JPH0674454B2 - Method for producing thick high-strength steel sheet with excellent low temperature toughness and weldability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, and in particular, 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 material used for these is 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. In particular, the low temperature toughness is greatly deteriorated. Further, since the constraint stress is maximized at the center of the plate thickness and the starting point of fracture is likely to occur, it is necessary to have uniform and excellent low temperature toughness even at the center of the plate thickness.

Also, 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, Japanese Patent Laid-Open No. 57- The one described in Japanese Patent No. 169019 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 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) and immediately below the surface, and especially when starting rolling in the non-recrystallized region, the temperature difference between the surface and the center 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 at the 1/4 t part where rolling can be achieved at the low temperature side of the best non-recrystallization region. On the other hand, if the rolling temperature is lowered, the low temperature toughness at the center of the plate thickness can be improved, but the temperature on the surface side will be too low and the transformed ferrite will be processed. With the decrease in toughness, the microstructure in the plate thickness direction is also non-uniform, and it is desired to develop a technology having a uniform whole 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 in a thick high-tensile steel plate, a manufacturing method capable of homogenizing and improving the low temperature toughness over the entire plate thickness direction For the purpose of providing.

In order to achieve the above object, the present invention provides (1) by weight ratio 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 stage thickness, and the rolling is temporarily interrupted and cooled or rolled. Without cooling, the surface temperature is Ar ₃
Heated uniformly coercive the steel to a temperature of Ar ₃ + 0.99 ° C. to Ar ₃ before dividing, and then subjected to rolling reduction ratio 50% to 70% in the Ar ₃ or more, after rolling, at a cooling rate 1 to 10 ° C. / sec Cool to 250-600 ℃,
A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, which is characterized by continuous 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 a basic component, Cr: 1.0% or less, Mo: 1.0% or less, V: 0.1% or less, Cu: 2.0%
A steel containing one or more of the following, Ni: 4.0% or less, Ti: 0.15% or less, Ca: 0.01% or less, and one or more of the following, with the balance Fe and inevitable impurities. 900 to 1150 was heated to ° C., intermediate stage or cool suspends rolled once rolled to a thickness of, or the cooling surface temperature remains slab state without rolling the steel before dividing the Ar ₃ Ar ₃ +150 ℃ ~ Ar ₃ to maintain a uniform temperature, then rolled with Ar ₃ or more at a rolling reduction of 50 ~ 70%, after rolling, cooling at a cooling rate of 1 ~ 10 ℃ / sec to 250 ~ 600 ℃, followed by air cooling A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, which is characterized by:

Is the gist.

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

However, as a result of further deep pursuit of the factors of the difference in toughness in the plate thickness direction, the inventors have found that the temperature distribution in the plate thickness direction before rolling is rather 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 cooling in the state of the slab or after some rolling, heat retention is performed to eliminate the temperature difference between the surface and the center of the plate thickness that occurs before the controlled rolling to make it uniform, and thereafter the controlled rolling-the plate after the controlled cooling. It was found that a uniform microstructure and mechanical properties can be obtained in the thickness direction, and the present invention is configured based on such knowledge.

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

C is an element that increases the strength at a low cost, and 0.03 is used to secure the strength.
%, But if added in a large amount, the toughness and weldability of steel are impaired, 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 needs 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 0.1%.

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 that improves the hardenability and is useful for 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 lowers 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 the 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 steel, but if it increases, inclusions in the steel increase, impairing toughness and weldability, so it is made 0.01% or less.

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. Further, Ti, Ni, and Ca are elements mainly useful for improving the 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 1150 ° C or lower for austenite grain refinement. On the other hand, if it is too low, the precipitation hardening element will not form a solid solution, so the lower limit is made 900 ° C or higher. 950 in terms of strength and toughness
Most preferred is the range of ℃ to 1050 ℃.

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 is almost uniform in the plate thickness direction, and the difference in characteristics in the plate thickness direction can be reduced, whereby a steel plate having excellent toughness at the center of the plate thickness can be manufactured.
The temperature during rolling should be in the range of Ar _{3 to} Ar ₃ + 150 ℃, but the total thickness is A
It is preferable to fall within the range of r _{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% or less at which the effect of controlled rolling is saturated.

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 10 ° C / sec or less that suppresses a sudden increase in surface hardness and does not produce an intermediate structure with poor toughness.

Next, the lower limit of the water 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, and H1 have small toughness differences in the plate thickness direction, and show good toughness just below the surface at 1 / 4t and 1 / 2t. On the other hand, A2 of the comparative example has a low water-cooling stop temperature and therefore has high strength but low toughness. Since B2 and H2 are not reheat-retained, 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.
For F2, the overall plate thickness is lower than Ar ₃ , and the toughness is poor. G2
Since the heating temperature is high, the toughness of the entire plate is poor.

Since D2 has a high heat retention temperature, it has high strength but low toughness. C2
Has a low surface toughness because heat retention begins at surface Ar ₃ or lower.

(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 view showing the temperature history of FIG. 2, and FIG. 2 is an explanatory view 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- 50427 (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 stage thickness, and the rolling is temporarily interrupted and cooled or rolled. Without cooling, the surface temperature is Ar ₃
Heated uniformly coercive the steel to a temperature of Ar ₃ + 0.99 ° C. to Ar ₃ before dividing, and then subjected to rolling reduction ratio 50% to 70% in the Ar ₃ or more, after rolling, at a cooling rate 1 to 10 ° C. / sec Cool to 250-600 ℃,
A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, which is characterized by continuous 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%
A steel containing one or more of the following, Ni: 4.0% or less, Ti: 0.15% or less, Ca: 0.01% or less, and one or more of the following, with the balance Fe and inevitable impurities. 900 to 1150 was heated to ° C., intermediate stage or cool suspends rolled once rolled to a thickness of, or the cooling surface temperature remains slab state without rolling the steel before dividing the Ar ₃ Ar ₃ +150 ℃ ~ Ar ₃ to maintain a uniform temperature, then rolled with Ar ₃ or more at a rolling reduction of 50 ~ 70%, after rolling, cooling at a cooling rate of 1 ~ 10 ℃ / sec to 250 ~ 600 ℃, followed by air cooling A method for producing a thick high-strength steel sheet excellent in low-temperature toughness and weldability, which is characterized by: