JPH0674457B2 - Method for manufacturing thick high-strength steel sheet excellent in low temperature toughness and weldability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a thick high-strength steel sheet excellent in low-tone toughness and weldability, and particularly, by controlling heating conditions, rolling conditions and subsequent cooling rates. The present invention relates to a method for producing a thick high-strength steel sheet which is uniform in the sheet thickness direction and has excellent low temperature toughness.

[Conventional technology]

In recent years, energy development has become polar and deep sea, and the marine structures used have become extremely large year by year, and it is necessary to use ice-breaking tankers for efficient energy transportation. The steel materials used for these are required to have a large plate thickness and excellent low temperature toughness. However, as the plate thickness increases, the material difference in the plate thickness direction increases, and the mechanical properties of the center part of the plate thickness are inferior to other parts. Particularly, the low temperature toughness is greatly deteriorated. Further, the constraint stress becomes maximum at the center of the plate thickness, which easily becomes a starting point of fracture, and therefore it is necessary to have excellent low temperature toughness from the surface to the center of the plate thickness.

Further, ensuring safety for these huge structures is an important issue, and it is necessary to keep the carbon equivalent low in order to improve weld cracking resistance and weld joint toughness.

In recent years, as a means for reducing the carbon equivalent to obtain high strength and high toughness, various material improvement techniques combining controlled rolling and controlled cooling have been studied and proposed, and for example, as a proposed method, JP-A-57- The one described in Japanese Patent No. 169019 is known. However, the technology described in the publication is intended for line pipes and general shipbuilding materials, and is also for relatively thin plates with a thickness of 50 mm or less. Is relatively uniform by nature.

[Problems to be solved by the invention]

However, when the plate thickness is increased to 50 mm or more, the material difference in the plate thickness direction becomes large, and the toughness particularly at the center of the plate thickness is significantly reduced. One of the causes for this is that in the conventional heating and rolling method, as shown in FIG. 2, the steel sheet temperature decreases with time during heating to 900 to 1150 ° C. in a heating furnace and then rough rolling to finish rolling. There is a large temperature difference between the center of the plate thickness (1 / 2t part) and immediately below the surface, and especially when starting rolling in the non-recrystallized region, the temperature difference between the surface and the center part of the plate thickness becomes large, and It is conceivable that crystallization will occur and rolling will occur on the high temperature side of the non-recrystallized region. Therefore, the toughness at the center part of the plate thickness is lower than that of the 1/4 t part where the best low temperature recrystallization in low temperature has been achieved. On the other hand, if the rolling temperature is lowered, it is possible to improve the low temperature toughness of the central portion of the plate thickness, but the temperature on the surface side is lowered too much,
The transformation-deposited ferrite is processed, and the low temperature toughness on the surface layer side is reduced. Therefore, it has been desired to develop a technology having excellent low temperature toughness over the entire portion of the plate thickness.

[Means for solving problems]

The present invention advantageously solves the above problems, a plate thickness 50 mm or more, a tensile strength 50 kgf / mm ₂ or more thick high-tensile steel plate, a manufacturing method capable of homogenizing and improving the toughness over the entire plate thickness direction. For the purpose of providing.

In order to achieve the above object, the present invention provides (1) a weight ratio of C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.50%, Nb:
Steel containing 0.001 to 0.10%, Al: 0.005 to 0.1% and the balance Fe and unavoidable impurities is heated to 900 to 1150 ° C and rolled to an intermediate thickness, and then the rolling is suspended and cooled. Or, without rolling, reheat the slab in the slab state before the center part of the plate thickness becomes Ar ₃ or less, heat the surface part that became Ar ₃ or less to Ac _{3 to} 1150 ° C, then restart the rolling and roll it. rolling reduction at a temperature of Ar ₃ + 100 ℃ ~Ar ₃ over the entire thickness when 50-7
After rolling 0%, after rolling 250 at a cooling rate of 1-10 ° C / sec
A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, characterized by cooling to ~ 600 ° C and then air cooling.

(2) By weight, C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.50%, Nb:
0.001 to 0.10%, Al: 0.005 to 0.1% as the basic component, and further C
r: 1.0% or less, Mo: 1.0% or less, V: 0.1% or less, Cu: 2.0% or less, and contains one or more, and further Ni: 4.0 or less, Ti:
Steel containing 1 or 2 or more of 0.15% or less and Ca: 0.01% or less with the balance Fe and unavoidable impurities 900 to 115
After heating to 0 ℃ and rolling to an intermediate thickness, the rolling is interrupted and cooled, or it is reheated in a slab state without rolling before the center of the plate thickness becomes Ar ₃ or less. ₃ follows become surface portion heated temperature to Ac ₃ to 1150 ° C., then resume rolling, the rolling reduction ratio 50% to 70% at a temperature of Ar ₃ + 100 ° C. to Ar ₃ over the entire thickness during rolling A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, which comprises performing cooling after rolling, cooling to 250 to 600 ° C at a cooling rate of 1 to 10 ° C / sec or more, and subsequently performing air cooling.

Is the gist.

As described above, in the conventional thick steel plate having a thickness of more than 50 mm, the material difference in the plate thickness direction, especially the difference in the low temperature toughness, is the fate because the control rolling process for controlling the rolling temperature is included. It has been considered as an unavoidable phenomenon.

However, the inventors of the present invention, as a factor of such a difference in toughness in the plate thickness direction, as a result of various studies, a surface layer reduced to Ar ₃ or less in a state where the plate thickness center is close to Ar ₃ before or during rolling. By forcibly heating the part and rolling it up to Ac ₃ or more, it is found that even after controlled rolling-controlled cooling, uniform and excellent mechanical properties in the plate thickness direction, particularly low temperature toughness, can be obtained, The invention is constituted by such knowledge.

Next, the reasons for limiting the components in the present invention will be described.

C is an element that inexpensively increases the strength and is required to be 0.03% or more to secure the strength. However, if added in a large amount, it impairs the toughness and weldability of steel, so the upper limit was made 0.20%.

Si is required to be 0.05% or more for deoxidizing the steel, but if it increases, it deteriorates the weldability, so the upper limit is made 0.60%.

Mn needs to be 0.50% or more to secure strength, but if it increases, it causes deterioration of weldability and toughness, so the upper limit is made 2.50%.

Nb is required to be 0.001% or more to prevent coarsening of austenite grains, suppress recrystallization, and secure strength, but if it increases, it hinders weldability, so the upper limit is 0.10%.

Al is required to be 0.005% or more for deoxidation, but if it increases, the toughness decreases significantly, so 0.1% is made the upper limit.

In the present invention, in addition to the above basic components, one or more of the following elements can be selectively added depending on the required properties of steel.

Cr is an element useful for improving the hardenability and increasing the strength, but if it increases, it hinders the toughness and weldability, so it is made 1.0% or less.

Mo is an element that improves the hardenability and is useful for increasing the strength, but if it increases, it decreases the weldability and toughness, so it is made 1.0% or less.

Cu is an element that is useful for increasing strength, but if it increases, it causes cracking during hot working and impairs weldability, so 2.0%
Below.

V is an element useful for increasing the strength due to precipitation hardening, but if it increases, it interferes with the weldability, so it is made 0.1% or less.

Ni is an element useful for improving toughness, but it is an expensive element, so the content is set to 4.0% or less.

Ti is an element that is useful for preventing coarsening of austenite grains and ensuring toughness, and is also useful for increasing strength by precipitation hardening, but if it increases, it impairs weldability, so it is made 0.1% or less.

Ca is effective in improving the material quality in the Z direction by controlling the morphology of sulfides in the steel, but if it increases, the inclusions in the steel increase and the content is made 0.01% or less, which impairs toughness and weldability.

Among these additive elements, V, Cu, Cr, and Mo are elements mainly useful for increasing strength, and one or more kinds are added as required. In addition, Ti, Ni, and Ca are elements that are mainly useful for improving toughness, and if necessary, one kind or two or more kinds are added.

Next, the reasons for limiting the heating, rolling and cooling conditions will be described.

The heating temperature is preferably 1150 ° C or lower for austenite grain refinement, but if it is too low, precipitation hardening elements do not form a solid solution, so it is 900 ° C or higher, but from the standpoint of strength and toughness, it is 950-1050. The range of ° C is most preferred.

After heating at these temperatures, after rolling to an intermediate thickness, the rolling is temporarily interrupted and cooled, or it is cooled in a slab state without rolling and reheated before the center part of the plate thickness becomes Ar ₃ or less. That is, as shown in FIG. 1, reheating is performed after extracting the heating furnace to reach an intermediate stage thickness, or cooling in a slab state without rolling and cooling to a surface of Ar ₃ or less by reheating Ac _{3 to} 1150. It heats up to ℃.
Here, the upper limit of reheating is set to 1150 ° C., because austenite becomes coarser above this temperature.

By such a heating method, the temperature during rolling is reduced to Ar
_{3 to} Ar ₃ +100 ℃, if more strictly controlled, Ar ₃ to Ar ₃ +5
The following non-recrystallization zone rolling can be performed at 0 ° C. As a result, the surface has the effect of grain refining by raising and lowering the transformation temperature (A ₃ ), and the central part of the plate thickness has the effect of lowering the rolling temperature. In comparison, it is possible to significantly improve the toughness of the entire plate thickness.

The cooling may be either water-cooling or air-cooling, and the reheating may be carried out by feeding back to a reheating furnace or a slab heating furnace attached to the rolling line for reheating.

Limiting the rolling temperature to these temperatures is that if the rolling temperature is too high, grain refining is not sufficient, and if rolling is performed at a temperature of less than Ar _{3, the} required strength will not be sufficiently quenched during the subsequent controlled cooling. This is because you cannot get

The reason why the rolling reduction at these temperatures is 50% or more is that if the temperature is less than this, grain refinement is not sufficient and the toughness is poor. The upper limit is preferably 70% at which the effect of controlled rolling begins to saturate.

Next, the cooling rate after hot rolling was set to 1 ° C / sec or more,
This is to ensure a predetermined strength by making a quenched structure up to the center of the plate thickness, and if it is less than this, the strength will be insufficient.

On the other hand, it is preferable that the upper limit is 10 ° C./sec that suppresses a rapid increase in surface hardness and does not exhibit an intermediate structure with poor toughness.

Next, the lower limit of the cooling stop temperature is set to 250 ° C. in order to prevent deterioration of toughness due to excessive increase in strength, and the upper limit is set to 60 ° C.
The temperature of 0 ° C means that the desired strength cannot be obtained above this temperature.
This is because the grain size becomes insufficient.

The air cooling after the cooling is stopped is to prevent the strength from increasing too much and the toughness to decrease due to the auto-tempering effect during the air cooling.

(Example) Next, the Example and comparative example of this invention are given.

The chemical composition of the test material is shown in Table 1, the manufacturing conditions are shown in Table 2, and the mechanical properties of the thick steel plate obtained are shown in Table 3.

As described above, the thick steel plates A1, B1, C1, D obtained by applying the method of the present invention
All of 1, E1, F1, G1, H1 have small toughness difference in the plate thickness direction,
Just under the surface, 1 / 4t and 1 / 2t both show good toughness. On the other hand, A2 of the comparative example has a low water-cooling stop temperature and therefore has high strength but low toughness. B2 has poor surface toughness because the surface has not reheated to Ac ₃ or higher. Since C2, D2 and H2 are not reheated, the rolling temperature at the center of the plate thickness is high and the toughness is poor.
Since E2 is controlled rolling only 38%, the toughness of the entire plate thickness is poor. F2 is the thickness center portion is lower than Ar _3,
Since the surface has not reheated to Ac ₃ or more, the toughness at the center of the plate thickness and the surface is poor. Since G2 has a high heating temperature, the toughness of the entire plate thickness is poor.

(Effects of the Invention) As described above, the present invention has a plate thickness of 50 mm or more and 50 kgf per 1 mm _2.
It is achieved by controlling the heating, rolling, and cooling of the grain refinement of the plate thickness center part of the steel plate having the above tensile strength,
It is possible to secure good low-temperature toughness including the center of thickness and to secure high strength under low carbon equivalents by appropriately limiting the component composition and content. is there.

[Brief description of drawings]

FIG. 1 shows just under the surface when reheated according to the present invention and 1 /
FIG. 2 is an explanatory diagram showing a temperature history of 2t, and FIG. 2 is an explanatory diagram showing a temperature history of a conventional method without reheating.

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-60-258410 (JP, A) JP-A-61-71105 (JP, A) JP-A-61-139622 (JP, A) JP-A-63- 50424 (JP, A)

Claims (2)

[Claims] 1. A weight ratio of C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.50%, Nb:
Steel containing 0.001 to 0.10%, Al: 0.005 to 0.1% and the balance Fe and unavoidable impurities is heated to 900 to 1150 ° C and rolled to an intermediate thickness, and then the rolling is temporarily stopped and cooled. Or, without rolling, reheat the slab in the slab state before the center part of the plate thickness becomes Ar ₃ or less, heat the surface part that became Ar ₃ or less to Ac _{3 to} 1150 ° C, then restart the rolling and roll it. rolling reduction at a temperature of Ar ₃ + 100 ℃ ~Ar ₃ over the entire thickness when 50-7
After rolling 0%, after rolling 250 at a cooling rate of 1-10 ° C / sec
A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, characterized by cooling to ~ 600 ° C and then air cooling. 2. By weight ratio, C: 0.03 to 0.20%, Si: 0.05 to 0.60%, Mn: 0.50 to 2.50%, Nb:
0.001 to 0.10%, Al: 0.005 to 0.1% as a basic component, Cr: 1.0% or less, Mo: 1.0% or less, V: 0.1% or less, Cu: 2.0
% Or less, containing 1 or 2 or more, and Ni: 4.0%
Below, Ti: 0.15% or less, Ca: 0.01% or less, containing 1 or 2 or more types, the balance Fe and inevitable impurities steel
After heating to 900-1150 ° C and rolling to an intermediate thickness, the rolling is interrupted and cooled, or it is reheated in a slab state without rolling before the center of the thickness becomes Ar ₃ or less. , The surface part that became Ar ₃ or less is heated to Ac _{3 to} 1150 ° C,
Then, the rolling was restarted, and Ar ₃ + 100 ° C was applied over the entire thickness during rolling.
Performs reduction ratio 50% to 70% rolling at a temperature of to Ar _3, after rolling,
A method for producing a thick and high-strength steel sheet excellent in low-temperature toughness and weldability, which comprises cooling to 250 to 600 ° C at a cooling rate of 1 to 10 ° C / sec or more and then air cooling.