JPH0674455B2 - Method for manufacturing thick high-strength steel sheet with excellent 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. Furthermore, since the restraint stress is maximized at the center of the plate thickness and is likely to become a starting point of fracture, it is necessary to have uniform and excellent low temperature toughness even at 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, JP-A-57-1690.
The method described in Japanese Patent Publication No. 19 is known. However, the technique described in the above publication is intended for line pipes and general shipbuilding materials, and is also a technique for relatively thin plates with a thickness of 50 mm or less. The material is relatively uniform by nature.

[Problems to be solved by the invention]

However, when the plate thickness becomes thicker than 50 mm, the material difference becomes large in the plate thickness direction, and the toughness particularly at the center part of the plate thickness remarkably decreases. 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 sheet thickness (1 / 2t) and immediately below the surface. Especially when rolling in the non-recrystallized region, the temperature difference between the surface and the center of the sheet 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 at the 1/4 t part where rolling can be achieved at the optimum low temperature side in the non-recrystallization region. On the other hand, if the rolling temperature is lowered, it is possible to improve the low temperature toughness of the central part of the plate thickness, but the temperature on the surface side is too low, and the transformed ferrite will be processed.
Along with the decrease in low temperature toughness on the surface side, the microstructure in the plate thickness direction is also non-uniform, and it is desired to develop a technology having a uniform plate thickness and excellent low temperature toughness.

[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,
Cooling or suspends temporarily rolled and rolled until an intermediate stage in thickness, or left to cool the slab state without rolling, the temperature of Ar ₃ + 0.99 ° C. to Ar ₃ of the steel before the surface temperature divided by Ar ₃ Heat evenly, then rolling with a reduction of 50 to 70% with Ar ₃ or more, cooling to 200 ° C or less at a cooling rate of 1 to 10 ° C / sec, and tempering at a temperature of 350 to 650 ° C after rolling. A method for producing a thick high-strength steel sheet excellent in toughness and weldability, which is characterized by the following.

(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 a basic component, Cr: 1.0% or less, Mo: 1.0% or less, V: 0.1% or less, Cu: 2.0
% Or less, 1 or 2 or more, Ni: 4.0 or less, Ti: 0.15% or less, Ca: 0.01% or less, 1 or 2 or more, and the balance Fe and unavoidable impurities 90
Heated to from 0 to 1,150 ° C., cooling or suspends temporarily rolled and rolled until an intermediate stage in thickness, or prior to cooling and the surface temperature remains slab state without rolling dividing the Ar ₃ Ar ₃ + 150 ℃ ~ uniformly heated holding a temperature of Ar _3, then reduction ratio 50% to 70% by Ar ₃ or more
After rolling, after rolling, 200 ℃ at a cooling rate of 1-10 ℃ / sec
A method for producing a thick and high-strength steel sheet excellent in toughness and weldability, which comprises cooling to the following temperature and tempering at a temperature of 350 to 650 ° C. Is the gist.

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

However, as a result of deeper pursuit of the factors of the toughness difference in the plate thickness direction, the inventors found that the temperature distribution in the plate thickness direction before rolling is more than the plastic deformation of each part of the plate thickness during rolling, the temperature change due to rolling, etc. We found that it had the most influence. Then, after the slab is cooled or slightly rolled, heat retention is performed to eliminate the temperature difference between the surface and the plate thickness center that occurs before the controlled rolling,
It has been found that a uniform microstructure and mechanical properties can be obtained in the sheet thickness direction even after the subsequent controlled rolling-controlled cooling by making them uniform, and the present invention is configured based on 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, inclusions in the steel increase and the toughness and weldability are impaired.

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 mainly useful for improving the toughness, and if necessary, one or more elements 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, rolling is interrupted before rolling to the intermediate stage thickness and the surface temperature lower than Ar ₃ , and the steel of the intermediate stage thickness or the steel in the slab state without rolling is Ar ₃ Charge into a furnace etc. set to a temperature of + 150 ° C to Ar ₃ ,
Keep the whole at a uniform temperature. Immediately after extraction, controlled rolling is performed in the non-recrystallized region. With this method,
As shown in the figure, the temperature difference between the central part (1 / 2t) of the plate thickness and immediately below the surface that occurred during rough rolling-finish rolling after extracting the heating furnace was eliminated, and at the start of controlled rolling in the unrecrystallized region. The rolling temperature at the center of the plate thickness can be directly above Ar ₃ without causing the surface to undergo two-phase rolling. That is, the temperature during rolling becomes substantially uniform in the plate thickness direction, and the characteristic difference in the plate thickness direction can be reduced,
This makes it possible to manufacture a steel sheet having excellent toughness even in the central portion of the plate thickness. The temperature during rolling is in the range of Ar _{3 to} Ar ₃ + 150 ° C,
It is preferred that the total thickness be in the range of Ar _{3 to} Ar ₃ + 50 ° C.

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 70% where 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. The upper limit is preferably 10 ° C./sec that suppresses a sudden increase in surface hardness and does not exhibit an intermediate structure with poor toughness.

The reason why the cooling stop temperature was set to 200 ° C or lower was to make the temperature to 250 ° C or less up to the center of the sheet thickness and to obtain a sufficiently quenched structure up to the center of the sheet thickness, and then to temper at 350 to 650 ° C. In order to improve the strength and toughness by hardening and tempering the plate thickness,
If the temperature is lower than 0 ° C, the strength remains high and the improvement of the toughness becomes insufficient, and the tempering exceeding 650 ° C causes a problem that the strength decreases largely and the required strength cannot be ensured, and should be avoided.

(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 high tempering temperature and thus has low strength. Since B2 and H2 do not retain heat, 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. In F2, the overall plate thickness is lower than Ar ₃ , and the overall toughness is poor. Since G2 has a high heating temperature, the toughness of the entire plate thickness is poor. C2 has a surface
Surface toughness is low because heat retention is started below Ar ₃ . D
No. 2 has a high heat retention temperature, and thus has high strength but low toughness.

(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 and 1 / 2t when heat is kept according to the present invention.
2 is an explanatory diagram showing the temperature history of FIG. 2, and FIG. 2 is an explanatory diagram showing the temperature history of the 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- 50426 (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,
Cooling or suspends temporarily rolled and rolled until an intermediate stage in thickness, or left to cool the slab state without rolling, the temperature of Ar ₃ + 0.99 ° C. to Ar ₃ of the steel before the surface temperature divided by Ar ₃ Heat evenly, then rolling with a reduction of 50 to 70% with Ar ₃ or more, cooling to 200 ° C or less at a cooling rate of 1 to 10 ° C / sec, and tempering at a temperature of 350 to 650 ° C after rolling. A method for producing a thick high-strength steel sheet excellent in toughness and weldability, which is characterized by the following. 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, a steel containing one or more of Ti: 0.15% or less and Ca: 0.01% or less and the balance Fe and unavoidable impurities,
Was heated to 900 to 1150 ° C., or cooled to interrupt temporarily rolled and rolled until an intermediate stage in thickness, or prior to cooling and the surface temperature remains slab state without rolling dividing the Ar ₃ Ar ₃ + 150 ℃ ~ Ar ₃
Uniform heat retention, then with Ar ₃ or more, reduction rate 50-70
% Rolling, and after rolling, 200 at a cooling rate of 1-10 ° C / sec
A method for producing a thick and high-strength steel sheet excellent in toughness and weldability, which comprises cooling to below ℃ and tempering at a temperature of 350 to 650 ℃.