JPH08283838A - Production of low yield ratio, high ductility steel excellent in strength, toughness and ductility - Google Patents

Abstract

PURPOSE: To produce a steel having a low yield ratio and excellent in ductility and toughness. CONSTITUTION: A slab contg., by weight, 0.01 to 0.4% C, 0.02 to 3.0% Mn, 0.001 to 0.75% Si and 0.001 to 0.1% Al, and the balance Fe with inevitable impurities is cast. After that, hot rolling is started without executing cooling, or after once executing cooling, it is heated again to the Ac3 point or above, subsequently, hot rolling is started and is finished at the Ar3 point or above, and after that, air cooling is executed. As for the subsequent cooling stage, it is executed from a temp. of the Ar3 -10 deg.C or below to >=700 deg.C at a cooling rate of 20 to 50 deg.C/sec to 0 to 350 deg.C, after that, its temp. is moreover raised to 750 to 850 deg.C, and it is held for 0 to 60min and is air-cooled or is cooled to 0 to 350 deg.C at a cooling rate of 0.1 to 50 deg.C/sec.

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 a low yield ratio and high ductility steel having excellent strength, toughness and ductility, which is used for building structures and the like.

[0002]

2. Description of the Related Art In recent years, steel sheets used for building structures,
For steel materials such as steel pipes and shaped steel, high-strength steel with a low yield ratio is required from the viewpoint of earthquake resistance.

In response to such a demand, for example, Japanese Patent Laid-Open No.
There are methods such as those described in JP-A-3-238817 and JP-A-1-75817, but none of them can reduce the yield ratio (hereinafter abbreviated as YR) to about 50%. Further, in order to significantly reduce YR, it is necessary to disperse a hard phase such as martensite in a soft phase such as ferrite, and in such a case, toughness and ductility are significantly reduced. There is also. As a means for improving toughness and ductility, there is a method described in Japanese Patent Application No. 6-66202. This method is to obtain low YR steel by treatment at α + γ two-phase region temperature, and is a mixture of bainite, martensite or high carbon martensite and retained austenite in fine ferrite structure (hereinafter abbreviated as M *). Is dispersed extremely finely, and YR is significantly reduced while maintaining toughness and ductility. Although it is possible to improve the toughness and ductility to some extent by such means, the low YR steel is often used as a building steel material, and from a structural mechanical viewpoint, it is an index showing ductility. It is required to further improve the uniform elongation.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the problems of the conventional method, and has high toughness and excellent ductility (uniform elongation, total elongation) while reducing YR to about 50%.
It is an object of the present invention to provide a method for producing a low-YR high-strength steel having a tensile strength of about 60 kgf / mm ² .

[0005]

The method of manufacturing the low YR high ductility steel of the present invention is as follows.

% By weight, C: 0.01 to 0.4
%, Mn: 0.02-3.0%, Si: 0.001-
0.75%, Al: 0.001-0.1% is contained, and the rest is made of Fe and unavoidable impurities. A steel slab that is composed of Fe and unavoidable impurities is cast and then hot rolling is started without cooling, or once cooling is performed again. After hot reheating to a temperature of Ac ₃ points or more, hot rolling is started, and after hot rolling is finished at a temperature of Ar ₃ points or more, the material is allowed to cool, and the subsequent cooling process is performed at a temperature of Ar ₃ -10 ° C or lower and 700 ° C or higher. To 750 ° C. at a cooling rate of 20 ° C./sec or more and 50 ° C./sec or less to a temperature of 0 to 350 ° C. or less.
It is characterized in that the temperature is raised to a temperature of ℃ to 850 ° C and held for 0 to 60 minutes, and is left to cool or is cooled to 0 to 350 ° C at a cooling rate of 0.1 ° C / sec to 50 ° C / sec. A method for producing a low YR high ductility steel excellent in strength, toughness and ductility.

Further, in% by weight, Nb: 0.001
~ 0.05%, Ti: 0.001-0.05%, V:
The method for producing a low YR high ductility steel excellent in strength, toughness, and ductility, characterized by containing any one or more of 0.001 to 0.1%.

Further, in weight%, Mo: 0.01-
1.0%, Ni: 0.01 to 2%, Cr: 0.01 to 1
%, Cu: 0.01 to 1%, B: 0.0001 to 0.
A method for producing a low YR high ductility steel excellent in strength, toughness and ductility, characterized by containing any one or more of 003%.

Further, in% by weight, REM: 0.00
2 to 0.10%, Ca: 0.0003 to 0.0030
% Of any one or two, and a method for producing a low YR high ductility steel excellent in strength, toughness and ductility according to any one of the above items.

[0010]

The method according to the present invention is applied to bainite in the ferrite structure,
By finely dispersing martensite and further extremely finely dispersing martensite or M * therein, a low YR steel having high toughness and excellent ductility is manufactured.

The basic idea of the present invention is as follows.

First, as a means for lowering the YR of steel, it is conceivable to disperse a hard phase such as bainite or martensite in a soft phase such as ferrite as a simple method. Such a metallographic state is rolled,
After cooling, the steel is heated to a temperature of Ac ₁ point or higher and a part of the metallographic structure is once made into a high-carbon austenite, which is then cooled to bainite, martensite or M *. As a result, the soft ferrite yields with low stress, while the hard bainite, martensite, and M * are dispersed, so that the tensile strength can be increased and a low-YR, high-strength steel can be obtained. However,
Such a mixed structure of the soft phase and the hard phase is generally inferior in toughness and ductility due to the nonuniformity.

On the other hand, in order to improve the toughness and ductility of ordinary steel, it is known that softening the metal structure and making the crystal grain size fine are effective. According to the method described in the specification 66202, in the low YR steel produced by the method of grain refining as described above, the ferrite grain size is made fine, and the martensite and M * dispersed in the ferrite structure are made fine. The toughness and ductility are improved by adjusting the precipitation rate appropriately.

In order to further improve the ductility of the low YR steel, as a result of investigating and examining the relationship between the metallographic state and the ductility, it was found that the ductility can be improved by directly precipitating the hard phase in the soft phase.
It has been found that it is extremely effective to once deposit a hard phase having a medium hardness (hereinafter abbreviated as a medium phase) in the soft phase, and further precipitate a hard phase therein.

Such a phenomenon is caused by the fact that when the hard phase is directly precipitated in the soft phase, the strain applied to the test piece is concentrated in the soft phase at the boundary between the hard phase and the soft phase, resulting in the formation of voids. It accelerates and leads to fracture due to ductile fracture at an early stage, whereas when there is a mesophase at the boundary between the soft phase and the hard phase, the strain smoothly disperses from the hard phase to the soft phase, and the strain is either boundary. It is considered that this is because the void formation and ductile fracture as described above are alleviated because the voids are not concentrated in the.

Next, a method for obtaining such a metallographic state will be described.

First, in order to obtain a ferrite structure as a soft phase, the austenite structure after rolling is allowed to cool to the Ar ₃ point or lower to cause ferrite transformation to an appropriate volume fraction of about 40 to 90%. At this time, from the viewpoint of improving the toughness, the ferrite grain size is preferably fine. Therefore, it is effective to fine-grain austenite by rolling prior to the precipitation of ferrite and to carry out rolling in the non-recrystallization temperature range of austenite.

Next, in order to disperse the intermediate phase in the ferrite structure which is the soft phase, the remaining austenite is converted to bainite and martensite having high hardness by carrying out forced cooling after the ferrite transformation as described above. Or transform into M *.

The hard phase having higher strength is finely dispersed in the medium-quality phase by heating the steel after forced cooling to a temperature of Ac ₁ point or higher to cause a part of the metal structure to have a high carbon content. After being transformed into austenite, it is allowed to cool further or forcedly cool depending on the composition of the steel. When austenite is formed by heating to the Ac ₁ point or higher, it is formed in a site with cementite or a high carbon metal structure before that,
This is because the formation of high-carbon austenite occurs from the part where the metallographic structure before temperature rise was bainite, martensite, or M * (the intermediate phase after). By appropriately controlling the amount of this generation, high-carbon austenite in bainite, martensite, and M * (medium phase after) before temperature rise is adjusted to an appropriate amount of about 2 to 10% by volume. It can be finely dispersed to 4 μm or less, and by allowing it to cool or forcibly cool it depending on the steel composition, high carbon austenite becomes high carbon martensite or M * (hard phase) with extremely high hardness. Be transformed. At the same time, the bainite, martensite, and M * (the middle phase) that have not been austenitized during the heat treatment at the Ac ₁ point or higher undergo the heat treatment corresponding to the tempering, so the tempered bainite It slightly softens because it changes to tempered martensite.

By the above treatment, tempered bainite and tempered martensite, which have a relatively high strength, are dispersed in the soft phase having the lowest ferrite structure. It is possible to finely disperse the hard phase such as or M *.

Next, a method for controlling the volume fraction of the medium phase and the hard phase in the soft phase will be described.

First, in the present invention, the soft phase is obtained mainly by forming ferrite from the end of rolling to the start of forced cooling. At this time, the volume ratio of the soft phase ferrite can be controlled by controlling the start temperature of the forced cooling.

Further, the remaining austenite portion at the start of forced cooling is transformed into bainite, martensite, and M * by forced cooling, and becomes a medium phase mainly by the subsequent heat treatment at the Ac ₁ point or higher.

The hard phase is finely dispersed in the intermediate phase by the heat treatment, and the amount of the hard phase in the intermediate phase is
It can be controlled by controlling the volume ratio of austenitizing at the time of reheating to Ac ₁ point or more to about 2 to 15%. Therefore, it is necessary to control the temperature and the holding time at this time. In the method of the present invention, 750 ° C or higher 8
It shall be 50 ° C or lower. When the reheating temperature is less than 750 ° C., low YR is obtained, but the M * fraction is too low as less than 2%, and ductility and toughness deteriorate. In the metallographic structure of such a steel, the number of martensites or M * s generated is small, and the generation sites (the sites where austenite is formed during the two-phase region treatment) are unevenly distributed. It is considered that this is because the carbon concentration of M * is too high and the hardness difference from the medium phase becomes too large. Further, at a temperature exceeding 850 ° C., the fraction of martensite or M * is considerably higher than 15%, spreads over a wide range, a fine dispersed state cannot be obtained, and the toughness and ductility also deteriorate. Further, if the holding time at 750 ° C. or higher and 850 ° C. or lower is too long, coarsening of the metal structure and carbides occurs, so it is set to 60 minutes or less. Further, after the end of the holding, it is necessary to perform cooling to transform the austenitized portion into martensite or M *.

FIG. 1 shows an example of the metal structure thus obtained. FIG. 1 shows 0.1% C, 0.25% Si,
Steel containing 1.3% Mn was rolled at 850 ° C., then allowed to cool to 750 ° C., ferrite was precipitated, and then about 3
Cool to room temperature at a cooling rate of 0 ° C / sec, then 750 ° C
After being kept at 30 ° C. for 30 minutes, the steel cooled to room temperature again at a cooling rate of about 30 ° C./sec is observed by repeller etching. According to this, dark gray tempered martensite (medium phase, volume ratio 21%) is distributed in a light gray ferrite structure (soft phase, volume ratio about 73%),
Furthermore, high-carbon martensite (hard phase, volume fraction 6%, grain size 2.5) with extremely high hardness in tempered martensite.
It can be seen that (μm) is extremely finely dispersed. Such a structure improves the ductility and toughness of the low YR steel. Further, FIG. 2 shows the tissue before the treatment of holding at 750 ° C. for 30 minutes in FIG. 1. Although a high-carbon martensite (hard phase) is observed in this case, it exists solely in the ferrite structure, has a large volume ratio of 30%, and has a coarse grain size of 15 μm, so that the desired structure cannot be obtained. Further, FIG. 3 shows the structure of the steel that was slowly cooled at a cooling rate of about 10 ° C./sec without being left to cool after rolling, and then was held and cooled at 750 ° C. for 30 minutes. With the phase of 92% and the hard phase of 8%, the middle phase portion of tempered martensite as shown in FIG. 1 is not formed. The reason for this is that since cooling was not performed after rolling, carbon was not sufficiently concentrated in the remaining austenite due to precipitation of ferrite, and the cooling rate was about 10 ° C./sec, which was slow cooling. This is because the remaining austenite did not become high-carbon martensite although it was made finer. Such a tissue corresponds to the tissue obtained by the method described in Japanese Patent Application No. 6-66202.

The above is the basic idea of the present invention, and is summarized as follows.

First, a low Y produced by austenitizing a part of steel at a temperature of Ac ₁ point or higher and further cooling it to disperse martensite and M * in the steel.
R steel generally deteriorates in toughness and ductility. However, according to the method of the present invention, the ductility can be remarkably improved by not precipitating the hard phase directly in the soft phase but by precipitating the medium phase once in the soft phase and further precipitating the hard phase therein. In order to obtain such a metallographic structure state, the austenite structure after rolling is allowed to cool to the Ar ₃ point or less and ferrite transformation is caused to a volume fraction of about 40 to 90%. Next, the remaining austenite is transformed into bainite, martensite, or M * having a high hardness by performing forced cooling, and a phase that becomes an intermediate phase is generated by the subsequent heat treatment, and then the steel after the forced cooling is performed. Is heated to a temperature of Ac ₁ point or higher to transform a part of the metallographic structure into a high-carbon austenite, and then allowed to cool or forcedly cool depending on the composition of the steel to obtain a higher hardness in the medium-grade phase. Austenite of carbon is finely dispersed in a particle size of 4 μm or less and a volume fraction of 2 to 15%.

The reasons for limiting each component will be described below.

C is an element effective for strengthening steel, and if it is less than 0.01%, sufficient strength cannot be obtained. On the other hand, if it exceeds 0.4%, the weldability is deteriorated.

Mn is an element effective for strengthening steel, and
If it is less than 02%, a sufficient effect cannot be obtained. On the other hand, 3.0
%, The workability of steel deteriorates.

Si is effective as a deoxidizing element and as a strengthening element for steel, but if less than 0.001%, it has no effect. On the other hand, if it exceeds 0.75%, the surface properties of steel are impaired.

Al is added to suppress the growth of austenite during reheating prior to deoxidation or rolling. 0.0
If it is less than 01%, there is no effect, and if it exceeds 0.1%, the surface properties of steel deteriorate.

All of Nb, Ti and V are effective in refining the crystal grains and strengthening the precipitation by adding a very small amount, so they may be used in a range that does not deteriorate the toughness of the welded portion. From this viewpoint, the addition amount of Nb and Ti is 0.001 to 0.
05% and 0.001 to 0.1% for V.

Mo, Ni, Cr, Cu and B are all elements that improve the hardenability of steel, and the strength of steel can be increased by adding them. However, since excessive addition impairs the toughness and weldability of steel, 0.01% is added to Mo.
% To 1.0%, Ni: 0.01% to 2
% Or less, 0.01% or more and 1% or less for Cr, Cu
0.01% or more and 1% or less for B, and 0.
It is limited to 0001% or more and 0.003% or less.

REM and Ca are effective for detoxifying S, but when added in a small amount, they have no effect, and excessive addition impairs toughness. Therefore, REM and 0.002.
10%, 0.0003-0.0030% for Ca
Limited to

In addition, the contents of P and S which are inevitable impurities are preferably 0.02% or less and 0.008% or less, respectively.

Next, the manufacturing conditions of the present invention will be described.

Since the present invention is effective for a steel slab cast under any casting conditions, it is not necessary to specify the casting conditions. Further, the hot rolling is started as it is without cooling the steel slab, or the once cooled steel slab is reheated to the Ac ₃ point or more and then the rolling is started. It should be noted that the rolling conditions are not specified except that the rolling is completed at a temperature of Ar ₃ or higher, because the effectiveness is not lost even if any rolling is performed. However, since the fine crystal grains of ferrite work effectively from the viewpoint of improving the toughness, it is necessary to adjust the rolling conditions so as to make the austenite finer, and in the unrecrystallized region of austenite by controlled rolling or the like. It may be rolled.

Next, the reasons for limiting the cooling conditions will be described.

In the present invention, soft phase ferrite, medium phase tempered bainite, tempered martensite, and hard phase high carbon martensite, M * is formed in an appropriate volume fraction of 2 to 15%. is necessary.

Therefore, first, in order to stably generate an appropriate amount of ferrite, some cooling is performed after the completion of rolling and before the start of forced cooling. The range of this cooling is from the end of rolling to a temperature of (Ar ₃ −10) ° C. or lower and 700 ° C. or higher. This is because the ferrite transformation just started at a temperature exceeding (Ar ₃ −10) ° C., the amount of ferrite produced was not sufficient, and the amount of ferrite produced was less than 700 ° C. Is too much.

Next, it is necessary to perform forced cooling to make bainite, martensite, or M * in order to make the rest of the ferrite formed into an intermediate phase. The starting temperature of this cooling is (Ar ₃ −10) ° C. or lower and 700 ° C. or higher for the above reason. Further, the cooling rate was set to 20 ° C./second or more in order to stably obtain bainite, martensite, and M *.
The cooling rate is set to 50 ° C./second or less because it is not easy to obtain a cooling rate higher than this with an ordinary cooling device. The cooling end temperature was set to 350 ° C or lower because austenite becomes pearlite, Widmanstetten ferrite, or bainite having a relatively low hardness at temperatures exceeding this temperature, and bainite having a high hardness, martensite, or M * is generated. This is because it cannot be stably generated.

Next, heat treatment is performed at an Ac ₁ point or higher. The reason why the treatment temperature is 750 ° C. or higher is that sufficient ductility cannot be obtained at a temperature lower than this. If the treatment temperature is too low, the volume fraction of austenite is less than 2%, which is too small, so that the austenite generation sites are unevenly distributed in the cast segregation portion of the steel, and if the volume fraction is too low, the austenite is generated. The carbon concentration of the converted part is too high,
It is estimated that the ductility deteriorates due to the large difference in height with ferrite. The reason why the temperature is 850 ° C. or lower is that at a temperature above this, the volume fraction of austenite is more than 15%, which is too large, and a fine dispersed state having a particle size of less than 4 μm cannot be obtained. The treatment time is set to 60 minutes or less because if the holding time is longer than this, coarsening of the metal structure and coarsening of carbides due to coalescence growth of ferrite grains and austenite grains occur, which is not preferable from the viewpoint of strength and toughness. Is.

The cooling rate after the end of holding is 0.1 ° C. /
Seconds or more means that at a cooling rate of less than 0.1 ° C / sec,
Martensite and M * are used for the austenitic part.
The reason is that the upper limit of the cooling rate is 50 ° C./sec.
This is because it is difficult to achieve a cooling rate exceeding / sec.

[0045]

EXAMPLES Table 1 shows the components of steel, and Tables 2 and 3 show the manufacturing conditions. In the table, underlined items are comparative examples, and items that do not satisfy the requirements of the present invention are underlined. The strength, YR, ductility, and toughness of the manufactured steel are shown in Tables 4 and 5. Yield strength YS (MPa) and tensile strength TS
(MPa) is shown. For YR, yield strength YS
/ Tensile strength TS, uniform elongation uEl (%) for ductility
And the elongation at break El (%) and toughness are the absorbed energy vE at 0 ° C in the Charpy impact test.
_It was evaluated by ₀ (J). The examples all showed clearly better characteristics than the comparative examples. According to the method of the present invention, it was possible to produce a steel having low YR and excellent ductility and toughness.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

[Table 4]

[0050]

[Table 5]

[0051]

According to the method of the present invention, it is possible to easily produce a steel having a low YR and excellent ductility and toughness.

[Brief description of drawings]

FIG. 1 is a photograph showing the metallographic structure of steel produced by the method of the present invention.

FIG. 2 is a photograph showing the metallurgical structure of steel that has been cooled to 750 ° C. after rolling and then water-cooled to room temperature.

FIG. 3 is a photograph showing the metallurgical structure of steel that was water cooled by holding controlled cooling steel at 750 ° C. for 30 minutes.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C22C 38/58 C22C 38/58 (72) Inventor Hidesato Mabuchi 20-1 Shintomi, Futtsu-shi Shinnihon Steel Engineering Co., Ltd.

Claims (4)

[Claims] 1. C .: 0.01 to 0.4%, Mn: 0.02 to 3.0%, Si: 0.001 to 0.75%, Al: 0.001 to 0. A steel slab containing 1% and the balance of Fe and unavoidable impurities is cast and then hot rolling is started without cooling, or once cooled, Ac is cooled again.
After reheating to a temperature of ₃ points or more, hot rolling is started and
After the hot rolling is finished at a temperature of r ₃ point or more, the material is allowed to cool, and the subsequent cooling process is performed at a cooling rate of 20 ° C./sec or more and 50 ° C./sec or less from a temperature of 700 ° C. or more in Ar ₃ to 0 to 350.
750 ℃ or more and 850 ℃
The temperature is raised to a temperature of 0 ° C or lower and held for 0 to 60 minutes, and then left to cool or 0 to 3 at a cooling rate of 0.1 ° C / sec or more and 50 ° C / sec or less.
A method for producing a low yield ratio and high ductility steel excellent in strength, toughness and ductility, characterized by cooling to 50 ° C. 2. Further, in weight%, any one or two of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, and V: 0.001 to 0.1%. The strength according to claim 1, characterized by containing at least one species,
A method for producing a low yield ratio and high ductility steel excellent in toughness and ductility. 3. Further, by weight%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2%, Cr: 0.01 to 1%, Cu: 0.01 to 1%, B : 0.0001 to 0.003% of any one kind or two kinds or more are contained, The manufacturing method of the low yield ratio and high ductility steel excellent in strength, toughness, and ductility according to claim 1 or 2, . 4. The composition further comprises, by weight, any one or two of REM: 0.002 to 0.10% and Ca: 0.0003 to 0.0030%. A method for producing a low yield ratio high ductility steel excellent in strength, toughness and ductility according to any one of 1 to 3.