JPH046214A - Production of high strength austenitic stainless steel excellent in seawater corrosion resistance - Google Patents

Abstract

PURPOSE:To provide superior seawater corrosion resistance and high fatigue strength in seawater by limiting components in a steel and carrying out controlled rolling and controlled cooling under the conditions suited to the above components, respectively. CONSTITUTION:A stainless steel has a composition consisting of, by weight, <=0.08% C, <= 2.00% Si, <= 2.0% Mn, > 21-30% Cr, 10-20%Ni, 0.5-3.0% Mo, > 0.3-0.5% N, and the balance Fe with impurity elements. This steel is heated to 1100-1300 deg.C and roughed at >= 1050 deg.C at >= 50% total reduction of area. Successively, finish rolling is performed at 800-1050 deg.C at >= 10% total reduction of area. After rolling, controlled cooling is carried out at >= 50 deg.C/min average cooling rate until 800-500 deg.C is reached, by which the recovery of working strain introduced by the finish rolling is inhibited. Further, the precipitation of Cr carbide in the above temp. region is inhibited. By this method, a high strength austenitic stainless steel excellent in seawater corrosion resistance can be obtained.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to austenitic stainless steel that has excellent seawater resistance, yield strength, and fatigue strength in seawater and is used for hull structures, such as hydrofoils of high-speed ships. This relates to a manufacturing method.

(Prior Art) Painted steel plates with heavy corrosion protection have conventionally been used for ship hull structures. Recently, there has been an increase in demand for high-speed ships equipped with hydrofoils, etc., and because these applications come into contact with high-speed seawater currents, materials with excellent seawater resistance that do not allow painting are required. Furthermore, high-strength materials are desired to reduce the weight of the ship.

Austenitic stainless steel is promising as a material with excellent seawater resistance, but in the conventional manufacturing method, it is subjected to solution annealing after hot rolling, which softens it and has a yield strength of 40 kg at most.
f/mj and low fatigue strength in seawater.

To solve this problem, do JP-A-62-267418 or JP-A-83-199851 propose a method to improve strength or corrosion fatigue strength by omitting solution annealing?Q, 296 Yield strength is 80kgf/-
If the fatigue strength in seawater is less than 40 kgf/-, the effect is not sufficient and it is difficult to use it for this purpose.

Furthermore, martensitic stainless steel, which has high strength even after solution annealing, has problems in corrosion resistance, formability, and weldability.

(Problems to be Solved by the Invention) An object of the present invention is to produce an austenitic stainless steel having excellent seawater resistance, yield strength, and fatigue strength in seawater required for hydrofoils of high-speed ships, etc.

In other words, the temperature at which pitting corrosion occurs is 30℃ or higher, and the yield strength is 60kg f.
/-1 An object of the present invention is to realize an austenitic stainless steel whose fatigue strength in seawater exceeds 40 kg f /mJ.

(Means for Solving the Problems) The present invention overcomes the problems of the prior art, has excellent seawater resistance,
In order to produce austenitic stainless steel with high yield strength and fatigue strength in seawater, we limited the ingredients and found effective controlled rolling and controlled cooling methods within that range. With this manufacturing method, solution annealing that causes a decrease in strength can be omitted.

In other words, weight: C0.08% or less, S+2.00% or less, Mn2.0% or less, Cr21% to 30%, N11
0~20%, Mo 0.5~3.0%, N003%~
Austenitic stainless steel containing 0.5% Fe and impurity elements was heated at 1100°C to 130°C.
It is heated to 0°C, rolled at 1050°C or higher to a total reduction of 50% or more, then finish rolled at 800°C to 1050°C to a total reduction of 10% or more, and further rolled at 800°C or higher. Average cooling rate from ℃ to 500℃ is 50'C
/win or more.

This manufacturing method does not deteriorate seawater resistance,
It is possible to effectively retain the strain introduced during hot rolling and improve yield strength and fatigue strength in seawater.

First, the reason for limiting the components in the present invention will be explained.

C is an element that increases strength, but if the content increases, Cr carbides are formed during hot rolling and the corrosion resistance deteriorates, so it is set to 0.08% or less.

St is usually added as a deoxidizing element, but if it exceeds 2.00%, hot workability deteriorates, so it is limited to 2.00% or less.

Mn is an unavoidable impurity element, but if it exceeds 2.0%, corrosion resistance decreases, so it is limited to 2.0% or less.

Cr is an essential element to maintain corrosion resistance in seawater, and in order to maintain sufficient corrosion resistance in seawater and furthermore prevent a decrease in fatigue strength in seawater, Cr must exceed 21%. It is necessary to add it. However, if the Cr content exceeds 3096, hot workability decreases and manufacturing becomes difficult.
The content was limited to more than 21% to 30%.

Ni is a basic element that maintains the structure as austenite, and if its content is less than 10%, austenite becomes unstable, ferrite crystallizes, and hot workability decreases. However, adding more than 20% has no effect and is only disadvantageous in terms of cost.

Therefore, the Ni content was limited to 10 to 20%.

Mo is an effective element for improving corrosion resistance, and must be added in an amount of 0.5% or more to ensure corrosion resistance and fatigue strength in seawater. However, if it is added in excess of 3.0%, hot workability will decrease, so the Mo content should be between 0.5 and 3.0%.
It was limited to 3.0%.

N is a solid solution in steel and is an essential element for increasing strength, and also has the effect of improving corrosion resistance in seawater. In order to ensure strength with the manufacturing method of the present invention, N is 0.
, Is it necessary to include more than 3%? Since adding more than 0.5% reduces manufacturability, the content of N is 0.5%.
It was limited to more than 3% to 0.5%.

In order to obtain high-strength austenitic stainless steel with excellent seawater resistance using the production method of the present invention, only the above-mentioned components are sufficient.
As other additive elements, Cu has the effect of pitting corrosion resistance, Ti and Zr have the effect of intergranular corrosion resistance, and Ajll, Ca, Mg, and lanthanoid rare earth elements have the effect of improving manufacturability.

The component ranges of the above additive elements will be described below.

Cu is effective in improving corrosion resistance, particularly pitting corrosion resistance, but excessive addition leads to an increase in cost, so it was limited to 2.0% or less.

Ti and Zr are effective in suppressing the formation of Cr carbides and improving intergranular corrosion resistance, and addition of a large amount leads to a decrease in manufacturability, so the content was limited to 0.5% or less.

Furthermore, addition of appropriate amounts of AN, Ca, Mg, and lanthanoid rare earth elements suppresses deterioration of hot workability and occurrence of ground defects due to S and O. However, if it is added in excess, the ground flaws will increase, so the content should be set at Ap 0. Old ~ 0.20
%, Ca 0.001-0.020%, Mg 0.00
1 to 0.020%, lanthanide rare earth elements 0.00
It was limited to 2 to 0.050%.

The lanthanoid rare earth element here is La.

Indicates a single or a mixture of lanthanum-based elements such as Cc.

Next, the reason for limiting the manufacturing conditions will be explained.

The controlled rolling of the present invention includes a rough rolling stage in which the steel ingot is heated to 1100°C to 1300°C and a total reduction of 50% or more at 1050°C or higher, followed by a total reduction at 800°C to 1050°C;
This consists of a finish rolling step in which the rolling stock is 10% or more.

The former is a stage in which the solidified structure is mainly broken and a uniform recrystallized structure is obtained, and the rear right is a stage in which working strain is introduced by rolling to increase the strength after rolling.

After rolling, the temperature is 50℃/ll1 from 800℃ to 500℃.
Controlled cooling at an average cooling rate of n or more suppresses recovery of processing strain introduced during finish rolling, and suppresses Cr carbide precipitation in this temperature range, resulting in high strength with excellent seawater resistance. Austenitic stainless steel can be obtained.

The reasons for limiting the conditions will be explained in more detail.

In order to enable rolling with a total reduction of 50% or more at 1050°C or higher, and to reduce deformation resistance and facilitate rolling, the steel ingot needs to be heated to 1100°C or higher. But 1300
The heating temperature was limited to 1100°C to 1300°C because heating above 130°C melts the grain boundaries and causes cracks during rolling.

In the rough rolling stage, in order to break the solidification structure and obtain a uniform recrystallized structure, the total rolling reduction must be at least 50% at 1050° C. or higher. If the rolling temperature is 1050° C. or less or the total reduction is 50% or less, a uniform recrystallized structure cannot be obtained, and component segregation during solidification remains, resulting in deterioration of corrosion resistance in seawater.

The finish rolling stage is the most important stage to obtain high strength austenitic stainless steel. Figure 1 shows 800℃~1
The relationship between the total reduction at 050°C and 0°2% yield strength is shown, and the relationship between the total reduction at 800°C to 1050°C and corrosion fatigue strength in seawater is shown in Fig. 2.

As shown in Table 1, A in the figure is within the composition range limited by the present invention, and J, L, and M are comparative steel types.

From this diagram, the target proof strength is 60kg f/mai
In order to obtain a steel that satisfies the above and the fatigue strength in seawater of 40 kg f/- or more, the total reduction at 800°C to 1050°C must be 10
% or more is required. Also 1050℃
If rolled at a temperature higher than 800° C., the steel recrystallizes and cannot accumulate processing strain, making it impossible to obtain sufficient strength. Rolling at a temperature lower than 800° C. increases the deformation resistance, making controlled rolling difficult. Controlled cooling after controlled rolling suppresses the decrease in strength due to recovery of processing strain, and also prevents deterioration of corrosion resistance due to carbide precipitation.

FIG. 3 shows the relationship between the average cooling rate from 800 DEG C. to 500 DEG C. and 0.2% proof stress for test steel A that satisfies the composition range defined in the present invention.

From this figure, if the average cooling rate is less than 50℃/win, strength failure will occur due to recovery of machining strain, and the 0.2% yield strength will be reduced to 60k.
In order to achieve g f / +*+i or more, the temperature must be 800℃~
Is it necessary to perform controlled cooling at an average rate of 50°C/min or more up to 500°C? Although it is possible to increase the strength by the above manufacturing method using ingredients other than the limited ingredients of the present invention, the effect is insufficient.

In order to ensure seawater resistance and obtain sufficient yield strength and corrosion fatigue strength, it is necessary to satisfy both the component range and the manufacturing method defined in the present invention.

(Example) Table 1 shows the chemical composition of the test steel. The content of unavoidable impurity elements other than those listed in the table is about the same as that of ordinary stainless steel.

In other words, the sulfur content is 0.01% or less, the phosphorus content is 0.05% or less, and the oxygen content is 0.01% or less. Further, REM in the table means a lanthanoid rare earth element, and the content indicates the total of these elements.

The above test steel was hot rolled under various conditions.

Table 2 shows product plate thickness, rolling end temperature, total rolling reduction above 1050°C, total rolling reduction between 800°C and 1050°C and 800°C.
Shows the average cooling rate up to ~500°C.

Note that heating before rolling was performed at 1140°C to 1270°C.

Numbers 1 to 14 in the table are the range of manufacturing conditions in the present invention, and 1
Numbers 5 to 22 are comparison conditions.

The seawater resistance, yield strength, and corrosion fatigue strength of the hot rolled steel sheets obtained under the above manufacturing conditions were evaluated. Seawater resistance was determined by immersing it in artificial seawater at 30°C for 1,000 hours and checking for pitting corrosion.

The yield strength was determined by cutting out a JIS No. 4 test piece from the center of the plate thickness at right angles to the rolling direction, and measuring the 02% offset yield strength. Corrosion fatigue strength was determined by performing a vibration axial fatigue test in artificial seawater.
The fatigue strength was evaluated based on the fatigue strength.

The method for collecting fatigue test pieces is the same as that for proof stress Mil+time, and the obtained corrosion fatigue strength is shown in the table in terms of amplitude stress range.

These evaluation results are also shown in Table 2.

As is known from the results in Table 2, Nos. 1 to 14, which are in the range of the present invention, do not cause pitting corrosion in artificial seawater at 30°C.
It is a high-strength austenitic stainless steel that satisfies a 0.2% yield strength of 60 kgf/IIIi or more and a corrosion fatigue strength of 40 kgf/ml11 or more, and exhibits excellent seawater resistance.

(Effects of the Invention) The present invention has made it possible to manufacture high-strength austenitic stainless steel with excellent seawater resistance by limiting the optimum components and performing controlled rolling and controlled cooling under conditions suitable for the components. .

The present invention has realized an austenitic stainless steel suitable for ship hull structures that satisfies the seawater resistance, yield strength, and fatigue strength in seawater required for hydrofoils of high-speed ships, and has an extremely large industrial contribution. .

[Brief explanation of the drawing]

Figure 1 shows the 80
A chart showing the relationship between total rolling reduction and 0.2% proof stress at 0°C to 1050°C, Figure 2 is for the test steels A, J, and M shown in Table 1.
Figure 3 shows the relationship between the total reduction at 800°C to 1050°C and the corrosion fatigue strength in seawater.
It is a chart showing the relationship between uniform cooling rate and 0.2% proof stress.

Claims (1)

[Claims] 1. C0.08% or less, Si 2.00% or less, Mn 2.0% or less, Cr more than 21% to 30%, Ni 10 to 20%, Mo 0.5 to 3.0%, Steel consisting of more than 0.3% to 0.5% N, balance Fe and impurity elements was heated at 1100°C.
Heating to ~1300℃, the total pressure reduction is 5 at 1050℃ or higher.
Rolled to 0% or more, and then heated at 800°C to 105°C.
A highly resistant steel sheet with excellent seawater resistance characterized by finish rolling at 0°C with a total reduction of 10% or more and an average cooling rate of 50°C/min or more from 800°C to 500°C after rolling. Method of manufacturing high strength austenitic stainless steel. 2. The high quality seawater-retaining material with excellent seawater resistance according to claim 1, which contains one or more of Cu 2.0% or less, Ti 0.5% or less, and Zr 0.5% or less in weight percent. Method of manufacturing high strength austenitic stainless steel. 3. One or two of Al0.01-0.20%, Ca0.001-0.020%, Mg0.001-0.020%, and lanthanoid rare earth elements 0.002-0.050% by weight. 2. The method for producing a high-strength austenitic stainless steel with excellent seawater resistance according to claim 1, characterized in that it contains at least one of the following. 4. One or more of Cu2.0% or less, Ti0.5% or less, Zr0.5% or less, Al0.0 in weight%
1 to 0.20%, Ca0.001 to 0.020%, Mg0.001 to 0.020%, and lanthanoid rare earth elements 0.002 to 0.050%. A method for producing a high-strength austenitic stainless steel with excellent seawater resistance according to claim 1.