JPH02285025A - Production of seawater corrosion resisting clad steel plate - Google Patents

Abstract

PURPOSE:To produce a seawater corrosion resisting clad steel plate having high corrosion resistance by laying a stainless steel having a composition in which the total content of Cr, Mo, and N and the relationship between Ni and Cr contents are specified, respectively, on a steel plate and then rolling the resulting clad steel plate while specifying temp. conditions. CONSTITUTION:A stainless steel having a composition which consists of <=0.030% C, 0.05-2.0% Mn, 18.0-27.0% Cr, 4.0-7.0% Mo, 18.4-30.0% Ni, 0.10-0.25% N, 0.001-0.20% Al, P and S as inevitable impurities in the amounts of <=0.050% and <=0.010%, respectively, and the balance Fe and in which the conditions of Cr+3Mo+17N>=41.0 and Ni>=0.8Ct+4.0 are satisfied is laid at least on one side of a plate of steel, such as carbon steel. The resulting clad steel is heated up to 1150-1250 deg.C and hot-rolled, and rolling is completed at >=800 deg.C, followed by cooling at >=1 deg.C/sec cooling rate. By this method, the clad steel plate having >=about 60 deg.C critical pitting corrosion temp. and excellent in corrosion resistance in seawater can be produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a seawater-resistant clad steel plate, and it is possible to produce a clad steel plate with high corrosion resistance by optimizing the component system of the laminated material and the rolling method. The purpose is to manufacture.

[Conventional technology]

As seawater resistant stainless steel, JP-A-52-9552
As typified by No. 4, austenitic stainless steels with increased Cr, Mo, and NJI content have been developed from the viewpoint of hot workability and corrosion resistance when rapidly cooling from a high temperature range (1100°C or higher) is assumed. ing. but.

Since large amounts of expensive Cr and Mo are added, when considering the manufacture of thick plates, cladding with carbon steel or low alloy steel is desirable from the economical point of view.

Therefore, when considering the production of seawater-resistant clad steel plates, rapid cooling from a high temperature range, as with conventional solid materials, can not be applied to the production of this type of clad steel plate, as it greatly impairs the toughness of the base material. On the other hand, with regard to the production of austenitic stainless clad steel, studies have been conducted from the viewpoint of suppressing precipitation of the σ phase during rolling cooling, as shown in JP-A No. 63-248583.

However, in the production of seawater-resistant clad steel sheets with an increased N content of 0.10% or more, Cr nitrides precipitate faster than the σ phase and greatly influence corrosion resistance.

It is thought that manufacturing such a seawater-resistant clad steel plate is possible by optimizing the composition of the laminated material and the rolling and cooling conditions, but at present no systematic research has been conducted in this direction.

[Problem that the invention seeks to solve]

Generally, in clad rolling, hot rolling is performed after heating to 1100°C or higher, rolling is finished in a temperature range of 800 to 1000°C, and cooling is performed by standing or accelerated cooling. However, it is difficult to secure a cooling rate of 10° C./S or more due to constraints on the base material. Figure 6 shows 20 Cr-1 which is a typical seawater resistant austenitic stainless steel corresponding to Japanese Patent Application Laid-open No. 52-95524.
After heating the 8Ni -6,2Mo -0,7Cu -0,2ON steel to 1200°C, the rolling finishing temperature was changed from 800°C to 1
The corrosion resistance when the temperature was changed to 000℃ and cooled at various cooling rates was evaluated using a pitting corrosion test (ASTM) using a ferric chloride solution.
G48. The results of evaluation using JISGO57g) are shown. The critical pitting temperature (CPT) at which pitting corrosion occurs has almost no dependence on the finishing rolling temperature, and increases as the cooling rate increases after rolling, and at 10"c/s, CPT = 55°C.Comparison As an example, assuming the manufacture of solid materials, the temperature is 120”C.
The CPT in the case of heating and then quenching is also shown, but it is 75°C, and when clad rolling is simulated, the corrosion resistance is thick (decreased).

On the other hand, FIG. 7 shows the relationship between the corrosion depth and the CPT in the ferric chloride test when a test piece with an artificial gap created by Teflon was immersed in natural seawater for one year. From the figure, a CPT of 60"C or higher is required to suppress crevice corrosion in natural seawater.

Judging from these results, 20cr-18Ni-6,2Mo-
0.7Cu -0.20N steel is insufficient as a component of a cladding material, and it is desired to develop a component that provides CPT≧60° C. even when taking the heat processing history of cladding production as a premise.

The present invention was made to solve the above-mentioned problems, and by optimizing the composition of the cladding steel and specifying the cladding rolling conditions, it is possible to create a seawater-resistant cladding steel plate with a CPT≧60°C. The purpose is to obtain.

[Means for solving problems]

In order to solve the above problems, the present inventors developed a component system based on the heat processing history of clad rolling, and investigated optimal rolling conditions based on these component systems. Details are given below.

FIG. 1 shows the CPT when steels 1 to 8 shown in Coating 1 below were heated to 1200°C and cooled at 1°C/s after finishing rolling at 900°C.

As shown in the figure, 20Cr-6Mo system and 2
5 In the 5Cr-4,5Mo system, CPT increases as Nii increases in all cases, and CP?, which is a condition for not causing crevice corrosion in natural seawater, increases. ≧60℃ (1
0% ferric chloride solution), 20 Cr
-20% or more for 6Mo series.

In the 25Cr-4,5Mo system, it is necessary to add 24% or more of Ni. It is generally believed that the pitting corrosion resistance and crevice corrosion resistance of austenitic stainless steel can be determined by the amounts of Cr, Mo, and N. For example, Cr+3Mo+17N is proposed as the plutting index. However, this experiment revealed that the amount of Ni plays an important role when cladding rolling is assumed.

In order to clarify how the minimum Nl amount to obtain kCPT≧60°C changes depending on the Cr amount, 0.2
cr with Cy −)-3Mo constant in ON,
100 after heating steel with different Mo balance to 1200℃
Rolling was completed at 0°C and cooled at 1°C/B. The results are shown in FIG. 2. The minimum amount of Ni required to satisfy CPT≧60° C. increases with the amount of Cr, and is expressed by the following equation as a function of the amount of Cr.

Nl (%) = 0.8 Cr (%) + 4 Thus, the present invention makes it possible to manufacture a seawater-resistant clad steel plate by optimizing the composition of the laminated material and the rolling conditions described below. In contrast, conventional seawater-resistant stainless steel (solid) is rapidly cooled from a high temperature range of 1100°C or higher, which suppresses the precipitation of intermetallic compounds such as Cr carbonitrides and sigma phase. Corrosion resistance in this case was mainly dominated by 1ccr, Mo, and Nt. However, in cases where rapid cooling is difficult due to the characteristics of the base material, such as in clad rolling, KNi plays a major role in addition to the above components.

As shown in Figure 2, the pitting corrosion resistance is improved by specifying the amount of Nl, which changes depending on the amount of Cr, because Ni suppresses the precipitation of Cr nitrides during the cooling process after rolling. It depends. Particularly in the case of a steel having a high N content like the steel of the present invention, it is important to suppress the precipitation of Cr nitrides.

Furthermore, FIG.
, Mo,, CPT when steel with varying amounts of N is heated to 1200°C and cooled at 3'C/a after finishing rolling at 900°C 4tCr (%) + 3 Mo (%) + 1
The results are shown organized by 7N (%). The ingredients of the 6 steels are shown in Table 2 below.

From the same figure, when Ni1i is set as the lower limit of the above equation, CPT
P i t t i ng necessary to make it ≧60℃
Index i.e. CrC%) + 3 Mo (%) +
It can be seen that 17 N (%) is 41. Also P
Even if it satisfies the Piffing Index≧41.0, steel with Cr added in excess of 27% and reduced Mo content [2] is significantly inferior in CPT compared to steel with the same Piffing Index. When Cr is added in excess of 27% in this way,
This is because the formation of the σ phase is significantly promoted. Note that Cr is required to be at least 18% from the viewpoint of improving pitting corrosion resistance and crevice corrosion resistance, and taking this into consideration, the cladding material component of seawater resistant clad steel plate should be 18%≦Cr≦27% and C'
r (%) + 3Mo (%) + 17N (%)≧41.
Ni that satisfies 0 and is greater than or equal to Ni=0.8Cr+4
It is necessary to contain the amount. however.

Considering the above formula that defines the amount of Ni and the lower limit of the amount of Cr, the lower limit of Ni is 18.4%, and since this element is expensive, the upper limit was decided to be 30%. Also Mo
is an effective element for improving pitting corrosion resistance and crevice corrosion resistance, so it is necessary to have a content of 4.0% or more, but if it exceeds 7.0%, the formation of the σ phase will be further promoted, so the upper limit is 7.0%
And so. Furthermore, N has the effect of increasing corrosion resistance.

It is necessary to add 0.10% or more, but it is difficult to add more than 0.25% within the scope of the present invention.

The reasons for limiting other components defined in the present invention will be described below.

The lower the C content, the more desirable it is from the viewpoint of corrosion resistance.

If the content exceeds 0.030%, corrosion resistance will be impaired.
It was set to 0.030% or less. Note that due to steel manufacturing constraints, 0.0005% is considered to be the lowest limit at present.

Sl is required to be O; 0.2% or more for deoxidation.

When it exceeds 1.0%, hot workability is significantly inhibited.

Therefore, the content was set in the range of 0.02% to 1.0%.

0.05% or more of Mn is required for deoxidation.

If it exceeds 2.0%, corrosion resistance will deteriorate. Therefore 0.
The range was 0.05% to 2.0%.

Al is required for deoxidation at 0.00% or more, but 0.00% or more is required for deoxidation.
If it exceeds 30%, corrosion resistance will be impaired.

Therefore, the content was set in the range of 0.001% to 0.30%.

The lower P and S are, the more desirable it is, and 0.050% for P.
If S exceeds 0.010%, hot workability will be impaired. Therefore, P≦0.050%, S≦
It was kept within the range of 0.010%. Note that, due to constraints in steel manufacturing, it is currently difficult to think that 0.0005% is the lowest limit for each.

Furthermore, in the second invention, in order to further improve hot workability or corrosion resistance, in addition to the above components, Cu≦2.0%.
, W≦0.5%, Ti≦1.0%1Nb≦1.0%, 7
61.0%, Zr≦1.0%, La≦0.02%,
One or more of Ce≦0.02% and C'a≦0.02% are included.

Next, the optimum rolling conditions for the cladding were investigated using Steel 7 having the above-mentioned components. FIG. 4 shows the change in CPT when the heating temperature and rolling finish temperature were changed and the material was cooled at 1° C./S. When compared on the assumption that 900℃ finishing is performed.

When heated at 1150°C or higher, a good value of CPT≧60°C was obtained, whereas when heated to 1100°C, CPT
The temperature decreased to PT=50°C.

In this way, when the heating temperature is lower than 1150°C, 1) In high Cr, high Mo, and high N steel such as the steel of the present invention, Cr
This is thought to be because carbonitrides or the σ phase are not completely dissolved in solid solution, which impairs pitting corrosion resistance. Furthermore, even when heated at 1200°C, the CPT of the material finished at 750°C decreased to 50'C.

This is because when the finishing rolling temperature is less than 800° C., even if the Ni content is increased, Cr nitrides precipitate during rolling, resulting in a loss of pitting corrosion resistance.

On the other hand, Fig. 5 shows the change in CPT when the steel 3 is heated once at 1200°C, then rolled at 900°C, and the subsequent cooling rate is varied widely. CPT decreases as the cooling rate decreases, but there is a particularly rapid decrease below 3"C/s. This is because Cr precipitates precipitate during cooling and pitting corrosion resistance is lost. .

The study of the heating, rolling, and cooling conditions shown above was conducted by simulating the thermal processing history that the laminate undergoes in clad rolling, but in actual clad rolling, it is necessary to also consider the characteristics of the base material, and the temperature exceeds 5250°C. Such heating is undesirable due to the toughness of the base material. Therefore, the rolling and cooling conditions for seawater-resistant stainless steel are to heat it to 1150°C or higher and 1250°C or lower, and after finishing rolling at 800°C or higher, 1°C/8
It is necessary to cool it down.

(Example 1) Table 3 below shows stainless steel plates whose components are shown from A to H.
．． Carbon steels having a composition of 20 C-0,25Si-0,70Mn were stacked together, heated to 1200°C and rolled for 8
The results are shown in which the pitting corrosion resistance of the laminated material was evaluated in a 10% ferric chloride solution when the test was completed at 50°C and then cooled at 2°C/3. The finished plate thickness in this experiment was 2 mm for the laminated material and 13 mm for the base material.
Steel A of m is the specified Ni amount (%) which is a feature of the present invention.
Since the condition ≧0.8Cr(%)+4 is not satisfied, the CPT is as low as 45°C. Also, although steels C and D satisfy the Ni content condition, steel C has Cr (%) + 3Mo (%) + 17N.
(%)≧41.0, and the steel does not satisfy the condition of N amount, so the CPT is low at 55°C. On the other hand, Steel B satisfies each prescribed condition regarding the components of the present invention, and therefore has a good CPT of 70°C. Similarly, among steels E-)I containing 25% or more of Cr, steel E, which does not meet the requirements for Ni, and steel H, which does not meet the requirements for Cr, have a CPT of 45°C and 50°C, respectively. low. On the other hand, steels F and G that meet the conditions of the present invention have a CPT of 65°C and 75°C, and satisfy the condition of CPT≧60°C, which is a condition for not causing crevice corrosion in natural seawater.

(Example 2) Table 4 below shows the steels B and F of the above examples that meet the compositional conditions of the present invention.
-0,03Nb -0,07V superimposed on low alloy steel with composition 33111111(
The pitting corrosion resistance of the laminate is 10 when rolled into 10W (2 laminates, base material SW).
% ferric chloride solution is shown.

Condition J is 1 in which the cooling rate after rolling is the cooling rate of the present invention.
Because the temperature does not meet ℃/8 or higher.

Conditions and O are heating temperatures within the temperature range of the present invention.
50'C or more and 1250°C or less, and conditions K and P do not satisfy the rolling finishing temperature of 800°C or more, which is the claimed range of the present invention, so both have a CPT of 5.
The temperature is low, below 5℃. In contrast, condition 1 satisfies the conditions of the present invention. Both M and N exhibit good corrosion resistance with CPT≧60°C.

(Example 3) Table 5 below shows stainless steel plates whose components are shown from Tenten Q to V.
The pitting corrosion resistance of the laminated material when laminated on a low alloy steel having a composition of 1Nb-0,01Ti, heated to 1230°C, finished rolling at 950°C, and cooled at 1.5°C/8 is 1.
The results of evaluation in 0% ferric chloride solution are shown. The finished plate thickness was 23 m in total, including 3 laminates and 20 mm base material.

Steel Q to which Cu is added to further improve corrosion resistance or hot workability, steel R to which W and La are added, Nb and Cm.
Steel S with added.

Since both the steel T with the addition of Zr and CE and the steel U with the addition of TI and V satisfy the conditions of the present invention,
It has good corrosion resistance of CPT≧60°C. This number ζ
On the other hand, steel that does not meet the Ni content regulations in the present invention
has a low CPT of 45°C.

〔Effect of the invention〕

As detailed above, when manufacturing seawater-resistant clad steel plates based on the thermal processing history of boat fishing rad rolling, one method is to optimize the composition system of the laminate and the rolling and cooling conditions as in the present invention. By doing so, it has become possible to produce a seawater-resistant clad steel sheet that has a critical pitting temperature of 60"C or higher in a 10% ferric chloride test and has excellent corrosion resistance in seawater Iζ.

[Brief explanation of drawings]

Figure 1 is a graph showing the change in critical pitting temperature with the amount of Ni, Figure 2 is a graph showing the change in critical pitting temperature with the amount of cr and Ni, and Figure 3 is the Pitting Index.
Figure 4 is a graph showing the relationship between critical pitting temperature and heating temperature, Figure 5 is a graph showing changes in critical pitting temperature due to heating temperature and finishing temperature, and Figure 5 is a graph showing changes in critical pitting temperature due to cooling rate after rolling. A graph showing changes in critical pitting temperature. Figure 6 is a graph showing changes in critical pitting temperature depending on rolling and cooling conditions. Figure 7 is a graph showing changes in critical pitting temperature.
FIG. 3 is a graph showing the relationship between the critical pitting temperature in the O% ferric chloride test and the maximum crevice corrosion depth due to seawater immersion. Patent applicant Nippon Kokan Co., Ltd. Inventor Sada Yamamoto Yasuo Butsuma Kobayashi

Claims (1)

[Claims] 1. C: 0.030% or less, Mn: 0.05% or more 2.
0% or less, Cr: 18.0% or more and 27.0% or less, Mo
: 4.0% or more and 7.0% or less, Ni: 18.4% or more 3
0.0% or less, N: 0.10% or more and 0.25% or less, A
l: Contains 0.001% or more and 0.30% or less, and C
r (%) + 3Mo (%) + 17N (%) ≧ 41.0, N
satisfies the condition of i (%) ≧ 0.8 Cr (%) + 4.0,
Unavoidable impurities include P: 0.050% or less, S: 0.
When rolling a clad steel plate in which stainless steel containing 0.010% or less and the balance is superimposed on at least one side of the steel plate, the temperature at 1150°C or higher is 12
A method for manufacturing a seawater-resistant clad steel sheet, which comprises heating to 50°C or lower, hot rolling, finishing rolling at 800°C or higher, and then cooling at 1°C/s or higher. 2.C: 0.030% or less, Mn: 0.05% or more2.
0% or less, Cr: 18.0% or more and 27.0% or less, Mo
: 4.0% or more and 7.0% or less, Ni: 18.4% or more 3
0.0% or less, N: 0.10% or more and 0.25% or less, A
l: Contains 0.001% or more and 0.30% or less, and C
u: 2.0% or less, W: 0.5% or less, Ti: 1.0%
Below, Nb: 1.0% or less, V: 1.0% or less, Zr:
1.0% or less, La: 0.02% or less, Ce: 0.02
% or less, including one or more of Ca: 0.02% or less, and Cr (%) + 3Mo (%) + 17N (%)
Stainless steel that satisfies the conditions of ≧41.0, Ni (%)≧0.8Cr (%) + 4.0, and contains P: 0.050% or less, S: 0.010% or less, and the balance Fe as unavoidable impurities. When rolling a clad steel plate in which steel is laminated on at least one side of the steel plate, hot rolling is performed after heating to 1150°C or more and 1250°C or less, finishing the rolling at 800°C or more, and then cooling at 1°C/s or more. A method for manufacturing a seawater-resistant clad steel sheet, characterized by: