JP2017078212A - Low yield ratio steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low yield ratio steel sheet having small variation in mechanical properties and excellent low-temperature toughness and a method for efficiently producing the same.SOLUTION: A low yield ratio steel sheet has a chemical composition comprising, in mass%, C: 0.03-0.10%, Si: 0.05-0.5%, Mn: 0.9-2.0%, P: 0.020% or less, S: 0.010% or less, Nb: 0.005-0.05%, Ti: 0.005-0.025%, sol.Al: 0.005-0.090%, N: 0.001-0.010%, Cu: 0-0.50%, Ni: 0-0.50%, Cr: 0-0.20%, Mo: 0-0.20%, V: 0-0.06%, B: 0-0.002%, Ca: 0-0.005%, Mg: 0-0.005%, with the balance being Fe and impurities, satisfying [0.10≤Cu+Ni+Cr+Mo≤1.0], the low yield ratio steel sheet having a metallographic structure in which: in a ferrite grain size distribution at a position of 1/4 of the sheet thickness, a percentage of the number of ferrite less than 10 μm is 70-85%, a percentage of the number of ferrite 10-20 μm is 13.5-30%, a percentage of the number of ferrite more than 20 μm is 1.5% or less, and a maximum ferrite grain size is 22 μm or less.SELECTED DRAWING: None

Description

  The present invention relates to a low yield ratio steel plate and a method for producing the same, and more particularly to a low yield ratio steel plate having excellent low temperature toughness and a method for producing the same.

  Steel sheets used as materials for tanks such as liquefied petroleum gas (LPG) or ammonia are required to have a low yield ratio (YR) in order to prevent stress corrosion cracking and the like. In order to obtain a low YR, it is necessary to ensure a certain amount of ferrite in the metal structure of the steel. In addition, steel plates used as structural members are required to reduce variations in mechanical properties from the viewpoint of reliability.

  Various studies have been made on methods for obtaining a steel material having a low YR. For example, Patent Document 1 discloses a method for producing a low-yield ratio high-tensile steel plate having excellent toughness. Patent Document 2 discloses a non-tempered low yield ratio high-tensile thick steel plate suitable for use in building structural members that require earthquake resistance and a method for manufacturing the same. Furthermore, in Patent Document 3, there is a method for economically producing steel with high yield and low yield ratio, with a small fluctuation range of yield strength, and capable of applying high-accuracy stress design. It is disclosed.

Japanese Patent Laid-Open No. 3-104820 JP 2014-177669 A JP 2000-87137 A

  Since a tank such as LPG needs to store a very low temperature liquid or gas, a steel sheet used as its material is required to have not only low YR but also excellent low temperature toughness.

  However, in the technologies described in Patent Documents 1 to 3, use in an extremely low temperature environment of −40 ° C. or lower is not assumed. Therefore, Patent Documents 1 to 3 do not discuss any improvement in low temperature toughness.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a low yield ratio steel sheet having excellent low temperature toughness and a method for efficiently producing the same.

  As a result of intensive studies on a method for obtaining a steel sheet having low YR and excellent low-temperature toughness, the present inventors have obtained the following knowledge.

  In order to lower YR, it is known that it is effective to make the steel structure a mixed structure of ferrite as a soft phase and bainite or martensite as a hard phase. At this time, in addition to these structures, pearlite or MA (mixed structure of martensite and austenite) may be present in the steel.

  It has been found that control of the crystal grain size of ferrite, which is a soft phase, is important in order to achieve both low YR and improved low-temperature toughness of steel materials. Refinement of ferrite leads to an increase in yield stress, which leads to an increase in YR. On the other hand, coarse ferrite causes deterioration of low temperature toughness. In consideration of these, it is important to control the particle size distribution of ferrite.

  The present invention has been made on the basis of the above findings, and the gist thereof is the following low yield ratio steel sheet and a method for producing the same.

(1) The chemical composition is mass%,
C: 0.03-0.10%,
Si: 0.05 to 0.5%,
Mn: 0.9 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.025%,
sol. Al: 0.005 to 0.090%,
N: 0.001 to 0.010%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.20%,
Mo: 0 to 0.20%,
V: 0 to 0.06%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Balance: Fe and impurities,
Satisfying the following formula (i)
In ferrite grain size distribution at 1/4 position of plate thickness,
70% to 85% of the number ratio of ferrite of less than 10 μm,
The number ratio of 10-20 μm ferrite is 13.5-30%,
The number ratio of ferrite exceeding 20 μm is 1.5% or less, and
Maximum ferrite grain size is 22μm or less,
A low yield ratio steel sheet having a metallographic structure.
0.10 ≦ Cu + Ni + Cr + Mo ≦ 1.0 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.

(2) The chemical composition is mass%,
Cu: 0.05 to 0.50%,
Ni: 0.05 to 0.50%,
Cr: 0.04-0.20% and Mo: 0.005-0.20%
The low yield ratio steel sheet according to (1) above, which contains one or more selected from the above.

(3) The chemical composition is mass%,
V: 0.005-0.06%
The low yield ratio steel sheet according to the above (1) or (2).

(4) The chemical composition is mass%,
B: 0.0002 to 0.002%,
Ca: 0.002-0.005% and Mg: 0.001-0.005%
The low yield ratio steel sheet according to any one of (1) to (3) above, which contains one or more selected from the above.

(5) After heating the slab having the chemical composition according to any one of (1) to (4) above to a temperature range of 1050 to 1200 ° C,
Rolling is performed under a condition that the cumulative rolling reduction in the temperature range of 900 ° C. or lower is 30% or more and the rolling end temperature T _FR (° C.) is Ar ₃ points or more at the steel sheet surface temperature,
After rolling, the steel sheet is subjected to cooling treatment under the conditions shown in the following (a) to (e) using an accelerated cooling device including the first cooling device and the second cooling device without stopping the steel sheet on the production line. A method for producing a specific steel sheet.
(A) cooling in the first cooling device, the steel sheet surface temperature _T FR _~T FR -50 ℃, and, starting with a range of _three or more points Ar, in the range of Ar ₃ point to Ar ₃ point -100 ° C. Stop.
(B) The average cooling rate in the first cooling device is 10 ° C./s or more.
(C) The moving time from the outlet of the first cooling device to the inlet of the second cooling device is 10 to 40 s.
(D) Cooling by the second cooling device is started when the steel sheet surface temperature is in the range of Ar ₃ point −15 ° C. to Ar ₃ point −100 ° C. and stopped in the range of 550 ° C. or less.
(E) The average cooling rate in the second cooling device is set to a rate exceeding 15 ° C./s.

  According to the present invention, it is possible to efficiently produce a low yield ratio steel sheet having small variations in mechanical properties and excellent low temperature toughness. The low yield ratio steel sheet according to the present invention is suitable for use as a tank material such as LPG or ammonia.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reason for limitation of each element is as follows. In the following description, “%” for the content means “% by mass”.

C: 0.03-0.10%
C is an element that increases the strength of the steel material. When the C content is less than 0.03%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.10%, the effect of refining the ferrite grain size increases, so the yield strength increases and low YR cannot be obtained. Therefore, the C content is 0.03 to 0.10%. The C content is preferably 0.04% or more, and preferably 0.07% or less.

Si: 0.05-0.5%
Si is an element having a function of deoxidizing steel and further promoting the formation of ferrite. If the Si content is less than 0.05%, these effects cannot be obtained. On the other hand, if the Si content exceeds 0.5%, the amount of MA produced increases remarkably and the toughness decreases. Therefore, the Si content is 0.05 to 0.5%. The Si content is preferably 0.1% or more, and preferably 0.23% or less.

Mn: 0.9 to 2.0%
Mn is an element that increases the hardenability of steel and increases the strength and toughness of the steel material. If the Mn content is less than 0.9%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 2.0%, the center segregation becomes remarkable and the toughness decreases. Therefore, the Mn content is set to 0.9 to 2.0%. The Mn content is preferably 1.2% or more, and preferably 1.6% or less.

P: 0.020% or less P is an impurity element, which lowers the mechanical properties of the steel material, particularly lowers the low temperature toughness. Therefore, the P content is 0.020% or less. The P content is preferably 0.015% or less, more preferably as low as possible.

S: 0.010% or less S is an impurity element, which combines with Mn to form MnS, thereby reducing the low temperature toughness of the steel material. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less, and more preferably as low as possible.

Nb: 0.005 to 0.05%
Nb is an element effective for expanding the austenite non-recrystallized region, contributes to refinement of crystal grains, and improves strength and toughness. If the Nb content is less than 0.005%, these effects cannot be obtained. On the other hand, if the Nb content exceeds 0.05%, the ferrite is remarkably refined and the YR cannot be reduced. Therefore, the Nb content is 0.005 to 0.05%. The Nb content is preferably 0.01% or more, and preferably 0.04% or less.

Ti: 0.005-0.025%
Ti combines with N in steel to form TiN, and is an element that improves the cleanliness of the slab surface and the steel material surface. Furthermore, it has the effect | action which suppresses the coarsening of an austenite crystal grain. If the Ti content is less than 0.005%, these effects cannot be obtained. On the other hand, if the Ti content exceeds 0.025%, the amount of precipitated carbide increases and the toughness decreases. Therefore, the Ti content is 0.005 to 0.025%. The Ti content is preferably 0.007% or more, and preferably 0.020% or less.

sol. Al: 0.005-0.090%
Al is an element having a function of deoxidizing steel. sol. When the content as Al (“acid-soluble Al”) is less than 0.005%, this effect cannot be obtained. On the other hand, sol. If the Al content exceeds 0.090%, the production amount of MA increases and coarse alumina is produced, so that the toughness is lowered. Therefore, sol. Al content shall be 0.005-0.090%.

N: 0.001 to 0.010%
N is an element having an action of binding to Ti to form TiN and suppressing austenite grain coarsening. If the N content is less than 0.001%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.010%, the solid solution N amount increases and the toughness decreases. Therefore, the N content is 0.001 to 0.010%. The N content is preferably 0.002% or more, and preferably 0.008% or less.

Cu: 0 to 0.50%
Since Cu is an element that enhances the strength and corrosion resistance of the steel material, it may be contained as necessary. However, when the Cu content is excessive, hot cracking is likely to occur. Therefore, the Cu content is 0.50% or less. The Cu content is preferably 0.40% or less. In order to obtain the above effect, the Cu content is preferably 0.05% or more.

Ni: 0 to 0.50%
Since Ni is an element that dissolves in steel and increases the strength and toughness of the steel, it may be contained if necessary. However, when the Ni content is excessive, not only this effect is saturated, but also the manufacturing cost increases. Therefore, the Ni content is 0.50% or less. The Ni content is preferably 0.45% or less. In order to obtain the above effect, the Ni content is preferably 0.05% or more.

Cr: 0 to 0.20%
Since Cr is an element that increases the strength of the steel material, it may be contained as necessary. However, if the Cr content is excessive, the strength of the steel material becomes too high. Therefore, the Cr content is 0.20% or less. The Cr content is preferably 0.15% or less. When the above effect is desired, the Cr content is preferably 0.04% or more.

Mo: 0 to 0.20%
Mo is an element that enhances the strength of the steel material, and contributes to the refinement of ferrite, so it may be contained as necessary. However, when the Mo content is excessive, the ferrite grains are remarkably refined, and the yield ratio is increased as the yield strength is increased. Therefore, the Mo content is 0.20% or less. The Mo content is preferably 0.15% or less. When it is desired to obtain the above effect, the Mo content is preferably 0.005% or more.

0.10 ≦ Cu + Ni + Cr + Mo ≦ 1.0 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.
In order to increase the strength of the steel material, it is necessary to contain one or more selected from the above Cu, Ni, Cr and Mo so that the total content is 0.10% or more. On the other hand, when these elements are contained in combination, the total content needs to be 1.0% or less. That is, it is necessary to satisfy the above-mentioned regulations for the content of each element and to satisfy the above formula (i) for the total content.

V: 0 to 0.06%
V has a function of forming carbonitride and precipitating and strengthening the steel material. Therefore, V may be contained as necessary. However, when the V content is excessive, not only the effect is saturated, but also the amount of fine precipitates generated increases and the toughness decreases. Therefore, the V content is 0.06% or less. The V content is preferably 0.05% or less. When it is desired to obtain the above effect, the V content is preferably 0.005% or more, and more preferably 0.01% or more.

B: 0 to 0.002%
B is an element that enhances the hardenability of the steel material in a small amount, and may be contained if necessary. However, when the B content is excessive, precipitates are generated and the toughness is lowered. Therefore, the B content is 0.002% or less. The B content is preferably 0.0015% or less. In order to obtain the above effect, the B content is preferably 0.0002% or more.

Ca: 0 to 0.005%
Ca combines with O or S in the steel to suppress the growth of austenite grains in the weld heat affected zone. Therefore, when the toughness of a welding heat affected zone is requested | required, you may make it contain as needed. However, this effect is saturated when the Ca content is excessive. Therefore, the Ca content is 0.005% or less. The Ca content is preferably 0.004% or less. In order to obtain the above effect, the Ca content is preferably 0.002% or more.

Mg: 0 to 0.005%
Mg combines with O or S in the steel to suppress the growth of austenite grains in the weld heat affected zone. If the toughness of the weld heat affected zone is required, it may be contained if necessary. However, this effect is saturated when the Mg content is excessive. Therefore, the Ca content is 0.005% or less. The Mg content is preferably 0.004% or less. In order to obtain the above effect, the Mg content is preferably 0.002% or more.

(B) Metallic structure In the present invention, it is necessary to control the ferrite grain size in order to achieve low YR and improved toughness of the steel material. Specifically, in the ferrite particle size distribution at the 1/4 position of the plate thickness, the number ratio of ferrite having a size of less than 10 μm is 70 to 85%, the number ratio of ferrite having 10 to 20 μm is 13.5 to 30%, and 20 μm. The metal structure is such that the number ratio of ferrite exceeding 1.5% is 1.5% or less and the maximum ferrite particle size is 22 μm or less.

  Although the metal structure other than ferrite is basically bainite, martensite, pearlite, and MA other than ferrite and ferrite may be present in a total area of 5% or less. If it exceeds 5%, the toughness is reduced. Moreover, it is preferable that a ferrite is 35 to 70% by area%.

Number ratio of ferrite less than 10 μm: 70 to 85%
When the number ratio of ferrite with a crystal grain size of less than 10 μm decreases and the number ratio of ferrite with a grain size of 10 μm or more increases, the YR decreases, but the fracture surface transition temperature (vTs) increases remarkably, and the toughness of the steel material is remarkably increased. Getting worse. In particular, in the ferrite particle size distribution at the 1/4 position of the plate thickness, when the number ratio of ferrite having a particle size of less than 10 μm is less than 70%, the deterioration of toughness becomes remarkable. On the other hand, when the number ratio of the ferrite having a particle size of less than 10 μm exceeds 85%, the yield stress increases due to the refinement of the ferrite, and the YR increases. Therefore, the number ratio of ferrite having a particle size of less than 10 μm is set to 70 to 85%.

Number ratio of 10-20 μm ferrite: 13.5-30%
As described above, in order to improve the toughness of the steel material, it is necessary to reduce the crystal grain size of the ferrite and ensure a certain amount of the ferrite. On the other hand, ferrite having a particle size of 10 μm or more needs to be suppressed to a certain amount or less. In addition, when a large number of ferrites having a crystal grain size exceeding 20 μm are present, the frequency of occurrence of low values of toughness in the Charpy test increases. Therefore, in the ferrite particle size distribution at the 1/4 position of the plate thickness, the number ratio of ferrite having a particle size exceeding 20 μm is set to 1.5% or less. The lower the number ratio of ferrite exceeding 20 μm, the better. From this, the ferrite number ratio with a particle size of 10 to 20 μm is 13.5 to 30%.

Maximum ferrite particle size: 22 μm or less Charpy test when ferrite with a particle size exceeding 22 μm is present at 1/4 position of the plate thickness even if the number ratio of ferrite exceeding 20 μm is 1.5% or less , A low value of toughness at −60 ° C. occurs. Therefore, the maximum value of the ferrite grain size is 22 μm or less.

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

(C) Manufacturing method As mentioned above, in order to obtain low YR, it is necessary to ensure a certain amount of ferrite in the metal structure of steel. As a method for securing a certain amount of ferrite, a method is often used in which the steel sheet is cooled to a predetermined temperature by allowing it to cool for a certain time after finish rolling, and then water cooling.

  However, in such a conventional method, it is necessary to temporarily stop the steel sheet on the production line, and thus there is a problem that the production efficiency is lowered. In addition, the cooling rate during water cooling changes locally due to a scale that is generated non-uniformly when allowed to cool, resulting in a large variation in the mechanical properties of the steel sheet.

  From the viewpoint of production efficiency, it is not preferable to temporarily stop the steel sheet on the production line. However, if the steel sheet is not stopped on the production line, it cannot be allowed to cool sufficiently, and accelerated cooling such as water cooling becomes difficult, so that it is difficult to secure a sufficient amount of ferrite.

  Therefore, as a result of repeated studies by the present inventors, the steel sheet is stopped on the production line by performing preliminary cooling (primary cooling) such as water cooling before accelerated cooling (secondary cooling) such as water cooling. Thus, it was found that a low YR steel plate can be manufactured.

  At this time, by performing primary cooling immediately after finish rolling, the transformation driving force of ferrite increases due to distortion in the austenite structure introduced by controlled rolling, and cooling between primary cooling and secondary cooling is performed. Sometimes it is possible to advance the ferrite transformation efficiently.

  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. For example, the method shown below provides high production efficiency. Can be manufactured.

First, after heating the slab which has the above-mentioned chemical composition to a temperature range of 1050-1200 degreeC with a heating furnace, it extracts from a heating furnace and implements hot rolling and manufactures a steel plate. At that time, rolling is performed under the condition that the cumulative rolling reduction in the temperature range of 900 ° C. or lower is 30% or more and the rolling end temperature T _FR (° C.) is Ar ₃ points or more at the steel sheet surface temperature. After rolling, the following (a) to (e) are performed using an accelerated cooling device including a first cooling device that performs primary cooling and a second cooling device that performs secondary cooling without stopping the steel sheet on the production line. The cooling process is performed under the conditions shown in FIG.

(A) cooling in the first cooling device, the steel sheet surface temperature _T FR _~T FR -50 ℃, and, starting with a range of _three or more points Ar, in the range of Ar ₃ point to Ar ₃ point -100 ° C. Stop.
(B) The average cooling rate in the first cooling device is 10 ° C./s or more.
(C) The moving time from the outlet of the first cooling device to the inlet of the second cooling device is 10 to 40 s.
(D) Cooling by the second cooling device is started when the steel sheet surface temperature is in the range of Ar ₃ point −15 ° C. to Ar ₃ point −100 ° C. and stopped in the range of 550 ° C. or less.
(E) The average cooling rate in the second cooling device is set to a rate exceeding 15 ° C./s.

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

Heating temperature before hot rolling: 1050 to 1200 ° C
When the heating temperature is lower than 1050 ° C., since the austenite crystal grains are refined, the ferrite crystal grains are refined. In this case, since the strength becomes too high, it is difficult to reduce YR. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite crystal grains are coarsened and the low temperature toughness may be lowered.

Cumulative rolling reduction RR ₉₀₀ in a temperature range of 900 ° C. or lower: 30% or more When the cumulative rolling reduction RR ₉₀₀ in a temperature range of 900 ° C. or lower is as low as less than 30%, the crystal grains may be coarsened and low temperature toughness may be reduced. is there. Therefore, RR ₉₀₀ is preferably 30% or more. RR ₉₀₀ is more preferably 40% or more, and further preferably 50% or more. The definition of RR ₉₀₀ is as follows.
RR ₉₀₀ = {(plate thickness at 900 ° C.−plate thickness after finish rolling) / plate thickness at 900 ° C.} × 100 (%)

Rolling end temperature T _FR : Ar ₃ points or more When the rolling end temperature T _FR is low and less than Ar ₃ points, proeutectoid ferrite is generated, and there is a possibility that the driving force for transformation cannot be obtained. Therefore, it is preferable that T _FR (° C.) be ₃ or more points of Ar at the steel sheet surface temperature. Further, the first cooling start temperature is due to the need to the Ar ₃ point or more, more preferably rolling end temperature is Ar ₃ point + 50 ℃ or higher.

  As described above, it is preferable that the cooling process is performed without stopping the steel sheet on the production line. In this way, by performing the cooling process under the conditions shown in the above (a) to (e) while transporting to the lower process side without stopping the steel plate or transporting in the opposite direction, the standby time (wasted time) The steel plate can be manufactured without providing). As a result, a steel plate can be manufactured efficiently. The conveyance speed in the cooling step is preferably 50 m / min or more. The upper limit of the conveyance speed is not particularly required, and generally depends on the conveyance capability of the conveyance device, the distance between the first cooling device and the second cooling device, etc., but the conveyance speed is generally 100 m / min or less.

(A) cooling in the first cooling device, the steel sheet surface temperature _T FR _~T FR -50 ℃, and, starting with a range of _three or more points Ar, in the range of Ar ₃ point to Ar ₃ point -100 ° C. Stop.
When the cooling start temperature in the first cooling device is less than T _FR −50 ° C., the driving force for ferrite transformation decreases. Therefore, the probability that coarse ferrite grains are generated increases. The cooling start temperature is more preferably T _FR −40 ° C. or higher. Further, when the cooling start temperature is lower than Ar ₃ point, pro-eutectoid ferrite is generated, and the amount of coarse ferrite generated increases. The cooling start temperature is more preferably Ar ₃ point + 20 ° C. or higher.

Further, when the cooling stop temperature exceeds the Ar ₃ point, the transformation driving force of the ferrite is not changed from that during the cooling, so the particle size distribution of the ferrite grains is shifted to the coarsening side and the toughness is lowered. On the other hand, if Ar ₃ point is less than −100 ° C., the transformation driving force of ferrite becomes too large, so that a large amount of fine ferrite is generated and the yield stress may be too high. Therefore, the cooling stop temperature is preferably in the range of Ar ₃ points to Ar ₃ points −100 ° C.

(B) The average cooling rate in the first cooling device is 10 ° C./s or more.
When the average cooling rate in the first cooling device is less than 10 ° C./s, ferrite transformation starts in the middle of cooling, so the particle size distribution of ferrite grains shifts to the coarsening side. The cooling rate is preferably set to a rate exceeding 15 ° C./s.

(C) The moving time from the outlet of the first cooling device to the inlet of the second cooling device is 10 to 40 s.
The moving time from the outlet of the first cooling device to the inlet of the second cooling device, that is, the cooling time between the primary cooling and the secondary cooling is preferably 10 to 40 s. When the moving time is less than 10 s, the ferrite fraction necessary for lowering YR cannot be obtained sufficiently. On the other hand, when the moving time exceeds 40 s, the ferrite crystal grains tend to be coarsened, and not only the toughness is lowered, but also the production efficiency is lowered. Further, if the moving time exceeds 40 s, there is a possibility that the second cooling cannot be started from the optimum temperature range. The travel time is more preferably 20 s or less.

(D) Cooling by the second cooling device is started when the steel sheet surface temperature is in the range of Ar ₃ point −15 ° C. to Ar ₃ point −100 ° C. and stopped in the range of 550 ° C. or less.
If the cooling start temperature in the second cooling device exceeds Ar ₃ point −15 ° C. due to reheating of the steel sheet, ferrite transformation does not proceed and ferrite required for low YR may not be obtained. On the other hand, if Ar ₃ point is less than −100 ° C., the transformation driving force of ferrite increases, so the ferrite fraction may increase too much or the ferrite grain size may become too fine. The cooling start temperature is more preferably Ar ₃ point −30 ° C. or lower, and more preferably Ar ₃ point −80 ° C. or higher.

  The cooling stop temperature is preferably 550 ° C. or lower. If the cooling stop temperature exceeds 550 ° C., a hard structure having a high strength necessary for low YR may not be obtained. However, if the cooling stop temperature is less than 400 ° C., a significantly low YR is achieved, but the toughness may be reduced. Therefore, when it is desired to achieve both toughness and a low yield ratio, the cooling stop temperature is preferably 400 ° C. or higher.

(E) The average cooling rate in the second cooling device is set to a rate exceeding 15 ° C./s.
When the average cooling rate is slow, the hardness of the hard structure is lowered, so that the tensile strength is lowered, and as a result, a low YR cannot be obtained. The average cooling rate is preferably set to a rate exceeding 15 ° C./s. The average cooling rate is more preferably 25 ° C./s or more.

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

  Steels having chemical components shown in Table 1 were melted, and slabs were produced with a continuous casting machine. The obtained slab was hot-rolled under the conditions shown in Table 2 and then cooled using the first cooling device and the second cooling device to obtain steel plates having the thicknesses shown in Table 3.

  After performing stress-relief annealing, a round bar tensile test piece having a parallel part length of 8.5 mm and a gauge distance of 42.5 mm was produced from each steel plate obtained. At this time, the test piece was cut out so that the length direction of the round bar tensile test piece was a direction (plate width direction) perpendicular to the rolling direction. Using a round bar tensile test piece, a tensile test was performed at normal temperature and atmospheric pressure, yield strength YS (MPa), tensile strength TS (MPa), yield ratio YR (= YS / TS × 100, unit is%) And total elongation EL (%) was calculated | required.

The low temperature toughness was evaluated by the Charpy impact test. The Charpy impact test was performed 3 times under the condition of −60 ° C. using a V-notch test piece defined in JIS Z 2242 (2005), and the lowest value was defined as the absorbed energy (vE- ₆₀ ). Furthermore, for the microstructure, after embedding in the resin so that the cross-section cut from the center part of the test piece becomes the test surface, mirror polishing, corroding with nital, observing the surface with an optical microscope, Each fraction (number ratio) was determined for each particle size after identification.

These results are summarized in Table 3. In addition, in this invention, when YR was 80% or less, it was evaluated that it has low YR, and when vE- ₆₀ was 100 J or more, it was determined that it was excellent in low temperature toughness.

  Referring to Tables 1 to 3, Test Nos. Satisfying all the chemical composition and metal structure defined in the present invention. 1 to 10 resulted in low YR and excellent low temperature toughness.

  On the other hand, test no. In Nos. 11 to 13, since the chemical composition was outside the specified range, a large number of fine ferrites having a size of 10 μm or less was generated, resulting in an increase in YR, and the desired characteristics could not be obtained. Test No. In No. 14, since the heating temperature was less than the lower limit, a large number of ferrites of 10 μm or less were generated, and the YR was increased. Test No. In No. 15, since the rolling reduction was less than the lower limit, coarse ferrite exceeding 22 μm was generated, resulting in poor low temperature toughness.

  Test No. In No. 16, since the rolling end temperature was less than the lower limit value, a large number of ferrites of 10 μm or less were generated and YR was increased. Test No. In No. 17, since the cooling start temperature in the first cooling device was less than the lower limit value, ferrite exceeding 22 μm was generated, resulting in poor low temperature toughness. Test No. In No. 18, since the cooling stop temperature in the first cooling device exceeded the upper limit, ferrite exceeding 22 μm was generated, the proportion of ferrite grains exceeding 20 μm was increased, and the low temperature toughness was inferior.

  Test No. In No. 19, since the cooling stop temperature in the first cooling device was less than the lower limit value, a large number of ferrites of 10 μm or less were generated, and YR was increased. Test No. In No. 20, since the cooling rate in the first cooling device was less than the lower limit value, coarse ferrite exceeding 22 μm was generated, resulting in high YR and low temperature toughness. Test No. In No. 21, since the moving time of the steel sheet was less than the lower limit, the amount of ferrite produced decreased and YR increased.

  Test No. No. 22, the moving time of the steel sheet exceeded the upper limit value, and the cooling start temperature in the second cooling device was less than the lower limit value, so the amount of ferrite produced increased, the proportion of ferrite grains exceeding 20 μm also increased, and YR was As the result increased, the low temperature toughness was inferior. Test No. In No. 23, since the cooling start temperature in the second cooling device exceeded the upper limit, the amount of ferrite produced decreased and YR increased. And the test number No. In No. 24, since the cooling rate in the second cooling device was less than the lower limit value, the hardness of the hard structure decreased, and the tensile strength decreased accordingly. As a result, YR was increased.

  According to the present invention, it is possible to efficiently produce a low yield ratio steel sheet having small variations in mechanical properties and excellent low temperature toughness. The low yield ratio steel sheet according to the present invention is suitable for use as a tank material such as LPG or ammonia.

Claims (5)

Chemical composition is mass%,
C: 0.03-0.10%,
Si: 0.05 to 0.5%,
Mn: 0.9 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.005 to 0.05%,
Ti: 0.005 to 0.025%,
sol. Al: 0.005 to 0.090%,
N: 0.001 to 0.010%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.20%,
Mo: 0 to 0.20%,
V: 0 to 0.06%,
B: 0 to 0.002%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
Balance: Fe and impurities,
Satisfying the following formula (i)
In ferrite grain size distribution at 1/4 position of plate thickness,
70% to 85% of the number ratio of ferrite of less than 10 μm,
The number ratio of 10-20 μm ferrite is 13.5-30%,
The number ratio of ferrite exceeding 20 μm is 1.5% or less, and
Maximum ferrite grain size is 22μm or less,
A low yield ratio steel sheet having a metallographic structure.
0.10 ≦ Cu + Ni + Cr + Mo ≦ 1.0 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate. The chemical composition is mass%,
Cu: 0.05 to 0.50%,
Ni: 0.05 to 0.50%,
Cr: 0.04-0.20% and Mo: 0.005-0.20%
The low yield ratio steel plate according to claim 1, comprising at least one selected from the group consisting of: The chemical composition is mass%,
V: 0.005-0.06%
The low yield ratio steel plate according to claim 1 or 2, comprising: The chemical composition is mass%,
B: 0.0002 to 0.002%,
Ca: 0.002-0.005% and Mg: 0.001-0.005%
The low yield ratio steel plate according to any one of claims 1 to 3, comprising one or more selected from the above. After heating the slab having the chemical composition according to any one of claims 1 to 4 to a temperature range of 1050 to 1200 ° C,
Rolling is performed under a condition that the cumulative rolling reduction in the temperature range of 900 ° C. or lower is 30% or more and the rolling end temperature T _FR (° C.) is Ar ₃ points or more at the steel sheet surface temperature,
After rolling, the steel sheet is subjected to cooling treatment under the conditions shown in the following (a) to (e) using an accelerated cooling device including the first cooling device and the second cooling device without stopping the steel sheet on the production line. A method for producing a specific steel sheet.
(A) cooling in the first cooling device, the steel sheet surface temperature _T FR _~T FR -50 ℃, and, starting with a range of _three or more points Ar, in the range of Ar ₃ point to Ar ₃ point -100 ° C. Stop.
(B) The average cooling rate in the first cooling device is 10 ° C./s or more.
(C) The moving time from the outlet of the first cooling device to the inlet of the second cooling device is 10 to 40 s.
(D) Cooling by the second cooling device is started when the steel sheet surface temperature is in the range of Ar ₃ point −15 ° C. to Ar ₃ point −100 ° C. and stopped in the range of 550 ° C. or less.
(E) The average cooling rate in the second cooling device is set to a rate exceeding 15 ° C./s.