JP7542326B2 - Steel plate and its manufacturing method - Google Patents

Description

This disclosure relates to a steel sheet and a method for manufacturing the same.

Steel plates used in steel structures such as buildings, bridges, storage tanks, and line pipes are required to meet various performance requirements (sufficient workability, strength, uniform elongation, toughness, and low yield ratio) and to be able to be manufactured inexpensively.

Patent Document 1 discloses a steel plate for welded structures in which the hardness and precipitates of the hard phase (i.e., one or more of tempered bainite and tempered martensite) and the soft phase (i.e., a structure in which bainite is tempered at high temperature and no cementite is present) are controlled.

Patent Document 2 discloses a welded steel pipe for high-strength line pipe in which the area ratio and location of a second phase, which is island martensite, are controlled in a matrix mainly composed of ferrite or bainite.

JP 2015-163730 A JP 2012-31509 A

However, the manufacturing method for the steel plate described in Patent Document 1 and the steel pipe described in Patent Document 2 requires that the steel be cooled after hot rolling and then heated again, which increases manufacturing costs.

The embodiments of the present invention have been made in consideration of these circumstances, and one of the objectives is to provide a steel sheet and a manufacturing method thereof that does not require cooling after hot rolling and then reheating, and that satisfies various properties (sufficient workability, strength, uniform elongation, toughness, and low yield ratio).

Aspect 1 of the present invention is
C: 0.08 to 0.16% by mass,
Si: 0.25 to 0.50% by mass,
Mn: 1.1 to 1.6% by mass,
P: more than 0% by mass and not more than 0.030% by mass,
S: more than 0% by mass and not more than 0.003% by mass,
Al: 0.020 to 0.080% by mass,
Ti: 0.008 to 0.020% by mass,
N: 0.0030 to 0.0070% by mass,
Ca: 0.0005 to 0.0050 mass%, and the balance: iron and inevitable impurities;
the sum of the area fractions of ferrite and bainite is 85% or more, and the area fraction of the bainite is 20 to 80%,
The hardness of the bainite is 220.0 to 380.0 HV,
The steel plate has a hardness distribution P _HV represented by the following formula (1) of 5.0 HV or less and a plate thickness of 12 mm or more.

P _HV = μ (HV _{cross-section average} ) - min (HV _{cross-section average} ) ... (1)

Here, the HV _{cross-sectional average} is the average hardness in the plate thickness direction at any position of the steel plate, and is calculated according to the following formula (2).

HV _{cross-sectional average} = ( _{1 mm below the HV surface} × 3 + HV _t/4 × (t/2-3) + HV _3t/4 × (t/2-3) + _{1 mm below the HV back surface} × 3) / t ... (2)

In the above formula (2), t is the thickness (mm) of the steel plate, HV _{1 mm below surface} is the hardness (HV) at a position 1 mm below the surface of the steel plate, HV _t/4 is the hardness (HV) at a position t/4 mm below the surface of the steel plate, HV _3t/4 is the hardness (HV) at a position 3t/4 mm below the surface of the steel plate, and HV _{1 mm below back surface} is the hardness (HV) at a position 1 mm below the back surface of the steel plate.
In the above formula (1), μ (HV _{cross-sectional average} ) is the average value of the HV cross-sectional _{average measured at 5 mm intervals in any 190 mm section excluding 50 mm at the width end section in a direction perpendicular to the rolling direction and thickness direction of the steel plate} , and min (HV _{cross-sectional average} ) is the minimum value of the HV _{cross-sectional average} measured at 5 mm intervals in the 190 mm section.

Aspect 2 of the present invention is
Cu: more than 0% by mass and not more than 0.50% by mass,
Ni: more than 0% by mass and not more than 0.50% by mass,
Cr: more than 0% by mass and not more than 0.50% by mass,
The steel sheet according to embodiment 1, further comprising one or more selected from the group consisting of Mo: more than 0 mass % and 0.50 mass % or less, and V: more than 0 mass % and 0.10 mass % or less.

Aspect 3 of the present invention is
The steel sheet according to embodiment 1 or 2, further comprising one or more selected from the group consisting of Nb: more than 0 mass % and 0.030 mass % or less, and B: more than 0 mass % and 0.002 mass % or less.

Aspect 4 of the present invention is
A step of preparing a steel having a chemical composition according to any one of aspects 1 to 3;
A step of performing hot rolling with a rolling end temperature of 830°C to 940°C;
and cooling the material from a cooling start temperature of Ar3 point (°C) -70°C or higher to a cooling stop temperature of 400°C to 600°C at an average cooling rate of 8 to 20°C/sec.
The method for producing a steel sheet according to any one of Aspects 1 to 3, wherein _PCR represented by the following formula (3) is 35 or less.

P _CR = ([C]+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/15+[Nb]+Sol.B×400)×CR _700-500 ...(3)

In the above formula (3), [C], [Mn], [Ni], [Cr], [Mo], [V] and [Nb] respectively represent the contents of C, Mn, Ni, Cr, Mo, V and Nb expressed in mass%, CR _700-500 is the average cooling rate on the surface of the steel sheet from 700°C to 500°C, and Sol. B is expressed by the following formula (4).

Sol. B=[B]-([N]-[Ti]/3.4)×10.8/14...(4)

In the above formula (4), [B], [N], and [Ti] represent the contents of B, N, and Ti, respectively, expressed in mass %. However, when the right side of the above formula (4) is less than 0, Sol. B = 0.

In an embodiment of the present invention, it is possible to provide a steel sheet and a manufacturing method thereof that does not require cooling after hot rolling and then reheating, and that satisfies various properties (sufficient workability, strength, uniform elongation properties, toughness, and low yield ratio).

FIG. 1 is a schematic diagram showing positions at which hardness of a steel plate is measured when determining the hardness distribution P _HV . FIG. 2A is a schematic diagram showing the change in hardness in the thickness direction in a cross section of a steel plate. FIG. 2B is a diagram illustrating a method for calculating the average hardness HV _{cross-sectional average} in the plate thickness direction according to an embodiment of the present invention. FIG. 3 is a graph showing the relationship between hardness distribution P _HV and TS×UEL in the examples. FIG. 4 is a graph showing the relationship between _PCR and hardness distribution P _HV in the examples.

The inventors of the present application conducted research from various angles to realize a steel sheet that does not require cooling after hot rolling and then reheating, and that satisfies various properties (sufficient workability, strength, uniform elongation characteristics, toughness, and low yield ratio).

As a result, it was found that a steel sheet that does not need to be cooled after hot rolling and then heated again and that satisfies various properties can be provided by setting a predetermined chemical composition, setting the sum of the area fractions of ferrite and bainite to 85% or more, the area fraction of bainite to 20 to 80%, setting the hardness of bainite to 220.0 to 380.0 HV, and setting the hardness distribution P _HV in the direction perpendicular to the rolling direction of the steel sheet surface to 5.0 HV or less. Furthermore, it was also found at the same time that in the manufacturing method of the steel sheet, the hardness distribution P _HV correlates with _PCR , which is a parameter derived from the chemical composition and the cooling rate.

The following provides details of each requirement stipulated by the embodiment of the present invention.

<1. Chemical composition>
The steel plate according to the embodiment of the present invention has a composition of C: 0.08 to 0.16 mass%, Si: 0.25 to 0.50 mass%, Mn: 1.1 to 1.6 mass%, P: 0 mass%. % or more and 0.030 mass% or less, S: more than 0 mass% and 0.003 mass% or less, Al: 0.020 to 0.080 mass%, Ti: 0.008 to 0.020 mass%, N: 0. 0.0030 to 0.0070 mass%, Ca: 0.0005 to 0.0050 mass%, and the balance: iron and unavoidable impurities.
Each element will be described in detail below.

(C: 0.08-0.16% by mass)
C is an element that increases the strength of a steel sheet. If the C content is less than 0.08 mass%, the bainite fraction decreases, making it difficult to ensure the necessary strength. The content is set to 0.08% by mass or more, preferably 0.09% by mass or more, more preferably 0.10% by mass or more, and further preferably 0.11% by mass or more.
On the other hand, if the C content exceeds 0.16 mass%, the HAZ toughness and weld cracking resistance deteriorate. Therefore, the C content is set to 0.16 mass% or less. The C content is preferably 0.155 mass% or less. , and more preferably 0.15 mass % or less.

(Si: 0.25-0.50% by mass)
Silicon is an element effective as a deoxidizer and for improving the strength of the base metal. In order to exert these effects, it is necessary to contain 0.25 mass% or more of silicon, preferably 0.30 mass% or more. It is.
On the other hand, if the Si content is excessive, the toughness of the base material, the HAZ toughness, and the weldability are deteriorated. Therefore, the Si content is set to 0.50 mass% or less, preferably 0.45 mass% or less, and more preferably 0.50 mass% or less. . 40% by mass or less.

(Mn: 1.1-1.6% by mass)
Mn is an element that is effective in improving the strength of the base metal and the welded portion. In order to achieve this effect, the Mn content must be 1.1 mass% or more, preferably 1.2 mass% or more. and more preferably 1.3 mass % or more.
On the other hand, if the Mn content is excessive, MnS is generated, which deteriorates the toughness of the base material and the HAZ toughness. Therefore, the Mn content is set to 1.6 mass% or less, preferably 1.5 mass% or less, and more preferably is 1.4 mass % or less.

(P: more than 0% by mass and 0.030% by mass or less)
P is an element that is inevitably contained in steel materials, and if the P content exceeds 0.030 mass%, the toughness of the base material and/or the HAZ portion is significantly deteriorated. Therefore, the P content is set to 0.030 mass% or less. The content is preferably 0.020% by mass or less, and more preferably 0.010% by mass or less.
Since it is industrially difficult to set the lower limit of the P content to 0 mass%, the lower limit may be more than 0 mass% and may be 0.002 mass% or more.

(S: more than 0% by mass and 0.003% by mass or less)
If the S content is too high, a large amount of MnS is generated, which deteriorates the toughness of the base material and/or the HAZ. Therefore, the S content is set to 0.003 mass% or less, and preferably 0.002 mass% or less. be.
The lower limit of the S content is set to more than 0 mass %, since it is industrially difficult to set it to 0%.

(Al: 0.020 to 0.080% by mass)
Al is a strong deoxidizing element, and in order to exhibit this effect, the Al content must be 0.020 mass % or more, and preferably 0.025 mass % or more.
On the other hand, if the Al content is excessive, Al oxides are generated in clusters, which deteriorates the toughness of the base material and/or the HAZ. Therefore, the Al content is set to 0.080 mass% or less. Preferably, It is 0.060 mass % or less, and more preferably 0.050 mass % or less.

(Ti: 0.008 to 0.020% by mass)
Ti is an effective element for improving the toughness of the HAZ because it precipitates in the steel as TiN and prevents the coarsening of austenite grains in the HAZ during welding and promotes ferrite transformation. It is particularly effective in ensuring HAZ toughness in high heat input welding. In order to achieve these effects, it is necessary to contain 0.008 mass% or more of Ti. More preferably, it is 0.009 mass% or more. More preferably, it is 0.010 mass % or more.
On the other hand, if the Ti content is excessive, solute Ti and TiC are precipitated, and the toughness of the base material and the HAZ portion is deteriorated. Therefore, the Ti content is set to 0.020 mass% or less. Preferably, it is set to 0.017 mass% or less. % by mass or less, more preferably 0.015% by mass or less.

(N: 0.0030 to 0.0070% by mass)
N is an element that precipitates as TiN in the steel structure, suppresses the coarsening of austenite grains in the HAZ, and promotes ferrite transformation to improve the toughness of the HAZ. It is effective in ensuring toughness. In order to exert these effects, it is necessary to contain N in an amount of 0.030 mass% or more, preferably 0.0035 mass% or more, and more preferably 0.0040 mass% or more. % or more.
On the other hand, if the N content is excessive, the HAZ toughness is deteriorated due to the presence of solute N. Therefore, the N content is set to 0.0070 mass% or less, preferably 0.0065 mass% or less. More preferably, it is 0.0060 mass % or less.

(Ca: 0.0005 to 0.0050% by mass)
Ca has the effect of controlling the morphology of sulfides, and has the effect of suppressing the formation of MnS by forming CaS. In order to exert this effect, the Ca content is set to 0.0005 mass% or more. It is necessary to add 0.0010% by mass or more, and more preferably 0.0015% by mass or more.
On the other hand, if the Ca content is excessive, the inclusions become coarse and the base material toughness and/or HAZ toughness deteriorates. Therefore, the Ca content is set to 0.0050 mass% or less, and preferably 0.0040 mass%. It is preferably 0.0030 mass % or less, and more preferably 0.0030 mass % or less.

The basic components are as described above, and in one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, the inclusion of elements brought in due to the circumstances of raw materials, materials, manufacturing facilities, etc. is permitted.
In addition, for example, there are elements such as P and S, whose content is usually the lower the better and therefore they are unavoidable impurities, but whose composition ranges are separately defined as above. Therefore, in this specification, when referring to "unavoidable impurities" that constitute the balance, this is a concept that excludes elements whose composition ranges are separately defined.

Furthermore, the steel sheet according to the embodiment of the present invention may selectively contain the following optional elements as necessary, and the properties of the steel sheet are further improved depending on the components contained.

(One or more selected from the group consisting of Cu: more than 0 mass% and 0.50 mass% or less, Ni: more than 0 mass% and 0.50 mass% or less, Cr: more than 0 mass% and 0.50 mass% or less, Mo: more than 0 mass% and 0.50 mass% or less, and V: more than 0 mass% and 0.10 mass% or less)
Cu, Ni, and Cr are all elements that are effective in improving hardenability and increasing strength without significantly affecting HAZ toughness. In order to achieve this effect, it is preferable that Cu is contained in an amount of more than 0 mass%, more preferably 0.01 mass% or more, even more preferably 0.05 mass% or more, and even more preferably 0.10 mass% or more. The same applies to Ni and Cr as to Cu.
On the other hand, since an excessive Cu content deteriorates toughness, the Cu content is preferably 0.50 mass% or less, more preferably 0.40 mass% or less, and further preferably 0.30 mass% or less. The same applies to Ni and Cr.

Mo is an element that improves hardenability and easily precipitates carbides, and is an element that is effective in increasing strength. In order to exert these effects, the Mo content is preferably 0 mass%, more preferably 0.01 mass% or more, even more preferably 0.05 mass% or more, and even more preferably 0.10 mass% or more.
On the other hand, if the Mo content is excessive, the HAZ toughness and weldability deteriorate, so the Mo content is preferably 0.50 mass% or less, more preferably 0.40 mass% or less, and further preferably 0.30 mass% or less.

V is an element that is effective in improving strength by forming carbides and/or nitrides. In order to exert this effect, it is preferable for V to be contained in an amount of more than 0 mass%, more preferably 0.003 mass% or more, and even more preferably 0.010 mass% or more.
On the other hand, an excessive V content deteriorates the weldability and the base metal toughness, so the V content is preferably 0.1 mass % or less, more preferably 0.08 mass % or less, and further preferably 0.06 mass % or less.

(One or more selected from the group consisting of Nb: more than 0 mass% and 0.030 mass% or less, and B: more than 0 mass% and 0.002 mass% or less)
Nb is an element effective in increasing strength and base metal toughness without deteriorating weldability. In order to exert this effect, the Nb content is preferably more than 0 mass%, more preferably 0.002 mass% or more, even more preferably 0.010 mass% or more, and even more preferably 0.020 mass% or more.
On the other hand, if the Nb content is excessive, the crystal grains become significantly finer, making it difficult to achieve low YR characteristics and deteriorating the toughness of the HAZ. Therefore, the Nb content is preferably 0.030 mass%, more preferably 0.025 mass% or less, and even more preferably 0.020 mass% or less.

B enhances hardenability and increases the strength of the base material and the weld, and during welding, when the heated HAZ cools, it combines with N to precipitate BN, promoting ferrite transformation from within the austenite grains and improving HAZ toughness. To achieve these effects, the B content is preferably more than 0 mass%, more preferably 0.0002 mass% or more, even more preferably 0.0005 mass% or more, and even more preferably 0.0010 mass% or more.
On the other hand, if the B content is excessive, the toughness of the base material and the HAZ portion deteriorates, and also the weldability deteriorates. Therefore, the B content is preferably 0.002 mass% or less, and more preferably 0.015 mass% or less.

(others)
In the chemical composition of the steel plate according to the embodiment of the present invention, the carbon equivalent (Ceq) is preferably 0.44 or less. By making Ceq 0.44 or less, deterioration of weldability can be suppressed, and it is preferably 0.42 or less, more preferably 0.40 or less.
Note that Ceq is calculated as shown in the following formula (5).

Ceq=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/15...(5)

In the above formula (5), [C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] respectively represent the contents of C, Mn, Si, Ni, Cr, Mo and V expressed in mass %.

<2. Metal structure>
In the steel plate according to the embodiment of the present invention, the sum of the area fractions of ferrite and bainite is 85% or more, the area fraction of bainite is 20 to 80%, and the hardness of bainite is 220.0 to 380.0 HV.

To obtain a low yield ratio, the sum of the area fractions of ferrite and bainite must be 85% or more. Structures other than ferrite and bainite may include pearlite, martensite, and retained austenite. The sum of the area fractions of ferrite and bainite is preferably 90% or more, more preferably 95% or more, and most preferably 100%, i.e., the metal structure of the steel plate consists of ferrite and bainite.

In order to obtain a desired strength and a low yield ratio, the area fraction of bainite must be 20 to 80%. If the area fraction of bainite is less than 20%, the strength of the steel plate is insufficient and the yield ratio is high. It is preferably 25% or more, and more preferably 30% or more.
On the other hand, if it exceeds 80%, the strength becomes too high and the workability deteriorates.The preferred range is 75% or less, and more preferably 70% or less.

In order to obtain strength and a low yield ratio, the hardness of the bainite must be 220.0 to 380.0 HV. If the bainite hardness is less than 220.0 HV, the strength of the steel plate is insufficient. It is preferably 230.0 HV or more, and more preferably 240.0 HV or more.
On the other hand, if the bainite hardness exceeds 380.0 HV, the strength may become too high and the toughness of the base material may deteriorate. The bainite hardness is preferably 360.0 HV or less, and more preferably 340.0 HV or less.

<3. Hardness distribution>
The inventors have found that in order to obtain sufficient uniform elongation properties, it is necessary to reduce the distribution of the average hardness HV _{cross-sectional average} in the sheet thickness direction (i.e., the average value - the minimum value). In other words, if the variation in the HV _{cross-sectional average} is large, the deformation of the part with a low HV _{cross-sectional average} becomes dominant, so that the hard part with the HV _{cross-sectional average} can hardly elongate, and the uniform elongation properties are reduced.
FIG. 1 is a schematic diagram showing the hardness measurement positions of a steel sheet when the hardness distribution P _HV is obtained. As shown in FIG. 1, an arbitrary 190 mm section 1B is selected in the X direction perpendicular to the rolling direction Y and the sheet thickness direction Z of the steel sheet 1. In the section 1B, the average hardness HV _{cross-sectional average} in the sheet thickness direction is measured at intervals of 5 mm in the X direction. As described later, the HV _{cross-sectional average} is obtained by measuring the hardness at a position D1 1 mm from the surface 1A of the steel sheet 1 in the Z direction, a position D2 of t (sheet thickness)/4 mm, a position D3 of 3t/4 mm, and a position D4 of t-1 mm (or a position 1 mm from the back surface) (respectively referred to as HV _{surface 1 mm} , HV _t/4 , HV _3t/4 , and HV _{back surface 1 mm} ). In FIG. 1, the hardness measurement positions are indicated by black circles ●.

Fig. 2A is a schematic diagram showing the change in hardness in the thickness direction in a certain cross section. As shown in Fig. 2A, the hardness in the thickness direction often changes in a downward convex curve.
2B is a diagram for explaining a method for calculating the average hardness HV _{cross-sectional average} in the plate thickness direction according to an embodiment of the present invention. In the embodiment of the present invention, it is assumed that the following relationships (A) to (D) hold between FIG. 2A and FIG. 2B.
(A) The integral value of the hardness (area S1) in the region R1 0 to 3 mm below the surface shown in FIG. 2A is approximately equal to the area (S1') represented by the position hardness HV _{1 mm below the surface} shown in FIG. 2B (i.e., the hardness at point P1 where the line 1 mm in the plate thickness direction from the surface 1A intersects with the hardness curve shown by the dashed dotted line) × 3 mm.
(a) The integrated value (area S2) of the hardness in the region R2 from 3 mm to t (plate thickness)/2 mm below the surface shown in FIG. 2A is approximately equal to the hardness HVt _/4 at a position t/4 mm below the surface shown in FIG. 2B (i.e., the hardness at point P2 where the line at position t/4 mm in the plate thickness direction from surface 1A intersects with the hardness curve shown by the dashed dotted line) × (t/2-3) mm area S2'.
(c) The integrated value (area S3) of the hardness in region R3 t/2 to t-3 mm below the surface (or 3 mm to t/2 mm below the back surface) shown in Figure 2A is approximately equal to the hardness HV _3t/4 at a position 3t/4 mm below the surface shown in Figure 2B (i.e., the hardness at point P3 where the line at position 3t/4 mm in the plate thickness direction from surface 1A intersects with the hardness curve shown by the dashed dotted line) x (t/2-3) mm area S3'.
(e) The integral value (area S4) of the hardness in the region R4 from 0 mm to 3 mm below the back surface shown in FIG. 2A is approximately equal to the hardness at a position 1 mm below the back _{surface shown in FIG. 2B (HV1 mm below the back surface} (i.e., the hardness at point P4 where the line at position t-1 mm in the plate thickness direction from the front surface 1A intersects with the hardness curve shown by the dashed dotted line) × 3 mm area S4'.
From the above, in an embodiment of the present invention, the average hardness HV _{cross-sectional average} in the plate thickness direction is calculated as shown in the following formula (2).

HV _{cross-sectional average} = ( _{1 mm below the HV surface} × 3 + HV _t/4 × (t/2-3) + HV _3t/4 × (t/2-3) + _{1 mm below the HV back surface} × 3) / t ... (2)

In addition, although FIG. 2A and FIG. 2B exemplarily show the case where HV _{surface 1 mm below} = HV _{back surface 1 mm below} and HV _t/4 = HV _3t/4 , these may be different.

In the HV _{cross-sectional average} obtained as described above, the average value μ (HV _{cross-sectional average} ) and the minimum value min (HV _{cross-sectional average} ) in a 190 mm section are obtained. The hardness distribution P _HV is calculated as shown in the following formula (1) using the average value μ (HV _{cross-sectional average} ) and the minimum value min (HV _{cross-sectional average} ).

P _HV = μ (HV _{cross-section average} ) - min (HV _{cross-section average} ) ... (1)

The inventors have found that uniform elongation properties, particularly the product of tensile strength (TS) and uniform elongation (UEL), correlate with P _HV , and that a steel sheet with a high TS×UEL can be obtained by setting P _HV to 5.0 HV or less. If the P HV exceeds 5.0 HV, a sufficient TS×UEL cannot be obtained.

The steel plate according to the embodiment of the present invention has a plate thickness of 12 mm or more, assuming its use in steel structures such as buildings, bridges, storage tanks, and line pipes. It is preferably 15 mm or more, and more preferably 19 mm or more.

The steel plate according to the embodiment of the present invention satisfies various properties (sufficient workability, strength, uniform elongation, toughness, and low yield ratio).
In order to exhibit sufficient workability, it is important that the strength is not too high, and the steel plate according to the embodiment of the present invention satisfies a yield strength (YS) of 500 MPa or less and a tensile strength (TS) of 670 MPa or less. Preferably, the YS is 450 MPa or less and the TS is 600 MPa or less.
The steel plate according to the embodiment of the present invention has high strength, specifically, a yield strength (YS) of 325 MPa or more and a tensile strength (TS) of 500 MPa or more. Preferably, the YS is 350 MPa or more and the TS is 550 MPa or more.
The steel sheet according to the embodiment of the present invention exhibits sufficient uniform elongation properties, specifically, a uniform elongation (UEL) of 5% or more and a TS×UEL of 4500 (MPa·%) or more. The UEL is preferably 10% or more, more preferably 11% or more. In addition, the TS×UEL is preferably 6000 MPa·% or more, more preferably 7000 MPa·% or more.
The steel plate according to the embodiment of the present invention has high toughness, specifically, a fracture appearance transition temperature (vTrs) of less than −30° C., preferably −45° C. or lower, more preferably −55° C. or lower.
The steel sheet according to the embodiment of the present invention has a low yield ratio, specifically, a yield ratio (YR) of 80% or less, preferably 78% or less.

<4. Manufacturing method>
The method for producing a steel sheet according to an embodiment of the present invention includes a step of performing hot rolling at a rolling end temperature of 830°C to 940°C, and a step of cooling from a cooling start temperature of Ar3 point (°C) -70°C or higher to a cooling stop temperature of 400°C to 600°C at an average cooling rate of 8 to 20°C/sec, and is a method for producing a steel sheet having _{a PCR} of 35 or less, as described below. As described above, reheating after cooling is not required to produce the steel sheet according to an embodiment of the present invention.

(Hot rolling process)
If the temperature at the end of hot rolling is low, the anisotropy of the structure occurs and the acoustic anisotropy is impaired. Therefore, the temperature at the end of hot rolling is set to 830° C. or higher, preferably 840° C. or higher, and more preferably 850° C. or higher.
On the other hand, from the viewpoint of grain refinement, the temperature during hot rolling should not be too high, and the temperature at the end of hot rolling is set to 940° C. or lower, preferably 930° C. or lower, and more preferably 920° C. or lower.
The temperature at the end of hot rolling refers to the surface temperature of the steel sheet immediately before the final hot rolling pass, and can be measured by a thermocouple or a radiation thermometer.

(Cooling step)
After the hot rolling is completed, cooling is started at a temperature equal to or higher than Ar3 point (°C) -70°C. If the cooling start temperature is set to less than Ar3 point (°C) -70°C, the bainite fraction may decrease. In addition, it is preferable that the cooling start temperature is set to the rolling end temperature or lower, since an additional heating step is not required.
The cooling start temperature refers to the surface temperature of the steel sheet at the start of cooling (i.e., immediately before the steel sheet is placed in the cooling device), and can be measured using a thermocouple or a radiation thermometer.
The Ar3 point (° C.) may be measured by a thermal expansion test or may be calculated according to the following formula (6).

Ar3 point (°C) = 910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]-0.35 (t-8)...(6)

In the above formula (6), [C], [Mn], [Cu], [Cr], [Ni], [Mo] and [V] represent the contents of C, Mn, Cu, Cr, Ni and Mo, respectively, expressed in mass%, and t represents the plate thickness.

The cooling stop temperature is set to 400 to 600°C, and the average cooling rate from the cooling start temperature to the cooling stop temperature must be set to 8 to 20°C/sec. If the average cooling rate is less than 8°C/sec, the bainite fraction decreases. It is preferably 9°C/sec or more, and more preferably 10°C/sec or more.
On the other hand, if the average cooling rate exceeds 20° C./sec, the bainite fraction becomes higher than necessary. The average cooling rate is preferably 18° C./sec or less, and more preferably 15° C./sec or less.
The cooling stop temperature means the temperature of the steel plate surface at the time when the reheating is completed after the cooling is completed. Since the surface temperature and the plate thickness center temperature converge to the average temperature in the plate thickness direction during the reheating, the surface temperature at the time when the reheating is completed after the cooling is completed may be considered to be synonymous with the average temperature in the plate thickness direction immediately after the cooling is stopped. The average temperature in the plate thickness direction may be obtained by measuring multiple points (at least 5 points) in the plate thickness direction at equal intervals using a thermocouple, or may be obtained from the steel plate surface temperature by a simulation taking into account the heat transfer characteristics. The average cooling rate can be obtained by dividing the difference between the cooling start temperature and the cooling stop temperature by the time required for cooling.

Even if the average cooling rate from the cooling start temperature to the cooling stop temperature is 8 to 20°C/sec, if the cooling stop temperature exceeds 600°C, the cooling will be completed before the bainite transformation progresses, resulting in a decrease in the bainite fraction. The cooling stop temperature is preferably 580°C or less, and more preferably 550°C or less.
On the other hand, even if the average cooling rate from the cooling start temperature to the cooling stop temperature is 8 to 20°C/sec, if the cooling stop temperature is less than 400°C, problems such as significant hardening of bainite and/or precipitation of harder martensite occur. The cooling stop temperature is preferably 430°C or higher, more preferably 450°C or higher.

In the manufacturing method of a steel sheet according to an embodiment of the present invention, it is necessary that the _PCR represented by the following formula (3) is 35 or less.

P _CR = ([C]+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/15+[Nb]+Sol.B×400)×CR _700-500 ...(3)

In the above formula (3), [C], [Mn], [Ni], [Cr], [Mo], [V] and [Nb] respectively represent the contents of C, Mn, Ni, Cr, Mo, V and Nb expressed in mass%, CR _700-500 is the average cooling rate on the surface of the steel sheet from 700°C to 500°C, and Sol. B is expressed by the following formula (4).

Sol. B=[B]-([N]-[Ti]/3.4)×10.8/14...(4)

In the above formula (4), [B], [N], and [Ti] represent the contents of B, N, and Ti, respectively, expressed in mass %. However, when the right side of the above formula (4) is less than 0, Sol. B = 0.

The cooling method in the cooling step may be, for example, water cooling.
When there is a large variation in the cooling rate within the steel plate during cooling, and when there is a large temperature deviation after cooling is stopped, the hardness distribution P _HV becomes large.
When a steel sheet is water-cooled, if a film boiling state in which a steam film exists between the cooling water and the steel sheet and a nucleate boiling state in which the cooling water and the steel sheet are in direct contact with each other occur simultaneously, particularly when the steel sheet surface is at 700 to 500°C, the heat extraction characteristics from the steel sheet surface will vary, resulting in variation in the cooling speed within the steel sheet (hereinafter, the state in which the film boiling state and the nucleate boiling state occur simultaneously will be referred to as a "transition boiling state"). In particular, when the amount of water used to cool the steel sheet is large, the steam film on the steel sheet surface is easily broken by the water pressure of the cooling water, resulting in a transition boiling state. As a result, the cooling speed within the steel sheet will vary and the temperature deviation after cooling is stopped will increase, resulting in a large hardness distribution P _HV .
Considering the above, the average cooling rate CR _700-500 from 700 to 500°C on the steel sheet surface is preferably 50°C/sec or less. More preferably, it is 40°C/sec or less. The temperature on the steel sheet surface may be measured using a thermocouple, or may be obtained by simulation taking into account the heat transfer coefficient and the like from the surface temperature immediately before cooling, the surface temperature at the time when the reheating is completed after cooling, and the time required for cooling. CR _700-500 can be obtained by dividing the difference (200°C) between 700°C and 500°C on the steel sheet surface by the time required for cooling.

Furthermore, when the value of ([C] + [Mn]/6 + [Ni]/40 + [Cr]/5 + [Mo]/4 + [V]/15 + [Nb] + Sol. B × 400) in the above formula (3) is large, the effect of the cooling rate on the hardness becomes sensitive, and even if the cooling rate variation within the steel sheet is small, hardness variation is likely to occur, resulting in a large hardness distribution P _HV .

As described above, the hardness distribution P _HV can be controlled by appropriately controlling the cooling rate and chemical composition of the steel sheet surface. Specifically, by making the P _CR represented by the above formula (3) 35 or less, the hardness distribution in the direction perpendicular to the rolling direction of the steel sheet surface can be reduced. It is preferably 30 or less, more preferably 25 or less, even more preferably 23 or less, and even more preferably 20 or less. On the other hand, from the viewpoint of ensuring strength, it is preferable to make the P _CR 3 or more.

There are no particular limitations on how long it takes to start cooling after the hot rolling process, and how long it takes to cool to room temperature after the cooling process, and it can be left to cool naturally, for example.

The following provides a more detailed explanation of the embodiments of the present invention, using examples. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the intent described above and below, and all such modifications are within the technical scope of the embodiments of the present invention.

A steel slab satisfying the chemical composition shown in steel type A in Table 1 was obtained by a conventional method. This steel slab was heated to 1100°C and hot-rolled, with the rolling end temperature being 849°C. Then, it was allowed to cool from the rolling end temperature to the cooling start temperature. Then, the cooling start temperature was 774°C, the cooling end temperature was 516°C, and water cooling was performed with an average cooling rate from 774°C to 516°C of 9.5°C/sec. The average cooling rate CR _700-500 from 700 to 500°C on the steel plate surface at that time was 17°C/sec, and the P _CR was calculated to be 6 according to the above formulas (3) and (4). Then, it was allowed to cool from 516°C to room temperature to obtain a steel plate of test No. 1. The plate thickness was 20 mm.
The rolling end temperature was the steel sheet surface temperature immediately before the final hot rolling pass, and was measured using a radiation thermometer. The cooling start temperature was measured using a radiation thermometer. The cooling end temperature was obtained by measuring the steel sheet surface temperature at the time when the reheating was completed after the cooling was completed using a radiation thermometer. The average cooling rate was obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the time required for cooling. The surface temperature during cooling (especially between 700 and 500 ° C) was obtained by simulation taking into account the heat transfer coefficient and the like from the cooling start temperature, the cooling end temperature, and the time required for cooling, and CR _{700 to 500} was obtained by dividing the difference (200 ° C) between 700 ° C and 500 ° C by the time required for cooling.
Furthermore, the steel type and manufacturing conditions were changed from Test No. 1 as shown in Tables 1 and 2 to obtain Test Nos. 2 to 10. In the chemical composition in Table 1, "-" indicates that it was not intentionally added. In Table 2, the Ar3 point (°C) was calculated based on the above formula (6), and since Test No. 5 was an example in which air cooling was performed to room temperature without cooling, there were no values corresponding to the cooling start temperature, cooling stop temperature, and average cooling rate, so they were marked with "-". In the case of air cooling as in Test No. 5, CR _{700 to 500} was calculated using the following formula (7).

CR _700-500 for air cooling = 12.508 x t ^{- 0.9797} (7)

In the above formula (7), t represents the plate thickness (mm).
In Test No. 6, CR _700-500 exceeded 100° C./sec, but since the simulation used could not calculate a quantitative value for a cooling rate exceeding 100/sec, the _PCR was calculated as CR _700-500 = 100.
In Tables 1 and 2 and Table 3 below, underlined values indicate values that are outside the range of the embodiments of the present invention.

Then, the metal structure, bainite hardness, hardness distribution, yield strength, tensile strength, yield ratio, uniform elongation, tensile strength × uniform elongation, and toughness of the obtained steel plate were evaluated.

[Metal structure evaluation]
The metal structure was observed as follows.
(a) A sample was taken from the above steel plate so that a plate thickness cross section including the front and back surfaces of the steel plate could be observed, parallel to a direction perpendicular to the rolling direction of the steel plate surface and perpendicular to the steel plate surface.
(b) The observation surface was mirror-finished by polishing with wet emery paper (#150 to #1500) or by polishing with an abrasive having an equivalent function, for example, polishing with an abrasive such as diamond slurry.
(c) The polished samples were etched with a 3% Nital solution to reveal the crystalline structure.
(d) The exposed structure was photographed at any location in the t (sheet thickness)/4 region at a magnification of 400. The structure fractions (ferrite, bainite, pearlite, martensite, and retained austenite) were calculated using the photographs taken at 400 times magnification (field of view area 150 μm×200 μm).

[Bainite hardness]
The hardness of the bainite structure at the t/4 portion of the corroded sample was measured using a micro Vickers hardness tester. The measurement load was 5 g in principle, and 3 g when the structure size was small. At least five points were measured, and the average value was used.

[Hardness distribution]
In any 190 mm section of the corroded sample, the HV _{surface 1 mm} , HV _t/4 , HV _3t/4 and HV _{back surface 1 mm} were measured at 5 mm intervals, and the HV cross _{-sectional average} was calculated using the above formula (2). Furthermore, the average value μ (HV _{cross-sectional average} ) and the minimum value min (HV cross-sectional _average ) of the HV cross-sectional _average in the 190 mm section were obtained, and the hardness distribution P _HV was calculated using the above formula (1).
[Yield strength (YS), tensile strength (TS), uniform elongation (UEL)]
A plate-shaped tensile test piece (same as the JIS No. 1A or No. 5 test piece) was taken from the portion of t (plate thickness)/4 so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a tensile test was carried out in accordance with JIS Z 2241 (2011). The yield point (YP) or 0.2% YS, TS and UEL were measured, and the yield ratio (YR) and TS×UEL were calculated.

[Toughness evaluation]
A full-size (10 mm x 10 mm x 55 mm) V-notch test piece was taken from the t (plate thickness)/4 region so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a Charpy impact test was carried out in accordance with JIS Z 2242 (2005) to measure the fracture transition temperature (vTrs). The results at each test temperature were the average of three pieces.
The results are shown in Table 3.

Consider the results in Tables 1 to 3.
Test Nos. 1 to 4 are examples produced under the production conditions specified in the embodiment of the present invention using steel types satisfying the chemical composition specified in the embodiment of the present invention. These satisfied the metal structure, bainite hardness, and hardness distribution specified in the present invention, and also satisfied various properties (yield strength 325 to 500 MPa, tensile strength 500 to 670 MPa, yield ratio 80% or less, uniform elongation 5% or more, TS x UEL 4500 MPa.% or more, and fracture transition temperature less than -30°C). In contrast, Test Nos. 5 to 10 did not satisfy the chemical composition and/or production conditions specified in the embodiment of the present invention, and at least one of the above-mentioned performances was inferior.

Test No. 5 is an example in which the material was air-cooled to room temperature. The average cooling rate was very slow, less than 8°C/sec, resulting in a structure consisting of ferrite and pearlite, with the sum of the area fractions of ferrite and bainite being less than 85% and the area fraction of bainite being less than 20%, resulting in a tensile strength of less than 500 MPa.

In Test No. 6, the average cooling rate from the rolling temperature to the cooling stop temperature exceeded 20°C/sec, so the bainite area fraction exceeded 80% and the yield strength exceeded 500 MPa. In addition, the _PCR exceeded 35, so the hardness distribution P _HV exceeded 5.0 and TS × UEL was less than 4500 MPa.

In test No. 7, the cooling stop temperature was less than 400°C, so the bainite hardness was more than 380.0 HV, the tensile strength was more than 670 MP, and the fracture transition temperature was -30°C, which meant that the toughness was insufficient.

In test No. 8, the C content was less than 0.08 mass%, so the sum of the area fractions of ferrite and bainite was less than 85% and the area fraction of bainite was less than 20%, resulting in a tensile strength of less than 500 MP and a yield ratio of more than 80%.

In test No. 9, the C content was less than 0.08 mass%, so the sum of the area fractions of ferrite and bainite was less than 85% and the area fraction of bainite was less than 20%, and the Nb content was also excessive, so the yield ratio was more than 80%.

In Test No. 10, the _PCR exceeded 35, so the hardness distribution _PHV exceeded 5.0 and TS×UEL was less than 4500 MPa·%.

In the above embodiment, a graph showing the relationship between hardness distribution P _HV and TS×UEL is shown in Figure 3. As shown in Figure 3, P _HV and TS×UEL are correlated, and it can be seen that they change in the shape of a dashed curve. It can be seen from the dashed curve that TS×UEL can be made 4500 MPa.% or more by setting P _HV to 5.0 HV or less.

In the above embodiment, a graph showing the relationship between the _PCR and the hardness distribution _PHV is shown in Figure 4. As shown in Figure 4, the _PCR and _PHV are correlated, and change in the shape of a curved broken line. From the curved broken line, it can be seen that by setting the _PCR to 35 or less, the _PHV can be set to 5.0HV or more.

The steel plate according to the embodiment of the present invention satisfies various performance requirements (sufficient workability, strength, uniform elongation characteristics, toughness, and low yield ratio), and can be manufactured inexpensively because it does not need to be cooled after hot rolling and then heated again. Therefore, the steel plate according to the embodiment of the present invention is suitable as a steel plate for use in steel structures such as buildings, bridges, storage tanks, and line pipes.

1 Steel plate 1A Surface of steel plate 1 1B 190 mm section D1 Position 1 mm from surface 1A of steel plate 1 in Z direction D2 Position t/4 mm from surface 1A of steel plate 1 in Z direction D3 Position 3t/4 mm from surface 1A of steel plate 1 in Z direction D4 Position t-1 mm from surface 1A of steel plate 1 in Z direction P1 Point where line 1 mm below surface intersects with hardness curve shown by dashed line P2 Point where line t/4 mm below surface intersects with hardness curve shown by dashed line P3 Point where line 3t/4 mm below surface intersects with hardness curve shown by dashed line P4 Point where line t-1 mm below surface intersects with hardness curve shown by dashed line R1 Region 0 to 3 mm below surface R2 Region 3 mm to t (plate thickness)/2 mm below surface R3 Region from t/2 to t-3 mm below the surface R4 Region from 0 mm to 3 mm below the back surface S1 Integral value of hardness from 0 to 3 mm below the surface S2 Integral value of hardness from 3 mm to t (sheet thickness)/2 mm below the surface S3 Integral value of hardness from t/2 to t-3 mm below the surface S4 Integral value of hardness from 0 mm to 3 mm _{below the back surface S1'Area expressed as 1 mm below the} HV surface × 3 mm S2'HV Area expressed as _t/4 × (t/2-3) mm S3'HV Area expressed as _3t/4 × (t/2-3) mm S4'Area expressed as _{1 mm below the back surface} × 3 mm X Direction perpendicular to the rolling direction Y and sheet thickness direction Z Y Rolling direction Z Sheet thickness direction

Claims (4)

C: 0.08 to 0.16% by mass,
Si: 0.25 to 0.50% by mass,
Mn: 1.1 to 1.6% by mass,
P: more than 0% by mass and not more than 0.030% by mass,
S: more than 0% by mass and not more than 0.003% by mass,
Al: 0.020 to 0.080% by mass,
Ti: 0.008 to 0.020% by mass,
N: 0.0030 to 0.0070% by mass,
Ca: 0.0005 to 0.0050 mass%, and the balance: iron and inevitable impurities;
In a portion that is 1/4 of the plate thickness, the sum of the area fractions of ferrite and bainite is 85% or more, and the area fraction of the bainite is 20 to 80%,
The hardness of the bainite is 220.0 to 380.0 HV,
A steel plate having a hardness distribution P _HV represented by the following formula (1) of 5.0 HV or less and a plate thickness of 12 mm or more.

P _HV = μ (HV _{cross-section average} ) - min (HV _{cross-section average} ) ... (1)

Here, the HV _{cross-sectional average} is the average hardness in the plate thickness direction at any position of the steel plate, and is calculated according to the following formula (2).

HV _{cross-sectional average} = ( _{1 mm below the HV surface} × 3 + HV _t/4 × (t/2-3) + HV _3t/4 × (t/2-3) + _{1 mm below the HV back surface} × 3) / t ... (2)

In the above formula (2), t is the thickness (mm) of the steel plate, HV _{1 mm below the surface} is the hardness (HV) at a position 1 mm below the surface of the steel plate, and HV _t/4 is the hardness HV _3t/4 is the hardness (HV) at a position 3t/4 mm below the surface of the steel plate, and HV _{1 mm below the back surface} is the hardness (HV) at a position 3t/4 mm below the surface of the steel plate. This is the hardness (HV) at a position 1 mm below.
In the above formula (1), μ (HV _{cross-sectional average} ) is the average value of the HV _{cross-sectional average} measured at 5 mm intervals in any 190 mm section perpendicular to the rolling direction and thickness direction of the steel plate. , min (HV _{cross-sectional average} ) is the minimum value of the HV _{cross-sectional average} measured at 5 mm intervals in the 190 mm section. Cu: more than 0% by mass and not more than 0.50% by mass,
Ni: more than 0% by mass and not more than 0.50% by mass,
Cr: more than 0% by mass and not more than 0.50% by mass,
The steel plate according to claim 1 , further comprising one or more selected from the group consisting of Mo: more than 0 mass % and 0.50 mass % or less, and V: more than 0 mass % and 0.10 mass % or less. The steel sheet according to claim 1 or 2, further comprising one or more selected from the group consisting of Nb: more than 0 mass% and 0.030 mass% or less, and B: more than 0 mass% and 0.002 mass% or less. A step of preparing a steel having a chemical composition according to any one of claims 1 to 3;
A step of performing hot rolling with a rolling end temperature of 830°C to 940°C;
and cooling the material from a cooling start temperature of Ar3 point (°C) -70°C or higher to a cooling stop temperature of 400°C to 600°C at an average cooling rate of 8 to 20°C/sec;
The method for producing a steel sheet according to any one of claims 1 to 3, wherein P _CR represented by the following formula (3) is 35 or less.

P _CR = ([C]+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/15+[Nb]+Sol.B×400)×CR _700-500 ...(3)

In the above formula (3), [C], [Mn], [Ni], [Cr], [Mo], [V] and [Nb] respectively represent the contents of C, Mn, Ni, Cr, Mo, V and Nb expressed in mass%, CR _700-500 is the average cooling rate on the surface of the steel sheet from 700°C to 500°C, and Sol. B is expressed by the following formula (4).

Sol. B=[B]-([N]-[Ti]/3.4)×10.8/14...(4)

In the above formula (4), [B], [N], and [Ti] represent the contents of B, N, and Ti, respectively, expressed in mass %. However, when the right side of the above formula (4) is less than 0, Sol. B = 0.

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