JP2003129133A - Method for manufacturing thick steel plate with high strength and high toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for a thick steel plate with high strength and high toughness. SOLUTION: This method for manufacturing the thick steel plate comprises heating a slab having a composition of, by mass%, 0.01-0.18% C, 0.01-0.5% Si, 0.6-3% Mn, 0.02% or less P, 0.02% or less S, 0.005-0.05% Ti, 0.01-0.05% Nb, 0.001-0.01% N, two or more of Mg, Al and Ca, and the balance Fe with unavoidable impurities, to a temperature of Ac3 or higher; omitting reduction, or stopping reduction after reducing it; cooling it to make an average temperature in a plate thickness direction to be Ar3 point +100 deg.C or lower but Ar3 point or higher, with a cooling rate of 2 deg.C/s or more but 50 deg.C/s or less at the surface of the steel plate; leaving it untill the temperature of the central part of the plate thickness falls to Ar3 point +100 deg.C or lower but Ar3 point or higher, and the surface layer recuperates heat at Ac1 point or higher after end of cooling; rolling it at a final temperature of Ar3 -200 deg.C or higher and with a cumulative rolling reduction of 30% or higher but 99% or lower, to make grain sizes of ferrite at the central part of the plate thickness to be 20 μm or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a method for manufacturing a high-strength and high-toughness steel plate having a plate thickness of mm or more, and is mainly applied in the steel industry.

[0002]

2. Description of the Related Art Thick steel plates used as structural members for offshore structures and bridges are required not only when the required strength is high but also when the environment in which they are used becomes severe. Is on the rise. Conventionally, as a method of manufacturing a thick steel plate excellent in strength and toughness, TMCP (Thermo-
Mechanical Controlled Pro
process technology has been developed, and the final structure has been refined by controlling the structure in heating, rolling and cooling. However, as the plate thickness increases and as it approaches the central part of the thick plate, the effect of microstructure refinement due to rolling and cooling decreases, and the strength and toughness of the central part of the plate thickness decrease. Therefore, in the case of high-strength steel, alloys are often added to increase the strength, but if the amount of alloy added is increased, the toughness further decreases.

On the other hand, as a method for improving the toughness, it is effective to apply a reduction in the non-recrystallization temperature range.
However, the reduction effect does not reach the center of the plate thickness sufficiently when the plate thickness is large, and the toughness improving effect due to the reduction is saturated. Therefore, a temperature difference is intentionally applied in the plate thickness direction to harden the surface part of the plate thickness from the center part of the plate thickness, and a reduction is applied while increasing the difference in deformation resistance in the plate thickness direction. The method of doing has been taken. Of course, in this case as well, it is still necessary to lower the temperature of the central part of the plate thickness to the unrecrystallized temperature range of austenite, but if the temperature drops too much, transformation starts at the surface part of the plate thickness. Ferrite appears. This ferrite has a problem that it is subjected to processing by subsequent rolling and becomes so-called processed ferrite, which impairs toughness.

In view of this, in Japanese Patent Laid-Open No. 63-307216, the surface layer portion of the sheet thickness is once cooled at the processing stage, and the temperature difference in the sheet thickness direction is adjusted by utilizing the heat recovery from the center portion of the sheet thickness. . That is, the rolling is stopped once in the middle of rolling, and the plate is cooled until the average temperature of the plate thickness reaches Ar ₃ point to Ar ₃ + 50 ° C., and the temperature difference between the surface layer part of the plate thickness and the surface part of the plate thickness is reduced by reheating. The temperature at the center of the plate thickness is Ar ₃ point to Ar ₃ +50.
The toughness is improved by allowing it to stand until it reaches ℃ and then applying reduction again. However, even with this method, when the plate thickness becomes large, it takes time for the plate thickness center to drop from Ar ₃ point to Ar ₃ + 50 ° C., and the austenite grains in the plate thickness center become coarse, so the toughness becomes a low value. . In addition, even in high strength steel, the Ar3 point decreases due to the addition of alloy,
It takes time for the central part of the plate thickness to drop from Ar ₃ point to Ar ₃ + 50 ° C., and the toughness becomes low. For this reason, it was difficult to secure sufficient toughness in the central part of the plate thickness in high strength thick materials.

[0005]

In view of the above situation, the present invention improves the toughness of the central part of the plate thickness without reducing the toughness of the surface layer part of the plate thickness even in high strength steel having a large plate thickness. It is an object of the present invention to provide a method for producing a high-strength and high-toughness thick steel plate that can be made.

[0006]

Means for Solving the Problems As a result of intensive studies by the present inventors, a combination of conditions of cooling during rolling and steel components such that the ferrite grain size in the central portion can be 20 μm even in a steel plate having a thickness of 50 mm or more. I found it. The present invention has been completed based on such findings, and the gist thereof is as follows. (1)% by mass, C: 0.01 to 0.18%, Si:
0.01-0.5%, Mn: 0.6-3%, P: 0.0
2% or less, S: 0.02% or less, Ti: 0.005-
0.05%, Nb: 0.01 to 0.05%, N: 0.0
01-0.01%, and further Mg: 0.000
1 to 0.004%, Al: 0.0001 to 0.1%, C
a: contains 0.0005 to 0.003% of two or more kinds,
The balance is a steel composition consisting of Fe and unavoidable impurities, and the slab is heated to a temperature of Ac ₃ or higher and not reduced, or the reduction is stopped after the reduction is applied, and the average sheet temperature is Ar ₃ points +
100 ° C. until the following Ar ₃ point or higher, and cooled to cooling rate of the steel sheet surface becomes less 2 ° C. / s or higher 50 ° C. / s, after the end of cooling, the temperature is Ar ₃ point of the thickness center portion +100
℃ or less Ar to a temperature of ₃ points or more, and the surface layer is Ac
_{It is} left standing until it is reheated to a temperature of ₁ point or more, and then rolling is performed at a cumulative rolling reduction of 30% or more and 99% or less at a temperature of Ar ₃ -200 ° C. or more, and the ferrite grain size at the center of the plate thickness is 20 μm. A method for manufacturing a high-strength, high-toughness thick steel sheet characterized by the following. (2) Steel, in mass%, Cu: 0.01 to 1%, Ni:
0.2-5%, Cr: 0.01-1%, Mo: 0.01
~ 1%, V: 0.001-0.5%, Zr: 0.01 ~
1%, Ta: 0.01 to 1%, B: 0.0001 to 0.
005%, REM: 0.0005-0.05% 1 type or 2 types or more are contained further, The manufacturing method of the high strength high toughness thick steel plate as described in said (1) characterized by the above-mentioned.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The problem with the conventional manufacturing method is that the time required for recuperation after cooling (dwelling time) that occurs during rolling is long due to the decrease in the Ar ₃ point due to the increase in plate thickness and the addition of alloys for higher strength. It means that the time becomes longer and the structure including the plate thickness center portion becomes coarser. Therefore, the present inventors diligently studied means for suppressing the coarsening of the structure at the time of recuperation and increasing the Ar ₃ point. As a result, coarsening of the structure during recuperation can be suppressed by dispersing a large amount of fine oxides, nitrides, and sulfides, and particularly in Ti-added steel, Mg, Ca,
It has been found that by adding two or more kinds of Al to form a composite oxide, the coarsening of the structure is significantly suppressed. Furthermore, the present inventors have found that these additions simultaneously raise the Ar ₃ point by 10 to 50 ° C. as compared with the case without addition.

After heating the slab having such a component system to a temperature of Ac ₃ or higher, a reduction is immediately applied and then the reduction is interrupted, and subsequently, the average sheet thickness temperature is Ar ₃ +100.
2 ° C / s or more 50 ° C or less until reaching a temperature of Ar ₃ points or more
After cooling the steel plate surface at a cooling rate of ℃ / s or less, the temperature at the center of the plate thickness decreases to a temperature of Ar ₃ points or more and Ar ₃ + 100 ° C. or less, and the surface layer has a temperature of Ac ₁ point or more. The ferrite grain size in the central part of the plate thickness can be suppressed to 20 μm or less by leaving it to reheat and then rolling at a cumulative reduction of 30% or more and 99% or less. Has been found to improve dramatically. The ferrite grain size at the center of the plate thickness is preferably 20 μm or less, but it is not necessary to obtain fine grains having a grain size of less than 1 μm, excluding rolling costs.

The sheet thickness average temperature is calculated from the heat conduction equation, but is simply (3T ₀ + T _s ) / 4 from the surface temperature T ₀ at the start of water cooling and the surface temperature T _s after water cooling. You may. In the present invention, by adding Nb, both the upper limit of the average plate thickness temperature that is the condition for stopping cooling and the upper limit of the temperature of the center part of the plate thickness that is the condition for starting rolling after recuperation are set to the conventional values. Ar ₃ + 50 ° C to Ar ₃ + 100 ° C
I was able to relax to.

Next, the details of the manufacturing method conditions will be described. Regarding heating before rolling, it is necessary that the steel sheet before rolling has a γ structure, that is, a temperature of Ac ₃ or higher. However, if it is Ac ₃ or more, it is not particularly necessary to turn it into a cold piece after casting and reheat it, and it may be during cooling after casting. Further, as a reduction for improving the toughness in the central portion of the plate thickness after cooling, a reduction of 30% or more in cumulative reduction is required, so it is necessary to interrupt the reduction with a certain thickness left before cooling. In this case, no particular reduction is required before cooling. Water cooling can be started below the slab heating temperature if no reduction is performed before cooling. The lower limit of the water cooling initiation temperature Ac ₃ -100 ° C. are preferred, it is better to start with even Ac ₃ -100 ° C. or more when the rolling before cooling.

Further, the temperature range to reach the cooling was set to Ar ₃ point + 100 ° C. or less Ar ₃ point or more in thickness average temperature, the Ar ₃ point + 100 ° C. than at recuperation process of the plate thickness center pending This is because it does not go down to the crystal region, and Ar ₃
This is because if the temperature is below the point, the plate surface temperature after recuperation is too low, and a large amount of worked ferrite is generated by the subsequent rolling, which adversely affects the toughness.

Further, it is necessary to transform the austenite from the austenite to the ferrite once by cooling the surface layer portion of the sheet thickness, and then to transform it to austenite again. However, the temperature range reached by cooling is limited to Ar ₃ or more in the sheet thickness average temperature. Since it is necessary, if the cooling rate during cooling is too low, the ratio of the portion transformed at the surface of the plate thickness becomes too small, and it becomes impossible to strongly reduce the central part of the plate thickness.
/ S or more. On the other hand, if the cooling rate is too high, it takes too long to recover the heat of the surface layer of the plate, so the upper limit of the cooling rate was set to 50 ° C / s.

After cooling, the temperature at the center of the plate thickness is Ar
₃The reason for leaving it to fall below the temperature range of + 100 ° C
The reason is that the temperature at the center of the plate thickness is the unrecrystallized region of austenite.
This is to reduce the temperature and then apply reduction. However,
If left unattended, transformation will start even in the center of the plate thickness.
Rolling is started after standing because it adversely affects the toughness.
The temperature at the center of the plate thickness is Ar₃The temperature was set to the point temperature or higher. Plate thickness
The core temperature is calculated from the heat conduction equation, but simply
Surface temperature T at the start of water cooling₀, Surface temperature T after water cooling_s1, Return
Surface temperature after heating T_s2From T₀+ T_s1-T _s2Calculate with
Good.

Further, the recuperation temperature of the surface layer portion of the plate thickness is set at the Ac ₁ point temperature or higher, because when the temperature is lower than the Ac ₁ point temperature, ferrite remains on the surface portion of the plate and undergoes processing, so This reduces the toughness of. The rolling end temperature is
The temperature may be lower than the rolling start temperature after cooling, but is set to Ar ₃ -200 ° C. or higher in order to avoid unnecessary ferrite processing.

Next, the reason for limiting the components will be described. C: C is an element that is indispensable as a basic element for improving the strength of the base material in steel, and its effective lower limit is to add 0.01% or more. Addition causes a decrease in the weldability and toughness of the steel material, so the upper limit was made 0.18%.

Si: Si is an element required as a deoxidizing element for steelmaking, and is added in an amount of 0.01% or more. However, if it exceeds 0.5%, the HAZ toughness decreases, so 0.5% is made the upper limit.

Mn: Mn is an element necessary for securing the strength and toughness of the base material, so it is necessary to add it at 0.6% or more. However, if it exceeds 3%, HA
In order to significantly inhibit Z toughness, the range is set to 0.6 to 3
%.

P: P is an element that affects the toughness of steel, and if it is contained in excess of 0.02%, the toughness is significantly impaired, so the upper limit of its content is set to 0.02%.

S: When S is added in excess of 0.02%, it causes formation of coarse sulfides and impairs toughness. Therefore, the upper limit of S content is set to 0.02%.

Ti: Ti is effective for refining crystal grains even if added in a trace amount, and is added in an amount of 0.005% or more. But,
Addition of more than 0.05% deteriorates the toughness of the weld, so
The upper limit was set to 0.05%.

Nb: Nb is the most important element in the present invention for expanding the non-recrystallization temperature range, and is an element that forms carbides and nitrides and is effective in improving strength. Addition of less than 01% has no effect, and
If added over 05%, the toughness is lowered, so the range is made 0.01 to 0.05%.

N: N forms a nitride together with Al and is effective for refining the crystal grains, and is added in an amount of 0.001% or more. However, since excessive N content impairs the toughness of the welded portion, it is 0.01% or less. And

Two or more of Mg, Al and Ca are added to form a complex oxide thereof. This composite oxide effectively acts as a nucleus of an oxide or nitride of Ti,
It becomes easier to disperse finely than when added alone.
The addition amounts of Mg, Al, and Ca are limited as follows on the premise of simultaneous addition of two or more kinds.

Mg: Mg is the main alloying element of the present invention, and is added mainly because it forms an oxide in steel and suppresses the growth of γ grains, but it is added in excess of 0.004%. And coarse oxides are more likely to be generated, and the base metal and HAZ
It causes a decrease in toughness. However, 0.0001%
If the addition amount is less than 0.1, it is not possible to sufficiently expect the formation of oxides necessary for the pinning particles.
It was limited to 0001 to 0.004%.

Al: Al is the main alloying element of the present invention, and is added mainly because it forms an oxide in steel and suppresses the growth of γ grains, but is added in excess of 0.1%. When,
Coarse oxides are likely to be formed, resulting in deterioration of the base material and HAZ toughness. However, if the addition amount is less than 0.0001%, it is not possible to sufficiently expect the formation of oxides necessary for the pinning particles.
˜0.1%.

Ca: Ca is the main alloying element of the present invention, and is added mainly because it forms an oxide in steel and suppresses the growth of austenite grains, but is added in excess of 0.003%. If so, a coarse oxide is likely to be generated, resulting in deterioration of the base material and HAZ toughness. However,
If the addition amount is less than 0.0005%, the oxide required for the pinning particles cannot be expected to be sufficiently generated, so the addition range is limited to 0.0005 to 0.003%.

In the present invention, as elements for improving strength and toughness, Cu, Ni, Cr, Mo, V,
In Ta, B, Zr, or REM, one or more elements can be added.

Cu: Cu is an element effective for increasing the strength, but if it is less than 0.01%, it has no effect, and if it exceeds 1%, cracking is likely to occur during heating of the steel slab or during welding. Therefore, the content is set to 0.01 to 1%.

Ni: Ni is an element effective in increasing the strength, but if it is less than 0.01%, it has no effect and excessive addition impairs weldability, so the upper limit was made 5%.

Cr: Cr is effective to be added in an amount of 0.01% or more in order to improve the strength of the steel by precipitation strengthening. However, if added in a large amount, hardenability is increased, bainite structure is generated, and toughness is increased. Lower. Therefore, the upper limit is set to 1%.

Mo: Mo is an element that improves hardenability and at the same time forms carbonitrides to improve strength. To obtain the effect, 0.01% or more must be added. The addition of a large amount exceeding 1% brings about a significant decrease in toughness as well as excessive strengthening, so the range was made 0.01 to 1%.

V: V is an element that forms carbides and nitrides and is effective in improving strength. Addition of less than 0.001% has no effect, and addition of more than 0.5% has the opposite effect. To lower the toughness,
It was set to 0.5%.

Zr, Ta: Zr and Ta are elements which form carbides and nitrides and are effective in improving strength, but 0.0
Addition of less than 1% has no effect, and addition of more than 1% causes deterioration of toughness.
It was set to 1 to 1%.

B: B is an element which generally increases the hardenability when it forms a solid solution, but also lowers the solid solution N as BN and improves the toughness of the weld heat affected zone. Therefore, 0.0
The effect can be utilized by adding 001% or more, but an excessive addition causes reduction in toughness, so the upper limit is set to 0.00
It was set to 5%.

REM: REM suppresses the formation of elongated MnS by forming sulfides and improves the properties of the steel material in the plate thickness direction, especially the lamella tear resistance. This effect cannot be obtained with less than 0.0005%, so this is made the lower limit. If it exceeds 0.05%, the number of coarse oxides increases and the number of ultrafine Mg-containing oxides decreases, so the upper limit was made 0.05%.

[0036]

EXAMPLE First, when the method of the present invention and the comparative method shown in Table 2 were applied to the present invention steel and the comparative steel having the components shown in Table 1, the final ferrite grain sizes shown in Table 3 were obtained.
As shown in Table 3, the strength and toughness of the steel of the present invention, which is 0 μm or less, clearly show excellent properties.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

As described above, according to the manufacturing method of the present invention, it is possible to provide a high-strength and high-toughness steel plate having excellent toughness over the entire plate thickness even when the plate thickness is large. It can be said that the value is extremely high.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Ryuji Uemori             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division (72) Inventor Akihiko Kojima             1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel shares             Company Kimitsu Works F term (reference) 4K032 AA01 AA04 AA05 AA08 AA16                       AA17 AA21 AA22 AA27 AA29                       AA31 AA35 BA01 CA00 CB01                       CB02 CC00

Claims (2)

[Claims] 1. In mass%, C: 0.01 to 0.18%, Si: 0.01 to 0.5%, Mn: 0.6 to 3%, P: 0.02% or less, S: 0.02% or less, Ti: 0.005 to 0.05%, Nb: 0.01 to 0.05%, N: 0.001 to 0.01%, and further Mg: 0.0001 to 0.004%, Al: 0.0001 to 0.1%, Ca: 0.0005 to 0.003%, two or more kinds are contained, and the balance is a steel component consisting of Fe and unavoidable impurities. Without heating after heating to a temperature of ₃ or more, or after applying the reduction, the reduction is interrupted, and the cold speed of the steel sheet surface is kept until the plate thickness average temperature reaches Ar ₃ point + 100 ° C or less Ar ₃ point or more. After cooling is completed at a temperature of 2 ° C / s or more and 50 ° C / s or less, the temperature at the center of the plate thickness is Ar ₃ point +100
℃ or less Ar to a temperature of ₃ points or more, and the surface layer is Ac
_{It is} left standing until it is reheated to a temperature of ₁ point or more, and then rolling is performed at a cumulative rolling reduction of 30% or more and 99% or less at a temperature of Ar ₃ -200 ° C. or more, and the ferrite grain size at the center of the plate thickness is 20 μm. A method for manufacturing a high-strength, high-toughness thick steel sheet characterized by the following. 2. Steel in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 5%, Cr: 0.01 to 1%, Mo: 0.01 to 1%, V: 0.001 to 0.5%, Zr: 0.01 to 1%, Ta: 0.01 to 1%, B: 0.0001 to 0.005%, REM: 0.0005 to 0.05%, 1 The method for producing a high-strength, high-toughness thick steel sheet according to claim 1, further comprising one or more kinds.