when the Ni content exceeds 0.5%, it is economical due to expensive element addition. In order to lose the amount of Ni, the amount of Ni added was 0.1-0.5%.

상기 Nb은 결정립을 미세화시키는데 아주 유용한 원소이며 동시에 강의 강도도 크게 향상시키는 역할을 하기 때문에 적어도 0.02%이상을 첨가하여야 하며 0.10%를 초과하는 경우에는 과도한 Nb 탄질화물의 석출에 기인되어 저온인성이 나빠지기 때문에 0.01-0.10%를 첨가범위로 하였다.Since Nb is a very useful element for refining grains and at the same time plays a role of greatly improving the strength of steel, at least 0.02% or more should be added. If it exceeds 0.10%, Nb may have low temperature toughness due to excessive precipitation of Nb carbonitride. Since it fell out, 0.01-0.10% was made into addition range.

상기 Ti도 결정립을 미세화시키는데 아주 유용한 원소로써 강중에 TiN으로 존재하여 열간압연을 위한 가열과정에서 결정립의 성장을 억제하는 효과가 있으며 또한 질소와 반응하고 남은 Ti은 강 중에 고용되어 있는 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 0.02%이상의 Ti이 첨가되어야 하며, 0.1%를 초과하여 첨가되면 강판을 용접하여 강관등으로 제조시 용융점까지 급열됨에 의해서 TiN이 재고용됨에 따라 용접 열영향부의 인성이 열화되기 때문에 Ti 첨가의 상한은 0.1%로 한다.The Ti is also a very useful element to refine the crystal grains and is present in the steel as TiN to inhibit the growth of the grains during the heating process for hot rolling. Also, the remaining Ti reacts with nitrogen and binds to the carbon dissolved in the steel. A precipitate of TiC is formed and the formation of TiC is very fine, which greatly improves 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 0.02% of Ti should be added. If the content exceeds 0.1%, the steel sheet is welded to the melting point when manufacturing the steel pipe. Since the toughness of the weld heat affected zone deteriorates as TiN is re-used by the rapid heating, the upper limit of Ti addition is made 0.1%.

상기 N의 성분 한정 사유는 상기의 Ti 첨가에 기인한 것이다. 일반적으로 N 은 강 중에 고용되었다가 석출되어 강의 강도를 증가시키는 역할을 하며 이러한 능력은 탄소보다도 훨씬 크다. 그러나 한편으로 강중에 질소가 존재하면 할수록 강의 인성은 크게 저하하는 것으로 알려져 있어 가능한한 질소 함유량을 감소시키려는 것이 일반적인 경향이다. 그러나 본 발명에서는 적정량의 질소를 존재케 하여 Ti과 반응시켜 TiN을 형성, 재가열 과정에서의 결정립 성장을 억제시키는 역할을 부여하였다. 그러나 Ti의 일부는 N과 반응하지 않고 남아 이후의 공정에서 탄소와 반응하여야 하기 때문에 그 상한을 0.01% 이하로 하였다.The reason for component limitation of said N is due to said Ti addition. In general, N is dissolved in steel and precipitated to increase the strength of the steel, which is much larger than carbon. On the other hand, however, it is known that the more nitrogen exists in the steel, the tougher the steel is. 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. However, since part of Ti does not react with N and must react with carbon in a subsequent process, the upper limit thereof is set to 0.01% or less.

상기 Ca은 개재물 형성제어 원소이며 저온인성의 개선 및 연성을 향상시킨다. 이러한 효과를 발휘하기 위해서는 적어도 0.001% 이상의 첨가가 필요하나, 0.005%를 초과하여 첨가하여도 그 효과가 향상되지 않는다.Ca is an inclusion formation controlling element and improves low temperature toughness and ductility. In order to achieve such an effect, at least 0.001% or more of addition is required, but addition of more than 0.005% does not improve the effect.

다음에서, 상기한 바와같은 강을 이용하여 열연강판을 제조하는 방법을 상세히 설명한다.Next, a method of manufacturing a hot rolled steel sheet using steel as described above will be described in detail.

일반적으로 총압하율은 700-1150℃에서 70-99%로 실시되는데, 본 발명에서는 상기와 같은 성분계의 강을 열간압연함에 있어 750-1000℃의 온도영역에서 총 압하율(TRR : total reduction ratio) 60% 이상의 압연을 실시하고 열간압연 마무리 온도(FDT : finish delivery temperature)를 750-850범위로 하는데, 이때 열간압연 마무리 온도와 총 압하율의 관계가 "TRR ≥ 0.13 x FDT-32"의 관계를 만족하도록 열간 압연한다.In general, the total reduction ratio is carried out at 70-99% at 700-1150 ° C. In the present invention, the total reduction ratio (TRR) in the temperature range of 750-1000 ° C in hot rolling of the component steel as described above. ) Rolling more than 60% and hot rolling finish temperature (FDT) is in the range of 750-850, where the relationship between hot rolling finish temperature and total rolling rate is "TRR ≥ 0.13 x FDT-32" Hot rolled to satisfy.

일반적으로, 열간압연 공정은 크게 두가지로 구분되어 상업 생산되고 있는데, 본 발명이 적용되는 공정과 같이 코일(coil)을 제조하는 열연공정과 플레이트(plate)를 제조하는 후판공정이 있다. 코일을 제조하는 열연공정에서는 마무리압연기가 6-7대 연속적으로 설치되어있으며, 따라서 열간압연 마무리 온도와 마무리 압연개시온도와의 차이는 통상적으로 최대 200℃를 벗어나지 않는다. 따라서 열간압연 마무리 온도를 본 발명의 최대온도인 850℃로 하여도 마무리 압연 개시온도는 1050℃로 나타난다.In general, the hot rolling process is largely divided into two commercially produced, there is a hot rolling process for producing a coil (coil) and a thick plate process for manufacturing a plate (plate) as the process to which the present invention is applied. In the hot rolling process for manufacturing coils, six to seven finishing mills are installed continuously. Therefore, the difference between the hot rolling finish temperature and the finish rolling start temperature does not normally exceed 200 ° C. Therefore, even if the hot rolling finish temperature is 850 ° C, which is the maximum temperature of the present invention, the finish rolling start temperature is 1050 ° C.

본 발명에서는 Nb, Ti가 복합첨가되어 있으며 이와 같은 석출원소는 마무리 열간압연 온도역에서 석출하여 오스테나이트의 재결정을 억제하므로써 결정립을 미세화시킴과 동시에 석출강화에 의한 강도 증대 효과가 크기 때문에 저온인성을 향상시키기 위해서는 열간압연 온도를 가능한한 낮게 함에 바람직하다. 따라서 본 발명에서는 오스테나이트의 미재결정 영역에서 압연을 하기 위하여 마무리 열간압연 개시온도와 종료온도의 범위를 750-1000℃로 하였으며 열간압연 마무리온도를 750-850℃의 범위로 정하였다. 열간압연 마무리온도가 750℃보다 낮으면, 열간압연 도중에 소재가 오스테나이트/페라이트 변태온도(Ar3) 이하로 떨어지게 되는 경우가 발생되며 이 경우 소재의 미세조직은 소위 '혼립'이 형성되어 저온인성을 급격히 저하시키기 때문에 그 하한을 750℃로 하였다. 한편 열간압연 마무리 온도를 850℃보다 높게 설정하면 마무리 압연 개시온도가 높게 되어 오스테나이트의 재결정이 유발되며 오스테나이트의 재결정은 직후의 급속한 결정립 성장을 일으키고 이 역시 저온인성을 해치는 작용을 한다.In the present invention, Nb and Ti are added in combination, and the precipitated element is precipitated at the final hot rolling temperature zone to suppress the recrystallization of austenite, thereby miniaturizing the grains and at the same time increasing the strength due to precipitation strengthening. In order to improve, it is desirable to make the hot rolling temperature as low as possible. Therefore, in the present invention, in order to roll in the unrecrystallized region of austenite, the finish hot rolling start temperature and finish temperature were set to 750-1000 ° C., and the hot rolling finish temperature was set to 750-850 ° C. If the hot rolling finish temperature is lower than 750 ° C, the material may fall below the austenite / ferrite transformation temperature (Ar 3 ) during the hot rolling, and in this case, the microstructure of the material may form a so-called 'mix', thereby causing low temperature toughness. The lower limit was made 750 degreeC in order to reduce rapidly. On the other hand, when the hot rolling finish temperature is set higher than 850 ° C, the finish rolling start temperature becomes high, which causes recrystallization of austenite, and recrystallization of austenite causes rapid grain growth immediately afterwards, which also impairs low temperature toughness.

따라서, 강의 인성을 향상시키기 위해서는 열간압연 마무리온도를 낮출수록 바람직하나 압연온도의 저하에 따라 압연기에 부가되는 부하가 크게 증가하기 때문에, 본 발명에서는 마무리 열간압연 역에서의 총 압하율과 마무리 압연온도와의 관계를 해석하여, "TRR ≥ 0.13xFDT-32"의 관계를 밝혀 내고 마무리압연온도의 변화에 따라 적절한 총압하율을 설정할 수 있도록 하였다.Therefore, in order to improve the toughness of the steel, it is preferable to lower the hot rolling finish temperature, but the load added to the rolling mill increases greatly as the rolling temperature decreases. Therefore, in the present invention, the total reduction ratio and the finish rolling temperature in the finish hot rolling region are increased. By analyzing the relationship with, the relationship between "TRR ≥ 0.13xFDT-32" was revealed and the appropriate total pressure reduction rate could be set according to the change of finish rolling temperature.

예를들어, 강관용 고장력강판은 일반적으로 두께 7-22mm 정도로 열연공정에서는 가장 두꺼운 제품에 속하기 때문에 총압하율이 60%이하가 될 가능성이 있으나, 마무리압연에 인입되는 슬라브의 두께를 조절하여 적어도 60% 이상의 총압하율을 유지하여야 저온인성이 우수한 강판을 제조할 수 있다.For example, high tensile steel sheet for steel pipe is generally 7-22mm thick, which is one of the thickest products in the hot rolling process, so the total reduction ratio may be less than 60%, but by adjusting the thickness of the slab that is introduced into the finish rolling A steel sheet having excellent low temperature toughness can be manufactured only by maintaining a total reduction ratio of at least 60%.

그러나, 무작정의 총압하율 증가는 전술한 바와같이 압연기에 부가되는 부하의 급속한 증가로 불가능하기 때문에 압연온도와 총압하율의 관계를 면밀히 분석하여 최적화시켜주는 것은 반드시 필요하다.However, it is necessary to closely analyze and optimize the relationship between the rolling temperature and the total pressure reduction rate because it is impossible to inadvertently increase the total pressure reduction rate by the rapid increase of the load added to the rolling mill as described above.

본 발명에서는 열간압연을 마무리한 후, 런-아웃 테이블 상에서 수냉각 등에 의한 급냉을 실시하므로써 오스테나이트의 결정립 성장을 억제하고 동시에 페라이트의 조대화를 방지하므로써 인성저하를 억제한다. 또한 권취온도는 500-630온도범위가 적정한데, 630℃보다 높으면 미세조직이 침상의 페라이트로 형성되지 않고 페라이트-펄라이트 조직으로 변화되고 석출물도 조대해져 소재의 강도가 크게 저하되며 500℃보다 낮으면 베이나이트 변태가 일어나 강도는 증가하나 소재의 인성이 크게 저하하고 석출도 불충분하기 때문이다.In the present invention, after hot rolling is finished, quenching by water cooling or the like on the run-out table suppresses grain growth of austenite and at the same time suppresses the decrease in toughness by preventing coarsening of ferrite. In addition, the winding temperature is appropriate in the temperature range of 500-630. If the temperature is higher than 630 ° C, the microstructure does not form as a needle-like ferrite, but changes into a ferrite-pearlite structure, and precipitates are coarsened. This is because bainite transformation occurs to increase strength, but the toughness of the material is greatly reduced and precipitation is insufficient.

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

실시예1Example 1

하기 표1에 나타낸 바와같은 화학성분을 지닌 발명강과 비교강을 용해하여 슬라브로 제조하였다.The inventive steel and the comparative steel with chemical components as shown in Table 1 were dissolved to prepare a slab.

강종Steel grade 화 학 성 분(wt%)Chemical component (wt%) CC SiSi MnMn MoMo NiNi NbNb TiTi NN CaCa 비교강AComparative Steel A 0.020.02 0.240.24 1.551.55 0.300.30 0.180.18 0.0970.097 0.0010.001 0.00070.0007 0.00220.0022 비교강BComparative Steel B 0.060.06 0.230.23 1.531.53 0.320.32 0.210.21 0.0980.098 0.0010.001 0.00400.0040 0.00280.0028 비교강CComparative Steel C 0.070.07 0.280.28 1.601.60 0.010.01 0.790.79 0.0440.044 0.0250.025 0.00600.0060 0.00230.0023 발명강DInventive Steel D 0.080.08 0.230.23 1.531.53 0.390.39 0.220.22 0.0520.052 0.0550.055 0.00450.0045 0.00210.0021 발명강EInventive Steel E 0.070.07 0.250.25 1.551.55 0.330.33 0.200.20 0.0790.079 0.0330.033 0.00340.0034 0.00220.0022

상기 표1의 비교강 A는 탄소량이 0.02%로 작고 Ti도 0.001%로 매우 작으며, 비교강 B는 Ti첨가량이 0.001%로 매우 작으며, 비교강 C는 Mo첨가량이 0.01%로 매우 작기 때문에, 비교강으로 선정된 것이다.Comparative steel A of Table 1 is very small carbon amount of 0.02% and Ti is 0.001%, Comparative steel B is very small Ti content of 0.001%, and comparative steel C is very small 0.01% Mo addition It was selected as a comparative steel.

상기와 같은 조성의 슬라브를 열간압연하여 판재로 제조하였다. 이때 열간압연 마무리온도는 750-900℃의 범위에서 변화시켰으며, 열간압연 후의 런아웃 테이블(run-out table)에서의 수냉각속도는 통상의 현장생산공정에서의 냉각유량을 사용하였는데 약 5℃/s 이상으로 측정되었다. 권취온도는 강종에 따라 다르게 적용하였으며 500-690℃의 범위에서 제어하였다. 즉, 하기 표2와 같은 마무리압연온도(FDT), 총압하율(TRR) 및 권취온도(CT)조건으로 열연강판을 제조하였다.The slab of the composition as described above was hot rolled to prepare a plate. At this time, the hot rolling finish temperature was changed in the range of 750-900 ℃, and the water cooling rate in the run-out table after hot rolling was about 5 ℃ / It was measured above s. The coiling temperature was applied differently according to the steel grade and controlled in the range of 500-690 ℃. That is, a hot rolled steel sheet was manufactured under the conditions of finish rolling temperature (FDT), total rolling reduction (TRR), and winding temperature (CT) as shown in Table 2 below.

강종Steel grade FDT(℃)FDT (℃) 총압하율(TRR), %Total Pressure Drop (TRR),% CT(℃)CT (℃) 비고Remarks 비교예1Comparative Example 1 비교강AComparative Steel A 820820 7575 550550 비교강-발명압연Comparative Steel-Invention Rolling 비교예2Comparative Example 2 비교강BComparative Steel B 800800 7474 580580 비교강-발명압연Comparative Steel-Invention Rolling 비교예3Comparative Example 3 비교강CComparative Steel C 810810 7676 510510 비교강-발명압연Comparative Steel-Invention Rolling 비교예4Comparative Example 4 발명강DInventive Steel D 900900 8585 580580 발명강-비교압연Inventive Steel-Comparative Rolling 비교예5Comparative Example 5 발명강DInventive Steel D 850850 6565 570570 발명강-비교압연Inventive Steel-Comparative Rolling 비교예6Comparative Example 6 발명강DInventive Steel D 830830 7676 660660 발명강-비교압연Inventive Steel-Comparative Rolling 발명예7Inventive Example 7 발명강DInventive Steel D 820820 8080 575575 발명강-발명압연Inventive Steel-Invention Rolling 발명예8Inventive Example 8 발명강DInventive Steel D 780780 7272 600600 발명강-발명압연Inventive Steel-Invention Rolling 비교예9Comparative Example 9 발명강EInventive Steel E 900900 8585 560560 발명강-비교압연Inventive Steel-Comparative Rolling 비교예10Comparative Example 10 발명강EInventive Steel E 800800 6060 530530 발명강-비교압연Inventive Steel-Comparative Rolling 비교예11Comparative Example 11 발명강EInventive Steel E 850850 8080 690690 발명강-비교압연Inventive Steel-Comparative Rolling 발명예12Inventive Example 12 발명강EInventive Steel E 850850 8080 600600 발명강-발명압연Inventive Steel-Invention Rolling 발명예13Inventive Example 13 발명강EInventive Steel E 800800 7373 560560 발명강-발명압연Inventive Steel-Invention Rolling

상기 표2에 나타낸 바와같이, 비교예(1)-비교예(3)은 비교강을 사용하여 본 발명조건으로 열간압연한 것이고, 그외의 비교예 및 발명예는 발명강을 사용하여 열간압연조건을 변화시킨 것이다. 즉, 비교예(4) 및 (9)는 발명강D 및 발명강 E을 열간압연함에 있어 열간압연 마무리온도가 본 발명조건 보다 높은 경우이며, 비교예(5) 및 비교예(10)은 마무리 압연역에서의 총압하율이 본 발명범위보다 작은 경우이고, 비교예(6) 및 (11)은 본 발명 범위의 권취온도보다 높은 것이다.As shown in Table 2, Comparative Example (1) -Comparative Example (3) was hot rolled under the present invention conditions using a comparative steel, and other Comparative Examples and Invention Examples were hot-rolled conditions using the inventive steel. Is a change. That is, Comparative Examples (4) and (9) are cases in which the hot rolling finish temperature is higher than the conditions of the present invention in hot rolling the inventive steels D and E, and the comparative examples (5) and (10) are finished. The total pressure drop rate in the rolling zone is smaller than the range of the present invention, and Comparative Examples (6) and (11) are higher than the winding temperature of the range of the present invention.

상기에서 얻어진 열연강판의 인장성질, 충격인성 및 미세조직 등을 측정 및 관찰하여 그 결과를 하기 표3에 나타내었다. 이때, 인장시험은 표점거리 50mm의 표준시험이고, 충격시험은 샤피 서브 사이즈를 7.5mmt로 하였다.Tensile properties, impact toughness and microstructure of the hot rolled steel sheet obtained above were measured and observed, and the results are shown in Table 3 below. At this time, the tensile test is a standard test of 50mm gauge, and the impact test was made 7.5mmt Charpy sub-size.

항복강도(YS)(kg/㎟)Yield strength (YS) (kg / mm2) 인장강도(TS)(kgf/㎟)Tensile Strength (TS) (kgf / ㎡) 총연신율(t.E1)(%)Total Elongation (t.E1) (%) 충격인성(vE-60℃)(Joules)Impact Toughness (vE -60 ℃ ) (Joules) 충격천이온도(℃)Impact Transition Temperature (℃) 미세조직Microstructure 비교예1Comparative Example 1 64.864.8 70.770.7 25.925.9 9595 -70-70 침상 페라이트Couch ferrite 비교예2Comparative Example 2 72.572.5 77.577.5 24.824.8 2525 -25-25 침상 페라이트Couch ferrite 비교예3Comparative Example 3 67.367.3 74.274.2 24.524.5 6565 -40-40 페라이트 + 베이나이트Ferrite + Bainite 비교예4Comparative Example 4 67.367.3 88.588.5 19.419.4 5555 -55-55 침상 페라이트Couch ferrite 비교예5Comparative Example 5 69.569.5 86.286.2 20.920.9 5555 -65-65 침상 페라이트Couch ferrite 비교예6Comparative Example 6 63.263.2 78.478.4 24.924.9 7070 -50-50 페라이트 + 펄라이트Ferrite + Pearlite 발명예7Inventive Example 7 74.274.2 92.092.0 19.119.1 9494 -75-75 침상 페라이트Couch ferrite 발명예8Inventive Example 8 70.370.3 86.986.9 22.322.3 108108 -100-100 침상 페라이트Couch ferrite 비교예9Comparative Example 9 68.468.4 87.787.7 23.223.2 5050 -50-50 침상 페라이트Couch ferrite 비교예10Comparative Example 10 71.971.9 91.291.2 18.418.4 7575 -60-60 침상 페라이트Couch ferrite 비교예11Comparative Example 11 62.962.9 77.277.2 25.725.7 6565 -55-55 페라이트 + 펄라이트Ferrite + Pearlite 발명예12Inventive Example 12 71.571.5 84.584.5 20.720.7 101101 -85-85 침상 페라이트Couch ferrite 발명예13Inventive Example 13 73.473.4 85.885.8 21.221.2 105105 -95-95 침상 페라이트Couch ferrite

상기 표 3에 나타난 바와같은 내용을 분석해 봄으로써, 다음과 같은 결과를 얻을 수 있었다.By analyzing the contents as shown in Table 3, the following results were obtained.

비교예(1)에 있어서는 다른 화학조성은 본 발명강과 유사하나 C 및 Ti이 첨가량이 작기 때문에 압연 및 냉각조건을 발명범위 내로 하더라도 본 발명강에 비해 인장강도가 작게 나타나고 있어 C 첨가에 의한 변태조직 강화효과가 소재의 강도를 비약적으로 향상시키고 있음을 알 수 있다.In Comparative Example (1), the other chemical composition is similar to that of the present invention steel, but since the addition amount of C and Ti is small, even if the rolling and cooling conditions are within the range of the invention, the tensile strength is smaller than that of the present invention. It can be seen that the reinforcing effect dramatically improves the strength of the material.

비교예(2)는 Ti 첨가량이 적은 비교강B를 본 발명 범위로 압연한 경우로써, 인장강도가 증가되어 77.5kg/㎟을 나타내고 있다. 그러나 아직 본 발명에서 목적하는 인장강도 값인 80kg/㎟에는 미달하고 있으며, 또한 Nb 단독 역할에 의한 과도한 석출경화로 항복비(=항복강도/인장강도)가 0.94로 매우 높고 충격인성 역시 저하되어 충격천이온도가 -25℃에 불과한 것을 알 수 있다. 이로부터 Nb 와 Ti의 적절한 조합에 의한 석출경화가 충격인성의 저하를 유발치 않으면서 강도를 극대화시킬 수 있음을 보여준다.Comparative Example (2) is a case where the comparative steel B with a small amount of Ti is rolled in the present invention range, and the tensile strength is increased to show 77.5 kg / mm 2. However, it is still less than 80kg / mm2, which is the target tensile strength value in the present invention, and the yield ratio (= yield strength / tensile strength) is very high as 0.94 due to excessive precipitation hardening by the role of Nb alone, and the impact toughness is also lowered. It can be seen that the temperature is only -25 ℃. From this, precipitation hardening by proper combination of Nb and Ti can maximize the strength without causing impact toughness deterioration.

비교예(3)은 Mo 첨가량이 작은 비교강C 를 본 발명 범위로 압연한 경우로써, 미세조직이 페라이트와 베이나이트로 혼재되어 있으며, 이에 따라 인장강도가 낮고 저온충격인성 값이 매우 작으며 충격천이온도로 높게 나타나고 있다. Mo 가 첨가되지 않으면 오스테나이트/페라이트 변태온도가 높게 되고 강의 소입성이 저하되어 소재의 강도는 변태강화에 의존되기 때문에 고온권취의 경우 페라이트+펄라이트 조직이 형성되어 강도가 낮으며 저온권취의 경우에는 강도는 증가되더라도 저온충격인성이 매우 나쁜 것이다.Comparative Example (3) is a case in which the comparative steel C having a small amount of Mo is rolled in the present invention, and the microstructure is mixed with ferrite and bainite, and thus the tensile strength is low, the low temperature impact toughness is very small, and the impact is low. The transition temperature is high. If Mo is not added, the austenite / ferrite transformation temperature is high, and the hardenability of the steel is lowered, and the strength of the material depends on the transformation strengthening. Therefore, in the case of high temperature winding, ferrite + pearlite structure is formed, and the strength is low. Even if the strength is increased, the low temperature impact toughness is very bad.

비교예(4) 및 비교예(9)는 본 발명조성강을 사용하여 제조조건을 변경한 경우이다. 발명강D에 있어 다른 압연 조건은 모두 본 발명범위로 하고 마무리 압연온도만을 900℃로 높인 비교예(4)와, 발명강E에 있어 마무리 압연온도 만을 900℃로 높인 비교예(9)의 경우에는 높은 마무리온도로 인하여 페라이트의 결정립 크기가 조대해지기 때문에 강도는 높으나 충격인성이 저하되어 충격천이온도가 불과 -50∼-55℃로 나타나고 있다.The comparative example (4) and the comparative example (9) are a case where manufacturing conditions are changed using this invention composition steel. In the case of Comparative Example (4) in which the other rolling conditions in the invention steel D were all within the scope of the present invention, and only the finish rolling temperature was increased to 900 ° C, and in Comparative Example (9) in which the finish rolling temperature was increased to 900 ° C in the invention steel E only. Due to the high finish temperature, the grain size of the ferrite becomes coarse, so the strength is high, but the impact toughness is lowered, and the impact transition temperature is only -50 to -55 ° C.

비교예(5) 및 비교예(10)은 마무리 압연온도 및 권취온도는 본 발명범위로 하고, 총압하율을 발명조건보다 작게 한 경우로써, 인강장도 및 항복강도는 본발명 조건과 유사한 값을 나타내고 있으나, 미재결정 영역에서의 누적압하율이 작기 때문에 침상 페라이트의 크기가 상대적으로 조대하여 -60℃에서의 충격흡수에너지가 각각 55, 75joules에 불과하며 충격 천이온도 역시-60∼-65℃에 불과한 것으로 나타나고 있다.In Comparative Example (5) and Comparative Example (10), the finish rolling temperature and the coiling temperature are the range of the present invention, and the total pressure reduction rate is smaller than the invention condition, and the tensile strength and the yield strength are similar to those of the present invention. However, because the cumulative reduction rate in the unrecrystallized region is small, the size of the needle-like ferrite is relatively coarse, so the shock absorption energy at -60 ° C is only 55 and 75joules, respectively, and the impact transition temperature is -60 ~ -65 ° C. It appears to be nothing.

비교예(6) 및 비교예(11)은 권취온도 만을 본 발명조건의 범위 외로 제조한 경우로써, 권취온도가 높기 때문에 미세조직이 침상 페라이트가 아닌 페라이트+펄라이트로 구성되어 있으며 이에 따라 항복강도 및 인장강도가 본 발명의 청구범위보다 낮고 동시에 충격인성도 낮게 나타나고 있다.Comparative Example (6) and Comparative Example (11) is a case in which only the winding temperature is manufactured outside the range of the present invention conditions, and because the winding temperature is high, the microstructure is composed of ferrite + pearlite rather than acicular ferrite, and thus yield strength and It is shown that the tensile strength is lower than the claims of the present invention and at the same time the impact toughness is low.

이에 반하여, 발명강을 이용하고, 본 발명의 제조조건으로 제조한 발명예(7)(8)(12) 및 (13)의 경우에는 항복강도:70.3-74.2kg/㎟, 인장강도:84.5-92.0kg/㎟으로 강도도 우수하며 -60℃에서의 충격흡수 에너지:94-108joules, 충격천이온도: -75∼-100℃로 충격인성도 매우 우수하게 나타나고 있다. 이와 같은 경향은 본 발명강 조성에서는 모두 동일하게 나타나고 있다.On the contrary, in the case of Inventive Examples (7) (8) (12) and (13) manufactured using the inventive steel under the manufacturing conditions of the present invention, the yield strength was 70.3-74.2 kg / mm2 and the tensile strength was 84.5-. Its strength is 92.0kg / mm2, and its impact absorption energy at -60 ℃: 94-108joules, impact transition temperature: -75 ~ -100 ℃ has excellent impact toughness. This tendency is the same in all of the inventive steel compositions.

결론적으로, 압연조건의 변화에 따른 소재의 기계적성질의 변화는 미세조직의 변화와 석출물의 변화에 기인된다고 볼 수 있다. 먼저 미세조직은 베이나이트, 침상 페라이트, 페라이트-펄라이트 등으로 변화되며 이러한 미세조직의 변화는 강도의 저하와 충격인성의 향상을 유발한다. 따라서 소재의 강도와 충격인성을 최대한 향상시키기 위해서는 미세조직을 미세화시킴과 동시에 석출물의 크기가 적절한 경우 이루어질 수 있다.In conclusion, it can be said that the change of mechanical properties of the material according to the change of rolling conditions is caused by the change of the microstructure and the precipitates. First, the microstructure is changed to bainite, acicular ferrite, ferrite-pearlite, and the like, and the change of the microstructure causes a decrease in strength and an improvement in impact toughness. Therefore, in order to improve the strength and impact toughness of the material as much as possible, it can be made if the size of the precipitate is appropriate while miniaturizing the microstructure.

실시예2Example 2

상기 실시예 1에 나타낸 발명강D를 이용하고, 열간압연조건을 다음의 (a)(b)(c)와 같이 변화시켜가면서 열연강판을 얻었다. (a)는 FDT:780℃, TRR:72%, CT:600℃, (b)는 FDT:820℃, TRR:80%, CT:575℃, (c)는 FDT:900℃, TRR:85%, CT:580℃이었다.Using the inventive steel D shown in Example 1, the hot rolled steel sheet was obtained while changing the hot rolling conditions as follows (a) (b) (c). (a) is FDT: 780 ° C, TRR: 72%, CT: 600 ° C, (b) is FDT: 820 ° C, TRR: 80%, CT: 575 ° C, (c) is FDT: 900 ° C, TRR: 85 % And CT: 580 deg.

얻어진 열연강판을 시험온도를 변화시켜가면서 충격인성시험을 하였다. 측정결과, -60℃에서의 샤피 V노치 충격흡수에너지(vE)는 (a)가 108J, (b)가 94J, (c)가 55J이었고, V노치 충격천이온도는 (a)가 -100℃, (b)가 -75℃, (c)가 -55℃이었다. 또한, 시험온도에 따른 충격흡수에너지를 측정하여 그 결과를 도 1에 나타내었다. 도 1에 나타낸 바와같이 발명강의 열간압연조건 변화에 따른 충격인성의 차이를 보면, 열간압연 마무리온도와 총 압하율의 관계가 "TRR≥0.13xFDT-32"의 관계를 만족할 때 가장 우수한 저온 충격인성을 나타내는 것을 알 수 있다.The resulting hot rolled steel sheet was subjected to an impact toughness test while varying the test temperature. As a result of the measurement, the Charpy V-notch shock absorption energy (vE) at -60 ° C was 108J, (b) was 94J, (c) was 55J, and the V-notch impact transition temperature was -100 ° C. , (b) was -75 ° C, and (c) was -55 ° C. In addition, the impact absorption energy according to the test temperature was measured and the results are shown in FIG. 1. As shown in FIG. 1, the difference in the impact toughness according to the change in the hot rolling condition of the inventive steel shows the best low temperature impact toughness when the relationship between the hot rolling finish temperature and the total rolling reduction satisfies the relationship of "TRR≥0.13xFDT-32". It can be seen that represents.

도 2는 상기와 같이 발명강D를 이용하여 충격인성을 나타내는 또 다른 형태의 시험법인 DWTT(Drop Weight Tear Test)에 의해서 소재의 크랙 전파 정도를 비교한 것이다. (e)는 TRR이 75%이고, FDT가 785℃, CT가 570℃로서, 본 발명조건 TRR≥0.13xFDT-32을 만족하여 압연하는 경우로써, 85%의 연성파면율(85% S.A ; Shear area)을 나타내는 온도가 -75℃였다. 이에, 반하여 (f)는 TRR이 65%이고, FDT가 825℃, CT가 580℃로서, 본 발명조건 TRR≥0.13xFDT-32을 만족하지 않는 경우로써, 85%의 연성파면율을 나타내는 온도가 불과 -45℃의 온도를 나타내고 있다.Figure 2 compares the degree of crack propagation of the material by the DWTT (Drop Weight Tear Test) which is another type of test method showing the impact toughness using the invention steel D as described above. (e) is TRR is 75%, FDT is 785 ℃, CT is 570 ℃, when the rolling conditions satisfy the present invention TRR≥0.13xFDT-32, 85% of the ductile fracture rate (85% SA; Shear area) was -75 ° C. On the contrary, (f) shows a TRR of 65%, an FDT of 825 ° C, and a CT of 580 ° C, which do not satisfy the present invention condition TRR≥0.13xFDT-32. The temperature of only -45 degreeC is shown.

이와 같이 본 발명조건으로 압연하는 경우, 인장강도 80kg/㎟이상, 충격천이온도 -75℃이하의 매우 우수한 저온인성을 지닌 고장력강을 제조할 수 있다.As described above, when rolling under the conditions of the present invention, it is possible to produce a high tensile strength steel having very excellent low temperature toughness of more than 80kg / mm 2 of tensile strength, less than -75 ℃ impact transition temperature.

상술한 바와같이, 본 발명에 의하면 강의 화학성분을 조절하고 열간압연 및 권취조건을 제어하므로써, 미세한 석출물이 기지조직에 분포된 침상 페라이트의 조직을 형성시킬 수 있고, 압연조건을 조절하므로써, 미세한 결정립의 형성과 석출에 의한 높은 강도를 쉬이 얻을 수 있으며 동시에 극히 우수한 저온인성을 발휘하는 특성을 나타내는 고장력 열연강판을 얻을 수 있는 효과가 제공된다. 또한, 강판으로의 제조함에 있어 비교적 정확한 열간압연 조건을 사용함으로써 압연 생산성을 향상시킬 수 있으며, 용접성이 매우 우수하므로 부품 제조시의 실수율 및 생산성을 향상시킬 수 있기 때문에, 제조원가 절감 등의 효과도 제공된다.As described above, according to the present invention, by adjusting the chemical composition of the steel and controlling the hot rolling and winding conditions, fine precipitates can form a needle-like ferrite structure distributed in the matrix, and by adjusting the rolling conditions, fine grains It is possible to easily obtain high strength by the formation and precipitation of, and at the same time, it is possible to obtain a high tensile strength hot rolled steel sheet exhibiting extremely excellent low temperature toughness. In addition, it is possible to improve the rolling productivity by using a relatively accurate hot rolling conditions in the production of steel sheet, and because the weldability is very excellent, it is possible to improve the error rate and productivity in manufacturing parts, thereby reducing the manufacturing cost. do.

Claims (1)

중량비로, C:0.03-0.10%, Si:0.01-0.50%, Mn:1.2-2.2%, P:0.015%이하, S:0.005%이하, Al:0.01-0.10%, Mo:0.2-0.7%, Ni:0.1-0.5%, Nb:0.02-0.10%, Ti:0.02-0.10%, N:0.01%이하, Ca:0.001-0.005%를 함유하고, 나머지 Fe 및 불가피한 불순물로 이루어진 강재를 750-1000℃의 온도에서 총압하율(TRR)에 대하여 60%이상의 압연을 실시하고, 마무리 압연온도(FDT)를 750-850℃의 온도에서 실시하는 열간압연을 행하는데, 이때 상기 마무리압연온도와 총압하율은 "TRR ≥0.13 x FDT - 32"의 관계를 만족하여 열간압연을 행하고, 압연후 급냉하여 500-630℃온도에서 권취함을 특징으로 하는 고장력 열연강판의 제조방법.By weight ratio, C: 0.03-0.10%, Si: 0.01-0.50%, Mn: 1.2-2.2%, P: 0.015% or less, S: 0.005% or less, Al: 0.01-0.10%, Mo: 0.2-0.7%, A steel material containing Ni: 0.1-0.5%, Nb: 0.02-0.10%, Ti: 0.02-0.10%, N: 0.01% or less, Ca: 0.001-0.005%, and the remaining Fe and unavoidable impurities is 750-1000 ° C. 60% or more of the total rolling reduction (TRR) is carried out at a temperature of, and hot rolling is performed at a finish rolling temperature (FDT) at a temperature of 750-850 ° C., wherein the finishing rolling temperature and the total rolling reduction are The hot rolling is performed by satisfying the relationship of "TRR ≥ 0.13 x FDT-32", and quenching after rolling to wind up at 500-630 ℃ temperature.
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KR100950962B1 (en) 2002-12-24 2010-04-02 주식회사 포스코 A shape control method of a strip in rolling mill
KR20150049659A (en) * 2013-10-30 2015-05-08 현대제철 주식회사 High strength steel and method of manufacturing the same
CN105908084A (en) * 2016-06-14 2016-08-31 舞阳钢铁有限责任公司 Tempering type high-strength steel plate for low-temperature container and production method for tempering type high-strength steel plate

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KR20010056228A (en) * 1999-12-06 2001-07-04 이구택 A method for manufacturing high strength hot rolled steel sheet for pressure vessel using pattern cooling
KR20020044879A (en) * 2000-12-07 2002-06-19 이구택 A hot-rolled steel sheet with excellent stretching workability, and a method for manufacturing it

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Publication number Priority date Publication date Assignee Title
KR100950962B1 (en) 2002-12-24 2010-04-02 주식회사 포스코 A shape control method of a strip in rolling mill
KR20150049659A (en) * 2013-10-30 2015-05-08 현대제철 주식회사 High strength steel and method of manufacturing the same
KR101586883B1 (en) * 2013-10-30 2016-01-19 현대제철 주식회사 High strength steel and method of manufacturing the same
CN105908084A (en) * 2016-06-14 2016-08-31 舞阳钢铁有限责任公司 Tempering type high-strength steel plate for low-temperature container and production method for tempering type high-strength steel plate

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