JPH05195058A - Production of thick steel plate having high toughness and high tensile strength - Google Patents

Abstract

PURPOSE:To produce a thick steel plate having high toughness and high tensile strength by subjecting a slab of a low carbon steel containing trace amounts of Nb, Ti, etc., to rolling at respectively specified heating temp. and draft, to cooling, and then to heat treatment. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.04-0.20% C, 0.05-0.50% Si, 0.70-2.0% Mn, <0.020% P, <0.010% S, 0.01-0.08% Al, 0.005-0.05% Nb, 0.005-0.030% Ti, and 0.0015-0.0080% N or further containing one or >=2 kinds among specific amounts of Cu, Ni, Mo, V, B, and Ca is heated up to a temp. not lower than (the Ac, transformation point of this steel)+100 deg.C and then rolled while regulating the cumulative draft from <=920 deg.C to finished plate thickness to >=30%. At this time, at the point of time when the thickness of the steel slab becomes <=1.2 times the finished plate thickness, the outermost layer parts, each in the range within 1/6 from the surface or the rear surface of this steel slab, are subjected to forced cooling until a temp. lower than the surface temp. before cooling by >=10 deg.C is reached. Subsequently, rolling is done, without recuperation, to finished plate thickness and rolling is finished at 680-850 deg.C. Further, within 200sec, the steel plate is cooled down to 500-800 deg.C at (0.4 to 12) deg.C/sec cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel plate having a plate thickness of 44 mm or more used for ships, marine structures and the like, and more particularly to a method for producing a high-toughness thick high-strength steel plate.

[0002]

2. Description of the Related Art In recent years, in order to increase the loading capacity of ships, especially container ships, heavy-walled high-tensile steel plates are used for construction, and the weight is reduced by devising the hull design without changing the ship width. Has come to be planned. However, the production of thick-walled high-strength steel sheets is required to be inexpensive in addition to ensuring high strength and toughness, and therefore there are many difficulties in the production method.

Conventionally, in the manufacture of thick high-strength steel sheets,
Generally, the quenching and tempering method has been adopted, but in recent years, the thermal processing control method has been widely used.

For example, there is a method such as the method described in Japanese Patent Application Laid-Open No. 60-169517, in which predetermined characteristics are secured by quenching twice. Also, a method of repeating quenching and tempering after repeating heating-rolling as in the method described in JP-A-59-80718, and further described in JP-A-60-169518 and JP-B-2-25968. There is also proposed a method of manufacturing a thick steel plate by a heat processing control method such as the above method.

[0005]

However, the above-mentioned Japanese Patent Laid-Open No.
In the method of quenching and tempering once or twice proposed in JP-A No. 60-169517 and JP-A-59-80718 and the method of performing quenching and tempering after repeating heating and rolling, the plate thickness exceeds 50 mm. It is difficult to impart a predetermined strength to a thick steel plate, and there is a problem that the heat treatment cost increases and an increase in manufacturing cost cannot be avoided.

Further, in the method of low temperature heating-rolling-accelerated cooling proposed in Japanese Patent Laid-Open No. 60-169518, when adopted for a member which is required to have high toughness at low temperature, the required characteristics are not satisfied. There's a problem.

Further, in the method of controlled rolling-accelerated cooling proposed in Japanese Examined Patent Publication No. 2-25968, quenching becomes excessive due to a low stop temperature of accelerated cooling, resulting in an increase in strength and a decrease in toughness. Not satisfied. In addition, there are problems that the tempering temperature is low and it is difficult to adjust the strength and recover the toughness, and the residual stress caused by accelerated cooling is not sufficiently reduced.

The present invention has been made in order to solve such a problem, and adjusts the chemical composition and controls the heating temperature, reduction rate, rolling finishing temperature and cooling rate of the steel bill, and then heat treatment. It is an object of the present invention to provide a method for manufacturing a high-toughness thick-walled high-strength steel sheet by carrying out.

[0009]

In order to solve the above problems, the present inventor has earnestly studied about a method for producing a thick high-strength steel having a plate thickness of 44 mm or more, which has high tensile strength and excellent toughness. As a result of repeated research, a small amount of Nb was added to the steel to expand the unrecrystallized region during rolling, and during rolling, control was performed by setting the cumulative rolling reduction from the lower part of the recrystallized region to 30% or more. Rolling is performed to reduce the crystal grains to finish rolling, then cooling is started within 200 seconds, accelerated cooling is finished at a temperature higher than 500 ° C, and fine bainite + pearlite + ferrite or a small amount of Perlite including bainite +
The present invention has been accomplished by finding that a structure of ferrite is generated, and the effects of increasing strength and improving toughness by controlled rolling and accelerated cooling are imparted to a thick-walled steel sheet.

The first invention is C: 0.04 to 0.20%, Si: 0.05 to
0.50%, Mn: 0.70 to 2.0%, P: 0.020 or less, S: 0.010%
Below, Al: 0.010 to 0.080%, Nb: 0.005 to 0.050%, Ti:
0.005 to 0.030%, N: 0.0015 to 0.0080%, balance F
Ac ₃ transformation point + 1
In the process of heating to a temperature of 00 ℃ or higher and rolling with a cumulative reduction ratio of 920 ℃ or lower to the finished plate thickness of 30% or more, when this billet thickness is 1.2 times or less of the finished plate thickness, the thickness direction After forcibly cooling the outermost layer that does not exceed 1/6 from the front and back surfaces of, to a temperature 10 ° C or more lower than the surface temperature before cooling,
Rolling immediately without recuperating, finish rolling within the temperature range of 680 to 850 ℃, start accelerated cooling within 200 seconds after rolling, and exceed 500 ℃ at a cooling rate of 0.4 to 12 ℃ / sec. This is a method of manufacturing high-toughness, high-strength steel sheets with a plate thickness of 44 mm or more, which is cooled to a temperature range of 800 ° C or less.

The second invention further comprises Cu:
0.90% or less, Ni: 1.20% or less, Mo: 0.50% or less, V: 0.08
0% or less, B: 0.0004 to 0.0030%, Ca: 0.0005 to 0.0050%
The method for producing a high-toughness thick-walled high-strength steel sheet according to claim 1, containing one or more selected from the above.

A third aspect of the invention is 600 ° C. after accelerated cooling.
The method for producing a high-toughness thick-walled high-strength steel sheet according to claim 1 or 2, wherein the tempering is performed once or twice or more in a temperature range of over 760 ° C.

[0013]

The present invention will be described in detail below. First, the reasons for limiting the chemical components in the present invention will be described. C is
It is a necessary and useful element to secure the strength of steel,
For this purpose, it is necessary to add 0.04% or more. But,
If the addition amount exceeds 0.20%, the toughness of steel deteriorates and the weldability also deteriorates. Therefore, the amount of C added is in the range of 0.04 to 0.20%.

Si is an element useful for deoxidizing and strengthening steel, and it is necessary to add at least 0.05%, but 0.50
If added in excess of%, the toughness deteriorates. Therefore, the addition amount of Si is set to the range of 0.05 to 0.50%.

Mn is an element necessary for securing the strength and toughness of steel and for preventing softening of the weld heat affected zone (HAZ). For this purpose, 0.70% or more is required to be added. However, if the addition amount exceeds 2.0%, the weldability and HAZ toughness deteriorate. Therefore, the amount of Mn added is 0.70 to 2.0.
The range is%.

[0016] P and S are contained in the steel as impurities, but if they are contained in large amounts, they deteriorate the toughness of the base material and HAZ, and are not preferable elements. Therefore, in the present invention, it is limited to 0.020% or less and 0.010% or less, respectively.

Al is an element necessary for deoxidizing steel and refining crystal grains, and for this purpose, addition of 0.010% or more is necessary. However, excessive addition produces Al oxide-based non-metallic inclusions and deteriorates toughness, so the upper limit of the addition is made 0.080%. Therefore, the added amount of Al is 0.010 ~
The range is 0.080%.

In the manufacturing process of the present invention, Nb is effective in preventing coarsening of austenite grains during heating, grain refinement during rolling, precipitation hardening during tempering treatment, and particularly expansion of the non-recrystallization temperature range. It is an important element that is effective and indispensable for the production of thick materials. To exert these effects effectively, it is necessary to add 0.005% or more.
The addition of more than 0% impairs the toughness of HAZ, so its upper limit is made 0.050%. Therefore, the amount of Nb added is 0.005
The range is from ~ 0.050%.

Ti is a strong nitride-forming element, and when added in a trace amount, TiN finely precipitates to form finer crystal grains and H.
It is effective in improving the toughness of AZ. 0 for this effect.
It is necessary to add more than 005%, but if a large amount is added, H
In order to deteriorate the toughness of AZ, the upper limit is made 0.030%.
Therefore, the amount of Ti added is in the range of 0.005 to 0.030%.

N is a kind of impurity, but when it is contained in an appropriate amount, TiN is formed together with Ti to form a base material and HA.
Improve the toughness of Z. In order to secure the toughness of HAZ, it is necessary to add 0.0015% or more, but if added in excess of 0.0080%, cracks may occur during the steel slab production, which will hinder the post-process. .. Therefore, N
The addition amount is 0.0015 to 0.0080%.

In addition to the above components, in the present invention,
If necessary, one or more selected from the elements Cu, Ni, Mo, V 2, B and Ca shown below can be added. Cu and Ni are elements that significantly improve the strength and toughness of the base material without adversely affecting the hardenability and toughness of the HAZ, and are extremely effective when used in the present invention,
If added excessively, the amount of bainite is excessively generated due to accelerated cooling after the end of rolling, which hinders the formation of ferrite, and an adverse effect due to an excessive increase in strength appears. Further, Cu needs to be limited because Cu cracks occur during rolling, which makes rolling difficult. Therefore, Cu is 0.90%
Below, Ni is 1.20% or less.

Mo is an element that remarkably improves the strength and toughness of the base metal, but if added in excess, it increases the hardenability, the base metal strength becomes excessive, and the base material toughness, HAZ toughness and weldability deteriorate. It is not preferable. Therefore, Mo
The amount added is 0.50% or less.

V is an element effective in securing toughness due to grain refinement of the structure after rolling and strength due to precipitation hardening, but since it is an expensive element, its addition amount is 0.080 from the viewpoint of economic efficiency.
% Or less.

B is an element effective in securing the base material strength or HAZ toughness, but if it is added less than 0.0004%, there is no effect of increasing the strength, and if it exceeds 0.0030%, the base material and HAZ toughness are increased.
Detrimental to the toughness of. Therefore, the amount of B added is 0.0004.
~ 0.0030% range.

Ca is effective in controlling the sulfide morphology, and is effective in improving the impact absorption energy in the direction perpendicular to the rolling direction and the HAZ toughness. It is also effective in reducing hydrogen defects in the steel sheet cooled by accelerated cooling after the completion of rolling. However, if the addition amount is less than 0.0005%, its effect is small, and if it exceeds 0.0050%, it causes an increase or accumulation of nonmetallic inclusions. Therefore, the amount of Ca added is 0.0005 to 0.0050%.
The range is.

Next, the reasons for limiting the hot rolling conditions will be described. According to the present invention, a steel piece melted by a conventional method is converted into Ac ₃ transformation point +
Heat to a temperature above 100 ° C. The heating temperature of the billet is sufficient to austenitize the billet uniformly, prevent coarsening of austenite grains, make grains fine during rolling, and especially make solid solution of Nb that is indispensable for the effect of expanding the unrecrystallized region. Temperature below the Ac ₃ transformation point + 100 ℃,
The remaining coarse crystal grains at the time of solidification remaining before austenitization cannot be made finer by rolling, and it is impossible to secure predetermined strength and toughness. Therefore, the heating temperature of the billet is Ac ₃
Limit the transformation point to + 100 ° C or higher.

The reason why the cumulative rolling reduction condition is 920 ° C. or lower and 30% or higher is as follows. Even if reduction is applied at a temperature higher than 920 ° C., recrystallization cannot prevent the austenite grains (γ grains) from significantly coarsening, and high strength and high toughness due to rolling cannot be obtained. Further, a cumulative rolling reduction of 30% or more is a rolling reduction necessary for refining γ grains.If the rolling reduction is less than 30%, the γ grains will not be finely refined and the predetermined strength and toughness will not be obtained. Cannot be secured. Therefore, it is necessary to add a cumulative reduction of 30% or more at a temperature of 920 ° C or less.

In the present invention, in the rolling process, when the thickness of the billet is 1.2 times or less the finished plate thickness,
After forcibly cooling the outermost layer in the range not exceeding 1/6 to a temperature that is 10 ° C or more lower than the surface temperature before cooling, rolling is performed to the finished sheet thickness without reheating.

In the above-mentioned forced cooling, the difference in hardness between the surface layer portion and the central portion of a thick steel plate is likely to be large. Therefore, cooling and rolling are combined to reduce the difference in hardness in the thickness direction of the steel plate. This is to reduce unevenness in strength and toughness. By rolling after this forced cooling, crystal grains become fine grains in the surface layer of the steel slab, and the temperature is lower than that in the center, so hardenability is reduced, and hardenability due to accelerated cooling after rolling is reduced, resulting in thick wall thickness. Nevertheless, it is possible to manufacture a steel sheet with a small difference in hardness in the thickness direction and a uniform material.

Next, the reasons for limiting the thickness of the billet during the forced cooling in the rolling process, the cooling range in the thickness direction, and the cooling temperature will be described. When the billet thickness exceeds 1.2 times the finished plate thickness, the number of rolling passes after forced cooling increases, and as a result, the surface layer of the billet is completely reheated. Since it hardens, in order to prevent this, the steel billet thickness should be 1.2 times or less of the finished plate thickness.

In order to reduce the hardness of the surface layer of the steel sheet,
If the cooling effect becomes excessive to the inside of the thickness of the billet, the quality of the material will deteriorate, so it is sufficient to cool only the surface layer portion where the hardening is most remarkable. Therefore, the cooling range is limited to the range that does not exceed 1/6 from the front and back of the billet thickness.

By forcibly cooling to a temperature that is lower than the surface temperature before cooling by 10 ° C. or more, the hardness of the surface layer portion of the above steel sheet can be lowered and the difference in hardness in the thickness direction of the steel sheet can be reduced. be able to. If the cooling temperature is less than 10 ° C., recuperation progresses in rolling after forced cooling, the crystal grain refined before accelerated cooling grows and the temperature rises, so that the quenching effect increases and hardening of the steel sheet surface layer portion occurs. Therefore, the surface temperature of the steel slab is limited to 10 ° C or more lower than that before cooling.

The reason why the rolling is performed immediately after the forced cooling without reheating is to ensure the effect of the forced cooling described above.

The reason why the rolling finishing temperature is limited to the range of 680 to 850 ° C. is that bainite + ferrite or a small amount of bainite produced by refining the austenite grains and accelerating cooling by sufficiently rolling in the non-recrystallization temperature range. This is to make the transformation structure of pearlite + ferrite containing a fine and uniform structure. Due to this fine structure, the toughness is significantly improved and the tensile strength can be secured. If the rolling end temperature is lower than 680 ° C, the γ + α (ferrite) two-phase rolling is excessively advanced, the ductility is deteriorated, and the α
Charpy impact absorption energy is significantly reduced due to the inclusion of particles. On the other hand, if the rolling end temperature is too high, the γ grains will not be refined by rolling, and the desired strength and toughness cannot be obtained. Therefore, the upper limit of the rolling end temperature is 8
Set to 50 ° C.

Next, the reasons for limiting the accelerated cooling conditions will be described. An important point in the present invention is to obtain a fine structure such as fine bainite + ferrite or a small amount of bainite + pearlite + ferrite by the controlled rolling in the limited temperature range described above and the subsequent accelerated cooling. To obtain a predetermined tensile strength and high toughness.

The reason why accelerated cooling is started within 200 seconds after rolling is that the above-mentioned fine transformation structure can be obtained by cooling the refined γ grains or γ + α grains within the above period. When it exceeds 200 seconds, the temperature drop from the rolling end temperature is remarkable, the effect of forced cooling performed in the rolling process is lost, the material becomes uneven, and the tensile strength also decreases. Water is usually a suitable medium for accelerated cooling.

If the cooling rate of accelerated cooling is less than 0.4 ° C./sec, the transformation of the microstructure is insufficient and the improvement in strength is small, and if it exceeds 12 ° C./sec, since it is a thick material, the surface layer part and the central part The difference in hardness between the two becomes large, and thus the difference in materials such as tensile strength and toughness becomes large. Therefore, the cooling rate is 0.4
Limit to ~ 12 ° C / sec.

If accelerated cooling is stopped in a temperature range where the steel sheet surface temperature exceeds 800 ° C., the cooling will be insufficient and a predetermined strength cannot be ensured, resulting in a large variation in the material within the sheet surface. on the other hand,
When stopped in a temperature range of 500 ° C or lower, the strength becomes higher than a predetermined strength and the strengths of the surface layer portion and the central portion increase. Further, as the strength increases, the toughness significantly decreases. Further, when cooled to a low temperature range, hydrogen defects peculiar to thick-walled materials are likely to occur, and sufficient precipitation hardening as a strength increasing effect cannot be obtained. In addition, the residual stress, which has an adverse effect such as deformation during welding, becomes large. Therefore, the cooling stop temperature is limited to the range above 500 ℃ and below 800 ℃.

The steel sheet thus acceleratedly cooled has sufficient strength and toughness, and can be used as it is depending on the purpose of use and the intended use. The tempering is performed once or twice or more for the purpose of imparting the uniformity of the material to the thickness direction of the steel sheet and removing the residual residual stress that causes deformation during welding.

When the tempering temperature is higher than 760 ° C., the tensile strength is largely reduced and a predetermined strength cannot be secured, while at a temperature of 600 ° C. or lower, the tempering effect is insufficient and the tensile strength is high. However, the toughness cannot be improved. Further, the removal of the residual stress inherent therein is also insufficient. Therefore, the tempering temperature exceeds 600 ° C and is 76
Limited to a temperature range of 0 ° C or lower.

Regarding the production plate thickness, a steel plate of less than 44 mm is
It can be sufficiently manufactured by a conventional technique, and the present invention is applied to manufacture a steel plate of 44 mm or more. The temperature of the Ac ₃ transformation point is determined by the following equation. Ac ₃ (℃) = 908-223.7C + 438.5P + 30.5Si + 37.9V-34.4Mn-23.0
Ni However, each alloying element is expressed by the content (%).

[0042]

EXAMPLES Examples of the present invention will be described below.
The test steel sheet was produced by melting and casting a low-carbon low-alloy steel containing the chemical components shown in Table 1 by a conventional method, and the obtained steel slab having a thickness of 60 to 120 mm was manufactured according to the production conditions shown in Table 2. It is a finished steel plate. Test pieces were sampled from these steel plates and subjected to a tensile test and a Charpy impact test. The results are shown in Table 3.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

Nos. 1, 3, 5, 7 of Tables 2 and 3
9, 11, 13, and 15 are the methods of the present invention, No. 2, 4, 6, 8, and 1
0, 12, 14, 16 are comparative methods. Examples will be described below in order of No.

The steel sheet according to the method of the present invention is as shown in Table 3 below.
It can be seen that each of them is a thick steel plate having high strength and high toughness, a small difference in hardness between the steel plate surface and the central portion, and having a uniform material in the plate thickness direction.

On the other hand, in the comparative method, since the forced cooling in the rolling process is not performed, the difference in hardness between the steel plate surface and the central portion is large, and it is considered that the steel plate is a thick steel plate having a uniform material in the plate thickness direction. I can't say.

More specifically, in No. 2, the accelerated cooling stop temperature is low and the tempering temperature is also low, so the Charpy impact characteristics (toughness) are poor and the difference in hardness is large. No.4 is
Since the rolling finish temperature is high and the accelerated cooling rate is high, the toughness is poor and the difference in hardness is large. No. 6 has a high rolling finish temperature and a low accelerating cooling stop temperature, and thus has poor toughness and a large difference in hardness.

No. 8 is inferior in toughness because the heating temperature is low and the accelerated cooling stop temperature is low. No. 10 has a small cumulative rolling reduction of 920 ° C or lower, and has a high rolling end temperature and a high stop temperature for accelerated cooling, and thus has poor tensile properties and toughness. No. 12
Since the time from the end of rolling to the start of accelerated cooling is long, the tensile properties are inferior despite the low stop temperature of accelerated cooling.

No. 14 has a low stop temperature of accelerated cooling and a low tempering temperature, so that the tensile strength is too high, the toughness is poor, and the difference in hardness is large. No. 16 has been tempered twice and then tempered, resulting in too high tensile strength and poor toughness.
The difference in hardness is also large.

The stepwise gradual cooling after the accelerated cooling is one in which a plurality of thick steel plates are stacked and gradually cooled, and is performed in order to prevent hydrogen defects peculiar to the thick steel plates.

[0053]

As described above, the method for producing a high yield strength, high toughness, thick wall and high strength steel sheet according to the present invention has a small amount of Nb.
Is added to expand the unrecrystallized region during rolling, and controlled rolling is performed with the cumulative rolling reduction from the lower part of the recrystallized region to 30% or more, and in this rolling process, the surface layer of the steel slab is forcibly cooled. Then, the crystal grains of the surface layer portion are made finer and the rolling is finished,
Immediately thereafter, accelerated cooling is performed to generate a fine bainite + pearlite + ferrite structure or a pearlite + ferrite structure containing a small amount of bainite, so that the difference in hardness between the surface layer portion and the center portion of the steel sheet is small. According to the present invention, which provides a tough and thick high-strength steel sheet, the difference in hardness between the surface layer portion and the central portion of the steel sheet is small, and the sheet thickness is 44 mm or more having a uniform material in the sheet thickness direction. It is possible to manufacture high-toughness, thick-walled, high-strength steel sheets.

Claims (3)

[Claims] 1. C: 0.04 to 0.20%, Si: 0.05 to 0.50%, M
n: 0.70 to 2.0%, P: 0.020 or less, S: 0.010% or less, Al:
0.010 to 0.080%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.0
A steel slab containing 30% and N: 0.0015 to 0.0080% and the balance Fe and unavoidable impurities is heated to a temperature of Ac ₃ transformation point + 100 ℃ or more, and the cumulative reduction ratio from 920 ℃ or less to the finish plate thickness. When the billet thickness is 1.2 times or less of the finished plate thickness in the process of rolling with 30% or more, the outermost surface part that does not exceed 1/6 from the front and back in the thickness direction is more than the surface temperature before cooling. After forced cooling down to 10 ° C or lower, rolling is immediately performed without reheating, rolling is completed within the temperature range of 680 to 850 ° C, accelerated cooling is started within 200 seconds after rolling, and 0.4 to A method of manufacturing high-toughness, high-strength steel sheets with a plate thickness of 44 mm or more, which is cooled to a temperature range of more than 500 ° C and less than 800 ° C at a cooling rate of 12 ° C / sec. 2. As chemical components, Cu: 0.90% or less, Ni: 1.20% or less, Mo: 0.50% or less, V: 0.080% or less, B: 0.0004 to 0.0030%, Ca: 0.0005 to 0.0050% The method for producing a high-toughness thick high-strength steel sheet according to claim 1, which contains one or more selected types. 3. After accelerated cooling, the temperature exceeds 600 ° C. and 760 ° C.
The method for producing a high-toughness thick-walled high-strength steel sheet according to claim 1 or 2, wherein the tempering is performed once or twice or more within the following temperature range.