KR101994784B1 - Thick-walled high-toughness high-strength steel plate and method for manufacturing the same - Google Patents

Abstract

Which is produced from a steel material having a specific component composition that provides a technique of achieving both toughness of the surface and strength and toughness in the steel sheet and casting under a condition that the cooling rate at the time of solidification of the surface is 1 DEG C / s or less, (VE-40) of 70 J or more and a plate thickness of 100 mm or more, which is excellent in toughness on the surface of the steel sheet and strength and toughness in the steel sheet.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high-

TECHNICAL FIELD The present invention relates to a high strength and high strength steel sheet used for steel structures such as buildings, bridges, shipbuilding, offshore structures, industrial buildings, tanks, penstocks, . The present invention is particularly excellent in toughness on the surface of the steel sheet, strength and toughness in the steel sheet. The steel sheet has a thickness of 100 mm or more and a yield strength of 620 MPa or more.

When steel is used in various fields such as construction, bridges, shipbuilding, offshore structures, dryers, tanks, penstocks, etc., the steel is usually welded to a desired shape. [0002] In recent years, the size of steel structures has been remarkably advanced, and the strength of the steel used and the thickness of the steel are considerably progressing.

Even if a steel plate having a plate thickness of 100 mm or more is a high strength steel plate and a steel plate having excellent strength and toughness at the center of the plate thickness is to be manufactured, the cooling rate is lowered at the center of the plate thickness so that a relatively low strength structure such as ferrite is likely to be formed . Therefore, in order to suppress the generation of such a structure, it is necessary to add a large amount of alloying elements.

Particularly, in order to satisfy the strength and toughness of the plate thickness center portion of the heavy metal material (steel plate having a thickness of 100 mm or more), it is important to form a mixed structure of bainite or bainite and martensite at the central portion of the plate thickness at the time of quenching . To this end, it is necessary to add a large amount of alloying elements such as Mn, Ni, Cr, and Mo.

Further, on the surface of the steel sheet, a martensite structure having a higher cooling rate and lower toughness than the central portion of the plate thickness is formed. Therefore, in a high-strength steel sheet having a thickness of 100 mm or more, it is difficult to achieve both toughness of the surface and strength and toughness in the steel sheet.

As a document describing the steel sheet related to this patent, there are, for example, the following two non-patent documents. In Non-Patent Document 1, there is a description of a material having a sheet thickness of 210 mm, and in Non-Patent Document 2, there is a description of a material having a sheet thickness of 180 mm.

 Shinnetsu Tetsuo Oh, 348 (1993), 10-16  Nihon Koukan Symbol O, 107 (1985), 21-30

In the above non-patent reference, it is described that the strength and toughness of the central portion of the plate thickness are good. However, there is no description about the toughness (Charpy impact property) of the steel sheet surface. Considering that a martensitic structure is formed on the surface of a steel sheet having a cooling rate that is higher than that of the central portion of the plate thickness and the toughness (Charpy impact property) of the surface of the steel sheet is lowered, The non-patent literature does not disclose that a steel sheet which stably satisfies the toughness of the steel sheet surface is produced.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a high strength and high strength steel sheet having both toughness of the surface, strength and toughness inside the steel sheet, and a method for producing the same.

In order to solve the above problems, the inventors of the present invention conducted a microstructure control (hereinafter, referred to as " microstructure control ") of a steel sheet having a yield strength of 620 MPa A careful study was made of the factors, and the following perceptions were obtained.

1. When the cooling rate of the steel material as a raw material during solidification exceeds 1 캜 / s, the formation of microsegregation and the solidification reaction compete. As a result, micro-segregation is alleviated. In the case of manufacturing a large-sized steel material, the cooling rate at the time of solidification of the steel material is lowered to 1 캜 / s or less, resulting in remarkable micro-segregation. Even in such a case, in order to obtain good toughness on the surface of the steel sheet that becomes the martensite structure at the time of quenching, it is important to reduce the micro-segregation upon solidification in addition to the reduction of the P content. Also, by setting the primary crystals at the time of solidification to the delta phase and setting the proportion of the delta phase at the start of the gamma phase to 30% or more, microsegregation is reduced and toughness is improved. In addition, the unit of the above ratio means volume%.

2. In order to obtain good strength and toughness at the center of the plate thickness, which is remarkably lower in cooling rate than the surface of the steel sheet during cooling after the hot working, the steel composition (component composition) is suitably selected, To be a martensite and / or bainite structure. For this purpose, it is necessary to appropriately select an alloy component, and it is particularly necessary to set the carbon equivalent (Ceq) to 0.65% or more. In addition to proper component design, the composition of the structure by hot working and heat treatment is also important.

3. In order to improve toughness, miniaturization of the prior γ grain size is effective. To miniaturize the spherical γ-particle size after the heat treatment, it is important to miniaturize the spherical γ-particle size before the heat treatment, that is, to make the spherical γ-particle size while being hot-worked finer. For this purpose, it is important to select appropriate hot working conditions and rolling conditions.

The present invention has been made in addition to the above-described recognition, and provides the following.

The steel according to any one of [1] to [4], wherein the steel contains, by mass%, 0.08 to 0.20% of C, 0.40% or less of Si, 0.5 to 5.0% Of Al, 0.010 to 0.080% of Al, 0.0070% or less of N and 0.0025% or less of O, satisfying the relations of the formulas (1) and (2), the balance being Fe and inevitable impurities, A high strength and high strength steel sheet having excellent toughness and strength and toughness inside the steel sheet, having a toughness (vE-40) of 70 J or more and a thickness of 100 mm or more on the surface of the steel sheet.

(1) Ceq ^IIW = C + Mn / 6 + Cu + Ni / 15 +

(C _L -C) / C _L × 100? 30 (2)

Here, C _L is defined by the following equation.

_{C L = 0.2 - (- 0.1} × (0.2-Si) -0.03 × (1.1-Mn) -0.12 × (0.2-Cu) -0.11 × (3-Ni) + 0.025 × (1.2-Cr) +0.1 (0.5-Mo) +0.2 x (0.04-V) -0.05 x (0.06-Al)) (3)

However, in the above formula, the symbol of the element is the content (mass%) of each alloy component, and the value not including it is zero.

[2] The steel sheet according to any one of [1] to [4], wherein the steel sheet contains one or more of Cu: not more than 0.50%, Mo: not more than 1.50%, V: not more than 0.400%, Nb: not more than 0.100% (1). ≪ RTI ID = 0.0 > (1) < / RTI >

[3] The steel sheet according to any one of the above items [1] to [3], wherein the steel sheet has a composition of Mg: 0.0001 to 0.0050%, Ta: 0.01 to 0.20%, Zr: 0.005 to 0.1%, Y: 0.001 to 0.01% [1] or [2], further comprising one or more of REM: 0.0005 to 0.0100%.

[4] A high strength, high strength steel sheet according to any one of [1] to [3], wherein the yield strength is 620 MPa or more.

[5] The high strength steel sheet according to any one of [1] to [4], wherein a reduction in area in the thickness direction of the plate thickness center is 40% or more.

[6] A method for producing a high strength high strength steel sheet according to any one of [1] to [5], wherein the steel material is heated to 1200 to 1350 캜 and subjected to hot forging at a cumulative rolling reduction of 25% , The hot rolled steel sheet is heated to a temperature not lower than the Ac 3 point and not higher than 1200 ° C and is subjected to hot rolling at a cumulative rolling reduction of not less than 40% And tempering is carried out at a temperature of 450 ° C to 700 ° C.

[7] A method for producing a high strength and high strength steel sheet according to any one of [1] to [5]

The steel material is heated to 1200 to 1350 캜 and subjected to hot forging at a cumulative reduction of not less than 25%, heated to Ac 3 point or more and 1200 캜 or less, and subjected to hot rolling at a cumulative rolling reduction of 40% Rapid cooling from a temperature of Ar3 or higher to a lower temperature of 350 占 폚 or lower or Ar3 point or lower and tempering at a temperature of 450 占 폚 to 700 占 폚.

[8] A method for producing a high strength, high strength steel sheet according to any one of [1] to [5]

The steel material is heated to 1200 to 1350 캜 and subjected to slabbing with a cumulative rolling reduction of not less than 40% and heated to not less than Ac 3 point and not more than 1200 캜 to obtain a hot rolled steel sheet having a cumulative rolling reduction of not less than 40% And is reheated to a temperature not lower than the Ac3 point and not higher than 1050 ° C and is quenched to a lower temperature of 350 ° C or lower or a temperature lower than the Ar3 point at a temperature of Ac3 point or higher and then tempered at a temperature of 450 to 700 ° C Which comprises the steps of:

[9] A method for producing a high strength, high strength steel sheet according to any one of [1] to [5]

The steel material is heated to 1200 to 1350 캜 and crushed to a cumulative reduction of 40% or more. Hot rolling is carried out by heating the steel to a temperature of Ac 3 point to 1200 캜 and a cumulative reduction of not less than 40% Rapid cooling from a temperature of Ar3 or higher to a lower temperature of 350 占 폚 or lower or Ar3 point or lower and tempering at a temperature of 450 占 폚 to 700 占 폚.

According to the present invention, a high strength and high strength steel sheet having a strength of 620 MPa or higher in yield strength and an excellent toughness and a thickness of 100 mm or more is obtained. By using this high strength and high strength steel sheet, a steel structure with high safety can be manufactured.

(Mode for carrying out the invention)

Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to the following embodiments.

<High-strength high-strength steel plate with high strength>

The composition of the present invention comprises 0.08 to 0.20% of C, 0.40% or less of Si (inclusive of 0%), 0.5 to 5.0% of Mn, 0.010% of P, (Including 0%), S: not more than 0.0050% (including 0%), Cr: not more than 3.0% (including 0%), Ni: 0.1 to 5.0% 0.010 to 0.080%, N: 0.0070% or less (including 0%), and O: 0.0025% or less (including 0%). Each component will be described below. In addition, &quot;% &quot; representing the content of the component means &quot; mass% &quot;.

C: 0.08-0.20%

C is a useful element for obtaining the strength required for structural steel at low cost. In order to obtain the effect, the C content needs to be 0.08% or more. On the other hand, when the C content exceeds 0.20%, the toughness of the base material and the welded portion remarkably deteriorates when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high hardness. Therefore, the upper limit of the C content was set at 0.20%. The preferred C content is 0.08% to 0.14%.

Si: 0.40% or less

Si is added for deoxidation. However, when other elements are added for deoxidation, the steel sheet of the present invention may not contain Si. When the Si content exceeds 0.40%, the toughness of the base material and the weld heat affected zone remarkably decreases when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high hardness. Therefore, the Si content is 0.40% or less. The preferable Si content is in the range of 0.05 to 0.3%. And more preferably in the range of 0.1 to 0.3%.

Mn: 0.5 to 5.0%

Mn is added in order to secure the strength of the base metal. If the Mn content is less than 0.5%, the effect is not sufficient. Also, when the Mn content exceeds 5.0%, center segregation is promoted to increase the casting defects of the slab, and when the steel structure is manufactured by welding by using a high strength steel sheet with a high strength and high strength, Degrade. Therefore, the upper limit of the Mn content is set to 5.0%. The Mn content is preferably in the range of 0.6 to 2%, more preferably 0.6 to 1.6%.

P: not more than 0.010%

If the P content exceeds 0.010%, the toughness of the base material and the weld heat affected zone remarkably deteriorates when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high hardness. Therefore, the smaller the P content is, the better (it does not need to be included) and is limited to 0.010% or less.

S: not more than 0.0050%

When the S content exceeds 0.0050%, the toughness of the base material and the weld heat affected zone remarkably deteriorates when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high hardness. Therefore, the smaller the S content is, the better (it does not need to be included), and it is made 0.0050% or less.

Cr: 3.0% or less

Cr is an element effective for increasing the strength of the base material. However, if the Cr content is excessive, the weldability decreases. Therefore, the Cr content should be 3.0% or less. The preferable Cr content is 0.1% to 2%. More preferably, it is in the range of 0.7% to 1.7%. The Cr content may be 0%.

Ni: 0.1 to 5.0%

Ni is a beneficial element for improving the strength of the steel and the toughness of the weld heat affected zone. To obtain this effect, the Ni content should be 0.1% or more. On the other hand, if the Ni content exceeds 5.0%, the economical efficiency remarkably decreases. Therefore, the upper limit of the Ni content is 5.0%. The Ni content is preferably 0.4 to 4%, and more preferably 0.8 to 3.8%.

Al: 0.010 to 0.080%

Al is added to sufficiently deoxidize the molten steel. When the Al content is less than 0.010%, the effect is insufficient. On the other hand, when the Al content exceeds 0.080%, the Al content to be solidified in the base material increases and the toughness of the base material decreases when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high hardness. Therefore, the Al content should be 0.080% or less. The Al content is preferably in the range of 0.030 to 0.080%, and more preferably in the range of 0.030 to 0.070%.

N: 0.0070% or less

N has an effect of improving the toughness of the base material and the weld heat affected zone when the steel structure is made by welding using a high-strength, high-strength steel sheet with a high hardness by forming a nitride with Ti or the like to make the structure finer. Since the effect of improving the toughness can be obtained by a constitution other than N, the steel sheet of the present invention may not contain N. However, from the viewpoint of obtaining this effect by N, it is preferable that the N content is 0.0015% or more. On the other hand, when the N content is more than 0.0070%, when the steel structure is manufactured by welding using the high-strength, high-strength steel sheet with high hardness, the amount of N dissolved in the base material increases and the toughness of the base material remarkably decreases, In addition, the coarse carbonitride is formed and the toughness is lowered. Therefore, the N content should be 0.0070% or less. It is preferably 0.006% or less, more preferably 0.005% or less.

O: 0.0025% or less

When the content of O exceeds 0.0025%, a hard oxide is generated in the steel and the toughness remarkably decreases. Therefore, the smaller the content of O is, the better (it does not need to be included), and the content is 0.0025% or less.

The high strength and high strength steel sheet of the present invention may contain at least one of Cu, Mo, V, Nb and Ti in addition to the above elements for the purpose of further increasing strength and / or toughness.

Cu: not more than 0.50%

When Cu is contained, the strength of the steel can be improved without impairing toughness. When the Cu content exceeds 0.50%, cracks may be generated on the surface of the steel sheet during hot working. Therefore, when Cu is contained, the content thereof is set to 0.50% or less.

Mo: 1.50% or less

Mo contributes to the strengthening of the base material when a steel structure is manufactured by welding using a high-strength, high-strength steel plate having a high strength. However, when the Mo content exceeds 1.50%, hardness is increased due to precipitation of alloy carbide, and toughness is lowered. Therefore, in the case of containing Mo, the upper limit of the Mo content is set to 1.50%. The preferable Mo content is in the range of 0.2% to 0.8%.

V: 0.400% or less

V contributes to the improvement of the strength and toughness of the base material when a steel structure is manufactured by welding using a high-strength, high-strength steel sheet having a high strength. Further, V is effective for lowering solute N by precipitating as VN. However, if the V content exceeds 0.400%, the toughness decreases due to the precipitation of hard VC. Therefore, when V is added, the V content is preferably 0.400% or less. More preferably, it is in the range of 0.01 to 0.1%.

Nb: 0.100% or less

Nb is effective because it is effective in improving the strength of the base material. If the Nb content exceeds 0.100%, the toughness of the base material significantly decreases. Therefore, the upper limit of the Nb content is set to 0.100%. Preferably, it is 0.025% or less.

Ti: 0.005 to 0.020%

Ti produces TiN during heating to effectively suppress the coarsening of austenite and improves the toughness of the base material and the weld heat affected zone when a steel structure is manufactured by welding using a high strength and high strength steel sheet. However, when the Ti content exceeds 0.020%, the Ti nitride is coarsened and toughness of the base material is lowered. Therefore, in the case of containing Ti, the Ti content is set in the range of 0.005% to 0.020%. , Preferably in the range of 0.008% to 0.015%.

The high strength and high strength steel sheet of the present invention may contain at least one of Mg, Ta, Zr, Y, B, Ca and REM for the purpose of further improving the material.

Mg: 0.0001 to 0.0050%

Mg is an element effective for forming a stable oxide at a high temperature and effectively suppressing the coarsening of the spherical γ of the weld heat affected zone and improving the toughness of the welded portion. In order to obtain this effect, the Mg content is set to 0.0001% or more. However, if the Mg content exceeds 0.0050%, the amount of intervening material increases and the toughness decreases. Therefore, when Mg is contained, the content thereof is preferably 0.0050% or less. More preferably, it is in the range of 0.0001% to 0.015%.

Ta: 0.01 to 0.20%

Addition of an appropriate amount of Ta is effective for improving the strength. Concretely, it is effective to make the Ta content 0.01% or more. However, when the content exceeds 0.20%, the toughness is lowered due to the formation of precipitates. Therefore, when Ta is contained, the content thereof is set to 0.01% to 0.20%.

Zr: 0.005 to 0.1%

Zr is an effective element for increasing the strength. In order to obtain this effect, it is effective to set the Zr content to 0.005% or more. On the other hand, when the Zr content exceeds 0.1%, coarse precipitates are formed and the toughness is lowered. Therefore, when Zr is contained, the content thereof is 0.005 to 0.1%.

Y: 0.001 to 0.01%

Y is an element effective for forming a stable oxide at a high temperature and effectively suppressing coarsening of the spherical γ of the weld heat affected zone and improving the toughness of the welded portion. In order to obtain this effect, it is effective to set the Y content to 0.001% or more. However, if the Y content exceeds 0.01%, the amount of intervening material increases and the toughness decreases. Therefore, when Y is contained, the content thereof is 0.001 to 0.01%.

B: not more than 0.0030%

B has an effect of suppressing ferrite transformation from the grain boundaries by segregation in the austenitic system, thereby increasing the quenching property. However, when the B content exceeds 0.0030%, B precipitates as a carbonitride to lower the quenching property and the toughness lowers. Therefore, the B content should be 0.0030% or less. When B is contained, the content thereof is preferably in the range of 0.0003 to 0.0030%. And more preferably in the range of 0.0005 to 0.002%.

Ca: 0.0005 to 0.0050%

Ca is an element useful for morphology control of sulfide inclusions. In order to exhibit the effect, it is necessary to set the Ca content to 0.0005% or more. However, when the Ca content exceeds 0.0050%, the cleanliness is lowered and the toughness is deteriorated. Therefore, when Ca is contained, the content thereof is preferably 0.0050% or less. And more preferably in the range of 0.0005% to 0.0025%.

REM: 0.0005 to 0.0100%

REM also has the effect of improving the quality of the steel by forming oxides and sulfides in the steel as in Ca. In order to obtain the effect, it is necessary to set the REM content to 0.0005% or more. However, even if the REM content exceeds 0.0100%, the effect becomes saturated. Therefore, when REM is contained, the content thereof is 0.0100% or less. The preferred REM content is in the range of 0.0005 to 0.005%.

When the content of the optional element is less than the lower limit value, these elements do not impair the effect of the present invention. Therefore, when the content of the arbitrary element is less than the lower limit value, it is assumed that these elements are included as inevitable impurities.

Ceq ^IIW ≥ 0.65%

In the present invention, it is necessary to add a proper alloy component in order to secure strength and good toughness at a yield strength of 620 MPa or more at the plate thickness center portion of the high-tensile high strength steel sheet having a thickness of 100 mm or more. Specifically, it is necessary to adjust the content of the alloy element so that the carbon equivalent (Ceq ^IIW ) is 0.65% or more as shown in the following formula (1).

(1) Ceq ^IIW = C + Mn / 6 + Cu + Ni / 15 +

The symbol of each element in the formula represents the content (mass%) of each element. In addition, the content not contained is 0.

(C _L -C) / C _L × 100? 30 (2)

As will be described later, according to the present invention, a steel sheet having good characteristics can be obtained even when the slab surface is manufactured from a steel material casted in a range of 1 DEG C / s or less in cooling rate at solidification. In the present invention, in order to satisfy satisfactory toughness (vE-40? 70 J) on the surface of a steel sheet of a high strength and high strength steel sheet having a thickness of 100 mm or more, it is particularly preferable that the cooling rate at the time of solidification of the slab surface is 1 캜 / It is necessary to reduce micro-segregation. For that purpose, it is necessary to set the initial phase at the time of solidification to the delta phase, and the ratio of delta phase ((C _L -C) / C _L x 100) at the start of the generation of the? Phase to 30% or more.

_{C L = 0.2 - (- 0.1} × (0.2-Si) -0.03 × (1.1-Mn) -0.12 × (0.2-Cu) -0.11 × (3-Ni) + 0.025 × (1.2-Cr) +0.1 (0.5-Mo) +0.2 x (0.04-V) -0.05 x (0.06-Al)) (3)

In the above formula (3), the symbol of the element is the content (mass%) of each alloy component, and the value not including it is zero.

For the formation of the delta phase, it is necessary to define the range of the amount of C in accordance with components other than C such as Si or Mn. The coefficient was determined based on the calculation of the effect of the alloying element on the solubility limit (C _L ) of δ using thermodynamic calculation software "Thermo-Calc". For example, &quot; -0.1 &quot; of the coefficient &quot; Si &quot; of Si shows a decrease of 0.1% in the solid content of C at the delta phase when Si is contained by 1%. In order to secure the required delta phase ratio, It is necessary to perform the operation. In the present invention, as a component to be used as a base for calculating C _L , 0.12% of C, 0.2% of Si, 1.1% of Mn, 0.2% of Cu, 1.2% of Cr, 3% of Ni, 0.5% of Mo, V was changed to 0.04%, and Al was changed to 0.06%, and the change from the amount of solid solution C when the content of each alloy element was varied was calculated and used as a coefficient. Thus, by calculating a percentage of δ-phase C is added with respect to the employment of a C of: by a (C _L -C) / C × 100 _L by more than 30%, the proportion of the δ phase at the time of start generation γ 30% or more.

Further, in the present invention, it is preferable that the drawing in the thickness direction of the center of the plate measured by the method described in the embodiment is not less than 40% from the viewpoint of securing safety during use of the steel material.

&Lt; Method for manufacturing high strength steel sheet with high strength and high strength >

Next, the manufacturing conditions of the present invention will be described. In the description, the temperature &quot; C &quot; means the temperature at the central portion of the plate thickness, excluding the quenching temperature in the case of quenching without cooling after rolling. The quenching temperature in the case of quenching without cooling after rolling is the steel sheet surface temperature. This is because the steel sheet temperature distribution in the sheet thickness direction becomes large at the time of rolling, and it is necessary to consider the temperature drop of the surface of the steel sheet. The temperature of the central portion of the plate thickness is obtained by simulation calculation or the like from the plate thickness, the surface temperature, and the cooling condition. For example, the plate thickness center temperature is obtained by calculating the temperature distribution in the plate thickness direction by using finite difference methods.

Steel material

The molten steel having the above composition is melted by a common method such as a converter, an electric furnace or a vacuum melting furnace, and is made into a steel material such as a slab or billet by a conventional casting method such as a continuous casting method and a roughing method. The cooling rate at the time of solidification at this time is determined by a simulation calculation by direct measurement using a thermocouple or the like and heat transfer calculation. As described above, in the present invention, it is preferable to use, as the steel material, those produced under the conditions that the cooling rate at the time of solidification of the surface is 1 占 폚 / s or less.

In addition, when there is a constraint such as a load on a forging machine and a rolling mill, the plate thickness of the work may be reduced by performing crushing rolling.

Hot forging conditions of steel material

The cast bloom or steel bloom having the above composition is heated to 1200 to 1350 占 폚. If the reheating temperature is less than 1200 ° C, the load for securing a cumulative reduction in the amount of predetermined hot working is increased, and a sufficient amount of reduction can not be ensured, and if necessary, And the production efficiency is lowered. Therefore, the reheating temperature should be 1200 ° C or higher. In addition, when the addition amount of the alloy element is high, such as the present steel having a carbon equivalent of 0.65% or more, casting defects such as center porosity and porous shrinkage cavity in the steel material remarkably converge. It is necessary to set the cumulative rolling reduction to 25% or more in order to make them harmless by squeezing them. On the other hand, when the reheating temperature exceeds 1350 DEG C, excessive energy is consumed, surface scratches are liable to be generated due to scale at the time of heating, and the repair load after hot forging is increased, Lt; / RTI &gt;

Crushing condition of steel material

The cast steel or the steel having the above composition is heated to 1200 to 1350 캜. If the reheating temperature is less than 1200 ° C, the load for securing a cumulative reduction in the amount of predetermined hot working is increased, and a sufficient amount of reduction can not be ensured, and if necessary, And the production efficiency is lowered. Therefore, the reheating temperature should be 1200 ° C or higher. In order to obtain the effects of the present invention by squeezing and making harmless the casting defects, the cumulative rolling reduction amount may be set to 30% or more. However, from the viewpoint of excellent drawing RA, desirable. On the other hand, when the reheating temperature exceeds 1350 DEG C, excessive energy is consumed, surface scratches are liable to occur due to scale at the time of heating, and the maintenance load after hot forging is increased.

Reheating of steel after forging or after crushing

The steel material after forging is heated to a temperature not lower than the Ac3 transformation point and not higher than 1200 ° C in order to uniformize the steel into one phase of the austenite structure. The heating temperature is preferably not lower than 1000 ° C and not higher than 1200 ° C.

The Ac3 transformation point uses a value calculated by the following equation (4).

Ac3 = 937.2-476.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo + 124.8V + 136.3Ti + 198.4Al + 3315B (4)

The symbol of each element in the formula (4) represents the content (mass%) of each alloy element.

Hot rolling condition

The steel material is processed to a desired plate thickness by hot rolling. In order to ensure the characteristics of the central portion of the thickness of the steel sheet having a thickness of 100 mm or more in order to sufficiently exhibit the effect of adjusting and refining the spherical γ-particle diameter by the heat treatment, Composition is needed. Concretely, when the accumulated rolling reduction amount in rolling is set to 40% or more, it is possible to achieve sizing in the rolling step even in the center of the plate thickness where recrystallization due to processing is hardly caused.

Heat treatment condition

In order to obtain strength and toughness at the center of the plate thickness, the steel sheet is quenched after hot rolling (for example, air cooling) or quenched at a temperature of Ar3 point or higher and 350 deg. C or lower without cooling after hot rolling. When it is cold, it is reheated from Ac3 point to 1050 deg. C and quenched until it becomes 350 deg. C or lower at a temperature of Ac3 point or higher. The reason why the reheating temperature is set to 1050 占 폚 or below is that when a steel structure is manufactured by welding using a high strength and high strength steel sheet by a coarsening of an austenite lips at a high temperature reheating exceeding 1050 占 폚, Is remarkably lowered. The reason why the reheating temperature is made higher than the Ac3 point is to make the entire steel plate an austenite structure. Further, at a temperature lower than the Ac3 point, a heterogeneous structure composed of ferrite and austenite is formed and necessary characteristics are not obtained. Therefore, the quenching temperature is set to Ac3 or higher. In the case of quenching without cooling, quenching temperature is set to Ar3 or higher because quenching is performed from a single phase of austenite. In addition, the quenching stop temperature is set to a lower temperature of 350 DEG C or lower or Ar3 point or lower in order to reliably obtain the structure after the transformation in the entire steel sheet. That is, the stop temperature needs to be not more than Ar3 point and not more than 350 deg.

The Ar3 transformation point uses a value calculated by the following formula (5).

Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (5)

The symbol of each element in the formula (5) represents the content (mass%) of each alloy element.

The quenching method is generally carried out by water cooling, but the cooling rate is preferably as fast as possible. For this reason, the cooling method may be other than water cooling, and for example, there is a method such as gas cooling.

Tempering conditions

The reason for tempering at 450 to 700 占 폚 after quenching is as follows. Below 450 캜, the effect of removing residual stress is small. On the other hand, when the steel structure is manufactured by welding by using a high-strength steel sheet having a high hardness and a high tensile strength, various carbides are precipitated at a temperature exceeding 700 DEG C, the structure of the base metal becomes coarse, .

In the industrial art, there is a case where it is repeatedly quenched for the purpose of strengthening the steel. In the present invention, it is also possible to perform repeated quenching. However, at the time of final quenching, it is necessary to quench after heating from Ac 3 point to 1050 캜 until it becomes 350 캜 or less, and then tempering at 450 to 700 캜.

Example

Nos. 1 to 30 shown in Table 1 were subjected to hot forging (other than Samples Nos. 5, 6 and 41) or crushing rolling (Sample Nos. 5, 6, and 41). Thereafter, steel sheets having the thicknesses shown in Table 2 were formed by hot rolling, and thereafter water quenching and tempering treatments were performed to produce steel sheets of Sample Nos. 1 to 38, Of the test. In this example, the reheating temperature is the quenching temperature in the case of reheating quenching.

The δ phase ratio is a value calculated by the formula (2) using the value of C _L obtained from the formula (3) and the value of the amount of the base material for each base metal component.

The cooling rate at the time of solidification at the time of manufacturing the steel material is a value calculated by heat transfer calculation based on data obtained by measuring the temperature of the mold surface by a radiation thermometer.

Tensile test

(YS) and tensile strength (TS) were measured by taking round bar tensile test specimens (? 12.5 mm, GL 50 mm) from the center of the plate thickness of each steel sheet in the direction perpendicular to the rolling direction.

Charpy Impact Test

Three 2 mm V notch Charpy test pieces were taken from the steel plate surface and the plate thickness center of each steel sheet in the rolling direction in the longitudinal direction. Each test piece was subjected to a Charpy impact test at -40 ° C at a test temperature of -40 ° C The absorbed energy was measured, and their average values were obtained (the mean values of the test pieces at the center of the plate thickness and the average values of the test pieces of the surface were obtained, respectively).

Tensile test in the thickness direction

A rod tensile test specimen (Φ 10 mm) rounded in the thickness direction was taken from a region including the central portion of the plate thickness of each steel sheet, and the drawing (RA) was measured. Drawing is a percentage of the minimum cross-sectional area after the breakage of the specimen and the difference between the original cross-sectional area and the fabric area.

The test results are shown in Table 2. From these results, it can be seen that the steel sheets (Sample Nos. 1 to 21 and 41) of Inventive Examples in which the composition of the steel composition is suitable for the present invention are all those having a YS of 620 MPa or more, a TS of 720 MPa or more, It is understood that the toughness (vE-40) at the center of the plate thickness is 70 J or more and the strength and toughness of the base material are excellent. Further, from the comparison of Nos. 5 and 6 and No. 41, it was confirmed that the drawing RA also becomes good when the collapse condition satisfies a specific condition.

On the other hand, the steel sheets (Sample Nos. 22 to 32) of comparative examples deviating from the composition of the present invention had a YS of less than 620 MPa, a TS of less than 720 MPa and a toughness (vE-40) of less than 70 J And the characteristics are inferior.

In addition, as shown in the samples Nos. 33 to 40, even if the composition of the steel is a steel sheet suitable for the present invention, the production conditions are the same as those of the present invention (No. 41 is 30% Of the YS, TS, and toughness (vE-40) is inferior to the case of not satisfying the conditions of the present invention.

Claims (9)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.08 to 0.20% of C, 0.40% or less of Si, 0.5 to 5.0% of Mn, 0.010% or less of P, : 0.010 to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, satisfying the relations of the formulas (1) and (2), the balance being Fe and inevitable impurities,
The toughness (vE-40) on the surface of the steel sheet is 70 J or more,
The plate thickness is 100 mm or more,
The reduction in area in the thickness direction of the center of the plate thickness is 40% or more,
Wherein the high strength steel sheet is a high strength high strength steel sheet produced from a steel material solidified at a cooling rate of 1 캜 / s or less at the time of solidification of the surface.
(1) Ceq ^IIW = C + Mn / 6 + Cu + Ni / 15 +
(C _L -C) / C _L × 100? 30 (2)
Here, C _L is defined by the following equation.
_{C L = 0.2 - (- 0.1} × (0.2-Si) -0.03 × (1.1-Mn) -0.12 × (0.2-Cu) -0.11 × (3-Ni) + 0.025 × (1.2-Cr) +0.1 (0.5-Mo) +0.2 x (0.04-V) -0.05 x (0.06-Al)) (3)
However, in the above formula, the symbol of the element is the content (mass%) of each alloy component, and the value not including it is zero. The method according to claim 1,
Further comprising, in% by mass, at least one of the following group A and group B:
A group: one or more of Cu: not more than 0.50%, Mo: not more than 1.50%, V: not more than 0.400%, Nb: not more than 0.100%, and Ti: 0.005%
B: 0.0001 to 0.0050% of Mg, 0.01 to 0.20% of Ta, 0.005 to 0.1% of Zr, 0.001 to 0.01% of Y, 0.0030% or less of B, 0.0005 to 0.0050% of Ca, 0.0005 to 0.0100% of Ca, One or more of The method according to claim 1,
High strength high strength steel plate with yield strength of 620 MPa or more. 3. The method of claim 2,
High strength high strength steel plate with yield strength of 620 MPa or more. A method for producing a high strength and high strength steel sheet according to any one of claims 1 to 4,
The steel material which has solidified under the condition that the cooling rate at the time of solidification of the surface is 1 占 폚 / s or less is heated to 1200 to 1350 占 폚, and hot forging is carried out at 25% Or less and then reheated to a temperature between Ac3 point and 1050 占 폚 and heated to a temperature of 350 占 폚 or lower or a temperature lower than Ar3 And rapidly tempering and tempering at a temperature of 450 to 700 占 폚. A method for producing a high strength and high strength steel sheet according to any one of claims 1 to 4,
The steel material which has solidified under the condition that the cooling rate at the time of solidification of the surface is 1 占 폚 / s or less is heated to 1200 to 1350 占 폚, and hot forging is carried out at 25% And the hot rolled steel sheet is subjected to hot rolling with a cumulative rolling reduction of 40% or more. The steel sheet is quenched to a lower temperature of 350 DEG C or lower or Ar3 point or lower at a temperature of Ar3 point or higher and tempered at a temperature of 450 DEG C to 700 DEG C Wherein the high strength steel sheet is manufactured by a method comprising the steps of: A method for producing a high strength and high strength steel sheet according to any one of claims 1 to 4,
The steel material which has solidified under the condition that the cooling rate at the time of solidification of the surface is not more than 1 占 폚 / s is heated to 1200 to 1350 占 폚 and crushed to a cumulative rolling reduction of not less than 40% Or less and then reheated to a temperature between Ac3 point and 1050 占 폚 and heated to a temperature of 350 占 폚 or lower or a temperature lower than Ar3 And rapidly tempering and tempering at a temperature of 450 to 700 占 폚. A method for producing a high strength and high strength steel sheet according to any one of claims 1 to 4,
The steel material which has solidified under the condition that the cooling rate at the time of solidification of the surface is not more than 1 占 폚 / s is heated to 1200 to 1350 占 폚 and crushed to a cumulative rolling reduction of not less than 40% And the hot rolled steel sheet is subjected to hot rolling with a cumulative rolling reduction of 40% or more. The steel sheet is quenched to a lower temperature of 350 DEG C or lower or Ar3 point or lower at a temperature of Ar3 point or higher and tempered at a temperature of 450 DEG C to 700 DEG C Wherein the high strength steel sheet is manufactured by a method comprising the steps of: delete