JP2019214751A - Low-yield-ratio thick steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio thick steel sheet excellent in low temperature toughness. A steel sheet has a predetermined chemical composition, Ceq is 0.30 to 0.39, and Csi is 0.11 or less, In the metal structure at the position of 1/4 of the plate thickness, the ferrite area ratio is 45 to 70%, and in the particle size distribution of the ferrite, the ferrite number ratio of less than 10 μm is 75 to 90%, and the ferrite of 10 to 22 μm. Is 8 to 25%, the number ratio of ferrite exceeding 22 μm is 2% or less, and a hard phase exists between ferrite grains. [Selection diagram] None

Description

  The present invention relates to a steel plate having a low yield ratio, for example, a steel plate for a tank used for a tank capable of carrying liquefied petroleum gas (LPG) and liquid ammonia in a mixed manner.

  With an increase in energy demand in recent years, an increase in the capacity of a tank of an energy transport ship has been demanded, and a demand for higher strength and thicker steel plates used for the tank has been increasing. In addition, a steel plate used as a material for tanks such as LPG and ammonia has excellent low-temperature toughness for transporting liquefied gas, and a low yield ratio (YR: yield) for suppressing stress corrosion cracking caused by ammonia. Strength (YS) / tensile strength (TS)) is also required. In general, low-temperature toughness and stress corrosion cracking resistance deteriorate as the strength and yield ratio of a steel sheet increase, and it is difficult to achieve compatibility with high strength.

Techniques for obtaining excellent low-temperature toughness while having a low YR have been studied. For example, in Patent Literature 1, after a slab having a predetermined chemical composition is heated, hot rolling is completed at _three or more points of Ar, the steel sheet is allowed to cool to 620 to 720 ° C, and then accelerated cooling is performed. Stop cooling at ° C. Further, in Patent Document 2, after the slab is heated, hot rolling is performed at a cumulative rolling reduction of 30% or more in a non-recrystallization temperature range, and stopped at 800 ° C. or more, (Ar ₃ temperature−50 ° C.) or more Accelerated cooling at a temperature of 5 to 50 ° C./sec.

JP 2008-261000 A JP-A-11-80832

  In the inventions of Patent Literatures 1 and 2, in order to generate initial ferrite, the steel sheet is allowed to cool to a predetermined temperature after completion of hot rolling, and then water cooling is started. The transformation of the initial ferrite that occurs while the steel sheet is being cooled mainly occurs at the triple point of the crystal grain boundaries, so it is difficult to obtain a fine metal structure uniformly, and the required low-temperature toughness of -60 ° C is required. It is difficult to secure. In addition, since cooling is considered to be performed on the production line, a long cooling time reduces productivity.

  The present inventors have studied to obtain a low-strength steel plate having high strength and excellent stress corrosion cracking resistance and low-temperature toughness for use in large tanks such as LPG and liquid ammonia. Specifically, the present inventors have studied a composition range and a production method which are high in tensile strength at 490 MPa or more, have a low yield ratio, and are also excellent in low-temperature toughness at -60 ° C.

  As a method of lowering the YR, the metal structure of the steel sheet is made to be a mixed structure of a soft phase such as ferrite and a hard phase such as bainite or martensite, so that when a tensile load is applied, the soft phase undergoes plastic deformation with low stress. It is known that a structure design in which the ratio between the yield strength and the breaking strength is increased by increasing the breaking strength of the hard phase is effective. On the other hand, the hard phase becomes a starting point of fracture during the impact test, making it difficult to ensure low-temperature toughness. Although refinement of ferrite crystal grains is effective as a method of improving toughness even when a hard phase is included, refinement of crystal grains causes an increase in yield stress, making it difficult to ensure low YR. .

  An object of the present invention is to provide a low-yield-ratio steel sheet having a tensile strength of 490 MPa or more in a wide thickness range of, for example, 16 to 40 mm and having both low YR and low-temperature toughness.

  The present inventors have conducted research to solve the above technical problem, and in order to achieve both low YR and low-temperature toughness, the contents of C and Si and the ratio of C and Si contents within a predetermined range. In addition, it has been newly found that it is effective to optimize the metallographic structure so that the ferrite area ratio, the ferrite grain size and the number ratio in the steel sheet fall within a predetermined range by controlling the rolling process.

First, the present inventors have defined the International Welding Society (IIW) shown in the following (Equation 1) in order to secure a desired tensile strength in the rolling process and obtain a range in which low-temperature toughness does not deteriorate. As a result of examining the carbon equivalent (Ceq) which is an index of hardenability, it was found that Ceq needs to be set to 0.30 to 0.39.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 (Formula 1)
In the formula (1), the element symbols with [] indicate the content (% by mass) of each element.

  Then, as a result of studying the composition and the metal structure for obtaining the low-temperature toughness and the low YR in this range, the following became clear.

  In a steel sheet having a metal structure in which a hard phase is precipitated, it is effective to increase the difference in hardness between the hard phase and the soft phase in order to obtain low YR. Here, the hard phase refers to each structure of martensite, bainite, pearlite, cementite, and MA (island martensite; a mixed structure of martensite and austenite), and the soft phase refers to a ferrite structure. In order to effectively use the difference in hardness, it is effective to form a hard phase between soft phases (ferrite), specifically, at a grain boundary of a ferrite structure or a triple point of a grain boundary. By arranging the hard phase between the ferrite grains in this way, while the ferrite structure is preferentially deformed, the hard phase therebetween can provide resistance to the deformation, which greatly contributes to the reduction in YR.

  In order to disperse the hard phase (hard phase including MA) between the ferrite grains, primary cooling is performed immediately after finish rolling in the rolling process to increase the transformation driving force, thereby increasing the nucleation frequency, thereby increasing the nucleation frequency. It is effective to form a hard phase at a grain boundary or a grain boundary triple point of a fine ferrite structure. On the other hand, refinement of the ferrite structure is also effective for improving toughness, but it is difficult to secure a low YR because the yield strength is increased. In order to obtain a desired low YR while maintaining low-temperature toughness, it is necessary to control the structure by mixing ferrite grains of an appropriate size for early yielding under a tensile load. For this purpose, the ferrite area ratio and the crystal grain size are controlled to be within predetermined ranges by appropriately setting a standby time between the primary cooling and the secondary cooling, and then the secondary cooling is performed, and the auto-tempering temperature range is set. It has been found that a desired metal structure can be obtained by stopping at. The present invention is as described below.

[1] The composition of the steel sheet is% by mass,
C: 0.03-0.07%,
Si: 0.28 to 0.65%,
Mn: 0.8-1.8%,
P: 0.015% or less,
S: 0.005% or less,
Al: 0.02 to 0.10%,
Nb: 0.005 to 0.040%,
Ti: 0.005 to 0.025%,
N: 0.0015 to 0.0060%,
O: 0.0040% or less,
Cu: 0 to 0.40%,
Ni: 0 to 0.80%,
Cr: 0 to 0.20%,
Mo: 0 to 0.10%,
V: 0 to 0.08%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
The balance has a chemical composition consisting of Fe and impurities,
Ceq defined by the following formula 1 is 0.30 to 0.39,
Csi defined by the following equation 2 is 0.11 or less;
In the metal structure at the position of 1/4 of the plate thickness,
Ferrite area ratio is 45-70%,
In the particle size distribution of the ferrite,
The number ratio of ferrite of less than 10 μm is 75 to 90%,
The number ratio of ferrite of 10 to 22 μm is 8 to 25%, and
The number ratio of ferrite exceeding 22 μm is 2% or less;
Hard phase exists between ferrite grains,
Low yield ratio steel plate.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 (Formula 1)
Csi = [C%] / [Si%] (Equation 2)
In (Formula 1) and (Formula 2), the symbol with [] indicates the content (% by mass) of each element.

  The composition of the present invention, if satisfying the requirements of the metallographic structure, the properties of the present invention can be obtained in the range of 16 to 40 mm in thickness of the thick steel plate, but the present invention is preferable in securing the shape and properties. The thickness range of the thick steel plate is 32 to 40 mm.

  According to the present invention, high tensile strength of 490 MPa or more, excellent low-temperature toughness at −60 ° C., and a low yield ratio can provide a high resistance to stress corrosion cracking. Therefore, it is possible to provide a steel material suitable for use in tanks such as LPG and ammonia where stress corrosion cracking is a concern, and the industrial effect is extremely large.

Hereinafter, the present invention will be described in detail.
First, the reason for limiting the composition of the steel plate according to the present invention as described above will be described in detail.

C: 0.03 to 0.07%
C is the most important element that determines the strength. Since it affects the hardness of the hard phase when the hard phase is finely dispersed, the C content is reduced. When the content of C is less than 0.03%, it becomes difficult to obtain required strength. On the other hand, if the content exceeds 0.07%, the hard phase is excessively hardened and the toughness is deteriorated. In order to suppress deterioration of the hard phase and stably maintain low-temperature toughness, C is preferably set to 0.03 to 0.05%.

Si: 0.28 to 0.65%
Si is an element effective for promoting the formation of the hardened phase MA and finely dispersing it. In order to obtain these effects, it is necessary to contain 0.28% or more of Si. On the other hand, if the content exceeds 0.65%, the surface properties of the steel sheet deteriorate, so this is made the upper limit. In order to promote the production of MA and stably obtain YR, the content of Si is preferably set to 0.32 to 0.65%.

Mn: 0.8-1.8%
Mn enhances the strength by increasing the hardenability, and reduces the adverse effect of S by forming MnS, thereby improving the elongation and toughness in a tensile test. To obtain these effects, Mn must be contained at 0.8% or more. On the other hand, if the content exceeds 1.8%, the hardenability becomes excessive and the toughness is deteriorated, so that the upper limit is set. Since the Mn content increases as the Mn content increases, the elongation increases. In order to stably secure the elongation, the Mn content is preferably set to 1.2 to 1.8%.

P: 0.015% or less P is present as an impurity, segregates at the crystal grain boundaries to deteriorate toughness, and also causes tempering embrittlement. Therefore, it is desirable that the P content be as low as possible. If the content of P exceeds 0.015%, the deterioration is remarkable, so the content of P is limited to 0.015% or less.

S: 0.005% or less Since S exists as an impurity and deteriorates the ductility and toughness of a steel sheet, its content is desirably as low as possible. If the content exceeds 0.005%, the adverse effect becomes remarkable, so the S content is limited to 0.005% or less.

Al: 0.02 to 0.10%
Al is an element added to obtain the cleanliness of the molten steel. To obtain the effect, it is necessary to contain 0.02% or more of Al. On the other hand, if the Al content exceeds 0.10%, coarse alumina is generated and the toughness is deteriorated, so that the upper limit is set.

Nb: 0.005 to 0.040%
Nb is an element effective in expanding the unrecrystallized region of austenite, and contributes to refinement of the structure by rolling, so that toughness can be improved. To obtain this effect, the content needs to be 0.005% or more. On the other hand, if the content exceeds 0.040%, the improvement in YP by refining the structure becomes remarkable, and it becomes difficult to reduce the YR. Therefore, the upper limit is set.

Ti: 0.005 to 0.025%
Ti forms a nitride and has an effect of suppressing coarsening of crystal grains due to heating, and thus contributes to improvement in toughness. To obtain the effect, it is necessary to contain 0.005% or more of Ti. On the other hand, if the content exceeds 0.025%, the carbide precipitates excessively and the toughness deteriorates.

N: 0.0015 to 0.0060%
N contributes to an improvement in toughness by forming a nitride to suppress coarsening of the structure during heating. In order to obtain the effect, N must be contained at 0.0015% or more. However, if N is excessively contained at more than 0.0060%, the nitride is coarsened and the toughness is deteriorated. This is the upper limit.

O: 0.0040% or less O exists as an impurity and forms an oxide in the steel sheet. If O is present in a large amount, the number of oxides increases and the toughness deteriorates. Therefore, the content of O is set to 0.0040% or less.

Cu: 0 to 0.40%
Since the strength of Cu can be improved by improving the hardenability, it may be contained as necessary. In order to obtain the above-mentioned effects, it is preferable to contain 0.05% or more. On the other hand, if it exceeds 0.40%, the toughness deteriorates, so this is made the upper limit.

Ni: 0 to 0.80%
Ni is a useful element that not only improves the hardenability to obtain strength but also improves the low-temperature toughness, and therefore may be contained as necessary. In order to obtain the above-mentioned effects, it is preferable to contain 0.05% or more. On the other hand, Ni is an expensive alloy element, and a content exceeding 0.80% does not meet economic rationality.

Cr: 0 to 0.20%
Since Cr can improve the strength by improving the hardenability, it may be contained as necessary. To obtain the above effects, the content is preferably 0.05% or more. On the other hand, if it exceeds 0.20%, it becomes difficult to obtain a low YR, so this is made the upper limit.

Mo: 0 to 0.10%
Mo can improve the quenchability and form a carbide to improve the strength. Therefore, Mo may be contained as necessary. In order to obtain the above effects, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.10%, it becomes difficult to obtain a low YR, so this is made the upper limit.

V: 0 to 0.08%
V generally improves hardenability and can improve strength by forming carbides, so that V may be contained as necessary. In order to obtain the above effects, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.08%, the toughness deteriorates, so this is made the upper limit.

B: 0 to 0.002%
B is effective for improving the hardenability and the strength with a small amount, so that B may be contained as necessary. On the other hand, if the content exceeds 0.002%, the toughness deteriorates, so this is made the upper limit. To obtain the above effects, the content is preferably 0.0004% or more.

Ca: 0 to 0.005%
Ca is effective in improving the toughness by reducing the adverse effect of S by forming a sulfide, and thus may be contained as necessary. On the other hand, if the content exceeds 0.005%, a coarse oxide is formed, which adversely affects toughness. Therefore, the upper limit is set. In order to obtain the above effects, the content is preferably 0.0008% or more.

  The balance in the chemical composition of the low yield ratio steel sheet of the present invention is Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially.

In order to secure the required tensile strength in the rolling process, the carbon equivalent (Ceq), which is an index of quenching hardness defined by the International Welding Society (IIW), is set to 0.30 to 0.30 as shown in (Equation 1) below. 0.39.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 (Formula 1)
In the formula (1), the element symbols with [] indicate the content (% by mass) of each element.

  Ceq defined by the above (Equation 1) is an index indicating the hardenability of the steel sheet. In order to secure the tensile strength, the content of C, Mn, Cu, Ni, Cr, Mo, and V contained in the steel sheet needs to be equal to or greater than 0.30 for Ceq defined by the above (Equation 1). There is. When Ceq is less than 0.30, sufficient tensile strength cannot be obtained due to insufficient hardenability. The tensile strength increases as Ceq increases, but if it exceeds 0.39, the tensile strength becomes excessive, and the energy absorbed by Charpy decreases significantly. Therefore, Ceq defined by the above (Equation 1) is set to 0.30 to 0.39.

Further, in the present invention, Csi defined by the following (Equation 2) is set to 0.11 or less.
Csi = [C%] / [Si%] (Equation 2)
In the formula (2), the element symbols with [] indicate the content (% by mass) of each element.

  Si concentrates C into untransformed austenite during ferrite transformation and promotes the formation of MA. In general, MA deteriorates toughness. However, if the C content is small and the degree of dispersion of MA is large, the amount of C contained in one MA becomes small, so that the deterioration of toughness is suppressed. The hard phase can be finely dispersed. For that purpose, Csi needs to be 0.11 or less. On the other hand, when Csi exceeds 0.11, the low-temperature toughness deteriorates due to excessive hardening of the hard phase. The smaller the Csi is, the lower the YR tends to be. It is more preferable to set the Csi to 0.10 or less in order to stably obtain a low YR.

  Further, in the present invention, it is necessary to control the metallographic structure in the rolling process in order to achieve a low YR and an improvement in the toughness of the steel material, and the metallographic structure at the 1/4 thickness position of the steel sheet is defined. The metal structure is basically ferrite, but the main structure other than ferrite is bainite, and a small amount of martensite, pearlite, and MA may be present. As described later, a hard phase such as martensite exists between ferrite grains.

Ferrite area ratio of metal structure: 45 to 70%
Ferrite has a small amount of C solid solution, and C is diffused into surrounding austenite during transformation to become a soft phase. Since the yield strength and tensile strength decrease as the area ratio of ferrite increases, the area ratio of ferrite must be 70% or less in order to obtain a predetermined strength and low YR. On the other hand, when the ferrite area ratio is less than 45%, the role of the soft phase becomes insufficient, and the yield strength increases, so that it is difficult to reduce the YR. Therefore, the ferrite area ratio is set to 45 to 70%.

  In the ferrite structure at the position of 1/4 of the plate thickness, the grain size distribution of the ferrite is as follows. Here, the particle diameter in the ferrite particle diameter is a so-called equivalent circle diameter (diameter of a circle when the projected area is regarded as a circle having the same area), and can be easily measured by using an image analyzer. In a measurement using an actual image analyzer, a ferrite having a very small particle size may be observed. When counting the number of all such ferrites and calculating the number ratio for each particle size, the number ratio of ferrite having a ferrite particle size of less than 10 μm may increase significantly. Since the total area occupied by the ferrite having such a small particle size in the whole is extremely small, it is not actually necessary to count it as a number. Specifically, the ferrite number ratio may be calculated by counting the number of ferrites having a circle equivalent diameter of 2 μm or more.

Ferrite number ratio of ferrite particle size less than 10 μm: 75 to 90%
The low-temperature toughness improves as the number ratio of ferrite having a small grain size increases. To ensure the required low-temperature toughness, the number ratio of ferrite having a grain size of less than 10 μm must be 75% or more. On the other hand, when the ferrite number ratio of less than 10 μm exceeds 90%, coarse ferrite grains having a particle size of more than 10 μm decrease, and the number of ferrites that yield early under a tensile load becomes insufficient. It becomes difficult to reduce the YR. Therefore, the ratio of the number of ferrites having a ferrite particle size of less than 10 μm is set to 75 to 90%.

Ferrite number ratio of ferrite particle size of 10 to 22 μm: 8 to 25%
If the number ratio of coarse ferrite having a particle size of 10 μm or more increases, yield occurs early under a tensile load, which is effective for securing a low YR. On the other hand, coarse ferrite has a problem of deteriorating low-temperature toughness. Ferrite having a grain size of 10 to 22 μm can achieve both low YR and low-temperature toughness. If the ferrite number ratio is less than 8%, the yield strength increases, while if it exceeds 25%, the low-temperature toughness deteriorates. The ratio of the number of ferrites having a diameter of 10 to 22 μm is 8 to 25%.

Ferrite number ratio exceeding 22 μm: 2% or less Ferrite having a particle size exceeding 22 μm remarkably deteriorates low-temperature toughness. Therefore, it is necessary to minimize the number of ferrite particles. Since low-temperature toughness can be obtained, the ratio of the number of ferrites having a ferrite grain size of more than 22 μm is set to 2% or less.

  The reason why the ferrite grain size at a quarter position of the sheet thickness is defined is to determine the ferrite grain size at an average position of the steel sheet. Further, when obtaining the ferrite grain size distribution at the 1/4 position of the plate thickness, the microstructure in a region of ± 2 mm around the 1/4 position of the plate thickness is observed, and based on the observation result, the size of each ferrite particle is determined. What is necessary is just to measure the number ratio.

  As described above, in order to obtain a low YR, it is necessary to secure required ferrite in the metal structure of steel and to generate a hard phase between ferrite grains. Here, the hard phase is a phase other than ferrite and bainite, and is a phase harder than ferrite excluding bainite, such as martensite and cementite in addition to MA. The hard phase here includes fine ones and those that are difficult to discriminate, and its area ratio and particle size are sometimes difficult to accurately quantify. When the steel plate of the present invention is manufactured by the manufacturing method shown in the figure, the area ratio of the hard phase generated between the ferrite grains is 2.0 to 9.0%, and the average grain size of the top 20 hard phases having a large grain size is obtained. The diameter is 2.0-8.0 μm.

  As a method for securing the ferrite ratio, a method is used in which the steel sheet is cooled for a certain period of time after the finish rolling, the steel sheet is cooled to a predetermined temperature to transform the ferrite, and then water-cooled.

  However, in the conventional method, it is necessary to allow the steel sheet to be cooled on the production line for a certain period of time, which is not preferable from the viewpoint of production efficiency, and the ferrite that is transformed from a high temperature during the cooling is easily coarsened. Ferrite number ratio is reduced.

  Therefore, as a result of repeated studies by the present inventors, preliminary cooling (primary cooling) such as water cooling is performed before accelerated cooling (secondary cooling) such as water cooling, thereby solving the problem of manufacturing efficiency. By increasing the transformation driving force of the ferrite by the primary cooling, the ferrite can be transformed in a short time and finely, so that the ferrite number ratio of less than 10 μm can be efficiently secured.

  The method for producing a low-yield-ratio steel sheet according to the present invention is not particularly limited as long as it is possible to produce a steel sheet having the above-described chemical composition and metal structure. Can be.

First, a slab having the above-mentioned chemical composition is heated in a heating furnace to a temperature range of 1050 to 1200 ° C., and then extracted from the heating furnace and subjected to hot rolling to manufacture a steel sheet. At this time, the conditions are such that the cumulative draft in the temperature range of 900 ° C. or less is 30% or more, and the rolling end temperature T _FR (° C.) is ₃ or more Ar points defined by the following (Equation 3) at the steel sheet surface temperature. To roll. After rolling, primary cooling and secondary cooling are performed.
Ar ₃ (° C.) = 910-310 × [C%] − 80 × [Mn%] − 20 × [Cu%] − 15 × [Cr%] − 55 × [Ni%] − 80 × [Mo%] + 0 .35 × ([plate thickness (mm)]-8) (Equation 3)
In (Equation 3), the element symbols with [] indicate the content (% by mass) of each element.

In the above-described primary cooling and secondary cooling, a cooling process is performed under the following conditions (a) to (e).
(A) a first cooling, the steel plate surface temperature _T FR _~T FR -50 ℃, and starts cooling in a range of _three or more points Ar, (Ar ₃ temperature -30 ℃) ~ (Ar ₃ temperature -130 (° C) range.
(B) The average cooling rate of the first cooling is set to 10 ° C./sec or more.
(C) The time from the first cooling to the second cooling is set to 10 to 40 sec.
(D) Stop the second cooling in the range of 350 ° C to 500 ° C.
(E) The average cooling rate of the second cooling is set to 15 ° C./sec or more.
As described above, the first cooling and the second cooling may be performed using one cooling device, or may be performed by continuously moving the steel plate using two cooling devices.

  Each step will be described in detail below. In addition, about the temperature shown below, it is a steel plate surface temperature unless there is particular notice.

Heating temperature of slab before hot rolling: 1050 to 1200 ° C
When the heating temperature is lower than 1050 ° C., the austenite crystal grains are refined, so that the ferrite crystal grains are also refined. In this case, since the yield strength becomes too high, it is difficult to secure a low YR. On the other hand, if the heating temperature exceeds 1200 ° C., austenite crystal grains may be coarsened and low-temperature toughness may be reduced.

Cumulative draft in the temperature range of 900 ° C or less: 30% or more Cumulative draft in the temperature range of 900 ° C or less refers to the reduction in the thickness of the sheet rolled to the thickness after finish rolling, based on the thickness at 900 ° C. Rate. If the cumulative rolling reduction is less than 30%, the crystal grains become coarse, and the low-temperature toughness may decrease.

Rolling end temperature T _FR : Ar ₃ points or more If the rolling end temperature T _FR is lower than Ar ₃ points, proeutectoid ferrite may be generated before cooling. For this reason, the rolling end temperature _TFR is set to _three points or more at the steel sheet surface temperature. Note that the Ar ₃ points are as shown in the above (Equation 3), and the Ar ₃ points in the cooling step described below are also the same.

Next, the cooling step will be described in detail below.
(A) a first cooling, the steel plate surface temperature _T FR _~T FR -50 ℃, and starts cooling in a range of _three or more points Ar, (Ar ₃ temperature -30 ℃) ~ (Ar ₃ temperature -130 (° C) range.
Starting temperature of the first cooling, since the rolling finishing temperature is T _FR, the following rolling end temperature T _FR. If the temperature drops to less than _TFR -50 ° C before the start of the first cooling, the dislocations introduced by rolling recover and the driving force for transformation decreases. For this reason, the number ratio of ferrite having a coarse particle diameter may increase. On the other hand, if the cooling start temperature is lower than the Ar ₃ point, proeutectoid ferrite may be generated before cooling. Therefore, the first cooling the steel plate surface temperature _T FR _~T FR -50 ℃, and cooling is started in a range of _three or more Ar.

On the other hand, if the cooling stop temperature of the first cooling is higher than (Ar ₃ temperature−30 ° C.), a sufficient transformation driving force cannot be obtained by cooling, and the toughness is reduced due to an increase in the number ratio of ferrite having a coarse grain size. . On the other hand, when the temperature is lower than (Ar ₃ temperature −130 ° C.), the transformation driving force is increased, so that the ferrite grains are excessively refined, and the yield strength is increased, so that it may be difficult to secure a low YR. Therefore, the first cooling is stopped in the range of (Ar ₃ temperature−30 ° C.) to (Ar ₃ temperature−130 ° C.).

(B) The average cooling rate of the first cooling is set to 10 ° C./sec or more.
When the average cooling rate of the first cooling is less than 10 ° C./sec, the ferrite transformation starts during the cooling, so that the number ratio of ferrite having a coarse grain size increases, and the toughness may decrease. For this reason, the average cooling rate of the first cooling is set to 10 ° C./sec or more. Although the upper limit of the average cooling rate of the first cooling is not specified, the average cooling rate of the first cooling is usually 40 ° C./sec or less from the viewpoint of the performance of the normal water cooling device.

(C) The time from the first cooling to the second cooling is set to 10 to 40 sec.
The time from the primary cooling to the secondary cooling is the cooling time from the stop of the primary cooling. If this time is less than 10 seconds, the ferrite transformation after the stop of the primary cooling becomes insufficient, and the ferrite area ratio becomes insufficient. . On the other hand, if it exceeds 40 seconds, the crystal grains of ferrite become coarse, so that the toughness may be deteriorated. For this reason, the time from the first cooling to the second cooling is set to 10 to 40 seconds. At this time, the surface temperature of the steel sheet at the start of the second cooling is in the range of (Ar ₃ temperature−30 ° C.) to (Ar ₃ temperature−150 ° C.) due to reheating of the steel sheet.

(D) Stop the second cooling in the range of 350 ° C to 500 ° C.
If the cooling stop temperature is 500 ° C. or higher, the hard phase required for obtaining a low YR may be insufficient. On the other hand, if the cooling stop temperature is less than 350 ° C., the hard phase is excessively hardened, and the toughness may be deteriorated. Therefore, the cooling stop temperature in the second cooling is set in the range of 350 to 500 ° C.

(E) The average cooling rate of the second cooling is set to 15 ° C./sec or more.
When the average cooling rate of the second cooling is low, a sufficient hard tissue cannot be obtained, so that it may be difficult to obtain a low YR. For this reason, the average cooling rate of the second cooling is set to 15 ° C./sec or more. Although the upper limit of the average cooling rate of the second cooling is not specified, the average cooling rate of the second cooling is 40 ° C./sec or less from the viewpoint of the performance of the normal water cooling device.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels having the chemical components shown in Tables 1 to 3 were melted, and cast pieces were produced with a continuous casting machine. The obtained slab was subjected to hot rolling under the conditions shown in Tables 4 to 6 and then subjected to first cooling and second cooling to obtain a thick steel plate having a predetermined thickness.

  About the obtained thick steel plate, the microstructure and the following various physical properties were investigated.

<Mechanical properties>
After performing stress relief annealing, a round bar tensile test piece having a parallel portion length of 8.5 mm and a gauge length of 42.5 mm was prepared from each of the obtained steel plates. At this time, the test piece was cut out such that the length direction of the round bar tensile test piece was perpendicular to the rolling direction (board width direction). Using a round bar tensile test piece, a tensile test is performed at normal temperature and atmospheric pressure, yield strength YS (MPa), tensile strength TS (MPa), yield ratio YR (= YS / TS × 100, unit is%) And the total elongation EL (%) was determined.

<Low temperature toughness>
A Charpy test piece (2 mm V notch test piece) based on JIS Z2242-2016 was sampled from a quarter position of the plate thickness. The notch position was in the thickness direction. Three tests were carried out at -60 ° C, and the lowest value was taken as absorbed energy (vE-60).

<Microstructure>
Microstructure, after polishing a cross section cut from the center of the Charpy test piece subjected to the test, corroding the surface with nital, observing a region of ± 2 mm from the center with an optical microscope, identifying the ferrite structure After removing ferrite grains having an equivalent circle diameter of less than 2 μm, each fraction (number proportion) was determined for each grain size. Although not shown in Tables 7 to 9, a hard phase is formed between the ferrite grains. At this time, the area ratio of the hard phase is 2.1 to 8.4%, and the upper 20 particles having the larger grain size are used. The average value of the particle sizes of the individual pieces was 2.3 to 7.2 μm.

  Tables 7 to 9 summarize these results. In the present invention, when TS is 490 MPa or more, YS is 355 MPa or more, YR is 78% or less, and EL is 21% or more, it is evaluated that the steel has the characteristics of a low YR steel, and vE-60 Was evaluated to be excellent in low-temperature toughness when it was 47 J or more, and the overall judgment was passed.

  With reference to Tables 1 to 9, test symbols A1 to A56, which are examples of the present invention satisfying all of the chemical composition, metallographic structure and production conditions defined in the present invention, have low YR and excellent low-temperature toughness. became.

  On the other hand, Test Nos. B1 to B26, which are comparative examples, did not satisfy any one or more of the chemical composition, the metal structure, and the manufacturing conditions, and as a result, desired characteristics were not obtained.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the low-yield-ratio steel plate which has excellent low temperature toughness suitable for using as a tank material, such as LPG or ammonia.

Claims (1)

The composition of the steel sheet is mass%
C: 0.03-0.07%,
Si: 0.28 to 0.65%,
Mn: 0.8-1.8%,
P: 0.015% or less,
S: 0.005% or less,
Al: 0.02 to 0.10%,
Nb: 0.005 to 0.040%,
Ti: 0.005 to 0.025%,
N: 0.0015 to 0.0060%,
O: 0.0040% or less,
Cu: 0 to 0.40%,
Ni: 0 to 0.80%,
Cr: 0 to 0.20%,
Mo: 0 to 0.10%,
V: 0 to 0.08%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
The balance has a chemical composition consisting of Fe and impurities,
Ceq defined by the following formula 1 is 0.30 to 0.39,
Csi defined by the following equation 2 is 0.11 or less;
In the metal structure at the position of 1/4 of the plate thickness,
Ferrite area ratio is 45-70%,
In the particle size distribution of the ferrite,
The number ratio of ferrite of less than 10 μm is 75 to 90%,
The number ratio of ferrite of 10 to 22 μm is 8 to 25%, and
The number ratio of ferrite exceeding 22 μm is 2% or less;
Hard phase exists between ferrite grains,
Low yield ratio steel plate.
Ceq = [C%] + [Mn%] / 6 + ([Cu%] + [Ni%]) / 15 + ([Cr%] + [Mo%] + [V%]) / 5 (Formula 1)
Csi = [C%] / [Si%] (Equation 2)
In (Formula 1) and (Formula 2), the symbol with [] indicates the content (% by mass) of each element.