JP4292864B2 - Structural Fe-Cr steel plate, method for producing the same, and structural steel - Google Patents

Description

【００５６】
【表２】

【００５７】
上記試験の結果を表２に併記した。本発明により製造した鋼板は、ＳＳ４００やＳＮ４００Ｂ並みの強度を有している。さらに溶接により成形したＨ形鋼の強度もＳＳ４００鋼並みの強度を有しており、電気抵抗溶接に伴う溶接部の脆化もなく、全てウェブ位置で破断した。また、大気中での溶接においても、良好な溶接を行うことができ、ペネトレータの未排出が原因で起きる溶接部破断は全く生じなかった。一方、本発明の成分組成から外れる比較例では、目的の強度が得られず、また引張試験においても溶接部で破断が生じ、十分な強度が得られなかった。
具体的には、No.１０は、粗圧延における強圧下圧延を行わなかったためにＨ形鋼溶接部の強度が十分ではなく、No.１１は、熱延後の冷却速度が大きいために鋼板強度が目標より高くなっている。No.１４，１５は、ＣあるいはＣ＋Ｎが高いため、溶接部の脆化が著しく、溶接強度が十分でない。また、No.１６は、Cu含有量が少ないためペネトレータの排出が不十分となり、No.１７は、Ｖ含有量が少ないために溶接熱影響部のフェライト粒が粗大化し、また、No.１８は、Mn含有量が多いために溶接熱影響部の硬化が大きく、いずれも引張試験で溶接部から破断が発生している。
【００５８】
（実施例２）
表３に示す成分組成を有する溶鋼を、転炉−２次精練工程を経て溶製し、連続鋳造法で200mm厚のスラブとした。これらのスラブを1170〜1220℃に再加熱後、表４に示した条件で、６パス目の圧下率を30〜45％、他パスの圧下率を30％未満とする計７パスの粗圧延を行った後、仕上圧延における圧延仕上温度が940〜1050℃となる７パスの仕上圧延により、4.5mmおよび6.0mm厚の熱延鋼板とし、815〜910℃の温度で巻き取ってコイルとした。巻取り後のコイルは、断熱材を敷き詰めた保熱ヤードへ搬送し、100mm厚の断熱材を内側にライニングした保熱カバーを被せてコイルの保熱を行い徐冷した。コイルの冷却速度の測定は、コイルの最外巻きのエッジ部近傍に熱電対を溶接して行った。また、一部のコイルに対してはコイル単重を調整しあるいは断熱材の厚さを変えることで、冷却速度を変化させた。これら熱延コイルの最外巻きエッジ部および最外巻き板幅方向1／4の部分から圧延方向に平行にＪＩＳ ５号引張試験片を切り出し、引張試験を実施した。
【００５９】
【表３】

【００６０】
上記試験の結果を表４に併記して示す。本発明に従い、巻取り後のコイルに保熱カバーを被せて徐冷した鋼板は、コイル最外巻きのエッジ部近傍の最冷点においても硬質化がほとんど起きず、ＳＳ４００鋼やＳＮ４００Ｂ並みの軟質な強度を得ることができた。一方、保熱を行っても、本発明の条件から外れる比較例ではエッジ部での強度上昇が大きく、さらに成分が外れている比較例では、1／4幅においても目的の強度が得られなかった。
具体的には、No.３０および３１は、巻取り後の冷却速度が本発明の範囲より速いため、コイル最外巻き部では軟質化が得られていない。また、No.３４はＣが、No.３５はＮが、さらにNo.３６はＣ＋Ｎが高いため、いずれも最外巻き部は高強度化している。さらに、No.３７はCu含有量が多く、No.３８はＶ含有量が多く、さらにNo.３９は、Mn含有量が多いため、いずれも最外巻き部は所望の鋼板強度より高い値となっている。
【００６１】
【表４】

【００６２】
【発明の効果】
以上説明したように、本発明によれば、鋼板の成分組成と熱間圧延条件および熱延後の冷却条件とを適正に組み合わせることによって、熱間圧延のままの状態でＳＳ４００鋼並みの強度を有し、しかもコイル先後端部および幅端部でも硬質化することのない構造用Fe−Cr系鋼板を得ることができるので、従来の製造ラインにおいても各種形鋼の製造をすることができる。また本発明の鋼板は、急熱、急冷を受ける溶接法による成形加工が可能であるため、構造用形鋼を電気抵抗溶接によって製造することができる。さらに、本発明の鋼板は、土木・建築用の構造物に使用しても充分な耐食性と耐久性を有するため、ライフサイクルコストの低減を図ることができ、その工業的利用価値は極めて大きい。
【図面の簡単な説明】
【図１】 巻取後の熱延コイルの温度履歴を計算した結果の１例を示すグラフである。
【図２】 巻取後の熱延コイルに保熱カバーを被せた際の温度履歴を計算した結果の１例を示すグラフである。
【図３】 保熱カバーによる保熱方法を模式的に示す図である。
【図４】 図２の冷却曲線と２℃／分の冷却曲線をＣＣＴ図に重ねて示したグラフである。
【符号の説明】
１．保熱カバー
２．熱延後コイル[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a structural Fe-Cr steel plate used for civil engineering and building structures such as bridges and residential structures that are required to have excellent corrosion resistance, durability, weldability and welded portion characteristics. Structurally Fe-Cr steel plate and its manufacture that does not cause a decrease in the strength of welded parts even if it is formed by electrical resistance welding where the strength variation in the coil is as small as SS400 steel and rapid heating and quenching during welding are significant. A method and structural steel obtained therefrom are provided.
[0002]
[Prior art]
Civil and architectural structures are required to have corrosion resistance and durability in addition to strength. Therefore, in these applications, in addition to SWH400, which is a welded lightweight H-section steel, conventional steels such as SS400, SN400B, etc., and high-strength steel, such as SM490, have been used. In order to further improve the corrosion resistance, steel materials that have been subjected to coating, plating treatment, cationic electrodeposition coating, etc. are also used. On the other hand, in recent years, the use of various materials has been studied with the diversification of design and the increasing awareness of environmental issues. Above all, Fe-Cr steels with excellent corrosion resistance and design properties require almost no maintenance for plating, rust-proof coating, drilling, touch-up treatment after welding, etc., so life cycle cost (LCC) Is a very attractive material.
[0003]
Among the above-mentioned Fe-Cr steels, what has been most studied as a structural material for civil engineering and construction has been widely used from the viewpoints of material strength, corrosion resistance, ease of welding, weld toughness, versatility, etc. Austenitic stainless steel represented by SUS304A. This austenitic stainless steel has characteristics that can be satisfactorily satisfied as a civil engineering / building material in characteristics such as strength, corrosion resistance, fire resistance and weld toughness. However, austenitic stainless steel is much more expensive than ordinary steel because it contains a large amount of alloying elements such as Ni and Cr. It was difficult in terms of cost to use as a substitute for general-purpose materials subjected to the treatment, and there was a problem that the application range was extremely narrow.
[0004]
In order to solve such problems, it has been studied to improve martensitic stainless steel which does not contain expensive Ni and has a relatively small Cr content and uses it for civil engineering and building materials. Martensitic stainless steels do not have the concerns of σ brittleness and 475 ° C brittleness, which are problematic in high Cr alloys, and there is no concern of stress corrosion cracking in an environment containing chloride, which is a problem with austenitic stainless steels There is an advantage.
[0005]
For example, Patent Document 1 contains Cr: 10 to 18 wt%, Ni: 0.1 to 3.4 wt%, Si: 1.0 wt% or less, and Mn: 4.0 wt% or less, and C: 0.03 wt% or less, N: There has been disclosed a martensitic stainless steel for welded structure in which the performance of the welded portion is improved by reducing the amount to 0.02 wt% or less and generating a massive martensite structure in the welded heat affected zone. Patent Document 2 contains Cr: 10 to 13.5 wt%, Si: 0.5 wt% or less, and Mn: 1.0 to 3.5 wt%, and C: 0.02 wt% or less and N: 0.02 wt% or less. In the above, a structural martensitic stainless steel having excellent weld toughness and workability that does not require preheating and postheating before and after welding by further limiting Ni to less than 0.1 wt% is disclosed. In Patent Document 3, Ni, Cu, Cr, Mo, etc. are added to a Fe-Cr alloy containing Cr in a range of more than 8 mass% and less than 15 mass% by adding Co, V, W in combination. A technique for improving initial rust resistance, workability, and weldability without increasing the amount, adding Ti, Nb, or excessively reducing C and N is disclosed. However, since the steel materials disclosed in Patent Documents 1 and 2 are too strong in the state of hot rolling, it is necessary to perform annealing after hot rolling, which leaves problems in terms of cost and delivery time. In the technique of Patent Document 3, it is essential to add Co, V, and W, and hot rolling annealing is recommended for softening.
[0006]
In view of this, development of techniques for reducing costs by reducing alloy elements and omitting hot-rolled sheet annealing has been carried out. For example, Patent Document 4 contains Cr: 8 to 16 wt%, Si: 0.05 to 1.5 wt%, Mn: 0.05 to 1.5 wt%, C: 0.005 to 0.1 wt%, N: 0.05 wt% or less, C + N : Steel material reduced to 0.1wt% or less is heated to 1100-1250 ° C, hot rolling is finished at 800 ° C or higher, and after winding at 700 ° C or higher, the average cooling rate to room temperature is 5 ° C / min. A technique for omitting hot-rolled sheet annealing by cooling below is disclosed. However, the steel material disclosed in Patent Document 4 has a tensile strength exceeding 450 MPa, and is used for soft steel materials of the SS400 steel class when forming or performing secondary processing or drilling on a shape steel or pipe. There is a problem that it is difficult to use the designed production line as it is.
[0007]
In addition, although the above-described conventional steel materials are excellent in arc weldability such as MIG using a welding rod, they are difficult to harden and embrittle the welded parts that are rapidly heated and cooled like electrical resistance welding. There are not enough measures taken. For example, for the production of a section steel by electric resistance welding, Patent Document 5 contains Cr: 3.5 wt% or more and less than 10.5 wt%, Si: 0.01 to 1.0 wt%, Mn: 0.01 to 2.5 wt%, and C: Disclosed is a manufacturing technology for steel materials reduced to 0.001 to 0.1 wt%, N: 0.001 to 0.10 wt%, and light-weight welded H-section steel for electrical resistance welding in a non-oxidizing atmosphere or reducing flame shield. ing. However, in this technology, when welding in the atmosphere, the oxide generated during welding heating remains without being discharged and generates a penetrator, which causes fracture at the weld. There is a problem that there is.
[0008]
[Patent Document 1]
Japanese Patent Publication No.51-013463
[Patent Document 2]
Japanese Patent Publication No.57-028738
[Patent Document 3]
JP 2002-053938 A
[Patent Document 4]
Japanese Patent Laid-Open No. 11-302737
[Patent Document 5]
Japanese Patent Laid-Open No. 02-305939
[0009]
[Problems to be solved by the invention]
As described above, many of the Fe-Cr hot-rolled steel sheets manufactured by the conventional technology have a tensile strength exceeding 450 MPa as they are hot-rolled. There was a problem in flowing it. In particular, the front and rear ends and the widthwise edge portions of hot-rolled coils having large material variations (high strength) must be cut off and used, leading to a decrease in yield. In addition, the steel plate of the prior art has not been sufficiently considered for the problem of hardening and embrittlement of the welded portion that is rapidly heated and rapidly cooled. There is a problem in using it as a material for ERW welded pipes (ERW).
[0010]
In view of the above-mentioned problems of the prior art, the object of the present invention is as it is hot-rolled, that is, without hot-rolled sheet annealing, and the tensile strength is 400 to 450 MPa over the entire length of the coil. The purpose is to propose a structural Fe-Cr steel sheet which does not cause embrittlement of the welded part even by rapid heating / cooling during welding and its inexpensive manufacturing method. Another object of the present invention is to propose a structural section steel that is formed by electrical resistance welding using the steel sheet.
[0011]
[Means for Solving the Problems]
The inventors are able to reduce the cost and use hot rolling based on a low alloy steel with a Cr content of 8 mass% or more and less than 10 mass% that provides sufficient corrosion resistance even when used in civil engineering and building structures. A hot-rolled steel sheet that has a tensile strength in the range of 400 to 450 MPa and has good electric resistance weldability was examined. As a result, first, in a steel sheet having a Cr content of 8 mass% or more and less than 10 mass%, the weld heat-affected zone has a fine martensite structure, so that excessive hardening of the martensite structure in the heat-affected zone can be prevented. It was found to be important in preventing the embrittlement of the weld.
[0012]
The hardness of martensite greatly depends on the amount of C and N dissolved in the steel material. Therefore, when adopting a welding method in which the cooling rate after welding is relatively slow, such as arc welding, the C and N content should be reduced and hot-rolled sheet annealing should be applied even with the conventional steel sheet. Thus, it is possible to obtain a steel sheet having a strength in the range of 400 to 450 MPa, excellent workability and weldability, and good toughness of the welded portion. However, when the electric resistance welding method used in the manufacture of welded lightweight H-shaped steel or ERW pipe is applied to the conventional steel plate, the heat-affected zone is markedly hardened and the weld zone has sufficient strength and toughness. The problem that cannot be obtained occurs. In particular, embrittlement of the welded portion is remarkable in the portion heated to 800 to 900 ° C. during welding.
[0013]
The reason is considered as follows. Conventional structural steel has a two-phase structure of ferrite (α) + austenite (γ) when heated to a temperature range of 800 to 900 ° C. Because of the large difference, C and N at higher concentrations are concentrated in the γ phase portion than in the case where the γ single phase is obtained. The γ phase enriched in C and N is transformed into hard martensite in the cooling process after welding, causing embrittlement of the weld. However, in ordinary arc welding, the vicinity of the weld is air cooled (cooled) after welding, so that the martensite is not so hardened. On the other hand, in electrical resistance welding, rapid heating and rapid cooling due to welding are significant compared to arc welding, and when the welding machine is water-cooled to prevent overheating of peripheral equipment such as welding tips, the vicinity of the welded part Since the steel material is cooled very rapidly immediately after welding, the martensite phase becomes harder and embrittlement becomes remarkable. For these reasons, it is important to control the components and the microstructure of the steel material in materials subjected to electrical resistance welding.
[0014]
The inventors first tried to reduce the C and N contents in order to solve the brittleness problem of the weld. However, excessive reduction of the amount of C and N is effective in lowering the martensite generation ability of the weld heat affected zone, but it causes the generation of so-called coarse ferrite, and the characteristics of the weld zone are reduced. . In addition, when a strong carbonitride-forming element such as Ti or Nb was added, the same tendency was observed because the amount of dissolved C and N decreased excessively.
[0015]
Therefore, in order to improve the electric resistance weldability, it is important for the inventors to improve the micro structure of the ferrite + martensite structure generated by heating and cooling in the two-phase region of α + γ during welding. In view of the above, detailed investigations were made focusing on the two points of suppressing the hardening of the martensite phase and improving the toughness by refining the crystal grains of the ferrite phase as a base material. As a result, by reducing the C and N contents and further adding an appropriate amount of V, it is possible to suppress an increase in the hardness of martensite generated in the two-phase zone heating section, and further in the hot rolling rough rolling By rolling at least one pass at a reduction ratio of 30% or more, the ferrite structure of the base metal can be refined, and as a result, the brittleness of the portion heated to the two-phase region by electric resistance welding can be greatly improved. all right. In addition to reducing the amount of Cr and Mn in steel, it was found that by adding an appropriate amount of Cu, the generation of undrained penetrators in the weld zone was suppressed, and electrical resistance welding could be performed well even in the atmosphere. .
[0016]
Furthermore, the inventors have studied a method for setting the strength in the range of 400 to 450 MPa over the entire length of the coil while still being hot-rolled. First, in order to know the cooling rate of the coil accurately, a thermocouple is attached to the hot rolled coil, the temperature change with time is measured, and based on this result, the portion Tmax (hereinafter “ It is called the “highest point.” Normally, the coil thickness and width direction center part) and the portion Tmin that cools the fastest (hereinafter called “cold point”. Usually the edge part in the width direction of the outermost winding of the coil) Calculations were made for changes over time. As an example, FIG. 1 shows the calculation results obtained under the conditions of coil weight: 12300 kg, coil width: 1450 mm, inner diameter: 760 mm, winding temperature: 850 ° C., and outside air temperature: 20 ° C. As is clear from FIG. 1, at the coldest point Tmin of the coil, the temperature dropped to about 400 ° C. in only about 30 minutes, and between 800 and 400 ° C. at a high rate of about 13 ° C./min. It became clear that it was cooled. For this reason, in conventional steel sheets, many hard phases such as martensite phase and bainite phase are generated and hardened at the leading and trailing ends of the coil (inner winding portion and outer winding portion) and the width direction edge portion, which have a high cooling rate. It was thought that.
[0017]
Therefore, the inventors collected metallurgical data such as continuous cooling transformation curve (CCT diagram) and isothermal transformation curve (TTT diagram) for alloys having a Cr content of less than 8 to 10 mass%, and maintained during cooling. The transformation behavior when heated was investigated. As a result, after coiling, if heat retention is performed by some means before the coil tip rear end or the width direction edge reaches a temperature of less than 400 ° C, the recuperation effect due to the internal heat of the coil and the slow cooling effect due to the heat retention Thus, it has been found that the average cooling rate between 800 ° C. and 400 ° C. can be made 2 ° C./min or less over the entire length of the entire coil even in the hot rolling, and that the desired softening can be achieved. In addition, the average cooling rate said by this invention is the average cooling rate in the whole time required to cool from 800 degreeC to 400 degreeC, and is not a temporary cooling rate in the middle of cooling.
[0018]
2 shows the coil 30 minutes after winding under the same conditions as in FIG. 1 when a heat insulation cover lining 100 mm thick heat insulating material as shown in FIG. The results of calculating the time-dependent changes in the temperatures of the coil maximum point Tmax and the coldest point Tmin are shown. From FIG. 2, by using the heat insulating cover, the cooling time from 800 ° C. to 400 ° C. at the coil coldest point Tmin with the fastest cooling rate is 400 minutes or more, that is, the average cooling rate is 1 ° C./minute or less. I understand that I can do it. FIG. 4 is a diagram in which the cooling curve of FIG. 2 and the cooling curve when continuously cooled at 2 ° C./min are superimposed on the CCT diagram. From FIG. 4, if the cooling time from 800 ° C. to 400 ° C. is set to a cooling rate of 12000 seconds (200 minutes) or more, that is, 2 ° C./minute or less, a soft ferrite single-phase structure is formed without forming bainite. It turns out that it is obtained. In addition, even at the coldest point Tmin of the coil, by starting the heat retention before being cooled to less than 400 ° C., the formation of a hard martensite phase is completely suppressed, and further, the cooling before the heat retention is started. It can be seen that the produced bainite can be transformed into a tempered bainite or a ferrite phase by the tempering effect by recuperation after heat retention, thereby softening. As described above, after the coil is wound, before the temperature of the coil coldest point Tmin is cooled to less than 400 ° C., some heat retention means is applied, and the average cooling rate of the steel sheet is set to 2 ° C./min or less. Thus, it was found that a soft Fe-Cr steel plate could be obtained over the entire length and width of the coil.
[0019]
The present invention developed on the basis of the above findings is C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass. %, Mn: 0.01 to 0.30 mass%, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 8 mass% or more and less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni : 0.01 to 1.0 mass%, V: 0.01 to 0.20 mass%, Al: 0.05 mass% or less, with the balance being Fe and inevitable impurities, Without hot-rolled sheet annealing A structural Fe-Cr steel sheet having a tensile strength of 400 to 450 MPa.
[0020]
In addition, the steel plate of this invention can contain Mo: 1.0 mass% or less further in addition to the said component composition, when high corrosion resistance is requested | required.
[0021]
In the present invention, C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 0 0.01 to 0.30 mass%, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 8 mass% or more and less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to A steel material containing 1.0 mass%, V: 0.01 to 0.20 mass%, Al: 0.05 mass% or less is heated to a temperature of 1100 to 1280 ° C, and hot rolling is performed at a temperature of more than 930 ° C. And by winding at a temperature of more than 810 ° C. and setting the average cooling rate between 800-400 ° C. inside the coil to 2 ° C./min or less, Without hot-rolled sheet annealing It is a method for producing a structural Fe-Cr steel sheet, characterized by obtaining a steel sheet having a tensile strength of 400 to 450 MPa.
[0022]
When high corrosion resistance is required, the steel material of the present invention can further contain Mo: 1.0 mass% or less in addition to the above component composition.
[0023]
In the production method of the present invention, it is preferable that at least one pass of rough rolling is performed at a temperature exceeding 1000 ° C. and a reduction rate of 30% or more.
[0024]
Further, in the above manufacturing method of the present invention, the average cooling rate between 800 to 400 ° C. at all positions of the coil is set to 2 ° C./min or less, and the means is to cool the coil with a heat insulating cover, It is preferable to use either a heat box or a heat-retaining furnace.
[0025]
Moreover, it is preferable that the structural steel of the present invention is a steel plate produced by the above-described steel plate or the above-described method by electric resistance welding.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described.
First, the reason for limiting the component composition of the steel sheet according to the present invention to the above range will be described.
C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, and C + N: 0.015 mass% or less
Although the weld heat affected zone of the steel of the present invention has a fine martensite structure, C and N greatly affect the hardness of the martensite phase generated in the weld heat affected zone. In order to improve the toughness and workability of the weld heat-affected zone and prevent weld cracking, it is effective to reduce C and N as conventionally known. However, excessively reducing the content of C and N not only raises the refining cost, but also reduces the martensite generation ability of the weld heat affected zone and promotes the formation of coarse ferrite, Significantly reduces the toughness. Therefore, C and N need to be contained by 0.0025 mass% or more, respectively. On the other hand, the upper limits of C, N, and C + N are 0.010 mass%, 0.010 mass%, and 0.015 mass%, respectively, to prevent embrittlement due to an increase in the extreme hardness of the martensite phase generated in the weld heat affected zone. Need to be restricted. Preferred ranges of C and N are C: 0.003 to 0.008 mass% and N: 0.0030 to 0.0060 mass%. C is more preferably 0.003 to 0.005 mass%.
[0027]
Si: 0.01-1.0mass%
Si is added as a deoxidizer and as a strengthening element. If the content is less than 0.01 mass%, a sufficient deoxidation effect cannot be obtained. On the other hand, excessive addition exceeding 1.0 mass% not only causes deterioration in toughness and workability, but also generates martensite in the weld heat affected zone. Reduce performance. Therefore, the amount of Si is limited to a range of 0.01 to 1.0 mass%. Preferably it is the range of 0.1-0.5 mass%.
[0028]
Mn: 0.01 ~ 0.30mass%
Mn is an austenite phase stabilizing element and contributes to improving the toughness of the weld zone by making the structure of the heat affected zone a martensite structure. However, if it is added excessively, the proportion of the hard phase as hot rolled increases, and the target tensile strength (400 to 450 MPa) cannot be obtained. In addition to increasing the hardness of martensite generated in the two-phase zone heating part of electric resistance welding and causing embrittlement, MnS is formed and corrosion resistance is reduced. Therefore, the upper limit of the Mn addition amount is limited to 0.30 mass%. On the other hand, Mn is also useful as a deoxidizer, as is Si, so the lower limit is set to 0.01 mass%. Preferably, it is the range of 0.10-0.30 mass%.
[0029]
P: 0.04 mass% or less
P is an element harmful not only to hot workability, formability and toughness but also to corrosion resistance. If the content of P exceeds 0.04 mass%, its adverse effect becomes remarkable, so it is limited to 0.04 mass% or less. Preferably it is 0.030 mass% or less.
[0030]
S: 0.03 mass% or less
S combines with Mn to form MnS, reducing corrosion resistance and durability. S is also a harmful element that segregates at the crystal grain boundary and promotes embrittlement of the grain boundary, and is preferably reduced as much as possible. In particular, when the content exceeds 0.03 mass%, the adverse effect becomes remarkable. Therefore, the S content is restricted to 0.03 mass% or less. Preferably it is 0.008 mass% or less.
[0031]
Cr: 8 mass% or more and less than 10 mass%
Cr is an element effective for improving the corrosion resistance, but if it is less than 8 mass%, it is difficult to ensure sufficient corrosion resistance. On the other hand, adding 10 mass% or more of Cr causes an increase in cost and makes it difficult to obtain a desired strength while hot rolling, so the amount added is limited to a range of 8 mass% or more and less than 10 mass%. To do.
[0032]
Cu: 0.01-1.0mass%
Cu is an element effective for improving corrosion resistance, and is added for the purpose of extending the life of building structures and the like. In the present invention, it is also an element that is particularly positively added in order to suppress the residual of the penetrator during welding and enable electric resistance welding in the atmosphere. The reason why the addition of Cu suppresses the penetration of the penetrator is not clear, but in addition to the reduction of elements that tend to form oxides in the welds such as Cr and Mn, Cu, a noble element that is harder to ionize than iron, is added. It is considered that the addition of an appropriate amount suppresses the generation of oxide in the melted portion. However, if the addition is less than 0.01 mass%, the above-mentioned addition effect is poor. On the other hand, excessive addition exceeding 1.0 mass% increases the cost and may increase the hot cracking susceptibility and cause embrittlement during hot rolling. is there. Therefore, Cu is limited to a range of 0.01 to 1.0 mass%. Preferably, the lower limit of Cu is preferably 0.1 mass% at which the effect of improving corrosion resistance becomes obvious, and the upper limit is preferably 0.7 mass% from the viewpoint of hot cracking prevention and workability.
[0033]
Ni: 0.01-1.0mass%
Ni is an element effective for improving ductility and toughness. In the present invention, it is added to improve the toughness of the weld heat affected zone and to improve the rust resistance. Furthermore, Ni is also effective in preventing brittle cracks during hot rolling due to Cu addition. However, if the content is less than 0.01 mass%, the addition effect is poor. On the other hand, if it exceeds 1.0 mass%, not only the addition effect is saturated, but also the material is hardened and the cost is increased. It is limited to the range of 0.01 to 1.0 mass%.
[0034]
V: 0.01 ~ 0.20mass%
V is an extremely important element in the present invention, and by adding an appropriate amount, V can prevent embrittlement of the weld heat-affected zone in electric resistance welding and also prevent coarsening of ferrite crystal grains. . However, if the addition amount is less than 0.01 mass%, the above-mentioned addition effect cannot be sufficiently obtained. On the other hand, if the addition amount exceeds 0.20 mass%, the martensite-forming ability in the weld heat affected zone is remarkably reduced, Toughness decreases. Moreover, it becomes difficult to obtain a desired tensile strength (400 to 450 MPa) with hot rolling. Therefore, the addition amount of V is limited to the range of 0.01 to 0.20 mass%. Preferably it is 0.03-0.20 mass%.
[0035]
Here, the mechanism by which the brittleness of the weld heat affected zone is improved by adding an appropriate amount of V is not clear, but is considered as follows. When elements with strong affinity for C and N, such as Ti and Nb, are added, the amount of solid solution C and N decreases significantly due to the formation of these carbonitrides. Productivity is extremely reduced. On the other hand, when V, which has a stronger affinity for C and N than Ti and Nb, is added, the carbonitride of V is unstable in a portion heated to a temperature above the austenite single phase region. Therefore, since the amount of solid solution C and N does not significantly decrease, the martensitic transformation ability of this portion can be sufficiently secured. On the other hand, in the portion heated to the temperature of the two-phase region, since the carbonitride of V is stable at that temperature, the amount of dissolved C and N is kept low, and the dissolved C and N in the γ phase are suppressed. Hardening of the martensite phase due to thickening is unlikely to occur. As a result, the hardness of the martensite phase of the two-phase region heating part can be kept low without reducing the martensite generation ability of the part heated to the γ single-phase region or higher, so that It becomes possible to provide excellent toughness.
[0036]
Al: 0.05 mass% or less
Al is not only useful as a deoxidizer, but also contributes effectively to improving the bending workability of the steel sheet. In order to acquire the effect, adding 0.003 mass% or more is preferable. However, if the Al content exceeds 0.05 mass%, the inclusions increase and the mechanical properties are deteriorated, so the content is limited to 0.05 mass% or less. This Al may not be contained in particular.
[0037]
Mo: 1.0 mass% or less
Mo is also an element effective for improving the corrosion resistance, and can be added as necessary in the present invention. In order to obtain the effect, 0.03 mass% or more is preferably added. However, if added over 1.0 mass%, the workability is remarkably lowered, and the desired tensile strength (400 to 450 MPa) cannot be obtained with hot rolling, so the addition amount is limited to 1.0 mass% or less. . In addition, from the viewpoint of balance between corrosion resistance and strength / workability, a range of 0.1 to 0.5 mass% is preferable.
[0038]
Next, the strength characteristics of the steel sheet according to the present invention will be described.
The steel sheet of the present invention needs to have a tensile strength in the range of 400 to 450 MPa. Conventionally, shape steel for civil engineering and building structures has been manufactured mainly by processing steel material of SS400 steel class, but in order to utilize the production line as it is, the strength and processing of the same level as SS400 steel etc. It is necessary to have sex. That is, if the tensile strength exceeds 450 MPa, it is not preferable because the processing load on the shape steel production line increases and the equipment needs to be strengthened, and the workability deteriorates. On the other hand, when the pressure is less than 400 MPa, excessive deformation occurs when forming the shaped steel, and the strength required for the product cannot be obtained.
[0039]
Next, the manufacturing method of the steel plate which concerns on this invention is demonstrated.
The molten steel adjusted to the above component composition is melted in a commonly known melting furnace such as a converter or electric furnace, and then vacuum degassing (RH method), VOD (Vacuum Oxygen Decarburization) method, AOD (Argon Oxygen Decarburization) ) And the like, and then a steel slab (steel material) is obtained by a continuous casting method or an ingot-bundling method. In this case, the slab thickness is preferably 100 mm or more in order to secure a reduction ratio in hot rough rolling described later.
[0040]
Subsequently, after heating the said steel slab to the temperature of 1100-1280 degreeC, it hot-rolls to make a hot-rolled steel plate. The slab heating temperature is preferably high in order to promote softening due to self-annealing after winding, but if it exceeds 1280 ° C, slab dripping will become remarkable, and crystal grains will become coarse and the toughness of the hot-rolled sheet will increase. Since it falls, it is not preferable. On the other hand, when the heating temperature is less than 1100 ° C., it is difficult to make the rolling finish temperature (FDT) of hot rolling exceed 930 ° C. Preferably it is 1100-1250 degreeC.
[0041]
In the hot rough rolling step, it is preferable to perform at least one pass of rolling with a rolling reduction of 30% or more in a temperature range exceeding 1000 ° C. By refining the crystal structure of the steel sheet by this strong rolling, it compensates for the deterioration of the base metal toughness due to the coarsening of ferrite crystal grains caused by heat retention after coil winding, which will be described later, which is a problem particularly in the coil central portion. be able to.
[0042]
In addition, the heavy rolling under rough rolling is also effective for improving the toughness of the portion heated to the α + γ two-phase region in electric resistance welding. This is because martensite generated in the two-phase zone heating section is generated from the ferrite grain boundaries of the steel sheet, but if the martensite is excessively hardened, it becomes a starting point of cracking and embrittlement occurs. Therefore, if the ferrite structure as a matrix is refined to improve the toughness of the ferrite phase, the propagation of cracks can be suppressed and embrittlement can be prevented. Therefore, in the present invention, the steel sheet is a γ single phase at a temperature exceeding 1000 ° C., but by performing at least one pass of rough rolling with a rolling reduction of 30% or more, the generation site of the ferrite phase is increased and crystallized. Refine the grain. The reason why the temperature of the rough rolling is limited to over 1000 ° C. is also to make the hot rolling finishing temperature over 930 ° C.
[0043]
In the present invention, in order to promote the softening effect due to self-annealing after coil winding, the rolling finishing temperature in finish rolling following hot rough rolling is over 930 ° C, and the coil winding temperature after rolling is limited to over 810 ° C. To do. The reason for limiting the rolling finishing temperature in finish rolling to over 930 ° C. is to prevent the introduction of processed ferrite by rolling in the α + γ two-phase region and to ensure a winding temperature of over 810 ° C. The reason why the coil winding temperature exceeds 810 ° C. is also to facilitate recuperation by heat retention after winding by keeping the temperature inside the coil high. Further, in order to set the temperature at the coil end at the start of heat retention to 400 ° C. or higher, the coiling temperature needs to exceed 810 ° C.
[0044]
Furthermore, in the present invention, in order to obtain the desired steel sheet strength, the cooling time from 800 to 400 ° C. inside the coil after winding is 200 minutes or more, and the average cooling rate inside the coil is 2 ° C./min or less. Is necessary. By setting this average cooling rate, the steel sheet structure can be a ferrite single phase (partially carbonitride), a tempered bainite single phase or a tempered bainite + ferrite structure, and completely suppress the formation of a hard martensite phase. It becomes possible.
[0045]
Here, the inside of the coil means a portion that is 50 mm or more inside from the edge in the central portion in the coil longitudinal direction and in the plate width direction. The method for measuring the cooling rate of this portion is most surely performed by inserting a thermocouple into the coil, but may be estimated by calculation.
[0046]
Note that the average cooling rate of the coil after winding is 2 ° C./min or less can be achieved relatively easily within the coil. However, the average cooling rate tends to be higher than 2 ° C./min at the front end portion (inner winding portion) and rear end portion (outer winding portion) of the coil and the end portion in the width direction of the coil (edge portion). And martensite phase is formed and hardened. Therefore, conventionally, this part of the coil has been cut off and used, which causes a decrease in yield.
[0047]
In order to solve this problem, the present invention starts heat retention by some means before the coldest point of the coil after winding is cooled to less than 400 ° C. In particular, a method is proposed in which the cooling time between 800 and 400 ° C. at all positions in the coil is 200 minutes or more and the average cooling rate is 2 ° C./minute or less. By performing this heat retention, the coldest point of the coil is sufficiently tempered, so that the desired strength can be obtained over the entire length of the coil. Preferably, the average cooling rate at all positions in the coil is 1 ° C./min or less. In addition, since the coldest point of the said coil generally hits the part corresponded to the width direction both ends of the coil outermost winding, a cooling rate can be measured by welding and attaching a thermocouple to this part. It is also possible to measure the temperature using a radiation thermometer.
[0048]
In addition, as a heat insulation method, for example, a method of covering a coil with a heat insulation cover lined with a heat insulating material inside an iron box, a pit-shaped hole is dug, and the heat insulation box is attached to the inner wall and stored in a heat insulation box Various methods such as a method and a heat-retaining furnace having a heating function can be applied, and a preferable heat-retaining means can be used according to the manufacturer's manufacturing equipment. In consideration of cooling from the lower part of the coil, it is necessary to devise such as placing the coil on the heat insulating material. Further, heating means by induction heating or the like may be used in combination for the front and rear ends and the width ends of the coil that is significantly cooled.
[0049]
By adopting the above heat-retaining method, the tensile strength can be in the range of 400 to 450 MPa over the entire length of the entire coil without performing hot-rolled sheet annealing, so the prior art Therefore, it is possible to suppress a reduction in yield due to the cutting off of the rear end of the coil tip and the trimming of the edge portion, which has been a problem, and a significant cost reduction can be achieved. Further, by making the tensile strength equal to that of SS400 steel, processing such as bending and drilling can be performed using the conventional production line as it is.
[0050]
In addition, the hot-rolled steel sheet according to the present invention not only has excellent workability and toughness in the state of hot rolling, but also in the heat-affected zone of the welding heat-affected zone even if electrical resistance welding with rapid heating / cooling is performed. It has excellent characteristics that it does not cause embrittlement. In addition, the steel sheet of the present invention can be used in the state of hot rolling, but after performing shape correction by skin pass rolling, de-scaling by shot blasting, pickling, etc., as necessary, or by polishing or the like It can also be used after adjusting to the desired surface properties. Moreover, it is also possible to use after applying a rust preventive agent etc. as needed. In addition, when performing pickling, you may anneal a hot-rolled sheet for the purpose of improving pickling property.
[0051]
In addition, the steel sheet of the present invention can be applied to various shapes of steel manufactured by bending, roll forming, etc., and is suitable for use in structural materials for civil engineering and construction, especially for structural steel for housing structures. It is. In addition, the steel sheet of the present invention can be used as a material for a shape steel assembled by various types of welding such as arc welding, but by utilizing the characteristic that there is no embrittlement of the weld due to rapid heating or rapid cooling, induction heating or direct It is suitable for use in welded lightweight H-shaped steels, electric-welded welding (ERW) pipes, square pipes and the like that are formed by an electric resistance welding method using electric heating.
[0052]
Furthermore, taking advantage of the above properties of the steel sheet of the present invention, it can be used as a material for various structures such as containers, coal wagons, and bus frames. In addition, the steel having the components of the present invention can be applied to various steel materials used in the civil engineering and construction fields such as thick steel plates, shaped steels, and bar steels manufactured by hot rolling.
[0053]
【Example】
(Example 1) Steel having the component composition shown in Table 1 was melted through a converter-secondary refining process, and a steel slab having a thickness of 200 mm was formed by a continuous casting method. After reheating these steel slabs to 1170 ° C, under the conditions shown in Table 2, a total of 7 passes of rough rolling in which the reduction rate of the sixth pass is 20 to 45% and the reduction rate of the other passes is less than 30%. , And then rolled into a hot rolled steel sheet with a thickness of 4.5mm and 6.0mm by 7-pass finish rolling with a finishing temperature of 940 to 1050 ° C in finish rolling, wound on a coil at a temperature of 815 to 910 ° C, and air-cooled did. A thermocouple was inserted into the coil after winding, and the cooling rate was measured. In addition, about some coils, the coiling weight after hot rolling was adjusted, the small coil was produced, and the cooling rate was enlarged.
[0054]
The cooled hot-rolled steel sheet was shot blasted and pickled, descaled, and a JIS No. 5 tensile test specimen was taken in parallel with the rolling direction from the vicinity of the temperature measuring position of the coil with a thickness of 4.5 mm. Tensile tests were performed and 0.2% yield strength, tensile strength, yield ratio and elongation were measured. In addition, a 4.5 mm thick coil is slit to a width of 300 mm to make a web material, and a 6.0 mm thick coil is slit to a width of 150 mm to make a flange material. The materials were joined by electric resistance welding to produce an H-section steel. The welding at this time was performed under the conditions of atmospheric gas: air or nitrogen gas purge, input power: 330 to 370 kW, welding speed: 20 to 40 m / min. From this welded H-shaped steel, in accordance with JIS G3353, cut out a 35mm wide H-shaped welded part tensile test piece in the welding direction, hold both flanges, conduct a tensile test, and measure the tensile strength and break position. Was conducted.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
The results of the above test are also shown in Table 2. The steel plate manufactured according to the present invention has the same strength as SS400 and SN400B. Further, the strength of the H-shaped steel formed by welding was as strong as that of SS400 steel, and there was no embrittlement of the welded part accompanying electrical resistance welding, and all fractured at the web position. Also, in welding in the atmosphere, good welding could be performed, and no weld breakage occurred due to the unpenetrated penetrator. On the other hand, in the comparative example deviating from the component composition of the present invention, the intended strength was not obtained, and also in the tensile test, fracture occurred in the welded portion, and sufficient strength was not obtained.
Specifically, No. 10 was not subjected to strong rolling under rough rolling, so the strength of the H-shaped steel weld was not sufficient, and No. 11 was steel plate strength because the cooling rate after hot rolling was large. Is higher than the target. Nos. 14 and 15 are high in C or C + N, so that the welded portion is significantly embrittled and the welding strength is not sufficient. In No. 16, the penetration of the penetrator is insufficient due to the low Cu content, and in No. 17, the ferrite content of the weld heat affected zone is coarsened due to the low V content. Since the Mn content is large, the heat-affected zone is greatly cured, and in all cases, the fracture occurs in the weld zone.
[0058]
(Example 2)
Molten steel having the composition shown in Table 3 was melted through a converter-secondary refining process, and a slab having a thickness of 200 mm was formed by a continuous casting method. After reheating these slabs to 1170-1220 ° C, under the conditions shown in Table 4, a total of 7 passes of rough rolling in which the reduction rate of the 6th pass is 30-45% and the reduction rate of other passes is less than 30%. , And then rolled into 4.5 mm and 6.0 mm thick hot rolled steel sheets at a temperature of 815 to 910 ° C. by 7-pass finish rolling with a finishing temperature of 940 to 1050 ° C. . The coil after winding was transported to a heat insulation yard covered with a heat insulating material and covered with a heat insulating cover lined with a 100 mm thick heat insulating material on the inside to heat the coil and gradually cooled. The cooling rate of the coil was measured by welding a thermocouple in the vicinity of the outermost winding edge of the coil. For some coils, the cooling rate was changed by adjusting the coil weight or changing the thickness of the heat insulating material. A JIS No. 5 tensile test piece was cut out in parallel to the rolling direction from the outermost winding edge portion of the hot-rolled coil and a portion of the outermost winding plate width direction 1/4, and a tensile test was performed.
[0059]
[Table 3]

[0060]
The results of the above test are shown together in Table 4. In accordance with the present invention, a steel sheet that has been gradually cooled by covering a coil after winding with a heat insulation cover is hardly hardened even at the coldest point near the edge of the coil outermost winding, and is as soft as SS400 steel or SN400B. Was able to obtain a good strength. On the other hand, even if heat retention is performed, in the comparative example that deviates from the conditions of the present invention, the strength increase at the edge portion is large, and in the comparative example in which the component is further out, the desired strength is not obtained even at 1/4 width. It was.
Specifically, in Nos. 30 and 31, the cooling rate after winding is faster than the range of the present invention, so that no softening is obtained at the outermost winding portion of the coil. Further, since No. 34 has C, No. 35 has N, and No. 36 has high C + N, the strength of the outermost winding portion is increased. Furthermore, No. 37 has a high Cu content, No. 38 has a high V content, and No. 39 has a high Mn content. It has become.
[0061]
[Table 4]

[0062]
【The invention's effect】
As described above, according to the present invention, by appropriately combining the component composition of the steel sheet, the hot rolling conditions, and the cooling conditions after hot rolling, the strength of SS400 steel can be obtained in the state of hot rolling. In addition, since a structural Fe-Cr steel plate that does not harden at the rear end portion and the width end portion of the coil tip can be obtained, various shape steels can be manufactured even in a conventional production line. Moreover, since the steel plate of the present invention can be formed by a welding method that undergoes rapid heating and rapid cooling, a structural shape steel can be produced by electric resistance welding. Furthermore, since the steel plate of the present invention has sufficient corrosion resistance and durability even when used in civil engineering and construction structures, it can reduce the life cycle cost, and its industrial utility value is extremely large.
[Brief description of the drawings]
FIG. 1 is a graph showing an example of a result of calculating a temperature history of a hot-rolled coil after winding.
FIG. 2 is a graph showing an example of a result of calculating a temperature history when a heat insulation cover is put on a hot rolled coil after winding.
FIG. 3 is a diagram schematically showing a heat retention method using a heat retention cover.
FIG. 4 is a graph showing the cooling curve of FIG. 2 and the cooling curve of 2 ° C./min superimposed on the CCT diagram.
[Explanation of symbols]
1. Thermal insulation cover
2. Coil after hot rolling

Claims (9)

C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 0.01 to 0.30 mass %, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 8 mass% or more and less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, V : 0.01 to 0.20 mass%, Al: 0.05 mass% or less, the balance is made of Fe and inevitable impurities, and the tensile strength is 400 to 450 MPa without hot-rolled sheet annealing. Featuring structural Fe-Cr steel sheet. The structural Fe-Cr steel sheet according to claim 1, further comprising Mo: 1.0 mass% or less in addition to the component composition. C: 0.0025 to 0.010 mass%, N: 0.0025 to 0.010 mass%, C + N: 0.015 mass% or less, Si: 0.01 to 1.0 mass%, Mn: 0.01 to 0.30 mass %, P: 0.04 mass% or less, S: 0.03 mass% or less, Cr: 8 mass% or more and less than 10 mass%, Cu: 0.01 to 1.0 mass%, Ni: 0.01 to 1.0 mass%, V : A steel material containing 0.01 to 0.20 mass% and Al: 0.05 mass% or less is heated to a temperature of 1100 to 1280 ° C, and hot rolling is finished at a temperature of more than 930 ° C, exceeding 810 ° C. coiling at temperature, by an average cooling rate of between eight hundred to four hundred ° C. inside the coil and 2 ° C. / min or less, a tensile without applying hot rolled sheet annealing strength 400~450MPa Structural Fe-Cr-based method for producing a steel sheet, characterized in that to obtain a steel plate. The manufacturing method according to claim 3, further comprising Mo: 1.0 mass% or less in addition to the component composition of the steel material. In the said manufacturing method, at least 1 pass of rough rolling is performed at the temperature over 1000 degreeC with the rolling reduction of 30% or more, The manufacturing method of Claim 3 or 4 characterized by the above-mentioned. In the above manufacturing method, the manufacturing method according to any one of claims 3-5, wherein the average child and the cooling rate 2 ° C. / min or less between 800 to 400 ° C. at all positions of the coils. The manufacturing method according to claim 6, wherein in the manufacturing method, the coil is cooled using any one of a heat insulating cover, a heat insulating box, and a heat insulating furnace. A structural steel, wherein the steel plate according to claim 1 or 2 is shaped into a shape steel by electric resistance welding. A structural steel, wherein the steel plate produced by the method according to any one of claims 3 to 7 is made into a steel shape by electric resistance welding.

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