JP5176885B2 - Steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material having high strength and a method for producing the same. Especially suitable for H-shaped steel, T-shaped steel, I-shaped steel, angle steel, channel steel, flat steel, steel sheet pile, etc. used in the fields of architecture, civil engineering, marine structures, etc. The present invention relates to a high-strength steel material excellent in deformation performance and a method for producing the same.

  In recent years, steel materials that are high in strength and excellent in deformation performance have been demanded as the height of buildings increases. Regarding these demands, Patent Documents 1 to 4 disclose a method for producing a steel having a tensile strength exceeding 750 MPa. Patent Document 5 discloses a low yield ratio H-section steel excellent in earthquake resistance with a yield ratio of 80% or less, and a method for producing the same.

Specifically, in Patent Document 1, by weight ratio, C: 0.05 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03 %: S: 0.01% or less, Cr: 0.10-0.70%, Mo: 0.10-0.60%, Nb: 0.08-0.15%, Ti: 0.005 0.025%, Al: 0.06% or less, N: 0.006% or less, and the steel which does not contain B substantially including iron and inevitable impurities is reheated to a temperature range of 1100 to 1250 ° C. Then, after rolling so that the cumulative reduction amount of 1000 ° C. or less is 50% or more, the steel is immediately quenched from a temperature of 750 ° C. or more to a normal temperature, and tempered in a temperature range of 450 ° C. to A _c1 transformation point. A manufacturing method of 80 kgf / mm ² grade high-strength steel excellent in weldability is disclosed.

In Patent Document 2, by weight ratio, C: 0.05 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03% or less, S: 0.005% or less, Mo: 0.10 to 0.60%, V: 0.15 to 0.65%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0 Re-heating steel containing substantially 012% or less, the balance being iron and inevitable impurities and substantially free of B to a temperature range of 1000 to 1250 ° C., and a cumulative reduction amount of 1000 ° C. or less is 50% or more After being rolled, the steel is immediately quenched from a temperature of 750 ° C. or higher to a normal temperature and tempered in a temperature range of 450 ° C. to A _c1 transformation point. A method for producing mm ² grade high strength steel is disclosed.

In Patent Document 3, C: 0.05 to 0.11% by weight, Si: 0.5% or less, Mn: 0.6 to 1.6%, P: 0.03% or less, S: 0 0.01% or less, Cu: 0.80 to 1.60%, Ni: 0.30 to 2.0%, Mo: 0.20 to 0.60%, Nb: 0.010 to 0.05%, Ti : 0.005 to 0.025%, Al: 0.06% or less, N: 0.005% or less The temperature of 1000 to 1250 ° C. is a steel containing the balance of iron and inevitable impurities and containing substantially no B. And then rolled so that the cumulative reduction at 1000 ° C. or less reaches 50% or more, and then immediately quenched from a temperature of 750 ° C. or higher to a normal temperature range of 400 ° C. to 550 ° C. A method for producing 80 kgf / mm ² grade high strength steel excellent in weldability and low temperature toughness characterized by reversion treatment is disclosed.

Patent Document 4 contains C: 0.06% or more, Ni: 0.5 to 3.0%, Nb: 0.02 to 0.04% by weight%, and Ceq: 0.48. Is heated at 950 ° C. or higher at a temperature where the amount of undissolved Nb is 0.005% or higher, then hot-rolled, and directly quenched after the end of rolling, and then 600 degrees below the Ac1 transformation point. A method for producing a tensile strength 780 N / mm ² grade steel excellent in toughness, characterized by tempering at a temperature of at least ° C. is disclosed. Here, Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (%).

Patent Document 5 discloses bainite and / or tempered martensite, which is a rolled H-section steel manufactured by hot rolling, in which one surface layer of the inner and outer surfaces of the flange of the rolled H-section steel has a volume ratio of 50% or more. The hard layer containing the soft layer containing 50% or more of ferrite by volume ratio on the other surface layer, the average particle diameter of the ferrite on the soft layer side is 5 to 40 μm, and the average value in the flange plate thickness direction Further, a low yield ratio rolled H-section steel excellent in earthquake resistance, characterized by having a structure containing 20 to 80% by volume of bainite and / or tempered martensite is disclosed.
JP 05-25542 A Japanese Patent Laid-Open No. 05-209222 JP 05-239548 A JP 11-61249 A JP 2005-264208 A

  As described above, Patent Documents 1 to 4 disclose a method for producing a steel having a tensile strength of 750 MPa or more, but all require a tempering treatment, resulting in an increase in production cost. In addition, the yield ratio is likely to be increased by tempering, and in the examples of Patent Documents 1 to 4, the yield ratio is 85.5 to 94.9%, and it is possible to obtain a steel material with a yield ratio of less than 85%. Have difficulty.

  On the other hand, as described above, Patent Document 5 discloses a low yield ratio H-section steel having a yield ratio of less than 80% and excellent in earthquake resistance, and a manufacturing method thereof. However, as described in paragraph 0001 of the document 5, the object is an H-section steel having a tensile strength of 490 MPa to 750 MPa, and there is nothing about stably obtaining a steel material having a tensile strength of more than 750 MPa. Not disclosed.

Moreover, in order to ensure safety when a large external force due to an earthquake or the like occurs in a steel structure, it is desirable that the steel has excellent toughness and deformation performance.
Accordingly, an object of the present invention is to provide a steel material having a tensile strength of over 750 MPa, a yield ratio of less than 85%, and excellent in toughness and elongation characteristics, and a method for producing the same.

The gist of the present invention resides in the following steel materials and methods for producing the same.
(1) By mass%, C: 0.04 to 0.2%, Si: 0.01 to 0.8%, Mn: 0.1 to 2.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.013 to 0.10%, N: 0.001 to 0.010% or less, Cu: 4% or less, Ni: 4% or less, Cr: 4% or less , Mo: 2% or less, V: 0.2% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.003% or less Is composed of Fe and impurities. In the impurities, O: 0.006% or less, Pcm value calculated by the following formula (1) is 0.21 to 0.40%, and bainite The total ratio of the structure and martensite structure is over 80%, the tensile strength is over 750 MPa, the yield ratio is less than 85%, and the tensile strength (MPa And steel, wherein a product of uniform elongation (%) is 4000 or more.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, the element symbol in the above formula (1) represents the steel content in mass% of the element.

(2) By mass%, C: 0.04 to 0.2%, Si: 0.01 to 0.8%, Mn: 0.1 to 2.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.003-0.10%, N: 0.001-0.010% or less, Cu: 4% or less, Ni: 4% or less, Cr: 4% or less , Mo: 2% or less, V: 0.2% or less, Nb: 0.1% or less, Ti: 0.1% or less, B: 0.003% or less Is made of Fe and impurities, and in the impurities, O: 0.006% or less, and a steel ingot having a chemical composition in which the value of Pcm calculated by the following formula (1) is 0.21 to 0.40% or The steel slab is heated so that the heating temperature is 1200 to 1350 ° C. and the heating time is 1 hour or more, and the cumulative rolling reduction at over 950 ° C. and below 1050 ° C. is 1%. The hot rolling is performed so that the cumulative rolling reduction at 950 ° C. or less is 20% or more, the rolling finishing temperature is 700 to 950 ° C., and the number of rolling passes is 10 or more, and the accelerated cooling start temperature is 650 to 900 ° C., A method for producing a steel material, characterized in that the accelerated cooling stop temperature is 200 to 690 ° C., the accelerated cooling rate (° C./s) is 1 to 20 ° C./second, and no tempering treatment is performed after the accelerated cooling. .
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, the element symbol in the above formula (1) represents the steel content in mass% of the element.

Further, “cumulative rolling reduction above 950 ° C. and below 1050 ° C.” means that the sheet thickness before the rolling pass when it becomes 1050 ° C. or less before rolling is t ₀ , and when it becomes 950 ° C. or less before rolling The value obtained by (t ₀ -t) / t ₀ × 100, where t is the thickness before the rolling pass.

“Cumulative rolling reduction at 950 ° C. or lower” means that the plate thickness before rolling pass when the temperature is 950 ° C. or lower before rolling is t ₀ , and the plate thickness at the end of hot rolling is t (t ₀ − t) / t _{0 The} value obtained by 100.

  The steel material of the present invention has a tensile strength of over 750 MPa, a yield ratio of less than 85%, a product of tensile strength (MPa) and uniform elongation (%) of 4000 or more, and good Charpy characteristics. It is suitable as H-shaped steel, T-shaped steel, I-shaped steel, angle steel, grooved steel, flat steel, steel sheet pile, etc. used in the fields of civil engineering, offshore structures and the like. Moreover, this steel material can be obtained comparatively easily by the production method of the present invention.

  The chemical composition, microstructure and mechanical properties of the steel material according to the present invention and the method for producing the steel material will be described in detail. In the present specification, “%” representing the chemical composition of steel means “% by mass” unless otherwise specified.

1. Chemical composition (1) C: 0.04 to 0.2%
C has the effect | action which raises the intensity | strength of a base material and a welding part. However, if the content is less than 0.04%, the effect of addition is poor. On the other hand, if the C content increases, and particularly if the C content exceeds 0.2%, the toughness of the base material and the welded portion is significantly reduced, and weld cracks increase. In order to obtain a greater effect, the C content is preferably 0.04 to 0.15%, and more preferably 0.04 to 0.09%.

(2) Si: 0.01 to 0.8%
Si has the effect | action which ensures the intensity | strength of a base material and a welding part. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, when the Si content increases, and particularly when the Si content exceeds 0.8%, the toughness of the base material and the welded portion is significantly reduced, and weld cracks increase. In order to obtain a greater effect, the Si content is preferably 0.05 to 0.6%, more preferably 0.1 to 0.4%.

(3) Mn: 0.1 to 2.5%
Mn is an indispensable element for ensuring the strength and toughness of the base material and the weld. However, if the Mn content is less than 0.1%, a sufficient addition effect cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, the toughness of the base metal and the welded portion is significantly reduced, and weld cracks increase. In order to obtain a greater effect, the Mn content is preferably 0.5 to 1.7%, more preferably 1.0 to 1.6%.

(4) P: 0.03% or less P is an element that is unavoidably present in steel as an impurity, and causes a decrease in toughness, and also causes hot cracking during welding. In particular, when its content exceeds 0.03%, the toughness is lowered and the occurrence of hot cracks during welding becomes significant. Since P is the more preferable impurity, the lower limit is not particularly specified. In order to obtain a greater effect, the P content is preferably 0.02% or less, and more preferably 0.01% or less.

(5) S: 0.015% or less S causes toughness deterioration of the base material and the welded portion. In particular, when the content exceeds 0.015%, the toughness deterioration of the base material and the welded portion becomes significant. Since the smaller the amount of S, the more preferable impurities, the lower limit is not particularly defined. In order to obtain a greater effect, the S content is preferably 0.01% or less, and more preferably 0.005% or less.

(6) Al: 0.003-0.10%
Al is an element effective for deoxidation during steelmaking. However, if the Al content is less than 0.003%, a sufficient addition effect cannot be obtained. On the other hand, if the Al content exceeds 0.10%, the amount of inclusions increases, and the toughness of the base metal and the welded portion is significantly deteriorated. In order to obtain a greater effect, the Al content is preferably 0.005 to 0.06%, and more preferably 0.01 to 0.05%.

(7) N: 0.001 to 0.010%
N is contained in the steel as an impurity by 0.001% or more. N forms TiN and BN, and when these nitrides are fine, it suppresses the coarsening of austenite grains during high-temperature heating and contributes to increasing the toughness of the base material and the weld. On the other hand, if the N content exceeds 0.010%, the toughness of the base metal and the welded portion is greatly reduced. In order to obtain a greater effect, the N content is preferably 0.002 to 0.008%, and more preferably 0.003 to 0.007%.

(8) Cu: 4% or less Cu is not required to be added, but is useful for ensuring the strength and toughness of the base material and the welded portion. When the Cu content is less than 0.1%, a sufficient addition effect cannot be obtained. Therefore, when adding, the content is preferably set to 0.1% or more. On the other hand, if the Cu content exceeds 4%, cracking is likely to occur during hot working, and the toughness of the base material and the welded portion is significantly deteriorated. In order to obtain a greater effect, the Cu content is preferably 0.2 to 1.5%, and more preferably 0.3 to 0.5%.

(9) Ni: 4% or less Ni is not required to be added, but is useful for ensuring the strength and toughness of the base material and the welded portion. If the Ni content is less than 0.1%, a sufficient addition effect cannot be obtained. Therefore, when Ni is added, the content is preferably set to 0.1% or more. On the other hand, if the Ni content exceeds 4%, the steel surface flaws become remarkable. In order to obtain a greater effect, the Ni content is preferably 0.2 to 2.5%, more preferably 0.3 to 1.0%. In order to prevent cracking during hot working due to the Cu content, it is preferable to contain 50% or more Ni of the Cu content, and to contain Ni of 75% or more of the Cu content. More preferred.

(10) Cr: 4% or less Although Cr does not need to be added, it is useful in securing the strength of the base material and the welded portion. If the Cr content is less than 0.1%, a sufficient addition effect cannot be obtained. Therefore, when it is added, the content is preferably set to 0.1% or more. On the other hand, if the Cr content exceeds 4%, weld cracks become prominent. In order to obtain a greater effect, the Cr content is preferably 0.2 to 2.0%, and more preferably 0.3 to 1.0%.

(11) Mo: 2% or less Although Mo does not need to be added, it is useful in securing the strength of the base material and the weld. If the Mo content is less than 0.1%, a sufficient addition effect cannot be obtained. Therefore, when it is added, the content is preferably set to 0.1% or more. On the other hand, if the Mo content exceeds 2%, weld cracks become prominent. In order to obtain a greater effect, the Mo content is preferably 0.2 to 1.0%, and more preferably 0.3 to 0.5%.

(12) V: 0.2% or less V is not necessary to be added, but is useful in securing the strength of the base material and the welded portion. When the V content is less than 0.01%, a sufficient addition effect cannot be obtained. Therefore, when V is added, the content is preferably 0.1% or more. On the other hand, if the V content exceeds 0.2%, the toughness of the base material is significantly reduced. In order to obtain a greater effect, the V content is preferably 0.02 to 0.1%, more preferably 0.03 to 0.09%.

(13) Nb: 0.1% or less Nb may be added, but is useful for improving strength and toughness. If the Nb content is less than 0.01%, a sufficient addition effect cannot be obtained. Therefore, when it is added, the content is preferably set to 0.01% or more. On the other hand, if the Nb content exceeds 0.1%, the weld toughness is significantly reduced. In order to obtain a greater effect, the Nb content is preferably 0.02 to 0.07%, and more preferably 0.03 to 0.05%.

(14) Ti: 0.1% or less Ti is not required to be added, but is useful for improving the surface properties of a steel ingot, especially a slab. Ti also has the effect of increasing the toughness of the base material and the weld. When the Ti content is less than 0.003%, a sufficient addition effect cannot be obtained. Therefore, when adding Ti, the content is preferably set to 0.003% or more. On the other hand, when the Ti content exceeds 0.1%, the toughness of the base material is significantly reduced. In order to obtain a greater effect, the Ti content is preferably 0.005 to 0.05%, more preferably 0.007 to 0.02%.

(15) B: 0.003% or less B may be added, but is useful in securing the strength of the base material and the welded portion. When the content of B is less than 0.0003%, a sufficient addition effect cannot be obtained. Therefore, when it is added, the content is preferably 0.0003% or more. On the other hand, if the B content exceeds 0.003%, the toughness of the base material is significantly reduced. In order to obtain a greater effect, the B content is preferably 0.0005 to 0.002%, more preferably 0.0008 to 0.0015%.

(16) Pcm: 0.21 to 0.40%
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Although the above formula (1) is a formula known as a weld cracking susceptibility composition, it is a useful parameter in the present invention in order to improve the characteristics of the base material. If the value of Pcm is less than 0.21%, it is difficult to ensure the target base material strength. On the other hand, when the value of Pcm exceeds 0.40%, the base material strength becomes too high, or the base material toughness is likely to be lowered. In order to obtain a greater effect, the value of Pcm is preferably 0.22 to 0.30%, and more preferably 0.23 to 0.27%.

(17) Others In order to ensure the effect of fine nitrides such as Ti in the base material and the welded portion, “Ti / N”, which is the ratio of Ti content to N content, should be 3 or less. Is desirable.

  O (oxygen) is an impurity inevitably contained in the steel. If the O content is too large, the toughness and ductility of the base metal and the welded portion will be significantly reduced, so the O content needs to be 0.006% or less. In order to obtain a greater effect, the O content is preferably 0.004% or less, and more preferably 0.002% or less.

2. Microstructure In order to provide the high-strength steel material of the present invention having the above chemical composition with the mechanical properties of a tensile strength of over 750 MPa and a yield ratio of less than 85%, the total ratio of the bainite structure and the martensite structure is set to 80. It is necessary to be over%. The total ratio of the bainite structure and the martensite structure may be 100%. In order to obtain a greater effect, the total ratio of the bainite structure and the martensite structure is preferably 90% or more, and more preferably 95% or more.

The type of tissue can be observed using an optical microscope or an electron microscope. Here, the ratio of a certain tissue refers to the ratio of the area of the tissue to the area of the observation field.
Note that the ratio of the structure is desirably determined by a ratio as an average value of a cross section in a cross section perpendicular to the rolling direction of the steel material.

  For convenience, a typical structural photograph of the steel material is taken with an optical microscope or electron microscope at representative positions such as 1/4 or 1/2 in the thickness direction, width direction, and length direction of the steel material. To do. The microstructure of the steel is classified into 1) ferrite, 2) pearlite, 3) bainite or martensite (including tempered martensite and island martensite in this case). Find the total area ratio.

3. Mechanical Properties The steel material of the present invention has a tensile strength of over 750 MPa, a yield ratio of less than 85%, and a product of tensile strength (MPa) and uniform elongation (%) of 4000 or more.

  In addition, although it says that the strength-elongation balance is good here that it is excellent in high strength and elongation property, the No. 4 test piece described in JIS Z 2201 "Metallic material tensile test piece" as the evaluation index. The tensile strength (TS, unit: MPa), total elongation (T.El, unit:%), uniform elongation (U.El, unit:%) were measured at room temperature using T.A. El, TS × U. Let El be obtained. TS × T. El, TS × U. The larger the value of El, the better. The steel material of the present invention is TS × U. The value of El is 4000 or more.

The steel material of the present invention preferably has the following mechanical properties in addition to the above mechanical properties.
0.2% proof stress: 600 MPa or more Conventionally, 0.2% proof stress of SM490B defined in steel materials that have been widely used, for example, JIS G 3106 “Rolled steel for welded structure”, plate thickness exceeds 40 mm, 75 mm or less In this case, it is 295 MPa or more. If the steel material of the present invention having a 0.2% proof stress of 600 MPa or more is used in place of SM490B, the weight of the member can be halved in general, depending on the design. Therefore, it can greatly contribute to the weight reduction of the structure.

Charpy characteristics: Transition temperature vTs of 0 ° C. or less, impact value vE0 at 0 ° C. of 100 J or more Conventionally, steel products such as SM490B defined by JIS G 3106 “Rolled steel for welded structure” Has an impact value of 27 J or more at a test temperature of 0 ° C. If the steel material of the present invention further has the above-mentioned characteristics, high toughness can be maintained even in a low temperature range by applying the steel material of the present invention to a member for which SM490B has been conventionally used. Therefore, not only the weight of the member can be reduced, but also the reliability of the member can be improved.

4). Manufacturing Conditions The steel material according to the present invention may be manufactured by any manufacturing method as long as it has the chemical composition, microstructure, and mechanical properties described above, but it is efficient and stable by adopting the following manufacturing method. It is realized to manufacture.

  That is, the steel having the above chemical composition, that is, the steel having the chemical composition defined in the present invention, for example, is melted in a converter and cast into a slab by a continuous casting method. Slab heating, rough rolling using hole rolling, intermediate rolling using edger rolling mill and rough universal rolling mill, and hot rolling consisting of finishing rolling using finishing universal rolling mill, By performing controlled cooling (accelerated cooling described below) after rolling, a steel material having a tensile strength of over 750 MPa, a yield ratio of less than 85%, and excellent elongation properties and toughness, specifically, for example, an H-section steel is manufactured. It is possible.

(1) Hot rolling The steel ingot or steel slab having the chemical composition according to the present invention is hot rolled into a predetermined shape and size.

  In the following description, temperature means the surface temperature of steel unless otherwise specified. In addition, when reheating after the steel surface temperature falls by heat removal by contact with a rolling roll or accelerated cooling, the surface temperature after reheating is meant.

A) Heating temperature: 1200-1350 ° C
When the heating temperature is set to 1200 ° C. or higher, hot working becomes easy, and Nb, V, Ti, B and the like are dissolved in the base, and the effect of increasing the tensile strength is obtained. Moreover, by making heating temperature 1350 degrees C or less, the coarsening of a crystal grain is suppressed and it contributes to ensuring favorable toughness. In addition, in order to acquire a bigger effect, it is preferable that heating temperature shall be 1200-1330 degreeC, and it is more preferable to set it as 1230-1320 degreeC.

B) Heating time: 1 hour or more Here, the sum of the temperature raising time and the holding time is defined as the heating time. If the heating time is too short, the effect of increasing the tensile strength may not be obtained because the internal temperature of the steel is low even if the surface temperature is high. Therefore, the heating time is set to 1 hour or longer. Note that if the heating time is too long, the toughness may be reduced due to the coarsening of the structure, so the heating time is desirably 10 hours or less. In order to obtain better performance, the heating time is more preferably 2 to 6 hours.

C) cumulative rolling reduction at 950 ° C. or less: 20% or more as "cumulative reduction rate at 950 ° C. or less" is the plate thickness before rolling pass when it becomes below 950 ° C. before rolling t _0, hot A value obtained by (t ₀ -t) / t ₀ × 100, where t is the thickness at the end of rolling and rolling.
By increasing the cumulative rolling reduction at 950 ° C. or lower, residual strain is given to the austenite phase, and the structure after phase transformation becomes fine. For this reason, the effect that toughness becomes favorable is acquired. In order to obtain this effect, the cumulative rolling reduction at 950 ° C. or lower is set to 20% or higher. In order to obtain a greater effect, the cumulative rolling reduction at 950 ° C. or less is preferably 40% or more, and more preferably 60% or more. The upper limit is not particularly limited, but if the cumulative rolling reduction at 950 ° C. or less is excessively large, the equipment load becomes large or the time required for the hot rolling process becomes long, so 90% or less is preferable. .

D) Cumulative rolling reduction above 950 ° C. and below 1050 ° C .: 1% or more “Cumulative rolling reduction above 950 ° C. and below 1050 ° C.” thickness of t _0, as rolling passes before plate thickness of t when it becomes below 950 ° C. before rolling is a value obtained by _{(t 0 -t) / t 0} × 100.
In addition to setting the cumulative reduction ratio below 950 ° C. to 20% or more, by increasing the cumulative reduction ratio above 950 ° C. and below 1050 ° C., the grain reduction and residual strain due to recrystallization of the austenite phase The structure after the phase transformation becomes fine, and the effect of improving the toughness is obtained. Therefore, the cumulative rolling reduction at 950 ° C. or more and 1050 ° C. or less is set to 1% or more. In order to obtain a greater effect, the cumulative rolling reduction at 950 ° C. or higher and 1050 ° C. or lower is preferably 5% or more, and more preferably 10% or more. If the austenite phase is recrystallized at a higher temperature, for example, 1250 ° C., grain growth is accelerated. However, even in such a case, the structure after phase transformation can be refined by increasing the cumulative rolling reduction at 1050 ° C. or lower. The upper limit of the cumulative reduction ratio above 950 ° C. and below 1050 ° C. is not particularly limited, but if it is excessively large, the time for maintaining in this temperature range becomes too long, and the energy efficiency and productivity of the hot rolling process decrease. Is concerned. Therefore, it is preferable that the cumulative rolling reduction at 950 ° C. or more and 1050 ° C. or less is 60% or less.

E) Rolling finishing temperature: 700-950 ° C
When the rolling finishing temperature is higher than 950 ° C., it becomes difficult to ensure good toughness. On the other hand, when the rolling end temperature is lower than 700 ° C., the ferrite transformation is likely to proceed before the accelerated cooling after hot, so that it becomes difficult to secure a desired microstructure and tensile strength. In order to obtain better strength and toughness, the rolling finishing temperature is preferably 750 to 920, more preferably 800 to 860.

F) Number of rolling passes: 10 passes or more When the number of rolling passes is small, the rolling load increases and the shape control of the steel material becomes difficult. Moreover, since the opportunity to perform temperature measurement and temperature control is reduced, the performance is likely to vary, and there is a high possibility that a good strength-elongation balance and toughness cannot be ensured. Therefore, the number of rolling passes is 10 passes or more. More preferably, the number of passes is 14 or more.

(2) Accelerated cooling When hot rolling is completed in this way, the obtained steel material is accelerated and cooled as follows to obtain a product. In the manufacturing method according to the present invention, in order to reduce the yield ratio, the steel piece after accelerated cooling is not tempered.

A) Accelerated cooling rate: 1 to 20 ° C./second Accelerated cooling is performed for the purpose of increasing tensile strength and toughness. In order to obtain this effect, the accelerated cooling rate is set to 1 ° C./second or more. In addition, when the accelerated cooling rate is too high, the toughness is deteriorated and further the shape is deteriorated such as bending and warping. Therefore, the accelerated cooling rate is set to 20 ° C./second or less. In order to obtain a greater effect, the accelerated cooling rate (° C./s) is preferably set to 1.5 ° C./second or more and not more than the Acr value calculated by the following formula (2). The surface is cooled quickly by heat transfer with water (or other liquid) used for accelerated cooling, while the inside of the steel is cooled relatively slowly by the heat conduction of the steel, so when the plate thickness increases, the surface of the steel The cooling rate difference between the inside and the inside becomes remarkable, and there is a possibility that the deterioration of toughness becomes remarkable near the surface cooled quickly. Since the Acr value is a function of the plate thickness, by using the Acr value as a reference, the difference in the cooling rate between the surface and the inside is made relatively small, and consequently the difference in the mechanical properties between the surface and the inside is made small. Is realized. The accelerated cooling rate is more preferably 2.0 ° C./second or more and 50% or less of the Acr value.

Acr = 300 / t (2)
Here, t represents the plate thickness (mm) of the cooling unit.
The accelerated cooling rate refers to a value obtained by dividing the difference between the accelerated cooling start temperature and the accelerated cooling stop temperature by the time from the accelerated cooling start to the accelerated cooling stop. The accelerated cooling stop temperature means the surface temperature after recuperation.

B) Accelerated cooling start temperature: 650-900 ° C
When the accelerated cooling start temperature is higher than 900 ° C., it becomes difficult to ensure good toughness. On the other hand, when the accelerated cooling start temperature is lower than 650 ° C., it is difficult to ensure desired tensile strength characteristics. In order to obtain better strength and toughness, the accelerated cooling start temperature is preferably 750 to 850 ° C, more preferably 780 to 830 ° C.

C) Accelerated cooling stop temperature: 200 to 690 ° C
When the accelerated cooling stop temperature is higher than 690 ° C., it becomes difficult to secure desired strength and toughness. On the other hand, when the cooling stop temperature is lower than 200 ° C., it becomes difficult to ensure desired impact characteristics or hydrogen cracking is likely to occur. In order to obtain better strength and toughness, the accelerated cooling stop temperature is preferably 400 to 600 ° C, and more preferably 400 to 550 ° C.
In addition, after accelerating cooling is stopped, it may be cooled to room temperature (room temperature) at a rate smaller than the above accelerated cooling rate. For example, it may be allowed to cool on a stand.

  Steels 1 to 42 having the chemical composition shown in Table 1 were melted in a vacuum melting furnace and cast into a mold to form a 180 kg steel ingot. Steels 1 to 36 are steels according to the present invention whose chemical composition is within the range defined by the present invention, and steels 37 to 42 are steels according to comparative examples in which the components deviate from the content range defined by the present invention.

  Each steel ingot was hot forged at 950 ° C. to 1250 ° C. to produce a steel piece having a thickness of 120 mm, a width of 170 mm, and a length of 180 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  The steel slab thus obtained was heated from room temperature to 1250 to 1360 ° C. over 1 hour and held at that temperature for another 1 hour. Therefore, the heating time at this time is 2 hours. In addition, about some steel slabs (test numbers 17, 19, and 34), the holding time was extended and the heating time was set to 4 hours. Moreover, about the steel slab of the test number 39, both heating time and holding time were 30 minutes, and heating time was 1 hour.

  The steel slab thus heated for a predetermined time was hot-rolled under the rolling conditions shown in Table 2. After the rolling finish, accelerated cooling using water and water-soluble quenching oil was performed under the accelerated cooling conditions shown in Table 2 as well. After completion of the accelerated cooling, it was taken out into the atmosphere and allowed to cool to room temperature (room temperature).

  Each steel plate obtained in this manner was examined for tensile properties and Charpy properties as microstructures and mechanical properties.

  The microstructure is obtained by collecting a test piece from a position that becomes a thickness direction 1/4, a width direction 1/2, and a length direction 1/2 of the steel sheet, and mirror-polishing the surface including the rolling direction and the plate thickness direction. It corroded with the night, and the structure was investigated by performing optical microscope observation and scanning electron microscope observation at a higher magnification at a magnification of 100 times or 500 times.

  The tensile test is performed at room temperature using a No. 4 test piece described in JIS Z 2201: 1998 “Metal material tensile test piece”, 0.2% proof stress (YP), tensile strength (TS), total elongation ( T.El), uniform elongation (U.El), yield ratio (YR), TS × T. El, TS × U. El was determined.

In addition, said tensile test piece was extract | collected in parallel with the rolling direction (namely, length direction of a steel plate) from the site | part used as the thickness direction 1/4 in the center part of the width direction of a steel plate.
The impact characteristics were obtained by collecting a V-notch test piece described in JIS Z 2242: 2005 from a portion of the steel sheet surface, which is 1/4 in the thickness direction, in parallel with the rolling direction, performing a Charpy impact test, and performing a ductile-brittle fracture surface. The transition temperature (vTs) and the absorbed energy (vE0) at 0 ° C. were measured.

  Table 3 shows the results of the above tests.

The steel composition of Test Nos. 1 to 38 of “Invention Example” satisfying the chemical composition and production conditions defined in the present invention has a total ratio of bainite structure and martensite structure exceeding 80% and a tensile strength of 750 MPa. The yield ratio is less than 85%, the product of tensile strength (MPa) and uniform elongation (%) is 4000 or more, other tensile properties (YP, TS × T.El), and Charpy properties (vTs, vE0) is also excellent.
On the other hand, the steel plates of test numbers 39 to 50 of “Comparative Example” have YP, TS, TS × T. El, TS × U. It is inferior to at least one of El, vTs, and vE0.

  The high-strength steel material of the present invention has a tensile strength of over 750 MPa and is excellent in deformation characteristics. It can be used as steel, channel steel, flat steel, steel sheet piles and the like. This high-strength steel material can be manufactured relatively easily by the manufacturing method of the present invention.

Claims (2)

In mass%, C: 0.04 to 0.2%, Si: 0.01 to 0.8%, Mn: 0.1 to 2.5%, P: 0.03% or less, S: 0.015 %: Al: 0.013-0.10 %, N: 0.001-0.010% or less, Cu: 4% or less, Ni: 4% or less, Cr: 4% or less, Mo: 2% or less, V: 0.2% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.003% or less It consists of impurities, and in the impurities, O: 0.006% or less, and having a chemical composition in which the value of Pcm calculated by the following formula (1) is 0.21 to 0.40%, a bainite structure and martensite The total ratio of the site structure is over 80%, the tensile strength is over 750 MPa, the yield ratio is less than 85%, and the tensile strength (MPa) is the same. Steel product of the elongation (%) is equal to or is more than 4,000.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, the element symbol in the above formula (1) represents the steel content in mass% of the element. In mass%, C: 0.04 to 0.2%, Si: 0.01 to 0.8%, Mn: 0.1 to 2.5%, P: 0.03% or less, S: 0.015 %: Al: 0.003-0.10%, N: 0.001-0.010% or less, Cu: 4% or less, Ni: 4% or less, Cr: 4% or less, Mo: 2% or less, V: 0.2% or less, Nb: 0.1% or less, Ti: 0.1% or less, and B: 0.003% or less A steel ingot or steel slab having a chemical composition consisting of impurities, wherein O is 0.006% or less, and the value of Pcm calculated by the following formula (1) is 0.21 to 0.40%. , Heated so that the heating temperature is 1200 to 1350 ° C. and the heating time is 1 hour or more, and the cumulative rolling reduction at 950 ° C. or more and 1050 ° C. or less is 1% or more, Hot rolling is performed so that the cumulative rolling reduction at 50 ° C. or less is 20% or more, the rolling finishing temperature is 700 to 950 ° C., and the number of rolling passes is 10 or more, and the accelerated cooling start temperature is 650 to 900 ° C., accelerated cooling. A method for producing a steel material, characterized in that accelerated cooling is performed so that the stop temperature is 200 to 690 ° C. and the accelerated cooling rate (° C./s) is 1 to 20 ° C./second, and no tempering treatment is performed after accelerated cooling.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, the element symbol in the above formula (1) represents the steel content in mass% of the element.