JP2011208222A - Method for manufacturing steel material for mixed loading of lpg and ammonium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel material to be used for a tank capable of storing and conveying both LPG and ammonium, the toughness of the steel being high and its yield strength being in compliance with the specification range.SOLUTION: In the method for manufacturing a steel material for the mixed loading of LPG and ammonium, a slab consisting of the steel containing, by mass, C: 0.02-0.10%, Si: 0.05-0.5%, Mn: 1.0-1.8%, P: ≤0.02, S: ≤0.01%, Ti: 0.005-0.02%, Nb: 0.005-0.06%, Cr: 0.05-0.20%, sol.Al: 0.015-0.08%, N: ≤0.008%, O: ≤0.005%, and the balance Fe with inevitable impurities, is heated, and hot-rolled. When executing the hot-rolling from the austenitic recrystallization temperature range to the austenitic-ferritic two-phase temperature range, the hot-rolling is started in the austenitic recrystallization temperature range, and the ratio of the pass number in the austenitic non-recrystallization temperature range to the total pass number in the hot rolling is set to be 40-60%.

Description

  The present invention relates to a method for producing a steel material for use in a tank capable of storing and transporting both LPG and ammonia, which has high toughness and yield strength conforms to a standard range.

  In view of the increase in energy demand in recent years, the tank capacity of energy transport ships is expanding.

  And from a viewpoint of eliminating the empty transport of an energy transport ship, not only LPG but also liquid ammonia may be stored at a low temperature and transported to the tank. Therefore, in addition to the low temperature toughness of the base metal part and the welded part of the tank being required as a basic characteristic from the viewpoint of low temperature storage, a steel material capable of preventing stress corrosion cracking due to ammonia is required. Various techniques have been proposed to meet such demands.

  For example, Patent Documents 1 to 3 describe techniques related to a steel material for LPG / ammonia mixed with excellent low-temperature toughness. All the steel materials described in these patent documents are manufactured by performing hot rolling at a temperature of 720 ° C. or higher with the cumulative reduction in the austenite non-recrystallization temperature range being 30% or higher. The purpose of such rolling in these techniques is to make austenite grains fine.

JP 2002-3983 A Japanese Patent Laid-Open No. 2002-3987 JP 2006-336065 A

  When a high pressure is applied in the austenite non-recrystallization temperature range, the austenite grains become finer and the structure of the steel material becomes finer in a form that succeeds. In this case, the strength of the steel material increases due to the fine grain effect. For this reason, in the techniques described in Patent Documents 1 to 3, since the steel material structure becomes finer, an increase in steel material strength is inevitable. As a result, although the toughness of the steel can be improved, the yield strength is also increased. Since the steel material for low temperature storage of ammonia requires stress corrosion cracking resistance due to ammonia loading, the upper limit of the standard of yield strength is stipulated as 440 MPa. Steel materials exceeding the standard range can be manufactured. Accordingly, it is possible to improve the characteristics (toughness) necessary for the steel material for low temperature storage of ammonia, while other characteristics (yield strength) cannot be obtained.

  The objective of this invention is providing the manufacturing method of the steel materials used for the tank which can store by low temperature both LPG and ammonia which have high toughness and the yield strength fits a specification range.

  The inventors of the present invention have obtained such a microstructure in consideration of the state of the microstructure during manufacture when manufacturing steel materials for use in LPG / ammonia transport tanks that have high toughness and yield strength that meets the standard range. The manufacturing method for this was examined.

  When the steel material is heated and the residual strain of the slab in the austenite region increases, the nucleation sites of ferrite transformation increase and the microstructure of the steel material is refined. Refinement of the microstructure causes an increase in yield strength as described above. Therefore, if the residual strain imparted by rolling can be controlled, an increase in yield strength can be avoided.

  The inventors calculated the yield strength predicted from the residual strain by simulation. As a result, it was found that when the residual strain is about 0.25 to 0.35, it conforms to the standard range of yield strength.

  On the other hand, a certain amount of residual strain promotes refinement of the microstructure and contributes to improvement of low temperature toughness. Therefore, a hot rolling reduction method was studied in order to impart a preferable residual strain to the slab.

  In conventional hot rolling, rolling is performed between the austenite-ferrite two-phase temperature region and the austenite non-recrystallization temperature region. In this case, since the amount of rolling in the austenite non-recrystallization temperature region becomes large, the austenite grain size becomes finer.

  Therefore, it is possible to avoid grain refinement due to recrystallization by rolling a lot in the austenite recrystallization temperature range where recrystallization occurs, and to adjust the crystal grains by performing partial rolling in the austenite non-recrystallization temperature range. Thought. Thus, the present invention was completed by obtaining the knowledge that an appropriate steel material can be obtained by controlling the reduction ratio in each temperature range.

  The gist of the present invention is as shown in any of the following (1) to (5). Hereinafter, they are referred to as “present invention (1)” to “present invention (5)”, respectively. Further, the present invention (1) to the present invention (5) may be collectively referred to simply as “the present invention”.

  (1) By mass%, C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.06%, Cr: 0.05 to 0.20%, sol. Al: 0.015-0.08%, N: 0.008% or less, O: 0.005% or less, LPG / ammonia mixed for heating and rolling hot slab made of Fe and impurities. This is a method for manufacturing steel products. When hot rolling is performed from the austenite recrystallization temperature range to the austenite-ferrite two-phase temperature range, hot rolling is started at the austenite recrystallization temperature range, and the total number of passes in the hot rolling. A method for producing a steel material for LPG / ammonia mixed use, wherein hot rolling is performed so that the ratio of the number of passes in the non-recrystallization temperature range of austenite to 40 to 60%.

  (2) The slab is characterized in that, instead of a part of Fe, by mass%, further, one or two of Cu: 0.30% or less and Ni: 0.50% or less are contained. The method for producing a steel material for LPG / ammonia mixed use according to (1) above.

  (3) The slab is characterized by containing, in place of a part of Fe, in mass%, and further containing one or two of Mo: 0.08% or less and V: 0.05% or less. The method for producing a steel material for LPG / ammonia mixed loading according to (1) or (2) above.

  (4) The slab is characterized in that, instead of a part of Fe, by mass%, and further containing one or two of Ca: 0.005% or less and Mg: 0.005% or less. The manufacturing method of the steel material for LPG / ammonia mixed loading in any one of said (1)-(3).

  (5) After hot rolling in the austenite recrystallization temperature range, the rolling is stopped, and after the slab temperature has decreased to the austenite non-recrystallization temperature range, the hot rolling is resumed. ) To (4) A method for producing a steel material for LPG / ammonia mixed use.

  (6) A method for producing LPG / ammonia mixed steel produced using two mills, a coarse mill and a finishing mill, in the austenite non-recrystallization temperature range with respect to the total number of passes in hot rolling in the finishing mill. The method for producing a steel material for LPG / ammonia mixed loading according to any one of (1) to (5) above, wherein the pass number ratio is 40 to 60%.

  According to the present invention, a steel material having a high toughness of −55 ° C. or less at a fracture surface transition temperature vTrs and conforming to a standard range of 440 MPa or less can be produced. By having such characteristics, it is possible to manufacture an LPG / ammonia mixed steel material that can withstand both low temperature storage of LPG / ammonia.

  Hereinafter, the constituent requirements of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of slab C: 0.02 to 0.10%
C is an element that is extremely effective in increasing the strength of steel. However, if the content is less than 0.02%, the desired strength cannot be ensured, so it is necessary to contain 0.02% or more. On the other hand, if the content exceeds 0.10%, the toughness of the welded joint is deteriorated and the SCC resistance is impaired due to the increase in hardness. For this reason, content of C shall be 0.02-0.10%. Preferably, it is 0.04 to 0.07%.

Si: 0.05-0.5%
Si is an element necessary as a deoxidizing material together with Al, and is extremely effective for increasing the strength of steel. In order to obtain a sufficient deoxidation effect and a sufficient steel strength, it is necessary to contain 0.05% or more. However, if the content exceeds 0.5%, it leads to abnormal hardening of the weld heat-affected zone and a decrease in joint toughness. For this reason, content of Si shall be 0.05-0.5%. Preferably, it is 0.1 to 0.4%.

Mn: 1.0 to 1.8%
Mn is an important element for improving the hardenability of steel and ensuring strength and toughness. In order to acquire this effect, it is necessary to make it contain 1.0% or more. However, if the content exceeds 1.8%, temper embrittlement becomes large and problems such as deterioration of weldability occur. For this reason, content of Mn shall be 1.0-1.8%. Preferably, it is 1.2 to 1.6%

P: 0.02% or less P is present in steel as an impurity. It is desirable to reduce as much as possible because it lowers the mechanical properties of the steel material, particularly low temperature toughness. However, since the removal of P is accompanied by a significant cost increase, the upper limit of the P content is set to 0.02% at which desired characteristics can be ensured. Preferably it is 0.015% or less.

S: 0.01% or less S is present as an impurity in steel. Since MnS is produced and low temperature toughness is lowered, it is desirable to reduce it as much as possible. However, since a significant increase in cost is inevitable for the removal of S, 0.01% that can ensure the desired characteristics is made the upper limit of the S content. Preferably it is 0.005% or less.

Ti: 0.005-0.02%
Ti is an extremely effective element for securing free N in steel and ensuring cleanliness of the slab surface and the steel material surface. And the effect becomes remarkable with content of 0.005% or more. However, if the content exceeds 0.02%, the impact properties of the steel material are lowered. For this reason, content of Ti shall be 0.005-0.02%. Preferably, it is 0.007 to 0.015%.

Nb: 0.005 to 0.06%
Nb improves the strength and toughness of the base material by refining and carbide precipitation. In order to obtain this effect, the Nb content needs to be 0.005% or more. However, if the content exceeds 0.06%, the cracking property during welding decreases. Therefore, the Nb content is 0.005 to 0.06% or less. Preferably it is 0.010 to 0.04%.

Cr: 0.05-0.20%
Cr is an element that contributes to increasing the strength of the steel material. In order to obtain this effect, the Cr content needs to be 0.05% or more. However, if the content exceeds 0.20%, not only this effect is saturated, but also the weldability is remarkably lowered, so the Cr content is 0.20% or less. Preferably it is 0.07 to 0.18%.

sol. Al: 0.015-0.08%
sol. Al fixes free N in steel as AlN and renders it harmless. It also has an effect as a deoxidizer. In order to acquire this effect, it is necessary to make it contain 0.015% or more. However, over 0.08% sol. Even if Al is contained, not only the effect is saturated, but also the toughness of HAZ (Heat Affected Zone) is deteriorated. For this reason, sol. Al content shall be 0.015-0.08%. Preferably it is 0.02 to 0.06%.

N: 0.008% or less N is present in steel as an inevitable impurity. N is sol. Although it is fixed as AlN by Al, it is desirable to reduce it as much as possible because it causes deterioration of the HAZ toughness when a large amount of N is present. Therefore, the N content is 0.008% or less. Preferably it is 0.006% or less.

O (oxygen): 0.005% or less O is present in steel as an impurity. If O exceeds 0.005%, oxide inclusions increase and the cleanliness and toughness of the steel are impaired. Accordingly, the Si content and the Al content must be regulated to appropriate amounts, and must be 0.005% or less. Preferably it is 0.003% or less.

  The slab used in the present invention is composed of Fe and impurities in addition to the above components. In addition, an impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing a slab industrially, and is permitted in the range which does not have a bad influence on this invention.

  In the slab used for this invention, you may contain 1 type, or 2 or more types among Cu, Ni, Mo, V, Ca, and Mg as needed.

Cu: 0.30% or less Cu has an effect of further improving strength and corrosion resistance, and can be contained as necessary. However, when Cu is contained exceeding 0.30%, hot cracking occurs. For this reason, Cu is 0.30% or less. Preferably it is 0.25% or less. In order to stably obtain the effect of improving the strength and corrosion resistance by Cu, it is preferable to contain 0.05% or more of Cu.

Ni: 0.50% or less Ni has an effect of increasing the toughness of the steel matrix (dough) in the solid solution state and further improving the toughness. Therefore, Ni can be contained as necessary. However, even if the content exceeds 0.50%, the effect of improving toughness is saturated, and improvement in characteristics commensurate with the increase in alloy cost cannot be obtained. For this reason, Ni is 0.50% or less. Preferably it is 0.45% or less. In order to stably obtain the effect of improving toughness by Ni, it is preferable to contain 0.05% or more of Ni.

Mo: 0.08% or less Since Mo is an element that contributes to an increase in the strength of the steel material, it can be contained as necessary. However, if Mo is contained in an amount exceeding 0.08%, the strength of the steel material becomes too large, the yield strength exceeds the standard range, and the weldability is also affected. Therefore, the Mo content is set to 0.08% or less. Preferably it is 0.05% or less. In order to stably obtain the effect of increasing the strength of the steel material due to Mo, it is preferable to contain 0.005% or more of Mo.

V: 0.05% or less V, like Mo, is an element that contributes to an increase in the strength of a steel material, and can be contained as necessary. However, if V exceeds 0.05%, the strength of the steel material becomes too high, and the yield strength exceeds the standard range. Therefore, the V content is 0.05% or less. Preferably it is 0.02% or less. In order to stably obtain the effect of increasing the strength of the steel material by V, it is preferable to contain V by 0.01% or more.

Ca: 0.005% or less Ca combines with S in steel to form a Ca-Mn-S compound, thereby preventing the evolution of the Mn-S compound and increasing the anisotropy of the mechanical properties of the steel. Since it is an element that can be reduced, it can be contained if necessary. However, even if Ca is contained in excess of 0.005%, the anisotropy reducing effect of the mechanical properties of the steel material is saturated, so the Ca content is set to 0.005% or less. Preferably it is 0.004% or less. In order to stably obtain the effect of reducing the anisotropy of the mechanical properties of the steel material due to Ca, it is preferable to add 0.002% or more of Ca.

Mg: 0.005% or less Mg has an effect of suppressing the growth of austenite grains in the weld heat affected zone and refines the structure, and is an element effective for improving the low temperature toughness of the weld zone. It can be contained if necessary. However, even if Mg is contained exceeding 0.005%, the effect of improving the low temperature toughness of the welded portion is saturated, so the content of Mg is made 0.005% or less. Preferably it is 0.004% or less. In order to stably obtain the low temperature toughness improvement effect of the welded portion by Mg, it is preferable to add 0.001% or more of Mg.

(B) Manufacturing method of steel material Below, the manufacturing conditions for obtaining the steel material for LPG and ammonia mixed loading whose yield strength is 440 Mpa or less and toughness is -55 degreeC or less with fracture surface transition temperature vTrs using said slab are described. To do.

  First, a slab having the above composition is prepared. Here, “slab” is used as a general term for steel ingots, blooms, billets and the like. Although a slab may be manufactured by an ingot method, it is preferable to manufacture a slab by a continuous casting method from a viewpoint of cost reduction. In this case, in order to control the inclusions at the center position of the plate thickness, the temperature of the molten steel is not excessively increased, and the difference is controlled within 50 ° C. with respect to the solidification temperature determined from the molten steel composition. It is preferable to perform electromagnetic stirring immediately before solidification and reduction during solidification. The slab thickness is preferably 200 mm or more.

The slab is heated to the austenite recrystallization temperature range. In order to perform sufficient rolling in the austenite recrystallization temperature range, it is preferable to heat to Ar ₃ point + 200 ° C. or higher. Specifically, it is preferable to heat to 1000 to 1180 ° C.

  Subsequently, hot rolling is performed. In the rolling step, hot rolling is started in the austenite recrystallization temperature range, and hot rolling is performed so that the ratio of the number of passes in the austenite non-recrystallization temperature range to the total number of passes in hot rolling is 40 to 60%. .

  First, recrystallization occurs when hot rolling is started in the austenite recrystallization temperature range, and the refinement of the structure due to rolling can be avoided. Here, it is preferable that the ratio of the number of passes in the austenite recrystallization temperature range to 50% to 30% in relation to the hot rolling in the austenite non-recrystallization temperature range relative to the total number of passes in hot rolling.

  After rolling in the austenite recrystallization temperature range, it is preferable to stop rolling and restart hot rolling after the slab temperature has dropped to the austenite non-recrystallization temperature range. When rolling is stopped, the slab temperature may be lowered by cooling or water cooling. After the slab temperature has dropped to the austenite non-recrystallization temperature range, hot rolling is performed in which the ratio of the number of passes in the austenite non-recrystallization temperature range to the total number of passes in hot rolling is 40 to 60%. By performing such hot rolling, the residual strain applied to the slab can be adjusted, yield strength can be adjusted, and toughness can be increased.

  Rolling in the austenite non-recrystallization temperature region is preferably started after becoming a low temperature side (below the intermediate temperature of the austenite non-recrystallization temperature region) in the austenite non-recrystallization temperature region. It is because it becomes easy to give a residual distortion by rolling in a low temperature range.

  Further, the hot rolling may be performed after the slab temperature falls in the austenite-ferrite two-phase temperature range. By performing hot rolling in the austenite-ferrite two-phase temperature range, the strength and toughness can be stabilized.

  If the above hot rolling is performed, the steel material used for the tank which can store and convey both LPG and ammonia which has high toughness and the yield strength conforms to the standard range can be produced.

  In addition, when manufacturing steel for LPG / ammonia mixed using two mills, a rough mill and a finishing mill, the number of passes in the rough mill is not counted, and the total number of passes in hot rolling in the finishing mill is not counted. The pass number ratio in the austenite non-recrystallization temperature range may be 40 to 60%. This is to control the applied amount of residual strain.

  Further, during the hot rolling, a descaling process may be performed in order to remove scale generated on the surface of the steel material by rolling. That is, the scale may be removed by spraying water onto the steel surface from a descaler installed adjacent to the mill. However, when the descaling process is performed, the steel material surface is temporarily cooled, the temperature difference between the steel material surface and the central portion is increased, and the variation in yield strength may be increased for each manufactured steel material. Therefore, it is desirable not to perform the descaling process as much as possible.

  What is necessary is just to manufacture steel materials by the manufacturing method normally performed especially after hot rolling. For example, after hot rolling, the steel material may be cooled to room temperature by air cooling or water cooling. In the case of water cooling, after completion of rolling, water cooling is started from a temperature of 600 ° C. or higher, cooled to a temperature of 500 ° C. or lower at a cooling rate of about 5 to 30 ° C./sec, and then water cooling is stopped. Good. By taking such a cooling pattern, it becomes easier to form a structure mainly composed of ferrite and satisfy the required standard without increasing the yield strength.

  Here, the heating temperature may be obtained from the atmospheric temperature in the furnace until the start of rolling, and may be obtained from the actually measured temperature on the steel material surface from the start of rolling to the completion of water cooling. Moreover, what is necessary is just to use the calculated value in a plate | board thickness center part for a cooling rate.

In addition, after water cooling, you may further heat-treat tempering as needed. For example, it may be reheated to a temperature of 450 ° C. or higher and Ac ₁ point or lower and kept soaked for a predetermined time, followed by air cooling or water cooling. By performing tempering, the transformation of the structure can be advanced and a stable structure can be obtained.

(C) Microstructure of steel material The microstructure of the steel material according to the present invention, that is, the microstructure of the steel material before welding when manufacturing the tank is mainly a ferrite structure.

  If the microstructure of the steel material is a ferrite structure, the yield strength is not so high, and therefore the SCC resistance is excellent. In addition, by carrying out constant controlled rolling in the austenite non-recrystallized region, an appropriate ferrite structure can be obtained, and sufficient low-temperature toughness to be used as a tank for LPG / ammonia carrier ships can be imparted to steel materials. .

  The microstructure does not have to be a complete ferrite structure. 60% or more of the area ratio should be a ferrite structure, and even if a pearlite structure or a bainite structure is partially formed in addition to the ferrite structure, it can be sufficiently used as a steel material for tanks for LPG / ammonia carrier ships. it can.

(D) Toughness and yield strength The steel material manufactured by the manufacturing method according to the present invention has a toughness of −55 ° C. or less at a fracture surface transition temperature vTrs and a yield strength of 315 to 440 MPa. In the present invention, by strictly controlling the rolling pass in the hot rolling, the yield strength becomes 440 MPa or less, the SCC (Stress Corrosion Cracking) characteristic can be secured, and high toughness is obtained. Can do.

A slab having a chemical composition shown in Table 1 and a thickness of 250 mm was prepared, and a steel material having a thickness t of 16 mm was manufactured according to the manufacturing method of the present invention. In manufacturing this steel material, two mills, a coarse mill and a finishing mill, were used. Detailed manufacturing conditions are as shown in Table 2. In Table 2, the non-recrystallized region two-phase region boundary temperature (Ar ₃ ) is a calculated value calculated from the following formula (1).
Ar ₃ = 868-396C + 24.6Si-68.1Mn-36.1Ni-20.9Cu-248Cr (1) Formula Here, the element symbol in the formula indicates the content (% by mass) of each element in the slab. .

  A steel material manufactured through a series of manufacturing processes has a cross section parallel to the rolling direction at a thickness of 1/4 (hereinafter referred to as “thickness (1/4) t”) from the steel surface of the thickness t. The microstructure was confirmed in two fields of view with an optical microscope (500 times magnification). Also, at the thickness (1/4) t position, take a tensile test piece (JISZ2241, 1A test piece) and a Charpy impact test piece (JIS Z2242, 2mmV notch test piece) from a cross section perpendicular to the rolling direction. And subjected to the test.

  The fracture surface transition temperature (vTrs) was determined by measuring the brittle fracture surface ratio from the specimen subjected to the Charpy impact test. The target of impact characteristics was vTrs of −55 ° C. or lower.

In the corrosion test, a test considering the environment loaded with ammonia and a test considering the environment loaded with LPG were performed. In the test considering the environment loaded with ammonia, stress corrosion cracking resistance against ammonia was evaluated. A test piece cut out from a part of the steel material was given a stress corresponding to 500 N / mm ² by four-point bending, and immersed in a corrosive solution (saturated NH ₄ CONH ₂ -liquid NH ₃ ) at a test temperature of 25 ° C. for 240 hours. Then, the presence or absence of a crack of each test piece was investigated using an optical microscope at a magnification of 200 times. As a result, the case where cracks were not observed was evaluated as good (◯), and the case where cracks were observed was evaluated as defective (x).

On the other hand, in the test considering the environment loaded with LPG, the sulfide contained as an impurity was considered and the stress corrosion cracking resistance against the sulfide was evaluated. Similarly, a test piece cut out from a part of steel material was subjected to stress corresponding to 500 N / mm ² by four-point bending, and immersed in a corrosive solution (pure water-2% H ₂ S) at a test temperature of 5 ° C. or less for 168 hours. Then, the presence or absence of cracking of each test piece was investigated using an optical microscope at a magnification of 200 times. As a result, the case where cracks were not observed was evaluated as good (◯), and the case where cracks were observed was evaluated as poor (x).

  Table 3 shows the results of the microstructure, tensile test (tensile strength TS, yield strength YP), fracture surface transition temperature vTrs, and stress corrosion cracking (in an ammonia environment and an LPG environment).

  From Table 3, the steel material manufactured by the manufacturing method specified in the present invention, that is, the manufacturing method No. 1-1, 1-2, 2-1, 3-1, 4-1 and 5-7 all have a yield strength of 315 to 440 MPa and are within the specified range, and the fracture surface transition temperatures are all below -55 ° C. And showed high toughness. The stress corrosion cracking characteristics were also good. On the other hand, a steel material manufactured by a manufacturing method that does not satisfy the manufacturing method specified in the present invention, that is, manufacturing method No. 1-3, 2-2, 3-2, 4-2, and 8 to 10 could not obtain desired characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the steel materials used for the tank which can store and convey both LPG and ammonia with high toughness and a yield strength suitable for a specification range can be provided. Therefore, the manufactured steel material not only improves safety when used as a steel material for low-temperature storage, but can also appropriately prevent stress corrosion cracking.

Claims (6)

  In mass%, C: 0.02-0.10%, Si: 0.05-0.5%, Mn: 1.0-1.8%, P: 0.02% or less, S: 0.01 %, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.06%, Cr: 0.05 to 0.20%, sol. Al: 0.015-0.08%, N: 0.008% or less, O: 0.005% or less, LPG / ammonia mixed for heating and rolling hot slab made of Fe and impurities. This is a method for manufacturing steel materials. When hot rolling is performed from the austenite recrystallization temperature range to the austenite-ferrite two-phase temperature range, the hot rolling is started in the austenite recrystallization temperature range, and the total number of passes in the hot rolling. A method for producing a steel material for LPG / ammonia mixed use, wherein hot rolling is performed so that the ratio of the number of passes in the non-recrystallization temperature range of austenite to 40 to 60%.   The slab contains, in place of a part of Fe, in mass%, and further contains one or two of Cu: 0.30% or less and Ni: 0.50% or less. Item 2. A method for producing an LPG / ammonia mixed steel material according to Item 1.   The slab is replaced by a part of Fe, and is in mass%, and further contains one or two of Mo: 0.08% or less and V: 0.05% or less. Item 3. The method for producing an LPG / ammonia mixed steel material according to Item 1 or 2.   The slab is replaced with a part of Fe, in mass%, and further contains one or two of Ca: 0.005% or less and Mg: 0.005% or less. Item 4. A method for producing an LPG / ammonia mixed steel material according to any one of Items 1 to 3.   After hot rolling in the austenite recrystallization temperature range, the rolling is stopped, and after the slab temperature is lowered to the austenite non-recrystallization temperature range, the hot rolling is restarted. The manufacturing method of the steel material for LPG and ammonia mixing in any one of these.   A method for producing an LPG / ammonia mixed steel material produced by using two mills, a coarse mill and a finishing mill, in which the number of passes in the austenite non-recrystallization temperature range relative to the total number of passes in hot rolling in the finishing mill. The method for producing a steel material for LPG / ammonia mixed loading according to any one of claims 1 to 5, wherein the ratio is 40 to 60%.