JP2006274388A - HIGH TENSILE STRENGTH STEEL SHEET SATISFYING YIELD STRENGTH OF >=650 MPa AND HAVING LOW ACOUSTIC ANISOTROPY, AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength steel sheet satisfying yield strength in a class of ≥650 MPa and having low acoustic anisotropy, and to provide a production method where off-line heat treatment for the steel sheet is not required. <P>SOLUTION: The steel sheet has a composition comprising 0.04 to 0.09% C, 0.01 to 0.6% Si, >2.00 to 3.00% Mn, and 0.0005 to 0.005% B, and comprising Nb and Ti so as to satisfy 0.045%≤Nb+2Ti≤0.090%, and in which A=(Nb+2Ti)×(C+N×12/14) satisfies 0.0025 to 0.0055, and has a structure composed of ≥80% bainite. In the method for producing the steel sheet, the componential steel is heated at T(=6300/(1.9-Log(A))-273) to 1,300°C, and is subjected to rolling so as to be rough-rolled at ≥1,020°C, thereafter to be rolled at a cumulative draft of ≤15% in the range from >920 to <1,020°C, to be rolled at a cumulative draft of 20 to 50% in the range from 860 to 920°C, and to be rolled at a finishing temperature of 860°C, successively, accelerated cooling is started from ≥800°C, and the cooling is stopped at 420 to 570°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength steel sheet having a yield strength of 650 MPa or higher with a small acoustic anisotropy and a method for producing the same, and in particular, an acoustic that can be produced with high productivity that does not require off-line heat treatment. The present invention relates to a high-tensile steel plate having a low anisotropy and a yield strength of 650 MPa or more and a method for producing the same. The steel of the present invention is used in the form of a thick steel plate as a structural member of a welded structure such as a building, a bridge, a ship, a construction machine, an industrial machine, an offshore structure, and a penstock.

  For welded structural members in the fields of architecture, bridges, construction machinery, industrial machinery, etc., with the recent increase in size of structures, for the purpose of reducing the weight of steel materials, higher strength than conventional ones, for example, yield strength 650 MPa. The above steel materials tend to be used.

  Conventionally, this class of high-strength steel has been generally manufactured by rolling a steel plate offline after re-heating and further performing tempering heat treatment by re-heating. Recently, there are many cases of so-called direct quenching, in which quenching is performed on-line after rolling of the steel sheet, but even in this case, tempering heat treatment by reheating is necessary.

  However, the need for an off-line heat treatment step inevitably hinders productivity, so to improve productivity, so-called non-tempered production that does not require off-line heat treatment by omitting tempering heat treatment. The method is desirable.

  Several methods for producing this class of high-strength steel with no tempering have been disclosed, all of which are techniques for producing an alloy by adjusting the alloy composition, heating, rolling conditions, and online water-cooling conditions.

  For example, Patent Document 1 discloses an invention relating to a manufacturing method in which steel having a specific composition is accelerated and cooled from a temperature range not lower than 500 ° C. to a temperature of 300 ° C. or less after hot rolling. However, by accelerated cooling to such a low temperature range, a structure containing martensite is obtained, so that it is easy to obtain strength, but the residual stress of the steel sheet increases. If the residual stress of the steel plate is large, the steel plate may be greatly deformed when it is cut, so that there is a problem in using it as a member for a welded structure.

  The invention described in Patent Document 2 also relates to a technique for producing this class of high-strength steel on-line. However, it requires complicated control of accelerating cooling after interrupting accelerating cooling and reheating. It is presumed that special equipment is required, and it cannot be said that it is a general technique.

  The invention described in Patent Document 3 sets the water cooling stop temperature to a relatively high temperature of 550 ° C. or higher in the process of stopping accelerated cooling in the middle. Thus, if accelerated cooling is stopped at a relatively high temperature, there is no problem of the residual stress as described above. However, in the present invention, it is essential to contain 1.0% or more of Cu, which increases the alloy cost. There's a problem.

  Further, Patent Document 4 also discloses an invention relating to a manufacturing method by an accelerated cooling-intermediate stopping process in which an accelerated cooling stop temperature is set to 580 ° C. to 450 ° C. However, controlled rolling at a low temperature is required to obtain strength, and the rolling completion temperature is 800 ° C. or lower (700 ° C. or lower in the examples). In particular, in applications such as bridges and buildings, it is required that the acoustic anisotropy of the steel sheet be small in order to affect the accuracy of the ultrasonic oblique flaw detection test of the weld, but the rolling is completed at a temperature of 800 ° C. or lower. In controlled rolling, since a texture is formed, the acoustic anisotropy of the steel sheet increases, and this does not necessarily match such applications.

  Furthermore, weldability is required for such high-tensile steel sheets, but in recent years, welding with particularly high heat input is often applied, and the toughness of the heat affected zone in high heat input welding is high. May be required.

  For high-strength steel with a yield strength of 650 MPa or more, it can be said that there has never been an economical non-tempered manufacturing method that comprehensively considers acoustic anisotropy, residual stress of a steel sheet, and weldability.

JP 2003-003233 A JP 2003-277829 A Japanese Patent Laid-Open No. 11-264017 Japanese Patent Laid-Open No. 06-093332

  Accordingly, in the present invention, a high-tensile steel sheet having a yield strength of 650 MPa or less with a small acoustic anisotropy, an economical component composition with a small amount of alloy addition, an accelerated cooling with a high productivity and a small residual stress of the steel sheet. -It was made into the subject obtained by the manufacturing method premised on the stop process on the way.

  In order to obtain a yield strength of 650 MPa or more, the influence of the steel structure is still great. When a structure containing martensite is accelerated by cooling to an Ms point (martensite transformation start temperature) or lower, the tensile strength increases, but the dislocation density becomes too high, and as a result, the number of movable dislocations increases, yield strength increases. May decline. On the other hand, if the water cooling stop temperature is raised and ferrite is produced, the yield strength decreases, which is disadvantageous for obtaining high strength. In order to obtain a higher yield strength, it is advantageous that the water cooling stop temperature is lowered to some extent and higher than the Ms point (martensite transformation start temperature), so that the structure is a bainite-based structure containing no martensite. is there.

  However, if an accelerated cooling-intermediate stop process is presupposed, relying solely on the strength of the substrate to obtain strength requires the addition of a large amount of C or alloy in order to ensure hardenability and cost. Or balance with weldability cannot be taken.

  As a strengthening means capable of strengthening with relatively few alloy components, there is a method using precipitation strengthening such as carbide or nitride of Nb, V, Ti, Mo, Cr. At that time, in order to obtain a large precipitation strengthening amount, it is important to form precipitates that are consistent with the substrate.

  In the accelerated cooling-intermediate stop process, the steel structure is austenite at the stage of rolling, and is transformed by accelerated cooling to a bainite-based structure. Precipitates precipitated in austenite before rolling or accelerated cooling lose their consistency with the substrate after transformation and the strengthening effect is reduced. In addition, precipitates precipitated at an early stage of rolling become coarse and cause toughness to decrease. Therefore, it is important to suppress the precipitation of precipitates during rolling and before accelerated cooling, and to precipitate the precipitates as much as possible in the bainite structure at the stage of slow cooling after the stop of accelerated cooling. If it is a process in which tempering heat treatment is performed by reheating after water cooling, a sufficient temperature and time for precipitation of the precipitate can be obtained, so that large precipitation strengthening can be easily obtained. On the other hand, in the case of the accelerated cooling-intermediate stop process without reheating and tempering, precipitation is expected during the slow cooling after stopping the accelerated cooling, but the accelerated cooling stop temperature is set to obtain a quenched structure. Since the temperature has to be lowered to some extent, both the temperature and time for precipitation are limited, which is generally disadvantageous for precipitation strengthening. For this reason, as described above, the non-tempering process is highly productive, but in order to obtain the same strength as the tempering process, accelerated cooling to a low temperature, a large amount of alloying elements are required, or a relatively low temperature is required. This is why the controlled rolling of the material has to be performed.

  Therefore, the inventors have obtained a high strength with a yield strength of 650 MPa or more without adding a large amount of alloy elements or by controlled rolling at a low temperature, assuming a highly productive accelerated cooling-interruption process. In addition, we have made extensive studies on how to make the most of precipitation strengthening while obtaining a bainite-based structure.

  First, in order to clarify the precipitation behavior in the slow cooling process after stopping the accelerated cooling, the precipitation rate and precipitation strengthening amount of carbide, nitride and carbonitride of each alloy element in the bainite structure, and the temperature and holding time The relationship was examined in detail. As a result, in the bainite structure, the precipitation rate of Nb carbonitride and Ti carbide is faster than other elements such as V, and these are precipitates that are consistent with the base material, so the amount of strengthening is large. Although the deposition rate is the fastest in the temperature range of 600 to 700 ° C., it is possible to form a substrate even in a relatively short time of holding at less than 600 ° C. by performing composite precipitation using Nb and Ti or Nb, Ti and Mo in combination. Consistent precipitates are finely dispersed. The inventors have found that, by setting the midway temperature of accelerated cooling to 420 ° C. to 570 ° C., considerable precipitation strengthening is possible while obtaining a bainite-based structure that does not contain martensite.

  However, if the amount of Nb and Ti added is too large, the generated precipitates tend to be coarse, and the number of precipitates is rather reduced, so that the amount of precipitation strengthening decreases. The precipitation rate and form of precipitates in the austenite and bainite of Nb and Ti carbides, nitrides and carbonitrides are greatly affected by the amounts of Nb and Ti added and the amounts of C and N. Through various experiments and analyses, the inventors have found that the precipitation rate and precipitation form of carbides, nitrides, and carbonitrides of Nb and Ti are parameters A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14) is well organized, and by setting this value within a certain range, it is possible to sufficiently obtain fine precipitation during slow cooling after stopping during water cooling while suppressing precipitation during rolling. Got. That is, it is necessary to decrease the addition amount of C and N as the addition amount of Nb and Ti increases. When the value of A is too small, the precipitation rate in bainite becomes slow, and sufficient precipitation strengthening cannot be obtained. Conversely, if the value of A is too large, the precipitation rate of carbides, nitrides and carbonitrides in austenite becomes too high and the precipitates become coarser, and the amount of matched precipitation during slow cooling after accelerating cooling is stopped, Again, the precipitation strengthening amount decreases. Specifically, Nb is added to be 0.02% or more, 0.08% or less, Ti is 0.005% or more, and [Nb] + 2 × [Ti] is 0.045% or more. Setting the value of parameter A to 0.0025 or more and 0.0055 or less is a condition for obtaining a yield strength of 650 MPa in the steel of the present invention.

  The bainite structure is easier to maintain a processed structure such as dislocation density than ferrite. In order to promote fine alignment precipitation, it is very effective that there are sufficient precipitation sites such as dislocations and deformation bands included in the processed structure. According to the study by the inventors, in order to obtain precipitation strengthening in addition to sufficient structure strengthening, the volume fraction of bainite is preferably at least 80% or more. The bainite structure here includes one or both of so-called upper bainite and lower bainite.

  Subsequently, the inventors examined specific manufacturing conditions for obtaining the maximum precipitation strengthening effect, and obtained the following knowledge.

The present invention aims to obtain strength by making the best use of precipitation strengthening of Nb, Ti, etc. in the accelerated cooling-intermediate stop process following rolling, in addition to the structural strengthening by bainite. It is necessary to sufficiently dissolve Nb and Ti when the piece or slab is heated. However, when Nb and Ti coexist, they tend to be harder to dissolve than when they exist alone, and these cannot always be sufficiently dissolved by heating to the solid solution temperature expected from the respective solubility products. I understood. The inventors investigated the heating temperature and the solid solution state of Nb and Ti in the steel of the present invention, and particularly analyzed the relationship between the A value and the solid solution state of Nb and Ti in detail thermodynamically. As a result, by making the heating temperature of the steel slab or slab higher than the temperature T (° C.) calculated by the conditional expression including the A value as shown below, Nb and Ti are sufficiently dissolved. I came to the conclusion that I could do it.
T = 6300 / (1.9-Log (A))-273
here,
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
[Nb], [Ti], [C], and [N] mean the contents expressed by mass% of Nb, Ti, C, and N, respectively.

  Since precipitation of Nb and Ti in the rolling stage is promoted by rolling strain, rolling conditions in a high temperature range of austenite, so-called rough rolling conditions, greatly influence the final precipitation strengthening effect. Specifically, the rough rolling is completed in a temperature range of 1020 ° C. or higher, and not rolling as much as possible in the temperature range of 1020 ° C. to 920 ° C. is a requirement for suppressing precipitation during rolling. However, if all rolling is completed in a temperature range of 1020 ° C. or higher, almost no work structure remains after accelerated cooling-interruption due to recovery and recrystallization, so there are sufficient precipitation sites such as dislocations and deformation bands. It does not exist and sufficient precipitation strengthening cannot be obtained. Therefore, it is an essential condition to perform necessary and sufficient rolling in the non-recrystallization temperature range and to perform accelerated cooling immediately after rolling. As a result of the study by the inventors, it has been found that it is a specific condition to perform relatively mild rolling with a cumulative rolling reduction of 20% to 50% within a limited range of 920 ° C to 860 ° C. Under these conditions, the rolling strain is not excessively large, so that unnecessary precipitation of Nb and Ti is suppressed and a strong texture is not formed, so that acoustic anisotropy does not increase. In addition, a necessary amount of rolling strain can be ensured in order to leave an appropriate precipitation site even after the accelerated cooling is stopped.

  The accelerated cooling / stopping temperature of the accelerated cooling / intermediate stopping process is set to 420 ° C. to 570 ° C. so as to be advantageous for precipitation of Nb and Ti while obtaining a bainite-based structure. In order to set it to 80% or more, the component composition of steel is limited to a specific range described later, and a cooling rate of 5 ° C./sec or more is required in accelerated cooling.

  The knowledge obtained here is that the acoustic anisotropy is reduced by rolling conditions that do not form a strong texture, the residual stress is reduced by obtaining a bainite-based structure that does not contain martensite, and Nb into the bainite structure. This is a new concept that controls the precipitation of Ti carbides or carbonitrides on-line from rolling, including high temperature range, to accelerated cooling, and to the slow cooling process after stopping cooling. Both structural strengthening and precipitation strengthening can be realized by an accelerated cooling-interruption process that does not require off-line heat treatment.

  Moreover, according to this manufacturing process, the weld cracking sensitivity index Pcm (Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [ Mo] / 15 + [V] / 10 + 5 [B]: Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B ] Means the content expressed by mass% of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively)), and yield strength is strong when Pcm ≦ 0.20. A strength of 650 MPa or more can be obtained. If Pcm ≦ 0.20 in the steel material of the present invention, sufficiently high toughness of the weld heat affected zone can be obtained for welding with a welding heat input of about 6 kJ / mm. However, the influence of Si on the toughness of the weld heat affected zone is also large. For example, in order to obtain a high weld heat affected zone toughness even in high heat input welding with a heat input of about 10 kJ / mm, the amount of Si is further increased. Must be 0.3% or less.

The present invention has been completed for the first time based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.04% or more, 0.09% or less, Si: 0.01% or more, 0.6% or less, Mn: more than 2.00%, 3.00% or less, Al : 0.003% or more, 0.10% or less, N: 0.002% or more, 0.008% or less, B: 0.0005% or more, 0.005% or less, and further Nb and Ti Nb: 0.02% or more, 0.08% or less, Ti: 0.005% or more, 0.03% or less, and 0.045% ≦ [Nb] + 2 × [Ti] ≦ 0.090% In addition, Nb, Ti, C, and N are contained within a range that satisfies the relationship that the value of A shown below is 0.0025 or more and 0.0055 or less, and is susceptible to weld cracking. Index Pcm ≦ 0.20, having a component composition consisting of the balance Fe and inevitable impurities, and having a steel structure High-tensile steel plate that the features, the more acoustic anisotropy is small yield strength 650MPa the volume ratio of bainite is 80% or more.
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
It is. Here, [Nb], [Ti], [C], [N], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] Means the content expressed by mass% of Nb, Ti, C, N, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively.
(2) Further, the composition contains Mo: 0.01% or more and 0.25% or less in terms of mass%, and the yield strength is 650 MPa or more with low acoustic anisotropy according to (1) above. High tensile steel plate.
(3) Further, by mass, Cu: 0.05% or more, 0.8% or less, Ni: 0.05% or more, 1% or less, Cr: 0.05% or more, 0.6% or less, V : 0.005% or more, 0.05% or less, W: 0.1% or more, 3% or less, 1 type or 2 types or more are contained, It is characterized by the above-mentioned (1) or (2) A high-tensile steel sheet with a yield strength of 650 MPa or more with low acoustic anisotropy.
(4) Further, it is characterized by containing one or two of Mg: 0.0005% or more and 0.01% or less and Ca: 0.0005% or more and 0.01% or less in mass%. The high-tensile steel sheet having a yield strength of 650 MPa or more with small acoustic anisotropy according to any one of (1) to (3) above.
(5) A steel slab or slab having the composition described in any one of (1) to (4) above is heated to T (° C.) or higher and 1300 ° C. or lower as shown below, and a temperature of 1020 ° C. or higher. After rough rolling in the range, the cumulative reduction rate in the range of less than 1020 ° C. and over 920 ° C. is suppressed to 15% or less, and the cumulative reduction rate in the range of 920 ° C. or less and 860 ° C. or more is 20% or more, 50 %, And finish rolling with a rolling finish temperature of 860 ° C. or higher is performed. Subsequently, accelerated cooling at a cooling rate of 5 ° C./sec or higher is started at 800 ° C. or higher, and at 570 ° C. or lower and 420 ° C. or higher. A method for producing a high-tensile steel plate having a low acoustic anisotropy and a yield strength of 650 MPa or more, wherein the accelerated cooling is stopped and then cooled at a cooling rate of 0.5 ° C./sec or less.
T = 6300 / (1.9-Log (A))-273
here,
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
[Nb], [Ti], [C], and [N] mean the contents expressed by mass% of Nb, Ti, C, and N, respectively.

  According to the present invention, a high-tensile steel sheet having a yield strength of 650 MPa or more with low acoustic anisotropy can be obtained by an economical component system with a small amount of alloy addition and a highly tempered manufacturing method with high productivity. The effect on the industry is extremely large.

  Below, the reason for limitation of each component and manufacturing method in this invention is demonstrated.

  C is an important element which obtains a bainite structure and forms carbides and carbonitrides with Nb and Ti and becomes a main element of the strengthening mechanism of the steel of the present invention. If the amount of C is insufficient, the amount of precipitation during slow cooling after the stop of accelerated cooling is insufficient, and the strength cannot be obtained. On the other hand, even if it is excessive, the precipitation rate in the γ-austenite region during rolling is increased, and as a result, the amount of consistent precipitation during slow cooling after the stop of accelerated cooling is insufficient, and the strength cannot be obtained. Therefore, the C content is limited to a range of 0.04% or more and 0.09% or less.

  Since Si is a deoxidizing element and has the effect of increasing the hardenability and increasing the strength, 0.01% or more is added. In the accelerated cooling-intermediate stop process, it is advantageous for improving the strength-toughness balance of the base material by suppressing the coarsening of cementite during the slow cooling after the cooling stop. However, in the weld heat-affected zone, the formation of island martensite may be promoted and the toughness may be lowered, so it is better to suppress the addition amount from the viewpoint of weldability. Therefore, for example, for a member that uses a welded structure by a general welding construction with a heat input of about 6 kJ / mm, the addition range of Si is 0.01% or more and 0.6% or less. In addition, for a member that uses a welded structure with a large heat input welding with a heat input of about 10 kJ / mm, the amount of Si added is 0.3% or less, more preferably less than 0.1%.

  Mn is an element necessary for increasing the hardenability and obtaining a bainite structure having a bainite fraction of 80% or more. For this purpose, addition of more than 2.00% is necessary, but if added over 3.00%, the toughness of the base metal may be lowered, so the upper limit is made 3.00%.

  Al is made 0.003% to 0.1% of the range usually added as a deoxidizing element.

  N combines with Ti to form TiN. If TiN is finely dispersed, the pinning effect suppresses the coarsening of the weld heat affected zone structure and improves the weld heat affected zone toughness. However, if N is insufficient, TiN becomes coarse and the pinning effect is increased. I can't get it. In order to finely disperse TiN, N is added in an amount of 0.002% or more, preferably more than 0.004%. Moreover, since it may reduce the toughness of a base material and a welding heat affected zone rather than containing N excessively, an upper limit shall be 0.008%.

  B is an essential element for economically obtaining a bainite-based structure because it enhances hardenability with a small amount of addition, and requires addition of 0.0005% or more, but it exceeds 0.005%. Even if the effect is not changed, the addition amount is set to 0.0005% or more and 0.005% or less.

  Nb and Ti form NbC, Nb (CN), TiC, TiN, Ti (CN), or a composite precipitate thereof, and a composite precipitate of these and Mo, and are the main elements of the strengthening mechanism of the steel of the present invention. Is an important element. As described in the section of the means for solving the problem, in order to obtain a sufficient composite precipitate in the accelerated cooling-intermediate stop process, it is necessary to add an appropriate range in consideration of the precipitation rate. That is, Nb is 0.02% or more, preferably 0.025% or more and 0.08% or less, Ti is 0.005% or more, and [Nb] + 2 × [Ti] is 0.045% or more. , Preferably 0.055% or more and 0.090% or less, and when A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14) The condition is that the value of A is 0.0025 or more and 0.0055 or less (where [Nb], [Ti], [C], and [N] are Nb, Ti, C, It means the content expressed by mass% of N.)

  Further, when Nb and Ti are added excessively, the toughness of the base material or the weld heat affected zone may be lowered. If Nb is 0.08% or less and [Nb] + 2 × [Ti] is 0.090% or less, these toughness reductions can be avoided, but if the added amount of Ti alone exceeds 0.03%, these toughnesses can be avoided. Since there is concern about the decline, the upper limit is made 0.03%.

  Mo improves hardenability and forms a composite precipitate with Nb and Ti, thereby greatly contributing to strengthening. In order to obtain this effect, 0.01% or more is added. However, if added in excess, the weld heat-affected zone toughness is impaired, so the addition is made 0.25% or less.

  When Cu is added as a strengthening element, 0.05% or more is required to exert its effect, but even if added over 0.8%, the effect is not great for the added amount. If added excessively, the weld heat-affected zone toughness may be hindered, so 0.8% or less.

  When Ni is added to increase the toughness of the base metal, 0.05% or more is required. However, if added excessively, the weld affected zone toughness may be hindered, and since it is an expensive element, the upper limit of addition is 1%.

  Cr, like Mn, has the effect of enhancing hardenability and making it easier to obtain a bainite structure. For that purpose, 0.05% or more is added, but if added excessively, the weld heat affected zone toughness is inhibited, so the upper limit is made 0.6%.

  V has less strengthening effect than Nb and Ti, but has an effect of increasing precipitation strengthening and hardenability to some extent. In order to obtain this effect, 0.005% or more of addition is necessary. However, if excessively added, the weld heat-affected zone toughness is lowered.

  W improves strength. When added, 0.1% or more is added. However, if added in a large amount, the cost increases, so the added amount is made 3% or less.

  By adding one or two of Mg and Ca, sulfides and oxides can be formed to increase the base metal toughness and the weld heat affected zone toughness. In order to obtain this effect, Mg or Ca needs to be added in an amount of 0.0005% or more. However, excessive addition over 0.01% may produce coarse sulfides and oxides, which may reduce toughness. Therefore, the addition amount is set to 0.0005% or more and 0.01% or less, respectively.

  In addition to the above components, P and S are harmful elements that lower the base material toughness as unavoidable impurities. Preferably, P is 0.02% or less, and S is 0.02% or less.

  Next, a manufacturing method will be described.

The heating temperature of the steel slab or cast slab is set higher than the temperature T (° C.) calculated by the conditional expression including the A value as shown below in order to sufficiently dissolve Nb and Ti.
T = 6300 / (1.9-Log (A))-273
here,
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
[Nb], [Ti], [C], and [N] mean the contents expressed by mass% of Nb, Ti, C, and N, respectively. However, if the heating temperature exceeds 1300 ° C., the austenite grain size becomes coarse and causes toughness reduction, so the heating temperature of the steel slab or slab during rolling is set to T (° C.) or more and 1300 ° C. or less.

  In order to suppress the precipitation of Nb and Ti during the rolling as much as possible, the rolling in the range below 1020 ° C. and above 920 ° C. is cumulative after rough rolling at an appropriate reduction rate in the temperature range of 1020 ° C. or higher. The rolling reduction is suppressed to 15% or less. Furthermore, in order to obtain a necessary and sufficient processed structure as a precipitation site, rolling is performed at a cumulative reduction ratio of 20% or more and 50% or less in a range of 920 ° C. or less and 860 ° C. or more. Furthermore, the rolling finishing temperature is 860 ° C. or higher. Under these rolling conditions, the formation of texture is suppressed, so that the acoustic anisotropy does not increase.

  In order to suppress the recovery of the processed structure and precipitation after the processing, accelerated cooling is performed immediately after finishing rolling. Cooling is performed from 800 ° C. or higher under conditions where the cooling rate is 5 ° C./sec or higher. A cooling rate of 5 ° C./sec or more is required in order to make the volume fraction of bainite 80% or more without containing martensite. Accelerated cooling is stopped halfway so that the steel sheet temperature is 570 ° C. or lower and 420 ° C. or higher, and then the cooling rate is set to 0.5 ° C./sec or lower by cooling. The purpose is to ensure sufficient temperature and time for Nb, Ti and their composite precipitation, and further for their composite precipitation with Mo. When the accelerated cooling stop temperature is too high, it is difficult to obtain a bainite structure, and at a low temperature, the precipitation is delayed and sufficient strengthening cannot be obtained. Since the temperature at the center of the steel sheet is higher than that of the surface immediately after the accelerated cooling is stopped, the temperature of the steel sheet surface once rises due to recuperation from the inside, and then starts cooling. The accelerated cooling stop temperature here means the highest temperature reached after recuperation.

  Moreover, according to this manufacturing process, the weld cracking sensitivity index Pcm of the steel material composition can be kept low, and a strength of 650 MPa or more can be obtained with Pcm ≦ 0.20. If Pcm ≦ 0.20 in the steel material of the present invention, sufficiently high toughness of the weld heat affected zone can be obtained for welding with a welding heat input of about 6 kJ / mm.

  The steel of the present invention is used in the form of a thick steel plate as a structural member of a welded structure such as a building, a bridge, a ship, a construction machine, an industrial machine, an offshore structure, and a penstock.

  Steel pieces obtained by melting steel having the component composition shown in Table 1 were made into steel plates having a thickness of 10 to 70 mm under the manufacturing conditions shown in Table 2. Among these, 1-A to 22-V are steels of the present invention, and 23-W to 62-B are comparative examples. In the table, underlined numbers indicate that the components or production conditions deviate from the scope of the present invention, or the characteristics do not satisfy the following target values.

Table 2 shows the measurement results of tensile strength, weld heat-affected zone toughness of heat input 6 kJ / mm, and acoustic anisotropy for these steel plates. Tensile strength was measured by taking a No. 10 round bar tensile test piece specified in JIS Z2201 and measuring it according to JIS Z2241. The base material toughness was evaluated by taking a test specimen specified in JIS Z2202 from the center of the thickness in the direction perpendicular to the rolling direction and obtaining the fracture surface transition temperature (vTrs) by the method specified in JISZ2242. The weld heat-affected zone toughness is the same as the original thickness of steel materials with a thickness of 32 mm or less, and steel plates with a thickness of more than 32 mm are reduced to 32 mm. / Mm2 carbon dioxide arc welding was performed, and the impact test piece specified in JIS Z2202 was taken so that the notch bottom was along the fusion line, and the absorbed energy at -5 ° C (vE _-5 ) evaluated. The acoustic anisotropy was evaluated as having a small acoustic anisotropy if the sound speed ratio was 1.02 or less in accordance with the NDIS 2413-86 standard of the Japan Nondestructive Inspection Association. The target values of the respective characteristics were set such that the yield strength was 650 MPa, vTrs was −20 ° C. or lower, vE ₋₅ was 70 J or higher, and the sound velocity ratio was 1.02 or lower. The volume fraction of the base material tissue was calculated by observing 10 visual fields in the range of 100 mm × 100 mm with a microscopic microstructure photograph at a magnification of 500 times and converting the average area ratio for each visual field into a volume fraction.

Example 1-A~22-V are both yield strength 650MPa greater, fracture appearance transition temperature of the base material Charpy test (vTrs) indicates -20 ° C. or less and a good base material toughness, vE _{- 5} is more than 100 J, indicating good weld heat-affected zone toughness, and the sound speed ratio is 1.02 or less and the acoustic anisotropy is small.

Table 3 shows the measurement results of the weld heat-affected zone toughness with a heat input of 10 kJ / mm for the steel plates in Table 1. Here again, a steel sheet with a thickness of 32 mm or less is kept at its original thickness, and a steel sheet with a thickness of more than 32 mm is prepared with a thickness reduced to 32 mm, and the submerged with a heat input of 10 kJ / mm is applied to the butt portion of the mold groove. Arc welding was performed, an impact test piece specified in JIS Z2202 was taken so that the bottom of the notch was along the melting line (fusion line), and evaluated by absorbed energy (vE _-5 ) at -5 ° C.

For submerged arc welding with a heat input of 10 kJ / mm, all examples with Si content less than 0.1% show good weld heat affected zone toughness with vE- ₅ of 70 J or more, but Si is 0.3%. Among these examples, Example 5-E, Example 8-H, Example 10-J, and Example 20-T have vE- ₅ of less than 70 J, and the weld heat affected zone toughness is somewhat low at this heat input.

  Since Comparative Examples 23-W and 24-X have low C, Comparative Examples 28-AB and 29-AC have low Mn, and Comparative Examples 32-AF and 33-AG have low Nb. Since -AJ and 37-AK have a value of parameter A (A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)) less than 0.0025, a comparative example Since 40-AN and 41-AO have a low B, Comparative Examples 47-D and 48-B have a heating temperature lower than T, so Comparative Examples 49-D and 50-B are less than 1020 ° C and more than 920 ° C. Since the cumulative reduction ratio in the range is high, Comparative Examples 51-D and 52-B have a low cumulative reduction ratio in the range of 920 ° C. or lower and 860 ° C. or higher, and Comparative Examples 57-D and 58-B have a cooling rate. Since the comparative example 59-D and 60-B have high stop temperature of accelerated cooling because it is small, comparative example 61-D and 62 Since the lower stop temperature of accelerated cooling is B, and yield strength is less than 650 MPa.

  Since Comparative Examples 25-Y and 26-Z have high C, Comparative Example 46-AT has an excessively high Mg addition amount, so that the yield strength is less than 650 MPa, respectively, and the toughness of the weld heat affected zone is also low.

  Since Comparative Example 27-AA has high Si and Comparative Example 43-AQ has high N, the toughness of the weld heat affected zone is low.

  Since Comparative Example 30-AD and 31-AE have high Mn, the base material toughness is low.

  Since Comparative Examples 34-AH and 35-AI have high Nb, the yield strength is less than 650 MPa, and the toughness of the base metal and the weld heat affected zone is low.

  Since Comparative Examples 38-AL and 39-AM have high Ti, Comparative Example 42-AP has low N, Comparative Example 44-AR has high Mo, and Comparative Example 45-AS has high V, respectively. The toughness of the material and weld heat affected zone is low.

  Since Comparative Examples 53-D and 54-B have a high cumulative rolling reduction in the range of 920 ° C. or lower and 860 ° C. or higher, the yield strength is less than 650 MPa, the toughness of the base material is low, and the acoustic anisotropy is large. .

  Since Comparative Examples 55-D and 56-B have a low rolling completion temperature, the acoustic anisotropy is large.

Claims (5)

% By mass
C: 0.04% or more, 0.09% or less,
Si: 0.01% or more, 0.6% or less,
Mn: more than 2.00%, 3.00% or less,
Al: 0.003% or more, 0.10% or less,
N: 0.002% or more, 0.008% or less,
B: 0.0005% or more and 0.005% or less, and further Nb and Ti,
Nb: 0.02% or more, 0.08% or less,
Ti: 0.005% or more and 0.03% or less, and
0.045% ≦ [Nb] + 2 × [Ti] ≦ 0.090%
In addition, Nb, Ti, C, and N are contained within a range that satisfies the relationship that the value of A shown below is 0.0025 or more and 0.0055 or less, and is susceptible to weld cracking. Yield strength with low acoustic anisotropy characterized by an index Pcm ≦ 0.20, having a component composition consisting of the balance Fe and inevitable impurities, and having a steel structure with a volume fraction of bainite of 80% or more A high-tensile steel plate with a thickness of 650 MPa or more.
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Here, [Nb], [Ti], [C], [N], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [B] Means the content expressed by mass% of Nb, Ti, C, N, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively. Furthermore, in mass%,
Mo: 0.01% or more and 0.25% or less, The high-tensile steel sheet having a low acoustic anisotropy and a yield strength of 650 MPa or more according to claim 1. Furthermore, in mass%,
Cu: 0.05% or more, 0.8% or less,
Ni: 0.05% or more, 1% or less,
Cr: 0.05% or more, 0.6% or less,
V: 0.005% or more, 0.05% or less,
W: 0.1% or more, 3% or less of 1 type or 2 types or more, characterized by low acoustic anisotropy and high yield strength of 650 MPa or more according to claim 1 or 2 Tensile steel plate. Furthermore, in mass%,
Mg: 0.0005% or more, 0.01% or less,
The yield strength with low acoustic anisotropy according to any one of claims 1 to 3, characterized by containing one or two of Ca: 0.0005% or more and 0.01% or less. A high-tensile steel plate of 650 MPa or more. The steel slab or slab having the component composition according to any one of claims 1 to 4 is heated to T (° C) or higher and 1300 ° C or lower as shown below, and subjected to rough rolling in a temperature range of 1020 ° C or higher. Later, the cumulative reduction in the range of less than 1020 ° C. and over 920 ° C. is suppressed to 15% or less, and the cumulative reduction in the range of 920 ° C. or less and 860 ° C. or more is set to 20% or more and 50% or less. Finish rolling is performed at a temperature of 860 ° C. or higher, and subsequently, accelerated cooling at a cooling rate of 5 ° C./sec or higher is started at 800 ° C. or higher, and the accelerated cooling is stopped at 570 ° C. or lower and 420 ° C. or higher. And then cooling at a cooling rate of 0.5 ° C./sec or less, a method for producing a high-tensile steel sheet having a low acoustic anisotropy and a yield strength of 650 MPa or more.
T = 6300 / (1.9-Log (A))-273
here,
A = ([Nb] + 2 × [Ti]) × ([C] + [N] × 12/14)
[Nb], [Ti], [C], and [N] mean the contents expressed by mass% of Nb, Ti, C, and N, respectively.