JP4897127B2 - Manufacturing method of high strength steel sheet for welded structure - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel sheet excellent in toughness, particularly applied to welded structures such as ships, buildings, bridges, tanks, and marine structures. Further, the steel sheet of the present invention may be distributed as a secondary processed product processed into a steel pipe, a column or the like, and these are also targeted.

  With the increase in size of steel structures in recent years, the demand for toughness has been increasing from the viewpoint of ensuring safety as well as increasing the thickness and strength of steel plates used. With the increase in thickness of such steel plates, high heat input welding such as submerged arc welding and electrogas welding has recently been frequently applied in order to improve the welding work efficiency. Therefore, ensuring the toughness of the base material and the weld heat affected zone (HAA) becomes a problem.

  TMCP (Thermo-Mechanical Control Process) is well known as a means for improving the toughness of the base metal. This is a method of refining the structure of a steel sheet by a combination of appropriate heating, hot rolling, cooling and heat treatment processes, as shown in Non-Patent Document 1, for example. In hot rolling, CR (Controlled Rolling) performed in a temperature range in which austenite (γ) does not recrystallize and ACC (Accelerated Cooling) that generates fine ferrite (α) grains are particularly important.

  Conventionally, as a technique for controlling the strength and toughness of a steel sheet, the chemical components are limited to a predetermined range, and in particular, the heating temperature of the hot rolling, the rolling temperature, the cumulative rolling reduction, the cooling rate, the heat treatment temperature, etc. are specified. A method has been proposed (for example, Patent Document 1). However, since the structure of steel in the manufacturing process changes in a complicated manner, the method described in Patent Document 1 may not provide desired characteristics.

  For example, in rough rolling, if the temperature and rolling reduction of each pass are not appropriate, the effect of refinement cannot be fully enjoyed due to local γ recrystallization and grain growth after recrystallization. There is. Also, in finish rolling, if the conditions are not appropriate, the cumulative strain becomes non-uniform, recrystallization occurs partially, and the strength and toughness of the steel sheet deteriorates or the productivity is significantly reduced. There is.

  On the other hand, some of the present inventors have proposed a method of controlling the rolling temperature and the rolling reduction for each rolling pass based on the sequential change of the metal structure during hot rolling (for example, patents). Reference 2). If this method is applied, a steel sheet having good strength and toughness can usually be produced efficiently. However, when high heat input welding is performed, securing the toughness of the HAZ becomes a problem.

  Until now, many means for improving the HAZ toughness of the high heat input welded joint have been proposed, but these are roughly classified into the following two according to the technical idea. One is an austenite (γ) grain coarsening prevention technology that uses the pinning effect that utilizes particles in steel, and the other is an effective grain refinement technology that uses γ intragranular ferrite (α) transformation. .

  As a technique corresponding to the former (pinning effect), a technique in which the effect of suppressing the growth of γ grains is studied for various nitrides, carbides, oxides, sulfides and the like generated in steel. For example, in steel to which Ti is added, fine particles of TiN are generated in the steel, and γ grain growth in the HAZ of the high heat input welded joint can be effectively suppressed (for example, Non-Patent Document 2).

On the other hand, as a technique corresponding to the latter (intragranular transformation), a Ti oxide having a particle diameter of 0.1 to 3.0 μm and a particle number of 5 × 10 ³ to 1 × 10 ⁷ particles / mm ³ , or Ti oxide There has been proposed a steel containing any of a composite of Ti nitride (for example, Patent Document 3). This is because particles such as Ti oxide or a composite of Ti oxide and Ti nitride act as γ intragranular ferrite (α) transformation nuclei in the welded HAZ of a high heat input welded joint to refine the HAZ structure. Technology to improve toughness.

JP 2008-214754 A JP 2004-269924 A JP-A-60-245768

Little finger soldier, “Controlled Rolling / Controlled Cooling”, Jinshokan (1997), p. 17 Kanazawa, Nakajima, Okamoto, Kanaya, “Improvement of weld bond toughness by fine TiN and development of steel for high heat input welding”, Iron and Steel, Vol. 61 (1975), p. 2589

  However, in particular, in order to improve the productivity while actively using Ti for structure control and improving the toughness of the HAZ of the base metal and the high heat input welded joint, the influence of Ti in hot rolling It is necessary to design the manufacturing process in consideration of the above. This invention is made | formed in view of such a situation, and makes it a subject to provide the manufacturing method of the efficient steel plate for welded structures excellent in a base material and HAZ toughness.

  In particular, the present invention is excellent in toughness of the base material and HAZ in which the process conditions are defined in order to precisely control the sequential change of the microstructure in the heating, rolling, cooling, and heat treatment processes in consideration of the influence of TiN. A method for producing high-strength steel for welded structures is provided. The strength of the steel sheet may be, for example, a yield strength of 315 MPa or more and 580 MPa or less, a tensile strength of 440 MPa or more and 720 MPa or less, a yield strength of 550 MPa or less, and a tensile strength of 470 MPa or more, 490 MPa or more, or 650 MPa or less or 620 MPa or less. The plate thickness is, for example, 10 to 100 mm, the lower limit may be 12 mm, 20 mm, or 30 mm, and the upper limit may be 70 mm or 50 mm. The gist of the present invention is as follows.

(1) In mass%,
C: 0.03-0.16%,
Si: 0.03 to 0.5%,
Mn: 0.3 to 2.0%,
Nb: 0.005 to 0.030%,
Ti: 0.003 to 0.050%,
Al: 0.002 to 0.10%,
N: 0.0020 to 0.0100%
Containing
P: 0.020% or less,
S: 0.010% or less, C, Mn, Nb content satisfies the following formula (1), Ti, N content satisfies the following formula (2), the balance is Fe and inevitable After heating a slab made of a general impurity under the conditions satisfying the following formulas (3) and (4), the rolling temperature, the rolling reduction, and the time between passes in each rolling pass should satisfy the following formulas (5) and (6): A method for producing a high-strength steel sheet for welded structures, characterized in that rough rolling is performed, followed by accelerated cooling at an average cooling rate in the thickness direction of 5 ° C./s or more.
0.32 ≦ [C] +0.15 [Mn] +3.8 [Nb] ≦ 0.39 (1)
1.5 ≦ [Ti] / [N] ≦ 3.0 (2)
56000 / (1.2−0.18 × log [Nb]) ≦ (T + 273) × {log (th _h ) +25} ≦ 91000 / (1.9−0.18 × log [Ti]) ( 3)
30 ≦ t _h (4)
72200 / [76.4 + A _j × ln {−ln (1-R _j )}] − 273 ≦ T _j ≦ 103000 / [87.6 + 8.1 × ln {−ln (1−R _j )}] − 273 ·・ (5)
B _j ≦ t _j ≦ B _j + 2700 / (T _j −680) (6)
However,
A _j = 8 + {25 × (R _j −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{B j = 6.45 × 10 -12 ×}} {-ln (1-R j)} -1.4 × exp {32800 / (T j +273)} × exp (73.1 × [Nb]),
And
[X]: the addition amount of the element X (mass%), T: heating temperature (° C.), _{t h:} retention time (min)
T _j : rolling temperature (° C.) of the j-th rolling pass,
t _j : time (seconds) between the j-th rolling pass and the (j + 1) -th rolling pass,
R _j : Reduction ratio of the j-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
It is.

(2) After the rough rolling and before the accelerated cooling, the rolling temperature and the rolling reduction of each rolling pass satisfy the following formulas (7) and (8), and the cumulative rolling reduction is the formula (9) or (10 The method for producing a high-strength steel sheet for welded structure according to (1) above, wherein finish rolling is performed so as to satisfy the formula.
62400 / [75.3 + 8.1 × ln {−ln (1-R _k )}] − 273 ≦ T _k ≦ 70200 / [77.3 + A _k × ln {−ln (1−R _k )}] − 273 (7)
t _k ≦ C _k (8)
0 ≦ ΣR _k ≦ h [h ≦ 20] (9)
3h / 4-15 ≦ ΣR _k ≦ h [h> 20] (10)
However,
A _k = 8 + {25 × (R _k −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{C k = 1.5 × 10 -12 ×}} {-ln (1-R k)} -1.4 × exp {32800 / (T k +273)} × exp (73.1 × [Nb])
And
T _k : rolling temperature (° C.) of the k-th rolling pass,
t _k : time (seconds) between the k-th rolling pass and the (k + 1) -th rolling pass,
R _k : rolling reduction ratio of k-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
h: plate thickness after finish rolling (mm),
ΣR _k : cumulative rolling reduction ratio of finish rolling = (sheet thickness after rough rolling−sheet thickness after finish rolling) / sheet thickness after rough rolling × 100
It is.

  (3) The method for producing a high-strength steel sheet for welded structure according to (2) above, wherein water cooling is performed between the end of the rough rolling and the start of the finish rolling.

  (4) The method for producing a high-strength steel sheet for welded structures according to (1) to (3) above, wherein water cooling is performed between one or both rolling passes of the rough rolling and the finish rolling.

  (5) The method for producing a high-strength steel sheet for welded structures according to (1) to (4) above, wherein heat treatment is performed at a temperature of 650 ° C. or less after the accelerated cooling.

(6) Furthermore, the said slab is mass%,
Cu: 1.5% or less,
Cr: 0.5% or less,
Mo: 0.5% or less,
Ni: 2.0% or less,
V: 0.10% or less,
B: 0.0030%
Mg: 0.0050% or less,
Ca: 0.0030% or less,
REM: The manufacturing method of the high strength steel plate for welded structures as described in said (1)-(5) characterized by containing 1 type or 2 types or less of 0.010% or less.

  By applying the present invention, a steel sheet having a tensile strength of 440 MPa or higher and a steel sheet having good HAZ toughness when welding with a base metal toughness and a welding heat input of about 100 to 300 kJ / cm is achieved. Therefore, the industrial effect is extremely large.

It is a figure which shows the relationship between Nb amount and Ti amount, and appropriate heating conditions. It is a figure which shows the relationship between Nb amount and suitable rough rolling conditions. It is a figure which shows the relationship between Nb amount and the lower limit of the time between passes of rough rolling. It is a figure which shows Nb amount and appropriate finish rolling conditions. It is a figure which shows the relationship between Nb amount and the upper limit of the time between passes of finishing rolling. It is a figure which shows the relationship between product thickness and the cumulative reduction rate of finish rolling.

  In general, a manufacturing method is important for improving the strength and toughness of a steel sheet. When the thickness of the steel sheet is thin or when the requirements for toughness are not strict, the slab is heated, hot rolled at a high temperature that promotes recrystallization, and recrystallized repeatedly to refine the crystal grain size. be able to. On the other hand, TMCP is applied when the plate thickness of the steel plate is large or when extremely excellent toughness is required.

  In TMCP, a slab is heated, and after rough rolling, finish rolling is generally performed, followed by accelerated cooling. Rough rolling is rolling in a high temperature range in order to refine the structure in a temperature range where γ recrystallizes (γ recrystallization temperature range). Finish rolling is performed in a temperature range (γ non-recrystallization temperature range) in which γ does not recrystallize, and is a rolling in a low temperature range for sufficiently stretching γ grains and accumulating strain, and controlled rolling (CR ). By accelerated cooling (ACC) after finish rolling, fine ferrite (α) grains are generated from γ grains with accumulated strain.

The basic guidelines for obtaining a fine structure can be summarized as follows.
(A) In the heating step, the temperature and holding time are appropriately controlled to make the γ grains uniform and fine.
(B) In the rough rolling step, the rolling reduction per pass and the cumulative rolling reduction are increased, and the γ grains are refined using recrystallization.
(C) In the finish rolling step, the rolling temperature is lowered, the cumulative rolling reduction is increased, the interfacial area is increased by stretching the γ grains, and the α nucleation sites such as deformation bands in which strain is accumulated are increased.
(D) In the accelerated cooling step, the transformation rate of the tissue is controlled by controlling the cooling rate, the cooling start temperature, and the cooling stop temperature, and an appropriate tissue is generated.
In the present invention, hot rolling in the γ recrystallization temperature region is defined as rough rolling, and hot pressure in the γ non-recrystallization temperature region is defined as finish rolling. For this reason, it is not necessary to perform rough rolling with a roughing mill, and it is not necessary to perform finish rolling with a finishing mill. For example, rough rolling and finish rolling may all be performed by one finishing mill.

  Nb produces precipitates such as carbides and nitrides, has the effect of delaying recrystallization and increasing the temperature of the non-recrystallization temperature, and contributes to precipitation strengthening. Therefore, a part of the present inventors, according to Patent Document 2, controls the rolling temperature and rolling reduction of each pass of finish rolling according to the amount of Nb added in order to improve the strength and toughness of the steel sheet. (Refer to Patent Document 2).

  On the other hand, in order to improve the HAZ toughness, it is necessary to refine the HAZ structure and reduce the brittle phase. As means for achieving finer HAZ structure, TiN particles and oxide / sulfide particles such as Ti, Ca and Mg are used to suppress γ grain coarsening and promote intragranular α transformation. Is common. Among these, TiN is easily finely dispersed in steel as compared with oxides and sulfides, and can also be used for controlling the structure of the base material.

  TiN promotes or delays recrystallization to affect the toughness of the base metal, and also acts as pinning particles to affect the HAZ toughness. Therefore, in order to improve the base metal and the HAZ toughness, it is necessary to control the heating temperature of hot rolling, the conditions of rough rolling, and further the conditions of finish rolling in consideration of the precipitation behavior of TiN.

  Therefore, the present inventors pay attention to the precipitation behavior of TiN, which is useful for preventing the coarsening of the HAZ structure and can also be used for the structure control of the base material, and details the sequential microstructural changes from the heating to the heat treatment process. We conducted a survey. Furthermore, the present invention was completed by clarifying the conditions of each process, particularly the heating conditions of hot rolling, the rolling reduction and the time between passes, in order to refine the structure and secure TiN that contributes to pinning. I let you. Hereinafter, the production conditions of the present invention will be described in detail.

  First, the heating process (slab heating) of hot rolling will be described. The heating temperature and holding time of hot rolling are extremely important for securing TiN that solidifies Nb having a large influence on the recrystallization behavior and improves HAZ toughness. In addition, the heating temperature and holding time of hot rolling are very important for controlling the microstructure of the steel, and in order to ensure the toughness of the steel sheet (base material), a uniform and fine γ structure is used. There is a need.

The important points in the heating process are the temperature and holding time that do not completely dissolve TiN, which is effective in suppressing the coarsening of γ grains, while sufficiently dissolving Nb that contributes to increasing the non-recrystallization temperature range and increasing strength. It is to do. The present inventors conducted various experiments and thermodynamic calculations on the precipitation behavior of Nb and Ti, and derived the following equations (3) and (4) based on the results.
56000 / (1.2−0.18 × log [Nb]) ≦ P _h ≦ 91000 / (1.9−0.18 × log [Ti]) (3)
30 ≦ t _h (4)
However, P _h = (T + 273) × (log (t _h ) +25), [X]: addition amount of element X (mass%), T: heating temperature (° C.), t _h : holding time (min) It is.

Functional form of P _h is in the tempering parameters used to carry out the conversion of the tempering temperature and time of the reference. The left side of the inequality is the lower limit of the heating condition that changes according to the amount of Nb, and the right side of the inequality is the upper limit of the heating condition that changes according to the amount of Ti. Each coefficient was experimentally determined from the limit conditions for generating coarse γ and the limit conditions for securing the amount of solute Nb. The holding time was set to 30 minutes or more, which is for uniformly dissolving a trace alloy element such as Nb. The holding time is defined as the time from when the temperature reaches 20 ° C. lower than the set furnace temperature until extraction, and the heating temperature is defined as the average temperature during that time.

FIG. 1 shows the lower limit of the heating conditions when Nb: 0.005% and 0.03%, and the upper limit of the heating conditions when Ti: 0.005% and 0.03%. If the lower limit of the heating temperature and holding time changes according to the Nb content, and slab heating is performed within a range that satisfies the conditions shown in FIG. Moreover, the upper limit of heating temperature and holding time changes according to the Ti content, and when the Ti amount is 0.005 to 0.03%, if slab heating is performed within the range shown in FIG. The coarsening can be suppressed. When the Ti amount is 0.003%, the curve indicated by the solid line in FIG. 1 moves downward, and when the Ti amount reaches 0.05%, the curve indicated by the dotted line in FIG. (Curve) moves upward. Thus, by performing slab heating under appropriate conditions according to the contents of Nb and Ti, solid solution Nb is secured and coarsening of γ grains is suppressed. As a result, hot rolling, accelerated cooling are performed. Thus, the base material toughness can be improved without increasing the manufacturing load in the subsequent process.
In addition, when a slab is heated too high, a very thick scale is generated, which may become a surface defect of the steel plate. For this reason, you may restrict | limit the heating temperature of a slab to 1300 degrees C or less, 1250 degrees C or less, 1200 degrees C or less, or 1180 degrees C or less. Although there is no need to provide an upper limit for the holding time, 500 minutes, 400 minutes, or 300 minutes may be set as the upper limit for the holding time in order to avoid a decrease in productivity due to long-time holding.

  After heating the slab, rough rolling is performed, and γ grains are refined as uniformly as possible by repeating recrystallization between rolling passes. If the steel plate is thin or the required toughness level is not high, accelerated cooling may be performed following the rough rolling. In order to refine the recrystallization γ in the rough rolling process, it is necessary to complete the recrystallization between rolling passes and to suppress the coarsening of the recrystallized γ grains.

The present inventors investigated in detail the relationship between the reduction temperature, reduction ratio, holding time and recrystallization and grain growth in the laboratory, and the following conditions ( 5) Formula and Formula (6) were derived.
72200 / [76.4 + A _j × ln {−ln (1-R _j )}] − 273 ≦ T _j ≦ 103000 / [87.6 + 8.1 × ln {−ln (1−R _j )}] − 273 (5)
B _j ≦ t _j ≦ B _j + 2700 / (T _j −680) (6)
However,
A _j = 8 + {25 × (R _j −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{B j = 6.45 × 10 -12 ×}} {-ln (1-R j)} -1.4 × exp {32800 / (T j +273)} × exp (73.1 × [Nb]),
And
T _j : rolling temperature (° C.) of the j-th rolling pass,
t _j : time (seconds) between the j-th rolling pass and the (j + 1) -th rolling pass,
R _j : Reduction ratio of the j-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
It is.

Here, the left side of the formula (5) represents the lower limit temperature at which recrystallization occurs, and the right side represents the upper limit temperature at which recrystallization γ does not grow. That is, in the function type of the equation (5), the distortion of each path, that is, the term of “−ln (1-R _j )” is larger, and the smaller the Nb amount, the easier the recrystallization occurs. This reflects the tendency that the recrystallized γ grains become smaller and the grains grow more easily if the strain of γ increases. The coefficient of each term in the formula (5) was experimentally determined from the limit conditions for causing recrystallization and grain growth.

  Further, the left side of the equation (6) represents the lower limit time necessary for completing recrystallization, and the right side represents the upper limit time necessary for preventing grain growth. The function type of the equation (6) expresses the tendency that the larger the strain, the higher the temperature, and the smaller the Nb, the faster the recrystallization is completed and the easier the grain growth. The coefficient of each term in the equation (6) was also experimentally determined.

  FIG. 2 shows the rough rolling temperature, the lower limit and the upper limit of the rolling reduction when Nb is 0.005% and 0.03%. FIG. 3 shows the minimum inter-path time for each Nb amount described above. By performing rough rolling under conditions that satisfy these conditions, the heated γ grains can be effectively recrystallized to obtain uniform and fine γ grains. In addition, air cooling may be performed between rough rolling passes, but water cooling using a descaler or the like may be performed. However, it is not necessary to limit cooling means and conditions from the viewpoint of tissue control.

  When the steel plate is thin and the structure is sufficiently refined by rough rolling, or when the demand for toughness is not severe, accelerated cooling is performed following the rough rolling. Since the temperature of the steel sheet is high after the end of the rough rolling and the structure is refined by the rough rolling, the recrystallization is completed in a relatively short time. If recrystallization is not completed before the start of accelerated cooling, recrystallization proceeds at a high temperature after the start of accelerated cooling. Therefore, the time from the end of rough rolling to the start of accelerated cooling is not specified. From the viewpoint of productivity, it is preferable to start accelerated cooling immediately after rough rolling, but it may take 30 to 90 s because of the time required for conveyance from the rolling equipment to the cooling equipment. Details of the processes after the accelerated cooling will be described later.

  When the thickness of the steel plate is large or when extremely excellent toughness is required, it is preferable to perform finish rolling subsequent to rough rolling. After rough rolling, as described above, the structure has been refined to some extent, and since the temperature is low and the cooling rate increases as the plate thickness decreases, grain growth and coarsening of TiN are suppressed. The Therefore, although the time until rough rolling is started after rough rolling is not particularly defined, 30 s to 180 s is required as time for adjusting the temperature of finish rolling (cooling time until finish rolling is possible). There is a case. Air cooling may be performed from the completion of rough rolling to the start of finish rolling, but water cooling may be performed in order to shorten the temperature waiting time from the viewpoint of productivity and to prevent recrystallization γ grain growth. .

  The finish rolling step introduces a work structure such as dislocations and deformation bands that become nucleation sites for α transformation in γ, promotes transformation in subsequent accelerated cooling, and significantly contributes to improvement of toughness. In order to introduce a large amount of α nucleation sites, it is effective to increase the cumulative rolling reduction as much as possible under the condition that no recovery or recrystallization occurs between passes. Therefore, it is preferable to increase the rolling reduction ratio of the rolling pass at a low temperature. However, due to the time required for the temperature to decrease, the finish rolling time becomes long, and thus the productivity is impaired.

The present inventors investigated in detail the relationship between the rolling temperature, rolling ratio, holding time, recrystallization and flatness of γ grains in the case of performing finish rolling after rough rolling, and further considered productivity. The following formula was derived.
62400 / [75.3 + 8.1 × ln {−ln (1-R _k )}] − 273 ≦ T _k ≦ 70200 / [77.3 + A _k × ln {−ln (1−R _k )}] − 273 (7)
t _k ≦ C _k (8)
0 ≦ ΣR _k ≦ h [h ≦ 20] (9)
3h / 4-15 ≦ ΣR _k ≦ h [h> 20] (10)
However,
A _k = 8 + {25 × (R _k −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{C k = 1.5 × 10 -12 ×}} {-ln (1-R k)} -1.4 × exp {32800 / (T k +273)} × exp (73.1 × [Nb])
And
T _k : rolling temperature (° C.) of the k-th rolling pass,
t _k : time (seconds) between the k-th rolling pass and the (k + 1) -th rolling pass,
R _k : rolling reduction ratio of k-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
h: plate thickness after finish rolling (mm),
ΣR _k : Cumulative rolling reduction ratio of finish rolling = (plate thickness before finish rolling−sheet thickness after finish rolling) / plate thickness before finish rolling × 100
It is. In addition, the plate thickness before finish rolling may be referred to as transfer thickness, and the plate thickness after finish rolling may be referred to as product thickness. The unit of the plate thickness before finish rolling and the plate thickness after finish rolling is mm. By multiplying the right side of ΣR _k by 100, the cumulative rolling reduction of finish rolling is substantially the cumulative rolling reduction in% units.

  Here, the left side of the equation (7) is a lower limit temperature necessary for uniformly extending γ and introducing a processed structure, and the right side is an upper limit temperature for suppressing recrystallization between passes. Equation (8) represents the upper limit of the time between passes necessary for recrystallization not to start. Note that the shorter the time between passes in the finish rolling process, the more advantageous is the suppression of recrystallization and the improvement of productivity. Therefore, it is not necessary to define the lower limit, and it is determined from the specifications of the rolling apparatus and the length of the steel plate. In a reverse rolling mill, since it is not easy to make the time between passes shorter than 1 s, the lower limit may be set to 1 s.

  Formulas (7) and (8) are also experimentally adopted in consideration of the effects of temperature, rolling reduction, and Nb amount on the recrystallization behavior, adopting a functional type according to formulas (5) and (6). The coefficient was determined. FIG. 4 shows the finish rolling temperature, the lower limit of the rolling reduction, and the temperature and the upper limit of the rolling reduction when Nb: 0.005% and 0.03%. FIG. 5 shows the maximum inter-path time for each Nb amount described above. By performing finish rolling under conditions that satisfy these conditions, it is possible to effectively flatten γ grains and accumulate strain in γ.

  Expressions (9) and (10) are conditions necessary for improving productivity while ensuring the strength and toughness of the steel sheet. This region is shown in FIG. If the relationship between the product thickness and the cumulative rolling reduction falls below this region, the amount of α nucleation sites introduced into γ is insufficient, and the base material toughness is not improved. On the other hand, if the relationship between the product thickness and the cumulative rolling reduction is deviated above the region shown in FIG. 6, the strength and toughness are improved, but the productivity of finish rolling is significantly reduced, and the shape of the thin material is also deteriorated. Also, between the finishing rolling passes, air cooling is usually sufficient, but water cooling may be performed.

  Accelerated cooling is performed after rough rolling or finish rolling. By performing accelerated cooling after rough rolling, growth of crystal grains and coarsening of precipitates can be prevented, and a decrease in toughness can be suppressed. Accelerated cooling after finish rolling is the process of generating a large number of α grains from γ (work hardening γ) flattened by finish rolling and strain accumulated by increasing the driving force of transformation. Very important from. The time required for accelerated cooling after finish rolling is not particularly specified, but it may take 30 to 90 seconds for transportation from the rolling equipment to the cooling equipment. It is preferable to shorten the time from the finish rolling to the start of accelerated cooling as short as possible in order to suppress dislocation recovery and recrystallization and improve productivity. If necessary, a lower limit of the cooling start temperature such as Ar3, Ar3-10 ° C or Ar3 + 10 ° C may be provided.

  In order to improve the toughness of the steel sheet, it is necessary to perform accelerated cooling at a cooling rate of 5 ° C./s or more on the average thickness. When the cooling rate is less than 5 ° C./s, not only the strength is insufficient, but the structure is not sufficiently refined, and the base material toughness is lowered. There is no need to specify the upper limit of the cooling rate. This is because if the chemical components and heating / rolling conditions of the present invention are applied, the hardenability will not be excessive and a coarse low-temperature transformation phase that adversely affects toughness will not be generated. However, the cooling rate has a limit depending on the thickness of the steel sheet and the capacity of the apparatus, and it is difficult to set the cooling rate above 100 ° C./s. The upper limit of the cooling rate may be limited to 75 ° C./s, 50 ° C./s, or 30 ° C./s.

  Further, the cooling stop temperature is not necessarily limited in the present invention, and may be determined according to the required characteristics of the steel sheet. In order to suppress the growth of crystal grains and precipitates and improve productivity, it is preferable to set the cooling stop temperature of accelerated cooling to 600 ° C. or lower. More preferably, it is 550 degrees C or less. Moreover, if the accelerated cooling is stopped at less than 200 ° C., the time required for the accelerated cooling becomes long and the productivity may be impaired. Therefore, the cooling stop temperature is preferably set to 200 ° C. or higher. The lower limit of the cold stop temperature may be set to 300 ° C., 400 ° C., or 450 ° C. in order to improve the strength.

After accelerated cooling, heat treatment (tempering treatment) may be performed at a temperature of 650 ° C. or lower in order to adjust strength and toughness. When the temperature exceeds 650 ° C., cementite and crystal grains are coarsened to promote the occurrence of brittle fracture, and the toughness of the base material may be lowered. Moreover, in order to improve the toughness of a steel plate, it is preferable that the temperature of heat processing shall be 400 degreeC or more. It may be 490 ° C. or higher for further improvement of toughness.
Note that the rolling productivity (ton / hr) depends not only on the dimensions of the rolled steel sheet such as the product thickness but also on the equipment specifications of the heating furnace, rolling mill, and accelerated cold speed apparatus. For this reason, in this invention, the target of rolling productivity cannot be defined clearly.

  Next, the reasons for limiting the components of the present invention will be described. Here, “%” for a component means mass%.

  C is an element essential for increasing the strength, and 0.03% or more is added. On the other hand, if the addition amount increases, it becomes difficult to ensure the HAZ toughness of the high heat input welded joint, so 0.16% is made the upper limit of the C amount. In order to improve the strength, the lower limit of C may be 0.05%, 0.06%, or 0.07%. In order to improve the HAZ toughness, the upper limit of C may be 0.14%, 0.13%, or 0.12%.

  Since Si is an inexpensive deoxidizing element and contributes to solid solution strengthening, 0.03% or more is added. On the other hand, if the Si content exceeds 0.5%, the weldability and the HAZ toughness are deteriorated, so the upper limit is made 0.5%. In order to perform reliable deoxidation, the lower limit of Si may be 0.05%, 0.08%, or 0.12%. In order to improve weldability and HAZ toughness, the upper limit of Si may be 0.40%, 0.35%, or 0.30%.

  Mn is effective as an element for improving the strength and toughness of the base material, so 0.3% or more is added. In order to improve hardenability, the lower limit of the amount of Mn is preferably 0.5% or 0.7%. More preferably, 0.9% or more or 1.0% or more is added. On the other hand, if Mn is added excessively, the HAZ toughness and weld cracking properties are deteriorated, so 2.0% is made the upper limit. The amount of Mn is preferably 1.8% or less, and more preferably 1.6% or less.

  P and S are inevitable impurities. In order to improve the toughness of the base material and the HAZ, the upper limit is 0.020% for P and 0.010% for S. In order to further improve toughness, the upper limit of P may be 0.017% or 0.015%, and the upper limit of S may be 0.008%, 0.006%, or 0.004%. The smaller the contents of P and S, the better. However, since it takes a great deal of cost to reduce industrially, the lower limit may be 0.001% for P and 0.0001% for S.

  Nb is an element that contributes to refinement of structure, transformation strengthening, and precipitation strengthening by addition of a small amount. In the present invention, 0.005% or more of Nb is added to ensure the strength of the base material. For further improvement in strength, etc., the content may be 0.008% or more or 0.010% or more. On the other hand, if Nb is added excessively, the HAZ hardens and deteriorates toughness, so 0.030% or less is made the upper limit. A more preferable upper limit of the Nb amount is 0.020%.

  Since Al is an important deoxidizing element, 0.002% or more is added. In order to perform deoxidation reliably, it is good also as 0.008% or more or 0.012% or more. However, excessive addition of Al impairs the surface quality of the slab and forms inclusions harmful to toughness, so the upper limit is made 0.10%. The upper limit with preferable Al amount is 0.07% or 0.05%.

  Ti is an extremely important element in the present invention, and is effective for improving the strength and toughness of the base metal and the HAZ toughness by refinement of the structure, precipitation strengthening, and formation of fine TiN when added in a small amount. Add more. A preferable lower limit of the amount of Ti is 0.005% or more, and more preferably 0.008% or more of Ti is added. On the other hand, when Ti is added excessively, the HAZ toughness is remarkably deteriorated, so 0.050% is made the upper limit. A preferable upper limit of the Ti amount is 0.040%. The upper limit may be 0.030%, 0.025%, or 0.020%.

  N is added in an amount of 0.0020% or more in order to form a nitride with Ti and improve the HAZ toughness. A preferable lower limit of the N amount is 0.0030% or more, and more preferably 0.0035% or more. On the other hand, when N is added excessively, embrittlement due to solute N occurs, so the content is limited to 0.0100% or less. In order to prevent embrittlement, it may be 0.0080% or less or 0.0060% or less.

  C, Mn, and Nb are elements that contribute to hardenability, and the added amount needs to satisfy the following formula (1) from the viewpoint of securing the base metal strength and the HAZ toughness.

0.32 ≦ [C] +0.15 [Mn] +3.8 [Nb] ≦ 0.39 (1)
[C], [Mn], and [Nb] in the above formula are addition amounts (mass%) of each element, and the coefficient was experimentally determined from the contribution to hardenability. If [C] +0.15 [Mn] +3.8 [Nb] is less than 0.32, the strength becomes insufficient. On the other hand, in particular, Mn and Nb are elements that are difficult to suppress center segregation, and when [C] +0.15 [Mn] +3.8 [Nb] exceeds 0.39, center segregation becomes significant. The HAZ toughness of the high heat input welded joint will decrease. 0.38 or 0.37 may be set as the upper limit for improving HAZ toughness, and 0.33 may be set as the lower limit for improving strength.

  When TiN particles are utilized for securing HAZ toughness, it is necessary to consider not only the addition amount of Ti and N alone but also the balance as shown in the following formula (2). That is, it is necessary to control the ratio of the addition amount of Ti and N within the range of 1.5 to 3.0. If the ratio Ti / N is less than 1.5, the amount of solute N becomes excessive, and the HAZ toughness of the high heat input welded joint decreases. On the other hand, when Ti / N exceeds 3.0, excess Ti forms coarse oxides and sulfides, or strength increases due to TiC precipitation, so that HAZ toughness is lowered.

1.5 ≦ [Ti] / [N] ≦ 3.0 (2)
However, [Ti] and [N] in the above formula are addition amounts (mass%) of each element.

  Furthermore, in order to improve strength and toughness, one or more of Cu, Cr, Mo, Ni, V, B, Mg, Ca, and REM may be added. On the other hand, in order to reduce alloy costs, it is preferable to avoid the addition of these elements.

  Cu, Cr, and Mo are all elements that improve the hardenability. Cu, Cr, and Mo may be added in an amount of 0.05% or more in order to increase the strength of the base material and prevent softening of the HAZ. On the other hand, excessive addition reduces HAZ toughness, so the upper limit is 1.5% for Cu and 0.5% for Cr and Mo. In order to avoid degradation of HAZ toughness, the upper limit of Cu is 0.5% or less, 0.35% or 0.20%, the upper limit of Cr is 0.3%, 0.2% or 0.1%. May be limited to 0.2%, 0.1%, and 0.08%.

  Ni is effective for securing strength, arrestability, and improving HAZ toughness, and may be added by 0.05% or more. On the other hand, an increase in the amount of Ni increases the alloy cost, so the upper limit is made 2.0%. In order to avoid an increase in alloy cost, the upper limit of Ni may be set to 0.8%, 0.6%, or 0.4%.

  V contributes to an increase in strength by precipitation strengthening, so 0.005% or more may be added. On the other hand, if V is added excessively, the HAZ toughness is lowered, so the upper limit is preferably made 0.10% or less. More preferably, it is 0.080% or less, More preferably, 0.05% or less is good.

  B is an element that improves hardenability, and 0.0002% or more may be added to increase the strength of the steel. On the other hand, excessive addition of B impairs weldability, so the upper limit of B is made 0.0030%. It is good also as 0.0020% or 0.0015%.

  Mg, Ca, and REM are elements that contribute to improving HAZ toughness by forming fine oxides and sulfides. Mg is 0.0003% or more, Ca is 0.0005% or more, and REM is 0.0005% or more. May be added. On the other hand, when these are added excessively, inclusions become coarse and lower toughness. Therefore, the upper limit of Mg amount is 0.0050% or less, the upper limit of Ca amount is 0.0030% or less, and the upper limit of REM is 0.010. % Or less is preferable. Note that REM is a rare earth metal such as La or Ce.

  Using a slab (steel piece) having the chemical components shown in Table 1, a steel plate having a thickness of 12 to 50 mm was made on a trial basis under the manufacturing conditions shown in Tables 2 to 11. Note that Ceq ′ in Table 1 is a calculated value of [C] +0.15 [Mn] +3.8 [Nb]. After rough rolling or finish rolling, the time required to start accelerated cooling and the time required to start finish rolling after rough rolling were 30 to 90 s.

Table 2 is the heating conditions of hot rolling, _{M L} is 56000 / Calculated (1.2-0.18 × log [Nb]) , P h is the (T + 273) × (log (t) +25) calc, _{M U} is the calculated value of 91000 / (1.9-0.18 × log [Ti ]). However, [X]: element addition amount (mass%), T: heating temperature (° C.), t: heating holding time (min) (where t ≧ 30). Table 3 shows the thickness of the slab, the thickness (transfer thickness) of the steel plate after rough rolling (before finish rolling), the thickness (product thickness) of the steel plate after product (finish rolling), between rolling passes and rough rolling. The presence or absence of water cooling between the steel and finish rolling, and the cumulative rolling reduction of finish rolling are shown. When accelerated cooling is performed after rough rolling, the transfer thickness is equal to the product thickness.

  Table 4 shows the sheet thickness on the exit side of each rolling pass. From the plate thickness shown in Table 4, the rolling reduction ratio of each rolling pass and the cumulative rolling reduction ratio of finish rolling were obtained and shown in Table 5. Table 6 and Table 7 show the rolling temperature of each pass of rough rolling, and the calculated values of the left side and the right side of Equation (5). Tables 8 and 9 show the rolling temperature of each pass of finish rolling, The calculated values of the left side and the right side of Equation (6) are shown. Tables 10 and 11 show the time between rolling passes of each pass of rough rolling and finish rolling, the calculated values of the left side and the right side of Equation (7), and the calculated value of the right side of Table (8). Table 12 shows accelerated cooling conditions and heat treatment temperatures.

  Table 13 shows the strength, toughness, HAZ toughness, and rolling productivity of the base material. For the base metal strength, a No. 1A full-thickness test piece (thickness of 40 mm or less) described in JIS Z 2201 or a No. 4 round bar test piece (thickness of more than 40 mm; thickness center) is taken in the direction perpendicular to the rolling direction. A tensile test was performed in accordance with JIS Z 2241, and the yield strength (YP) and the tensile strength (TS) were measured. Based on JIS Z 2242, the base metal toughness was obtained by taking a 2 mm V notch Charpy test piece in the rolling direction from the center of the plate thickness of the steel sheet, performing a Charpy impact test at various temperatures, and then performing a fracture surface transition temperature (vTrs ) Was evaluated. The base material toughness was set to be −50 ° C. or less in vTrs.

  For HAZ toughness, submerged arc welding or electrogas welding was performed under conditions of a heat input of about 100 to 300 kJ / cm, and a Charpy specimen with a notch in HAZ 1 mm away from the melt line at the center of the plate thickness was collected. The test was performed and evaluated by vTrs. The target value of HAZ toughness was set to −40 ° C. or less in vTrs. The rolling productivity was evaluated by a value (Ton / h) obtained by dividing the rolling weight by the rolling time (here, the time from the start of rough rolling to the end of finish rolling). Rolling productivity generally decreases as the plate thickness increases. This is because when waiting for temperature occurs from the end of rough rolling to the start of finish rolling, it is usually necessary to increase the waiting slab thickness (transfer thickness) as the plate thickness increases, and wait for it. This is because time becomes longer. The productivity target is 240, 230, 220, 210, 200, 190, 180, and 170 Ton when the plate thickness is 15 mm or less, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm, and 50 mm, respectively. / H or more.

  As shown in Tables 1-12, No. of the present invention example. Nos. 1 to 16 have chemical components within a predetermined range and were manufactured under predetermined conditions, and as shown in Table 13, all have sufficient strength as steel having a tensile strength of 440 MPa or higher in a thickness range of 12 to 50 mm. The base metal toughness was −50 ° C. or less in vTrs and the high heat input HAZ toughness was −40 ° C. or less in vTrs, both of which were good and the rolling productivity also satisfied the target value. On the other hand, no. 17 to 36, as shown in Tables 1 to 12, since either chemical components or production conditions deviated from the scope of the present invention, as shown in Table 13, the base material strength, base material toughness, HAZ Either toughness or productivity has fallen.

  No. 20, 22, and 30 are comparative examples in which the heating conditions for hot rolling are out of the scope of the present invention. No. In No. 20, since the Ph exceeded the upper limit, the heating γ was coarsened, a fine structure was not obtained, and the base material toughness was lowered. No. No. 22 had a Ph of less than the lower limit, so that the solid solution of Nb was insufficient and the chemical components were the same. Compared with 7 and 8, the strength is reduced and the base metal toughness is also insufficient. No. Since No. 30 had a short heating and holding time, the solution of the alloy element became insufficient, and the base material toughness was lowered.

  No. Reference numerals 17, 19, 23, and 26 are comparative examples in which the rough rolling conditions are out of the scope of the present invention. No. No. 17 has a long inter-pass time between 1-2 passes and 2-3 passes of rough rolling. No. 19 has a high rolling temperature of 2 passes and 3 passes, so that the recrystallization γ is coarsened and the toughness of the base material is lowered. No. No. 23 has a short time between 1 and 2 passes and between 2 and 3 passes. In No. 26, the rolling temperature of 8 pass and 9 pass was low, so that recrystallization was not completed and a mixed grain structure was formed, so that the toughness was lowered.

  No. 18, 21, 24, 25, 28 and 31 are comparative examples in which the finish rolling conditions are outside the scope of the present invention. No. In No. 24, since the rolling temperature was high, γ was partly recrystallized to form a mixed grain structure and toughness was reduced. No. In No. 28, the time between passes was long, and recrystallization γ occurred, resulting in a decrease in toughness. No. No. 21 has a low rolling temperature in the third pass and the fourth pass. Since No. 18 had a large cumulative rolling reduction, the base metal toughness was good, but the rolling productivity decreased. No. In No. 25, the rolling temperature was too low and excessive two-phase rolling occurred, so that both the base metal toughness and productivity decreased. No. No. 31 had a small cumulative rolling reduction, so that a fine structure was not obtained and toughness was lowered.

  No. In No. 29, accelerated cooling was not performed after finish rolling, so the structure was not refined and toughness was reduced. No. In No. 27, since the heat treatment temperature was high, cementite and the structure became coarse, and the toughness decreased.

  No. 32 to 36 are comparative examples in which the chemical components are outside the scope of the present invention. No. In No. 32, since the index Ceq ′ composed of C, Mn, and Nb exceeded the upper limit value, the center segregation became remarkable, and the HAZ toughness particularly decreased. No. In No. 33, the index Ceq 'was less than the lower limit value, so the base material strength decreased. No. Since No. 34 had high Ti / N, coarse Ti oxide remained, and particularly HAZ toughness was lowered. No. Since No. 35 had a low Ti / N ratio, the HAZ toughness particularly deteriorated due to the effect of solid solution N. No. Since 36 had a large amount of C, the strength increased, and the base metal and the HAZ toughness decreased.

Claims (6)

% By mass
C: 0.03-0.16%,
Si: 0.03 to 0.5%,
Mn: 0.3 to 2.0%,
Nb: 0.005 to 0.030%,
Ti: 0.003 to 0.050%,
Al: 0.002 to 0.10%,
N: 0.0020 to 0.0100%
Containing
P: 0.020% or less,
S: 0.010% or less, C, Mn, Nb content satisfies the following formula (1), Ti, N content satisfies the following formula (2), the balance is Fe and inevitable After heating a slab made of a general impurity under the conditions satisfying the following formulas (3) and (4), the rolling temperature, the rolling reduction, and the time between passes in each rolling pass should satisfy the following formulas (5) and (6): A method for producing a high strength steel for welded structures, characterized in that rough rolling is performed, followed by accelerated cooling at an average cooling rate in the thickness direction of 5 ° C./s or more.
0.32 ≦ [C] +0.15 [Mn] +3.8 [Nb] ≦ 0.39 (1)
1.5 ≦ [Ti] / [N] ≦ 3.0 (2)
56000 / (1.2−0.18 × log [Nb]) ≦ (T + 273) × {log (th) +25} ≦ 91000 / (1.9−0.18 × log [Ti]) (3 )
30 ≦ t _h (4)
72200 / [76.4 + A _j × ln {−ln (1-R _j )}] − 273 ≦ T _j ≦ 103000 / [87.6 + 8.1 × ln {−ln (1−R _j )}] − 273 (5)
B _j ≦ t _j ≦ B _j + 2700 / (T _j −680) (6)
However,
A _j = 8 + {25 × (R _j −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{B j = 6.45 × 10 -12 ×}} {-ln (1-R j)} -1.4 × exp {32800 / (T j +273)} × exp (73.1 × [Nb]),
And
[X]: the addition amount of the element X (mass%), T: heating temperature (° C.), _{t h:} retention time (min)
T _j : rolling temperature (° C.) of the j-th rolling pass,
t _j : time (seconds) between the j-th rolling pass and the (j + 1) -th rolling pass,
R _j : Reduction ratio of the j-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
It is. After the rough rolling and before the accelerated cooling, the rolling temperature and rolling reduction of each rolling pass satisfy the following formulas (7) and (8), and the cumulative rolling reduction is formula (9) or formula (10). Finishing rolling so that it may satisfy | fill, The manufacturing method of the high strength steel for welded structures of Claim 1 characterized by the above-mentioned.
62400 / [75.3 + 8.1 × ln {−ln (1-R _k )}] − 273 ≦ T _k ≦ 70200 / [77.3 + A _k × ln {−ln (1−R _k )}] − 273 (7)
t _k ≦ C _k (8)
0 ≦ ΣR _k ≦ h [h ≦ 20] (9)
3h / 4-15 ≦ ΣR _k ≦ h [20 <h] (10)
However,
A _k = 8 + {25 × (R _k −0.2) +5} × {1−exp (−160 × [Nb])},
^{_{C k = 1.5 × 10 -12 ×}} {-ln (1-R k)} -1.4 × exp {32800 / (T k +273)} × exp (73.1 × [Nb])
And
T _k : rolling temperature (° C.) of the k-th rolling pass,
t _k : time (seconds) between the k-th rolling pass and the (k + 1) -th rolling pass,
R _k : rolling reduction ratio of k-th rolling pass = (entry side plate thickness−outer side plate thickness) / entry side plate thickness,
h: plate thickness after finish rolling (mm),
ΣR _k : cumulative rolling reduction ratio of finish rolling = (plate thickness after rough rolling−sheet thickness after finish rolling) / plate thickness after rough rolling × 100.   The method for producing high-strength steel for welded structures according to claim 2, wherein water cooling is performed after the rough rolling until the finish rolling is started.   The method for producing high-strength steel for welded structures according to any one of claims 1 to 3, wherein water cooling is performed between one or both rolling passes of the rough rolling and the finish rolling.   5. The method for producing high-strength steel for welded structures according to claim 1, wherein heat treatment is performed at a temperature of 650 ° C. or less after the accelerated cooling is finished. Further, the slab is in mass%,
Cu: 1.5% or less,
Cr: 0.5% or less,
Mo: 0.5% or less,
Ni: 2.0% or less,
V: 0.10% or less,
B: 0.0030%
Mg: 0.0050% or less,
Ca: 0.0030% or less,
REM: 0.010% or less of 1 type or 2 types or more is contained, The manufacturing method of the high strength steel for welded structures of any one of Claims 1-5 characterized by the above-mentioned.

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