JP4254550B2 - High strength steel plate with excellent weld heat affected zone toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel plate excellent in welding heat affected zone toughness used in the fields of line pipes, architecture, offshore structures, shipbuilding, civil engineering, construction machinery and the like, and a method for producing the same.

  From the viewpoints of high-pressure operation of gas pipelines, increasing the size of welded steel structures, and reducing material costs, demand for steel sheets with higher strength and higher toughness is increasing. Usually, a high-strength and high-toughness steel plate is manufactured by quenching and tempering treatment or controlled cooling subsequent to controlled rolling, so-called TMCP method. For example, a method is known in which steel containing Nb or V is used, precipitated as Nb or V carbonitride during rolling, and directly quenched and tempered after rolling or accelerated cooling (for example, Patent Document 1, (See Patent Document 2). Also known is a method of precipitating VN at the time of air cooling or tempering by using high-V, high-N content steel and air-cooling after controlled rolling-accelerated cooling or directly quenching after controlled rolling and then tempering. (For example, see Patent Document 3).

On the other hand, a technique is known in which rolling to quenching and tempering are performed on the same line, and rapid heating and tempering without a holding time are performed (see, for example, Patent Document 4 and Patent Document 5). 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 and then rapidly heated and tempered to cause supersaturated carbon to precipitate as fine cementite and further tempered without holding time. Since cementite does not become coarse due to the treatment, it has excellent strength and toughness.
JP-A-57-126916 JP 55-41927 A JP-A-4-235218 Japanese Patent No. 3015923 Japanese Patent No. 3015924

  According to the steel sheet manufacturing methods described in Patent Literature 1, Patent Literature 2, Patent Literature 3, and the like, a certain degree of strength increase is obtained by precipitation strengthening, but the effect is not sufficient because the size of the precipitate is relatively large. In order to further increase the strength, it is necessary to add a large amount of alloying elements, and deterioration of the toughness of the weld heat affected zone becomes a problem. Furthermore, since a normal off-line tempering process is performed, the heat treatment takes time and the manufacturing cost is high. Moreover, in the techniques described in Patent Document 4, Patent Document 5, and the like, the production efficiency and the production cost can be greatly improved. However, in order to obtain high-strength steel, as shown in the Examples, the carbon content of the steel material Since it is necessary to increase the amount or increase the addition amount of other alloy elements, not only the cost of the material is increased, but also the deterioration of the weld heat affected zone toughness becomes a problem.

  As described above, in the conventional technique, it has been difficult to efficiently produce a steel plate having high strength and excellent weld heat affected zone toughness.

  Therefore, an object of the present invention is to solve such problems of the prior art, and to produce a high-strength steel plate excellent in welding heat-affected zone toughness that can be produced at a low cost without adding a large amount of alloying elements, and a method for producing the same. It is to provide.

The features of the present invention for solving such problems are as follows.
(1), in mass%, C: 0.02% or more, less than 0.06%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.8%, Mo: 0.05 to 1 containing 0.3%, Al: 0.07% or less, Ti: 0.005-0.04%, Nb: 0.005-0.07%, V: 0.005-0.10% [C] / ([Mo] +, which is a ratio of the amount of C in atomic% and the total amount of Mo, Ti, Nb, and V, containing seeds or two or more species, the balance being Fe and inevitable impurities [Ti] + [Nb] + [V]) is 0.5 to 3, Ceq represented by the formula (a) is 0.38 or less, and the bainite phase has a metal structure of 90% or more. A high-strength steel sheet excellent in weld heat-affected zone toughness, characterized in that a carbide containing one or more of Ti, Nb, V and Mo and Mo is dispersed and precipitated.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (a)
However, the element symbol of the formula (a) indicates mass% of each contained element.
(2) Furthermore, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, B: 0.005% or less The high-strength steel sheet having excellent weld heat affected zone toughness according to (1) , characterized by containing at least a seed.
(3) After heating the steel having the component composition described in (1) or (2) to a temperature of 1000 to 1300 ° C. and hot rolling at a rolling finish temperature not lower than the Ar 3 temperature, 10 ° C./s or higher. Accelerated cooling to a temperature of less than 300 ° C. at a cooling rate of 1 ° C./s, followed by reheating to 550-700 ° C. at a rate of temperature rise of 1 ° C./s or higher. A method for producing a strength steel plate.
(4) A solenoid type induction heating device is installed on the same production line as a rolling mill for hot rolling of steel and a cooling device for accelerated cooling, and the solenoid type induction heating device is used to recycle the steel plate. Heating is performed, The manufacturing method of the high strength steel plate excellent in the weld heat affected zone toughness as described in (3).
(5) A steel plate manufactured using the manufacturing method according to (3) or (4), wherein the total amount of carbon constituting carbides newly precipitated by reheating (excluding cementite) A high-strength steel sheet excellent in weld heat-affected zone toughness, characterized in that the concentration in the steel sheet is 10 to 300 ppm.

  According to the present invention, a high-strength steel sheet excellent in weld heat affected zone toughness can be produced at low cost without adding a large amount of alloying elements. For this reason, steel plates used for welded structures such as line pipes, buildings, marine structures, shipbuilding, civil engineering, construction machinery, etc. can be manufactured stably in large quantities at low cost, and productivity and economy are significantly increased. Can do.

  As a result of intensive studies on a method for producing a steel sheet having high strength and excellent weld heat affected zone toughness with high efficiency, the present inventors have obtained the following findings (A) to (D).

  (A) Steel containing carbide and Mo, Ti, Nb, and V in a certain range is subjected to reheating treatment from a state where C is supersaturated by accelerated cooling after controlled rolling. It precipitates as a fine composite carbide containing one or more of Ti, Nb, and V and Mo.

  (B) In order to obtain a sufficient amount of precipitates at the time of reheating, it is necessary to suppress the precipitation of carbides during cooling after controlled rolling. For this purpose, accelerated cooling at a cooling rate of a certain rate or more is required. is necessary.

  (C) Furthermore, very fine precipitates can be obtained by performing reheating at a heating rate of a certain rate or higher, and high strength can be achieved.

  (D) As a result, the alloy components can be greatly reduced, and high weld heat affected zone toughness can be achieved at the same time.

  The present invention relates to a high-strength steel sheet in which fine precipitates are dispersed and precipitated by rapid heating after performing controlled rolling-accelerated cooling as described above, and a method for producing the same, and accelerates at a high cooling rate. Since precipitation strengthening is utilized to the maximum in addition to transformation strengthening by cooling, it is not necessary to add a large amount of alloy elements, and high strength can be achieved without impairing the weld heat affected zone toughness.

  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.

  The metal structure of the steel sheet of the present invention is substantially a bainite single phase. When one or more of different metal structures such as ferrite, martensite, or pearlite are mixed in the bainite phase, HIC tends to occur due to hydrogen accumulation and stress concentration at the heterogeneous interface. The smaller the fraction, the better. However, since the influence is negligible when the volume fraction of the structure other than bainite is low, other metal structures in a total volume fraction of 10% or less, preferably 5% or less, that is, ferrite, martensite, pearlite, You may contain 1 type, or 2 or more types of cementite.

  Next, the precipitate that is dispersed and precipitated in the steel sheet in the present invention will be described. In the steel sheet according to the present invention, carbide containing one or more of Ti, Nb, and V and Mo and Mo is dispersed and precipitated in the bainite phase. Mo and Ti are elements that form carbides in steel, and strengthening steel by precipitation of MoC and TiC has been conventionally performed. However, in the present invention, Mo and one of Ti, Ti, Nb, and V are used. In the case of precipitation strengthening of MoC and / or TiC by adding two or more compounds in combination and finely precipitating composite carbide containing one or more of Ti, Nb, V and two and Mo in steel. Compared to the above, it is characterized in that a greater strength improvement effect can be obtained. This unprecedented strength improvement effect is that a composite carbide containing one or more of Ti, Nb, V and Mo and Mo is stable and has a slow growth rate, and therefore, extremely fine precipitation with a particle size of less than 10 nm. This is because things are obtained.

  In the present invention, a composite carbide containing one or more of Ti, Nb, and V, and Mo, which is a precipitate that is dispersed and precipitated in a steel sheet, includes the following steel materials and production methods of the present invention. It can be obtained by being dispersed in the bainite phase by using it to produce a steel plate. When the high-strength steel sheet of the present invention contains precipitates other than the composite carbide containing one or more of Ti, Nb, and V and Mo, one or more of Ti, Nb, and V The effect of strengthening by the composite carbide containing Mo is not impaired, and the HIC resistance is not deteriorated. The number of precipitates of less than 10 nm is 95% or more of the total number of precipitates excluding TIN. It is preferable that

  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%.

  C: Not less than 0.02% and less than 0.06%. 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 0.06% or more, the toughness is deteriorated, so the C content is 0.02% or more. It is specified to be less than 0.06%.

  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%.

  Mn: 0.5 to 1.8%. Mn is added for strength and toughness, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 1.8%, the weldability deteriorates, so the Mn content is 0.5 to 1.8%. Stipulate. Preferably, it is 0.5 to 1.5%.

  Mo: 0.05 to 0.3%. Mo is an important element in the present invention, and by containing 0.05% or more, it suppresses pearlite transformation at the time of cooling after hot rolling, and is as fine as one or more of Ti, Nb, and V. A composite precipitate is formed, which greatly contributes to an increase in strength. However, if it exceeds 0.3%, the weld heat-affected zone toughness is deteriorated, so the Mo content is specified to be 0.05 to 0.3%.

  Al: 0.07% or less. Al is added as a deoxidizer, but if it exceeds 0.07%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.07% or less. Preferably, it is 0.01 to 0.07%.

  In addition to the above chemical components, the present invention contains one or more of Ti, Nb, and V.

  Ti: 0.005 to 0.04%. Addition of 0.005% or more forms a composite precipitate with Mo, which greatly contributes to an increase in strength. However, since addition exceeding 0.04% causes deterioration of the weld heat affected zone toughness, the Ti content is specified to be 0.005 to 0.04%. Furthermore, when it is less than 0.02%, more excellent toughness is exhibited. For this reason, when adding Nb and / or V, it is preferable to make Ti content into 0.005 to less than 0.02%.

  Nb: 0.005 to 0.07%. Nb improves toughness by refining the structure, but forms a composite precipitate together with Mo and further Ti and V, 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%, the toughness of the weld heat-affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.

  V: Set to 0.005 to 0.1%. V, like Nb and Ti, forms a composite precipitate with Mo and contributes to an increase in strength. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates, so the V content is specified to be 0.005 to 0.1%.

  In the present invention, in order to make the best use of precipitation strengthening, C / (Mo + Ti + Nb + V), which is a ratio of the amount of C in atomic% and the total amount of Mo, Ti, Nb, and V, is set to 0.5-3. Strengthening according to the present invention results in a composite precipitate (mainly carbide) containing one or more of Ti, Nb, and V and Mo. At this time, when the value of C / (Mo + Ti + Nb + V), expressed by the atomic% content of each element, is 0.5 to 3, a very fine and stable precipitate is obtained, and thus a large increase in strength is obtained. . However, when the value of C / (Mo + Ti + Nb + V) is less than 0.5 or exceeds 3, the amount of any element is excessive, and the precipitate becomes thermally unstable and easily becomes coarse. Furthermore, a hardened structure such as island martensite is formed in the weld heat affected zone due to excessive elements, leading to deterioration of the weld heat affected zone toughness. Therefore, the value of C / (Mo + Ti + Nb + V) is preferably 0.5-3. When the value of C / (Mo + Ti + Nb + V) is 0.7-2, it is more preferable because finer precipitates having a particle size of 5 nm or less can be obtained. Each element symbol of C / (Mo + Ti + Nb + V) is the content of each element in atomic%, and when using a mass% content, (C / 12.01) / (Mo / 95.9 + TI / 47.9 + Nb / 92.91 + V / 50.94).

  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.

  Cu: 0.5% 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 0.5%.

  Ni: 0.5% or less. Ni is an element effective for improving toughness and increasing strength, but if added in a large amount, it is disadvantageous in terms of cost, and the weld heat affected zone toughness deteriorates, so when added, the upper limit is 0.5%. To do.

  Cr: 0.5% or less. Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. However, if a large amount is added, weldability deteriorates, so when added, the upper limit is 0.5%.

  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.

  Moreover, it is preferable to prescribe | regulate the upper limit of Ceq defined by the following (1) Formula from a viewpoint of weldability according to an intensity | strength level. When the yield strength is 448 MPa or more, Ceq is 0.32% or less, when the yield strength is 482 MPa or more, Ceq is 0.35% or less, and when the yield strength is 551 MPa or more, Ceq is set to 0. By making it 38% or less, good weldability can be secured.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
However, the element symbol of the formula (1) indicates mass% of each contained element.

  In addition, about the steel material of this invention, there is no board thickness dependence of Ceq in the range of about 10 mm to 30 mm, and it can design with the same Ceq to about 30 mm.

  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.

  Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.

  The present invention uses a combination of transformation strengthening due to bainite transformation during accelerated cooling and precipitation strengthening due to fine carbides precipitated during reheating after accelerated cooling, thereby increasing strength without adding a large amount of alloying elements. Is a possible technology.

  The high-strength steel sheet of the present invention uses steel having the above-mentioned composition, and is hot-rolled at a heating temperature of 1000 to 1300 ° C., a rolling end temperature of Ar 3 temperature or higher, and then at a cooling rate of 10 ° C./s or higher. Accelerated cooling to a temperature of less than 1 ° C., and then reheating to a temperature of 550 to 700 ° C. at a temperature increase rate of 1 ° C./s or more, and one or more of Ti, Nb, and V, Mo and The composite carbide containing can be dispersed and precipitated. Here, the temperature is the average temperature of the steel sheet. Hereinafter, each manufacturing condition will be described in detail.

  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.

Rolling end temperature: Ar3 temperature or higher. Ar3 temperature means the ferrite transformation start temperature during cooling, and can be determined by the following equation (2).
Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (2)
However, the element symbol of the formula (2) indicates mass% of each contained element.
When the rolling end temperature is equal to or lower than the Ar3 temperature, the subsequent ferrite transformation rate is lowered, so that sufficient precipitation of fine precipitates cannot be obtained during ferrite transformation by reheating, and the strength is lowered. Above the temperature.

  Immediately after the completion of rolling, cooling is performed at a cooling rate of 10 ° C./s or more. If the cooling rate is less than 10 ° C./s, ferrite is generated during cooling, so that not only strengthening by bainite is not obtained, but also precipitation occurs during cooling and the precipitate easily coarsens, so that sufficient strength is obtained. I can't. Therefore, the cooling rate after the end of rolling is specified to be 10 ° C./s or more. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process.

  Cooling stop temperature: less than 300 ° C. In the present invention, precipitation strengthening due to fine precipitates can be obtained during the subsequent reheating treatment by forming a bainite single phase in which C is supersaturated by accelerated cooling after the end of rolling. However, when the cooling stop temperature is 300 ° C. or higher, the bainite transformation is not completed, pearlite is precipitated during air cooling after cooling stop, and sufficient strength cannot be obtained after accelerated cooling. Is insufficient, the amount of fine carbides deposited becomes insufficient, and the strength after reheating cannot be obtained. Therefore, the accelerated cooling stop temperature is specified to be less than 300 ° C.

  After accelerated cooling, reheating is performed to a temperature of 550 to 700 ° C. at a temperature rising rate of 1 ° C./s or more. This process is an important manufacturing condition in the present invention.

Temperature increase rate: 1 ° C./s or more. By heating rapidly from the state in which C is supersaturated, while suppressing the precipitation of coarse carbides such as cementite and Mo ₂ C, one or more of very fine Ti, Nb, V and more It becomes possible to precipitate the composite carbide containing Mo. However, if the rate of temperature increase is less than 1 ° C./s, it takes a long time to reach the target reheating temperature, so that coarse carbides such as cementite and Mo ₂ C are precipitated, and strengthening by fine precipitates cannot be obtained. .

  Reheating temperature: 550 to 700 ° C. The fine carbide used in the present invention can be most stably generated in the temperature range of 550 to 700 ° C. When the reheating temperature is less than 550 ° C., the diffusion is slow, so that a sufficient amount of precipitation cannot be obtained. When the reheating temperature exceeds 700 ° C., the precipitate is coarsened and sufficient strength cannot be obtained. It is specified in ° 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, a sufficient amount of precipitation can be obtained, so that the strength can be increased. However, the temperature can be kept within 30 minutes in order to reliably finish the precipitation of fine carbides. If the temperature is maintained for more than 30 minutes, the precipitates may become coarse and the strength may be reduced. In addition, since precipitation proceeds in the cooling process after reheating, the cooling rate after reheating is basically air cooling. However, it is also possible to perform cooling at a rapid cooling rate that does not inhibit precipitation.

  FIG. 1 is a photograph of a steel sheet of the present invention (0.05C-0.25Si-1.3Mn-0.15Mo-0.014Ti) manufactured using the above manufacturing method, observed with a transmission electron microscope (TEM). Indicates. According to FIG. 1, it can be confirmed that very fine precipitates are randomly precipitated, and the size of the precipitates is as very small as 10 nm or less. The precipitate is a carbide containing Mo and Ti, and this was confirmed by analysis using energy dispersive X-ray spectroscopy (EDX) or the like.

  In order to obtain a high production efficiency with a fast heating rate, it is effective to perform reheating by an induction heating device installed on the same production line as the rolling mill and the cooling device. When the plate thickness is thin, a fast heating rate can be obtained even when reheating is performed using a gas combustion furnace. However, when the plate thickness is thick or to obtain a faster heating rate, an induction heating device should be used. Is preferred. Solenoid induction heating devices are particularly preferable because they are easier to control the temperature than a soaking furnace or the like, have a relatively low cost, and can quickly heat the steel plate after cooling. In addition, by arranging a plurality of induction heating devices in series, even if the line speed and the type and size of the steel sheet are different, the number of induction heating devices to be energized and the supply power can be set by arbitrarily setting them. It is possible to freely control the temperature rate and the reheating temperature.

  An example of equipment for carrying out the production method of the present invention is shown in FIG. As shown in FIG. 2, 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 the upstream side toward the downstream side. 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.

  As described above, in the present invention, by performing the reheating treatment, fine composite carbide containing one or more of Ti, Nb, and V and Mo and Mo is precipitated, thereby achieving high strength of the steel sheet. In order to sufficiently increase the strength, it is preferable that the precipitation amount at the time of reheating is a certain level or more, and the steel sheet of the present invention is a concentration in the steel sheet of the total amount of carbon constituting carbides newly precipitated by reheating. Is preferably 10 to 300 ppm. However, cementite, which is an iron carbide, is not included in newly precipitated carbide by reheating.

  The amount of carbide precipitated during reheating after rolling and cooling is very important for increasing the strength of the material. If this value is less than 10 ppm, the amount of carbide precipitated during reheating is too small and the strength becomes low. Moreover, when it exceeds 300 ppm, the quantity of a carbide | carbonized_material will increase too much and toughness will fall.

  The amount of carbon constituting the carbide containing Mo and one or more selected from Ti, Nb, and V newly precipitated at the time of reheating is, for example, by the following methods (A) and (B): Can be sought.

  (A) These metals newly deposited by reheating are measured by measuring the precipitation amount of Ti, Nb, V, and Mo after reheating and the precipitation amount of Ti, Nb, V, and Mo after cooling. A method for obtaining the total concentration of carbon constituting the element (Ti, Nb, V, Mo) and carbide.

  The carbon content of the carbide after cooling and reheating of the steel sheet can be obtained by calculation by analyzing the amounts of Nb, Ti, V, and Mo contained in the precipitates at each stage. Δ [CasNbC] is the amount of C newly precipitated during reheating combined with Nb, and Δ [CasMoC] is the amount of newly precipitated C during reheating combined with Mo. When the amount of newly precipitated C is Δ [CasTiC] and the amount of C newly precipitated during reheating combined with V is Δ [CasVC], Δ [CasNbC] + Δ [CasMoC] + Δ [CasTiC] + Δ [Ca [ CasVC] is a carbon amount constituting a carbide containing one or more selected from Nb, Ti, V, and Mo newly precipitated during reheating, and Δ [CasNbC], Δ [CasMoC], Δ [ CasTiC] and Δ [CasVC] are obtained by the following equations (3) to (6).

Δ [CasNbC] = 12/93 × ((Nb precipitation after reheating) − (Nb precipitation after cooling)) (3)
Δ [CasMoC] = 12/96 × ((Mo precipitation amount after reheating) − (Mo precipitation amount after cooling)) (4)
Δ [CasTiC] = 12/48 × ((Ti precipitation amount after reheating) − (Ti precipitation amount after cooling)) (5)
Δ [CasVC] = 12/51 × ((V precipitation amount after reheating) − (V precipitation amount after cooling)) (6)
The amount of precipitation of each metal element is a value obtained by measuring the residue obtained by electrolytic extraction of 10% acetyl-acetone using a part of the steel sheet after cooling and reheating as a sample (in terms of ppm) May be used.

  (B) A method for obtaining the total concentration of carbon constituting carbides newly deposited by reheating by measuring the amount of Ti, Nb, V, and Mo in the precipitate after reheating (determining the P value) To obtain the total concentration of carbon constituting the newly precipitated carbide by reheating).

  Analyzing the carbon content of the carbides after cooling and reheating of the steel sheet at each production stage takes time and is not desirable in operation. Therefore, in the present invention, only the steel sheet after manufacture is analyzed to examine a method for obtaining the carbon content of the carbide after reheating, and the amount of Ti, Nb, V, and Mo in the precipitate is measured after reheating. Thus, it has been found that it is possible to estimate the total concentration of carbon constituting the newly precipitated carbide by reheating.

  In the production method of the present invention, the steel sheet is hot-rolled at a rolling finish temperature of Ar3 temperature or higher, and then accelerated cooling is performed to a temperature of less than 300 ° C at a cooling rate of 10 ° C / s or more. Since it is the manufacturing conditions which cool to the low temperature which is not, the precipitation amount of the carbide after cooling is calculable from the component of a steel plate. Hereinafter, the calculation principle for each carbide forming element will be described.

  For Nb, even in the case of rapid cooling, one-fifth of the added amount is precipitated from the initial stage of cooling and does not contribute to the increase in strength, so that four-fifth of the added amount is newly precipitated carbide by reheating. It becomes an element that constitutes.

  About V and Mo, the quantity which precipitates as a precipitate at the time of accelerated cooling is small, and is about 1/100 of the addition amount. Therefore, 99/100 of the added amount becomes an element constituting carbide newly precipitated by reheating.

  Regarding Ti, a certain amount of TiN has already been deposited in the steelmaking stage, and one-fourth of the remaining Ti that has not precipitated as TiN precipitates during cooling and does not contribute to an increase in strength. Therefore, three-quarters of the remaining Ti that did not precipitate as TiN is an element constituting carbide newly precipitated by reheating.

In summary, the P value represented by the following formula (7) is the total amount of carbide newly precipitated by reheating. However, [M] ppt is the ppm conversion value of the amount of the metal element M that forms carbide in the manufactured steel plate (corresponding to after reheating), and [M] mat is the added amount of the metal element M (component of the steel plate) It is a ppm conversion value of (composition). [TiasTiN] is a ppm conversion value of the amount of Ti forming TiN in the steel sheet.
P = 12/93 × ([Nb] ppt− [Nb] mat / 5) + 12/51 × ([V] ppt− [V] mat / 100) + 12/96 × ([Mo] ppt− [Mo] mat / 100) + 12/48 × ([Ti] ppt − ([Ti] mat− [TiasTiN]) / 4− [TiasTiN]) (7)
Therefore, if the P value represented by the formula (7) is 10 to 300, it is understood that a steel sheet having a total concentration of carbon constituting the newly precipitated carbide by reheating is 10 to 300 ppm, By analyzing only the steel sheet after production and measuring the amounts of Ti, Nb, V, and Mo in the precipitate, it is possible to determine the total concentration of carbon constituting the newly precipitated carbide by reheating. .

  [M] ppt is a value obtained by measuring the residue obtained as a carbide by electrolytic extraction of 10% acetyl-acetone, for example, by ICP emission analysis (ppm conversion value) using a part of the steel sheet as a sample, as described above. That's fine. [M] mat can be obtained by using an ordinary steel component analysis method. The amount of [TiasTiN] can be determined as 3.4 times the [N] mat, which is the nitrogen concentration of the steel sheet, or as the amount of nitrogen that forms nitrides quantified by bromine-methanol extraction or the like.

  Therefore, the steel plate having a concentration of 10 to 300 ppm in the steel plate of the total amount of carbon constituting the carbide newly precipitated by the reheating of the present invention is a steel plate having the P value of 10 to 300, and the above (A ), (B) and other methods to obtain the total concentration of carbon that constitutes the newly precipitated carbide by reheating, and by adjusting the manufacturing conditions while feeding back to the manufacturing conditions, the steel sheet is more efficiently strengthened. It becomes possible to make it.

  Steels (steel types A to I) having chemical components shown in Table 1 were made into slabs by a continuous casting method, and thick steel plates (Nos. 1 to 16) having a thickness of 12 to 28 mm were produced using the slabs.

  After the heated slab was rolled by hot rolling, it was immediately cooled using a water-cooled accelerated cooling facility, and then 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 manufacturing conditions of each steel plate.

  The microstructure of the steel sheet produced as described above was observed with a transmission electron microscope (TEM). The components of the precipitate were analyzed by energy dispersive X-ray spectroscopy (EDX). Moreover, the tensile property of each steel plate was 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 580 MPa or more was determined as the strength required for the present invention. For the weld heat affected zone (HAZ) toughness, a Charpy test was performed using a test piece to which a heat history corresponding to a heat input of 40 kJ / cm was added by a reproducible heat cycle apparatus. And the thing whose Charpy absorbed energy in 0 degreeC is 80J or more was made favorable.

  The measurement of the precipitation amount of the precipitate was No. The test was carried out on 2, 3, 7, 8, 11, and 14 steel plates. Carbide was extracted from the steel plate after reheating using 10% acetyl-acetone, and metal elements (Mo, Ti, V, Nb) forming the carbide were quantitatively measured and evaluated. The amount of Ti that forms nitride is the difference between the amount of nitrogen [NasNitride] that forms nitride from the amount of nitrogen [NasNitride] that forms nitride from the residue extracted using bromine-methanol electrolyte, and the amount of TiN The amount of nitrogen to be formed was set to [TiasTiN] by multiplying 48/14 which is the ratio of the mass number of Ti and N. No. For the steel sheets 2, 3, 7, 8, 11, and 14, the material that was subsequently cooled from the cooling stop temperature to room temperature was prepared as a sample after cooling, and the precipitation amount of the precipitate was measured. From the measurement results, the carbon amount of the carbide after cooling (Ti, Mo, V, Nb carbide precipitation amount in terms of precipitation amount of Ti, Mo, V, Nb carbide) and the carbon amount of the carbide after reheating (Ti, Mo, V, Nb carbide precipitation amount in ppm carbon equivalent amount) and the amount of carbon constituting Mo, Ti, V, Nb of the steel plate after reheating, and Mo, Ti, V, Nb of the steel plate after cooling. The difference from the amount of carbon to be formed is also shown in Table 2 as the amount of carbide forming Mo, Ti, V, and Nb formed during the reheating and formed in the reheating process. In addition, Table 2 also shows the amount of carbon constituting Mo, Ti, V, and Nb obtained in the final stage (carbon amount of carbide after reheating) and the P value obtained from the steel components (for No. 8) This is a case where the P value cannot be applied because the cooling rate is less than 10 ° C./s).

  In Table 2, Nos. 1 to 7 as examples of the present invention all have chemical components and production methods within the scope of the present invention, high strength of tensile strength of 580 MPa or more, and good weld heat affected zone toughness. In addition, in the structure of the steel sheet, precipitates of carbides having a fine particle size including one or more of Ti, Nb, and V and Mo and Mo were dispersed and precipitated.

  Nos. 8 to 12 were insufficient in strength because the chemical components were within the scope of the present invention, but the production method was outside the scope of the present invention, and thus precipitation strengthening by fine carbides was not sufficient. Nos. 13 to 16 had chemical components outside the scope of the present invention, so that sufficient strength was not obtained or welding heat affected zone toughness was inferior.

  In Table 2, the precipitation amount of Ti, Mo, V, Nb carbides precipitated during reheating and the P value show very close values, and the P value of Ti, Mo, V, Nb precipitated during reheating. It was confirmed that the amount of carbide precipitation was shown. No. About 2, 3, and 7, P value was 10 or more and sufficient intensity | strength was obtained. No. About 11 and 14, P value was less than 10, and sufficient intensity was not obtained because the amount of carbide precipitation at the time of reheating was small. On the other hand, no. For No. 8, the production method was outside the scope of the present invention, the amount of carbide precipitation after cooling was small, and sufficient strength was not obtained.

The photograph which observed the steel plate of the present invention with the transmission electron microscope (TEM). Schematic which shows an example of the manufacturing line for enforcing the manufacturing method of this invention.

Explanation of symbols

1 Rolling line 2 Steel plate 3 Hot rolling mill,
4 Accelerated cooling device 5 In-line induction heating device 6 Hot leveler

Claims (5)

In mass%, C: 0.02% or more, less than 0.06%, Si: 0.01 to 0.5%, Mn: 0.5 to 1.8%, Mo: 0.05 to 0.3% Al: 0.07% or less, Ti: 0.005-0.04%, Nb: 0.005-0.07%, V: 0.005-0.10%, 1 type or 2 types [C] / ([Mo] + [Ti] + which is the ratio of the amount of C in atomic% and the total amount of Mo, Ti, Nb, and V, with the balance being Fe and inevitable impurities . [Nb] + [V]) is 0.5 to 3, Ceq represented by the formula (1) is 0.38 or less, the metal structure is a bainite phase having a volume fraction of 90% or more , and Ti A high-strength steel sheet excellent in weld heat affected zone toughness, characterized in that a carbide containing Mo, one or more of Nb, V, and Mo is dispersed and precipitated.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
However, the element symbol of the formula (1) indicates mass% of each contained element.   Furthermore, by mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, B: 0.005% or less selected from one or more The high-strength steel sheet having excellent weld heat-affected zone toughness according to claim 1.   The steel having the component composition according to claim 1 or 2 is heated to a temperature of 1000 to 1300 ° C, hot-rolled at a rolling finish temperature of Ar3 temperature or higher, and then at a cooling rate of 10 ° C / s or higher. Production of a high strength steel plate with excellent weld heat affected zone toughness characterized by accelerated cooling to a temperature of less than 300 ° C. and then reheating to 550 to 700 ° C. at a heating rate of 1 ° C./s or more. Method.   A solenoid induction heating device is installed on the same production line as a rolling mill for hot rolling steel and a cooling device for accelerated cooling, and the steel plate is reheated by the solenoid induction heating device. The manufacturing method of the high strength steel plate excellent in the weld heat affected zone toughness of Claim 3 characterized by these.   The steel sheet manufactured using the manufacturing method according to claim 3 or claim 4, wherein the concentration of carbon in the total amount of carbon constituting carbides newly precipitated by reheating (excluding cementite) is determined. Is a high-strength steel sheet excellent in weld heat-affected zone toughness, characterized by being 10 to 300 ppm.

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