JPH09256056A - Production of high strength thick steel plate good in uniformity with high efficiency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a tough and homogeneous thick steel plate with high efficiency by subjecting a steel plate contg. specified metals by weight % within specified ranges to specified heat treatment. SOLUTION: This is a method for producing a high strength thick steel plate good in uniformity with high efficiency in which a steel having a compsn. contg., by weight, 0.02 to 0.25% C, 0.03 to 2.0% Si, 0.30 to 3.5% Mn and 0.002 to 0.10% Al or furthermore contg. one or two kinds of 0.002 to 0.10% Nb and 0.002 to 0.10% Ti, and the balance Fe with inevitable impurities is subjected to hot rolling and is thereafter directly quenched, or in the case a thick steel plate subjected to heating and quenching after the temp. reduces to a room temp. is continuously carried and is subjected to tempering in a furnace, the inclination of temps. is given to the inside of the furnace, that on the inlet side of the furnace is set higher than the prescribed tempering temp. by >=200 deg.C within the range of 350 deg.C to the Ac1 point + 100 deg.C, the set furnace temp. is stepwise reduced toward the outlet side of the furnace, and the set temp. of the furnace in front of the outlet of the furnace is regulated to the above tempering temp. ±50 deg.C or below.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a strong thick steel plate.

[0002]

2. Description of the Related Art With the increase in size of steel structures, the development of stronger steel is required. Normal tensile strength 60kgf / mm ²
The above steels are manufactured by a method in which martensite or a lower bainite transformation is caused by quenching and supersaturated solid solution carbon is precipitated as carbides by a subsequent tempering treatment. Such a manufacturing method requires a long manufacturing time and a large manufacturing cost.

In recent years, in order to make up for the drawbacks of such a normal quenching and tempering process, a direct quenching technique has been developed in which quenching is carried out as it is after rolling. This method has produced some effects in terms of reducing manufacturing costs and strengthening steel.
As such a manufacturing method, for example, Japanese Patent Publication No. 53-661.
No. 6, JP-B-55-49131, JP-B-58
There is a publication such as -3011. However, in such a technique, since the tempering process is the same as the conventional one, the manufacturing cost is basically high due to its low productivity. It is hard to say that the optimal metallographic state has been obtained from the metallurgical point of view.

Further, in recent years, Japanese Patent Laid-Open No. 015753/1990 has been reported as a technique for increasing the toughness more than before by increasing the rate of temperature rise during tempering. However, in this method, since the material is annealed directly from the rolling unrecrystallized temperature range, it is inevitable that the material anisotropy becomes large. Deterioration cannot be avoided.

Further, increasing the rate of temperature rise during tempering by ordinary furnace heating inevitably causes the periphery of the plate to be excessively heated as compared with the center of the plate, resulting in a high temperature. For this reason, there is a drawback that material fluctuations inside the plate cannot be avoided. For this reason, there has been a strong demand for a method capable of highly efficiently manufacturing a steel having a uniform material inside the plate and being strong.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a strong and homogeneous thick steel plate with high efficiency.

[0007]

The present invention provides a manufacturing method capable of advantageously eliminating the above-mentioned drawbacks of the conventional method, in which a thick steel plate quenched under predetermined conditions is continuously quenched. When returning, it is a method of manufacturing a tough and homogeneous thick steel plate by uniformly tempering at a high temperature rising rate by controlling the method of setting the furnace temperature, and the summary thereof is as follows.

(1) C: 0.02% to 0.2% by weight
5%, Si: 0.03% to 2.0%, Mn: 0.30%
~ 3.5%, Al: 0.002% to 0.10%, the balance consisting of Fe and unavoidable impurities is hot-rolled and then directly quenched or heated to room temperature before heating. When a thick steel plate that has been quenched is continuously conveyed and tempered in the furnace, a temperature gradient is applied inside the furnace, and the furnace entrance side is within a range of 350 ° C or higher and Ac ₁ point + 100 ° C or lower. Is set to 200 ° C or more higher than the predetermined tempering temperature, the set furnace temperature is gradually decreased toward the exit side of the furnace, and the set temperature of the furnace before the exit of the furnace is set to the above-mentioned tempering temperature ± 50 ° C. A method for producing a tough steel plate with high efficiency and good uniformity, characterized in that

(2) Further, by weight%, Nb: 0.002%
To 0.10%, Ti: 0.002% to 0.10%, or a combination of two or more of them. Law.

(3) Further, by weight%, Cu: 0.05% to
3.0%, Ni: 0.05% to 10.0%, Cr: 0.
05 to 10.0%, Mo: 0.05% to 3.5%, C
o: 0.05% to 10.0%, W: 0.05% to 2.0
%, And 1 or 2 or more of them are contained, The manufacturing method of the highly efficient and tough steel plate with high uniformity as described in (1) or (2) characterized by the above-mentioned. (4) High efficiency and good uniformity according to any one of (1) to (3), characterized by further containing V: 0.002% to 0.10% by weight. Manufacturing method of tough steel plate.

(5) Further, by weight%, B: 0.0002%
~ 0.0025% is contained, The manufacturing method of the highly efficient and tough thick steel plate as described in any one of (1) to (4). (6) Further by weight%, Rem: 0.002% to 0.10.
%, Ca: 0.0003% to 0.0030% is contained, and the method for producing a high-strength steel plate having high efficiency and good uniformity according to any one of (1) to (5). .

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The technical idea underlying the present invention is as follows. Steels having a tensile strength of 60 kg / mm ² or more are often produced by quenching and tempering or direct quenching after rolling and tempering. It is known that the balance of strength and toughness is best when the metal structure is a mixed structure of martensite and lower bainite.

Further, the dislocations inherited in the structure after transformation disappear under normal tempering conditions after quenching, but dislocations are increased by increasing the temperature rising rate up to the tempering temperature. It is possible to precipitate solid solution carbon as carbides while leaving a large amount of carbon, and further strengthening is possible.

However, since the strength and toughness of thick steel plates are highly dependent on the tempering temperature, there is a serious problem in practical use that if there is a temperature difference inside the plate after tempering, the material fluctuation will increase. That is, if the set temperature of the heat treatment furnace is increased in order to increase the rate of temperature increase up to the tempering temperature, the temperature difference inside the plate inevitably increases, so that the temperature increase up to the tempering temperature is actually increased. It was impossible to increase the speed.

However, the present inventors have found a method of controlling the set furnace temperature of the heat treatment furnace so that the temperature rising rate up to the tempering temperature can be increased and the temperature difference inside the plate can be reduced. This is because only the furnace temperature on the inlet side of the heat treatment furnace is set to a high temperature, and the temperature rising rate in the initial stage of tempering is significantly increased, and the furnace temperature on the outlet side of the heat treatment furnace is made close to the target tempering temperature. This is a method of setting and converging the temperature difference inside the plate.

As a result, it becomes possible to precipitate solute carbon as carbides while leaving a large amount of transformation dislocations substantially equally over the entire plate, and it is possible to make the thick steel sheet after tempering extremely homogeneous and tough. Became. As a result of detailed investigation of the chemical composition of steel and the production conditions of steel based on such new knowledge, the present inventors have led to a production method of high-strength steel as set forth in the claims.

The reasons for limiting the manufacturing method will be described in detail below. First, the reasons for limiting the components of the starting material in the present invention will be described. C is an element advantageous for strengthening steel,
If it is less than 0.02%, sufficient strength cannot be obtained. On the other hand, if its content exceeds 0.25%, the weldability is deteriorated.
Si is effective as a deoxidizing element and as a strengthening element for steel, but it is not effective when the content is less than 0.03%.
On the other hand, if it exceeds 2.0%, the surface properties of steel are impaired.

Mn is an element effective in strengthening steel, and
If it is less than 30%, a sufficient effect cannot be obtained. On the other hand, if its content exceeds 3.5%, the workability of steel deteriorates. A
l is added as a deoxidizing element. If the content is less than 0.002%, there is no effect, and if the content exceeds 0.1%, the surface properties of steel are impaired.

Nb and Ti both effectively function in terms of crystal grain refinement and precipitation hardening when added in a small amount.
You may add in the range which does not deteriorate the toughness of a welded part. From this viewpoint, the upper limit of the addition amount of Nb and Ti is 0.10.
%. Both are ineffective if the addition amount is too small, so the lower limit of the addition amount is set to 0.002%.

Cu, Ni, Cr, Mo, Co and W are all elements for improving the hardenability of steel. In the case of the present invention, the addition thereof can increase the strength of the steel, but addition of an excessive amount impairs the weldability of the steel.
Cu ≦ 3.0%, Ni ≦ 10.0%, Cr ≦ 10.0
%, Mo ≦ 3.5%, Co ≦ 10.0%, W ≦ 2.0%
Limited to. In addition, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.05% for each element.

V is effective in increasing the strength of the steel by precipitation hardening, but excessive addition impairs the toughness of the steel.
The upper limit is set to 0.10%. Further, if the addition amount is too small, there is no effect, so the lower limit of the addition amount is made 0.002%.

B is an element that improves the hardenability of steel. In the case of the present invention, the addition thereof can increase the strength of the steel, but excessive addition increases the precipitates of B and impairs the toughness of the steel, so the upper limit of the content is 0.0.
025%. Further, if the addition amount is too small, there is no effect, so the lower limit of the addition amount is made 0.0002%.

Rem and Ca are effective for detoxifying S, but if the addition amount is small, S remains harmful and excessive addition impairs toughness, so Rem: 0.002% to 0.10.
%, Ca: 0.0003% to 0.0030%.

Next, the manufacturing conditions in the present invention will be described. Since the present invention is effective for a slab cast under any casting conditions, it is not necessary to limit the casting conditions. The hot rolling may be started as it is without cooling the slab, or the slab that has been once cooled may be reheated to a temperature of Ac ₃ point or higher and then rolled. Further, it is not necessary to specify rolling conditions in particular.

The effect of the present invention is effective both in the case of direct quenching after rolling and in the case of quenching by reheating after the temperature of the plate has once dropped to room temperature after rolling. In order to make the structure after quenching sufficiently martensite, bainite and a mixed structure thereof, it is necessary to raise the cooling temperature at the time of quenching, and usually cooling is performed at a cooling rate of 2 ° C./s or more. If the cooling end temperature is 550 ° C. or higher, it is not possible to sufficiently transform into martensite and bainite, so it is desirable to cool to 550 ° C. or lower.

When the tempering temperature is lower than 350 ° C., solid solution carbon is not sufficiently precipitated, and when the Ac ₁ point + 100 ° C. or higher,
On the contrary, since the rate of transformation into austenite becomes too high and the strength decreases, it is desirable to set the target tempering temperature to 350 ° C. or higher and less than Ac ₁ point + 100 ° C.

If the temperature rising rate during tempering is low, dislocations disappear before precipitation of solute carbon and the toughness deteriorates. Therefore, the temperature rising rate in the initial stage of tempering is 0.5 ° C./s or more. It is desirable to do. Therefore, the set furnace temperature of the heat treatment furnace is set to 200 ° C. or more higher than the predetermined tempering temperature on the entrance side of the furnace. When the tempering temperature is set to a predetermined tempering temperature + 200 ° C or lower, the temperature rising rate is 0.5 when the plate thickness is large.
It is often less than ° C / s.

By setting the temperature to a predetermined tempering temperature + 200 ° C. or more, a temperature rising rate of 0.5 ° C./s or more can be achieved, but the plate edge is excessively heated from the plate center and excessively tempered. It causes the strength to decrease. Therefore, in order to reduce the temperature difference between the plate edge and the plate center,
The set furnace temperature is gradually decreased toward the exit side of the furnace, and the set temperature of the furnace before the exit of the furnace is set to a predetermined tempering temperature ± 50 ° C.
Within the range, the temperature difference inside the plate gradually converges, and the fluctuation of the material of the finally obtained plate can be almost eliminated.

The set temperature of the furnace before the exit of the furnace is a predetermined tempering temperature + 50 ° C. or more or a predetermined tempering temperature −
If the temperature is lower than 50 ° C., the temperature difference inside the plate does not converge sufficiently, and the difference of 10 ° C. or more remains, so that there is a practical problem.

[0030]

Next, the present invention will be described in detail with reference to examples. First, for steels having the components shown in Table 1, hot rolling is performed on a thick plate having the dimensions shown in Table 2 and then direct quenching or reheating / quenching is performed, and then Table 3 (specification table of heat treatment furnace)
Alternatively, heat treatment was performed using a furnace shown in Table 4 (list of specifications of heat treatment furnace). When the method of the present invention and the comparative method are applied under the heat treatment conditions shown in Table 5 as the furnace temperature setting conditions at the time of heat treatment, the center of the thick steel plate as shown in Table 6 (comparison result of heat treatment time and mechanical properties of steel materials) The strength and toughness of parts and corners were obtained.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

According to Table 6, when the set furnace temperature at the initial stage of tempering is high and the temperature rising rate is high, the strength / toughness balance is improved. However, there is a large difference between the plate corner and the plate center in the material of the thick steel plate with the set furnace temperature kept high.
It can be seen that the method of the present invention in which the set furnace temperature is set to a high temperature in the initial stage and a low temperature in the latter stage can avoid material fluctuations at the plate corner and the plate center while maintaining the toughness effect.

If the set furnace temperature in the initial stage of heat treatment is less than the predetermined tempering temperature + 200 ° C., the toughening effect is insufficient. In addition, the furnace temperature immediately before the furnace exit is the prescribed tempering temperature ± 50 ° C.
When the value is out of the range, there remains a material difference between the plate corner and the plate center.

Further, when the heat treatment times of steel materials of the same size are compared, the heat treatment time required for the heat treatment method of the present invention, which is set by gradually lowering the furnace atmosphere temperature, sets the furnace temperature to a low constant temperature. It can be seen that the heat treatment time required by the conventional heat treatment method is obviously shorter, and the heat treatment efficiency is improved by the present invention.

[0040]

According to the present invention, a thick steel plate which is tough and homogeneous can be manufactured with high efficiency, and has a great industrial effect.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58

Claims (6)

[Claims] 1. By weight%, C: 0.02% to 0.25%, Si: 0.03% to 2.0%, Mn: 0.30% to 3.5%, Al: 0.002. % To 0.10%, the balance consisting of Fe and unavoidable impurities is hot-rolled and then directly quenched, or a thick steel sheet that has been heated and quenched after the temperature has dropped to room temperature is placed in a furnace. In the case of continuous transportation and tempering, the temperature inside the furnace is graded, and the furnace inlet side is 350 ° C or higher Ac ₁ point +100
Within the range of ℃ or less, set the tempering temperature 200 ° C or more higher than the predetermined tempering temperature, gradually decrease the set furnace temperature toward the exit side of the furnace, and set the temperature of the furnace before the exit of the furnace to the tempering temperature ±
A high-efficiency and highly-uniform tough steel plate manufacturing method characterized in that the temperature is within 50 ° C. 2. The composition further comprises, by weight, one or more of Nb: 0.002% to 0.10% and Ti: 0.002% to 0.10%. 1. A method for producing a strong and thick steel plate having high efficiency and good uniformity according to 1. 3. Further, by weight%, Cu: 0.05% to 3.0%, Ni: 0.05% to 10.0%, Cr: 0.05% to 10.0%, Mo: 0. 05% -3.5%, Co: 0.05% -10.0%, W: 0.05% -2.0% 1 type, or 2 or more types of containing, 1 or 2 characterized by the above-mentioned. 2. A method for producing a tough steel plate having high efficiency and good uniformity according to 2. 4. Further, by weight%, V: 0.002% to 0.
10% is contained, The highly efficient and highly uniform tough steel plate manufacturing method of any one of Claim 1 thru | or 3 characterized by the above-mentioned. 5. Further, by weight%, B: 0.0002% to
0.0025% is contained, The highly efficient and highly uniform tough steel plate manufacturing method of any one of Claim 1 thru | or 4 characterized by the above-mentioned. 6. The composition according to claim 1, further comprising Rem: 0.002% to 0.10% and Ca: 0.0003% to 0.0030% by weight. The method for manufacturing a strong and thick steel plate having high efficiency and good uniformity as described in 1.