JP2016148105A - Steel sheet for lpg tank and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet for an LPG tank having excellent arrest property and low temperature toughness and a manufacturing method which makes it possible to manufacture the steel sheet stably at low cost.SOLUTION: There is provided a steel sheet for an LPG tank having a chemical composition containing, by mass%, C:0.03 to 0.08%, Si:0.05 to 0.50%, Mn:1.00 to 1.80%, P≤0.015%, S≤0.005%, Ni:0.10 to 0.80%, Nb:0.008 to 0.060%, Ti:0.005 to 0.050%, sol.Al:0.002 to 0.050%, N≤0.0070%, Cu≤0.50%, Cr≤0.50%, Mo≤0.50%, V≤0.060%, B≤0.0050%, Ca≤0.0040%, Mg≤0.0020%, REM≤0.0020% and the balance:Fe with impurities and a metallic structure consisting of, by area fraction, 65 to 90% of ferrite, 5 to 35% of bainite and 5% or less of cementite with average crystal particle diameter of ferrite of 10 to 20 μm.SELECTED DRAWING: None

Description

  The present invention relates to a steel sheet for LPG tank and a method for producing the same. In particular, the present invention relates to a steel sheet for an LPG tank having excellent characteristics (arrest characteristics) for stopping the brittle crack propagation that has occurred in order to prevent collapse of the entire structure when a brittle crack has occurred, and a method for manufacturing the same.

  When brittle fracture occurs in any structure, a crack is propagated to the entire structure at an extremely high speed and the entire structure collapses instantaneously, so enormous damage is assumed. Therefore, brittle fracture is a fracture mode that should be avoided. Therefore, structures such as storage tanks are designed to avoid the occurrence of brittle fracture. However, in such a building, it is necessary to consider the case where brittle fracture occurs due to an abnormal situation unexpected by the designer (such as an action of external force exceeding the design standard or a defect caused by construction). is there. Therefore, even if a brittle crack is generated, a characteristic capable of stopping the propagation of the generated crack is required. This characteristic is generally called “arrest characteristic”.

  A structure in which members having arrest characteristics are arranged in place can not only prevent the occurrence of a brittle crack, but also stop the propagation of the crack even if a brittle crack occurs. That is, safety measures are taken in each of the stage of occurrence of the brittle crack and the stage of propagation of the crack. Taking a plurality of safety measures in this way is extremely important as the design philosophy of the structure.

  Conventionally, steel plates for LPG tanks have been used as members having arrest characteristics. The steel sheet for LPG tank is mainly used as a constituent member of an LPG tank for storing LPG (Liquefied Petroleum Gas) at a low temperature. In addition, low temperature means the temperature which can store LPG as a liquid, ie, the temperature below -60 degreeC.

  As described above, the LPG tank stores LPG at a low temperature. For this reason, the steel sheet for LPG tanks is required to have excellent fracture toughness at low temperatures from the viewpoint of ensuring safety. In other words, both the base material and the welded joint are required to have excellent arrest characteristics at a low temperature of −60 ° C. or lower.

  The simplest method for imparting arrest properties to a steel material is to contain Ni, which is an element that significantly improves toughness. It has been conventionally known that the effect of improving the arrest characteristics by Ni is large. As a steel material having arrest properties in a low temperature environment, a so-called 9% Ni steel containing 9% Ni is generally used, and is also defined in Japanese Industrial Standards (JIS). However, Ni is a very expensive element. If 9% of Ni is contained, the manufacturing cost of the steel material increases. Therefore, there is a problem in cost in improving the arrest characteristics by Ni.

  On the other hand, the technique which improves the arrest characteristic of steel materials, without making Ni contain abundantly is also proposed. For example, in the steel sheets disclosed in Patent Documents 1 and 2, the surface layer structure is made extremely fine. In these steel sheets, the formation of shear lip can be promoted at the time of brittle crack propagation, so that the arrest characteristics can be improved without relying on expensive elements such as Ni. Further, in the steel sheet disclosed in Patent Document 3, the arrest characteristics are improved by defining the main body of the microstructure as a ferrite structure and defining the average crystal grain size of the ferrite within a specific range.

JP-A-3-2322 JP 7-126798 A JP2013-245360A

  However, although the steel sheets disclosed in Patent Documents 1 and 2 have sufficient strength, the toughness at low temperatures is not sufficient as a steel sheet for LPG tanks. On the other hand, the steel sheet disclosed in Patent Document 3 has sufficient low temperature toughness. However, in the technique described in the cited document 3, a steel sheet having excellent arrest characteristics and low temperature toughness cannot be stably produced, and the characteristics of the steel sheet may vary.

  The present invention has been made to solve the above-described problems, and is a high-strength LPG tank steel sheet having excellent arrest characteristics and low-temperature toughness, and a manufacturing method capable of stably manufacturing the steel sheet for low-cost. The purpose is to provide.

  In order to improve the arrest characteristics of the steel material without containing a large amount of Ni, it is necessary to compensate for the characteristic deterioration accompanying the reduction of Ni by other elements and to compensate for the characteristic deterioration by the structure control. In order to realize a steel structure necessary to compensate for the characteristic deterioration, the manufacturing method is important.

  In the conventional manufacturing method of the steel sheet for LPG tanks, finish rolling is performed in a relatively low temperature range, and immediately after that, accelerated cooling by water cooling is performed. In this manufacturing method, the metallographic structure of the steel sheet can be refined by performing finish rolling at a low temperature, and the strength, toughness, and arrest characteristics (hereinafter also simply referred to as the characteristics of the steel sheet) of the steel sheet can be improved. . On the other hand, the present inventors reviewed the conventional manufacturing method so that the characteristic of a steel plate can be improved more stably. As a result, the following findings (a) to (c) were obtained.

  (A) In constructing a new production method, finish rolling in a low temperature region is essential from the viewpoint of improving the properties of the steel sheet. On the other hand, the present inventors considered not only improving the properties of the steel sheet only by finish rolling, but also improving the properties by rolling before finish rolling.

  Specifically, by rolling the steel sheet in the austenite non-recrystallized region, the metal structure of the steel sheet can be easily refined. Thereby, it becomes easy to improve the characteristic of a steel plate. Therefore, in order to stably improve the properties of the steel sheet, it is important to control the rolling in the austenite non-recrystallized region.

  (B) In addition, the rolling in a low temperature region tends to cause variations in the characteristics of the steel sheet. Therefore, it is necessary to appropriately control the rolling temperature of not only the final pass (finish rolling pass) but also the final three passes (three passes including finish rolling).

  (C) On the other hand, with respect to water cooling after finishing rolling, the amount of transformation of the metal structure can be controlled by controlling the start temperature, end temperature and cooling rate within a certain range. Thereby, a certain amount of ferrite can be secured in the steel sheet, and the ferrite grain size can also be controlled. As a result, the characteristics of the steel sheet can be stabilized.

This invention is made | formed based on the said knowledge, and makes a summary the following steel plate for LPG tanks, and its manufacturing method.
(1) The chemical composition is mass%,
C: 0.03-0.08%,
Si: 0.05 to 0.50%,
Mn: 1.00-1.80%,
P: 0.015% or less,
S: 0.005% or less,
Ni: 0.10 to 0.80%,
Nb: 0.008 to 0.060%,
Ti: 0.005 to 0.050%,
sol.Al: 0.002 to 0.050%,
N: 0.0070% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.060%,
B: 0 to 0.0050%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0020%,
REM: 0 to 0.0020%,
Balance: Fe and impurities,
LPG tank characterized in that the metal structure is composed of 65 to 90% ferrite, 5 to 35% bainite and 5% or less cementite in area fraction, and the average crystal grain size of ferrite is 10 to 20 μm. Steel plate.

(2) The chemical composition is mass%,
Cu: 0.10 to 0.50%,
Cr: 0.05 to 0.50%,
Mo: 0.02 to 0.50%,
V: 0.010-0.060% and B: 0.0003-0.0050%
The steel sheet for LPG tanks according to (1) above, which contains one or more selected from the above.

(3) The chemical composition is mass%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0002 to 0.0020%, and REM: 0.0002 to 0.0020%
The steel sheet for LPG tanks according to (1) or (2) above, which contains one or more selected from the above.

(4) The chemical composition is mass%,
C: 0.03-0.08%,
Si: 0.05 to 0.50%,
Mn: 1.00-1.80%,
P: 0.015% or less,
S: 0.005% or less,
Ni: 0.10 to 0.80%,
Nb: 0.008 to 0.060%,
Ti: 0.005 to 0.050%,
sol.Al: 0.002 to 0.050%,
N: 0.0070% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.060%,
B: 0 to 0.0050%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0020%,
REM: 0 to 0.0020%,
The remainder: when heating, rolling and cooling the slab which is Fe and impurities,
In the rolling process, the temperature of the slab when rolled to a thickness three times the finish thickness is 810 ° C. or less, and the temperature of the slab when rolled to the pass two steps before the finish pass is 720 to 770. And rolling the slab so that the finishing temperature is 700 to 750 ° C.
In the cooling step, the cooling start temperature is set to 650 to 720 ° C., the cooling stop temperature is set to 350 to 500 ° C., and water cooling is performed at an average cooling rate of 20 ° C./s or more.

(5) The chemical composition is mass%,
Cu: 0.10 to 0.50%,
Cr: 0.05 to 0.50%,
Mo: 0.02 to 0.50%,
V: 0.010-0.060% and B: 0.0003-0.0050%
The manufacturing method of the steel plate for LPG tanks of said (4) containing 1 or more types selected from.

(6) The chemical composition is mass%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0002 to 0.0020%, and REM: 0.0002 to 0.0020%
The manufacturing method of the steel plate for LPG tanks of said (4) or (5) containing 1 or more types selected from these.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength LPG tank steel plate which has the outstanding arrest characteristic and low-temperature toughness can be manufactured stably at low cost.

  Hereinafter, a steel sheet for LPG tank (hereinafter, also simply referred to as a steel sheet) and a manufacturing method thereof according to the present invention will be described in detail. The steel sheet for LPG tank of the present invention has a target thickness of 6 to 40 mm and a tensile strength of 490 MPa or more. In the method for manufacturing a steel sheet for LPG tank according to the present invention, the sheet thickness is particularly 6 to 40 mm. It is suitable for manufacturing a steel sheet for LPG tank having a strength of 490 MPa or more. Moreover, in the following description, a base material and a weld heat affected zone (hereinafter referred to as HAZ) mean a base material and HAZ in a welded structure made of a steel plate manufactured by the manufacturing method according to the present invention.

1. Chemical composition of steel sheet for LPG tank and chemical composition of slab used for manufacturing steel sheet for LPG tank First, the chemical composition of steel sheet for LPG tank according to the present invention and the chemical composition of slab used in the manufacturing method will be described. “Slab” is a general term for steel ingots, blooms, billets and the like. In the following description, “%” indicating the content of each element means “mass%”.

C: 0.03-0.08%
C is an element necessary for ensuring the strength of the steel sheet. In order to acquire this effect, it is necessary to contain 0.03% or more of C. On the other hand, when the C content exceeds 0.08%, the toughness deteriorates significantly in the bainite transformation region, and the HAZ toughness also deteriorates. Therefore, the C content is 0.03 to 0.08%. From the viewpoint of balance between strength and arrest properties, the preferable lower limit of the C content is 0.04%, and the preferable upper limit is 0.07%.

Si: 0.05 to 0.50%
Si is an element necessary for deoxidation at the refining stage and contributes to an increase in the strength of the steel sheet. In order to obtain these effects, it is necessary to contain Si by 0.05% or more. However, if the Si content exceeds 0.50%, the formation of island martensite in the HAZ is promoted and the toughness is adversely affected. Therefore, the Si content is set to 0.05 to 0.50%. The minimum with preferable Si content is 0.10%, and a preferable upper limit is 0.40%.

Mn: 1.00-1.80%
Mn is an element necessary for ensuring the strength of the steel sheet. In order to obtain this effect, it is necessary to contain 1.00% or more of Mn. On the other hand, if the Mn content exceeds 1.80%, the toughness of the HAZ is significantly deteriorated. Therefore, the Mn content is 1.00 to 1.80%. The minimum with preferable Mn content is 1.20%, and a preferable upper limit is 1.60%.

P: 0.015% or less P is present in steel as an impurity and causes grain boundary cracking in HAZ. If the P content exceeds 0.015%, the occurrence of the above-mentioned grain boundary cracks becomes remarkable, so the upper limit of the P content is set to 0.015%. The P content is preferably 0.010% or less. The P content is preferably as low as possible.

S: 0.005% or less S is an element which exists in steel as an impurity and forms MnS which becomes a starting point of brittle fracture, thereby degrading arrest properties. When the S content exceeds 0.005%, the arrest characteristics are remarkably deteriorated, so the upper limit of the S content is set to 0.005%. The S content is preferably less than 0.003%. Note that the S content is preferably as low as possible.

Ni: 0.10 to 0.80%
Ni is an element having an effect of improving the arrest characteristics of the steel sheet. In order to acquire this effect, it is necessary to contain 0.10% or more of Ni. However, if the Ni content exceeds 0.80%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Ni content is set to 0.10 to 0.80%. The minimum with preferable Ni content is 0.30%, and a preferable upper limit is 0.60%.

Nb: 0.008 to 0.060%
Nb is an element effective for refining the structure, improving hardenability, and increasing strength by precipitation hardening. In order to acquire these effects, it is necessary to contain Nb 0.008% or more. In particular, it is important when applying a TMCP (Thermo-Mechanical Control Process) method because the effect of expanding the non-recrystallized region is great. However, when the Nb content exceeds 0.060%, the increase in precipitates causes toughness deterioration. Therefore, the Nb content is set to 0.008 to 0.060%. The minimum with preferable Nb content is 0.010%, and a preferable upper limit is 0.030%.

Ti: 0.005 to 0.050%
Ti has the effect of increasing the toughness by refining the structure. In order to obtain this effect, it is necessary to contain 0.005% or more of Ti. Ti is particularly effective in refining the structure when producing a base material by online accelerated cooling. However, if the Ti content exceeds 0.050%, the toughness of the welded joint is significantly reduced. Therefore, the Ti content is 0.005 to 0.050%. The minimum with preferable Ti content is 0.007%, and a preferable upper limit is 0.030%.

sol. Al: 0.002 to 0.050%
Al is an element necessary for deoxidation of steel. In order to obtain this effect, sol. It is necessary to contain 0.002% or more of Al (acid-soluble Al). However, sol. When the Al content exceeds 0.050%, precipitates increase and the deterioration of arrest characteristics becomes remarkable. Therefore, sol. The Al content is 0.002 to 0.050%. sol. The minimum with preferable Al content is 0.005%, and a preferable upper limit is 0.040%.

N: 0.0070% or less N is present in the steel as an impurity, forms precipitates, and deteriorates toughness. When the N content exceeds 0.0070%, the deterioration of arrest characteristics becomes remarkable, so the N content is set to 0.0070% or less. The N content is preferably 0.0060% or less. In order to secure low temperature toughness, the N content is preferably as low as possible.

  In addition to the above elements, the steel plate according to the present invention may further contain one or more selected from Cu, Cr, Mo, V, B, Ca, Mg, and REM as described below, as necessary. Good.

Cu: 0 to 0.50%
Cu has the effect of improving strength without degrading the toughness of the steel sheet. However, if the Cu content exceeds 0.50%, precipitates increase and the arrest characteristics deteriorate, and further, micro cracks are generated on the surface during hot working. Therefore, the upper limit of the Cu content is 0.50%. The upper limit with preferable Cu content is 0.30%. In order to stably obtain the above-mentioned effects due to Cu, it is preferable to contain 0.10% or more of Cu.

Cr: 0 to 0.50%
Cr has the effect of increasing the strength of the steel sheet. However, if the Cr content exceeds 0.50%, a hardened structure is formed in the HAZ and the toughness is deteriorated. Therefore, the upper limit of Cr content is 0.50%. The upper limit with preferable Cr content is 0.30%. In order to stably obtain the above-described effects due to Cr, it is preferable to contain 0.05% or more of Cr.

Mo: 0 to 0.50%
Mo has the effect of increasing the hardenability and improving the strength of the steel sheet. However, the production cost increases by containing Mo. On the other hand, if the Mo content exceeds 0.50%, the HAZ toughness is deteriorated. Therefore, the upper limit of the Mo content is 0.50%. The upper limit with preferable Mo content is 0.30%. In order to stably obtain the above-described effects due to Mo, it is preferable to contain 0.02% or more of Mo.

V: 0 to 0.060%
V has the effect of improving the strength of the steel sheet by improving hardenability and precipitation hardening. However, if the V content exceeds 0.060%, the toughness is significantly deteriorated. Therefore, the upper limit of V content is 0.060%. In order to stably obtain the above-described effects due to V, it is preferable to contain V by 0.010% or more.

B: 0 to 0.0050%
B can suppress the ferrite transformation from the austenite grain boundary to improve the hardenability and increase the strength of the steel sheet. However, if the B content exceeds 0.0050%, the toughness deteriorates. Therefore, the upper limit of the B content is 0.0050%. The upper limit with preferable B content is 0.0015%. In order to stably obtain the above-described effect due to B, it is preferable to contain 0.0003% or more of B. In addition, when B is contained, it is important to sufficiently balance the content of other elements given to the hardenability in consideration of the carbon equivalent in order to appropriately secure the ferrite content in the center portion of the plate thickness. It is.

Ca: 0 to 0.0040%
Ca has the effect of controlling the form of inclusions and improving the arrest characteristics. However, if the Ca content exceeds 0.0040%, the cleanliness of the steel sheet itself is greatly reduced. Therefore, the upper limit of the Ca content is 0.0040%. The upper limit with preferable Ca content is 0.0020%. In order to stably obtain the above effects due to Ca, it is preferable to contain 0.0003% or more of Ca.

Mg: 0 to 0.0020%
Mg has the effect of improving the toughness of HAZ by increasing the dispersion density of fine oxides in steel. However, if the Mg content exceeds 0.0020%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. Therefore, the upper limit of the Mg content is 0.0020%. The upper limit with preferable Mg content is 0.0015%. In order to stably obtain the above-described effects due to Mg, it is preferable to contain Mg in an amount of 0.0002% or more. Mg is preferably contained in the molten steel before Al.

REM: 0 to 0.0020%
Similar to Mg, REM (rare earth element) has the effect of improving the toughness of HAZ by increasing the dispersion density of fine oxides in steel. Furthermore, REM also has the effect of fixing excess S as sulfides. However, if the REM content exceeds 0.0020%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced. Therefore, the upper limit of the REM content is 0.0020%. The upper limit with preferable REM content is 0.0015%. In order to stably obtain the above effect by REM, it is preferable to contain REM 0.0002% or more. REM is preferably contained in the molten steel before Al.

  “REM” is a generic name for 17 elements of Y and Sc combined with 15 elements of lanthanoid, and the content of REM means the total content of one or more elements of REM. . REM may be added as misch metal and contained so that the amount of REM falls within the above range.

  The slab used in the manufacturing method of the steel sheet for LPG tank according to the present invention and the steel sheet for LPG tank according to the present invention contains the above-mentioned elements, and the balance consists of Fe and impurities. "Impurity" means a component that is mixed due to various factors in raw materials such as ore and scrap, or in the manufacturing process when a steel sheet is industrially manufactured.

2. Metal structure of steel sheet for LPG tank The metal structure of the steel sheet for LPG tank according to the present invention is composed of 65 to 90% ferrite, 5 to 35% bainite and 5% or less cementite in terms of area fraction. The crystal grain size is 10-20 μm.

  When the metal structure is a fine-grained ferrite structure, the toughness of the base metal and the welded portion is improved. Therefore, it is necessary to increase the ferrite structure by a manufacturing method described later. At this time, in order to use as a steel sheet for LPG tank having sufficient toughness, the ferrite structure of the steel sheet needs to be 65% or more. However, when the ferrite structure is too much, a strength such as a tensile strength of 490 MPa cannot be obtained. Therefore, the ferrite structure of the steel sheet is 90% or less. On the other hand, strength is obtained by making a part of the metal structure a bainite structure. At this time, the bainite structure needs to be 5% or more. A steel sheet having higher strength can be obtained as the structure fraction of the bainite structure is larger. However, the bainite structure is set to 35% or less from the relationship with the structure fraction of the ferrite structure. As a structure other than the ferrite structure and the bainite structure, inevitably formed cementite may be contained in an amount of 5% or less. If the cementite content exceeds 5%, the toughness of the base material decreases, which is not preferable.

  On the other hand, toughness is not sufficient if the ferrite structure is 65% or more, and it can be obtained by making the ferrite structure finer. At this time, sufficient toughness can be obtained by setting the average crystal grain size of ferrite to 20 μm or less. The smaller the ferrite crystal grain size, the higher the toughness, but the ferrite crystal grain size is at least 10 μm even if it is small.

  In addition to the optical microscope, the tissue fraction may be evaluated based on observation using a scanning electron microscope and a transmission electron microscope having an acceleration voltage of 100 to 200 kV. Here, the tissue fraction is evaluated by the area ratio. Specifically, for 100 visual fields observed by these observation methods, after calculating the area ratio of ferrite to the total visual field area in each visual field, an average value of the area ratio of ferrite of 100 visual fields may be obtained.

  In addition, when the crystal grain size is quantified based on the grain boundary recognized by an optical microscope or scanning electron microscope, the correspondence with the fracture surface unit is poor when the orientation difference between adjacent crystal grains is small. It cannot be a number that represents the organization size. Therefore, the “average crystal grain size” in the present invention means a crystal grain size of a portion surrounded by a structure boundary having an orientation difference of 15 ° or more when evaluated by EBSP. That is, by using EBSP (Electron Backscatter Diffraction Pattern) method, observation of 5 fields or more is performed at a magnification of 300 times, a structure boundary having an orientation difference of 15 ° or more is regarded as a grain boundary, What is necessary is just to obtain | require the area which calculated | required the area inside a crystal | crystallization, and converted the area into the equivalent circle diameter as an average crystal grain diameter.

3. Manufacturing method of slab The slab can be manufactured by, for example, an ingot method or a continuous casting method, but is not limited to these methods. From the viewpoint of cost reduction, it is preferable to manufacture by a continuous casting method. In this case, in order to control the inclusion at the center position of the plate thickness, it is preferable not to make the temperature of the molten steel excessively high. Specifically, it is preferable to manage the difference between the solidification temperature determined from the chemical composition of the molten steel and the actual temperature of the molten steel within 50 ° C. Furthermore, it is preferable to perform electromagnetic stirring immediately before the molten steel solidifies, and it is preferable to perform reduction during solidification of the molten steel.

  In the manufacturing method according to the present invention, the temperature of the slab when it is rolled to a thickness that is three times the finished thickness is controlled in the rolling process described later. Therefore, the thickness of the slab before rolling is at least 3 times the finished thickness, and preferably 5 times the finished thickness. From the viewpoint of manufacturing efficiency of the steel sheet, the thickness of the slab before rolling is preferably 8 times or less of the finished thickness.

4). Hereinafter, the manufacturing method of the LPG tank steel plate which concerns on this invention is demonstrated. In the present invention, a steel sheet for an LPG tank is produced by heating, rolling and cooling a slab having the above chemical composition. This will be specifically described below.

(Heating process)
The heating temperature of the slab is not particularly limited, and the slab may be heated so that rolling can be performed in a predetermined temperature range in the rolling process after the heating process. The higher the slab temperature, the softer the slab becomes and the easier it is to roll. Therefore, the heating temperature of the slab in the heating process is preferably 950 ° C. or higher. On the other hand, if the temperature of the slab is too high, the time until the temperature of the slab decreases to a temperature suitable for rolling becomes long, and the productivity decreases. For this reason, it is preferable that the heating temperature of the slab in a heating process is 1100 degrees C or less. A preferable lower limit of the heating temperature of the slab is 1000 ° C., and a preferable upper limit is 1050 ° C.

(Rolling process)
In the rolling process, the slab is rolled by a plurality of passes so as to satisfy the following conditions (i) to (iii).
(I) The temperature of the slab when rolled to a thickness three times the finished thickness: 810 ° C. or less (ii) The temperature of the slab when rolled to the pass immediately before the finish pass: 720 to 770 ° C.
(Iii) Finishing temperature: 700-750 ° C
As long as these conditions (i) to (iii) are satisfied, the rolling start temperature of the slab is not particularly limited. Therefore, as long as the above conditions (i) to (iii) are satisfied, for example, the slab taken out from the heating furnace may be rolled without providing a waiting time for temperature reduction. That is, rolling may be started immediately after the heating step. Hereinafter, the conditions (i) to (iii) will be described in more detail.

First, the condition (i) will be described.
In the steel having the above composition, 810 ° C. or lower is the austenite non-recrystallized region. The metal structure is refined by rolling the slab in the austenite non-recrystallized region. Thereby, the toughness of a steel plate can be improved. Therefore, in the present invention, the temperature of the slab when rolled to a thickness three times the finished thickness is limited to 810 ° C. or less. Thereby, the slab can be rolled in the austenite non-recrystallized region from a thickness of 3 times the finished thickness to the finished thickness. As a result, the metal structure can be sufficiently finely divided. The lower limit of the temperature of the slab when rolled to a thickness three times the finished thickness is not particularly limited, but the lower limit is preferably 750 ° C. from the relationship with the conditions (ii) and (iii). Note that the temperature of the slab when the thickness exceeds three times the finished thickness is not particularly limited.

In general, it is considered that the slab has a thickness that is three times the finished thickness during rolling by an arbitrary pass. In this case, it is difficult to actually measure the temperature at the moment when the thickness of the slab becomes three times the thickness. Therefore, in such a case, during rolling by an arbitrary pass, it is assumed that the thickness of the slab decreases linearly as the temperature of the slab decreases. Based on the following formula (I), the above three times Find the temperature of the slab at the moment of thickness.
_{T = (T B -T A)} / (t B -t A) × (t-t A) + T A ··· (I)

In the above formula (I), t indicates a thickness (mm) that is three times the finished thickness, and T indicates the temperature of the slab at the moment when the thickness becomes three times the above thickness. T _A indicates the thickness (mm) of the slab after being rolled in an arbitrary pass A and before being rolled in a pass B (pass after the pass A). t _B indicates the thickness (mm) of the slab after being rolled in pass B and before being rolled in pass C (pass after pass B). T _A indicates the temperature (° C.) of the slab immediately before the path B, and T _B indicates the temperature (° C.) of the slab immediately after the path B. In the above formula, t _A , t _B , T _A and T _B are actually measured values.

For example, if t is 30 mm (ie, the finishing thickness is 10 mm), T _A is 812 ° C., T _B is 808 ° C., t _A is 31 mm, and t _B is 27 mm, the thickness is three times the above. The temperature T of the slab at the moment is 811 ° C. In this case, the requirements of the present invention are not satisfied.

For example, when t is 30 mm, T _A is 812 ° C., T _B is 808 ° C., t _A is 33 mm, and t _B is 29 mm, the temperature of the slab at the moment when the thickness becomes three times the above. T is 809 ° C. In this case, the requirements of the present invention are satisfied.

Next, conditions (ii) and (iii) will be described.
The higher the temperature of the slab, the smaller the deformation resistance of the slab and the easier the rolling. However, in the present invention, the metal structure is refined by performing rolling at a relatively low temperature in the finishing pass (the last pass), the pass before the finishing pass, and the pass before the finishing pass. To do. Thereby, the intensity | strength and toughness of a steel plate can be improved. Specifically, rolling in the pass immediately before the finishing pass is performed at 720 to 770 ° C. (Condition ii), and rolling in the finishing pass is performed at a temperature (finishing temperature) immediately before rolling of 700 to 750 ° C. (Condition iii). Thus, by finely defining the rolling temperature in the finishing pass and the pass close to the finishing pass, it is possible to prevent variations in the characteristics of the steel sheet. From the viewpoint of stabilizing the properties of the steel sheet, the preferable lower limit of the slab temperature of condition (ii) is 740 ° C, and the preferable upper limit is 750 ° C. Moreover, the minimum with a preferable finishing temperature of condition (iii) is 720 degreeC, and a preferable upper limit is 740 degreeC.

(Cooling process)
In the cooling step, water cooling is performed so as to satisfy the following conditions (iv) to (vi).
(Iv) Cooling start temperature: 650-720 ° C
(V) Cooling stop temperature: 350-500 ° C
(Vi) Average cooling rate: 20 ° C./s or higher By performing water cooling satisfying the above conditions (iv) to (vi), the metal structure refined by the rolling process can be maintained. Thereby, the metal structure of a steel plate can be refined. In addition, the metal structure can be appropriately transformed. As a result, the strength and toughness of the steel sheet can be improved. Hereinafter, the conditions (iv) to (vi) will be described in more detail.

First, the condition (iv) will be described.
In the steel having the chemical composition described above, when the water cooling start temperature exceeds 720 ° C., the metal structure becomes coarse before the water cooling starts. On the other hand, when the water cooling start temperature is less than 650 ° C., the ferrite transformation starts before the water cooling starts. Also in this case, the metal structure cannot be sufficiently finely divided. In these cases, the properties of the steel sheet cannot be sufficiently improved. Therefore, in this invention, water cooling start temperature shall be 650-720 degreeC. A preferable lower limit of the water cooling start temperature is 680 ° C., and a preferable upper limit is 710 ° C.

Next, the condition (v) will be described.
In the steel having the above chemical composition, when the water cooling stop temperature exceeds 500 ° C., the cooling is stopped in the middle of the ferrite transformation. In this case, the metal structure cannot be sufficiently finely divided. On the other hand, when the water cooling stop temperature is less than 350 ° C., a hardened structure is generated and the toughness of the steel sheet is lowered. Therefore, in this invention, water cooling stop temperature shall be 350-500 degreeC. The preferable lower limit of the water cooling stop temperature is 400 ° C, and the preferable upper limit is 450 ° C.

Next, the condition (vi) will be described.
In the steel having the above-described chemical composition, the strength cannot be sufficiently ensured when the average cooling rate is less than 20 ° C./s. A preferable lower limit of the average cooling rate is 25 ° C./s. The upper limit of the average cooling rate is not particularly defined, but is about 40 ° C./s because of the cooling capacity of the cooling device.

  By producing a steel sheet through the above steps, for example, the metal structure is mainly composed of ferrite, and is composed of 65 to 90% ferrite, 5 to 35% bainite, and 5% or less cementite. A steel plate having a thickness of 10 to 20 μm can be obtained.

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

  250 mm-thick slabs 1 to 48 having the chemical composition shown in Table 1 below were prepared, and steel plates 1 to 48 were manufactured according to the manufacturing conditions described in Table 2.

  As an evaluation of the strength characteristics of the obtained steel plates 1 to 48, in order to measure the tensile strength (MPa) and the yield strength (MPa), in accordance with the test method described in JIS Z 2241 (2011), for the tensile test The test piece was collected. The sampling position was the (1/4) t position of the sheet thickness t and the C direction (direction perpendicular to the rolling direction). The yield point was determined as a test speed of 10 N / (mm · s), and the yield point was 0.2% proof stress when no clear yield point appeared. The target value of strength was 490 MPa or more in terms of tensile strength.

  Moreover, the Charpy impact test and the NRL drop weight test were implemented as the evaluation method of the toughness and arrest property of the obtained steel plates 1-48. In the Charpy test, a 2 mmV notch test piece was collected according to the test method described in JIS Z 2242 (2005) and used for the test. In the Charpy impact test, those having a fracture surface transition temperature (vTrs) of −80 ° C. or lower were determined to be acceptable. The NRL drop weight test was performed using a P-2 test piece according to the test method described in ASTM E208, and an NDT temperature (Nil-Ductility-Transition Temperature: NDTT) of −90 ° C. or less was determined to be acceptable.

  The ferrite structure fraction (%), the bainite structure fraction (%), and the ferrite average crystal grain size (μm) were obtained by taking a specimen from the obtained steel sheet and (1/4) t of the sheet thickness t. The vicinity of the position was measured based on the measurement method described above.

  The above evaluation results are summarized in Table 3. In addition, the temperature of the slab when rolled to a thickness three times the finished thickness was obtained based on the above-described formula (I). In addition, the evaluation results shown in Table 3 show the intermediate values of three samples taken for each steel plate and tested.

  As can be seen from the evaluation results shown in Table 3, the chemical composition is within the range defined by the present invention, the metal structure satisfies the conditions defined by the present invention, and the manufacturing process is also within the range defined by the present invention. All of the steel plates 1 to 31 have strength, toughness, and arrest characteristics necessary for a steel plate for an LPG tank. In this example, three samples were tested for each steel plate as described above, and it was confirmed that there was no variation in the characteristics and that stable production was possible.

  On the other hand, in the steel plates 32 to 40 whose chemical composition is within the range specified by the present invention but whose manufacturing process is outside the range specified by the present invention, the structure fraction of ferrite, the structure fraction of bainite or ferrite Any of the average grain sizes does not meet the requirements of the present invention. And in this steel plates 32-40, either intensity | strength, a fracture surface transition temperature, or NDT temperature does not satisfy the criteria of a pass determination, At least among the intensity | strength, toughness, and arrest characteristic required as a steel plate for LPG tanks Do not have one.

  Further, the steel plate 41 and 43 to 48 whose manufacturing process is within the range specified by the present invention but whose chemical composition is outside the range specified by the present invention, and the manufacturing process and chemical composition are outside the range specified by the present invention. Even in a certain steel plate 42, any one of strength, fracture surface transition temperature, or NDT temperature does not satisfy the criteria for acceptance determination, and at least one of strength, toughness, and arrest properties necessary for a steel plate for an LPG tank is obtained. I don't have it.

  As described above, the steel sheet of the present invention has not only arrest properties but also high strength and excellent low-temperature toughness, and if manufactured by the method for manufacturing a steel sheet of the present invention, a certain amount of ferrite can be secured in the steel sheet. At the same time, the ferrite grain size can be controlled, and the characteristics of the steel sheet can be stabilized. Therefore, it is suitable as a steel sheet for an LPG tank.

Claims (6)

Chemical composition is mass%,
C: 0.03-0.08%,
Si: 0.05 to 0.50%,
Mn: 1.00-1.80%,
P: 0.015% or less,
S: 0.005% or less,
Ni: 0.10 to 0.80%,
Nb: 0.008 to 0.060%,
Ti: 0.005 to 0.050%,
sol.Al: 0.002 to 0.050%,
N: 0.0070% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.060%,
B: 0 to 0.0050%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0020%,
REM: 0 to 0.0020%,
Balance: Fe and impurities,
LPG tank characterized in that the metal structure is composed of 65 to 90% ferrite, 5 to 35% bainite and 5% or less cementite in area fraction, and the average crystal grain size of ferrite is 10 to 20 μm. Steel plate. The chemical composition is mass%,
Cu: 0.10 to 0.50%,
Cr: 0.05 to 0.50%,
Mo: 0.02 to 0.50%,
V: 0.010-0.060% and B: 0.0003-0.0050%
The steel sheet for LPG tanks according to claim 1, comprising one or more selected from the above. The chemical composition is mass%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0002 to 0.0020%, and REM: 0.0002 to 0.0020%
The steel sheet for LPG tanks according to claim 1 or 2, which contains one or more selected from the above. Chemical composition is mass%,
C: 0.03-0.08%,
Si: 0.05 to 0.50%,
Mn: 1.00-1.80%,
P: 0.015% or less,
S: 0.005% or less,
Ni: 0.10 to 0.80%,
Nb: 0.008 to 0.060%,
Ti: 0.005 to 0.050%,
sol.Al: 0.002 to 0.050%,
N: 0.0070% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.060%,
B: 0 to 0.0050%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0020%,
REM: 0 to 0.0020%,
The remainder: when heating, rolling and cooling the slab which is Fe and impurities,
In the rolling process, the temperature of the slab when rolled to a thickness three times the finish thickness is 810 ° C. or less, and the temperature of the slab when rolled to the pass two steps before the finish pass is 720 to 770. And rolling the slab so that the finishing temperature is 700 to 750 ° C.
In the cooling step, the cooling start temperature is set to 650 to 720 ° C., the cooling stop temperature is set to 350 to 500 ° C., and water cooling is performed at an average cooling rate of 20 ° C./s or more. The chemical composition is mass%,
Cu: 0.10 to 0.50%,
Cr: 0.05 to 0.50%,
Mo: 0.02 to 0.50%,
V: 0.010-0.060% and B: 0.0003-0.0050%
The manufacturing method of the steel plate for LPG tanks of Claim 4 containing 1 or more types selected from. The chemical composition is mass%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0002 to 0.0020%, and REM: 0.0002 to 0.0020%
The manufacturing method of the steel plate for LPG tanks of Claim 4 or 5 containing 1 or more types selected from these.