JP3153980B2 - Low yield ratio steel plate with good toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate having good toughness and a low yield ratio, which is used as a structural member.

[0002]

2. Description of the Related Art It has recently been found that it is effective to use a steel material having a low yield ratio as a structural member in order to enhance the earthquake resistance of a building when an earthquake is encountered. Therefore, aiming at a low yield ratio, for example,
No. 126882, Japanese Unexamined Patent Publication No. Sho 59-212528, and the like have been developed. Each of these methods is a production method in which the thickness of the steel plate is cooled to a predetermined temperature after completion of hot rolling and then quenched, and the yield ratio is reduced by forming a mixed structure of ferrite and bainite / martensite. It lowers. However, in such a production method, ferrite is generated during air cooling, so that the grain size becomes coarse and the toughness is poor. Another manufacturing method is disclosed in
As disclosed in Japanese Patent Publication No. 517, there is a method of performing a heat treatment in a two-phase temperature range of _one or more Ac after completion of rolling. Although this method is also effective for lowering the yield ratio, there is a problem that the original metal structure becomes coarse due to recrystallization and transformation by high-temperature heat treatment, and the toughness also deteriorates.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a low yield ratio steel sheet having good toughness.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention utilizes a work hardening characteristic of retained austenite by defining the state of a metal structure including retained austenite, and provides a low yield ratio steel plate having good toughness. The main points are as follows.

(1) C: 0.02 to 0.35% by weight Si: 0.02 to 2.5% Mn: 0.30 to 3.5% Al: 0.002 to 0.10% Remainder Is steel consisting of Fe and unavoidable impurities, consisting of lath-like ferrite and cementite, and having a volume fraction of 1
A low yield ratio steel sheet with good toughness containing retained austenite of at least 30%.

(2) C: 0.02 to 0.35% by weight Si: 0.02 to 2.5% Mn: 0.30 to 3.5% Al: 0.002 to 0.10% Remainder Is a steel comprising Fe and unavoidable impurities, and is composed of lath-like ferrite and cementite having an area ratio of 1% or more and 40% or less in the form of a layer between the ferrite laths. Low yield ratio steel plate with good toughness including austenite.

(3) The above item (1) or (2), wherein one or two types of Nb: 0.002 to 0.10% Ti: 0.002 to 0.10% by weight are contained. ) Low yield strength steel plate with good toughness as described.

(4) Cu: 0.05 to 3.0%, Ni: 0.05 to 10.0% Cr: 0.05 to 10.0%, Mo: 0.05 to 3.5% by weight % Co: 0.05 to 10.0%, W: 0.05 to 2.0%, one or more of the preceding items (1) to (3). A low-yield specific thickness steel sheet having good toughness as described in 1).

(5) V: 0.002-0.1% by weight
The low yield ratio steel sheet having good toughness according to any one of the above items (1) to (4), which contains 0%.

(6) B: 0.0003-0.
(1) to (1) characterized by containing 0025%.
(5) The low yield ratio specific thickness steel sheet according to any one of (5). (7) Rem: 0.002-0.10%, Ca: 0.0003% by weight
The steel sheet having good toughness and a low yield ratio according to any one of the above items (1) to (6), which contains one or two kinds of steel sheets of from 0.001 to 0.0030%.

Here, the lath-like ferrite is a lath-like ferrite structure observed when a quenched structure is observed with a transmission electron microscope, and since it has been subjected to a heat treatment such as tempering, solute C precipitates as carbide. Cementite and ferrite are separated. Cementite existing in layers between ferrite laths means cementite precipitated along the ferrite lath as shown in FIG. In each of the structural fractions, for the structural fractions of ferrite and cementite, an area replica was measured based on a photograph taken with a transmission electron microscope of an extracted replica created from a sample taken from a thick steel plate. The amount of residual γ was quantified by X-ray (wide-angle goniometer).

[0012]

The present invention will be described below in detail. The technical idea underlying the present invention is as follows. Tensile strength 6
Thick steel sheets of 0 kg / mm ² or more are usually manufactured by quenching and tempering or direct quenching and tempering after rolling, and the metal structure is often martensite, bainite, or a mixed structure thereof. Usually, the yield ratio after tempering is as high as about 90%, which is disadvantageous for earthquake resistance. Therefore, a method has been adopted in which a certain amount of ferrite is formed before quenching to reduce the yield ratio as a mixed structure of soft ferrite and hard bainite or martensite. However, in this method, the ferrite grain size that appears during air cooling is coarse, and the toughness is deteriorated. Therefore, after quenching or tempering, a heat treatment is performed in a temperature range of ₁ point or more of Ac to recrystallize the lath-like structure in the tempered bainite or martensite to lower the yield point, thereby lowering the yield ratio. Have been. However, in order to maintain good toughness in such a lath-like structure, the state of the lath must be preserved. If the lath is recrystallized and its shape is broken, the toughness is also deteriorated.

The tensile strength is 50 kg / mm ² or more and 60 k or more.
Steels having a g / mm ^{2 or} less are usually produced as they are rolled or by normalizing or accelerated cooling after rolling.
The metal structure is often a mixed structure of ferrite-pearlite or ferrite-bainite. The yield ratio of such a steel is generally about 70%, which is relatively low. However, in order to further decrease the yield ratio, coarse ferrite is generated at the time of transformation as described above, or heat treatment is performed in a temperature range of ₁ point or more of Ac. Therefore, a method of coarsening ferrite is adopted, which causes deterioration of toughness of steel. The present inventors have conducted various studies and found that steel having a certain metal structure has a remarkably low yield ratio. The metallographic structure of this steel is basically a steel composed of lath-like ferrite and cementite and containing a predetermined amount of retained austenite. The effect is more remarkable when the cementite is dispersed inside the lath-like ferrite and inside or on the lath boundary. Normally, in the metal structure after quenching and tempering, residual γ is decomposed and hardly recognized, but, for example, the above lath shape is obtained by heating just above the Ac ₁ point and then cooling at a predetermined cooling rate. If the structure composed of ferrite and cementite contains 1% or more by volume of residual γ, the yield ratio is significantly reduced. This is thought to be because the γ phase, which originally has a strong tendency to work hardening, lowers the yield point and increases the tensile strength.

The inventors of the present invention have conducted detailed investigations on the chemical composition of steel and the metal structure of steel based on the above-mentioned new findings regarding the reduction of the yield ratio of thick steel plates. Low yield ratio steel plate was found. Hereinafter, the reasons for limiting the chemical components and the metal structure will be described in detail. First, the reasons for limiting the components of the steel of the present invention will be described.

C is an element effective for strengthening steel,
If it is less than 0.02%, sufficient strength cannot be obtained. On the other hand, when the content exceeds 0.35%, the weldability deteriorates. Although Si is effective as a deoxidizing element and as a strengthening element for steel, it is not effective at a content of less than 0.02%. On the other hand, if it exceeds 2.5%, the surface properties of the steel are impaired.

Mn is an element effective for strengthening steel.
If it is less than 30%, a sufficient effect cannot be obtained. On the other hand, if the 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%, the effect is not obtained, and if it exceeds 0.10%, the surface properties of steel are impaired. Both Ti and Nb function effectively in terms of crystal grain refinement and precipitation hardening when added in trace amounts. However, if the added amount is small, the effect cannot be obtained. Nb, Ti
In both cases, the amount of addition is limited to the range of 0.002 to 0.10%.

Cu, Ni, Cr, Mo, Co, and W are all elements that improve the hardenability of steel. In the case of the present invention, the addition of the steel can increase the strength of the steel.However, if the addition is small, the effect of improving the hardenability is not obtained, and the addition of an excessive amount impairs the weldability of the steel. With Cu: 0.05-3.0%, Ni: 0.05-1
0.0%, Cr: 0.05 to 10.0%, Mo: 0.0
5 to 3.5%, Co: 0.05 to 10.0%, W: 0.
The range is limited to the range of 0.05 to 2.0%.

V is effective in increasing the strength of the steel by precipitation hardening, but the effect is not obtained if the addition amount is small, and the addition of an excessive amount impairs the toughness of the steel. Is limited to the range of 0.002 to 0.10%. 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 if the addition amount is small, the hardenability does not improve, and excessive addition increases the precipitates of B and impairs the toughness of the steel. The content is in the range of 0.0003 to 0.0025%.

Rem and Ca are effective in detoxifying S. However, if the added amount is small, S remains harmful, and excessive addition impairs toughness.
0%, Ca: 0.0003 to 0.0030%. Next, conditions for limiting the metallographic structure in the present invention will be described. As described above, the present invention basically comprises a mixed structure of lath-like ferrite and cementite, and contains therein 1 to 30% by volume of residual γ. A structure in which carbides such as cementite are not precipitated like martensite as quenched has excessive hardness and poor toughness. The effect of ferrite is remarkable when the form of ferrite is lath-like with a width of about 0.5 μm or less, as is particularly observed when martensite is tempered. Also, the shape of cementite is arranged in a layered manner between ferrite laths, and the yield ratio is remarkably reduced when the area ratio is 1% or more. However, if it is present in an amount of 40% or more, the toughness may deteriorate. As described above, the effect when the cementite is arranged in a layered manner between the ferrite laths is also recognized in the case of the pearlite structure. The reason that the yield ratio decreases in this way is that the residual point γ having a high work hardening index lowers the yield point and increases the tensile strength. The yield ratio decreases when the residual γ and cementite are contained in a certain amount, but significantly decreases when they are present at each boundary of the fine lath. If the residual γ content is less than 1% by volume, the effect is small, and if it exceeds 30%, the variation in strength becomes large.

[0020]

Next, the present invention will be described in detail with reference to examples. First, with respect to the steels having the components shown in Table 1, when a mechanical test is performed on steels having the metal structures of the present invention steels and comparative steels shown in Tables 2 and 3, tensile properties and impact properties as shown in Tables 2 and 3 are obtained. Becomes However, both the tensile test and the impact test are JIS
No. 4 test piece was used. Further, among the metal structures, the structure fraction of ferrite and cementite was determined by preparing an extraction replica from a sample taken from a thick steel plate and measuring the area ratio based on a photograph taken with a transmission electron microscope. The amount of residual γ was quantified by X-ray (wide-angle goniometer). Obviously, the yield ratio is reduced without deteriorating the toughness according to the present invention, and the present invention is effective. FIG. 2 shows the relationship between the tensile strength and the yield ratio of the example. In general, the yield ratio depends on the structure of the steel,
In many cases, the ferrite-based structure exhibits a value of about 70%, and the tempered martensite-based structure exhibits a value of more than 90%.
That is, the higher the tensile strength, the higher the yield ratio tends to be. Therefore, when discussing the level of the yield ratio of a certain thick steel plate, it is necessary to make a comparison in the case where the same steel exhibits substantially the same tensile strength, and the absolute value cannot be uniformly discussed. At the same tensile strength level, the yield ratio of the steel according to the invention shows clearly lower values.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

As described in detail above, by defining the metal structure according to the present invention, it is possible to obtain a steel having the same level of toughness and a reduced yield ratio when compared at the same tensile strength level. The steel of the present invention has excellent performance as a structural member, and is extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a transmission electron micrograph of a quenched structure.

FIG. 2 is a diagram showing the relationship between the tensile strength and the yield ratio of the steel shown in the examples. The steel of the present invention and the comparative steel are clearly separated,
The straight line in the figure indicates the boundary. The steel of the present invention has a significantly lower yield ratio at the same tensile strength as compared with the comparative steel. The symbol ▲ in the figure is a comparative steel whose toughness is significantly deteriorated even at a low yield ratio.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Martensite lath 2 Lath layered cementite 3 Retained austenite 4 Lath cementite (intragranular cementite)

Claims (7)

(57) [Claims] C. 0.02 to 0.35% Si: 0.02 to 2.5% Mn: 0.30 to 3.5% Al: 0.002 to 0.10% by weight% A steel consisting of Fe and unavoidable impurities, consisting of lath-like ferrite and cementite.
A low yield ratio steel sheet with good toughness containing retained austenite of at least 30%. 2. C: 0.02 to 0.35% Si: 0.02 to 2.5% Mn: 0.30 to 3.5% Al: 0.002 to 0.10% by weight% Steel consisting of Fe and unavoidable impurities, consisting of lath-like ferrite and cementite having an area ratio of 1% or more and 40% or less between the ferrite laths, and a residual volume of 1% or more and 30% or less. Low yield ratio steel plate with good toughness including austenite. 3. The composition according to claim 1, further comprising one or two of Nb: 0.002 to 0.10% Ti: 0.002 to 0.10% by weight.
Or a low yield strength steel plate having good toughness according to 2. 4. Cu: 0.05 to 3.0%, Ni: 0.05 to 10.0%, Cr: 0.05 to 10.0%, Mo: 0.05 to 3.5% by weight%. The toughness according to any one of claims 1 to 3, wherein one or more of Co: 0.05 to 10.0% and W: 0.05 to 2.0% are contained. Good low yield ratio steel plate. 5. V: 0.002 to 0.10% by weight%
5. The method according to claim 1, wherein
Low yield ratio specific thickness steel sheet with good toughness as described in Item. 6. B: 0.0003 to 0.00% by weight
The low yield ratio steel sheet having good toughness according to any one of claims 1 to 5, which contains 25%. 7. Rem: 0.002 to 0.10%, Ca: 0.0003% by weight
2. The composition according to claim 1, wherein the content of one or two kinds is 0.001 to 0.0030%.
7. The low-yield specific thickness steel sheet having good toughness according to any one of Nos.