JP5811032B2 - Steel sheet for LPG tank - Google Patents

Description

  The present invention is an excellent steel sheet for LPG tanks with excellent toughness, especially in order to prevent the collapse of the whole structure when a brittle crack is generated, and is excellent in the characteristic (arrest characteristic) for stopping the generated brittle crack. The present invention relates to a steel sheet for an LPG tank. In this case, the steel sheet for LPG tanks is for steel plates with a thickness of 6 to 40 mm, and the strength class is for steels with a tensile strength of 450 MPa or more.

  The main purpose of the LPG tank steel sheet is a storage tank for storing LPG (Liquefied Petroleum Gas) in a low temperature range. The low temperature means a temperature of −60 ° C. or less at which LPG can be stored in a liquid temperature range.

  In all structures, the collapse due to brittle fracture is the type of fracture that should be avoided because the entire structure collapses instantly and enormous damage is assumed. Therefore, although structures such as storage tanks are designed to avoid the occurrence of brittle fracture, brittle fracture due to abnormal situations unexpected by the designer, such as the effects of external force exceeding the design and defects caused by construction, etc. It is also necessary to consider the case where this occurs. When brittle fracture occurs, brittle fracture spreads throughout the structure due to extremely high-speed crack propagation, and the entire structure is destroyed. Therefore, even if a brittle crack is generated, a characteristic capable of stopping the generated brittle crack is required. This characteristic is generally called “arrest characteristic”.

  A structure with arrested members in place not only avoids the occurrence of brittle cracks, but even if a brittle crack occurs, it can stop the crack that has propagated due to propagation. It has double safety (double integrity) at the stage of initiation and propagation of brittle cracks. This is extremely important as the design concept of the structure.

  And the steel sheet for LPG tanks made into a thing with a board thickness of 6-40 mm is calculated | required the fracture fracture toughness excellent in low temperature from the surface of ensuring safety | security. 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 add Ni, which is an element that significantly improves toughness. It has been found that the effect of improving the arrest characteristics by the addition of Ni is large, and the arrest characteristics can be improved. As a steel material having arrest properties in a low temperature environment, a so-called 9% Ni steel added with 9% Ni is generally used and is also defined in Japanese Industrial Standards (JIS).

  For example, Patent Documents 1 and 2 disclose a method of manufacturing a steel sheet that promotes shear lip formation during brittle crack propagation by making the surface layer structure very fine. According to this method, it is possible to improve arrest characteristics without relying on expensive elements such as Ni.

JP-A-3-2322 JP 7-126798 A

  As described above, the effect of improving the arrest characteristics by Ni is large, and the arrest characteristics can be improved. However, Ni is a very expensive element, and if Ni is added as much as 9%, the steel material cost will rise. Therefore, improvement of arrest characteristics by adding Ni has many problems in terms of cost.

  On the other hand, according to the invention disclosed in Patent Documents 1 and 2, it is possible to improve arrest characteristics without adding expensive elements such as Ni. However, as an LPG tank steel plate, high strength is obtained, but toughness at low temperatures is not sufficient.

  In view of such circumstances, an object of the present invention is to provide a low-cost steel sheet for LPG tank that has not only arrest properties but also high strength and excellent low-temperature toughness.

  In order to solve the above-mentioned problems, various studies and experiments were conducted on strength and low-temperature toughness in addition to arrest characteristics (brittle crack propagation stopping characteristics) as steel sheets for LPG tanks. As a result, the following findings (a) to (e) were obtained.

  (a) The arrest properties improve as Ni is added.

  (b) By adding an appropriate amount of Ti, the structure becomes finer and the arrest characteristics of the base metal and the welded joint are improved.

  (c) By adding an appropriate amount of Nb, strength is improved while securing arrest properties by precipitation hardening of Nb. However, when there is too much content, a hardened deposit will increase and an arrest characteristic will deteriorate on the contrary.

  (d) In order to ensure arrest properties and obtain high toughness, the microstructure should be mainly composed of a soft ferrite structure. A pearlite structure containing a large amount of C deteriorates the toughness of the welded portion. However, if the fraction of the ferrite structure is 90% or more, it may be mixed if the fraction of the pearlite structure is less than 5%. Therefore, the microstructure is a ferrite-based structure having a ferrite structure of 90% or more and a pearlite structure of less than 5%. When a structure such as bainite is generated, the strength increases excessively and the toughness deteriorates.

  (e) Even in such a structure mainly composed of ferrite, when the crystal grain size becomes coarse, the strength, toughness and arrest characteristics are lowered. Therefore, the average crystal grain size is set to 10 to 20 μm.

  The present invention has been made on the basis of the above findings, and the gist thereof is the following steel plates for LPG tanks (1) to (3).

(1) By mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.50%, Mn: 1.0 to 1.8%, P: 0.015% or less, S: 0.005% or less, Ni: 0.1 to 0.8%, Nb: 0.008 -0.06%, Ti: 0.005-0.05%, Sol.Al:0.002-0.05% and N: 0.007% or less, consisting of the balance Fe and impurities, ferrite structure in the microstructure is 90% or more and pearlite structure A steel sheet for an LPG tank having a thickness of 6 to 40 mm, having a ferrite average crystal grain size of 10 to 20 μm and a pearlite average crystal grain size of 10 to 20 μm, less than 5%.

  (2) It is characterized by containing at least one of Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.06% or less, and B: 0.005% or less by mass%. The steel sheet for an LPG tank according to (1) above.

  (3) In the above (1) or (2), the composition further contains at least one of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less by mass%. Steel sheet for LPG tank.

  It is possible to provide a low-cost steel sheet for an LPG tank having not only arrest properties but also high strength and excellent low-temperature toughness.

  Hereinafter, the steel sheet for LPG tank according to the present invention will be described in detail for each requirement. In addition, "%" regarding content means "mass%" unless otherwise indicated.

A. Regarding chemical composition C: 0.03 to 0.08%
C is an element necessary for ensuring strength. Unless 0.03% or more is contained, steel having practical strength cannot be produced. On the other hand, if the content exceeds 0.08%, the toughness deterioration of the bainite transformation region becomes remarkable and the toughness of the weld heat affected zone is also impaired. Therefore, the C content is 0.03 to 0.08%. From the standpoint of balance between strength and arrest characteristics, the preferred range is 0.04 to 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 strength. For this reason, it is necessary to contain 0.05% or more of Si. However, if the Si content exceeds 0.50%, the formation of island martensite in the weld heat affected zone is promoted, which adversely affects toughness. Therefore, the Si content is set to 0.05 to 0.50%. A preferable range is 0.10 to 0.40%.

Mn: 1.0 to 1.8%
Mn is an element necessary for ensuring strength. However, if the content is less than 1.0%, this effect cannot be obtained. On the other hand, if it exceeds 1.8%, the toughness of the weld heat affected zone is significantly deteriorated. Therefore, the Mn content is 1.0 to 1.8%. A preferred range is 1.2-1.6%.

P: 0.015% or less P is present in steel as an impurity and causes intergranular cracking in the weld heat affected zone. If the P content exceeds 0.015%, the occurrence of intergranular cracks in the weld heat affected zone becomes significant, so the upper limit of the P content is 0.015%. The P content is preferably 0.010% or less. In addition, it is preferable to make the mixing amount 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 base point of brittle fracture and impairs arrest properties. When the S content exceeds 0.005%, the arrest characteristics are remarkably deteriorated. Therefore, the upper limit of the S content as the impurity element is set to 0.005%. The S content is preferably less than 0.003%. In addition, it is preferable to make the mixing amount as low as possible.

Ni: 0.1 to 0.8%
When Ni is contained, the arrest properties of the steel sheet can be improved, and therefore the content of 0.1% or more is necessary. However, since the Ni content causes an increase in cost, the content is made 0.8% or less. In addition, the minimum with preferable content of Ni is 0.3%.

Nb: 0.008 to 0.06%
Nb is an element effective for refining the structure, improving hardenability, and increasing the strength by precipitation hardening. For that purpose, Nb needs to be contained in an amount of 0.008% or more. This is particularly necessary when applying the TMCP (Thermo-Mechanical Control Process) method because the effect of expanding the non-recrystallized region is great. However, if its content exceeds 0.06%, the increase in precipitates leads to deterioration of toughness. Therefore, the Nb content is set to 0.008 to 0.06%. Preferably it is 0.01 to 0.03%.

Ti: 0.005 to 0.05%
Ti has the effect of increasing the toughness by refining the structure. In order to exhibit this effect, it is necessary to contain 0.005% or more. In particular, it is effective for refining the structure when manufacturing the base material by online accelerated cooling. However, when the content of Ti is too large, the toughness of the welded joint is reduced. In particular, when it exceeds 0.05%, the toughness of the welded joint is significantly reduced. Therefore, the Ti content is set to 0.005 to 0.05%. A more desirable Ti content is 0.005 to 0.03%.

sol. Al: 0.002 to 0.05%
Al is an element necessary for deoxidation of steel. Therefore, 0.002% or more of sol. Al content is required. However, when the content exceeds 0.05%, the deterioration of arrest properties becomes remarkable through the increase of precipitates. Therefore, sol. The Al content is 0.002 to 0.05%. Preferably it is 0.002 to 0.04%

N: 0.007% or less N exists as an impurity and causes toughness deterioration by forming a precipitate. When the content of N exceeds 0.007%, the deterioration of arrest characteristics becomes remarkable, so the content of N is set to 0.007% or less. Preferably it is 0.006% or less. In addition, in order to ensure low temperature toughness, the lower one is better.

  In the LPG tank steel plate according to the present invention, the balance consists of Fe and impurities. Here, the impurity means a component that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet for LPG tank is industrially manufactured.

  The steel sheet for LPG tank according to the present invention may further contain one or more of Cu, Cr, Mo, V, B, Ca, Mg and REM in addition to the above components.

Cu: 0.5% or less Cu can be contained as necessary. When Cu is contained, the strength can be improved without deteriorating toughness. However, if the content exceeds 0.5%, the arrest properties are deteriorated due to the increase in precipitates, and further, when the processing is hot, micro cracks are generated on the surface. The upper limit is 0.5%. A preferable upper limit of Cu is 0.3%. In order to stably develop the strength improvement effect by Cu, it is preferable to contain Cu by 0.1% or more.

Cr: 0.5% or less Cr can be contained as necessary. When Cr is contained, the strength can be increased. However, if the content exceeds 0.5%, the toughness is deteriorated on the contrary, and further, a hardened structure is formed in the weld heat affected zone and the toughness is deteriorated. Therefore, the upper limit of the content is 0.5%. And A preferable upper limit of Cr is 0.3%. In order to stably develop the strength improvement effect by Cr, it is preferable to contain 0.05% or more of Cr.

Mo: 0.5% or less Mo can be contained as necessary. When Mo is contained, the hardenability can be improved and the strength can be improved. However, the content of Mo becomes a cost increase factor, and if the content exceeds 0.5%, the toughness of the weld heat affected zone is deteriorated, so the upper limit of the content is 0.5%. A preferable upper limit of Mo is 0.3%. In order to stably develop the hardenability and strength improvement effect of Mo, it is preferable to contain 0.02% or more of Mo.

V: 0.06% or less V can be contained as necessary. Inclusion of V is effective for improving hardenability and improving strength by precipitation hardening. However, if the V content exceeds 0.06%, the toughness is significantly deteriorated. Therefore, the upper limit of the content is set to 0.06%. In order to stably exhibit the effect of improving the hardenability and strength by V, it is preferable to contain V by 0.01% or more.

B: 0.005% or less B can be contained if necessary. When B is contained, the ferrite transformation from the austenite grain boundary is suppressed, the hardenability is improved, and the strength can be increased. However, since the toughness deteriorates when the B content exceeds 0.005%, the upper limit of the content is set to 0.005%. A preferable upper limit of B is 0.0015%. In order to stably express the effect of improving hardenability and strength by B, it is preferable to contain B by 0.0003% or more. When B is contained, it is important to sufficiently consider the balance with the hardenability indicated by the carbon equivalent in order to ensure the ferrite content at the center of the plate thickness.

Ca: 0.004% or less Ca can be contained as necessary. When Ca is contained, it has an effect of controlling the shape of inclusions and contributes to improvement of arrest characteristics. However, if its content exceeds 0.004%, the cleanliness of the steel itself is greatly reduced, so the upper limit of its content is 0.004%. A preferable upper limit of Ca is 0.002%. In order to stably express these effects by Ca, it is preferable to contain 0.0003% or more of Ca.

Mg: 0.002% or less Mg can be contained as necessary. When Mg is contained, the dispersion density of the fine oxide can be increased, and the toughness of the weld heat affected zone is improved. However, if its content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced, so the upper limit of its content is made 0.002%. A preferable upper limit of Mg is 0.0015%. In order to stably exhibit the effect of improving the fine oxide dispersion density by Mg, it is preferable to contain 0.0002% or more of Mg. Here, the step of containing Mg in the molten steel is preferably performed before Al is contained in the molten steel.

REM: 0.002% or less REM (rare earth element) can be contained as required. When REM is contained, like Mg, the dispersion density of the fine oxide can be increased, and the toughness of the weld heat affected zone is improved. Furthermore, the effect of fixing excess S as sulfides can also be obtained. However, if its content exceeds 0.002%, fine oxides cannot be obtained, and the cleanliness of the steel is greatly reduced, so the upper limit of its content is made 0.002%. A preferable upper limit of REM is 0.0015%. In addition, in order to stably express these effects by REM, it is preferable to contain REM 0.0002% or more. Here, the step of incorporating REM in the molten steel is preferably performed before Al is contained in the molten steel. In addition, REM means 17 elements in which Y and Sc are combined with 15 elements of lanthanide, and one or more of these elements can be contained. You may make it contain in steel in the state mixed as misch metal. Note that the content of REM means the total content of these elements.

B. Regarding the microstructure of a steel sheet In general, the pearlite structure and the bainite structure have lower toughness of the base material and the welded part than the fine-grained ferrite structure. Therefore, it is necessary to increase the ferrite structure. Examples of methods for increasing the ferrite structure include an increase in γ grain boundaries due to low-temperature heating and an expansion of the ferrite generation range due to processing induction. As a result of testing on steels having various ferrite ratios, the present inventors have found that steels having a ferrite structure fraction of 90% or more possess excellent arrest properties. Therefore, the ferrite structure fraction in steel is defined as 90% or more. It was also found that if the pearlite structure was less than 5%, there was almost no decrease in toughness. Therefore, the pearlite structure fraction is less than 5%.

  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.

C. Regarding the average crystal grain size of the steel sheet The toughness of the high-strength thick steel sheet is improved by setting the average crystal grain size to 20 μm or less in addition to defining the ferrite structure fraction of the steel sheet obtained after rolling as 90% or more. I found out that

  In addition, when measuring the crystal grain size 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.

  A slab having a thickness of 250 mm made of a steel type having the chemical composition shown in Table 1 was prepared, and a steel plate was produced by the TMCP method.

  The conditions of the TMCP method are as described in Table 2. That is, heating, rolling, and water cooling were performed to obtain a steel plate having a thickness t of 16 mm.

  Specimens are collected from the obtained steel sheet, and the ferrite structure fraction (%), pearlite structure fraction (%), ferrite average particle diameter (μm) and pearlite average particle diameter (μm) are measured by the above-described measuring method. Measured based on

  In order to measure the tensile strength TS (MPa) and the yield strength YS (MPa) as an evaluation of the strength characteristics of the obtained steel sheet, a specimen for a tensile test was taken according to the test method described in JIS Z2201. did. The sampling position was the (1/4) t position of the sheet thickness t and the C 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 for strength was 490 MPa or more in terms of tensile strength TS.

  In addition, a Charpy impact test and an NRL drop weight test were performed as methods for evaluating the arrest properties of the obtained steel sheet. In the Charpy test, a 2 mmV notch test piece was collected according to the test method described in JIS Z2242, and used for the test. In the Charpy impact test, a specimen having a fracture surface transition temperature (vTrs) of −80 ° C. or lower was 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 above survey results are summarized in Table 3.

  As can be seen from the characteristic evaluation results shown in Table 3, the chemical composition is within the range specified in the present invention, and the ferrite structure fraction, the pearlite structure fraction, the ferrite average particle diameter and the pearlite average particle diameter (μm) are The steel grades within the range defined in the present invention, that is, the steel Nos. 1, 2, 3, 6, 7, 9, 12, and 13 are all required strength properties and arrest properties as steel plates for LPG. Is satisfied.

  On the other hand, steel No. 4 is outside the range of Nb content. Therefore, the structure refinement was insufficient and the toughness deteriorated. In addition, the strength deteriorated.

  Steel Nos. 5 and 8 are outside the specified range of the C content. Accordingly, none of the ferrite structure fraction, the pearlite fraction, the ferrite particle size, and the pearlite particle size reach the standard, and as a result, the toughness is insufficient.

  Steel Nos. 10 and 11 have compositions within the specified range of the present invention, but any one of ferrite structure fraction, pearlite fraction, ferrite particle size, and pearlite particle size does not reach the specified value. , Lack of toughness. Here, Steel No. 10 has a bainite-based structure because the rolling finishing temperature is too high, and Steel No. 11 has a too high heating temperature, so that the crystal grain size becomes coarse during heating. The toughness is deteriorated.

  As described above, the steel sheet according to the present invention is suitable as a steel sheet for an LPG tank because it has not only arrest properties but also high strength and excellent low-temperature toughness.

Claims (3)

In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.50%, Mn: 1.0 to 1.8%, P: 0.015% or less, S: 0.005% or less, Ni: 0.1 to 0.8%, Nb: 0.008 to 0.06% , Ti: 0.005 to 0.05%, Sol.Al: 0.002 to 0.05% and N: 0.007% or less, consisting of remaining Fe and impurities, ferrite structure in the microstructure is 90% or more and pearlite structure is less than 5% A steel sheet for an LPG tank having a thickness of 6 to 40 mm, wherein the ferrite average crystal grain size is 10 to 20 μm and the pearlite average crystal grain size is 10 to 20 μm.   Further, it is characterized by containing at least one of Cu: 0.5% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.06% or less, and B: 0.005% or less. The steel sheet for LPG tanks according to claim 1.   The steel sheet for an LPG tank according to claim 1 or 2, further comprising at least one of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less in mass%. .