JP7827154B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate and a manufacturing method thereof, and in particular to a thick steel plate for cryogenic use that can consistently maintain excellent cryogenic toughness over a wide range of plate thicknesses. The steel plate of the present invention can be suitably used as a steel for structures used in cryogenic environments, such as liquefied gas storage tanks for ships and on land.

When hot-rolled steel plates are used in structures such as liquefied gas storage tanks, the environment in which they are used is extremely low, and so they are required to have not only excellent strength but also excellent toughness at extremely low temperatures (cryogenic toughness). For example, when hot-rolled steel plates are used in liquefied natural gas storage tanks, they need to ensure excellent toughness at extremely low temperatures of -164°C or lower, which is the boiling point of liquefied natural gas. If the cryogenic toughness of the steel material is poor, there is a risk that the safety of the cryogenic storage structure cannot be maintained, and therefore there is a strong demand for improved cryogenic toughness of the steel plates used.
For marine applications where the tank volume is relatively small, a steel plate with a relatively small thickness is required, while for land applications where the tank volume is relatively large, a steel plate with a larger thickness is required. To meet these requirements, 7% Ni or 9% Ni steel plates have conventionally been used.

Steel sheets containing 7 to 9% Ni are proposed in, for example, Patent Documents 1 and 2.
Patent Document 1 discloses a steel plate for cryogenic use containing, by mass%, more than 5.0% but less than 10.0% Ni and predetermined amounts of C, Si, Mn, and Al. The steel plate of Patent Document 1 has an average absorbed energy per unit area vE _-196°C of 1.25 J/ ^mm2 or more over a plate thickness range of 6 to 50 mm.
Patent Document 2 discloses a low-temperature Ni-containing steel containing, by mass%, 7.0 to 10.5% Ni and predetermined amounts of C, Si, Mn, and Al. The steel of Patent Document 2 has an average absorbed energy vE at _−196°C of 150 J or more over a plate thickness range of 30 to 60 mm.

JP 2011-219848 A JP 2011-214099 A

As a result of extensive research by the present inventors into 7% Ni steel thick plates, they discovered a problem of variation in cryogenic temperature toughness within the steel plate. They also found that this variation in cryogenic temperature toughness is caused by variations in the amount and size of retained γ (retained austenite). In other words, during impact tests at cryogenic temperatures, retained γ transforms into martensitic metal due to the applied impact strain, thereby relieving stress and suppressing the occurrence of brittle cracks. However, if the amount of retained γ is insufficient, this effect cannot be fully achieved.
In this specification, "stable retained γ" refers to the tendency of retained austenite to be unlikely to transform into a martensite structure even when strain of a certain amount or more is applied at -196°C. Conversely, "unstable retained γ" refers to the tendency of retained austenite to be easily transformed into a martensite structure by strain at -196°C.

However, in both Patent Documents 1 and 2, the cryogenic toughness is only examined as an average value of absorbed energy, and no consideration is given to the variation in cryogenic toughness within the steel plate.

The present invention has been developed in consideration of these problems, and aims to provide a highly stable steel plate and manufacturing method thereof that ensures high strength while preventing variation in cryogenic toughness within the steel plate regardless of the thickness of the steel plate.

In order to solve the above problems, the present inventors have conducted extensive research into the chemical composition and structure of 7% Ni steel sheets and have obtained the following findings.
(1) In order to stably ensure cryogenic toughness, it is important to generate a certain amount of retained γ. If the volume fraction of the retained γ phase is less than 2.0%, the absorbed energy in the impact test will not be stable.

(2) It is important that the residual γ is finely dispersed. If the circular equivalent diameter of the residual γ phase exceeds 5.0 μm, it will become coarse martensite, reducing the resistance to crack propagation.

The present invention was completed based on the above findings, and its gist is as follows:

1. In mass %,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 0.60% or less,
Ni: 6.0% or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.05% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less, and N: 0.0010% or more and 0.0080% or less,
The balance has a composition consisting of Fe and unavoidable impurities,
A steel sheet in which, at a position 1/4 depth from the surface of the steel sheet in the sheet thickness direction, a gamma phase has a volume fraction of 2.0% or more and an equivalent circle diameter of the gamma phase is 5.0 μm or less.

2. The component composition further comprises, in mass %,
Al: 0.008% or more and 0.10% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
2. The steel plate according to 1 above, containing one or more selected from the group consisting of Ti: 0.03% or less, and B: 0.0030% or less.

3. The component composition further comprises, in mass %,
Ca: 0.007% or less,
3. The steel sheet according to item 1 or 2 above, further comprising one or more selected from the group consisting of REM: 0.010% or less, and Mg: 0.070% or less.

4. C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 0.60% or less,
Ni: 6.0% or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.05% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less, and N: 0.0010% or more and 0.0080% or less,
A steel material having a composition with the balance being Fe and unavoidable impurities,
Heating to a temperature of 900°C or higher and 1200°C or lower,
The heated steel material is hot-rolled to form a hot-rolled steel sheet;
The hot-rolled steel sheet is subjected to a first accelerated cooling in which the average cooling rate is 1°C/s or more in a temperature range of 550°C or less and 300°C or more at a temperature at a quarter-thickness position of the sheet, and the cooling stop temperature is 300°C or less at a temperature at a quarter-thickness position of the sheet,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating in which the average heating rate of 500°C or more is less than 1°C/s and the temperature at a quarter-thickness position is in a temperature range of A _c1 point or more and less than A _c3 point;
The hot-rolled steel sheet after the two-phase region heating is subjected to second accelerated cooling in which the average cooling rate at the temperature at a quarter position of the sheet thickness is 1°C/s or more and the cooling stop temperature is 300°C or less;
The hot-rolled steel sheet after the second accelerated cooling is subjected to tempering at a tempering temperature of 300°C or more and 500°C or less at a temperature at a position of 1/2 of the sheet thickness.
Steel plate manufacturing method.

5. The component composition further comprises, in mass %,
Al: 0.008% or more and 0.10% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
5. The method for producing a steel sheet according to 4 above, further comprising the step of adding one or more elements selected from the group consisting of Ti: 0.03% or less, and B: 0.0030% or less.

6. The component composition further comprises, in mass %,
Ca: 0.007% or less,
6. The method for producing a steel sheet according to 4 or 5 above, further comprising adding one or more selected from the group consisting of REM: 0.010% or less, and Mg: 0.070% or less.

The present invention makes it possible to provide a steel plate that maintains high strength while exhibiting uniformly excellent cryogenic toughness throughout the steel plate, regardless of the plate thickness. By using the steel plate of the present invention in steel structures used in cryogenic environments, such as tanks for storing liquefied gas, the safety of the steel structure can be improved, bringing about significant industrial benefits.

The following describes in detail an embodiment of the present invention. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to this.

[Component composition]
The steel sheet of the present invention has a predetermined chemical composition. Furthermore, it is preferable that the steel material used to manufacture the steel sheet of the present invention also has the above-mentioned predetermined chemical composition. Hereinafter, each element contained in this chemical composition will be explained. Unless otherwise specified, in this specification, "%" as a unit of content of each element means "mass %."

C: 0.01% or more and 0.15% or less C is an element that has the effect of improving the strength of the steel plate. To achieve this effect, the C content is set to 0.01% or more, preferably 0.03% or more. On the other hand, if the C content exceeds 0.15%, the cryogenic toughness of the steel plate decreases. Therefore, the C content is set to 0.15% or less, preferably 0.12% or less.

Si: 0.01% or more and 0.50% or less Si is an element that contributes to improving the strength of steel sheet and also acts as a deoxidizer. To achieve these effects, the Si content is set to 0.01% or more. On the other hand, if the Si content is excessively high, toughness decreases. Therefore, the Si content is set to 0.50% or less, preferably 0.30% or less.

Mn: 0.05% or more and 0.60% or less Mn is an element that improves the hardenability of steel and is effective in increasing the strength of steel sheets. To achieve this effect, 0.05% or more of Mn is added. On the other hand, if the Mn content exceeds 0.60%, the temper embrittlement susceptibility increases and toughness begins to vary. Therefore, the Mn content is limited to 0.60% or less. Specifically, if the Mn content exceeds 0.60%, the Mn concentration in the γ structure increases, making it easier for γ, which does not contribute to ensuring toughness, to form. The Mn content is preferably less than 0.40%, more preferably less than 0.30%, even more preferably less than 0.20%, and even more preferably less than 0.17%.

Ni: 6.0% or more and 7.5% or less Ni is an element extremely effective in improving the cryogenic toughness of steel sheet. Specifically, if the Ni content is less than 6.0%, the γ phase is difficult to form, and the γ content cannot be set to 0.5 volume % or more. Furthermore, if the Ni content is less than 6.0%, the strength of the steel sheet also decreases. Therefore, the Ni content is set to 6.0% or more. On the other hand, since Ni is an expensive element, the cost of the steel sheet increases as the Ni content increases. Therefore, in the present invention, the Ni content is set to 7.5% or less.

Cr: 0.01% or more and 1.00% or less Cr is an element that can improve the strength of steel plate without significantly impairing the cryogenic toughness. To achieve the above effect, the Cr content is set to 0.01% or more, preferably 0.30% or more. However, if the Cr content exceeds 1.00%, the cryogenic toughness of the steel plate decreases. Therefore, the Cr content is set to 1.00% or less.

Mo: 0.05% or more and 0.50% or less Like Cr, Mo is an element that can improve the strength of steel plate without significantly impairing cryogenic toughness. If the Mo content is less than 0.05%, it is difficult to ensure the desired strength and toughness, and strength in particular cannot be achieved. In particular, in the present invention, even if strength is likely to decrease due to the suppression of cryogenic toughness variation by reducing the Mn content, the desired strength can be ensured by adding a predetermined amount of Mo. Therefore, the Mo content is 0.05% or more, preferably more than 0.10%. On the other hand, if the Mo content exceeds 0.50%, the cryogenic toughness actually decreases. Therefore, the Mo content is 0.50% or less, preferably 0.30% or less, and more preferably 0.25% or less.

P: 0.03% or less P is an inevitable impurity and a harmful element that adversely affects the cryogenic toughness of steel sheets. For example, in order to obtain a sound base material and welded joint when steel sheets are welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is limited to 0.03% or less. Furthermore, from the viewpoint of cryogenic toughness, the lower the P content, the better. Therefore, the lower limit is not particularly limited, and it may be 0%, but even in this case, it is acceptable for P to be contained as an inevitable impurity. On the other hand, excessive reduction in P content causes an increase in cost, so from the viewpoint of cost, it is preferable that the lower limit of the P content be 0.001%.

S: 0.005% or less S forms MnS in steel and significantly deteriorates cryogenic toughness, so the upper limit of the S content is 0.005%, and it is desirable to reduce it as much as possible. The S content is preferably 0.002% or less. On the other hand, the lower the S content, the better, so there is no particular restriction on the lower limit, and it may be 0%, but even in that case, it is acceptable for S to be contained as an unavoidable impurity.

N: 0.0010% or more and 0.0080% or less N forms precipitates in steel, and if the content exceeds 0.0080%, it causes a decrease in the toughness of the base material. However, N is also an element that contributes to the refinement of the base material grains by forming AlN, and this effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content is set to 0.0010% or more and 0.0080% or less. The N content is preferably set to 0.0020% or more and preferably 0.0060% or less.

In one embodiment of the present invention, the component composition can consist of the above-mentioned specified amounts of elements, with the remainder consisting of Fe and unavoidable impurities.

In another embodiment of the present invention, the above component composition may optionally further contain one or more elements selected from the group consisting of Al, Cu, Nb, V, Ti and B, preferably in the amounts described below.

Al: 0.008% or more and 0.10% or less Al is an element contained in a deoxidizer. If the Al content is less than 0.008%, the effect as a deoxidizer is poor. Al is also an element that contributes to the refinement of the base material grain by forming AlN. Therefore, when Al is contained, the Al content is preferably 0.008% or more, more preferably 0.02% or more. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the steel is impaired. Therefore, the Al content is preferably 0.10% or less, more preferably 0.05% or less.

Cu: 0.40% or less Cu is an element that has the effect of increasing the strength of steel sheet by improving hardenability. However, if the Cu content exceeds 0.40%, the cryogenic toughness of the steel sheet decreases, and the surface properties of the steel material (slab) after casting deteriorate. Therefore, when Cu is added, the Cu content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, although there is no particular lower limit for the Cu content, in order to obtain the above effect, the Cu content is preferably 0.10% or more.

Nb: 0.05% or less Nb is an effective element for increasing the strength of steel sheet through precipitation strengthening. However, if the Nb content is excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when Nb is added, the Nb content is preferably 0.05% or less, and more preferably 0.03% or less. On the other hand, although there is no particular lower limit for the Nb content, in order to obtain the above-mentioned effect, the Nb content is preferably 0.010% or more.

V: 0.05% or less Like Nb, V is an effective element for increasing the strength of steel sheet by precipitation strengthening. However, if the V content is excessively high, the cryogenic toughness of the steel sheet decreases. Therefore, when V is added, the V content is preferably 0.05% or less, more preferably 0.04% or less. On the other hand, although there is no particular lower limit for the V content, in order to obtain the above-mentioned effect, the V content is preferably 0.010% or more.

Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld without degrading the mechanical properties of the base material when steel plates are welded to form a welded structure. For this purpose, it is preferable to add Ti at a content of 0.003% or more. On the other hand, if the Ti content exceeds 0.03%, the toughness will actually decrease. Therefore, it is preferable to add Ti at a content of 0.03% or less.

B: 0.0030% or less B is an element that improves hardenability when added in small amounts. To effectively exert this effect, B can be contained in an amount of 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, toughness deteriorates. Therefore, when B is contained, the content is preferably 0.0030% or less.

In another embodiment of the present invention, the above component composition may optionally further contain one or more elements selected from the group consisting of Ca, REM, and Mg, preferably in the amounts described below.

Ca: 0.007% or less Ca is an element that has the effect of improving the cryogenic toughness of steel sheets by controlling the morphology of inclusions in steel. However, excessive Ca impairs the cleanliness of steel. Therefore, when Ca is added, the Ca content is preferably 0.007% or less, more preferably 0.004% or less. On the other hand, although there is no particular lower limit for the Ca content, it is preferably 0.001% or more to obtain the above effect.

REM: 0.010% or less Like Ca, REM (rare earth metals) are elements that have the effect of improving the cryogenic toughness of steel sheets by controlling the morphology of inclusions in steel. However, excessive REM impairs the cleanliness of steel. Therefore, when REM is added, the REM content is preferably 0.010% or less, and more preferably 0.008% or less. On the other hand, although there is no particular lower limit for the REM content, to obtain the above effect, the REM content is preferably 0.001% or more.
Here, REM is a collective term for 17 elements, including 15 lanthanoid elements, Y, and Sc, and these elements can be contained alone or in combination. The REM content means the total content of these elements.

Mg: 0.070% or less Like Ca and REM, Mg is an element that controls the morphology of inclusions in steel, thereby improving the cryogenic toughness of the steel plate. However, excessive Mg impairs the cleanliness of the steel. Therefore, when Mg is added, the Mg content is preferably 0.070% or less, and more preferably 0.004% or less. On the other hand, although there is no particular lower limit for the Mg content, in order to obtain the above-mentioned effect, the Mg content is preferably 0.001% or more.

[Microstructure]
The steel plate of the present invention is characterized in that the gamma phase has a volume fraction of 2.0% or more and a circle equivalent diameter of 5.0 μm or less at a depth position of 1/4 of the plate thickness t from the surface of the steel plate in the plate thickness direction (sometimes referred to as "(1/4)t"). The presence of a predetermined amount of gamma with the above-mentioned predetermined composition in the steel plate makes it possible to achieve stable and high cryogenic toughness.

The gamma (austenite) present in steel sheets transforms under strain during impact testing, providing a stress relief effect and suppressing the occurrence of brittle fracture. To consistently achieve this effect, the gamma phase must have a volume fraction of 2.0% or more. Here, the volume fraction of the gamma phase can be calculated, for example, by determining the diffraction intensity of alpha-Fe and gamma-Fe using an X-ray diffraction test.

On the other hand, if the γ phase is coarse, martensite transformed by strain may reduce the resistance to crack propagation. Therefore, the equivalent circle diameter of the γ phase must be 5.0 μm or less. The equivalent circle diameter of the γ phase is preferably 4.5 μm or less, and more preferably 4.0 μm or less. Note that the equivalent circle diameter of the γ phase is the average value counted for γ phases of 0.1 μm or more.

Furthermore, the steel plate preferably has a structure in which the total area ratio of bainite and martensite is 85% or more. As described above, a structure mainly composed of bainite and martensite can easily obtain sufficient strength while ensuring excellent cryogenic toughness. Here, the ratio of bainite to martensite can be any ratio.
Here, the term "structure mainly composed of martensite and bainite" refers to a structure in which the total area ratio of martensite and bainite exceeds 50%.

The thickness of the steel plate is not particularly limited and can be any thickness, but is preferably 6 mm or more and 50 mm or less. In particular, even for thin steel plates, which have traditionally been more susceptible to variation in cryogenic toughness within the steel plate, the thickness can be less than 30 mm in order to effectively suppress variation in cryogenic toughness and to better enjoy the benefits of the present application.

[Mechanical properties]
(Tensile strength)
The lower limit of the tensile strength of the steel sheet is not particularly limited and can be any value, but is preferably 700 MPa, more preferably 720 MPa. On the other hand, the upper limit of the tensile strength is not particularly limited and can be any value, but is preferably 930 MPa, more preferably 900 MPa.
The tensile strength can be measured by the method described in the examples below.

(Cryogenic toughness)
The toughness of the steel plate is such that the Charpy absorbed energy at −196° C. (vE _{−196° C.} ) in a full-size Charpy impact test is preferably 250 J or more, more preferably 280 J or more, and may be 350 J or less.

[Manufacturing method]
Next, an example of a manufacturing method that can suitably manufacture the steel sheet of the present invention will be described. In the following description, unless otherwise specified, temperature refers to the temperature at the center of the sheet thickness. The temperature at the center of the sheet thickness can be calculated by heat transfer calculation from the surface temperature of the steel sheet measured with a radiation thermometer, for example.

As a specific example of the manufacturing method, the steel sheet of the present invention can be suitably manufactured by sequentially carrying out the following steps (1) to (7).
(1) Heating of steel material (2) Hot rolling (3) First accelerated cooling (4) Heating in the two-phase region (5) Second accelerated cooling (6) Tempering (7) Air cooling

(1) Heating of Steel Material First, it is preferable to heat a steel material having the above-described composition to a temperature of 900°C or higher and 1200°C or lower. The method for producing the steel material is not particularly limited, but for example, the steel material can be produced by melting molten steel having the above-described composition by a conventional method and casting it. Melting can be carried out by any method, such as a converter, an electric furnace, or an induction furnace. Furthermore, from the viewpoint of productivity, casting is preferably carried out by a continuous casting method, but it can also be carried out by an ingot making-splitting rolling method. For example, a steel slab can be used as the steel material.
Here, the heating of the steel material may be carried out after the steel material obtained by a method such as casting has been cooled once, or the obtained steel material may be directly subjected to heating without being cooled.

If the heating temperature of the steel material is below 900°C, the steel material's high deformation resistance increases the load on the rolling mill in the subsequent hot rolling, making it difficult to perform the hot rolling. Therefore, it is preferable to set the heating temperature of the steel material to 900°C or higher. On the other hand, if the heating temperature of the steel material is higher than 1200°C, oxidation of the steel becomes significant, and losses due to removing the oxide film caused by oxidation increase, resulting in lower yields. Therefore, it is preferable to set the heating temperature of the steel material to 1200°C or lower.

(2) Hot Rolling After the heating, the heated steel material can be hot rolled to form a hot-rolled steel sheet. The final thickness of the hot-rolled steel sheet is not particularly limited, but as described above, it is preferably 6 mm or more and 50 mm or less.

(3) First Accelerated Cooling The hot-rolled steel sheet after the hot rolling can be subjected to accelerated cooling (first accelerated cooling). In the first accelerated cooling, it is preferable that the average cooling rate in the temperature range of 550°C or less and 300°C or more at a temperature at a position of the plate thickness (¼)t of the steel sheet is 1°C/s or more, and the cooling stop temperature is preferably 300°C or less at a temperature at (¼)t. By performing the first accelerated cooling under these conditions, the hot-rolled steel sheet is well quenched, and a desired structure mainly composed of martensite and bainite is easily obtained.

In the first accelerated cooling, if the average cooling rate in the temperature range of 550°C or less and 300°C or more at (1/4)t is less than 1°C/s, it is difficult to obtain the desired transformed structure and sufficient strength. Furthermore, unstable γ tends to remain in the steel, making it difficult to reduce the rate of decrease in the amount of residual γ before and after deep cooling. As a result, cryogenic toughness is likely to decrease. On the other hand, while there is no particular upper limit for the average cooling rate, if the average cooling rate is higher than 200°C/s, it becomes difficult to control the temperature at each position within the steel plate, and material properties tend to vary in the plate width direction and rolling direction. As a result, material properties such as tensile properties and toughness tend to vary. Therefore, it is preferable to set the average cooling rate to 200°C/s or less.

The first accelerated cooling can be performed by any method without any particular limitation. For example, air cooling and/or water cooling can be used. For water cooling, any cooling method using water (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used.

(4) Two-Phase Region Heating Next, the hot-rolled steel sheet cooled after hot rolling can be subjected to two-phase region heating. Specifically, the cooled hot-rolled steel sheet is preferably heated at a temperature of 500°C or higher at a thickness (¼)t position at an average heating rate of less than 1°C/s and to a temperature range of A _c1 point or higher and less than A _c3 point. By performing two-phase region heating, it is preferable to reverse transform a portion of the structure of the hot-rolled steel sheet from bainite and/or martensite to form an austenite mixed structure having an alloy-enriched phase enriched with C, Ni, and Mn. However, as described above, in this austenite mixed structure, it is preferable to suppress the enrichment of Mn to less than 2 mass% and to increase the enrichment of Ni to 12 mass% or higher.
Here, the average temperature rise rate refers to the average rate from 500° C. until the temperature reaches the two-phase region heating temperature.
If the average heating rate in the high-temperature region during two-phase heating is 1°C/s or more, the formation of the above-mentioned alloy-enriched phase tends to be insufficient. In particular, the Ni concentration in the γ structure of the steel sheet cannot be sufficiently increased. As a result, the stability of γ decreases, the rate of decrease in residual γ before and after deep-cooling increases, and it becomes difficult to ensure excellent cryogenic toughness. Furthermore, the toughness tends to vary.

If the heating temperature in the two-phase region is less than the _Ac1 point, the above-mentioned reverse-transformed austenite is hardly obtained, making it difficult to obtain the desired microstructure by subsequent accelerated cooling. As a result, it is difficult to obtain the desired cryogenic temperature toughness in the finally obtained steel plate. On the other hand, if the heating temperature in the two-phase region is more than the _Ac3 point, the reverse transformation rates of bainite and martensite tend to be excessively high, making it difficult to form the above-mentioned alloy-enriched phase. As a result, it is difficult to ensure the amount of retained γ after deep-cooling, making it difficult to ensure excellent cryogenic temperature toughness. In addition, the toughness tends to vary.
The A _c1 point (A _c1 transformation point) and the A _c3 point (A _c3 transformation point) can be calculated by the following formulas (1) and (2), respectively.
A _c1 point (℃) = 750.8 - 26.6 x C + 17.6 x Si - 11.6 x Mn - 22.9 x Cu - 23 x Ni + 24.1 x Cr + 22.5 x Mo - 39.7 x V - 5.7 x Ti + 232.4 x Nb - 169.4×Al...(1)
A _c3 point (℃) = 937.2 - 436.5 x C + 56 x Si - 19.7 x Mn - 16.3 x Cu - 26.6 x Ni - 4.9 x Cr + 38.1 x Mo + 124.8 x V + 136.3 x Ti - 19.1 x Nb + 198.4×Al...(2)
However, the element symbols in the above formulas (1) and (2) represent the content (mass%) of each element, and if the element is not contained, it is set to 0.

Any heating method can be used for two-phase heating, as long as it can control the heating temperature as described above. One example of a heating method is furnace heating. There are no particular limitations on the furnace heating method, and a general heat treatment furnace can be used.

After the two-phase heating temperature is reached, the next accelerated cooling may be started immediately, or the material may be held at the two-phase heating temperature for any desired time before starting the next accelerated cooling. When holding at the two-phase heating temperature, there are no particular restrictions on the holding time, but it is preferable to hold it for 5 minutes or more.

(5) Second Accelerated Cooling Next, the hot-rolled steel sheet after the two-phase region heating can be subjected to accelerated cooling (second accelerated cooling). In the second accelerated cooling, the average cooling rate at the position of the steel sheet thickness (¼)t is preferably 1°C/s or more, and the cooling stop temperature at the temperature at (¼)t is preferably 300°C or less.

In the second accelerated cooling, if the average cooling rate at the temperature at (1/4)t is less than 1°C/s, unstable γ tends to remain in the steel, making it difficult to reduce the amount of γ. As a result, the cryogenic toughness of the steel sheet obtained at the end is reduced. On the other hand, although there is no particular upper limit for the average cooling rate, if the average cooling rate is higher than 200°C/s, it becomes difficult to control the temperature at each position in the steel sheet, and material properties tend to vary in the sheet width direction and rolling direction. As a result, material properties such as tensile properties and toughness tend to vary. Therefore, it is preferable that the average cooling rate be 200°C/s or less.
Here, the average cooling rate refers to the average rate at which the temperature decreases per unit time from the start of accelerated cooling to the end of accelerated cooling in the second accelerated cooling step.

Furthermore, in the second accelerated cooling, if the cooling stop temperature exceeds 300°C at (1/4)t, unstable austenite is likely to remain, and the cryogenic toughness is likely to decrease.

The second accelerated cooling can be performed by any method without any particular limitations. For example, air cooling and/or water cooling can be used. For water cooling, any cooling method using water (e.g., spray cooling, mist cooling, laminar cooling, etc.) can be used.

(6) Tempering Next, the hot-rolled steel sheet cooled after heating in the two-phase region can be tempered. The tempering temperature at the half-thickness position is preferably 300°C or higher and 500°C or lower, more preferably 350°C or higher, and more preferably 450°C or lower. If the tempering temperature is lower than 300°C, tempering is insufficient and toughness is likely to decrease. On the other hand, if the tempering temperature exceeds 500°C, coarse γ is generated and toughness is likely to decrease.

Any heating method can be used in the tempering process as long as it can control the heating temperature as described above. One example of a heating method is furnace heating. There are no particular limitations on the furnace heating method, and a general heat treatment furnace can be used.

After the tempering temperature is reached, the material may be held at the tempering temperature for any desired time, after which optional cooling may begin. When holding at the tempering temperature, there are no particular restrictions on the holding time, but it is preferable to hold it for 5 minutes or more.

(7) Air Cooling The steel sheet after tempering can be subjected to any cooling method as described above. Although the cooling method is not particularly limited, air cooling is preferred from the viewpoint of ease of operation during production and cost.

Steel plates were manufactured using the procedure described below and their properties were evaluated.

First, molten steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and then continuously cast into steel slabs (thickness: 200 mm) as steel materials. Table 1 also lists the A _c1 point (°C) calculated using the above-mentioned formula (1) and the A _c3 point (°C) calculated using the above-mentioned formula (2).

Next, the obtained steel material (slab) was heated and hot-rolled according to the conditions shown in Table 2 to obtain hot-rolled steel sheets having various thicknesses (final thicknesses).
Next, according to the conditions shown in Table 2, the obtained hot-rolled steel sheets were subjected to a heat treatment including a first accelerated cooling, a two-phase region heating, and a second accelerated cooling.
The heat-treated hot-rolled steel sheets were then tempered according to the conditions shown in Table 2. In all examples, air cooling was carried out after tempering to obtain steel sheets having various thicknesses ranging from 6 mm to 50 mm.
A heat treatment furnace was used for heating in each of the above steps.

Next, the microstructure, Mn and Ni concentrations in γ, tensile strength (TS), and Charpy absorbed energy at −196° C. (vE _{−196° C.} ) of each of the obtained steel sheets were evaluated according to the following methods.

[Microstructure]
A test piece for microstructure observation was taken from each steel plate so that the position of the plate thickness (1/4) t was the observation position. This test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror polished. Next, after performing nital etching, the specimen was observed with a scanning electron microscope at magnifications of 2000 and 10,000 times, and images of the structure were taken. The obtained images were analyzed to identify the microstructure.
Of the steel plates Nos. 1 to 34 shown in Table 2, all except for Comparative Example No. 5 had a lath-shaped microstructure, and were either a structure of tempered martensite alone or a mixed structure of tempered martensite and bainite.

(Tensile strength)
A JIS No. 4 tensile test piece was taken from the position of the steel plate at (¼) the plate thickness t. Using this tensile test piece, a tensile test was carried out in accordance with the provisions of JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. Here, a TS of more than 700 MPa was considered to be acceptable. The results are shown in Table 2.

(Cryogenic toughness)
V-notch test specimens were taken from the steel plate at a position equal to (1/4) the plate thickness t in accordance with JIS Z 2242. Using these V-notch test specimens, Charpy impact tests were conducted in accordance with JIS Z 2242 to determine the Charpy absorbed energy at -196°C (vE _-196°C ). The Charpy absorbed energy can be considered an index of the cryogenic toughness of the steel plate. For the Charpy impact test, three test specimens were taken from each steel plate at different positions along the rolling direction. More specifically, test specimens were taken from three locations: one at each end of the steel plate in the rolling direction (length direction), 1000 mm inward, and one at the center of the steel plate in the rolling direction (length direction). Each test specimen was measured three times, once for each test specimen. The individual measurement results are shown in Table 2.
For Nos. 6 and 22, which had small plate thicknesses, half-size Charpy impact tests were conducted using half-size test pieces (sub-size test pieces), and for the other examples, full-size Charpy impact tests were conducted using full-size test pieces.

As can be seen from Tables 1 and 2, the steel plate according to the present invention has high strength and excellent cryogenic toughness while effectively suppressing the variation in toughness within the steel plate. Furthermore, this effect was achieved even in relatively thin steel plates, for example, with thicknesses of 6 to 25 mm. On the other hand, in the comparative examples outside the scope of the present invention, the Charpy absorbed energy was lower than 200 J in at least one of the three measurements. In other words, in the comparative examples, the cryogenic toughness varied within the steel plate, resulting in areas with low cryogenic toughness, and the above-mentioned target performance was not met.

According to the present invention, steel plates of various thicknesses can be made to exhibit uniform and excellent cryogenic toughness while maintaining high strength.

Claims (6)

In mass%,
C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 0.60% or less,
Ni: 6.0% or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.05% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less, and N: 0.0010% or more and 0.0080% or less,
The balance has a composition consisting of Fe and unavoidable impurities,
A steel sheet, wherein a gamma phase has a volume fraction of 2.0% or more at a position 1/4 depth from the surface of the steel sheet in the sheet thickness direction, and the gamma phase has an equivalent circle diameter of 1.4 μm or more and 5.0 μm or less. The component composition further comprises, in mass%,
Al: 0.008% or more and 0.10% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
The steel plate according to claim 1, further comprising one or more selected from the group consisting of Ti: 0.03% or less, and B: 0.0030% or less. The component composition further comprises, in mass%,
Ca: 0.007% or less,
The steel sheet according to claim 1 or 2, further comprising one or more selected from the group consisting of REM: 0.010% or less, and Mg: 0.070% or less. C: 0.01% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 0.60% or less,
Ni: 6.0% or more and 7.5% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.05% or more and 0.50% or less,
P: 0.03% or less,
S: 0.005% or less, and N: 0.0010% or more and 0.0080% or less,
A steel material having a composition with the balance being Fe and unavoidable impurities,
Heating to a temperature of 900°C or higher and 1200°C or lower,
The heated steel material is hot-rolled to form a hot-rolled steel sheet;
The hot-rolled steel sheet is subjected to a first accelerated cooling in which the average cooling rate is 1°C/s or more in a temperature range of 550°C or less and 300°C or more at a temperature at a quarter-thickness position of the sheet, and the cooling stop temperature is 300°C or less at a temperature at a quarter-thickness position of the sheet,
The hot-rolled steel sheet after the first accelerated cooling is subjected to two-phase region heating in which the average heating rate of 500°C or more is less than 1°C/s and the temperature at a quarter-thickness position is in a temperature range of A _c1 point or more and less than A _c3 point;
The hot-rolled steel sheet after the two-phase region heating is subjected to second accelerated cooling in which the average cooling rate at the temperature at a quarter position of the sheet thickness is 1°C/s or more and the cooling stop temperature is 300°C or less;
The hot-rolled steel sheet after the second accelerated cooling is subjected to tempering at a tempering temperature of 300°C or more and 500°C or less at a temperature at a position of 1/2 of the sheet thickness,
Obtain a steel sheet having a gamma phase of 2.0% or more in volume fraction at a position of 1/4 depth from the surface of the steel sheet in the sheet thickness direction, and the gamma phase has an equivalent circle diameter of 1.4 μm or more and 5.0 μm or less .
Steel plate manufacturing method. The component composition further comprises, in mass%,
Al: 0.008% or more and 0.10% or less,
Cu: 0.40% or less,
Nb: 0.05% or less,
V: 0.05% or less,
The method for producing a steel sheet according to claim 4, further comprising the step of adding one or more selected from the group consisting of Ti: 0.03% or less, and B: 0.0030% or less. The component composition further comprises, in mass%,
Ca: 0.007% or less,
The method for producing a steel sheet according to claim 4 or 5, further comprising the step of adding one or more selected from the group consisting of REM: 0.010% or less, and Mg: 0.070% or less.