JP2013255936A - Method for manufacturing austenitic stainless-clad steel excellent in marine corrosion resistance and low-temperature toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless-clad steel excellent in marine corrosion resistance and low-temperature toughness.SOLUTION: In a method for manufacturing austenitic stainless-clad steel with 30 mm or more total thickness excellent in marine corrosion resistance and low-temperature toughness, a carbon steel contains, by mass%, 0.10-0.15% C; 0.25-0.40% Si; 0.45-2.0% Mn; ≤0.015% P; ≤0.004% S; ≤0.006% N; 0.01-0.1% Cr; 0.005-0.05% Nb; and 0.005-0.05% Al, and is composed of the balance Fe and unavoidable impurities, regarding the component composition. A clad steel material including the carbon steel as a base material and austenitic stainless steel as a laminated material is heated to temperature of 1,000-1,250°C before starting hot rolling, in which a cumulative rolling reduction of control rolling in a temperature range of 730-950°C is set to be 30% or more, and rolling finishing temperature is set to be 730-850°C.

Description

  The present invention relates to an austenitic stainless clad steel excellent in seawater corrosion resistance and low temperature toughness used in various applications such as marine structures, shipbuilding and seawater desalination facilities.

  In recent years, as the needs of industrial facilities and structures, durability, long life, and maintenance free have been directed, and stainless steel is attracting attention as a material suitable for these needs. On the other hand, alloy elements typified by Ni, Mo, and Cr, which are the main raw materials for stainless steel, have soaring prices and up / down movements in price. Therefore, instead of pure stainless steel, the superior corrosion resistance of stainless steel is more enhanced. Recently, stainless steel clad steel has attracted attention as a steel material that can be economically used, is stable in price, and inexpensive.

  Stainless steel clad steel is a composite steel material in which two kinds of different metals are joined, stainless steel as a laminated material and ordinary steel as a base material. The clad steel is obtained by metallographically bonding dissimilar metals, and unlike the plating, there is no fear of peeling, and it can have new characteristics that cannot be achieved by a single metal and alloy. As described above, the stainless clad steel requires only a small amount of stainless steel, and can secure the same corrosion resistance as that of a solid material (full thickness stainless steel).

  From the above, stainless clad steel is considered to be a very useful functional steel material, and in recent years, its needs are increasing in various industrial fields. Especially when stainless steel clad steel is used in various applications such as seawater desalination equipment, shipbuilding (FPSO: Floating Production, Storage and Offloading system) used in environments where it contacts with ocean structures and seawater. Because it is used in a corrosive environment, seawater corrosion resistance is required.

  The passive film of stainless steel is easily broken by chloride ions, and its corrosion form takes the form of Pitting Corrosion or Crevice Corrosion. Therefore, while corrosion forms in acids such as sulfuric acid and hydrofluoric acid exhibit general corrosion, pitting corrosion resistance, which is the starting point of local corrosion, is an important index in seawater.

  The pitting corrosion resistance of stainless steel is generally arranged by the pitting corrosion index (PRE) (represented by Cr (mass%) + 3.3Mo (mass%) + 16N (mass%)). It is said to be excellent in food quality. However, this is applicable to solid stainless steel that has been heat-treated to dissolve precipitates, etc., and is not directly applicable to the pitting corrosion resistance of stainless clad steel composite material that is a composite material with carbon steel. .

  Stainless steel clad steel is usually used after so-called normalization in which it is heated to a temperature range of 850 to 950 ° C. and air-cooled for the purpose of ensuring mechanical properties such as strength and toughness of the base material. When stainless steel is subjected to a thermal history such as a heat-affected zone caused by inappropriate heat treatment or welding, the corrosion resistance may be significantly reduced. One of the causes is precipitation of intermetallic compounds such as carbide and σ phase. The plate thickness of the clad steel also affects the thermal history, and particularly in the clad steel of 30 mm or more, the cooling rate becomes low, and intermetallic compounds such as carbides and σ phases are more likely to precipitate.

  In order to ensure the mechanical properties of the base material and prevent the corrosion resistance from deteriorating, for example, in Patent Document 1, the austenitic stainless steel of the clad material is 850 to 950 ° C. by specifying the components of the laminated material. A technique for maintaining excellent corrosion resistance even after normalizing treatment is disclosed.

  Patent Document 2 discloses a method of manufacturing a stainless clad steel pipe that defines a solution heat treatment condition and components of a base material carbon steel and uses stainless steel having excellent seawater resistance as a combination material and carbon steel as a base material. It is disclosed.

  Patent Document 3 discloses a method for securing the base material toughness by defining the carbon content of the base material carbon steel and by prescribing the joint rough reduction rate and the reduction rate by controlled rolling.

JP-A-63-125615 Japanese Patent No. 4179133 Japanese Examined Patent Publication No. 2-41400

However, the corrosion resistance of the stainless clad steel obtained by the method disclosed in Patent Document 1 has a problem that the corrosion rate in the ferric chloride corrosion test is about ² g / m ² · h, which is not sufficient as seawater corrosion resistance. is there.

  On the other hand, in the method disclosed in Patent Document 2, in order to ensure the seawater corrosion resistance required for each use of a stainless clad steel pipe (for example, an offshore structure, etc.), the stainless steel used as a bonding material is selected for each use. Must. That is, only the method of adjusting only with the components of stainless steel is shown. In the case of stainless clad steel, the reliability of the bonding interface is improved in terms of soundness (bondability) and the performance of the base material and the laminated material (corrosion resistance and It is difficult to maintain high mechanical properties at the same time for high-grade steel materials and all kinds of products.

  In addition, the method disclosed in Patent Document 3 has a problem that the corrosion resistance of the laminated stainless steel has not been studied.

  An object of this invention is to provide the austenitic stainless clad steel excellent in seawater corrosion resistance and low temperature toughness which solves the above-mentioned problems.

  In order to solve the above-mentioned problems, a study was made on a manufacturing method excellent in pitting corrosion resistance and low-temperature toughness in stainless clad steel completed from rolling to heat treatment with a plurality of components (steel composition) and a plurality of histories. And when it examined considering the component of steel, rolling conditions, pitting corrosion resistance, and low temperature toughness, C: 0.10-0.15% by mass%, Si: 0.25-0.40%, Mn: 0.45 to 2.0%, P: 0.015% or less, S: 0.004% or less, N: 0.006% or less, Cr: 0.01 to 0.1%, Nb: 0.0. A clad steel material having a carbon steel in a component range of 005 to 0.05% and Al: 0.005 to 0.05% as a base material and an austenitic stainless steel as a combination material in a temperature range of 730 to 950 ° C. By performing hot rolling at a cumulative rolling reduction of 30% or more and a finish rolling temperature of 730 to 850 ° C., the critical pitting temperature (CPT) is 30 mm or more and ASTM G48-03 Method E is used. ) Is 45 ° C. or higher, and JIS Z 2242 An austenitic stainless clad steel excellent in seawater corrosion resistance and low-temperature toughness, characterized in that the absorbed energy in a Charpy impact test at −40 ° C. is 100 J or more.

  This invention is made | formed based on the above knowledge, The summary is as follows.

  [1] Component composition is mass%, C: 0.10 to 0.15%, Si: 0.25 to 0.40%, Mn: 0.45 to 2.0% P: 0.015% or less S: 0.004% or less, N: 0.006% or less, Cr: 0.01 to 0.1%, Nb: 0.005 to 0.05%, Al: 0.005 to 0.05% Containing a remaining steel and carbon steel composed of unavoidable impurities as a base material and a clad steel material made of austenitic stainless steel as a combined material, heated to 1000 to 1250 ° C., and then started hot rolling, Sea rolling corrosion resistance and low temperature toughness with a total thickness of 30 mm or more, characterized by performing hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of 950 ° C. and a finishing temperature of 730 to 850 ° C. Method for producing austenitic stainless clad steel with excellent resistance.

  [2] The composition of the carbon steel further includes, in mass%, Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, Ti: 0.005 to 0.015%. The method for producing an austenitic stainless clad steel excellent in seawater corrosion resistance and low-temperature toughness having a total thickness of 30 mm or more as described in the above [1], comprising one or more selected from the above.

  [3] Using the clad steel material according to the above [1] or [2], after heating to 1000 to 1250 ° C, hot rolling is started, and cumulative rolling of controlled rolling in a temperature range of 730 to 950 ° C is started. The rate is 30% or more, and after performing hot rolling with a rolling finishing temperature of 730 to 850 ° C., accelerated cooling with a cooling rate of 3 to 40 ° C./s and a cooling stop temperature of 500 ° C. or more is performed. A method for producing an austenitic stainless clad steel having a total thickness of 30 mm or more and excellent in seawater corrosion resistance and low temperature toughness.

  [4] In the method for producing a stainless clad steel according to any one of [1] to [3], the pitting corrosion index (PRE) defined by the following formula (1) of the austenitic stainless steel of the laminated material is: A method for producing an austenitic stainless clad steel having a total thickness of 30 mm or more and excellent in seawater corrosion resistance and low temperature toughness, characterized by being 40 or more.

PRE = Cr + 3.3Mo + 16N (1)
In addition, each element symbol is the content expressed by mass% of each element.

  According to the present invention, a seawater resistant stainless clad steel having a total thickness of 30 mm or more excellent in seawater corrosion resistance and low temperature toughness can be obtained. Thereby, it can use suitably in the use for which the marine structure and the shipbuilding field represented by FPSO, and the seawater corrosion resistance represented by the seawater desalination apparatus are requested | required.

  The stainless clad steel production method of the present invention has a total thickness of 30 mm or more, a critical pitting corrosion temperature (CPT) according to ASTM G48-03 Method E of 45 ° C. or more, and a Charpy impact at −40 ° C. according to JIS Z 2242. The chemical composition and production conditions of the base carbon steel are specified so that the absorbed energy in the test is 100 J or more.

1. About component composition First, the reason which prescribed | regulated the component composition of the base material of this invention is demonstrated. In addition, all component% means the mass%.

C: 0.10 to 0.15%
C is an effective component for improving the strength of the base material, and needs to contain 0.10% or more in order to ensure the strength as a structural steel material. However, if the content exceeds 0.15%, the toughness of the base metal is deteriorated and the weldability is also adversely affected, so the C content is in the range of 0.10 to 0.15%. Preferably it is 0.11 to 0.13% of range.

Si: 0.25 to 0.40%
Si is a component necessary for securing the strength of the base material and deoxidation, and in order to obtain the effect, it is necessary to contain 0.25% or more. On the other hand, if the content exceeds 0.40%, the toughness of the base material is remarkably deteriorated, so the Si content is in the range of 0.25 to 0.40%. Preferably it is 0.30 to 0.35% of range.

Mn: 0.45-2.0%
Mn needs to be contained in an amount of 0.45% or more as an effective component for securing the strength and toughness of the base material. In order to increase, the amount of Mn is made into the range of 0.45-2.0%. In addition, from a viewpoint of base material toughness and weld heat affected zone toughness, it is preferably in the range of 1.0 to 1.6%.

P: 0.015% or less P is an inevitable impurity in steel, and the smaller the content, the better. However, in order to reduce industrially, the cost is high, so the P amount is 0.015% or less.

S: 0.004% or less S is an unavoidable impurity in the steel, and it is desirable that the content is small.

N: 0.006% or less N combines with Al in the steel, adjusts the crystal grain size at the time of rolling, and strengthens the steel. However, if the content exceeds 0.006%, the toughness deteriorates. The N content is 0.006% or less. Preferably it is 0.003 to 0.005% of range.

Cr: 0.01 to 0.1%
Cr is effective for improving the strength and toughness of the base material, and is preferably contained in an amount of 0.01% or more. On the other hand, the content exceeding 0.1% lowers the weld heat affected zone toughness, so the Cr content is in the range of 0.01 to 0.1%. In addition, Preferably, it is 0.02 to 0.07% of range.

Nb: 0.005 to 0.05%
Nb has the effect of improving the toughness due to precipitation strengthening by generating NbC and the refinement of crystal grains. Therefore, when performing controlled rolling with a cumulative reduction of 30% or more in the temperature range of 730 to 950 ° C. as in the present invention, it contributes to an increase in strength and toughness. The effect is exhibited when the content is 0.005% or more. If the content exceeds 0.05%, the effect is not only saturated, but surface flaws are likely to occur in the steel slab. Therefore, the Nb content is in the range of 0.005 to 0.05%. In addition, Preferably it is 0.025 to 0.050% of range.

Al: 0.005 to 0.05%
Al is an element effective as a deoxidizer, but if less than 0.005%, the effect cannot be obtained, and if it exceeds 0.05%, the toughness deteriorates. The range is 0.05%. For the same reason, the range is preferably 0.005 to 0.015%.

  The above is the basic component of the base material of the present invention, but in order to further improve the characteristics, in addition to the above components, one or more selected from Cu, Ni and Ti are selectively contained in the following range. Also good.

Cu: 0.01 to 0.3%
Cu is effective in improving the toughness of the base material and increasing the strength, and is preferably contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.3%, the toughness of the base metal and the toughness of the heat affected zone are deteriorated. preferable. More preferably, it is 0.20 to 0.3% of range.

Ni: 0.01 to 0.3%
Ni is effective for improving the strength and toughness of the base material, and is preferably contained in an amount of 0.01% or more. On the other hand, the content exceeding 0.3% saturates the effect, so when Ni is contained, the Ni content is preferably in the range of 0.01 to 0.3%. More preferably, it is 0.20 to 0.28% of range.

Ti: 0.005 to 0.015%
Ti forms TiN and has the effect of suppressing grain growth during slab heating and welding heat affected zone, resulting in refinement of the microstructure and improving toughness. If it is less than 0.005%, the effect is small, so 0.005% or more is contained. On the other hand, if the Ti content exceeds 0.015%, the weld heat-affected zone toughness is deteriorated. Therefore, when Ti is contained, the Ti content should be in the range of 0.005 to 0.015%. Is preferred. More preferably, it is 0.010 to 0.015% of range.

  In addition, although the austenitic stainless clad steel of the present invention uses an austenitic stainless steel excellent in seawater corrosion resistance, it is necessary to satisfy the pitting corrosion index defined in the following formula (1) as a condition of the laminated material.

Pitting Corrosion Index (PRE): 40 or less The Pitting Corrosion Index (PRE) is defined by the following formula (1).

PRE = Cr + 3.3Mo + 16N (1)
In addition, each element symbol is the content expressed by mass% of each element.

  As seawater corrosion resistance, pitting corrosion resistance and crevice corrosion resistance are important. These local corrosions are arranged by the pitting corrosion index defined by the above formula (1), and the higher the numerical value, the better the pitting corrosion resistance. In the clad steel using the austenitic stainless steel as a laminated material, the pitting corrosion index is preferably 40 or more in order to obtain sufficient seawater corrosion resistance.

2. Production Method The production method of the austenitic stainless clad steel excellent in seawater corrosion resistance and low temperature toughness according to the present invention will be described below.

  The base material of the clad steel of the present invention is carbon steel, and its component composition is adjusted to the above-described component range and can be melted by a conventional method or the like. As the base material, austenitic stainless steel excellent in seawater corrosion resistance is selected as a laminated material to become a clad steel material, which is hot-rolled and austenite excellent in seawater corrosion resistance and low-temperature toughness with a total thickness of 30 mm or more. Stainless steel clad steel.

Heating temperature: 1000-1250 ° C
When the heating temperature of the clad steel material is less than 1000 ° C., the rolling efficiency decreases, and when it exceeds 1250 ° C., the austenite grains of the base material become coarse and the toughness of the base material decreases, so the range is 1000 to 1250 ° C. A preferable heating temperature range is 1000 to 1150 ° C. from the viewpoint of base material toughness.

Controlled rolling: Cumulative rolling reduction of 30% or more in the temperature range of 730 to 950 ° C. As a condition on the base metal side, in order to improve strength and toughness, the temperature range of 730 to 950 ° C., that is, the austenite low temperature range, or two phases It is necessary to finish rolling in the area. The rolling in the low temperature range of austenite is to make austenite grains elongated and to introduce a deformation band into the austenite grains.

  This is because the nucleation side of ferrite increases due to the elongation of austenite grains and the introduction of deformation bands, eventually the structure becomes finer and the strength and toughness of the steel improve. Rolling in a two-phase region (temperature range of 730 to 950 ° C.) is to improve the strength by introducing strain into the ferrite grains, and at the same time, introduce a deformed texture to achieve improved toughness. Is aimed at.

  The reason why the cumulative rolling reduction is 30% or more is that if the cumulative rolling reduction is less than 30%, the austenite is not sufficiently refined and the toughness is not improved.

Rolling finishing temperature: 730-850 ° C
As rolling conditions on the base material side, required strength and low temperature toughness can be obtained at a rolling finishing temperature of 700 to 850 ° C. When the temperature exceeds 850 ° C., it is difficult to stretch the austenite grains and introduce deformation bands due to recrystallization of the austenite grains. On the other hand, when the temperature is lower than 700 ° C., excessive strain is introduced into the ferrite grains and work hardening is remarkable. It is.

  On the other hand, the rolling condition on the laminated material side requires that the finishing temperature be 730 ° C. or higher because the corrosion resistance deteriorates when the rolling finishing temperature is less than 730 ° C.

  Accordingly, the rolling finishing temperature is set to 730 to 850 ° C. in order to ensure the characteristics of both the base material and the laminated material.

Cooling rate: 3-40 ° C / s
By performing accelerated cooling after rolling, the austenite → ferrite transformation occurs at a low temperature in the base material (carbon steel), and part of the bainite transformation also occurs. As a result, higher strength and higher toughness are achieved. Moreover, also in a laminated material, it is preferable from a viewpoint of ensuring corrosion resistance to implement cooling with the said cooling rate.

  When the cooling rate is less than 3 ° C./s, the cooling effect is not remarkable. When the cooling rate exceeds 40 ° C./s, the cooling effect is saturated. Therefore, the cooling rate is preferably in the range of 3 to 40 ° C./s.

Overall thickness of clad steel: 30 mm or more Because clad steel for marine structures and shipbuilding uses a thick material of 30 mm or more. In the present invention, a clad steel excellent in seawater corrosion resistance and low-temperature toughness is provided even for a material having a total thickness of 30 mm or more with a low cooling rate.

  Examples of the present invention will be described in detail below.

  A carbon steel having the composition shown in Table 1 was used as a base material, and an austenitic stainless steel shown in Table 2 was used as a combination material and assembled into a slab. Subsequently, the slab was heated to the temperature shown in Table 3 and hot-rolled to produce clad steel having each thickness shown in Table 3.

  With respect to the stainless clad steel obtained as described above, the corrosion resistance of the laminated material was evaluated by ASTM G48-03 Method E shown below.

  Further, a tensile test and a Charpy impact test at −40 ° C. according to JIS Z 2242 were performed to obtain YS (MPa), TS (MPa), and absorbed energy (J).

The critical pitting corrosion temperature (CPT) was determined by “immersion test at 5 ° C. for 24 hours in 6% FeCl ₃ + 1% HCl solution” defined in ASTM G48-03 Method E.

  The immersion test is performed at a pitch of 5 ° C., and is performed three times at each temperature. If a pitting corrosion with a depth of 0.025 mm or more is generated even once, the pitting corrosion is rejected, and the pitting corrosion is generated and is rejected. The highest test temperature was CPT (° C.). The target temperature of CPT was 45 ° C. or higher, preferably 50 ° C. or higher.

  The evaluation results are shown in Table 3.

  From Table 3, No. which is an invention example. Nos. 1 to 12 and 22 have a CPT of 45 ° C. or higher of the target temperature and a −40 ° C. Charpy absorbed energy of 100 J or higher, and exhibit excellent seawater corrosion resistance and low temperature toughness.

No. 13-21 shows a comparative example. No. The base metals of 14 to 18 were all comparative steels, and the -40 ° C. Charpy absorbed energy was below the target value in all cases.
No. 13, no. Although the base materials 19 to 21 are all invented steels, the finishing temperatures of 13 and 20 are too higher than the specified temperature and -40 ° C Charpy is inferior. Below target temperature.

Claims (4)

  Ingredient composition is mass%, C: 0.10-0.15%, Si: 0.25-0.40%, Mn: 0.45-2.0%, P: 0.015% or less, S : 0.004% or less, N: 0.006% or less, Cr: 0.01 to 0.1%, Nb: 0.005 to 0.05%, Al: 0.005 to 0.05% In addition, a carbon steel composed of the balance Fe and inevitable impurities is used as a base material, and a clad steel material including austenitic stainless steel as a combined material is heated to 1000 to 1250 ° C., and then hot rolling is started, and 730 to 950 ° C. It is excellent in seawater corrosion resistance and low temperature toughness with a total thickness of 30 mm or more, characterized by performing hot rolling with a cumulative rolling reduction rate of 30% or more in a temperature range of 730 to 850 ° C. A method for producing austenitic stainless clad steel.   The component composition of the carbon steel is further selected from Cu: 0.01 to 0.3%, Ni: 0.01 to 0.3%, and Ti: 0.005 to 0.015% by mass%. The method for producing an austenitic stainless clad steel having a total thickness of 30 mm or more and excellent in seawater corrosion resistance and low-temperature toughness according to claim 1.   Using the clad steel material according to claim 1 or 2, hot rolling is started after heating to 1000 to 1250 ° C., and the cumulative rolling reduction of controlled rolling in the temperature range of 730 to 950 ° C. is set to 30% or more. The total thickness of 30 mm or more is characterized by performing accelerated cooling with a cooling rate of 3 to 40 ° C./s and a cooling stop temperature of 500 ° C. or higher after performing hot rolling with a rolling finishing temperature of 730 to 850 ° C. Of austenitic stainless clad steel with excellent seawater corrosion resistance and low temperature toughness. In the manufacturing method of the stainless clad steel in any one of Claims 1 thru | or 3, the pitting corrosion index (PRE) defined by following formula (1) of the austenitic stainless steel of a laminated material is 40 or more. A method for producing an austenitic stainless clad steel having a total thickness of 30 mm or more and excellent in seawater corrosion resistance and low temperature toughness.
PRE = Cr + 3.3Mo + 16N (1)
In addition, each element symbol is the content expressed by mass% of each element.