JPS62192530A - Manufacture of high cr and high mo austenitic stainless steel plate - Google Patents

Abstract

PURPOSE:To obtain a stainless steel plate having superior corrosion resistance, ductility and toughness by soaking a slab having a specified composition according to the composition, hot rolling the soaked slab to a prescribed thickness and subjecting it to final heat treatment. CONSTITUTION:The composition of an austenitic stainless steel slab is composed of, by weight, <0.1% C, <1% Si, <2% Mn, <0.05% P, <0.03% S, 10-30% Ni, 16-25% Cr, 2-6% Mo, <3% Cu <0.3% N and the balance Fe with inevitable impurities and the thickness is regulated to >=100mm. The slab is soaked at a temp. between T1( deg.C) represented by formula I and T2( deg.C) represented by formula II for a time between t1(sec) represented by formula III and t2(sec) represented by formula IV. The soaked slab is hot rolled to a prescribed thickness and subjected to final heat treatment at 1,010-1,150 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] The present invention relates to a method for manufacturing a high Cr/high Mo austenitic stainless steel sheet, which has a 1010 to 115
The purpose is to produce a stainless steel plate with a thickness of about 4 to 100 mm that has excellent corrosion resistance and high elongation toughness even after final heat treatment at 0°C.

Industrial Applications Technology for manufacturing austenitic stainless steel, which has a higher degree of corrosion resistance and has excellent strength and toughness as a structural member, such as materials for chemical tankers and nuclear fusion reactors.

Conventional technology Austenitic stainless steel (hereinafter referred to as stainless steel) is used for a wide range of applications such as chemical machinery, towers and tanks, and decorative items due to its corrosion resistance, but recently it has been used as a material for chemical tankers and nuclear fusion reactors. Increasingly, materials are required to have higher corrosion resistance than the above-mentioned uses, as well as strength and toughness as structural members. For this reason, the composition of stainless steel is increasing the amounts of Cr and Mo to improve corrosion resistance, and the trend is to add N to increase strength.

However, the general manufacturing method for stainless steel today is to melt the material in an electric furnace and form it into a slab using a continuous casting method or an ingot-forming method.
After heating to the above temperature and hot rolling to a predetermined thickness, 1
This is a step in which final heat treatment is performed at a temperature of 0.010° C. or higher. Considering the production of thick plates of 50 ton or more in terms of yield and slab operation, etc., it is desirable to use slabs with a large cross section of 100 tn or more in thickness.

Problems to be Solved by the Invention However, in the conventional technology as described above, when the cross section of the slab is made large and thick, delta ferrite in which elements such as Cr--MO are concentrated is formed inside the steel ingot during solidification. These delta ferrites remain in the steel sheet after rolling and promote the precipitation of second phases including the sigma phase. For example 1
Since the temperature is 150° C. or higher, even if the precipitates are dissolved in the final heat treatment, they are easily reprecipitated during use or by heating during welding.

However, the precipitates of Somaru et al. create localized Cr and Mo depletion areas around them, leading to general corrosion, pitting corrosion, and carbon depletion of stainless steel.
Not only does it cause corrosion along the r-deficient portion and stress corrosion cracking starting from the chromium-deficient portion, but it also significantly deteriorates its ductility and toughness. In addition, since a high temperature of 1150°C or higher is required for complete solid solution of the precipitates, not only will the crystal grain size become coarse and the high yield strength due to N addition ■ may be reduced, but the surface will become rough during processing and the beauty of the surface will be affected. 1 member calls.

"Structure of the invention" Means for solving the problem C: 0.10 wt% or less, Si: 1. 00wL
% or less, Mn: 2.00wt% or less, P: 0.06w
t% or less, S: 0.03wt% or less, Ni: 10
．． 00~30.00wt%, Cr: L6.00~2
5.00wt%, Mo: 2.00-6-6-0O%
, Cu: 3.00wt% or less, N: 0.30w
t% or less, the balance consists of iron and unavoidable impurities, and Cr equivalent ffl: L 8 to 28 wt%, Ni equivalent: white, 4X Cr equivalent! 15.2wt%) ~ (1,4
×Cr ft 8.2wt%) is 1
00m or more austenite stainless steel slab T, (
'C) or more, Tt ('c) or less at a temperature of t7 seconds or more1
．． A method for producing a high Cr, high Mn austenitic stainless steel sheet, which comprises soaking for a time of less than seconds, then hot rolling to a predetermined thickness, and then final heat treatment at 1010 to 1150°C.

However, Cr is applicable! =%Cr +%Mo+l, 5X%Si+0
．． 5X (%Nb+%Ti) Ni hit! =%Ni +3Qx (%C0%N)+
0.5
tt+273)}-6,25 t2 (seconds)=1.8XlO' (=50 hours) Action High Cr, high M having the above-mentioned component composition.

When manufacturing austenitic stainless steel Ntffl from a large thick section, the concentrated precipitates that appear inside the steel plate are soaked according to the composition of the stainless steel, and then hot rolled into a steel plate with a predetermined thickness and 10IQ. By final heat treatment at ~1150°C, it becomes a form that has little effect on solid solution, material quality, and corrosion resistance.

By making the slab thickness 100 or more, the board thickness is 20~
100 nm thick stainless steel plate is produced with high yield and favorable slab operation.

EXAMPLE To further explain the present invention as described above, the inventors of the present invention have repeatedly studied how to solve the problems of the conventional products as described above, and as a result, carried out soaking treatment on slabs according to their composition. By doing so, it is possible to produce a stainless steel plate with a thickness of 4 to 100 mm that has excellent corrosion resistance and high elongation toughness even after hot rolling and final heat treatment (1010 to 1150°C) equivalent to 5US304 (L) and 316 (L). succeeded in.

That is, the product according to the present invention has a wt% (hereinafter simply referred to as %)
C: 0.10% or less Si: 1.00% or less Mn: 2.00% or less P: 0.06% or less S: 0.03% or less Ni: 10.00% or more, 30. O0% or less Cr:1
6.00% or more, 25.00% or less Mo: 2.00 or more, 6.00% or less Cu: 3.00% or less N: 0.20% or less and Ti: 0.05% or more, IO x %C Below Nb: 0.05
% or more and 10x% or less of C, the balance consists of iron and unavoidable impurities, and the %Cr equivalent according to the following formulas (1) and (2): 18% or more and 28% or less% Ni equivalent; (11-15.2 per 1,4XlCr
%) or more and less than (1-8.2% per 1.4 x Cr) and has a thickness of 100 m or more and is manufactured by the ingot-forming method or continuous casting. 'c) Above, below (
4) According to the formula 'rt, at a temperature below ('c), the following (5
) of formula 1 + (seconds) or more, tz according to formula (6) below
(seconds) Perform soaking treatment under the following conditions. Soaking treatment under these conditions can be carried out before, during, or after blooming if blooming is required, or during hot soaking if blooming is not required. This can be carried out before rolling or when heating the slab during hot rolling.

The soaked slab can be used as it is, or once cooled and then reheated and hot rolled to a specified thickness to make a stainless steel sheet.
Final heat treatment is carried out below 150°C.

%Cr equivalent; %Cr+%Mo+1.5X%Si+0.5
．． X(%Nb+%Ti)
(11%Ni equivalent=%Ni+30X (%C0%N)
+0.5
(3) Tz ('C) = -450X (%Mo
/Cr current view)+12Q5(4)Log(t+) = 1
．． 25X10' X (1/(tz+273)} -6
, 25T2: Heating temperature ('C) tz (seconds) = 1.8 X 10', (-50
Time) (6) The reasons for limiting the range of steel composition according to the present invention in terms of wt% (hereinafter simply referred to as %) are as follows.

C is an element that increases the intergranular corrosion susceptibility of stainless steel, so it is desirable to reduce it as much as possible, but on the other hand, it is an austenite stabilizing element, and it also increases stress corrosion cracking resistance in chlorides. , it is necessary to contain an appropriate amount. Based on these facts, the C content was set to 0.10% or less.

Since Si is an element necessary for deoxidation and effective for improving acid resistance, the content was set to 1.00% or less.

Since Mn is also an austenite stabilizing element and an effective element for deoxidation, the content was set to 2.00% or less.

In order to improve intergranular corrosion resistance and stress corrosion cracking resistance in chloride, it is desirable to have a small amount of P, but 0.0
If it is 6% or less, there are few practical problems, so the content is set to 0.06% or less.

S maintains good hot workability and pitting corrosion resistance,
The content was set to 0.03% or less.

Cr is a basic component for maintaining the corrosion resistance of stainless steel. For corrosion resistance, it is necessary to contain 16.00% or more, but considering the saturation of the effect on corrosion resistance and its manufacturability, 25. Made sure not to exceed 0%.

Ni is an austenite stabilizing element that is effective for general corrosion resistance and stress corrosion cracking resistance in chlorides, but it is expensive, so the content is limited to or from 0 to 100% in consideration of its economic efficiency.

% or more, 30. 0% or less.

Mo is an element that improves overall corrosion resistance, pitting corrosion resistance, intergranular corrosion resistance, etc., and is necessary in an amount of 2.00% or more, but since it has a negative effect on manufacturability, the upper limit is set to 6.00%. The content was set to 2600 to 6.00%.

Cu is effective in general corrosion resistance and stress corrosion cracking resistance, but the effect is saturated if it exceeds 3.00%, so the content is set to 3.00% or less.

N is an element that increases yield strength and pitting corrosion resistance by 0.2%, and can be added up to 0.3% if necessary. If it exceeds 0.3%N, not only the effect on pitting corrosion resistance is saturated, but also weldability is further deteriorated. For steel types in which ri and Nb are added singly or in combination, the addition amount is limited to 0.2% or less, as this may adversely affect the material quality and corrosion resistance, such as the formation of nitrides.

Ti and Nb can be used to fix C and N in the form of carbides and nitrides, making it a substantially low-C material, and improve intergranular corrosion resistance, so they can be used in addition to the above elements as necessary. One or two types can be added. For the purpose of such an effect, Ti and Nb are individually contained at 0.05% or more and (10x%C)% or less. C)%
Above that, the effect is saturated.

The reasons for limiting the Cr equivalent and Nt equivalent are as follows.

That is, from the viewpoint of corrosion resistance, a Cr equivalent of 18% or more is required in total of Cr and Mo. Corrosion resistance increases as the equivalent weight of Cr and Mo increases, but the effect is 28
If it exceeds 28%, it becomes saturated, and adding more than that is not necessary for corrosion resistance and is not economical, so the upper limit was set at 28%. Furthermore, this C
An increase in r-equivalent increases corrosion resistance, but promotes the precipitation of delta ferrite.

On the other hand, increasing the Ni equivalent stabilizes the austenite group during solidification and suppresses the precipitation of delta ferrite.

Therefore, the precipitation of delta ferrite is caused by Ni equivalent and C
It can be related to r equivalent. In other words, in the region where the Cr concentration is 18% or more and 28% or less, the Nt equivalent is (1
,4 However, Ni
In order to increase the equivalent weight, expensive Ni must be used due to restrictions on additive elements such as C, N, and Mn, which is not economical. In addition, the Ni equivalent is (1,4XCr equivalent - 15
, 2%) or less, precipitates remain on the steel sheet no matter what final heat treatment is performed.

Next, it is necessary that the thickness of each slab is 10,088 or more. In other words, in order to manufacture a thick stainless steel plate with a thickness of 20 to 100 mm, a slab with a thickness of 100 mm or more is required.
Thick, large-section slabs improve yields and slab operations. However, in slabs with a thickness of 100 mm or more, Cr, Mo
Local concentration of elements that promote the precipitation of delta ferrite occurs, resulting in the formation of precipitates that impair the ductility and corrosion resistance of stainless steel sheets, so it is necessary to adopt the special manufacturing method of the present invention. For slabs with a thickness of less than 100 mm, there is no such problem and the conventional method is sufficient.

The final heat treatment temperature needs to be 1010°C to 1150°C. In other words, in order to remove strain during hot rolling and to form a solid solution of precipitates that affect material quality and corrosion resistance, or to form precipitates that minimize the effect on material quality and corrosion resistance, the final rolling process is carried out at a temperature of 1010°C or higher and 1150°C or lower as described above. Perform heat treatment.

Furthermore, the reasons for limiting the slab soaking conditions are as follows.

That is, austenitic stainless steel (A-J steel) having the chemical composition shown in Table 1 below is melted in an electric furnace, cast in a 26-ton large mold or continuous casting machine, and then bloomed to a thickness of 15088 or more, I made it a slab of 250 dragons or less.

However, the delta ferrite ratio in the slab obtained as described above was measured with a ferrite scope, and the Ni equivalent
The results of determining the relationship between the delta ferrite percentage and the Cr equivalent are shown in FIG. 1. The delta ferrite percentage in the slab of the sample steel tends to be high at the center of the slab and low at the surface layer.

A, D, E, F, G, 1 steels with Ni equivalents close to (1,4, , 3.0% or less), the delta ferrite percentage of B-1, B-2, B-3, C, J, and H steels with a composition in which the Ni equivalent is close to (1,4XCr equivalent M-8,2) is Low (0.3% or more, 0.5% or less)
.

~ J steel respectively at 1150℃ or higher and 1275℃ or lower (
Delta ferrite ratio after soaking for 30 hours at 25℃ step) and further hot rolling and final heat treatment (A-G steel: 1050℃, H-J steel: 11
Figure 2 shows the number of precipitates in the steel plate heated to 00°C. In other words, the slab soaking conditions of 1,150°C or higher and 1,275°C or lower were investigated because, at a heating temperature of less than 1,150°C,
This is because the effect of slab soaking cannot be obtained, and at temperatures exceeding 1275° C., the yield decreases due to scale loss.

The ferrite rate is determined using a ferrite scope and expressed in %. After electrolytically etching the precipitates in a 10% ammonium persulfate solution, a total of 15 cm was measured in the thickness direction of 1/2 part of the plate using an optical microscope. The number of precipitates perpendicular to the line (2) was determined and expressed as the number of precipitates per unit length [/dragon].

If %Mo/%Cr is 0.1 or less, the delta ferrite rate can be made 0.2% or less at any temperature between 1150°C and 1250°C, but %Mo/%
C "Delta ferrite rate increases by 0.2% as the equivalent weight increases.
The maximum heating temperature will be lowered to below. %Mo/%
For J'FA with a Cr equivalent of 0.24, the temperature is 1150°C or higher,
A delta ferrite ratio of 0.2% or less is possible only at temperatures below 1175°C. In addition, if the delta ferrite ratio is 0.2% or less, the number of precipitates in the steel plate will be 2/number or less. From the above, reducing the delta ferrite in the flage,
In addition, in order to suppress precipitates in the steel plate, the slab soaking temperature should be set below the diagonal line shown in Fig. 2 according to %Mo/%Cr, i.e. -450X(%Mo/Cr!#,)+129
It is necessary to determine it using equation (4) in Section 5.

Determine the soaking temperature of A-J steel from Figure 2 and calculate the respective percentages.
Slabs aged isothermally at a temperature corresponding to Mo/%Cr equivalent and these slabs are hot rolled and final heat treated (A-G steel:
Fig. 3 shows the results of examining delta ferrite and precipitates in steel sheets heated at 1050°C and H-J steel: 1100°C. In other words, when looking for slab soaking conditions where the delta ferrite in the slab is 0.2% or less and the precipitates in the steel plate is 2/2 or less, at a soaking temperature of 1250°C, delta ferrite is 0 after soaking for 3 hours or more. Although it is possible to achieve a precipitate of .2% or less and a steel plate precipitate of 2/fin or less, a soaking temperature of 1150° C. requires 30 hours or more. In other words, the relationship between the soaking water temperature determined by the above formula in Figure 2 and the time required for the delta ferrite to become 0.2% or less and the precipitate to be 2/2% or less is Log(t
+)□1.25X10'X (1/(Tz+273)}
It is shown in equation (5) of 6.25.

The effect persists even after the heating time given by equation (5) is exceeded, but at heating temperatures exceeding 1200°C, scale loss increases (more than 31 scales on one side) and the yield decreases, so it can last up to 50 hours. Do not exceed.

Regarding A-J steel as described above, steel sheets manufactured by the method of the present invention and the conventional method were examined for secondary sum, material quality, and corrosion resistance. Note that the final heat treatment was performed at 1050°C for A-GE and 1100°C for H-J steel. The following tests were conducted on these slabs and steel plates. Hereinafter, the slabs and steel plates manufactured by the method of the present invention are referred to as the present invention material.
Slabs and steel plates manufactured using conventional methods are called conventional materials.

The delta ferrite ratio of the slab and the number of precipitates in the steel plate were determined by the methods described above. The material has been subjected to a Z-direction tensile test (aperture of area at break: RAz%), which sensitively reflects the influence of precipitates.
) was investigated. Also, the corrosion resistance is J Is GO573.
The test was performed using the Huey test according to ASTM A262 practice C. That is, this Huey test detects carbides, sigma phases, Cr-deficient layers, etc., and therefore clearly shows the influence of precipitates on the corrosion resistance of stainless steel.

The results of each test are summarized in Table 2 below.

That is, according to the results in Table 2 above, ASB-1 with a small %Mo/%Cr equivalent (0.08 or more and 0.12 or less);
B-2, B-3, CSD, ■Conventional steel materials have delta ferrite in the slab of 0.3% or more and 3.0% or less, and precipitates inside the steel plate of 2.7/s or more, 5 ,0/
In contrast, AlB-1, B of the material of the present invention
-2 and B-3 steels, the delta ferrite in the slab and the precipitates in the steel plate disappeared, and in the inventive materials C, D, [1, the delta ferrite in the slab was 0.1% or more, 0.2 % or less, and the precipitates on the steel plate are more than 1.0/ms,
It has decreased to below 1.9/m-. For that purpose, A.
The RAz of B-1B-2, B-3, C, D, and l steel plates is 10% or more and 24% or less for conventional materials, but 40% for the present invention materials.
% or more and 65% or less, and the Huey test results were 0.7 g for conventional materials, 0.9 g for 7 m”/h or more.
/m"/h or less to 0.3g/m"/h for the present invention material
As mentioned above, it decreased to 0.4 g/mt/h or less, and good material quality and corrosion resistance were obtained.

%Mo/%C "Equivalence is large (0.15 or more, 0.2
4 or less) In conventional materials of E, F, G, H, and J steel, delta ferrite in the slab is 0.3% or more and 2.5% or less, and precipitates in the steel plate are 4.2/am or more, 7 .0 /
ms or less, whereas in the material of the present invention, delta ferrite in the slab is 0.1% to 0.2%, and precipitates in the steel plate are 1.9/■■ or more and 2.07m or less. By applying the method of the present invention, delta ferrite in the slab and precipitates in the steel plate are significantly reduced compared to conventional materials. Since a small amount of precipitates remain in E, F, G, H1Jw4, RAz shows the same value (50% or more, 70% or less) as in A, B-1, B-2, B-3, and C steels. However, while the conventional material has a value of 4% or more and 15% or less, the present invention material shows a high value of 35% or more and 40% or less. The Huey test results are 0.8g/m”/h or higher than conventional materials,
1.3 g/m"/h or less, the material of the present invention has a
Indicates 3 g/m2/h or more and 0.4 g/m"/h or less,
The presence of precipitates similar to steels E, G, H, and J does not affect corrosion resistance, and the material has good material quality and corrosion resistance similar to steels A, B-1, B-2, B-3, C, and D. was gotten.

``Effects of the Invention'' According to the present invention as explained above, it is possible to manufacture stainless steel sheets that exhibit good material quality and corrosion resistance that could not be obtained using conventional techniques, even from thick-walled, large-section slabs. This is an invention with great industrial effects.

[Brief explanation of drawings]

The drawings show the technical contents of the present invention, and FIG. 1 is a chart showing the delta ferrite percentage in the slab in relation to Ni equivalent and C equivalent, and FIG. 2 is a chart showing the delta ferrite percentage in the slab in relation to Ni equivalent and C equivalent. and % on the number of precipitates in one tin plate
A chart showing the relationship between Mo/%Cr equivalent and soaking temperature, and FIG. 3 is a chart showing the relationship between the soaking condition of the slab (heating temperature x time) and the delta ferrite percentage in the slab. No. / Circle

Claims (1)

[Claims] C: 0.10 wt% or less, Si: 1.00 wt% or less,
Mn: 2.00wt% or less, P: 0.06wt% or less,
S: 0.03wt% or less, Ni: 10.00-30.0
0wt%, Cr:16.00-25.00wt%, Mo
: 2.00 to 6.00 wt%, Cu: 3.00 wt% or less, N: 0.30 wt% or less, and the balance consists of iron and inevitable impurities, and Cr equivalent: 18 to 28 wt%, Ni equivalent. : (1.4 x Cr equivalent - 15.2 wt%) ~ (
1.4 × Cr equivalent - 8.2 wt%) An austenitic stainless steel slab with a thickness of 100 mm or more is soaked at a temperature of T_1 (℃) or more and T_2 (℃) or less for a time of t_1 seconds or more and t_2 seconds or less. , then after hot rolling to a predetermined thickness, 10
A method for producing a high Cr, high Mn austenitic stainless steel sheet, the method comprising performing final heat treatment at 10 to 1150°C. However, Cr equivalent=%Cr+%Mo+1.5×%Si+0.
5×(%Nb+%Ti) Ni equivalent = %Ni+30×(%C+%N) +0.5×%Mn T_1(℃)=1150 T_2(℃)=-450×(%Mo/Cr equivalent)+12
95 log(t_1)=1.25×10^4×{1/(t_
2+273)}-6.25 t_2 (seconds) = 1.8 x 10^5 (=50 hours)