JP2002332521A - Method for producing steel with ultrafine ferritic structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel with a ultrafine ferritic structure which does not cause separation in a Charpy test while maintaining sufficient strength and toughness as steel for structural purposes, and has no reduction of shelf energy. SOLUTION: A steel stock containing precipitate forming elements is heated at the Ac3 point or higher. After that, the steel stock is rolled at a draft of <=40%, or is not rolled, and is cooled to the temperature lower than the Ac3 point. Next, heating to the Ac3 point or higher and cooling to the Ac1 point or lower are repeated, respectively, for one or more times. Finally, the steel stock is heated to the Ac3 point or higher and rolled at the Ar3 point or higher, and is thereafter cooled. The cooling is carried out preferably by a method of performing rapid cooling at >=3 deg.C/s directly after the rolling, and stopping the cooling at >=500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultrafine ferritic steel (steel material having an ultrafine ferrite structure), and particularly to a method for shipbuilding, bridges, buildings, marine structures, penstocks, tank materials and the like. The present invention relates to a method for manufacturing an ultrafine ferritic steel used for a structure using a thick steel plate.

[0002]

2. Description of the Related Art Ultra-fine ferrite grain refinement of steel is the only method that increases the strength of steel and improves toughness without adding a large amount of elements such as Ni and Mn that have the effect of increasing the strength of steel. Means. Not adding a large amount of these elements is also excellent in recyclability. In recent years, vigorous research has been conducted on ultra-fine ferrite grains in steel, for example, CAMP-ISIJ,
A method using mechanical milling as shown in 12 (1999), 357. and a method using large austenite under reduced pressure using a small test piece as shown in CAMP-ISIJ, 12 (1999), 365. Has been published.

[0003] However, only small samples can be obtained by these methods, and it has not been possible to produce industrially usable thick steel plates. In Japanese Patent Application Laid-Open No. 2000-309850, although a 11.8 mm square bar having a high shelf energy in a Charpy test is obtained by warm working in a ferrite region or the like, it has not yet been manufactured from a plate material. Also,
CAMP-ISIJ, 13 (2000), 1136. succeeded in creating a 5mm-thick ultrafine-grained ferritic steel, but did not solve the problem that separation occurred in the Charpy test and the shelf energy decreased. Materials that cause separation and reduce shelf energy lack reliability as structural steel and cannot be used with confidence.

[0004] It is known that rolling must not be performed or rolling must be terminated in the austenite region in order not to lower the Charpy shelf energy. However, conventionally, if rolling is not performed or rolling is stopped in the austenite region, the ferrite grain size will decrease.
An ultrafine ferritic steel with a thickness of 0.5 μm to 3 μm was not obtained.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned problems in the prior art, the present invention has sufficient strength and toughness as structural steel, but does not cause separation in a Charpy test.
An object of the present invention is to provide a method for producing an ultrafine ferritic structure steel in which shelf energy is not reduced.

[0006]

Means for Solving the Problems The inventors of the present invention have been keenly engaged in the development of a material in which the above-mentioned ferrite grains are made ultra-fine, in which no separation occurs in the Charpy test and the shelf energy does not decrease. Then, the present inventors have conceived a method for ultrafine-refining ferrite grains finally obtained by ultrafine-refining austenite grains before transformation.

That is, by heating to a temperature of _three or more points of Ac where the phase becomes completely austenite single phase and cooling to a temperature of _one point or less of Ar at which austenite completely transforms, one grain of ferrite grains is obtained by the heating. Austenite is refined by nucleating a plurality of austenites from the boundaries. Conversely, in the cooling, a plurality of ferrites are nucleated from a grain boundary of one austenite grain to be refined. It has been found that ultrafine austenite grains can be finally obtained.

However, this alone cannot provide a sufficient ultrafine ferrite grain structure. Subsequent skillful combination of rolling and cooling results in an ultrafine ferrite grain structure. That is, the ultrafine austenite grains are crushed by rolling in the austenite non-recrystallized region, the austenite grain boundary density is increased, and the finally obtained ferrite grains are formed into ultrafine ferrite grains of 0.5 to 3 μm. In this case, since the strain energy introduced by rolling promotes the grain growth of ferrite, it is cooled immediately after rolling,
It is preferable to stop cooling in a temperature range where bainite and martensite are not generated.

[0009] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) After heating a steel material containing a precipitate-forming element to _three or more points of Ac, and rolling or not rolling at a rolling reduction of 40% or less,
Cool to less than Ac ₃ points, then heat to more than Ac ₃ points and Ar ₁
After repeating the cooling to below the point one or more times,
and heated above c ₃ points was rolled by Ar ₃ point or more, then the production method of ultra-fine ferrite structure steel, characterized by cooling.

[0010] (2) After heating a steel material containing the precipitate-forming elements in _three or more points Ac, without rolling or after rolling at a reduction ratio of 40% or less, cooled to Ac less than ₃ points, then Ac ₃ point Heating to above and cooling to Ar ₁ point or less are repeated at least once each, then finally heating to Ac ₃ points or more and rolling at Ar ₃ points or more, then immediately quenching at 3 ° C / s or more. Quenching at 500 ℃
A method for producing an ultrafine ferritic steel characterized by stopping as described above.

(3) The temperature at which the steel material is first heated is a temperature at which a precipitate formed by a precipitate-forming element in the steel material can be dissolved. The method for producing an ultrafine ferritic steel according to (1). (4) The steel material is expressed as mass%, C: 0.01 to 0.40%, S
i: 0.05 to 0.80%, Mn: 0.2 to 2.5%, Al: 0.01 to 0.08
%, Nb: 0.005 to 0.10%, and Ti: 0.005%
(1) to (3), wherein one or two of V: 0.005 to 0.10% are contained.
The method for producing an ultrafine ferritic steel according to any one of the above.

(5) The steel material is further added by mass%
Cu: 0.1 to 2.0%, Ni: 0.1 to 2.0%, Cr: 0.1 to 1.0
%, Mo: 0.05-0.5%, B: 0.0005-0.0025%
(4) The method for producing an ultrafine ferritic steel according to (4), wherein the steel contains one or more kinds.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for using a steel material (slab, bloom, billet, etc.) containing a precipitate-forming element in the present invention is that pinning of austenite grains or ferrite grains by precipitates during repeated transformation treatment. This is for suppressing grain growth. In the case of a steel material containing no precipitate-forming element, ultra-fine austenite grains cannot be obtained even after repeated transformation treatment. Examples of the precipitate forming element include Ti, Nb, and V.

Here, the repetitive transformation process means a process in which heating to _three or more points of Ac and cooling to _one or less points of Ar are alternately performed at least once. The first stage of the repetitive transformation process is heating to the point of Ac ₃ or more, and the final stage is cooling to the point of Ar ₁ or less. In the first heating (initial heating) of the steel material, the attained temperature is set to _three points or more for the austenitization. Among them, the precipitates already existing in the steel material before the initial heating (the steel material) Is preferably set at a temperature at which at least the precipitate formed by the precipitate-forming element contained in the compound can be dissolved. This is because once the precipitate is dissolved, a fine precipitate having a high pinning effect is precipitated during the repeated transformation treatment, so that the austenite grains can be ultrafine uniformly.

The temperature at which the precipitate can be dissolved: T
(K) is log [% Ti] [% N] =-8000 / T + 0.322 log [% Nb] [% N] =-10230 / T + 0.04 log [% V] [% N] =-8330 / T + 3.06 log [% Ti] [% C] =-7000 / T + 2.75 log [% Nb] [% C] = − 7510 / T + 2,96 log [% V] [% C] = -10800 / T + 7.18, can be determined experimentally, or can be determined by using thermodynamic calculation software.

Rolling (primary rolling) after the initial heating may or may not be performed. In the case where the primary rolling is not performed, cooling is performed, for example, until the next transformation process starts. Whether or not the primary rolling is performed is appropriately determined according to the situation. For example, when it is desired to obtain a thick steel plate, the primary rolling can be omitted. In addition, when primary rolling is performed, austenite grains are recrystallized and refined, so that the number of repetitions of the repeated transformation treatment can be reduced as compared with a case where the same austenite grain size is obtained without primary rolling. .

However, in the case of performing primary rolling, if the rolling reduction exceeds 40%, sufficient rolling reduction cannot be achieved in rolling (secondary rolling) after repeated transformation treatment, and crystal grain refinement is insufficient. Therefore, it is necessary to make it 40% or less. The material that has been subjected to primary rolling (or cooling, etc.) after the initial heating is once cooled to less than Ac _three points in order to be subjected to repeated transformation processing (the first stage of the processing is heating to _three or more Ac points). . This cooling may be left to cool.

The repetitive transformation treatment is performed to make the austenite grain structure ultra-fine. First, when heated to _three or more Ac points, the structure becomes an austenitic single phase structure. The heating temperature at this time becomes a part of the austenite principal tissue ferrite remained If it is Ac less than ₃ points, when immediately cooled from the Ac less than ₃ points, the remaining ferrite grows preferentially, new from austenite grain boundaries Nucleation of ferrite becomes insufficient. Therefore, the finally obtained ferrite structure is not refined.

For the same reason, when the cooling is stopped at more than _one point of Ar, the structure becomes a ferrite-based structure in which austenite remains. Immediately after heating in this state, the remaining austenite grows preferentially and grows from the ferrite grain boundary. Nucleation of new austenite is suppressed, and the micronization of the structure becomes insufficient. Heating to above ₃ points of Ac and Ar ₁
The more the repetition of cooling to below the point and the transformation are repeated, the more the structure becomes finer, but the effect is saturated even if it is repeated more than 6 times, and it takes time and cost, so the number of repetitions is 2 times. Six times is appropriate. The definition of the number of repetitions here is defined as one repetition of one temperature increase to _three points or more of Ac or one temperature decrease to _one point or less of Ar (in the case of one temperature increase and one temperature decrease, (The number of repetitions = 2).

After the repeated transformation, rolling (secondary rolling) is performed. In the secondary rolling, in order to perform rolling in the austenite region, it is necessary to set the rolling start temperature to at least _three points of Ac and the rolling end temperature to be at least _three points of Ar. For this purpose, the use of an element (such as Nb) that expands the austenite unrecrystallized region is particularly effective. As a result, the ultrafine austenite grains are further crushed, so that not only the austenite grain boundaries as ferrite nucleation sites increase but also the number of ferrite nucleation sites such as deformation bands can be expected. In order to obtain these effects sufficiently, the rolling temperature of the secondary rolling is (Ar ₃ points + 10
0 ° C.) to Ar ₃ points, and the rolling reduction is preferably 80% or more.

[0021] Incidentally, Ac ₃ point, Ar ₁ point, Ar ₃ point can be measured by a thermal expansion test. Note that the Ac ₃ point and the Ar ₃ point depend on the chemical composition, and the dependence can be accurately described by a primary bond formula of the component content as shown in the following formula (unit: transformation point is ° C., component content) Is mass%). Ac ₃ points = 961.6−311.9C + 49.5Si−36.4Mn−51Cu−29Ni
−8.7Cr + 13.5Mo−140V + 318.9Ti + 308.1Nb + 12.7Al
+ 611.2B Ar ₃ points = 910−273C−74Mn−5Cu−56Ni−16Cr−9Mo However, there is a concern that grain energy of ferrite is promoted by the strain energy introduced by rolling. The solution to this is accelerated cooling following secondary rolling. That is, 2
Rapid cooling from the processed austenite structure state immediately after the next rolling,
It is important to suppress the ferrite nucleation and growth within a certain temperature range, and then to perform ferrite nucleation all at once in the lowest temperature range where ferrite nucleation is possible. For this purpose, it is preferable to perform quenching immediately after the secondary rolling, and to stop the quenching at 500 ° C. or higher, and it is preferable that the quenching cooling rate be 3 ° C./s or higher. The cooling rate is more preferably 5 ° C./s or more. After the rapid cooling is stopped, cooling may be performed as usual.

Next, the preferred chemical composition of the steel material used in the present invention will be described. In addition,% concerning the concentration (content) of each chemical component means mass%. C: 0.01 to 0.40% C is 0.01% from the viewpoint of securing strength and precipitation of carbide.
More preferably, the content is 0.40% from the viewpoint of preventing toughness deterioration.
The following is preferred.

Si: 0.05 to 0.80% Si is preferably contained in an amount of 0.05% or more in steel making, but if it exceeds 0.80%, the toughness of the base material is deteriorated. Mn: 0.2 to 2.5% Mn is preferably 0.5% or more in order to secure the strength of the base material, but if it exceeds 2.0%, the toughness of the welded portion is significantly deteriorated.

Al: 0.01 to 0.08% Al is preferably 0.01% or more in terms of deoxidation of steel, but if it exceeds 0.08%, the toughness of the base material is reduced, and at the same time, the weld metal is diluted to deteriorate the toughness of the same. Let it. Nb: 0.005 to 0.10% Nb precipitates as Nb (C, N), has the effect of suppressing the growth of austenite grains or ferrite grains during the repeated transformation treatment, and expands the austenite unrecrystallized region. The content is preferably 0.005% or more, but if it exceeds 0.10%, the toughness of the heat affected zone deteriorates. It is more preferably 0.050% or less.

Ti: 0.005 to 0.025% Ti precipitates as Ti (C, N) and has the effect of suppressing the growth of austenite grains or ferrite grains during the repeated transformation treatment. To obtain this effect, 0.005% The above is preferable, but if it exceeds 0.025%, the expected effect cannot be obtained due to the coarsening of the Ti (C, N) particles.

V: 0.005-0.10% V precipitates as V (C, N) and has the effect of suppressing the growth of austenite grains or ferrite grains during the repeated transformation treatment. To obtain this effect, 0.005% The above is preferable, but if it exceeds 0.10%, toughness is deteriorated. Cu: 0.1 to 2.0% Cu increases the strength due to solid solution strengthening and precipitation strengthening at 0.1% or more, but deteriorates toughness at more than 2.0%.

Ni: 0.1 to 2.0% Ni increases the strength while maintaining the high toughness of the base material at 0.1% or more, but is expensive and hinders the recyclability, so the upper limit is preferably 2.0%. . Cr: 0.1 to 1.0%, Mo: 0.05 to 0.5% Cr and Mo are effective in increasing the strength of the base material when the content is 0.1% or more and 0.05% or more, respectively. It is better to set them to 1.0% and 0.5%, respectively.

B: 0.0005% to 0.0025% B is 0.0005% or more, which segregates at the austenite grain boundaries during the initial heating and suppresses ferrite transformation, thereby contributing to high strength. However, if it exceeds 0.0025%, hardening is remarkable. There is a possibility that the toughness may be deteriorated.

[0029]

EXAMPLE A steel plate having a thickness of 20 to 30 mm was manufactured using steel materials having the chemical composition shown in Table 1 and under the heating, rolling and cooling conditions shown in Table 2. With respect to these steel sheets, the strength and toughness of the base metal were investigated using JIS No. 14A tensile and JIS No. 4 Charpy test pieces collected from the center of the sheet thickness. Table 2 shows the results.

[0030]

[Table 1]

[0031]

[Table 2]

As can be seen from Table 2, in the invention example (Example of the present invention), higher strength and higher toughness were obtained as compared with the comparative example, and the shelf energy in the Charpy test was at the conventional level ( 100J or less).
Although the comparative example shows the same level of shelf energy as the inventive example, the fracture surface transition temperature is much higher (ie, worse).

[0033]

According to the present invention, there is obtained an excellent effect that an ultrafine ferritic steel having sufficient strength and toughness as a structural steel, not causing separation in a Charpy test, and not reducing the shelf energy can be obtained. To play.

Continued on the front page F term (reference) 4K032 AA01 AA02 AA04 AA05 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA24 AA31 AA35 AA36 BA01 CA01 CA02 CA03 CB02 CC02 CC03 CC04 CD02 CD03 CF02 CF03

Claims (5)

[Claims] A steel material containing a precipitate-forming element is made of Ac ₃
After heating above the point, rolling to a rolling reduction of 40% or less, or without rolling, cooling to less than Ac ₃ points, then heating to Ac ₃ points or more and cooling to Ar ₁ point or less is repeated at least once each. And finally rolling to at least _three points of Ac and rolling at _three or more points of Ar, followed by cooling. 2. The steel material containing a precipitate-forming element is made of Ac ₃
After heating above the point, rolling to a rolling reduction of 40% or less, or without rolling, cooling to less than Ac ₃ points, then heating to Ac ₃ points or more and cooling to Ar ₁ point or less is repeated at least once each. After that, finally, the material is heated to _three or more points of Ac and rolled at _three or more points of Ar, and then immediately cooled at 3 ° C / s or more, and the rapid cooling is stopped at 500 ° C or more. Manufacturing method of structured steel. 3. The temperature at which the steel material is first heated is a temperature at which at least a part of the precipitates formed by the precipitate-forming elements in the steel material can be dissolved. Or the method for producing an ultrafine ferritic steel according to item 2. 4. The steel material is expressed in mass%, C: 0.01 to 0.2.
40%, Si: 0.05 to 0.80%, Mn: 0.2 to 2.5%, Al: 0.01
0.08%, Nb: 0.005 to 0.10%, and further, Ti:
2. The method according to claim 1, wherein one or two of 0.005 to 0.025% and V: 0.005 to 0.10% are contained.
The method for producing an ultrafine ferritic steel according to any one of claims 1 to 3. 5. The steel material according to claim 1, further comprising:
0.1-2.0%, Ni: 0.1-2.0%, Cr: 0.1-1.0%,
5. The method for producing an ultrafine ferritic steel according to claim 4, wherein one or more of Mo: 0.05 to 0.5% and B: 0.0005 to 0.0025% are contained.