JP2020117796A - Ultra-low yield ratio high-strength steel plate and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultra-low yield ratio high tension thick steel sheet which can stably secure an ultra-low yield ratio (yield ratio less than 80%) and has strength and toughness, and a method for manufacturing the same. SOLUTION: In mass%, C: more than 0.08%, 0.20% or less, Si: 0.01 to 0.50%, Mn: 0.5 to 3.0%, P: 0.015%. Hereinafter, S: 0.0050% or less, Al: 0.005 to 0.10%, and N: 0.0015 to 0.0065% are contained, and the balance has a component composition consisting of Fe and unavoidable impurities. Cementite is contained in one or both tissues of baynite and martensite, and the total area fraction of baynite and martensite is 50.0. % Or more and less than 95.0%, the area fraction of island-shaped martensite is 5 to 20%, the average circle equivalent diameter of island-shaped martensite is less than 5.0 μm, and the area fraction of cementite is An ultra-low yield ratio high-tensile thick steel plate having a microstructure of more than 0.5% and 5% or less and having an average equivalent circle diameter of cementite of less than 0.5 μm. [Selection diagram] None

Description

TECHNICAL FIELD The present invention relates to an ultra-low yield ratio high-tensile steel plate, and more particularly to an ultra-low yield ratio high-tensile steel plate having a lower yield ratio than conventional and excellent in deformation performance and suitable for construction. The present invention also relates to a method for manufacturing the ultra-low yield ratio high-strength thick steel plate.

In recent years, as building structures have become taller and spans have increased, thickening and strengthening of the steel materials used have been demanded. From the viewpoint of the safety of steel structures, they have a high allowable stress and yield stress. It is required to reduce the ratio (=the ratio of yield strength to tensile strength).

When the yield ratio is reduced, even if a stress equal to or higher than the yield point is applied, the stress allowed up to the fracture is increased, and the uniform elongation is increased, so that the steel material has excellent plastic deformability. Therefore, if the yield ratio can be reduced as compared with the conventional one, a steel material having more excellent deformability can be obtained.

Conventionally, as a manufacturing process of a steel plate with a low yield ratio and a high tensile strength, a multi-step heat treatment is generally performed in which a ferrite+austenite two-phase region is reheated and quenched and then tempered. However, the microstructure of the thick steel sheet obtained by the multi-stage heat treatment is one in which bainite or martensite as the hard second phase is dispersed in the ferrite phase as the main phase, and therefore 980 MPa depending on the volume fraction of the ferrite phase. It is difficult to stably achieve an ultra-high tensile strength. In addition, the yielding point rises due to the tempering process, and it is difficult to stably obtain a low yield ratio for higher strength steel.

In Patent Document 1, after quenching the steel sheet after hot rolling, the steel is strengthened and the yield ratio (YR) is lowered to 85% or less by quenching by heating again to the two-phase region of ferrite+austenite. Achieving a yield ratio is described.

According to Patent Document 2, the microstructure after quenching is made into a bainite phase or a martensite phase by a direct quenching method of immediately quenching after rolling, and then normalizing is performed by heating again to a two-phase region of ferrite+austenite. It is described that high strength and low yield ratio are thereby achieved.

In Patent Document 3, after rolling for a certain period of time, ferrite is precipitated, and then a direct quenching method of quenching is used to form a two-phase structure of a ferrite phase and a martensite phase to improve strength and reduce a yield ratio. Achievements are described.

Japanese Patent Laid-Open No. 06-248337 JP 05-230530A JP, 07-097626, A

However, in the technique described in Patent Document 1, the hard phase effective for reducing the yield ratio is decomposed by tempering, and it is difficult to stably obtain an ultra-low yield ratio and high tensile strength. The techniques described in Patent Documents 2 and 3 require rapid heating of the steel sheet, which imposes a heavy load on the heat treatment operation, making it particularly difficult to manufacture thick-walled materials. Also, it is difficult to obtain excellent toughness.

In view of such circumstances, the present invention has a low yield ratio and a high tensile strength steel plate having an ultra-low yield ratio (yield ratio of less than 80%) and having both strength and toughness, regardless of whether the material is thin or thick. The purpose is to provide a method.

In order to achieve the above-mentioned subject, the present inventors earnestly studied and acquired the following knowledge.

(1) In the conventional process, after the two-phase region heating and quenching, as a final step, a tempering treatment for improving toughness is performed. As a result, island martensite (MA), which is effective for lowering the yield ratio, is decomposed, the number of mobile dislocations that can suppress an increase in yield strength (YP) is reduced, and an ultra-low yield ratio can be achieved. Can not.

(2) Instead of the conventional quenching and tempering steps, after heating in the two-phase region, quenching is stopped at a temperature of 200° C. or higher and below the bainite transformation start temperature (Bs point), and then air cooling is performed to obtain self-tempered bainite containing MA. A structure having a matrix of self-tempered martensite is obtained.

(3) Furthermore, by performing heating at 900 to 1000° C. after hot rolling and before heating in the two-phase region, it is possible to further improve strength and toughness while maintaining an ultra-low yield ratio. As a result, a thick steel plate having both high strength and an ultra-low yield ratio can be manufactured.

The present invention has been completed by further studies based on the above findings. The gist of the present invention is as follows.

1. In mass %,
C: more than 0.08%, 0.20% or less,
Si: 0.01 to 0.50%,
Mn: 0.5-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.005 to 0.10%, and N: 0.0015 to 0.0065%,
The balance has a composition of Fe and inevitable impurities,
Including bainite containing island martensite, martensite, and cementite,
Cementite is contained in the structure of one or both of bainite and martensite,
The total area fraction of bainite and martensite is 70.0% or more and less than 95.0%,
The average equivalent circle diameter of bainite and martensite is less than 50 μm,
The area fraction of island martensite is 2 to 10%,
The average equivalent circle diameter of the island-shaped martensite is less than 5.0 μm,
An ultra-low yield ratio high-tensile thick steel plate having a microstructure in which the area fraction of cementite is more than 0.5% and 5% or less and the average equivalent circular diameter of cementite is less than 0.5 μm.

2. The component composition is mass%,
Ti: 0.004 to 0.03%,
Cu: 1.0% or less,
Ni: 3.0% or less,
Cr: 2.0% or less,
Mo: 1.0% or less,
B: 0.0050% or less,
Nb: 0.1% or less, and V: 0.2% or less, further containing 1 or 2 or more selected from the group consisting of, ultra-low yield ratio high tensile thick steel plate according to 1 above.

3. The component composition is mass%,
Ca: 0.005% or less,
The ultra-low yield ratio high tensile strength steel plate according to the above 1 or 2, further containing 1 or 2 or more selected from the group consisting of REM: 0.02% or less and Mg: 0.005% or less.

4. The ultra-low yield ratio high-tensile thick steel plate according to any one of 1 to 3 above, which has a tensile strength of more than 980 MPa and a yield ratio of less than 80%.

5. In mass %,
C: more than 0.08%, 0.20% or less,
Si: 0.01 to 0.50%,
Mn: 0.5-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.005 to 0.10%, and N: 0.0015 to 0.0065%,
A hot rolling step of hot rolling a steel material having a composition with the balance being Fe and inevitable impurities into a thick steel plate;
The thick steel sheet after the hot rolling is reheated to a reheating temperature of 900° C. or higher and 1000° C. or lower, held at the reheating temperature for a holding time of 10 minutes or longer, and cooled to a cooling stop temperature of 400° C. or lower. A first reheating step of cooling,
Second reheating step of reheating the thick steel plate after the first reheating step to a reheating temperature of Ac1 point+80° C. or more and less than Ac3 point and holding the reheating temperature for a holding time of 10 minutes or more. When,
The thick steel plate after the second reheating step is accelerated cooled to an accelerated cooling stop temperature of 200° C. or higher and lower than the bainite transformation start temperature at an average cooling rate of 1 to 200° C./s at a plate thickness 1/4 position. And then cooling with air. A method for manufacturing an ultra-low yield ratio high-strength steel plate.

6. The component composition is mass%,
Ti: 0.004 to 0.03%,
Cu: 1.0% or less,
Ni: 3.0% or less,
Cr: 2.0% or less,
Mo: 1.0% or less,
B: 0.0050% or less,
Nb: 0.1% or less, and V: 0.2% or less, 1 or 2 or more selected from the group which consists further, The manufacturing method of the ultra-low yield ratio high tension thick steel plate of said 5 is further contained.

7. The component composition is mass%,
Ca: 0.005% or less,
7. The method for producing an ultra-low yield ratio high strength thick steel plate according to 5 or 6, further containing 1 or 2 or more selected from the group consisting of REM: 0.02% or less and Mg: 0.005% or less. ..

ADVANTAGE OF THE INVENTION According to this invention, the ultra-low yield ratio (yield ratio less than 80%) can be ensured stably, and the ultra-low yield ratio high tension thick steel plate which has both strength and toughness can be obtained. Further, according to the present invention, the ultra-low yield ratio high-tensile thick steel plate can be stably manufactured regardless of the plate thickness. Therefore, the present invention greatly contributes to the upsizing of a steel structure and the improvement of earthquake resistance, and has an industrially remarkable effect.

Hereinafter, embodiments of the present invention will be described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description.

[Ingredient composition]
The steel material used for producing the ultra-low yield ratio high-tensile steel plate and the ultra-low yield ratio high-tensile steel plate of the present invention needs to have the above-described composition. Hereinafter, each component contained in the component composition will be described. In addition, "%" showing the content of each component means "mass %" unless otherwise specified.

C: more than 0.08% and 0.20% or less C is an element having the effect of increasing the strength of steel and ensuring the strength required as a structural steel material. In order to obtain the above effect, the C content is set to more than 0.08%. The C content is preferably 0.10% or more. On the other hand, when the C content exceeds 0.20%, the formation of island-shaped martensite and cementite is promoted, and the toughness of the base material is reduced. Therefore, the C content is 0.20% or less. The C content is preferably 0.15% or less.

Si: 0.01 to 0.50%
Si is an element that functions as a deoxidizer and has the effect of increasing the strength of the base material. In order to obtain the above effect, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 0.50%, the formation of island martensite is promoted, and the deterioration of toughness and weldability becomes apparent. Therefore, the Si content is 0.50% or less. The Si content is preferably 0.35% or less.

Mn: 0.5-3.0%
Mn is an element that has the effect of increasing the strength of steel. In order to secure the tensile strength of the base material, the Mn content needs to be 0.5% or more. The Mn content is preferably 0.8% or more. On the other hand, when the Mn content exceeds 3.0%, island-like martensite is excessively formed, and the toughness of the base material is significantly deteriorated. Therefore, the Mn content is set to 3.0% or less. The Mn content is preferably 2.8% or less.

P: 0.015% or less P is an element that deteriorates the low temperature toughness of the base material, and it is desirable to reduce P as much as possible. Therefore, it is desirable to reduce P in order to improve the toughness of the base material. Therefore, the P content is 0.015% or less.

S: 0.0050% or less S is an element that deteriorates the low temperature toughness of the base material, and it is desirable to reduce S as much as possible. If the S content exceeds 0.0050%, the low temperature toughness is significantly deteriorated, so the S content is set to 0.0050% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of high-strength steel. Further, Al fixes N in the steel as AlN and contributes to the improvement of the toughness of the base material. In order to obtain the above effect, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more. On the other hand, if the Al content exceeds 0.10%, the toughness of the base material decreases. Therefore, the Al content is 0.10% or less. The Al content is preferably 0.07% or less.

N: 0.0015 to 0.0065%
N combines with Al and Ti to precipitate and form carbonitrides, suppress coarsening of austenite grains, and improve the base material toughness. In order to obtain the effect, the N content is set to 0.0015% or more. The N content is preferably 0.0030% or more. On the other hand, when the N content exceeds 0.0065%, the toughness of the base material and the welded portion is significantly reduced due to the increase in the amount of solute N. Therefore, the N content is 0.0065% or less. The N content is preferably 0.0060% or less.

In one embodiment of the present invention, the ultra-low yield ratio, high-strength steel plate can have a composition of the above elements, and the balance Fe and unavoidable impurities.

Further, in another embodiment of the present invention, the component composition further contains one or more selected from the group consisting of Ti, Cu, Ni, Cr, Mo, B, Nb, and V. can do.

Ti: 0.004 to 0.030%
Ti has a strong affinity with N and precipitates as TiN during solidification. By suppressing the coarsening of the austenite crystal grains in the weld heat affected zone by the pinning effect of TiN that is stable even at high temperature, the toughness of the weld heat affected zone can be improved. When Ti is added to obtain the above effect, the Ti content is 0.004% or more. The Ti content is preferably 0.006% or more. On the other hand, when the Ti content exceeds 0.030%, the TiN particles become coarse and the effect of suppressing coarsening of the austenite grains becomes saturated. Therefore, the Ti content is 0.030% or less. The Ti content is preferably 0.025% or less.

Cu: 1.0% or less Cu is an element capable of increasing strength while maintaining high toughness, and can be optionally contained according to desired strength. However, if the Cu content exceeds 1.0%, hot brittleness occurs and the surface properties of the steel sheet deteriorate. Therefore, when Cu is contained, the Cu content is 1.0% or less. The Cu content is preferably 0.7% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but in order to sufficiently obtain the above effect, the Cu content is preferably 0.01% or more, more preferably 0.10% or more, It is more preferably 0.20% or more.

Ni: 3.0% or less Like Cu, Ni is an element capable of increasing strength while maintaining high toughness, and can be optionally contained according to the desired strength. However, when the Ni content exceeds 3.0%, the effect of addition is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Ni is contained, the Ni content is 3.0% or less. The Ni content is preferably 1.7% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but in order to sufficiently obtain the above effect, the Ni content is preferably 0.01% or more, more preferably 0.10% or more, It is more preferably 0.20% or more.

Cr: 2.0% or less Cr is an element that contributes to the strength improvement of steel, and can be optionally contained according to the desired strength. However, if the Cr content exceeds 2.0%, the toughness deteriorates. Therefore, when Cr is contained, the Cr content is 2.0% or less. On the other hand, the lower limit of the Cr content is not particularly limited, but it is preferable that the Cr content be 0.05% or more from the viewpoint of sufficiently obtaining the strength improving effect of Cr.

Mo: 1.0% or less Mo, like Cr, is an element that contributes to the improvement of the strength of steel, and can be optionally contained depending on the desired strength. However, if the Mo content exceeds 1.0%, the toughness deteriorates. Therefore, when Mo is contained, the Mo content is set to 1.0% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but the Mo content is preferably 0.05% or more from the viewpoint of sufficiently obtaining the strength improving effect of Mo.

B: 0.0050% or less B is an element having an action of improving the strength of steel by improving the hardenability. However, if the B content exceeds 0.0050%, the hardenability becomes excessively high, and the toughness and ductility of the base material deteriorate. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0020% or less. On the other hand, the lower limit of the B content is not particularly limited, but it is preferable that the B content is 0.0003% or more from the viewpoint of sufficiently obtaining the effect of adding B.

Nb: 0.1% or less Nb, like Cr and Mo, is an element that contributes to the improvement of the strength of steel, and can be arbitrarily contained depending on the desired strength. However, if the Nb content exceeds 0.1%, the base material toughness deteriorates. Therefore, when Nb is contained, the Nb content is set to 0.1% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but the Nb content is preferably 0.005% or more from the viewpoint of sufficiently obtaining the strength improving effect of Nb.

V: 0.2% or less V, like Cr, Mo, and Nb, is an element that contributes to the improvement of the strength of steel, and can be optionally contained depending on the desired strength. However, if the V content exceeds 0.2%, the toughness deteriorates. Therefore, when V is contained, the V content is set to 0.2% or less. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of sufficiently obtaining the strength improving effect of V, the V content is preferably 0.01% or more.

Further, in another embodiment of the present invention, the component composition may optionally further contain one or more selected from the group consisting of Ca, REM, and Mg.

Ca: 0.005% or less Ca is an element having an effect of improving toughness by refining crystal grains, and can be optionally contained according to desired characteristics. However, if the Ca content exceeds 0.005%, the effect of addition is saturated. Therefore, when Ca is contained, the Ca content is set to 0.005% or less. On the other hand, although the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.001% or more from the viewpoint of sufficiently obtaining the toughness improving effect of Ca.

REM: 0.02% or less REM (rare earth metal) has a toughness-improving effect similarly to Ca, and can be arbitrarily contained according to desired characteristics. However, if the REM content exceeds 0.02%, the effect of addition is saturated. Therefore, when REM is contained, the REM content is 0.02% or less. On the other hand, although the lower limit of the REM content is not particularly limited, the REM content is preferably 0.002% or more from the viewpoint of sufficiently obtaining the toughness improving effect by REM.

Mg: 0.005% or less Mg is an element having the effect of improving the toughness by refining the crystal grains similarly to Ca, and can be optionally contained according to the desired characteristics. However, if the Mg content exceeds 0.005%, the effect of addition is saturated. Therefore, when Mg is contained, the Mg content is set to 0.005% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but it is preferable that the Mg content be 0.001% or more from the viewpoint of sufficiently obtaining the toughness improving effect of Mg.

[Microstructure]
The ultra-low yield ratio high tensile strength steel plate of the present invention has a microstructure that satisfies all of the following conditions (1) to (7).
(1) Bainite containing martensite islands, martensite, and cementite are contained.
(2) Cementite is contained in the structure of one or both of bainite and martensite.
(3) The total area fraction of bainite and martensite is 70.0% or more and less than 95.0%.
(4) The average equivalent circle diameter of bainite and martensite is less than 50 μm.
(5) The island-like martensite has an average equivalent circle diameter of less than 5.0 μm.
(6) The area fraction of island martensite is 2 to 10%.
(7) The average equivalent circle diameter of cementite is less than 0.5 μm.
(8) The area fraction of cementite is more than 0.5% and 5% or less.

The reason why the microstructure is limited to the above range will be described below. In addition, the "area fraction" in the following description refers to the area fraction with respect to the entire microstructure unless otherwise specified. The above-mentioned microstructure refers to the microstructure at the plate thickness 1/4 position of the steel sheet.

B+M total area fraction: 70.0% or more and less than 95.0% If the total area fraction of bainite (B) and martensite (M) is less than 70.0%, sufficient strength may be obtained. Can not. Therefore, from the viewpoint of securing strength, the total area fraction of bainite and martensite is set to 70.0% or more. On the other hand, if the total area fraction is 95.0% or more, the proportion of the soft phase such as ferrite decreases, and the area fraction of island martensite also decreases, making it difficult to achieve an ultra-low yield ratio. Therefore, the total area fraction is less than 95.0%. In the present specification, bainite and martensite, which occupy 50.0% or more of the microstructure, may be collectively referred to as “matrix”.

In the microstructure of the present invention, bainite contains island martensite. However, the total area fraction does not include the area fraction of the island martensite. Similarly, in the present invention, cementite is included in the structure of one or both of bainite and martensite, but the total area fraction does not include the area fraction of cementite. The total area fraction of bainite and martensite can be measured by the method described in the examples.

Average equivalent circle diameter of bainite and martensite: less than 50 μm In order to obtain desired strength and toughness, the average equivalent circle diameter of bainite and martensite in the above microstructure must be less than 50 μm. Therefore, the average equivalent circle diameter of bainite and martensite is less than 50 μm, preferably 30 μm or less. On the other hand, the smaller the average circle equivalent diameter, the better, so the lower limit is not particularly limited. However, if the average equivalent circle diameter is made to be excessively small, the time required for the heat treatment increases and the productivity decreases. Therefore, from the viewpoint of productivity, it is preferable that the average circle equivalent diameter is, for example, 20 μm or more.

The average equivalent circle diameters of bainite and martensite are obtained by performing picric acid corrosion on a steel plate as a sample, then observing with an optical microscope at a magnification of 200, and analyzing the photographed image with an image analyzer. It can be obtained by

(Island martensite)
Area fraction of MA: 2-10%
If the area fraction of island-shaped martensite (MA) is less than 2%, the above-described effects of high strength and ultra-low yield ratio cannot be obtained. Therefore, the area fraction of MA is set to 2% or more. The area fraction of MA is preferably 4% or more. On the other hand, when the area fraction of MA exceeds 10%, the ductility and toughness of the base material deteriorate. Therefore, the area fraction of MA is 10% or less. The area fraction of MA is preferably 8% or less.

Average circle equivalent diameter of MA: less than 5.0 μm If the average circle equivalent diameter of MA is 5.0 μm or more, the toughness of the welded portion deteriorates. Therefore, the average equivalent circle diameter of MA is set to less than 5.0 μm. On the other hand, the lower limit of the average equivalent circle diameter of MA is not particularly limited, but is preferably 0.5 μm or more.

The area fraction and the average equivalent circle diameter of MA were determined by using a scanning electron microscope (SEM) after subjecting a steel plate as a sample to repeller corrosion (Journal of Metals, March, 1980, p.38-39). It can be obtained by observing at a magnification of 1000 and analyzing the photographed image using an image analysis device.

(Cementite)
Area fraction of cementite: more than 0.5% and 5% or less In the present invention, in order to secure toughness, cementite is precipitated in the structure of at least one of bainite and martensite as a matrix by a self-tempering treatment described later. Let When the area fraction of cementite is 0.5% or less, it means that the structure has not undergone self-tempering or is insufficient, and the toughness cannot be secured. Therefore, the area fraction of cementite is set to more than 0.5%. On the other hand, if the area fraction of cementite exceeds 5%, it means that the structure has undergone excessive tempering. In such a case, the desired ultra-low yield ratio cannot be obtained because the MA is decomposed by excessive tempering and the working dislocations are reduced. Therefore, the area fraction of cementite is set to 5% or less. The area fraction of cementite is preferably 3% or less.

Average circle equivalent diameter of cementite: less than 0.5 μm If the average circle equivalent diameter of cementite is 0.5 μm or more, it becomes a starting point of brittle fracture and the toughness of the base material decreases. Therefore, the average equivalent circular diameter of cementite is less than 0.5 μm.

In addition, the area fraction and the average equivalent circle diameter of cementite were obtained by observing a steel plate as a sample with Nital (an ethanol solution of nitric acid) at a magnification of 5000 times and observing the photographed image, It can be determined by analysis using an image analysis device.

If the area fraction of bainite, martensite, MA, and cementite satisfies the above conditions, the microstructure may contain other structure such as ferrite. When ferrite is present, the area fraction of the ferrite is preferably 30% or less.

[Thickness]
The plate thickness of the ultra low yield ratio high tensile strength steel plate of the present invention is not particularly limited and may be any thickness, but is preferably 6 mm or more, and more preferably 12 mm or more. On the other hand, the upper limit is preferably 100 mm or less.

[Mechanical properties]
(Yield strength)
The yield strength (YP) of the ultra-low yield ratio high-strength thick steel plate of the present invention is not particularly limited and may be any value, but preferably 500 MPa or more.

(Tensile strength)
The tensile strength (TS) of the ultra-low yield ratio high-tensile thick steel sheet of the present invention is not particularly limited and may be any value, but it is preferably more than 980 MPa.

(Yield ratio)
The yield ratio (YR) of the ultra-low yield ratio high tensile strength steel plate of the present invention is not particularly limited and may be any value, but is preferably less than 80%. Here, the yield ratio means a value representing the ratio of the yield strength (YP) to the tensile strength (TS) in percentage, that is, YP/TS×100 (%).

[Production method]
Next, a method for manufacturing an ultra-low yield ratio high-strength thick steel plate according to an embodiment of the present invention will be described. In addition, in the following description, the temperature refers to the temperature at the center of the plate thickness unless otherwise specified. The temperature at the center of the plate thickness can be obtained by heat transfer calculation from the steel plate surface temperature measured by a radiation thermometer. The temperature condition in the cooling condition after the hot rolling is the temperature at the 1/4 position of the plate thickness, and the cooling rate also means the average cooling rate calculated based on the temperature at the 1/4 position of the plate thickness.

The ultra low yield ratio high tensile strength steel plate of the present invention can be manufactured by sequentially performing the following steps.
(1) A steel material having the above-described composition is hot rolled into a thick steel plate (hot rolling step).
(2) The thick steel sheet after the hot rolling is reheated to a reheating temperature of 900° C. or higher and 1000° C. or lower, kept at the reheating temperature for a holding time of 10 minutes or longer, and cooled to 400° C. or lower. Cool to the stop temperature (first reheating step).
(3) The thick steel plate after the first reheating step is reheated to a reheating temperature of Ac1 point+80° C. or higher and less than Ac3 point and held at the reheating temperature for a holding time of 10 minutes or more ( Second reheating step).
(4) The thick steel plate after the second reheating step has an average cooling rate at a position of 1/4 of the plate thickness: 1 to 200° C./s, up to 200° C. or more and an accelerated cooling stop temperature that is less than the bainite transformation start temperature. Accelerated cooling and then air cooling (cooling step).

Hereinafter, each step will be specifically described.

(1) Hot rolling step A steel material having the above-described composition is hot rolled into a thick steel plate. The method for producing the steel material is not particularly limited, but, for example, molten steel having the above-described composition can be melted by a conventional method and cast to manufacture. The melting can be performed by any method such as a converter, an electric furnace and an induction furnace. The casting is preferably performed by a continuous casting method from the viewpoint of productivity, but can also be performed by an ingot-decomposition rolling method. As the steel material, for example, a steel slab can be used.

The steel material is heated prior to rolling. The heating may be performed after the steel material obtained by a method such as casting is once cooled, or may be directly subjected to the heating without cooling the obtained steel material. In the present invention, since the microstructure and characteristics of the thick steel sheet are controlled in the reheating step and the cooling step after hot rolling, the heating temperature is not particularly limited and can be any temperature. However, if the heating temperature is lower than 900° C., the deformation resistance of the steel material is high, so that the load on the rolling mill in hot rolling increases, and it may be difficult to perform hot rolling. Therefore, the heating temperature is preferably 900° C. or higher. On the other hand, if the heating temperature is higher than 1250° C., the oxidation of the steel becomes remarkable and the loss due to the oxidation increases, resulting in a decrease in yield. Therefore, the heating temperature is preferably 1250° C. or lower.

After the above heating, the heated steel material is hot rolled into a thick steel plate. Although the final plate thickness of the thick steel plate is not particularly limited, it is preferably 6 mm or more, more preferably 12 mm or more, and preferably 100 mm or less.

After the hot rolling is finished, reheating is performed as described later, but the thick steel plate can be cooled between the hot rolling and the reheating step. The conditions for performing the cooling are not particularly limited, but the cooling can be performed by any method such as air cooling and water cooling. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used. The cooling temperature is not particularly limited, but may be, for example, normal temperature (20° C. or higher) or higher and 300° C. or lower.

The thick steel plate after the hot rolling process is reheated, held, and accelerated cooled. The reheating treatment partially reverse-transforms the bainite and martensite structures of the hot-rolled steel sheet into austenite, and tempers the untransformed bainite and martensite structures. Subsequent accelerated cooling transforms some of the reverse-transformed austenite into martensite and bainite. Next, the accelerated cooling is stopped at a temperature of 200° C. or higher and lower than the bainite transformation start temperature (Bs point), and air-cooled to transform untransformed austenite into island martensite, and bainite and martens newly formed by accelerated cooling. The site can be tempered.

(2) First Reheating Step The thick steel sheet obtained in the hot rolling step is reheated to a reheating temperature of 900° C. or higher and 1000° C. or lower, and a holding time of 10 minutes or longer at the reheating temperature. Hold for a period of time and then cool to a cooling stop temperature of 400°C or less. By performing the first reheating step, the austenite grain size is reduced, and the average grain size of bainite and martensite in the finally obtained ultra-low yield ratio high-strength thick steel plate can be set to a desired size. As a result, the strength and toughness of the ultra-low yield ratio high-tensile steel plate can be improved.

Reheating temperature: 900-1000°C
In order to secure hardenability and prevent the formation of coarse upper bainite and ferrite, the reheating temperature in the first reheating step is set to 900° C. or higher. Further, the reheating temperature is set to 1000° C. or lower so that the average equivalent circle diameter of bainite and martensite in the finally obtained ultra-low yield ratio high tensile strength steel plate is less than 50 μm. If the reheating temperature is higher than 1000° C., the austenite grain size becomes coarse, and the average grain size of bainite and martensite also becomes coarse, so that the desired size cannot be obtained.

Holding time: 10 minutes or more After heating the thick steel plate to the reheating temperature, it is held at the reheating temperature. The holding time in the above holding is set to 10 minutes or more in order to reduce variations in austenite grain size. On the other hand, the upper limit of the holding time is not particularly limited, but the effect is saturated even if it is excessively lengthened. Therefore, in view of productivity, it is preferably 100 minutes or less, and more preferably 60 minutes or less.

In the first reheating step, any heating method can be used as long as the reheating temperature and the holding time can be controlled as described above. One example of a heating method that can be used is furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

Cooling stop temperature: 400° C. or lower In the first reheating step, after the holding, the thick steel plate is cooled. The cooling stop temperature in the cooling is 400° C. or lower. By cooling the austenite generated by heating to the reheating temperature described above to 400° C. or lower, a low temperature transformation phase can be obtained, and high strength and a low yield ratio can be realized. The cooling stop temperature is preferably 300° C. or lower, and more preferably room temperature.

The method for cooling is not particularly limited, and any method such as air cooling or water cooling can be used. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used.

(3) Second reheating step Reheating temperature: Ac1 point +80°C or higher and less than Ac3 point By heating the thick steel plate after the first reheating to a reheating temperature of Ac1 point +80°C or higher and less than Ac3 point. Most of the structure of the hot-rolled steel sheet is a mixed structure of bainite and austenite reverse-transformed from martensite. If the reheating temperature is less than Ac1 point+80° C., the amount of reverse transformed austenite is small, and desired amounts of martensite and bainite cannot be obtained in the thick steel sheet finally obtained. Further, when the reheating temperature is Ac3 or higher, bainite and martensite are all reverse transformed to austenite, and thus the desired amount of martensite and bainite cannot be obtained in the finally obtained thick steel sheet.

The Ac1 point and the Ac3 point can be calculated by the following equations (1) and (2).
Ac1 (℃) = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo- 39.7V-5.7Ti + 232.4Nb-169.4Al-894.7B (1)
Ac3(℃)=937.2-436.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo + 124.8V + 136.3Ti-19.1Nb + 198.4Al + 3315B …(2)
However, the element symbol in the above formulas (1) and (2) represents the content (mass %) of each element, and is set to 0 when the element is not contained.

Holding time: 10 minutes or more The holding time for holding the reheating temperature is 10 minutes or more. This is because if the holding time is less than 10 minutes, the variation in the austenite grain size becomes large. On the other hand, the upper limit of the holding time is not particularly limited, but productivity is lowered when holding for an excessively long time, so it is preferably set to 180 minutes or less.

Any heating method can be used for the reheating as long as the reheating temperature and the holding time can be controlled as described above. Furnace heating is an example of a heating method. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

(4) Cooling process average cooling rate: 1 to 200° C./s
After the reheating step, accelerated cooling is performed at an average cooling rate of 1/4 position of the plate thickness: 1 to 200° C./s. If the average cooling rate in the accelerated cooling step is less than 1°C/s, the desired quenched structure, that is, bainite and martensite cannot be obtained, and the strength decreases. Therefore, the average cooling rate is set to 1° C./s or more. On the other hand, if the average cooling rate is higher than 200°C/s, it becomes difficult to control the temperature at each position in the steel sheet, and the material tends to vary in the sheet width direction and the rolling direction. Variation occurs. Therefore, the average cooling rate is set to 200° C./s or less.

The method of accelerated cooling is not particularly limited, but cooling can be performed by any method such as air cooling or water cooling. As the water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be used.

Accelerated cooling stop temperature: 200° C. or higher and lower than Bs point By stopping accelerated cooling at a temperature of 200° C. or higher and lower than Bs point and air cooling, untransformed austenite is transformed into island martensite, and bainite and martensite. To self-temper. When the accelerated cooling stop temperature is the Bs point or higher, the coarse upper bainite mainly forms the structure. Further, excessive tempering causes excessive formation of cementite, and even if island-like martensite is generated, most of it is decomposed, so that desired strength, toughness, and ultra-low yield ratio cannot be obtained. On the other hand, when the accelerated cooling stop temperature is less than 200° C., the bainite and martensite do not self-temperate and the desired toughness cannot be obtained. The Bs point can be calculated by the following equation (3).
Bs (℃) = 830-270C-90Mn-37Ni-70Cr-83Mo (3)
However, the element symbol in the above formula (3) represents the content (mass %) of each element, and is set to 0 when the element is not contained.

The cooling conditions in the temperature range after the accelerated cooling is stopped do not substantially affect the structure and the like of the thick steel plate. Therefore, the air cooling after stopping the accelerated cooling can be performed under any condition without particular limitation, but generally, it is preferable to perform the air cooling at a cooling rate of less than 1° C./s.

(Example 1)
Molten steel having the composition shown in Table 1 was melted in a converter and was made into a steel slab (thickness: 250 mm) as a steel material by a continuous casting method. Table 1 also shows the Ac1 transformation point (° C.) obtained by the above equation (1), the Ac ₃ transformation point (° C.) obtained by the equation (2), and the Bs point obtained by the equation (3).

After heating the said steel slab to 1150 degreeC, it hot-rolled and obtained the thick steel plate. Table 2 shows the rolling end temperature and the final plate thickness in the hot rolling.

Then, the thick steel plate after hot rolling was cooled to 200° C. by the method shown in Table 2.

Next, the thick steel plate was subjected to first reheating, second reheating, and accelerated cooling under the conditions shown in Table 2, and air-cooled after the accelerated cooling was stopped. A heat treatment furnace was used for the first reheating and the second reheating. The cooling rate in the air cooling was 0.5 to 0.01°C/s, although it depends on the plate thickness and the accelerated cooling stop temperature.

The microstructure and mechanical properties of each of the thick steel plates obtained as described above were evaluated. The evaluation was performed by the method described below.

(Microstructure)
From the thick steel plate, a test piece for microstructure observation was taken so that the plate thickness 1/4 position was the observation position. The test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Then, after repeller corrosion was performed, observation with a scanning electron microscope at a magnification of 1000 times was performed to take an image of the structure, and an island martensite structure was identified. Images taken for 5 fields of view were analyzed by an image analyzer to determine the area fraction of the island martensite structure and the average equivalent circle diameter.

Next, the resin-embedded sample after the observation of the island martensite structure was mirror-polished again and subjected to nital corrosion, and then observed with a scanning electron microscope at a magnification of 5000 times to take an image of the structure. Images taken for 10 fields of view were analyzed by an image analyzer to determine the area fraction of the cementite structure and the average equivalent circle diameter.

Then, the magnification of the scanning electron microscope was changed to 200 times and the image of the tissue was taken. Images taken for 5 fields of view were analyzed by an image analyzer to determine the area fraction of bainite, martensite, and ferrite structure.

The average equivalent circle diameters of bainite and martensite were measured by the following procedure. First, another test piece used for observing another microstructure was mirror-polished again, and then picric acid was corroded, and then observed with an optical microscope at a magnification of 200 times to take an image of the structure. The captured images of 5 fields of view were analyzed by an image analyzer to determine the average equivalent circle diameters of bainite and martensite.

(Mechanical properties)
A JIS No. 4 tensile test piece was sampled from the plate thickness center of the thick steel plate (position at the plate thickness 1/2 position). Using the tensile test piece, a tensile test was carried out according to JIS Z 2241 to evaluate the yield strength (YP), tensile strength (TS), and yield ratio (YR) of the thick steel plate.

In addition, a V-notch test piece was sampled from the plate thickness center of the thick steel plate (position at the plate thickness 1/2 position) in accordance with JIS Z 2202. Using the V-notch test piece, a Charpy impact test was performed at 0° C. according to JIS Z 2242, and Charpy absorbed energy (vE ₀ ) was determined to evaluate toughness.

Table 3 shows the obtained evaluation results. The tensile strength (TS) was more than 980 MPa, the yield strength (YP) was 500 MPa or more, the yield ratio (YR) was less than 80%, and the absorbed energy (vE ₀ ) at 0°C was 100 J or more.

As can be seen from the above results, all of the steel plates satisfying the conditions of the present invention have a tensile strength of more than 980 MPa, a yield strength of 500 MPa or more, a yield ratio of less than 80%, and an absorbed energy vE ₀ at 0°C. : 100 J or more, high strength, ultra-low yield ratio, and excellent toughness. On the other hand, the thick steel plate that does not satisfy the conditions of the present invention is inferior in at least one of the properties of strength, yield ratio, and toughness.

Claims (7)

In mass %,
C: more than 0.08%, 0.20% or less,
Si: 0.01 to 0.50%,
Mn: 0.5-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.005 to 0.10%, and N: 0.0015 to 0.0065%,
The balance has a composition of Fe and inevitable impurities,
Including bainite containing island martensite, martensite, and cementite,
Cementite is contained in the structure of one or both of bainite and martensite,
The total area fraction of bainite and martensite is 70.0% or more and less than 95.0%,
The average equivalent circle diameter of bainite and martensite is less than 50 μm,
The area fraction of island martensite is 2 to 10%,
The average equivalent circle diameter of the island-shaped martensite is less than 5.0 μm,
An ultra-low yield ratio high tensile strength steel plate having a microstructure in which the area fraction of cementite is more than 0.5% and 5% or less and the average equivalent circular diameter of cementite is less than 0.5 μm. The component composition is mass%,
Ti: 0.004 to 0.03%,
Cu: 1.0% or less,
Ni: 3.0% or less,
Cr: 2.0% or less,
Mo: 1.0% or less,
B: 0.0050% or less,
The ultra low yield ratio high tensile strength steel plate according to claim 1, further containing 1 or 2 or more selected from the group consisting of Nb: 0.1% or less and V: 0.2% or less. The component composition is mass%,
Ca: 0.005% or less,
The ultra-low yield ratio high tensile strength steel plate according to claim 1 or 2, further containing 1 or 2 or more selected from the group consisting of REM: 0.02% or less and Mg: 0.005% or less. The ultra-low yield ratio high tensile steel plate according to any one of claims 1 to 3, which has a tensile strength of more than 980 MPa and a yield ratio of less than 80%. In mass %,
C: more than 0.08%, 0.20% or less,
Si: 0.01 to 0.50%,
Mn: 0.5-3.0%,
P: 0.015% or less,
S: 0.0050% or less,
Al: 0.005 to 0.10%, and N: 0.0015 to 0.0065%,
A hot rolling step of hot rolling a steel material having a composition with the balance being Fe and inevitable impurities into a thick steel plate;
The thick steel sheet after the hot rolling is reheated to a reheating temperature of 900° C. or higher and 1000° C. or lower, held at the reheating temperature for a holding time of 10 minutes or longer, and cooled to a cooling stop temperature of 400° C. or lower. A first reheating step of cooling,
Second reheating step of reheating the thick steel plate after the first reheating step to a reheating temperature of Ac1 point+80° C. or more and less than Ac3 point and holding the reheating temperature for a holding time of 10 minutes or more. When,
The thick steel plate after the second reheating step is accelerated cooled to an accelerated cooling stop temperature of 200° C. or higher and lower than the bainite transformation start temperature at an average cooling rate of 1 to 200° C./s at a plate thickness 1/4 position. And then cooling with air. A method for manufacturing an ultra-low yield ratio high-strength steel plate. The component composition is mass%,
Ti: 0.004 to 0.03%,
Cu: 1.0% or less,
Ni: 3.0% or less,
Cr: 2.0% or less,
Mo: 1.0% or less,
B: 0.0050% or less,
The method for producing an ultra-low yield ratio high-strength thick steel plate according to claim 5, further comprising 1 or 2 or more selected from the group consisting of Nb: 0.1% or less and V: 0.2% or less. The component composition is mass%,
Ca: 0.005% or less,
Production of the ultra-low yield ratio high tensile strength steel plate according to claim 5 or 6, further containing 1 or 2 or more selected from the group consisting of REM: 0.02% or less and Mg: 0.005% or less. Method.