Patent Application
20250207232
The present disclosure relates to a steel plate, particularly a steel plate that can stably ensure excellent strength, low-temperature toughness, and low yield ratio and is suitable for use at low temperatures, and a method of producing the same. The steel plate according to the present disclosure can be suitably used in large low-temperature liquefied gas storage tanks because, for example for liquefied gas storage tanks for ships, stress relief can be performed mechanically instead of by post weld heat treatment.
When carbon steel and/or carbon manganese steel is used for tanks that are classified as independent Type-C tanks and have a design temperature below −10° C. among liquefied gas storage tanks for ships, stress relief after welding is required. Stress relief is usually performed by post weld heat treatment (PWHT), but mechanical stress relief is also possible if the yield ratio of the steel material is 0.8 or less. For large tanks, PWHT is difficult, and accordingly a low yield ratio material that can be subjected to mechanical stress relief is desired. For example, in the case where carbon steel is used for large storage tanks for liquefied CO2, high-strength steel that ensures excellent toughness at a low temperature of −50° C. and has a tensile strength (hereafter also referred to as TS) of 690 MPa or more is needed. Thus, a steel material having low yield ratio, high strength, and excellent low-temperature toughness is required. Since weld defects are checked by ultrasonic testing during welding process in some cases, the material may also be required to have low acoustic anisotropy.
For steel plates with low-temperature toughness and low yield ratio, for example, JP 2016-507649 A (PTL 1) discloses a steel plate having an impact toughness of 150 J or more at −75° C., a yield ratio of 0.8 or less, and a tensile strength of 530 MPa or more.
However, the TS of the steel plate described in PTL 1 is 620 MPa at a maximum, and a steel plate with a TS of 690 MPa or more is not yet provided. Conventionally, since carbon steel with low yield ratio, excellent low-temperature toughness, and high strength of 690 MPa or more is unavailable, expensive nickel steel such as 9% Ni steel needs to be used, causing an increase in material costs.
It could therefore be helpful to provide a steel plate having high strength of TS≥690 MPa, excellent low-temperature toughness, low yield ratio, and low acoustic anisotropy, and a method of producing the same.
Upon careful examination on the chemical composition and microstructure of high-strength steel to achieve low-temperature toughness and low yield ratio, we discovered the following, especially regarding the microstructure: An effective way of achieving desired property improvement is that, in the microstructure at a depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction, the ferrite fraction is 5% to 95% and the martensite austenite constituent fraction is 1% to 30% with the residual microstructure consisting of tempered martensite and/or bainite, the average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of degrees or more is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250/mm2 or less.
Herein, the term “ferrite” refers to BCC phase that remains without reverse transformation even when martensite and bainite are heat-treated to a temperature of Ac1 point or more and inherits the original lath-like microstructure. By setting this relatively soft ferrite phase to 5% to 95% and finely distributing martensite austenite constituent, low yield ratio can be achieved.
Moreover, when regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more are defined as crystal grains, by suppressing the formation of coarse grains so that the number density of crystal grains having an equivalent circular diameter of more than 30 μm will be 250/mm2 or less and limiting the residual microstructure other than ferrite and martensite austenite constituent to be tempered martensite and/or bainite phase, low-temperature toughness can be achieved.
Furthermore, by setting the average aspect ratio of crystal grains to less than 2.0, reduction in acoustic anisotropy can also be achieved.
The present disclosure is based on these discoveries. We thus provide the following.
The steel plate according to the present disclosure, despite its material being carbon steel or carbon manganese steel, can be used for steel structures that are used in low-temperature environments, for example, large low-temperature storage tanks for ships such as liquefied CO2 tanks and LPG tanks. The steel plate according to the present disclosure also contributes to lower construction costs than nickel steel. This yields industrially great advantageous effects.
Embodiments of the present disclosure will be described in detail below. The following description shows preferred embodiments of the present disclosure, and the present disclosure is not limited to such.
A steel plate according to the present disclosure has a certain chemical composition. Preferably, the steel material used for producing the steel plate according to the present disclosure also has the certain chemical composition. Each element contained in the chemical composition will be described below. In this specification, “%” as a unit of content of each element denotes “mass %” unless otherwise specified.
Cis an element that has the effect of improving the strength of the steel plate. In order to achieve this effect, the C content is 0.02% or more. The C content is preferably 0.03% or more. If the C content is more than 0.15%, the amount of martensite austenite constituent in the steel plate increases and low-temperature toughness decreases. The C content is therefore 0.15% or less. The C content is preferably 0.12% or less.
Si is an element that acts as a deoxidizer. In order to achieve this effect, the Si content is 0.01% or more. The Si content is preferably 0.03% or more. If the Si content is excessively high, toughness decreases. The Si content is therefore 0.50% or less. The Si content is preferably 0.30% or less.
Mn is an element that enhances the hardenability of the steel and is effective in increasing the strength of the steel plate. In order to achieve this effect, the Mn content is 0.05% or more. The Mn content is preferably 0.10% or more. If the Mn content is more than 2.50%, toughness degrades. The Mn content is therefore 2.50% or less. The Mn content is preferably 2.00% or less.
Ni: 0.5% or More and Less than 5.0%
Ni is an element effective in improving the low-temperature toughness of the steel plate. In order to achieve this effect, the Ni content is 0.5% or more. Since Ni is an expensive element, the steel plate costs increase as the Ni content increases. Accordingly, in the present disclosure, the Ni content is less than 5.0%. The Ni content is preferably 0.8% or more. The Ni content is preferably 3.5% or less.
P is an inevitable impurity, and is a harmful element that adversely affects the low-temperature toughness of the steel plate. For example, in order to obtain a sound base metal and weld joint when welding the steel plate to produce a welded structure, it is preferable to reduce the P content as much as possible. The P content is therefore 0.03% or less. Since lower P content is better from the viewpoint of low-temperature toughness, the lower limit of the P content is not set and may be 0%. Excessively reducing the P content, however, causes an increase in cost. Accordingly, the lower limit of the P content is preferably 0.001% from the viewpoint of cost.
S forms MnS in the steel and significantly degrades low-temperature toughness, and accordingly it is desirable to reduce the S content as much as possible with the upper limit being 0.005%. The S content is preferably 0.002% or less. Since lower S content is better, the lower limit of the S content is not set and may be 0%.
N forms precipitates in the steel. If the N content is more than 0.0080%, the toughness of the base metal decreases. N is also an element that forms AlN and thus contributes to grain refinement of the base metal. This effect is achieved when the N content is 0.0010% or more. The N content is therefore 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. The N content is preferably 0.0060% or less.
In one embodiment of the present disclosure, the chemical composition contains the certain amounts of the elements described above with the balance consisting of Fe and inevitable impurities.
In another embodiment of the present disclosure, the chemical composition may optionally further contain one or more selected from Cr, Mo, Al, Cu, Nb, V, Ti, B, Ca, REM, and Mg preferably in the following amounts.
Cr is an element that can improve the strength of the steel plate without significantly impairing low-temperature toughness. In order to achieve this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.30% or more. If the Cr content is more than 2.00%, the low-temperature toughness of the steel plate may decrease. The Cr content is therefore preferably 2.00% or less. The Cr content is more preferably 0.80% or less.
Mo is an element that contributes to improve strength of the steel, and may be optionally added depending on the desired strength. If the Mo content is more than 1.0%, toughness may degrade. Accordingly, in the case where Mo is added, the Mo content is preferably 1.0% or less. From the viewpoint of achieving the strength improving effect by Mo, the Mo content is preferably 0.01% or more.
Al is an element that acts as a deoxidizer, and is most widely used in the molten steel deoxidation process for high tensile strength steel. In order to achieve this effect, the Al content is preferably 0.001% or more. The Al content is more preferably 0.010% or more. If the Al content is more than 0.100%, the toughness of the base metal may decrease. The Al content is therefore preferably 0.100% or less. The Al content is more preferably 0.07% or less.
Cu is an element that can increase strength while maintaining high toughness, and may be optionally added depending on the desired strength. If the Cu content is more than 2.0%, hot brittleness may occur and degrade the surface characteristics of the steel plate. Accordingly, in the case where Cu is added, the Cu content is preferably 2.0% or less. The Cu content is more preferably 1.0% or less. In order to achieve the foregoing effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more, and further preferably 0.20% or more.
Nb is an element that contributes to improve strength of the steel, and may be optionally added depending on the desired strength. If the Nb content is more than 0.1%, the toughness of the base metal may degrade. Accordingly, in the case where Nb is added, the Nb content is preferably 0.1% or less. From the viewpoint of achieving the strength improving effect by Nb, the Nb content is preferably 0.005% or more.
V is an effective element that enhances the strength of the steel plate through strengthening by precipitation. If the V content is excessively high, the low-temperature toughness of the steel plate may decrease. Accordingly, in the case where V is added, the V content is preferably 0.05% or less. The V content is more preferably 0.04% or less. Although no lower limit is placed on the V content, the V content is preferably 0.010% or more in order to achieve the foregoing effect.
Ti is an element that has the effect of enhancing the toughness of the weld without degrading the mechanical properties of the base metal when welding the steel plate to produce a welded structure. In order to achieve this effect, the Ti content is preferably 0.003% or more. A Ti content of more than 0.03%, however, may cause a decrease in toughness. The Ti content is therefore preferably 0.03% or less.
B is an element that enhances hardenability when added in a small amount. In order to sufficiently achieve this effect, the B content is preferably 0.0003% or more. If the B content is more than 0.0030%, toughness may degrade. Accordingly, in the case where B is added, the B content is preferably 0.0030% or less.
Ca is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel. If the Ca content is excessively high, the cleanliness of the steel may be impaired and the Charpy absorbed energy at low temperatures (hereafter also referred to as Charpy toughness) may decrease. Accordingly, in the case where Ca is added, the Ca content is preferably 0.007% or less. The Ca content is more preferably 0.004% or less. Although no lower limit is placed on the Ca content, the Ca content is preferably 0.001% or more in order to achieve the foregoing effect.
REM (rare earth metal) is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca. If the REM content is excessively high, the cleanliness of the steel may be impaired and Charpy toughness may decrease. Accordingly, in the case where REM is added, the REM content is preferably 0.010% or less. The REM content is more preferably 0.008% or less. Although no lower limit is placed on the REM content, the REM content is preferably 0.001% or more in order to achieve the foregoing effect.
Herein, REM is a generic term for 17 elements including 15 lanthanoid elements and Y and Sc, and these elements may be contained singly or in combination. The REM content is the total content of these elements.
Mg is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca and REM. If the Mg content is excessively high, the cleanliness of the steel may be impaired and Charpy toughness may decrease. Accordingly, in the case where Mg is added, the Mg content is preferably 0.007% or less. The Mg content is more preferably 0.004% or less. Although no lower limit is placed on the Mg content, the Mg content is preferably 0.001% or more in order to achieve the foregoing effect.
(Ferrite Fraction: 5% to 95%, Martensite Austenite Constituent Fraction: 1% to 30%, Residual Microstructure: Tempered Martensite and/or Bainite)
In the microstructure at a depth position of ¼ of the plate thickness from the surface of the steel plate according to the present disclosure in the plate thickness direction, the ferrite fraction is 5% to 95% and the martensite austenite constituent fraction is 1% to 30% with the residual microstructure consisting of tempered martensite and/or bainite. If the ferrite fraction in the microstructure is less than 5%, a yield ratio of 0.80 or less cannot be achieved. If the ferrite fraction is more than 95%, the fraction of hard phase such as martensite austenite constituent is low and a yield ratio of 0.80 or less cannot be achieved. If the martensite austenite constituent fraction is less than 1%, a yield ratio of 0.80 or less cannot be achieved. If the martensite austenite constituent fraction is more than 30%, toughness degrades. If the residual microstructure is not tempered martensite and/or bainite, for example, if the residual microstructure is as-quenched martensite or bainite, the desired toughness cannot be achieved.
(Average Aspect Ratio of Crystal Grains:Less than 2.0, Number Density of Crystal Grains Having Equivalent Circular Diameter of More than 30 μm:250/Mm2 or Less)
In the microstructure at a depth position of ¼ of the plate thickness from the surface of the steel plate according to the present disclosure in the plate thickness direction, when regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more are defined as crystal grains, the average aspect ratio of the crystal grains is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250/mm2 or less. If the average aspect ratio of the crystal grains is 2.0 or more, the orientation of the microstructure tends to be high, causing an increase in acoustic anisotropy. If the number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is more than 250/mm2, toughness decreases. The number density of crystal grains having an equivalent circular diameter of more than 30 μm is preferably 150/mm2 or less.
The plate thickness of the steel plate is not limited and may be any thickness. For example, the plate thickness of the steel plate is preferably 6 mm or more and 50 mm or less.
The tensile strength of the steel plate is not limited, but is preferably 690 MPa or more. With a tensile strength of 690 MPa or more, the plate thickness can be reduced when the steel plate is used for tanks. The tensile strength of the steel plate is more preferably 720 MPa or more. Although no upper limit is placed on the tensile strength, the tensile strength is preferably 1000 MPa or less.
The tensile strength can be measured by the method described in the EXAMPLES section below.
The yield ratio of the steel plate is not limited, but is preferably 0.80 or less. With a yield ratio of 0.80 or less, mechanical stress relief can be performed instead of post weld heat treatment.
The toughness value of the steel plate is not limited, but the Charpy absorbed energy at −50° C. (vE−50° C.) is preferably 100 J or more in a full-size Charpy impact test. vE−50° C. of the steel plate is more preferably 150 J or more.
A method of producing the steel plate according to the present disclosure (production method) will be described below. In the following description, the term “temperature” refers to the temperature at the center of the plate thickness unless otherwise specified. The temperature at the center of the plate thickness can be obtained, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.
The production method is a steel plate production method comprising subjecting a steel material having the above-described chemical composition to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment. In the hot rolling, the finish temperature is 900° C. or more. In the first heating retention, the heating temperature is in the temperature range of Ac3 point or more and 1000° C. or less. In the quenching, the average cooling rate from 600° C. to 300° C. at a depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction is 3° C./s or more, and the cooling end temperature is 300° C. or less. In the second heating retention, the heating temperature is in the temperature range of Ac1 point or more and less than Ac3 point. In the cooling treatment, the average cooling rate from 700° C. to 500° C. at a depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction is 3° C./s or more, and the cooling end temperature is 500° C. or less and 200° C. or more. The steel plate according to the present disclosure can be suitably produced under these conditions.
Each step will be described in detail below.
First, the steel material having the above-described chemical composition may be heated before hot rolling. In this case, the heating temperature of the steel material is preferably 900° C. or more and 1250° C. or less. The method of producing the steel material is not limited. For example, molten steel having the above-described chemical composition may be prepared by steelmaking using a conventional method and cast to produce the steel material. The steelmaking may be performed using any method such as a converter, an electric furnace, or an induction furnace. The casting is preferably performed by continuous casting from the viewpoint of productivity, but may be performed by ingot casting and blooming. An example of the steel material is a steel slab.
The steel material obtained as a result of casting and the like may be heated after cooling, or directly heated without cooling.
If the heating temperature of the steel material is less than 900° C., due to high deformation resistance of the steel material, the load on the mill in the subsequent hot rolling may increase and hinder the hot rolling. Therefore, the heating temperature of the steel material is preferably 900° C. or more. If the heating temperature of the steel material is more than 1250° C., due to noticeable oxidation of the steel, the loss incurred by the removal of the oxide film caused by the oxidation may increase, resulting in a decrease in yield rate. Therefore, the heating temperature of the steel material is preferably 1250° C. or less.
The steel material having the above-described chemical composition is subjected to hot rolling to obtain a hot-rolled steel plate having a final plate thickness. The finish temperature in the hot rolling is 900° C. or more. If the finish temperature is less than 900° C., acoustic anisotropy may degrade. The final plate thickness of the hot-rolled steel plate is not limited, but is preferably 6 mm or more and 50 mm or less as mentioned above. After the hot rolling, the hot-rolled steel plate may be optionally cooled. The cooling method is not limited, and may be, for example, air cooling or water cooling.
After the hot rolling, the hot-rolled steel plate is subjected to heating retention in an austenite range (first heating retention). Thus, the heating temperature in the first heating retention is Ac3 point or more. If the heating temperature of the hot-rolled steel plate is more than 1000° C., crystal grains coarsen and toughness decreases. Accordingly, the heating temperature in the first heating retention is 1000° C. or less.
The hot-rolled steel plate after the first heating retention is subjected to quenching (i.e. accelerated cooling). It is important that, in the quenching, the average cooling rate from 600° C. to 300° C. at a depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction (hereafter such a depth is also denoted as ¼t) is 3° C./s or more.
If the average cooling rate in the quenching is less than 3° C./s, it is difficult to obtain the desired transformed microstructure, so that sufficient strength cannot be achieved. Although no upper limit is placed on the average cooling rate, if the average cooling rate is more than 200° C./s, it is difficult to control the temperature at each position in the steel plate, so that the material quality tends to vary in the plate transverse direction and the rolling direction. This is likely to cause variation in material properties such as tensile property and toughness. Therefore, the average cooling rate is preferably 200° C./s or less.
If the cooling stop temperature (i.e. cooling end temperature) at a position of ¼t in the quenching is more than 300° C., the desired transformed microstructure cannot be obtained. The cooling stop temperature at a position of ¼t in the quenching is therefore 300° C. or less. By accelerated cooling under such conditions, the hot-rolled steel plate is quenched well.
The quenching (accelerated cooling) treatment may be performed by any method without limitation. For example, one or both of air cooling and water cooling may be used. For water cooling, any cooling method (for example, spray cooling, mist cooling, laminar cooling, etc.) using water is available.
The hot-rolled steel plate after the quenching is then subjected to heating retention in a dual-phase temperature range (second heating retention). The heating temperature in the second heating retention is Ac1 point or more and less than Ac3 point. If the heating temperature is less than Ac1 point, martensite austenite constituent cannot be obtained and low yield ratio cannot be achieved. If the heating temperature is Ac3 point or more, ferrite phase will be less than 5% and tempered martensite phase will be more than 90%, and low yield ratio cannot be achieved.
For heating retention in the dual-phase temperature range, any heating method may be used as long as the heating temperature can be controlled in the foregoing manner. An example of the heating method is furnace heating. The furnace heating is not limited, and a typical heat treatment furnace may be used.
After the heating temperature is reached, the hot-rolled steel plate may be held in the dual-phase temperature range for any period of time before the below-described cooling treatment is started. The holding time is not limited, but is preferably 5 minutes or more.
Ac1 point can be calculated according to the following formula (1):
Ac3 point can be calculated according to the following formula (2):
Cooling treatment is then performed. In the cooling treatment, the average cooling rate from 700° C. to 500° C. at a position of ¼t is 3° C./s or more, and the cooling end temperature is 500° C. or less and 200° C. or more. If the average cooling rate is less than 3° C./s, there is a possibility that the desired transformed microstructure cannot be obtained and strength and toughness decrease. If the cooling stop temperature is more than 500° C., there is a possibility that bainite formed after the cooling stop becomes a main component and low-temperature toughness is not satisfied. If the cooling stop temperature is less than 200° C., the desired tempering effect cannot be achieved in the end and toughness degrades.
After the cooling stop (after the end of the cooling treatment), air cooling may be performed in order to improve toughness by self-tempering. The cooling rate in the air cooling is not limited. For example, the cooling rate is typically 1° C./s or less when the plate thickness is 6 mm to 50 mm.
Steel plates were each produced according to the following procedure, and their properties were evaluated.
First, molten steel having the chemical composition shown in Table 1 was prepared by steelmaking using a converter, and subjected to continuous casting to obtain a steel slab (thickness: 200 mm) as a steel material. Ac1 point (° C.) calculated according to the foregoing formula (1) and Ac3 point (° C.) calculated according to the foregoing formula (2) are shown in Table 1 (and Table 2).
N
0.17
O
0.01
P
0.55
Q
0.25
R
0.03
S
2.70
T
0.04
U
0.0070
V
0.0095
W
0.0005
N
O
P
Q
R
S
T
U
V
W
Subsequently, the obtained steel slab was heated and then hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel plate having the corresponding plate thickness (final plate thickness). After this, each treatment was performed under the conditions shown in Table 2 to obtain a steel plate.
750
1050
680
860
0.5
(Air
cool-
ing)
0.5
(Air
cool-
ing)
100
550
450
N
O
P
Q
R
S
T
U
V
W
For each of the obtained steel plates, the microstructure, tensile strength (TS), yield ratio (YR), low-temperature toughness (vE−50° C.), and acoustic anisotropy (sound velocity ratio) were evaluated in the following manner. The evaluation results are shown in Table 3.
A test piece for microstructure observation was collected from the steel plate so that a position of ¼t would be the observation position. The test piece was embedded in resin so that a cross section perpendicular to the rolling direction would be the observation plane, and mirror-polished. After this, nital etching was performed, and then observation was made using a scanning electron microscope with 5000 magnification and an image of microstructure was taken. The obtained image was analyzed to identify microstructure fractions. Microstructures identified are as follows:
Further, microstructure analysis was conducted by EBSD. Defining regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more as crystal grains, for a total area of 1 mm×1 mm, the crystal grain size distribution was calculated and the number density of crystal grains having an equivalent circular diameter of more than 30 μm was measured. In addition, the average value (average aspect ratio) of the aspect ratios (=the length in the rolling direction/the length in the plate thickness direction) of crystal grains obtained in an area of 1 mm×1 mm was calculated.
A JIS No. 4 tensile test piece perpendicular to the rolling direction was collected from a position of ¼t of the steel plate. Using the tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength was 690 MPa or more, the steel plate was evaluated as having high strength and rated as “pass”. The yield ratio (YR) was also evaluated based on the tensile test result. If the yield ratio was 0.80 or less, the steel plate was rated as “pass”.
[Low-Temperature Toughness (vE−50° C.)]
V-notched test pieces parallel to the rolling direction were collected from a position of ¼t of the steel plate in accordance with JIS Z 2202. Using the V-notched test pieces, Charpy impact test was conducted in accordance with JIS Z 2242 to determine the Charpy absorbed energy at −50° C. (vE−50° C.). The Charpy absorbed energy can be regarded as an index of the low-temperature toughness of the steel plate. In the Charpy impact test, three test pieces (A, B, and C) were collected per steel plate and measured. The measurement results are shown in Table 3. In the full-size Charpy impact test, if vE−50° C. of each test piece was 100 J or more, the steel plate was evaluated as having excellent Charpy toughness and rated as “pass”.
In order to evaluate the acoustic anisotropy of the steel plate, the transverse sound velocity ratio specified in JIS Z 3060 was evaluated. The transverse sound velocity ratio herein is a value defined as CSL/CSC, that is, the ratio of the sound velocity CSL (m/sec) when the vibration direction of the transverse wave is the rolling direction (L direction) to the sound velocity CSC (m/sec) when the vibration direction of the transverse wave is a direction (C direction) orthogonal to the rolling direction. The measurement results are shown in Table 3. If CSL/CSC was 1.02 or less, the steel plate was evaluated as having low acoustic anisotropy and rated as “pass”.
2.5
280
100
260
260
bainite
260
260
35
260
As can be seen from these tables, in each Example, a steel plate having a certain chemical composition and microstructure and having high strength, high low-temperature toughness, low yield ratio, and low acoustic anisotropy was obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-069704 | Apr 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/000107 | 1/5/2023 | WO |
