JP2004269924A - High efficient producing method of steel sheet excellent in strength and toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently producing a steel sheet excellent in strength and toughness by making fine of metallurgical structure. <P>SOLUTION: This steel sheet having excellent strength and toughness can efficiently be obtained, with which a rolling condition is set so as to satisfy a relation between a prescribed rolling temperature and rolling reduction in each rolling pass, and accelerated cooling after rolling is combined according to the addition of Nb into this steel slab or no and the temperature zones in the hot-rolling, and thus, γ grain is efficiently made to fine by repeating perfect recrystallization in the hot temperature zone and the good effect of rolling in non-crystallized zone is maximumly received in the low temperature zone and the last structure after cooling is made to fine. For satisfying the relation between the prescribed rolling temperature and rolling reduction, it is desirable to cool between the high temperature zone rolling and the low temperature zone rolling in each pass interval. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００４５】
【表２】

【００４６】
【表３】

【００４７】
【発明の効果】
本発明によれば、生産性を極度に阻害することなく強度・靭性の良好な鋼板を製造することが可能であり、製造コスト低減、工期短縮等、産業上の効果は極めて大きい。
【図面の簡単な説明】
【図１】再結晶域圧延における各パスの圧延温度と圧下率の関係を示す図。
【図２】未再結晶域圧延における各パスの圧延温度と圧下率の関係を示す図。
【図３】未再結晶域圧延における製品厚と累積圧下率の関係を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for efficiently producing a hot-rolled steel sheet, particularly a steel sheet having excellent strength and toughness used for ships, bridges, middle- and high-rise buildings, marine structures, and the like.
[0002]
[Prior art]
In recent years, as ships, bridges, middle and high-rise buildings, marine structures, and the like have become larger, demands for thicker and tougher steel materials have been increasing. At the same time, demands for shorter delivery times for demand have been increasing year by year, and there is a strong demand for improvement in productivity in steel material production.
[0003]
As a means for improving strength and toughness, TMCP (Thermo-Mechanical Control Process) is well known. This is a method of refining the structure of a steel material by a suitable combination of heating, rolling, cooling, and heat treatment steps. As a general measure, austenite (hereinafter abbreviated as γ) is refined by rolling and recrystallizing in a high-temperature region, and γ is sufficiently stretched in an unrecrystallized state in a low-temperature region, and strain is accumulated. To obtain fine ferrite (hereinafter abbreviated as α).
[0004]
However, in conventional rolling methods, as described in Patent Literatures 1 to 4 below, although there was a method of controlling the temperature during rolling, the rolling temperature and the rolling temperature were changed based on the sequential change of the metal structure during rolling. There was no method for controlling the rolling reduction for each rolling pass. Therefore, if the temperature and the rolling reduction are not appropriate in the high-temperature rolling, the recrystallization of γ will only partially proceed, or γ that has been refined by recrystallization will continue to be coarsened by grain growth. In some cases, the refining effect by rolling recrystallization cannot be sufficiently enjoyed.
[0005]
Also, when rolling in the γ non-recrystallized state, if the conditions are not appropriate, partial recrystallization occurs due to cumulative strain and ultimately the strength and toughness are deteriorated, and the strength and toughness are good. However, there is a problem that productivity is significantly reduced. Even in the case of the accelerated cooling method in which cooling is performed after rolling, these problems are not basically solved.
[0006]
As other means, low-temperature heating, low-temperature large rolling and the like may be performed. However, it is necessary to secure a temperature of Ac3 or higher because the heating temperature is required to sufficiently increase γ during heating, and there is a limit to miniaturization by low-temperature heating, and toughness deterioration due to segregation of alloy elements becomes apparent. Sometimes. In low-temperature rolling, rolling is often performed in a two-phase temperature range. Excessive low-temperature rolling not only deteriorates the material, but also reduces the rolling efficiency (rolling weight per unit time: Ton / h 2). It also has an adverse effect on the rolling shape and may increase the refining load.
[0007]
As a special method for refining the crystal grains of a steel material, there is a method of increasing the number of times of transformation until reaching a final structure by repeating heating and cooling as described in Patent Document 5 below. However, even if the number of transformations is increased simply by controlling the temperature, the effect of refining the crystal grains is not only saturated, but also the productivity is significantly reduced, so that the possibility of application to an actual process is extremely low.
[0008]
Further, a method of cooling the slab surface before rolling or after the end of rough rolling and producing fine grains α in the surface layer by rolling while maintaining a temperature difference from the inside in the surface recuperation process, the following Patent Document 6, 7 etc. However, since these rolls are necessarily rolled in the α region or the two-phase region, the rolling reduction per pass must be reduced, which greatly impairs productivity.
[0009]
[Patent Document 1]
JP-A-53-40620 [Patent Document 2]
JP-A-53-40621 [Patent Document 3]
JP-A-59-182916 [Patent Document 4]
JP-A-60-56017 [Patent Document 5]
JP-B-49-7293 [Patent Document 6]
Japanese Patent Publication No. 6-4903 [Patent Document 7]
JP-A-5-271860 [0010]
As described above, various TMCPs aimed at miniaturization of the structure have been proposed. However, an attempt to improve the material usually causes a decrease in production efficiency and also increases the load of the refining process. That is, it has been a very difficult task to achieve both improvement in material quality and improvement in productivity.
[0011]
[Problems to be solved by the invention]
The present invention provides a method for efficiently producing a steel material having excellent strength and toughness without requiring a complicated hot rolling step and a heat treatment step with low productivity.
[0012]
[Means for Solving the Problems]
The feature of the present invention is to achieve uniform miniaturization of γ by appropriately controlling the rolling temperature and rolling reduction of each pass in the rolling process, and to achieve a finer final structure by combining with a cooling process, thereby improving productivity. This makes it possible to manufacture a steel sheet having excellent strength and toughness without extremely hindering the steel sheet.
The summary is as follows.
[0013]
(1) In mass%,
C: 0.03 to 0.20%, Si: 0.05 to 1.0%,
Mn: 0.40 to 2.0%, P: ≦ 0.025%,
S: ≦ 0.020%, Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting, or after being heated to a temperature range of not less than the Ac3 point, and the following formula is satisfied in each rolling pass. A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after rolling in a recrystallization zone in a relationship between a rolling temperature and a rolling reduction, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {72.4 + 8.1 × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness.
[0014]
(2) In mass%,
C: 0.03 to 0.20%, Si: 0.05 to 1.0%,
Mn: 0.40 to 2.0%, P: ≦ 0.025%,
S: ≦ 0.020%, Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After the casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting or after being heated to a temperature range of not less than the Ac3 point. Rolling in the recrystallization zone according to the relationship between the rolling temperature and the rolling reduction that satisfies the formula, followed by unrecrystallization in the relationship between the rolling temperature and the rolling reduction that satisfies the formulas (2) and (3) or (2) and (4). A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after the zone rolling, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {72.4 + 8.1 × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}...
61400 / {75.3 + 8.1 × ln (−ln (1-Rk))} ≦ Tk
≦ 72000 / {75.3 + 8.1 × ln (−ln (1-Rk))} ▲▲ 2 ▼
0 <ΣRk ≦ 3h / 5 [h ≦ 40] ... ▲ 3 ▼
h / 2-20 ≦ ΣRk ≦ 3h / 5 [40 <h]… ▲ 4 ▼
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Tk: rolling temperature (K) of the k-th rolling pass,
Rk: rolling reduction of k-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness, j <k,
h: Product thickness (mm)
ΣRk: Cumulative rolling reduction in non-recrystallization zone rolling = (transfer thickness−product thickness) / transfer thickness × 100 (%).
[0015]
(3) In mass%,
C: 0.03 to 0.20%, Si: 0.05 to 1.0%,
Mn: 0.40 to 2.0%, Nb: 0.003 to 0.050%,
P: ≦ 0.025%, S: ≦ 0.020%,
Al: 0.005 to 0.10%, N: 0.0005 to 0.0080%
After casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting, or after being heated to a temperature range of not less than the Ac3 point, and the following formula is satisfied in each rolling pass. A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after rolling in a recrystallization zone in a relationship between a rolling temperature and a rolling reduction, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {74.4 + Aj × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Aj = 8 + {25 × (Rj−0.2) +5} × {1-exp (−160 × Nb)},
Nb: Nb addition amount (% by mass).
[0016]
(4) In mass%,
C: 0.03 to 0.20%, Si: 0.05 to 1.0%,
Mn: 0.40 to 2.0%, Nb: 0.003 to 0.050%,
P: ≦ 0.025%, S: ≦ 0.020%,
Al: 0.005 to 0.10%, N: 0.0005 to 0.0080%
After the casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting or after being heated to a temperature range of not less than the Ac3 point. Rolling in the recrystallization zone according to the relationship between the rolling temperature and the rolling reduction that satisfies the formula, followed by unrecrystallization in the relationship between the rolling temperature and the rolling reduction that satisfies the formulas (2) and (3) or (2) and (4). A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after the zone rolling, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {74.4 + Aj × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}...
61400 / {75.3 + 8.1 × ln (−ln (1-Rk))} ≦ Tk
≦ 72000 / {77.3 + Ak × ln (−ln (1-Rk))}.
0 <ΣRk ≦ 3h / 5 [h ≦ 40] ... ▲ 3 ▼
h / 2-20 ≦ ΣRk ≦ 3h / 5 [40 <h]… ▲ 4 ▼
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Tk: rolling temperature (K) of the k-th rolling pass,
Rk: rolling reduction of k-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness, j <k,
Aj = 8 + {25 × (Rj−0.2) +5} × {1-exp (−160 × Nb)},
Ak = 8 + {25 × (Rk−0.2) +5} × {1-exp (−160 × Nb)},
Nb: Nb addition amount (% by mass),
h: Product thickness (mm)
ΣRk: Cumulative rolling reduction in non-recrystallization zone rolling = (transfer thickness−product thickness) / transfer thickness × 100 (%).
[0017]
(5) The steel sheet having excellent strength and toughness according to any one of the above (2) to (4), wherein the steel sheet is water-cooled after the end of the recrystallization zone rolling and before the start of the non-recrystallization zone rolling. High efficiency manufacturing method.
(6) The steel sheet having excellent strength and toughness according to any one of the above (1) to (5), wherein the steel sheet is water-cooled between rolling passes of recrystallization zone rolling or unrecrystallization zone rolling. High efficiency manufacturing method.
(7) In addition to the above composition,
Cu: 0.05 to 1.5%, Cr: 0.05 to 1.0%,
Mo: 0.05-0.5%, Ni: 0.05-3.5%,
Ti: 0.003 to 0.10%, V: 0.005 to 0.10%,
B: 0.0002 to 0.0030%,
Ca: 0.0005 to 0.0030%,
REM: 0.0005-0.0060%
The method for producing a steel sheet having excellent strength and toughness according to any one of the above (1) to (6), wherein a steel slab containing one or more of the following is used.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components of the present invention will be described.
C has a lower limit of 0.03% as an effective component for improving the strength of the steel, and an excessive addition significantly lowers the weldability and HAZ toughness of the steel material, so the upper limit is 0.20%.
[0019]
Si is an element necessary for deoxidation at the time of smelting, and is added in an amount of 0.05% or more in order to strengthen the matrix by solid solution when added in an appropriate amount. On the other hand, if more than 1.0% is added, the toughness decreases due to the hardening of the HAZ, so the upper limit was made 1.0%.
[0020]
Mn needs to be added in an amount of 0.40% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is set to 2.0% within an allowable range of the toughness and cracking property of the welded portion.
[0021]
It is desirable that the contents of P and S are as small as possible. However, since it costs a great deal to reduce them industrially, the upper limits are set to 0.025% and 0.020%, respectively.
[0022]
Since Al is an important deoxidizing element, the lower limit is set to 0.005%. In addition, if a large amount of Al is present, the surface quality of the slab is deteriorated, so the upper limit is set to 0.10%.
[0023]
N has an effect of improving HAZ toughness by precipitating as TiN. However, an increase in solute N due to excessive addition leads to a decrease in HAZ toughness. Therefore, N is limited to the range of 0.0005 to 0.0080%.
[0024]
The selective additive elements are limited for the following reasons.
Nb is a small element that suppresses the recrystallization of γ in a trace amount, greatly contributes to the refinement of the structure by rolling in the non-recrystallized region (hereinafter abbreviated as CR), and is an element effective for improving hardenability and strengthening precipitation. Although 0.003% or more is added, excessive addition significantly lowers the HAZ toughness, so the upper limit is 0.050%.
[0025]
Cu, Cr, and Mo are added in an amount of 0.05% or more because they are effective for improving the strength of the steel material. However, when added in large amounts, the weldability and the HAZ toughness are reduced. The upper limits were 0% and 0.5%.
[0026]
Ni is added in an amount of 0.05% or more to improve the strength and toughness of the steel material. However, since an increase in the amount of Ni increases the cost, the upper limit is 3.5%.
[0027]
Ti is a deoxidizing element and, at the same time, is added with N in an amount of 0.003% or more because it combines with N to form a Ti nitride, which has a certain effect on the heating γ and the grain refinement of HAZ. However, when the amount of solid solution Ti increases, the HAZ toughness decreases, so the upper limit is 0.10%.
[0028]
V is added in an amount of 0.005% or more to improve hardenability and form carbonitrides to contribute to high strength. However, a large amount of V deteriorates HAZ toughness, so the upper limit is 0.10%.
[0029]
B is added in an amount of 0.0002% or more because it is effective in suppressing the growth of grain boundary ferrite and ferrite side plates and increasing the strength, which is detrimental to HAZ toughness, but 0.0030% because excessive addition deteriorates toughness. Was set as the upper limit.
[0030]
Ca is effective in controlling the form of sulfides, and it is necessary to add 0.0005% or more in order to refine the HAZ structure by generating Ca-based oxides and improve toughness. However, since excessive addition generates coarse inclusions, the upper limit is 0.0030%.
[0031]
REM is an element effective for refining the HAZ structure and increasing the toughness, but since the excessive addition deteriorates the toughness, the content was limited to the range of 0.0005 to 0.0060%.
[0032]
Next, the technical concept forming the basis of the present invention will be described.
The present inventors aim at microstructure refinement by rolling in the γ region and subsequent accelerated cooling, with the aim of efficiently obtaining a steel sheet excellent in strength and toughness, the time between normal hot rolling passes, The relationship between the reduction ratio, the rolling temperature, and the metal structure was investigated in detail.
As a result, the recrystallization of γ is completed within the normal time between rolling passes, remarkable coarsening until the next pass is engaged is suppressed, and γ after recrystallization zone rolling (hereinafter abbreviated as OR) is completed. It has been found that the relationship between the rolling temperature and the rolling reduction is expressed by the following formulas (1) and (2) depending on the presence or absence of Nb as a condition for efficiently reducing the size of Nb.
[No Nb] 72200 / {72.4 + 8.1 × ln (-ln (1-R))} ≦ T
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-R))}｝ 1
[With Nb] 72200 / {74.4 + A × ln (−ln (1-R))} ≦ T
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-R))}｝ 2
[0033]
Further, they found that the relationship between the rolling temperature and the rolling reduction is expressed by the following formulas (3) and (4) as conditions for preventing recrystallization from starting within the normal time between rolling passes.
[No Nb] 61400 / {75.3 + 8.1 × ln (-ln (1-R))} ≦ T
≦ 72000 / {75.3 + 8.1 × ln (−ln (1-R))} ▲▲ 3 ▼
[With Nb] 61400 / {75.3 + 8.1 × ln (-ln (1-R))} ≦ T
≦ 72000 / {77.3 + A × ln (−ln (1-R))} ▲▲ 4 ▼
However,
T: rolling temperature (K),
R: reduction rate of each pass = (inlet side thickness-outside thickness) / inlet side thickness,
A = 8 + {25 × (R−0.2) +5} × {1-exp (−160 × Nb)},
Nb: Nb addition amount (% by mass).
[0034]
That is, in the case of aiming at γ refinement by rolling recrystallization, by selecting conditions satisfying the relationship between the rolling temperature and the rolling reduction in the formulas (1) and (2), γ can be recrystallized within the time between passes. After completion, grain growth until the next pass can be substantially avoided, and the γ grain size at the time of completion of OR can be efficiently reduced. On the other hand, when aiming for γ stretching by CR, by selecting conditions that satisfy the relationship between the rolling temperature and the rolling reduction in the formulas (3) and (4), it is possible to efficiently maintain γ in an unrecrystallized state and efficiently Can be rolled. By controlling the rolling temperature and rolling reduction for each pass, the number of rolling passes can be reduced without lowering the strength and toughness, and the temperature waiting time when performing CR can be reduced.
[0035]
Subsequently, the reasons for limiting the manufacturing conditions of the present invention will be described in detail.
In the present invention, the cast slab may be directly rolled without casting, and the cast slab may be reheated and rolled. The heating temperature is set to the Ac3 point or higher, and there is no particular need to set an upper limit. In the high-temperature rolling, conditions satisfying the relationship between the rolling temperature and the rolling reduction in the expressions (1) and (2) are selected in order to reduce the γ by recrystallization. This region is shown in FIG.
[0036]
If the rolling temperature and the rolling reduction fall outside the lower limits of the formulas (1) and (2), recrystallization of γ will not be completed between passes, and some unrecrystallized γ will remain. As a result, the grains grow remarkably between passes, and in any case, a uniform and fine structure is not finally obtained, and the strength and toughness are reduced. When performing CR following OR, conditions that satisfy the relationship between the rolling temperature and the rolling reduction in the equations (3) and (4) are selected. This region is shown in FIG.
If the rolling temperature and the rolling reduction fall outside the upper limits of the formulas (3) and (4), partial recrystallization occurs between the passes, resulting in a mixed grain structure. Not only can they not be accumulated, but productivity also drops significantly. Further, in order to improve the rolling efficiency while securing the strength and toughness, it is necessary to satisfy the expressions (5) and (6).
0 <ΣR ≦ 3h / 5 [h ≦ 40]… ▲ 5 ▼
h / 2−20 ≦ ΔR ≦ 3h / 5 [40 <h] ... [6]
However,
ΣR: Cumulative rolling reduction in unrecrystallized zone rolling = (transfer thickness−product thickness) / transfer thickness × 100 (%)
h: Product thickness (mm).
[0037]
This region is shown in FIG. If the relationship between the product thickness and the cumulative rolling reduction deviates below this area, the strain accumulation of the thick material will be insufficient and the toughness will decrease, and if it deviates upward, the strength and toughness will improve, but the productivity will be remarkable. And the rolling shape also deteriorates.
In order to satisfy the expressions (1) to (4), it is desirable to appropriately cool between rolling passes and between OR and CR, but it is not necessary to limit the cooling means from the viewpoint of controlling the metallographic structure.
[0038]
In the present invention, accelerated cooling after rolling is essential. This is because, even if γ-graining and elongation are performed by rolling, if air cooling is thereafter performed, the structure-refining effect is greatly impaired. The time until the start of cooling is not particularly limited, but it is desirable to start cooling as soon as possible after recrystallization in the case of OR and after rolling in the case of CR.
The average cooling rate in the thickness direction in the accelerated cooling needs to be 2 to 30 ° C./s. When the cooling rate is less than 2 ° C./s, the driving force for α transformation does not increase, and the structure is not sufficiently refined. If the cooling rate is more than 30 ° C./s, the hardness of the surface layer portion increases significantly, the elongation decreases, and the toughness also deteriorates. The cooling stop temperature need not be limited in the present invention, and may be determined according to the required characteristics of the steel material.
[0039]
As described above, by manufacturing under the conditions within the scope of the present invention, microstructure refinement is achieved without relying on extremely low-productivity low-temperature rolling and heat treatment steps, and strength / toughness and productivity can be improved. .
[0040]
【Example】
Trial production was performed using ropes having the chemical components shown in Table 1. Tables 2 and 3 show the manufacturing conditions of the steel sheet, the mechanical properties of the base material, and the productivity. YP and TS were values based on a total thickness tensile test piece (plate thickness ≦ 50 mm) or average values of １ ／ t and ｔt parts of a JIS No. 4 round bar test piece. vTrs was an average value of JIS No. 4 impact test pieces (2 mm V notch Charpy) taken from 1/4 t and 1/2 t parts. The specimen sampling direction was perpendicular to the rolling direction.
[0041]
As is clear from Table 3, Examples 1 to 8 of the present invention were manufactured under predetermined conditions, so that strength and toughness were good and productivity was high.
On the other hand, Comparative Examples 1 to 8 are inferior in any of strength, toughness, and productivity for the following reasons, as compared with Inventive Examples 1 to 8 of the same steel type.
[0042]
In Comparative Example 1, since the temperature and rolling reduction after the fifth pass deviated from the upper limits of the conditions described in claim 1, γ remarkably grew after recrystallization, and the final structure was refined. However, the strength and toughness were reduced.
In Comparative Example 2, since the temperature and rolling reduction after the ninth pass deviated from the lower limit of Claim 3, γ was partially recrystallized to form a mixed grain structure, and the strength and toughness were deteriorated.
In Comparative Example 3, the temperature and rolling reduction in the 7th and 8th passes were low, and the lower limit of the CR condition of Claim 2 was exceeded, so that the material was almost the same as that of the invention example, but the productivity was greatly reduced.
[0043]
Comparative Example 4 did not satisfy the condition of the CR cumulative rolling reduction shown in claim 4 and was manufactured by OR, so that the toughness was significantly deteriorated.
In Comparative Example 5, since the temperature and rolling reduction of the CR in the fifth pass were high and deviated from the upper limit of the condition set forth in claim 4, partial recrystallization proceeded to form a mixed grain structure and the toughness was deteriorated. .
In Comparative Examples 6 and 7, since the cooling rate deviated from a predetermined range due to air cooling after rolling or excessive water cooling, strength and toughness were reduced.
In Comparative Example 8, because the temperature was outside the upper limit of the CR cumulative rolling reduction in claim 2, the temperature waiting time and the number of passes increased, and the productivity was significantly reduced.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
[Table 3]

[0047]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, it is possible to manufacture a steel plate with good strength and toughness without extremely impairing the productivity, and the industrial effects such as reduction of production cost and shortening of the construction period are extremely large.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between a rolling temperature and a rolling reduction of each pass in recrystallization zone rolling.
FIG. 2 is a view showing a relationship between a rolling temperature and a rolling reduction of each pass in a non-recrystallization region rolling.
FIG. 3 is a diagram showing a relationship between a product thickness and a cumulative rolling reduction in unrecrystallized region rolling.

Claims (7)

In mass%,
C: 0.03 to 0.20%,
Si: 0.05 to 1.0%,
Mn: 0.40-2.0%,
P: ≦ 0.025%,
S: ≦ 0.020%,
Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting, or after being heated to a temperature range of not less than the Ac3 point, and the following formula is satisfied in each rolling pass. A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after rolling in a recrystallization zone in a relationship between a rolling temperature and a rolling reduction, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {72.4 + 8.1 × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness. In mass%,
C: 0.03 to 0.20%,
Si: 0.05 to 1.0%,
Mn: 0.40-2.0%,
P: ≦ 0.025%,
S: ≦ 0.020%,
Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After the casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting or after being heated to a temperature range of not less than the Ac3 point. Rolling in the recrystallization zone according to the relationship between the rolling temperature and the rolling reduction that satisfies the formula, followed by unrecrystallization in the relationship between the rolling temperature and the rolling reduction that satisfies the formulas (2) and (3) or (2) and (4). A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after the zone rolling, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {72.4 + 8.1 × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}...
61400 / {75.3 + 8.1 × ln (−ln (1-Rk))} ≦ Tk
≦ 72000 / {75.3 + 8.1 × ln (−ln (1-Rk))} ▲▲ 2 ▼
0 <ΣRk ≦ 3h / 5 [h ≦ 40] ... ▲ 3 ▼
h / 2-20 ≦ ΣRk ≦ 3h / 5 [40 <h]… ▲ 4 ▼
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Tk: rolling temperature (K) of the k-th rolling pass,
Rk: rolling reduction of k-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness, j <k,
h: Product thickness (mm)
ΣRk: Cumulative rolling reduction in non-recrystallization zone rolling = (transfer thickness−product thickness) / transfer thickness × 100 (%). In mass%,
C: 0.03 to 0.20%,
Si: 0.05 to 1.0%,
Mn: 0.40-2.0%,
Nb: 0.003 to 0.050%,
P: ≦ 0.025%,
S: ≦ 0.020%,
Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting, or after being heated to a temperature range of not less than the Ac3 point, and the following formula is satisfied in each rolling pass. A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after rolling in a recrystallization zone in a relationship between a rolling temperature and a rolling reduction, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {74.4 + Aj × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Aj = 8 + {25 × (Rj−0.2) +5} × {1-exp (−160 × Nb)},
Nb: Nb addition amount (% by mass). In mass%,
C: 0.03 to 0.20%,
Si: 0.05 to 1.0%,
Mn: 0.40-2.0%,
Nb: 0.003 to 0.050%,
P: ≦ 0.025%,
S: ≦ 0.020%,
Al: 0.005 to 0.10%,
N: 0.0005 to 0.0080%
After the casting, the steel consisting of Fe and unavoidable impurities is heated without cooling to a temperature of not more than the Ar3 point after casting or after being heated to a temperature range of not less than the Ac3 point. Rolling in the recrystallization zone according to the relationship between the rolling temperature and the rolling reduction that satisfies the formula, followed by unrecrystallization in the relationship between the rolling temperature and the rolling reduction that satisfies the formulas (2) and (3) or (2) and (4). A high-efficiency production method of a steel sheet having excellent strength and toughness, characterized in that after the zone rolling, the steel sheet is water-cooled at an average cooling rate in a thickness direction of 2 to 30 ° C./s.
72200 / {74.4 + Aj × ln (−ln (1-Rj))} ≦ Tj
≦ 106000 / {85.6 + 8.1 × ln (−ln (1-Rj))}...
61400 / {75.3 + 8.1 × ln (−ln (1-Rk))} ≦ Tk
≦ 72000 / {77.3 + Ak × ln (−ln (1-Rk))}.
0 <ΣRk ≦ 3h / 5 [h ≦ 40] ... ▲ 3 ▼
h / 2-20 ≦ ΣRk ≦ 3h / 5 [40 <h]… ▲ 4 ▼
Here, Tj: the rolling temperature (K) of the j-th rolling pass,
Rj: rolling reduction of the j-th rolling pass = (inlet-side sheet thickness-outlet-side sheet thickness) / inlet-side sheet thickness,
Tk: rolling temperature (K) of the k-th rolling pass,
Rk: rolling reduction of k-th rolling pass = (incoming plate thickness-outgoing plate thickness) / incoming plate thickness, j <k,
Aj = 8 + {25 × (Rj−0.2) +5} × {1-exp (−160 × Nb)},
Ak = 8 + {25 × (Rk−0.2) +5} × {1-exp (−160 × Nb)},
Nb: Nb addition amount (% by mass),
h: Product thickness (mm)
ΣRk: Cumulative rolling reduction in non-recrystallization zone rolling = (transfer thickness−product thickness) / transfer thickness × 100 (%). The high-efficiency method for producing a steel sheet having excellent strength and toughness according to any one of claims 2 to 4, wherein water cooling is performed after the recrystallization zone rolling is completed and before the unrecrystallization zone rolling is started. The high-efficiency method for producing a steel sheet excellent in strength and toughness according to any one of claims 1 to 5, wherein water cooling is performed between each rolling pass of recrystallization zone rolling or unrecrystallization zone rolling. In addition to the above composition,
Cu: 0.05-1.5%,
Cr: 0.05 to 1.0%,
Mo: 0.05-0.5%,
Ni: 0.05 to 3.5%,
Ti: 0.003 to 0.10%,
V: 0.005 to 0.10%,
B: 0.0002 to 0.0030%,
Ca: 0.0005 to 0.0030%,
REM: 0.0005-0.0060%
The method for producing a steel sheet having excellent strength and toughness according to any one of claims 1 to 6, wherein a steel slab containing one or more of the following is used.

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