JPH0735538B2 - Method for manufacturing high strength and high toughness thick steel plate with excellent weldability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a manufacturing technique of a high-strength and high-toughness thick steel plate subjected to welding.

(Prior Art and Problems to be Solved) As a material for a large structure, a thick steel plate having high strength, high toughness and good weldability is used. For example, low carbon is used to improve weldability. ASTM A710 is known as a steel that utilizes the age precipitation hardening of Cu in an equivalent amount of the component system. This steel is
It contains 1.0 to 1.3% Cu and is precipitating Cu by aging treatment to obtain high strength.

However, in the case of the above steel, when a thick steel plate having a plate thickness of 35 mm or more is used, it is difficult to secure excellent low temperature toughness (fracture transition temperature vTrs of −80 ° C. or less) by the conventional manufacturing method. .

On the other hand, there has been proposed a method of performing controlled rolling and then directly quenching when manufacturing a steel sheet of this type that requires high strength and high toughness. However, when such a method is applied to the above steel, it is necessary to apply a large cumulative reduction in the low temperature range of austenite. As a result, the rolling finish temperature was a very low temperature of 800 ° C or lower, and therefore, even if direct quenching was performed immediately after rolling, the effect of quenching was not sufficiently obtained, and the aging precipitation hardening of Cu was used. Also has a tensile strength of 70 kgf /
It is difficult to obtain high strength of mm ² or more. Moreover, as the thickness of the steel sheet increases, Mn, Cr, Mo
It is necessary to add a large amount of alloying elements such as, as a result, the carbon equivalent of the steel becomes high, and there is a problem that the weldability is significantly impaired. For example, JP-A-60-59018 discloses only the case where the plate thickness is 30 mm or less.

For the above reasons, it has been very difficult to produce a high-strength, high-toughness steel sheet having a plate thickness of 35 mm or more and excellent weldability by utilizing Cu precipitation hardening.

However, a method in which the rolling finishing temperature is set relatively high, such as 800 ° C or higher, and quenching is directly performed after the rolling is completed (Japanese Patent Laid-Open No. 62-25 / 1987)
No. 6915, No. 62-149845), or a method of adding heat quenching has been tried (JP-A Nos. 61-149430 and 62-14).
9845), in the case of direct quenching, there is a problem in whether strength and low temperature toughness can be obtained in a well-balanced manner at a plate thickness of 35 mm or more, and there is a problem of cost increase due to reheating.

The present invention has been made under such circumstances,
Utilizing the precipitation hardening of Cu in a low carbon equivalent component system, and applying controlled rolling to further improve low temperature toughness, it has a high strength with a plate thickness of 35 mm or more and a tensile strength of 70 kgf / mm ² or more. However, the fracture surface transition temperature at the center of the plate thickness has high toughness of -80 ° C or less,
Moreover, it is an object of the present invention to provide a method for obtaining a high-strength, high-toughness thick steel plate having excellent weldability.

(Means for Solving the Problem) In order to achieve the above-mentioned object, the present inventors have an effect of enhancing the effect of controlled rolling, promoting transformation strengthening during direct quenching, and further causing a precipitation effect during tempering. Focusing on Nb, we conducted intensive research on how to effectively utilize it.

As a result, by appropriately adjusting the chemical composition and, particularly, by appropriately controlling the rolling conditions in the rough rolling stage, the above object can be achieved, and a high strength and high toughness thick steel plate having excellent weldability can be manufactured. I found a way to do it. Specifically, (1) the amount of C is reduced to 0.01 to 0.10% to improve weldability, (2) 0.8 to 1.5% of Cu is added for high strength, and controlled rolling is performed. Enhances the effect, promotes transformation strengthening, and contains 0.010 to 0.060% of Nb which has a precipitation hardening action. (3) A constant cooling rate without applying reduction in a specific temperature range at the rough rolling stage. The above is the cooling.

Based on these findings, the inventors have conducted experimental research on various conditions and have reached the present invention.

That is, the present invention, C: 0.01 ~ 0.10%, Si: 0.10 ~ 0.50
%, Mn: 0.5 to 2.0%, Al: 0.01 to 0.10%, Cu: 0.8 to 1.5
%, Nb: 0.01 to 0.06%, Ti: 0.005 to 0.020% and N: 0.015
% Or less, and if necessary, Ni: 3.0% or less, C
r: 1.0% or less, Mo: 0.5% or less, V: 0.1% or less, B: 0.0030%
For the steel containing 1 or 2 or more of the following and Ca: 0.0050% or less, the balance being iron and unavoidable impurities, and heating to a temperature range of 1000 to 1150 ° C., and then performing hot rolling. In rough rolling, no reduction is applied while the slab surface temperature T satisfies the following formula 900 ° C ≤ T + t ≤ 1000 ° C (t: slab thickness), and the cooling rate at the slab surface temperature is 12 ° C / min or more during this period. Cool to
A plate characterized by being subjected to a reduction of 60% or more at 800 to 700 ° C in finish rolling and completed rolling at (Ar ₃ transformation point -30 ° C) or more, and immediately being directly quenched and further tempered. It requires a method for producing a high strength, high toughness thick steel plate having a thickness of 35 mm or more and excellent weldability.

(Operation) The present invention will be described in more detail below.

First, the reasons for limiting the chemical components in the present invention are as follows.

C: C is an element effective for increasing strength, and 0.01% for that
The above is necessary. However, it is necessary to regulate the upper limit to 0.10% from the viewpoint of securing toughness and preventing deterioration of weld crack resistance. Therefore, the amount of C is set to the range of 0.01 to 0.10%.

Si: Si is a deoxidizing element, and it is necessary to add 0.10% or more, but excessive addition deteriorates the toughness, so the upper limit is 0.5%.
And Therefore, the amount of Si is set in the range of 0.10 to 0.50%.

Mn: Mn is effective in increasing strength, and therefore 0.5% or more is necessary, but if added in excess of 2.0%, toughness deteriorates. Therefore, the amount of Mn is set in the range of 0.5 to 2.0%.

Al: Al is a deoxidizing element and is required to be 0.10% or more, but excessive addition forms inclusions and deteriorates toughness, so the upper limit is made 0.10%. Therefore, the amount of Al is set to 0.01 to 0.10%.

Nb: Nb is an important element which is the point of the present invention. That is, Nb is a solid solution state, suppresses recrystallization of austenite, enhances the effect of controlled rolling, improves transformation strengthening by direct quenching, and precipitates during tempering to contribute to the improvement of strength and toughness. Is. Such an effect is exhibited at 0.01% or more, but if it exceeds 0.06%, the toughness of the welded portion is deteriorated, which is not preferable. Therefore, the amount of Nb 0.01-0.06
The range is%.

Ti: Ti is an element which forms a refractory carbonitride and suppresses the growth of austenite grains during slab heating or during welding, and thus has the effect of improving the base material and the toughness during welding. Also, by fixing C and N in steel, solid solution Nb
It also has an effect of suppressing precipitation as a nitride. For that purpose, 0.005% or more is required, but if added in excess of 0.020%, coarse inclusions are formed and the toughness deteriorates.
Therefore, the Ti content is in the range of 0.005 to 0.020%.

N: If N is added in an amount of more than 0.015%, the toughness of the base material and the welded portion is significantly deteriorated, so the N content is 0.015% or less.

Cu: Cu is an element effective for solid solution strengthening, precipitation strengthening, or transformation strengthening by improving hardenability. In order to exert these effects, it is necessary to add 0.8% or more. However, excessive addition lowers the toughness, so the upper limit is made 1.5%. Therefore, the Cu content is set to 0.8 to 1.5%.

Each of the above elements is an essential component, but if necessary, one or more of Ni, Cr, Mo, V, B and Ca shown below can be added in an appropriate amount.

Ni has the effect of improving the low temperature toughness, but is expensive, and is 3.0% or less from the viewpoint of economy. In addition, Cr is 1.0% or less, Mo is 0.5% or less, and V is 0.1% mainly for the purpose of strengthening.
Below, 0.0030% or less of B can be added. Further, Ca can be added in an amount of 0.0050% or less for the purpose of improving toughness. It should be noted that if these elements are excessively added in excess of the upper limits, the toughness or the weldability is deteriorated, which is not preferable.

Next, the reasons for limiting the conditions of ingot heating and hot rolling in the present invention will be described.

The ingot heating temperature must be 1150 ° C. or lower to prevent deterioration of toughness due to coarsening of austenite grains. However, if it is less than 1000 ° C., it is difficult to secure the solid solution Nb that effectively acts to improve the strength and toughness, which is not preferable.

When hot rolling an ingot heated to the above temperature range, it is necessary to control the conditions in rough rolling and finish rolling.

That is, first, in the middle of the rough rolling, the slab coating temperature T is not reduced within a range satisfying the following formula 900 ° C. ≦ T + t ≦ 1000 ° C. (t: slab thickness), and during this period, 12 ° C./min It is necessary to cool at the cooling rate of. This is the most important point of the present invention. As described above, the temperature range limited by the present invention is the temperature at which Nb is most likely to precipitate in austenite, and rolling during this period remarkably promotes the precipitation of Nb. Therefore, without rolling during this period, and by cooling at a higher cooling rate than usual, precipitation of Nb during hot rolling can be suppressed as much as possible. As a result, the effect of controlled rolling in the low temperature range of austenite is improved, the toughness is improved, the transformation strengthening during direct quenching is promoted, and the precipitation strengthening amount of Nb by tempering can be increased. Furthermore, according to this method, the growth of austenite grains in this temperature range can be suppressed, and the effect of improving toughness increases.

For example, FIG. 1 shows 0.04% C-0.25% Si-1.4% Mn-0.8.
% Cu-0.4% Ni-0.2% Mo-0.045% Nb-0.010% Ti-0.00
It shows the influence of the cooling rate (horizontal axis) in the rough rolling interruption temperature range on the strength and toughness (vertical axis) of steel having a composition of 45% N-0.035% Al. The finish rolling was carried out at a rolling reduction of 65% at 800 to 700 ° C, and after tempering, tempered at 600 ° C. In the figure, the strength increases as the cooling rate increases, and tends to saturate at 12 ° C / min or more.
On the other hand, the toughness is gradually improved as the cooling rate is increased. Therefore, it is understood that the cooling rate during this period needs to be 12 ° C./min or more. The cooling rate during this period is preferably set to 15 ° C./min to more stably obtain high strength.

Furthermore, in finish rolling, in order to improve toughness, 80
Perform reduction of 60% or more in austenite low temperature range of 0 to 700 ℃. This temperature range is a non-recrystallization temperature range of austenite, and the reduction during this gives a working strain in austenite, promotes the refinement of the structure after transformation, and improves the strength and toughness. Therefore, a reduction of at least 60% is required. In addition, the finish rolling finish temperature is (Ar ₃ transformation point −30 in order to fully exert the effect of the subsequent direct quenching.
℃) or more.

After rolling, the steel sheet is directly quenched. This is to maximize the effect of transformation strengthening by improving the hardenability of solute Nb. Furthermore, this steel sheet is subjected to a tempering treatment in order to obtain high strength by precipitating Nb existing in a solid solution state after transformation.

Next, examples of the present invention will be described.

(Example) A sample steel having the chemical composition shown in Table 1 was melted and cast by a conventional method, and the obtained ingot was heated to the temperature shown in Table 2 and then under the conditions shown in the same table. Then, rough rolling and finish rolling were performed, quenching was immediately performed, and further tempering at 600 ° C was performed.

The mechanical properties (yield strength YS, tensile strength TS, surface transition temperature at the center of thickness) of the obtained steel sheet were examined. The results are also shown in Table 2.

As is clear from Table 2, No. 1 (steel) which is an example of the present invention
A1), No.7 (Steel B), No.11 (Steel F), No.12 (Steel G),
No. 13 (Steel H) has a tensile strength of 70 kgf / mm ² or more, and the surface transition temperature vTrs at the central part of the plate thickness is -80 ° C or less, and has an excellent strength-toughness balance. You can see that Of course, since the chemical component, especially the amount of C, is low, the weldability is excellent.

On the other hand, even if the steel has the same chemical composition as the present invention example,
If the heating and rolling conditions are outside the scope of the present invention, that is, No. 2 (Steel A) to No. 6 (Steel A), the strength is 70 kgf / mm ^2.
Is not reached or the surface transition temperature at the center of the plate thickness is higher than -80 ° C.

Further, Comparative Examples No. 8 (Steel C), No. 9 (Steel D), N relating to Steels C to E having excellent components from the chemical composition range of the present invention.
In o.10 (Steel E), even if the heating and rolling conditions are within the range defined by the present invention, the tensile strength is 70 kgf / mm ² or less, or the surface transition temperature is −80 ° C. As above, the targeted characteristics have not been obtained.

(Effects of the Invention) As described in detail above, according to the present invention, in the steel in which the chemical composition is appropriately adjusted, the rolling is interrupted in the specific temperature range in the rough rolling stage, and during this period, the cooling rate is higher than usual. Cool down
Furthermore, finish rolling under specific conditions, quenching immediately, and further tempering, so tensile strength 70 kgf / mm ² or more, surface transition temperature -80 ° C or less, excellent strength-toughness balance, and weldability An excellent high-strength and high-toughness thick steel plate can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of the cooling rate when the rough rolling is interrupted on the tensile strength and the transition temperature of the surface to be covered in the central portion of the plate thickness.

Claims (2)

[Claims] 1. In weight% (hereinafter the same), C: 0.01 to 0.10.
%, Si: 0.10 ~ 0.50%, Mn: 0.5 ~ 2.0%, Al: 0.01 ~ 0.10
%, Cu: 0.8 to 1.5%, Nb: 0.01 to 0.06%, Ti: 0.005 to 0.02
For steel containing 0% and N: 0.015% or less, the balance being iron and inevitable impurities, after heating to a temperature range of 1000 to 1150 ° C., when performing hot rolling, slab surface temperature T in rough rolling Does not apply any reduction while satisfying the following formula 900 ° C ≤ T + t ≤ 1000 ° C (t: slab thickness), and during this time, cooling is performed so that the cooling rate at the slab surface temperature is 12 ° C / min or more, and then ,
A plate characterized by being subjected to finish rolling at a temperature of 800 to 700 ° C with a reduction of 60% or more (Ar ₃ transformation point -30 ° C) or higher, immediately followed by direct quenching and tempering. A method for producing high strength, high toughness thick steel plates with a thickness of 35 mm or more and excellent weldability. 2. The steel contains Ni: 3.0% or less, Cr: 1.0% or less,
Mo: 0.5% or less, V: 0.1% or less, B: 0.0030% or less and Ca: 0.
The method according to claim 1, wherein the method contains one or more of up to and including 100%.