JP4660034B2 - A non-water-cooled manufacturing method of an X70 grade steel plate having a high impact absorption energy and a thickness of 15 mm or less. - Google Patents

Description

【００３４】
【表２】

【００３５】
【表３】

【００３６】
鋼１〜８は本発明鋼であり、化学成分と製造条件を特定の狭い範囲に最適化することで、Ｘ７０級の強度と２５０Ｊ以上の高いｖＥ_-20を同時に満足している。非水冷圧延ままであるからこのときの生産性は高い。一方、鋼９〜２４は従来鋼であり、化学成分あるいは製造条件が最適な範囲から外れるために、上記の強度あるいは吸収エネルギーを達成することができない。鋼９はＣが少ないためにＹＳが不足している。鋼１０はＣが多いためにｖＥ_-20が不足している。鋼１１はＭｎが少ないためにＹＳが不足している。鋼１２はＭｎが多いためにｖＥ_-20が不足している。鋼１３はＭｏが少ないためにｖＥ_-20が不足している。鋼１４はＭｏが多いためにｖＥ_-20が不足している。鋼１５はＮｂが少ないためにＹＳ、ＦＡＴＴ、ＳＡ_-20、ｖＥ_-20が劣化している。鋼１６はＮｂが多いためにｖＥ_-20が不足している。鋼１７はＰが多いためにＦＡＴＴとＳＡ_-20が劣化してｖＥ_-20も不足している。鋼１８はＳが多いためにｖＥ_-20が不足している。鋼１９は脱酸元素であるＳｉが少ないためにＯが多くなってしまい、ｖＥ_-20が不足している。鋼２０は圧延前の鋼片厚みが圧延後の鋼板厚みに対して小さいためにＦＡＴＴとＳＡ_-20が劣化してｖＥ_-20が不足している。鋼２１は加熱温度が低いためにＹＳ、ＦＡＴＴ、ＳＡ_-20、ｖＥ_-20が劣化している。鋼２２はＡｒ₃〜Ａｒ₃＋１００℃での累積圧下量が少ないためにＹＳ、ＦＡＴＴ、ＳＡ_-20、ｖＥ_-20が劣化している。鋼２３はＡｒ₃〜Ａｒ₃＋１００℃での累積圧下量が多いためにｖＥ_-20が劣化している。鋼２４は圧延終了温度がＡｒ₃未満であるためにｖＥ_-20が不足している。
【００３７】
【発明の効果】
本発明により、板厚が１５ｍｍ以下で、Ｘ７０級の強度を有し、２５０Ｊ以上のｖＥ_-20を有する鋼板を高い生産性のもとで製造することが可能になった。その結果、鋼板製造者は製造コストを低く抑え、製造納期を短縮することが可能となった。本発明によって製造された鋼板は、原油や天然ガス等の輸送用ラインパイプをはじめ、延性破壊特性が重視される各種の鋼構造物に適用され、鋼構造物の安全性を高めることに貢献する。
【図面の簡単な説明】
【図１】Ｃ量を変化させた１２ｍｍ厚みＸ７０級鋼板におけるシャルピー衝撃特性の遷移曲線を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is a technique for producing an X70 grade steel sheet having a plate thickness of 15 mm or less having high absorbed energy at the time of ductile fracture with high productivity. In the steel industry, it is applied to the plate manufacturing process. The steel sheet produced according to the present invention is mainly used for transportation line pipes for crude oil, natural gas and the like. In addition, it can be applied to various steel structures where ductile fracture characteristics are important.
[0002]
[Prior art]
When line pipes are welded on site, there is a demand for thinning that requires fewer welding passes from the viewpoint of welding efficiency. On the other hand, there is a demand for higher strength in order to increase the operating pressure of the line pipe from the viewpoint of transportation efficiency. In addition, in recent years, there is a demand to increase resistance to unstable ductile fracture from the viewpoint of safety of line pipes. As steel plates for line pipes that are required at a high level, for example, provision of steel plates that satisfy the following specifications is required.
Plate thickness ≤15mm
X70 grade strength (API standard)
Absorbed energy (vE _-20 ) ≧ 250 J in 2 mm V notch full size Charpy impact test at −20 ° C.
[0003]
The present invention is a technique for manufacturing a steel plate satisfying the above three specifications with high productivity.
[0004]
In the plate manufacturing process, applying accelerated cooling after rolling in order to increase the strength and toughness of the steel sheet is widely performed. By making full use of such accelerated cooling technology, it is not impossible to manufacture a steel sheet having the above required characteristics. However, when accelerated cooling is applied to a very thin steel plate having a plate thickness of 15 mm or less, there is a problem that the cooling becomes uneven and the shape of the steel plate deteriorates. As a result, an extra work process for correcting the shape of the steel sheet occurs, and the problem is that productivity at the production site is significantly hindered. Against this background, there is a need for a new mass production technology for manufacturing steel plates that have the above required characteristics in high production without using accelerated cooling without using accelerated cooling. A production method without cooling is referred to as non-water-cooled rolling or non-water-cooled rolling).
[0005]
[Problems to be solved by the invention]
The present invention provides a method for producing a steel sheet having a plate thickness of 15 mm or less, an X70 grade strength, and a vE- ₂₀ of 250 J or more as it is non-water-cooled.
[0006]
[Means for Solving the Problems]
The present inventor can make the ductile fracture surface ratio at −20 ° C. 100% by controlling the metal structure of the steel sheet from both the steel component and rolling conditions, and the ductile fracture surface ratio is 100%. The inventors have found that the fracture resistance can be increased, and as a result, an X70 grade steel sheet having a thickness of 15 mm or less having high impact absorption energy can be produced, and the present invention has been completed.
[0007]
The gist of the present invention is as follows.
[0008]
(1) C: 0.03 to 0.06% by mass%,
Mn: 1.4 to 2.0%,
Mo: 0.05-0.5%
Nb: 0.01 to 0.1%,
P: ≦ 0.01%
S: ≦ 0.003%,
O: ≦ 0.005%
Further Si: 0.05-0.5%,
Al: 0.001 to 0.05%,
Ti: 0.005 to 0.05%
In the case of carrying out hot rolling by heating a steel slab of chemical components containing one or more of the above, the balance being iron and inevitable impurities to 1000 ° C. or higher, the thickness of the steel slab before rolling is rolled. The rolling is finished so that the cumulative reduction amount is 20% or more and less than 60% in the temperature range of Ar _{3 to} Ar ₃ + 100 ° C., and then air-cooled. A non-water-cooled manufacturing method of an X70 grade steel plate having a high impact absorption energy and a thickness of 15 mm or less.
[0009]
(2) Cu: ≦ 1% by mass%
Ni: ≦ 1%
Cr: ≦ 1%
V: ≦ 0.1%
B: ≦ 0.005%
1 type or 2 types or more of these, The non-water-cooled type manufacturing method of X70 grade steel plate with a plate | board thickness of 15 mm or less which has the high impact absorption energy as described in said (1) characterized by the above-mentioned.
[0010]
(3) By mass%, Ca: ≦ 0.005%,
Mg: ≦ 0.005%,
REM: ≦ 0.01%
Zr: ≦ 0.01%
1 type or 2 types or more of these, The non-water-cooled type manufacturing method of the X70 grade steel plate with a plate | board thickness of 15 mm or less which has the high impact absorption energy as described in said (1) or (2) characterized by the above-mentioned.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The greatest technical problem in the present invention is to stably acquire vE- ₂₀ of 250 J or more under mass production. The technical idea for this purpose will be broadly divided into (1) and (2).
[0012]
(1) First, making the fracture mode at −20 ° C., which is the Charpy test temperature, completely ductile fracture is the minimum condition for obtaining high vE- ₂₀ . That is, the ductile fracture surface rate at -20 ° C must be 100%. For this purpose, in the steel material to which the present invention is directed, the fracture surface transition temperature needs to be −60 ° C. or lower. In order to realize this, the present invention refines the metal structure of the steel sheet under the following three conditions.
(1) A steel slab to which 0.01% or more of Nb has been added is heated to 1000 ° C. or higher and then hot-rolled.
(2) The cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C. in hot rolling is set to 20% or more.
(3) The thickness of the steel slab before hot rolling is at least 10 times the thickness of the steel plate after hot rolling.
[0013]
By simultaneously satisfying these three conditions, the austenite (γ) structure at the time of rolling is refined, the transformation structure generated in the subsequent cooling process is refined, and a fracture surface transition temperature of −60 ° C. or less is achieved. The ductile fracture surface rate at 20 ° C. is stably 100%. If these three conditions are not met, the fracture surface transition temperature of the steel sheet becomes higher than −60 ° C., and it becomes difficult to stably ensure a ductile fracture surface ratio of 100% at −20 ° C. Regarding (1), if Nb is less than 0.01% or the steel slab heating temperature is less than 1000 ° C., the amount of Nb that dissolves in γ is insufficient, so the γ recrystallization temperature range and γ unrecrystallization The γ structure is not sufficiently refined through rolling in the temperature range. Regarding {circle around (2)}, when the cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C. is less than 20%, γ is not sufficiently refined because the degree of processing of γ in the γ non-recrystallization temperature range is insufficient. Regarding (3), if the thickness of the steel slab before rolling is less than 10 times the thickness of the steel sheet after rolling, the processing amount at a high temperature of Ar ₃ + 100 ° C. or more is insufficient and γ is not sufficiently refined.
[0014]
(2) Next, the fracture resistance must be increased with a ductile fracture surface ratio of 100%. For this purpose, the present invention has been devised to control the metal structure from both the steel component and rolling conditions, focusing on the suppression of separation. Separation is a crack perpendicular to the fracture surface and parallel to the rolling surface. It is widely known that the absorption energy decreases when separation occurs. When manufacturing a thin steel sheet of 15 mm or less targeted by the present invention as it is with non-water-cooled rolling, if strong controlled rolling is performed with emphasis on the fracture surface transition temperature as in the prior art, a large number of separations occur and ductile fracture occurs. Even if the area ratio is 100%, it has been difficult to stably obtain high absorbed energy exceeding 250 J. As described above, for example, “Iron and Steel, 68 (1982), 435” is widely known as a cause of separation. In order to suppress the development of the rolling texture, it is known that it is effective for suppressing the separation to end the rolling at Ar ₃ or more. However, there was no knowledge regarding the influence of steel components on the occurrence of separation.
[0015]
Thus, the inventors have studied in detail the influence of the steel component on the separation, and as a result, have found experimental facts in which the C content has a very large effect. FIG. 1 shows a transition curve of Charpy impact characteristics in a 12 mm thick X70 grade steel sheet with varying C content. These steel sheets were rolled and air cooled at the same temperature higher than Ar _{3 of} all steels.
[0016]
All these steels have a fracture surface transition temperature of −60 ° C. or lower, and the ductile fracture surface ratio at −20 ° C. is 100% for all steel plates. However, the value of vE- ₂₀ varies greatly depending on the amount of C. When the C content is 0.055% or less, it exceeds 300J, and when the C content is 0.074% or more, it is less than 250J. The reason why vE _-20 varies depending on the amount of C in this way is that the amount of separation generated depends on the amount of C.
[0017]
When the amount of separation generated at −20 ° C. (Is: the total length of the separation length appearing on the fracture surface) is observed, no separation occurs at a low C amount of 0.055% or less. On the other hand, separation occurs when the amount of C is as high as 0.074% or more. When the temperature is −40 ° C., it is clear that the amount of separation generated increases with increasing C content. That is, it was found that reducing the amount of C is extremely effective in order to suppress separation under a ductile fracture surface ratio of 100% and obtain high absorbed energy.
[0018]
When the amount of C decreases, the development of the rolling texture is suppressed and separation is less likely to occur. The second effect of reducing the amount of C is that the separation that occurs along the center segregation portion of the steel sheet is suppressed. Since this type of separation is deeply generated perpendicular to the fracture surface, the absorbed energy is greatly impaired. When the C content is reduced, the center segregation in the continuously cast steel slab is reduced, so that the development of the band structure generated at the center segregation portion of the steel sheet is suppressed, and the deep separation caused by the center segregation is less likely to occur.
[0019]
Thus, it has been found that by reducing the amount of C, it is possible to strongly suppress the occurrence of separation over the entire region from the surface layer in the thickness direction to the inside. Based on this completely new knowledge, in the present invention, it is a technical pillar to control the C amount to 0.06% or less.
[0020]
Furthermore, as a result of examining the relationship between the rolling conditions and the separation, even if it is attempted to avoid the rolling texture by ending rolling at Ar ₃ or higher as conventionally known, the cumulative reduction of Ar _{3 to} Ar ₃ + 100 ° C. It has been found that when the amount exceeds 60%, the rolled texture of γ is inherited to the transformed structure, and separation is likely to occur. Therefore, in the present invention, the cumulative reduction amount in this temperature range is suppressed to less than 60%, and the occurrence of separation is thoroughly suppressed.
[0021]
Next, the reasons for limiting each chemical component will be described.
[0022]
C is the most important element in the present invention. In order to ensure the strength, C of 0.03% or more is necessary. However, when C increases, separation easily occurs and the absorbed energy decreases. Further, when C increases, center segregation is promoted, deep separation resulting from this occurs, and the absorbed energy decreases. Furthermore, when the amount of C increases, the volume fraction of cementite particles and pearlite phase increases, and these become the buds of void generation in ductile fracture, which promotes destruction and the absorbed energy decreases. From the above, the upper limit of C must be 0.06%.
[0023]
Mn is an element indispensable for securing strength and toughness, and it is necessary to actively add it instead of the low C of the present invention, particularly from the viewpoint of strength. In order to ensure the strength of X70 grade under low C, it is necessary to add 1.4% or more of Mn. When Mn exceeds 2.0%, center segregation is promoted, deep separation resulting from this occurs, and the absorbed energy decreases. Therefore, the upper limit of Mn is set to 2.0%.
[0024]
Mo is an extremely important element in the present invention. Mo enhances hardenability in the transformation after rolling, and promotes the formation of a fine structure in which acicular ferrite and bainite are mixed. At the same time, the formation of a band structure parallel to the rolling direction is prevented, and the generation of a more isotropic structure is promoted. The dispersion state of the cementite particles is also refined. As a result, the fracture surface transition temperature decreases due to the refinement of the structure, and the strength increases. Furthermore, separation and voids are less likely to occur and the absorbed energy is improved. In order to enjoy these effects, 0.05% or more of Mo is necessary. When Mo exceeds 0.5%, the hardenability becomes excessive, and a hardening phase called MA (Martensite Austenite constituent) increases and the absorbed energy decreases. Therefore, the upper limit of Mo is 0.5%.
[0025]
Nb is an important element in the present invention. Nb promotes refinement of the γ structure by rolling to refine the transformation structure. As a result, the fracture surface transition temperature is lowered and the strength is increased. Strength is also increased by precipitation hardening. For these, 0.01% or more of Nb is necessary. When Nb exceeds 0.1%, center segregation is promoted, deep separation resulting from this occurs, and the absorbed energy decreases. Therefore, the upper limit of Nb is set to 0.1%.
[0026]
P is an element which is not preferable in the present invention. P promotes center segregation or causes grain boundary segregation to cause significant deterioration in toughness. In order to obtain high absorbed energy, P must be 0.01% or less.
[0027]
S and O are undesirable elements in the present invention. These form non-metallic inclusions, promote the generation of voids, and reduce the absorbed energy. S must be 0.003% or less, and O must be 0.005% or less.
[0028]
Si, Al, and Ti act as deoxidizing elements. In order to make O into 0.005% or less, it is necessary to add one or more of these. Therefore, it is necessary that Si is 0.05% or more, Al is 0.001% or more, and Ti is 0.005% or more. If there are too many of these deoxidizing elements, the oxide becomes coarse and adversely affects the starting point of destruction, so the upper limit is set to 0.5% for Si, 0.05% for Al, and 0.05% for Ti.
[0029]
Cu, Ni, Cr, V, and B are effective for increasing the strength. If these amounts are too large, the HAZ toughness is impaired, so the upper limit is 1% for Cu, 1% for Ni, 1% for Cr, 0.1% for V, and 0.005% for B.
[0030]
Ca, Mg, REM, and Zr form sulfides in preference to Mn, and form spherical inclusions that are difficult to stretch by rolling. As a result, separation and voids are less likely to occur and the absorbed energy is improved. If too much of these desulfurization elements are present, the sulfides become coarse and adversely affect the starting point of destruction. Therefore, Ca is 0.005%, Mg is 0.005%, REM is 0.01%, and Zr is 0.01%. Is the upper limit.
[0031]
The conditions at the time of hot rolling the steel slab characterized by the above low C-high Mn-Mo-Nb component are demonstrated. First, the heating temperature of the steel slab is set to 1000 ° C. or higher. The reason is that if the heating temperature is less than 1000 ° C., Nb that is solid-solved in γ is insufficient, so that the γ structure is not sufficiently refined by rolling and the precipitation hardening of Nb is also reduced. . That is, both strength and fracture surface transition temperature deteriorate. Next, the cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C. is set to 20% or more and less than 60%. This is because when the cumulative reduction amount in this temperature region is less than 20%, the γ structure is not sufficiently refined by rolling and the fracture surface transition temperature is deteriorated. Further, when the cumulative reduction amount in this temperature range is 60% or more, the texture of γ due to rolling develops and is inherited in the transformation structure, so that the separation easily occurs and the absorbed energy deteriorates. Next, rolling is completed at Ar ₃ or more. The reason for this is that when rolling is finished at less than Ar ₃ , the development of the rolled texture becomes remarkable due to the formation of processed ferrite, a large amount of separation occurs, and the absorbed energy decreases dramatically. Next, air cooling is performed after rolling. The reason for this is that if accelerated cooling is applied to a very thin steel plate with a plate thickness of 15 mm or less, the cooling becomes non-uniform and the steel plate shape deteriorates, and an extra work step is required to correct this. This is because productivity is significantly inhibited. In the above hot rolling, the thickness of the steel slab before rolling is at least 10 times the thickness of the steel plate after rolling. The reason for this is that if the thickness of the steel slab before rolling is less than 10 times the thickness of the steel sheet after rolling, the processing amount at Ar ₃ + 100 ° C. or more will be insufficient and γ will not be sufficiently refined. This is because of deterioration.
[0032]
【Example】
A steel plate having a thickness of 15 mm or less was manufactured as a non-water-cooled rolling under the thick plate manufacturing conditions shown in Table 2 using a continuously cast steel piece having chemical components shown in Table 1 as a raw material. Table 3 shows the mechanical properties of the steel sheet.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
[Table 3]

[0036]
Steels 1 to 8 are steels of the present invention, and by simultaneously satisfying X70 grade strength and high vE- _{20 of} 250 J or more by optimizing chemical components and production conditions in a specific narrow range. Productivity at this time is high because it is still non-water-cooled. On the other hand, steels 9 to 24 are conventional steels, and the above-described strength or absorbed energy cannot be achieved because the chemical components or production conditions deviate from the optimum range. Since Steel 9 has a small amount of C, YS is insufficient. Steel 10 has a large amount of C, so vE- ₂₀ is insufficient. Steel 11 is deficient in YS due to its low Mn content. Steel 12 is deficient in vE- ₂₀ due to its high Mn content. Since the steel 13 has a small amount of Mo, vE- ₂₀ is insufficient. Steel 14 is deficient in vE- ₂₀ due to its high Mo content. Since Steel 15 has a small amount of Nb, YS, FATT, SA- ₂₀ , and vE- ₂₀ are deteriorated. Steel 16 lacks vE _-20 due to the high Nb content. Since Steel 17 has a large amount of P, FATT and SA- ₂₀ deteriorate and vE- ₂₀ is also insufficient. Since steel 18 has a large amount of S, vE- ₂₀ is insufficient. Steel 19 has a small amount of Si, which is a deoxidizing element, and therefore O is increased, and vE- ₂₀ is insufficient. In Steel 20, the thickness of the steel slab before rolling is smaller than the thickness of the steel plate after rolling, so that FATT and SA- ₂₀ deteriorate and vE- ₂₀ is insufficient. Since the heating temperature of the steel 21 is low, YS, FATT, SA- ₂₀ , and vE- ₂₀ are deteriorated. Since the steel 22 has a small cumulative reduction amount at Ar _{3 to} Ar ₃ + 100 ° C., YS, FATT, SA ₋₂₀ , and vE ₋₂₀ are deteriorated. In Steel 23, vE- ₂₀ is deteriorated due to a large cumulative rolling amount at Ar _{3 to} Ar ₃ + 100 ° C. Steel 24 has an insufficient vE _-20 for rolling end temperature is less than Ar _3.
[0037]
【The invention's effect】
According to the present invention, it is possible to manufacture a steel plate having a plate thickness of 15 mm or less, an X70 grade strength, and a vE- ₂₀ of 250 J or more with high productivity. As a result, the steel sheet manufacturer can keep the production cost low and shorten the production delivery time. The steel sheet produced by the present invention is applied to various steel structures where ductile fracture characteristics are important, including transportation line pipes for crude oil, natural gas, etc., and contributes to improving the safety of steel structures. .
[Brief description of the drawings]
FIG. 1 is a diagram showing a transition curve of Charpy impact characteristics in a 12 mm thick X70 grade steel plate with varying C content.

Claims (3)

C: 0.03 to 0.06% by mass%,
Mn: 1.4 to 2.0%,
Mo: 0.05-0.5%
Nb: 0.01 to 0.1%,
P: ≦ 0.01%
S: ≦ 0.003%,
O: ≦ 0.005%
Further Si: 0.05-0.5%,
Al: 0.001 to 0.05%,
Ti: 0.005 to 0.05%
In the case of carrying out hot rolling by heating a steel slab of chemical components containing one or more of the above, the balance being iron and inevitable impurities to 1000 ° C. or higher, the thickness of the steel slab before rolling is rolled. The rolling is finished so that the cumulative reduction amount is 20% or more and less than 60% in the temperature range of Ar _{3 to} Ar ₃ + 100 ° C., and then air-cooled. A non-water-cooled manufacturing method of an X70 grade steel plate having a high impact absorption energy and a thickness of 15 mm or less. Cu in the mass%: ≦ 1%,
Ni: ≦ 1%
Cr: ≦ 1%
V: ≦ 0.1%
B: ≦ 0.005%
The non-water-cooled manufacturing method of X70 grade steel plate with a plate | board thickness of 15 mm or less which has the high impact absorption energy of Claim 1 characterized by containing 1 type, or 2 or more types of these. In mass%, Ca: ≦ 0.005%,
Mg: ≦ 0.005%,
REM: ≦ 0.01%
Zr: ≦ 0.01%
The non-water-cooled manufacturing method of the X70 grade steel plate with a plate | board thickness of 15 mm or less which has the high impact absorption energy of Claim 1 or Claim 2 characterized by containing 1 type, or 2 or more types of these.

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