JP3891030B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

【００４８】
加熱したスラブを熱間圧延により圧延した後、直ちに水冷型の加速冷却設備を用いて冷却を行い、誘導加熱炉またはガス燃焼炉を用いて再加熱を行った。誘導加熱炉は加速冷却設備と同一ライン上に設置した。各鋼板（Ｎｏ.１〜３０）の製造条件を表２に示す。
【００４９】
以上のようにして製造した鋼板のミクロ組織を、光学顕微鏡、透過型電子顕微鏡（ＴＥＭ）により観察した。析出物の成分はエネルギー分散型Ｘ線分光法（ＥＤＸ）により分析した。また各鋼板の引張特性、母材靭性を測定した。測定結果を表２に併せて示す。引張特性は、圧延垂直方向の全厚試験片を引張試験片として引張試験を行い、引張強度を測定した。引張強度７００MPa以上（API X80グレード以上）を本発明に必要な強度とした。母材靭性（ｖＥ）については、−１０℃でのシャルピー吸収エネルギーが１００J以上の物を良好とした。
【００５０】
【表２】

【００５１】
表２において、本発明例であるＮｏ.１〜１８はいずれも、化学成分および製造方法が本発明の範囲内であり、引張強度７００MPa以上の高強度であり、かつ鋼板の組織は、実質的にフェライト＋ベイナイト２相組織であり、ＴｉとＭｏと、一部の鋼板についてはさらにＮｂおよび／またはＶとを含む粒径が１０nm未満の微細な炭化物の析出物が分散析出していた。
【００５２】
Ｎｏ.１９〜２５は、化学成分は本発明の範囲内であるが、製造方法が本発明の範囲外であるため、組織がフェライト＋ベイナイト２相組織にならない場合や、微細炭化物が分散析出しない場合があり、強度不足であった。Ｎｏ.２６〜３０は化学成分が本発明の範囲外であるので、十分な強度が得られないか、靭性が劣っていた。
【００５３】
【発明の効果】
以上述べたように、本発明によれば、引張強度７００MPa以上の高強度を有する高強度鋼板を、多量の合金元素を添加することなく、低コストで製造することができる。このため建築、海洋構造物、造船、土木、建設機械等の溶接構造物に使用する鋼板を、安価で大量に安定して製造することができ、生産性および経済性を著しく高めることができる。
【図面の簡単な説明】
【図１】本発明の製造方法を実施するための製造ラインの一例を示す概略図。
【符号の説明】
１：圧延ライン、
２：鋼板、
３：熱間圧延機、
４：加速冷却装置、
５：インライン型誘導加熱装置、
６：ホットレベラー[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel plate used in the fields of architecture, offshore structures, shipbuilding, civil engineering, construction machinery and the like, and a method for producing the same.
[0002]
[Prior art]
From the viewpoint of increasing the size of welded steel structures and reducing costs, there is an increasing demand for steel sheets having higher strength and higher toughness. Usually, a high-strength and high-toughness steel sheet is manufactured by a so-called TMCP method using quenching and tempering treatment or controlled rolling / controlled cooling. However, the quenching and tempering treatment requires time and labor and is expensive to manufacture. Further, when increasing the strength of a steel material using the TMCP method, it is necessary to add a large amount of alloy elements to the steel material, which raises a problem of cost increase due to the addition of alloy elements.
[0003]
In order to compensate for the shortcomings of quenching and tempering, Japanese Patent Publication No. 53-6616 and Japanese Patent Publication No. 58-3011 disclose direct quenching techniques in which quenching is performed as it is after rolling. Since the process is performed separately from the line, there is no significant difference from the conventional type, and manufacturing efficiency and manufacturing cost are not improved.
[0004]
On the other hand, Japanese Patent No. 3015923 and Japanese Patent No. 3015924 disclose a technique in which rolling to quenching and tempering are performed on the same line, and rapid heating and tempering without holding time are performed. Since all the processes are performed on the same line, the manufacturing time is shortened, so that the manufacturing efficiency and the manufacturing cost are greatly improved. In addition, the steel produced by this technique is rapidly tempered to form a bainite or martensite structure, followed by rapid heating and tempering, so that supersaturated solid solution carbon precipitates as fine cementite, and further tempering without holding time. Since cementite does not become coarse due to the treatment, it has excellent strength and toughness.
[0005]
[Problems to be solved by the invention]
However, in the techniques described in Japanese Patent Nos. 3015923 and 3015924, the manufacturing efficiency and manufacturing cost can be greatly improved. However, in order to obtain high-strength steel, as shown in the examples, the carbon of the steel material is used. Since it is necessary to increase the content or increase the amount of other alloy elements added, the material cost increases. As described above, in the conventional technique, it is difficult to manufacture a high-strength steel sheet without adding a large amount of alloy elements.
[0006]
Accordingly, an object of the present invention is to solve such problems of the prior art and provide a high-strength steel sheet that can be manufactured at a low cost without adding a large amount of alloy elements and a method for manufacturing the same.
[0007]
[Means for Solving the Problems]
The features of the present invention for solving such problems are as follows.
(1), in mass%, C: 0.02% or more, less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.0%, Mo: 0.05 to 0.5%, Ti: 0.04%, greater than 0.10%, Al: containing from 0.01 to 0.08%, the balance being Fe and inevitable impurities or Rannahli, the C content in atomic% Mo, C / is the total amount of the ratio of Ti (Mo + Ti) is 0.5 to 3.0, a metal structure gaff ferrite and bainite dual phase structure of the Ti in the ferrite phase, Mo A high-strength steel sheet, characterized in that a carbide containing is dispersed and precipitated.
[0008]
(2) Further, by mass%, Nb: 0.005 to 0.07% and / or V: 0.005 to 0.10%, C amount in atomic% and Mo, Ti, Nb, C / (Mo + Ti + Nb + V), which is the ratio of the total amount of V, is 0.5 to 3.0, and carbides containing Ti, Mo, Nb and / or V are dispersed and precipitated in the ferrite phase. The high-strength steel sheet according to (1), characterized in that
[0009]
(3) Furthermore, by mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, B: 0.005% or less The high-strength steel sheet according to (1) or (2), which contains seeds or more.
[0010]
(4) After heating the steel having the component composition according to any one of (1) to (3) to a temperature of 1000 to 1300 ° C and hot rolling at a rolling end temperature of 750 ° C or higher, 5 ° C A high-strength steel sheet characterized by performing accelerated cooling to 300 to 600 ° C. at a cooling rate of not less than / s, and immediately reheating to 550 to 700 ° C. at a temperature rising rate of not less than 0.5 ° C./s. Production method.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors diligently studied a method for producing a high-strength steel sheet that can be produced at a low cost without adding a large amount of alloying elements. In the production process of accelerated cooling after controlled rolling and subsequent reheating, the bainite transformation is performed. By reheating in the middle, in addition to strengthening due to bainite transformation during accelerated cooling, precipitation strengthening due to fine precipitates precipitated during ferrite transformation from untransformed austenite during reheating reduces the number of alloy elements and We obtained the knowledge that high strength can be achieved in steel. Then, by using steel containing Mo and Ti, a dispersion precipitate of a composite carbide of very fine Mo and Ti is obtained. Even when Nb and V are added in combination, Ti, Mo and Nb and The inventors have found that the ferrite phase can be strengthened by dispersing and precipitating a precipitate containing V.
[0012]
The present invention has a two-phase structure composed of a bainite phase generated by accelerated cooling after rolling and a ferrite phase in which precipitates containing Ti and Mo generated by subsequent reheating are dispersed and precipitated. The present invention relates to a high-strength steel sheet and a method for producing the same. In order to make the best use of precipitation strengthening in addition to transformation strengthening, it is not necessary to add a large amount of alloy elements, and high strength can be achieved.
[0013]
Hereinafter, the high-strength steel sheet of the present invention will be described in detail. First, the structure of the high-strength steel sheet of the present invention will be described.
[0014]
The metal structure of the steel sheet of the present invention is substantially a two-phase structure of ferrite and bainite. In the present invention, a combination of transformation strengthening by bainite transformation during accelerated cooling and precipitation strengthening by fine precipitates reheated after accelerated cooling and precipitating in ferrite can be used to add a large amount of alloying elements. The strength can be increased. The ferrite phase is rich in ductility and is generally soft, but in the present invention, high strength can be achieved by the fine precipitates described below. On the other hand, when a large amount of the alloy element is not added, the strength is insufficient only with the bainite single layer structure obtained by accelerated cooling, but the two-phase structure with the precipitation strengthened ferrite phase has sufficient strength. It will be a thing. When two or more different metal structures such as martensite and pearlite are mixed in the two-phase structure of ferrite and bainite, the strength decreases, so the smaller the fraction of the structure other than the ferrite and bainite phases. good. However, if the volume fraction of the structure other than the ferrite phase and the bainite phase is low, the influence can be ignored. Therefore, other metal structures of 5% or less in total volume fraction, that is, one type of martensite, pearlite, etc. You may contain 2 or more types. Further, it is desirable that the ferrite fraction is 5% or more from the viewpoint of securing strength, and the bainite fraction is 10% or more from the viewpoint of securing toughness of the base material.
[0015]
Next, the precipitate that is dispersed and precipitated in the ferrite phase will be described.
In the steel sheet of the present invention, precipitation strengthening by precipitates containing Mo and Ti in the ferrite phase as a basis is utilized. Mo and Ti are elements that form carbides in steel, and it has been conventionally practiced to strengthen steel by precipitation of MoC and TiC. However, in the present invention, Mo and Ti are added together to form Mo and Ti. It is a feature that a greater strength improvement effect can be obtained by finely depositing a composite carbide containing the above in steel as compared with the case of precipitation strengthening of MoC or TiC. This unprecedented strength improvement effect is due to the fact that a composite carbide containing Mo and Ti as a basis is stable and has a slow growth rate, so that an extremely fine precipitate having a particle size of less than 10 nm can be obtained. .
[0016]
When the composite carbide containing Mo and Ti as a base is composed of only Mo, Ti, and C, the total of Mo and Ti and C are combined in an atomic ratio of about 1: 1. Yes, it is very effective for increasing strength. In the present invention, it has been found that by adding Nb and / or V in combination, the precipitate becomes a composite carbide containing Mo, Ti and Nb and / or V, and the same precipitation strengthening can be obtained.
[0017]
In the present invention, the composite carbide mainly composed of Mo and Ti, which is a precipitate dispersed and precipitated in the steel sheet, is produced by manufacturing the steel sheet using the manufacturing method of the present invention to steel having the components described below. It can be obtained by dispersing in.
[0018]
Next, chemical components of the high-strength steel sheet of the present invention will be described. In the following description, all units represented by% are mass%.
[0019]
C: Not less than 0.02 and less than 0.07%. C is an element that contributes to precipitation strengthening as a carbide. However, if it is less than 0.02%, sufficient strength cannot be secured, and if it is 0.07% or more, the toughness is deteriorated, so the C content is 0.02 or more, 0 It is specified to be less than 0.07%.
[0020]
Si: 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is 0.01 to 0.00. Specify 5%.
[0021]
Mn: 0.5 to 2.0%. Mn is added for strength and toughness, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.0%, the weldability deteriorates, so the Mn content is 0.5 to 2.0%. Stipulate.
[0022]
Al: 0.01 to 0.08%. Al is added as a deoxidizer, but if it is less than 0.01%, there is no effect, and if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.01 to It is specified as 0.08%.
[0023]
Mo: 0.05 to 0.5%. Mo is an important element in the present invention, and by containing 0.05% or more, fine composite precipitates with Ti are formed while suppressing pearlite transformation during cooling after hot rolling, thereby increasing strength. A big contribution. However, if it exceeds 0.5%, the cost is increased and the weldability is deteriorated, so the Mo content is specified to be 0.05 to 0.5%.
[0024]
Ti: More than 0.04% and not more than 0.10%. Ti, like Mo, is an important element in the present invention. By adding over 0.04%, a composite precipitate is formed with Mo, which greatly contributes to an increase in strength. However, it is up to 0.10% that contributes to the strength increase, and addition beyond that causes a cost increase, so the Ti content is specified to be more than 0.04% and not more than 0.10%.
[0025]
The high-strength steel sheet of the present invention can produce fine precipitates of composite carbide containing Ti and Mo by using the steel of the above components, but in order to make maximum use of precipitation strengthening, the carbide is formed. It is desirable to limit the proportion of element content as follows. That is, C / (Mo + Ti), which is a ratio of the amount of C in atomic% and the total amount of Mo and Ti, is preferably 0.5 to 3.0. The increase in strength according to the present invention is due to precipitates containing Ti and Mo. In order to effectively use the precipitation strengthening by this composite precipitate, the relationship between the amount of C and Mo and Ti which are carbide forming elements is important. By adding these elements in an appropriate balance, A thermally stable and very fine composite precipitate can be obtained. At this time, when the value of C / (Mo + Ti) represented by the atomic% content of each element is less than 0.5 or more than 3.0, either element amount is excessive, and Ti of the present invention Since precipitates other than fine precipitates containing Mo and a hardened phase are generated and toughness is deteriorated, the value of C / (Mo + Ti) is preferably set to 0.5 to 3.0. However, each element symbol is the content of each element in atomic%. In addition, when content of mass% is used, it is represented by (C / 12.01) / (Mo / 95.9 + Ti / 47.9).
[0026]
Since Nb and / or V forms a fine composite carbide with Ti and Mo, the steel sheet of the present invention may contain Nb and / or V.
[0027]
Nb: 0.005 to 0.07%. Nb improves toughness by refining the structure, but forms a composite precipitate with Ti and Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, weld cracks are likely to occur, so the Nb content is specified to be 0.005 to 0.07%.
[0028]
V: Set to 0.005 to 0.10%. V, like Nb, forms a composite precipitate with Ti and Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.10%, weld cracking tends to occur, so the V content is specified to be 0.005 to 0.10%.
[0029]
When Nb and / or V are contained, C / (Mo + Ti + Nb + V), which is the ratio of the amount of C in atomic% and the total amount of Mo, Ti, Nb, and V, is 0.5 to 3.0. Is preferred. Strengthening according to the present invention depends on precipitates containing Ti and Mo, but when Nb and / or V are contained, they become composite precipitates (mainly carbides) containing them. At this time, when the value of C / (Mo + Ti + Nb + V) represented by the content of atomic% of each element is less than 0.5 or exceeds 3.0, the amount of any element is excessive, and Ti of the present invention Since precipitates other than fine precipitates containing Mo and a hardened phase are generated and the toughness is deteriorated, the value of C / (Mo + Ti + Nb + V) is preferably set to 0.5 to 3.0. However, each element symbol is the content of each element in atomic%. In addition, when content of mass% is used, it is represented by (C / 12.01) / (Mo / 95.9 + Ti / 47.9 + Nb / 92.91 + V / 50.94).
[0030]
In the present invention, for the purpose of further improving the strength toughness of the steel sheet, one or more of Cu, Ni, Cr and B shown below may be contained.
[0031]
Cu: 0.55% or less. Cu is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates, so when added, the upper limit is 0.50%.
[0032]
Ni: It shall be 0.50% or less. Ni is an element effective for improving toughness and increasing strength, but adding a large amount is disadvantageous in terms of cost, so when added, the upper limit is 0.50%.
[0033]
Cr: 0.55% or less. Like Mn, Cr is an element effective for obtaining sufficient strength even at low C. However, if added in a large amount, the weldability deteriorates, so when added, the upper limit is 0.50%.
[0034]
B: Set to 0.005% or less. B is an element that contributes to strength increase and HAZ toughness improvement, but if added over 0.005%, weldability deteriorates, so when added, the content is made 0.005% or less.
[0035]
The remainder other than the above consists essentially of Fe. The balance substantially consisting of Fe means that an element containing an inevitable impurity and other trace elements can be included in the scope of the present invention unless the effects of the present invention are lost.
[0036]
Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
[0037]
The present invention uses a combination of transformation strengthening by bainite transformation during accelerated cooling and precipitation strengthening by fine carbides precipitated in ferrite during reheating after accelerated cooling, without adding a large amount of alloying elements. This is a technology that can increase strength. In the present invention, the ferrite transformation can be completed without maintaining the temperature during the subsequent reheating by supercooling to the bainite transformation region by accelerated cooling.
[0038]
The high-strength steel sheet of the present invention uses steel having the above component composition, and is hot-rolled at a heating temperature of 1000 to 1300 ° C. and a rolling end temperature of 750 ° C. or higher, and then 300 ° C. at a cooling rate of 5 ° C./s or higher. Accelerated cooling to ˜600 ° C., and then immediately reheating to a temperature of 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more, thereby converting the metal structure into a two-phase structure of ferrite and bainite. Fine composite carbides mainly composed of Ti and Ti can be dispersed and precipitated in the ferrite phase. Here, the temperature is the average temperature of the steel sheet. Hereinafter, each manufacturing condition will be described in detail.
[0039]
Heating temperature: 1000-1300 ° C. If the heating temperature is less than 1000 ° C., the solid solution of the carbide is insufficient and the required strength cannot be obtained, and if it exceeds 1300 ° C., the toughness deteriorates, so the temperature is set to 1000 to 1300 ° C.
[0040]
Rolling end temperature: 750 ° C. or higher. If the rolling end temperature is low, the subsequent ferrite transformation rate decreases, so that sufficient precipitation of fine precipitates cannot be obtained during ferrite transformation by reheating, and the strength decreases, so the rolling end temperature is 750 ° C. or higher. To do.
[0041]
Immediately after the end of rolling, it is cooled at a cooling rate of 5 ° C./s or more. If the cooling rate is less than 5 ° C./s, ferrite is generated during cooling, so that not only strengthening by bainite cannot be obtained, but also precipitates generated during ferrite transformation in a high temperature region of 700 ° C. or higher easily coarsen. A sufficient strength cannot be obtained. Therefore, the cooling rate after the end of rolling is specified to be 5 ° C./s or more. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process.
[0042]
Cooling stop temperature: 300 to 600 ° C. After completion of rolling, the bainite phase is rapidly cooled to 300 to 600 ° C. by accelerating cooling, thereby generating a bainite phase and stopping the cooling in the middle of the bainite transformation so that the untransformed austenite is subjected to subsequent reheating. It becomes possible to transform into ferrite. Furthermore, since the driving force is increased by the supercooling, the ferrite transformation in the reheating process is promoted, and the ferrite transformation can be completed by a short reheating. When the cooling stop temperature is less than 300 ° C., the bainite transformation is almost completed, so that not only a sufficient amount of ferrite can be obtained by the subsequent reheating, but also island-shaped martensite (MA) is generated, and thus the reheating is not performed. Precipitation of fine carbide becomes insufficient, and if it exceeds 600 ° C., the driving force for ferrite transformation is not sufficient, and ferrite transformation does not complete during reheating, so that precipitation of fine carbide is insufficient. Since the strength cannot be obtained, the accelerated cooling stop temperature is set to 300 to 600 ° C.
[0043]
Immediately after accelerated cooling, reheating is performed to a temperature of 550 to 700 ° C. at a heating rate of 0.5 ° C./s or more. This process is an important manufacturing condition in the present invention. Fine precipitates that contribute to strengthening of the ferrite phase are deposited simultaneously with the ferrite transformation during reheating. In order to obtain such fine precipitates, it is necessary to reheat to a temperature range of 550 to 700 ° C. immediately after accelerated cooling. When the heating rate is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency is deteriorated and pearlite transformation occurs, so that the fine precipitates cannot be dispersed and precipitated. A sufficient strength cannot be obtained. If the reheating temperature is less than 550 ° C., the ferrite transformation does not proceed and bainite transformation occurs, so that sufficient precipitation strengthening cannot be achieved, and if it exceeds 700 ° C., the precipitate becomes coarse and sufficient strength cannot be obtained. The temperature range of heating is specified at 550 to 700 ° C. There is no need to set the temperature holding time at the reheating temperature. If the production method of the present invention is used, even if it is cooled immediately after reheating, the ferrite transformation proceeds sufficiently, so that high strength due to fine precipitation can be obtained. However, temperature retention within 30 minutes can be performed in order to reliably complete the ferrite transformation. If the temperature is maintained for more than 30 minutes, the precipitates may become coarse and the strength may be reduced. Also, since the ferrite transformation proceeds in the cooling process after reheating, the cooling rate after reheating is basically air cooling. However, cooling can also be performed at a rapid cooling rate that does not inhibit the ferrite transformation.
[0044]
As equipment for performing reheating after accelerated cooling, a heating device can be installed on the downstream side of the cooling equipment for performing accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or induction heating device capable of rapid heating of the steel sheet. The induction heating device is particularly preferable because temperature control is easier than in a soaking furnace, the cost is relatively low, and the cooled steel sheet can be heated quickly. In addition, by arranging a plurality of induction heating devices continuously in series, even if the line speed and the type and dimensions of the steel sheet are different, the number of induction heating devices to be energized can be set arbitrarily, the heating rate, The reheating temperature can be freely controlled.
[0045]
Moreover, an example of the equipment for implementing the manufacturing method of this invention is shown in FIG. As shown in FIG. 1, a hot rolling mill 3, an acceleration cooling device 4, an in-line induction heating device 5, and a hot leveler 6 are arranged in the rolling line 1 from upstream to downstream. By installing the in-line type induction heating device 5 or other heat treatment device on the same line as the hot rolling mill 3 as a rolling facility and the accelerated cooling device 4 as a subsequent cooling facility, the rolling and cooling can be quickly performed. Since a reheating process can be performed, it can heat without reducing the steel plate temperature after rolling cooling too much.
[0046]
【Example】
Steels (steel types A to P) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 30) having a thickness of 18 and 26 mm were manufactured using the slabs.
[0047]
[Table 1]

[0048]
After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility and reheated using an induction heating furnace or a gas combustion furnace. The induction furnace was installed on the same line as the accelerated cooling equipment. Table 2 shows the production conditions of each steel plate (No. 1 to 30).
[0049]
The microstructure of the steel sheet produced as described above was observed with an optical microscope and a transmission electron microscope (TEM). The components of the precipitate were analyzed by energy dispersive X-ray spectroscopy (EDX). In addition, the tensile properties and base metal toughness of each steel plate were measured. The measurement results are also shown in Table 2. Tensile properties were measured by performing a tensile test using a full thickness test piece in the vertical direction of rolling as a tensile test piece, and measuring the tensile strength. The tensile strength of 700 MPa or more (API X80 grade or more) was determined as the strength required for the present invention. Regarding the base material toughness (vE), a material having Charpy absorbed energy at −10 ° C. of 100 J or more was considered good.
[0050]
[Table 2]

[0051]
In Table 2, all of Nos. 1 to 18 as examples of the present invention have chemical components and production methods within the scope of the present invention, have a high tensile strength of 700 MPa or more, and the structure of the steel sheet is substantially In addition, it has a ferrite + bainite two-phase structure, and fine carbide precipitates having a particle size of less than 10 nm containing Ti, Mo, and some steel sheets and further containing Nb and / or V were dispersed and precipitated.
[0052]
In Nos. 19 to 25, the chemical components are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, and therefore, the structure does not become a ferrite + bainite two-phase structure or fine carbides are not dispersed and precipitated. In some cases, the strength was insufficient. Nos. 26 to 30 had chemical components outside the scope of the present invention, so that sufficient strength could not be obtained or toughness was poor.
[0053]
【The invention's effect】
As described above, according to the present invention, a high-strength steel sheet having a high tensile strength of 700 MPa or more can be manufactured at a low cost without adding a large amount of alloy elements. For this reason, the steel plate used for welding structures, such as a building, a marine structure, shipbuilding, a civil engineering, a construction machine, can be manufactured stably in large quantities cheaply, and productivity and economical efficiency can be improved significantly.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an example of a production line for carrying out the production method of the present invention.
[Explanation of symbols]
1: rolling line,
2: Steel plate,
3: Hot rolling mill,
4: Accelerated cooling device,
5: Inline type induction heating device,
6: Hot leveler

Claims (4)

In mass%, C: 0.02% or more, less than 0.07%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.0%, Mo: 0.05 to 0.5% , Ti: 0.04%, greater than 0.10%, Al: contains 0.01 to 0.08 percent, the balance being Fe and inevitable impurities or Rannahli, C amount in atomic percent Mo, a Ti is the ratio of the total amount C / (Mo + Ti) is 0.5 to 3.0, a metal structure gaff ferrite and bainite dual phase structure of carbide containing and Ti in the ferrite phase, and Mo A high-strength steel sheet characterized in that is dispersed and precipitated. Further, in mass%, Nb: 0.005 to 0.07% and / or V: 0.005 to 0.10%, and the total amount of C, Mo, Ti, Nb, and V in atomic% The ratio of C / (Mo + Ti + Nb + V) is 0.5 to 3.0, and carbides containing Ti, Mo, Nb and / or V are dispersed and precipitated in the ferrite phase. The high-strength steel plate according to claim 1. Furthermore, by mass%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, and B: 0.005% or less are contained. The high-strength steel sheet according to claim 1 or 2, wherein the steel sheet has high strength. The steel having the component composition according to any one of claims 1 to 3 is heated to a temperature of 1000 to 1300 ° C, hot-rolled at a rolling finish temperature of 750 ° C or higher, and then 5 ° C / s or higher. A method for producing a high-strength steel sheet, characterized in that accelerated cooling is performed at a cooling rate of 300 to 600 ° C, and then reheating is performed immediately at 550 to 700 ° C at a heating rate of 0.5 ° C / s or more.

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