JP2016047953A - Stainless clad steel sheet having weld zone excellent in sea water corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless clad steel sheet having a clad material weld zone excellent in sea water corrosion resistance.SOLUTION: There is provided a stainless clad steel sheet having a weld zone excellent in sea water corrosion resistance having a clad material containing, by mass%, C:0.030% or less, Si:0.02 to 1.50%, Mn:0.02 to 2.0%, P:0.040% or less, S:0.030% or less, Ni:22.0 to 25.0%, Cr:22.0 to 26.0%, Mo:3.5 to 5.0%, N:0.10 to 0.25% and the balance Fe with inevitable impurities and having pitting-corrosion index (PI) defied by the formula (1) of 40 or more and austenite average crystal grain diameter of 25 μm or less. PI=Cr+3.3Mo+16 N≥40 (1), where each element symbol represents mass% of each element.SELECTED DRAWING: None

Description

  The present invention relates to a stainless clad steel plate excellent in seawater corrosion resistance used in various applications represented by harbor 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.

  A stainless steel clad steel plate is a composite steel material in which two kinds of metals having different properties are joined, such as 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 sheet requires only a small amount of a stainless steel material, and can secure the same corrosion resistance as that of a solid material (full thickness stainless steel).

  From the above, it is considered that the stainless clad steel plate is a very useful functional steel material, and in recent years, its needs are increasing more and more in various industrial fields. In particular, harbor structures and shipbuilding used in environments where stainless steel clad steel is in contact with seawater, floating offshore oil and gas production storage and loading equipment (FPSO: Floating Production, Storage and Offloading system), seawater When used in various applications typified by desalination facilities, etc., seawater corrosion resistance is required because it is used in severe seawater corrosion environments.

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

  The pitting corrosion resistance of stainless steel is affected by the amount of Cr, Mo, and N in the steel. Generally, Cr (mass%) + 3.3 Mo (mass) as the pitting corrosion index (PI) or Pitting resistance equivalent (PRE). %) + 16N (mass%), Cr (mass%) + 3Mo (mass%) + 10N (mass%), etc., and the higher the pitting corrosion index, the better the pitting corrosion resistance.

  Moreover, it is necessary to consider also about the corrosion resistance of the laminated material of a welded part in stainless steel clad steel.

  In Patent Document 1, the laminated material is “C: 0.02% or less, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, Ni: 23.0 to 28.0%. Cr: 19.0 to 21.0%, Mo: 4.0 to 5.0%, Cu: 1.0 to 2.0%, N: 0.15 to 0.30%, Ni equivalent / The clad steel sheet made of austenitic stainless steel with a Cr equivalent value of 1.25 or more is heated to 1050 ° C. or less, and then solution heat treatment is performed at a cooling rate of 30 ° C./min or more to provide excellent corrosion resistance. A technique for manufacturing a stainless clad steel plate is disclosed.

  In Patent Document 2, stainless steel clad steel plate is formed, seam welded to form a steel pipe, and then subjected to solution heat treatment at a heating temperature of 1100 to 1200 ° C. and a cooling rate of 1 ° C./second or more. A method for manufacturing a clad steel pipe is disclosed.

JP 09-104953 A JP 2005-133125 A

  However, the technique described in Patent Literature 1 performs solution heat treatment to ensure the corrosion resistance of the laminated material, but does not consider the seawater corrosion resistance of the laminated material in the weld zone.

  The technology described in Patent Document 2 defines the solution heat treatment conditions after pipe forming in order to recover the corrosion resistance deterioration of the seam welded part in the production of the stainless clad steel pipe. As a result, the tube is distorted, and there is a problem that correction is necessary and the number of manufacturing processes is increased.

  In view of such circumstances, an object of the present invention is to provide a stainless clad steel plate excellent in seawater corrosion resistance of a welded portion of a laminated material.

  In order to solve the above-mentioned problems, the present invention examines the steel components that affect the seawater corrosion resistance of the welded joints of stainless steel clad steel plates that have been completed from rolling to heat treatment with multiple components (steel composition) and multiple histories. Going to complete the present invention.

  The gist of the present invention is as follows.

  [1] The laminated material of the stainless clad steel plate is mass%, C: 0.030% or less, Si: 0.02-1.50%, Mn: 0.02-2.0%, P: 0.040 %: S: 0.030% or less, Ni: 22.0-25.0%, Cr: 22.0-26.0%, Mo: 3.5-5.0%, N: 0.10 It contains 0.25%, has a pitting corrosion index (PI) defined by the following formula (1) of 40 or more, consists of the balance Fe and inevitable impurities, and has an austenite average crystal grain size of 25 μm or less. Stainless steel clad steel plate with excellent seawater corrosion resistance.

PI = Cr + 3.3Mo + 16N ≧ 40 (1)
Each element symbol represents mass% of each element.

  [2] Further, the laminated material of the stainless clad steel plate is mass% and contains B: 0.0010 to 0.0055%. The seawater corrosion resistance of the welded portion according to [1] is excellent. Stainless clad steel plate.

  [4] Further, the laminated material of the stainless clad steel plate is mass% and contains Cu: 0.20% or less. Excellent seawater corrosion resistance of the welded portion according to [1] or [2] Stainless steel clad steel plate.

  According to the present invention, since a seawater-resistant stainless clad steel plate having both seawater corrosion resistance of the welded portion of the laminated material and the mechanical properties of the base material can be obtained, the shipbuilding field represented by harbor structures and FPSO, freshwater seawater equipment The present invention can be suitably used in applications that require seawater corrosion resistance, such as

  The configuration of the present invention will be described below. The stainless clad steel sheet of the present invention defines the component composition, the average particle diameter, etc. in order to satisfy the seawater corrosion resistance of the welded part of the laminated material.

1. About the component composition of a laminated material The reason for limiting the component composition of the stainless steel of a laminated material is demonstrated below. In addition, unless otherwise indicated, the component% of each element means the mass%.

C: 0.030% or less C is preferably as low as possible from the viewpoint of corrosion resistance, particularly the corrosion resistance of the weld heat-affected zone, and the C content is 0.030% or less. Preferably, it is 0.020% or less.

Si: 0.02-1.50%
Si is a necessary component for deoxidation, and in order to obtain the effect, it is necessary to contain 0.02% or more. However, if the content exceeds 1.50%, the hot workability is remarkably deteriorated, so the Si content is in the range of 0.02 to 1.50%. Preferably, it is 0.02 to 0.60% of range.

Mn: 0.02 to 2.0%
Mn is a component necessary for deoxidation, and in order to obtain the effect, the Mn content needs to be 0.02% or more. However, if the content exceeds 2.0%, the corrosion resistance is deteriorated, so the Mn content is in the range of 0.02 to 2.0%. Preferably, it is 0.20 to 0.60% of range.

P: 0.040% or less, S: 0.030% or less P, S is preferably as low as possible from the viewpoint of hot workability, P content exceeds 0.040%, S content exceeds 0.030% Since hot workability is impaired, the P content is 0.040% or less and the S content is 0.030% or less. Preferably, the P content is 0.030% or less, and the S content is 0.010% or less.

Ni: 22.0-25.0%
From the viewpoint of the stability of the austenite phase, Ni is made 22.0% or more mainly due to the balance with Cr and Mo. On the other hand, since the hot deformation resistance contained in excess of 25.0% is increased and the economic efficiency is deteriorated, the Ni content is set in the range of 22.0 to 25.0%. The Ni content is preferably in the range of 22.0 to 24.5%, more preferably in the range of 22.5 to 24.5%, from the viewpoint of achieving both the stability and economic efficiency of the austenite phase.

Cr: 22.0-26.0%
Cr is effective for improving pitting corrosion resistance and crevice corrosion resistance, and needs to be contained at 22.0% or more. On the other hand, if the content exceeds 26.0%, precipitation of the σ phase is remarkably promoted during production as a laminated material and during clad rolling and cooling, and the corrosion resistance and hot workability are hindered. The range is 22.0 to 26.0%. In addition, from the viewpoint of improving pitting corrosion resistance and crevice corrosion resistance and suppressing sigma phase precipitation, it is preferably in the range of 23.0 to 26.0%, more preferably in the range of 24.0 to 25.5%. is there.

Mo: 3.5-5.0%
Mo is effective for improving pitting corrosion resistance and crevice corrosion resistance, and is 3.5% or more. On the other hand, if the content exceeds 5.0%, precipitation of the σ phase is remarkably promoted during production as a laminated material and during clad rolling and cooling, and the corrosion resistance and hot workability are hindered. The range is 3.5 to 5.0%. In addition, from the viewpoint of improving pitting corrosion resistance and crevice corrosion resistance and suppressing σ phase precipitation, it is preferably in the range of 4.0 to 5.0%, more preferably in the range of 4.2 to 4.8%. is there.

N: 0.10 to 0.25%
N has an effect of improving the corrosion resistance, and is made 0.10% or more in order to obtain the effect. On the other hand, if it exceeds 0.25%, it becomes difficult to contain from the relationship of the content of other components, so the N content is in the range of 0.10 to 0.25%. Preferably it is 0.15 to 0.25% of range, more preferably 0.17 to 0.23% of range.

Pitting corrosion index (PI): 40 or more The pitting corrosion index (Pitting Index: PI) is defined by the following formula (1).

PI = Cr + 3.3Mo + 16N (1)
Each element symbol represents mass% of each element.

  As the seawater corrosion resistance, pitting corrosion resistance is important, and the local corrosion is arranged by the pitting corrosion index defined by the above formula (1). 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.

  However, in the weld heat affected zone, even if the pitting corrosion index is 40 or more, if the amount of Cr or Mo is large, the σ phase is precipitated and the pitting corrosion resistance is deteriorated. There is a need.

Austenite average grain size: 25 μm or less Next, the austenite average crystal grain size of the stainless steel of the laminated material is set to 25 μm or less. It is known that in an austenitic stainless steel, a σ phase, which is an intermetallic compound, is generated due to a thermal history, thereby deteriorating corrosion resistance. When the σ phase, which is an intermetallic compound, is generated, the amount of Cr and Mo around the σ phase is reduced, so that the corrosion resistance is degraded.

  In the present invention, it is possible to suppress the formation of the σ phase in the weld heat affected zone by optimizing the austenite average crystal grain size of the stainless clad steel laminated material, and the average crystal grain size is 25 μm or less as an index thereof. To do. When the austenite average crystal grain size exceeds 25 μm, the corrosion resistance at the weld heat affected zone deteriorates. Preferably it is 20 micrometers or less.

  The austenite average crystal grain size can be measured by the microscope test method of JIS G0551.

  The balance other than the above is Fe and inevitable impurities. Further, in the present invention, other trace elements can be contained when the above-mentioned effects are not diminished.

  Furthermore, in addition to the above components, the following restrictions can be further provided for B and Cu.

B: 0.0010 to 0.0055%
B is effective for improving corrosion resistance and hot workability, and is preferably 0.0010% or more. On the other hand, when it contains exceeding 0.0055%, corrosion resistance and hot workability will deteriorate. Therefore, the B content is preferably in the range of 0.0010 to 0.0055%. Preferably it is 0.0015 to 0.0035% of range.

Cu: 0.20% or less Cu is preferably as low as possible from the viewpoint of corrosion resistance, and should be limited to 0.20% or less. Preferably, it is 0.10% or less. More preferably, it is 0.05 or less.

  Carbon steel or low alloy steel can be used as the base material of the stainless clad steel plate of the present invention.

  The stainless clad steel plate of the present invention is obtained by clad stainless steel having the above component range as a laminated material on one side or both sides of this base material.

2. About a manufacturing method A manufacturing method is demonstrated below. The laminated material of the stainless clad steel plate is adjusted to the above component range and can be melted by a conventional method or the like. The material of the base material is selected depending on the application and the like, and is made into a clad steel plate by clad rolling. The heating temperature is preferably high so as to dissolve the σ phase in the laminated material. Preferably it is 1200-1250 degreeC. When the heating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. The austenite average crystal grain size of the laminated material can be reduced to 25 μm or less by performing thermal processing control (TMCP) that combines controlled rolling and accelerated cooling.

  The rolling end temperature is preferably a high temperature in order to suppress σ phase precipitation. Preferably it is 1000 degreeC or more. In addition, the rolling is immediately cooled after the end of rolling to suppress the precipitation of the σ phase. The cooling rate is preferably as high as possible, preferably 2 ° C./s or more, more preferably 10 ° C./s.

Examples of the present invention will be described below.
An austenitic stainless steel having an ingredient composition shown in Table 1 and an SM490 ingredient steel (hereinafter may be abbreviated as “normal steel”) were used. A slab having a thickness of (60 + 5 + 5 + 60) mm was assembled by combining an SM490 steel having a thickness of 60 mm as a base material and an austenitic stainless steel having a thickness of 5 mm as a laminated material.

  Next, the base plate is heated to 1240 ° C., subjected to hot finish rolling at 1000 to 1050 ° C., then cooled with water, the cooling stop temperature is set to 600 ° C., and the cooling is accelerated at a cooling rate of 10.0 ° C./s. A stainless clad steel plate having a thickness of 12 mm and a laminated material plate thickness of 1 mm was produced.

  In addition, in order to change the average austenite crystal grain size, the stainless clad steel was heat treated at 1100 to 1250 ° C. to produce a stainless clad steel plate.

  Using these, the groove shape is a V groove, the base metal side is SAW with a maximum heat input of 20 kJ / cm, and the laminated material is an ALL625-based welding material with a maximum heat input of 20 kJ / cm MAG to produce a welded joint. did.

  Corrosion test pieces were collected from the welded portion of the laminated material of the stainless clad steel plate obtained as described above, and the pitting corrosion resistance of the laminated material was evaluated according to JISG0578 shown below.

The pitting corrosion critical temperature (CPT: Critical Pitting Temperature Test) was determined by “24-hour immersion test at 5 ° C. intervals in a 6% FeCl ₃ + 1 / 20N HCl solution” defined in JIS G0578.

  In the immersion test, the temperature was increased at a pitch of 5 ° C., and each temperature was repeated three times. When the maximum pitting depth among the pitting corrosion generated even once reached 0.025 mm, the immersion test was rejected. In all three cases, the maximum pitting corrosion depth was less than 0.025 mm, and the highest test temperature at which pitting corrosion was rejected was defined as CPT (° C.). The CPT pass temperature was 60 ° C. or higher, preferably 65 ° C. or higher.

Pitting corrosion index PI = Cr (mass%) + 3.3Mo (mass%) + 16N (mass%) requires 40 or more as a stainless clad steel plate used in seawater resistant applications.
The austenite average crystal grain size of the laminated material was measured by a microscope test method of JIS G0551.

  From Table 1, in the example of the present invention, the CPT is 60 ° C. or more, which is an excellent resistance to seawater corrosion.

Claims (3)

The laminated material of the stainless clad steel plate is mass%, C: 0.030% or less, Si: 0.02-1.50%, Mn: 0.02-2.0%, P: 0.040% or less, S: 0.030% or less, Ni: 22.0-25.0%, Cr: 22.0-26.0%, Mo: 3.5-5.0%, N: 0.10-0.25 %, The pitting corrosion index (PI) defined by the following formula (1) is 40 or more, the balance is Fe and inevitable impurities, and the austenite average crystal grain size is 25 μm or less. Stainless steel clad steel plate with excellent seawater corrosion resistance.
PI = Cr + 3.3Mo + 16N ≧ 40 (1)
Each element symbol represents mass% of each element.   The stainless clad steel plate with excellent seawater corrosion resistance of the welded portion according to claim 1, wherein the laminated material of the stainless clad steel plate is mass% and contains B: 0.0010 to 0.0055%. .   Furthermore, the stainless clad steel plate excellent in seawater corrosion resistance of the weld part according to claim 1, wherein the laminated material of the stainless clad steel plate contains, by mass%, Cu: 0.20% or less.