JPH09279231A - Production of ferritic stainless steel excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a ferritic stainless steel excellent in corrosion resistance by executing the modification of nonmetallic inclusions causing the generation of rusting in a ferritic stainless steel used in an atmospheric wet environment without deteriorating its productivity. SOLUTION: A steel contg. 0.001 to 0.080% C, 0.001 to 0.050% N, 15.0 to 35.0% Cr, <=0.010% S, <=0.04% P, 0.06 to 0.25% Ti, 0.01 to 1.0% Mn, 0.01 to 1.0% Si and 0.01 to 0.050% Al also so as to satisfy 0.15×Ti or above, <=0.010% O, and the balance inevitable impurities is heated at a heating temp. satisfying the inequality, 0.25>=Ti(%)>=(1.28×10<-3> .HT-1.37)×Mn(%)+0.06 (wherein, HT is heating temp. ( deg.C) before hot rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ferritic stainless steel used in a humid atmospheric environment for exterior materials for building construction materials such as roofs and walls, and outer panels of transportation equipment such as railway vehicles. Is.

[0002]

2. Description of the Related Art Ferritic stainless steel is widely used in kitchen appliances and the like because it has a lower Ni content and is less expensive than austenitic stainless steel.
Further, in addition to low cost, the coefficient of thermal expansion is small, so Mo
In recent years, the materials containing s have been increasingly used as building materials in beach areas and the like.

In order to secure the corrosion resistance in the atmospheric humid environment including such beach areas, the basic components such as high Cr and M are contained.
It is known that addition of o is effective. In addition, in order to stabilize the corrosion resistance, it is also known that it is necessary to increase the corrosion resistance of the basic components and at the same time control non-metallic inclusions that tend to be the starting point of rusting. In ferritic stainless steel, as disclosed in Iron and Steel (1979, P.S329),
There is known a method of improving the rust resistance by modifying non-metallic inclusions by adding Ti in an amount of 0.3% or more. However, when Ti is contained in an amount of 0.3% or more, nozzle clogging and flaws are generated during casting, and the toughness is lowered, resulting in markedly poor manufacturability, and a method that does not use a large amount of Ti has been desired.

[0004]

SUMMARY OF THE INVENTION The present invention does not cause problems such as nozzle clogging during casting and flaw generation and toughness reduction during casting, which are problems during the production of ferritic stainless steel used in an atmospheric humid environment. An object of the present invention is to provide a ferritic stainless steel having excellent corrosion resistance, which is capable of modifying a non-metallic inclusion that becomes a starting point of rusting.

[0005]

The present inventors have found that a large amount of T
A method for controlling non-metallic inclusions without using i was examined.
First, from the results of the air exposure test material, among the non-metallic inclusions as the rust initiation point, especially MnS is the main component, and when a large amount of Ti is added, the sulfide becomes a Ti-based sulfide instead of MnS. It was confirmed that it did not become the starting point of rusting. From this, the study was conducted so as not to precipitate MnS, which is likely to be a rust initiation point. At the same time, in order to ensure manufacturability, nozzle clogging, scratches, and toughness of the hot rolled sheet were also examined. As a result, by controlling the amounts of Mn, Ti and S used as deoxidizing elements and the heating temperature before hot rolling, MnS, which is the starting point of rusting, did not precipitate, and it was found that rust resistance can be greatly improved. did.

The present invention is based on such knowledge, and its constitution is as follows. (1) By weight%, C: 0.001 to 0.080%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, S: 0.010% or less, P: 0.04% or less, Cr: 15.0 to 35.0%, Ti: 0.06 to 0.25%, N: 0.001 to 0.050%, Al: 0.01 to 0.050% Steel containing 0.15 x Ti or more and O: 0.010% or less and the balance being Fe and unavoidable impurities is heated at a heating temperature satisfying the following formula before hot rolling. A method for producing a ferritic stainless steel having excellent corrosion resistance. 0.25 ≧ Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn
(%) +0.06 Here, HT: heating temperature (° C) (2) In addition to the ferritic stainless steel component described in the above item (1) in weight%, Mo: 0.5 to 5.0%, Ni : 0.1-5.0%, Cu: 0.1: 3.0%, 1 type, or 2 or more types are contained, The ferritic stainless steel excellent in corrosion resistance as described in (1) above. Manufacturing method. (3) In addition to the ferritic stainless steel component according to the above (1) or (2), further by weight%, Nb: 0.01 to 0.5%, Zr: 0.01 to 0.5% V: 0 0.01: 0.5% of 1 type or 2 types or more is contained, The manufacturing method of the ferritic stainless steel excellent in corrosion resistance as described in said (1) or (2) characterized by the above-mentioned. According to the present invention, MnS, which is a starting point of rusting, can be modified and the corrosion resistance can be stabilized without reducing nozzle clogging, scratches and toughness which are problems in manufacturing ferritic stainless steel.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
In order to prevent the corrosion resistance due to MnS-based sulfides, the inventors of the present application studied in detail the structural change of sulfides in high Cr ferritic stainless steel.

26% Cr-2% Mo-30 ppm C-60
The effect of alloying elements on the microstructural change during high temperature heating was investigated using ferritic stainless steel containing ppm N steel as a basic component. 0.05 to 3% Mn and 0 Ti in vacuum melting
~ 1.0%, stainless steel with S changed by 0.0005 to 0.05% was cast into a 15 kg ingot, and after taking a sample, precipitates after heating at a temperature of 1000 ° C to 1300 ° C for 1 hour It was investigated by SEM-EDS and electron microscopy. As a result, it was found that not only the Mn content and Ti content change the precipitates, but also the precipitation phase changes from Mn-based to Ti-based by changing the heat treatment temperature.

The results when the S amount is 100 ppm or less are shown together in FIG. The K value on the horizontal axis is a value determined by the amount of Mn and the heating temperature, and the M value increases as the K value increases.
It indicates that the amount and the heating temperature increase. By determining the amount of Mn, the heating temperature and the amount of Ti so as to be in the shaded area in FIG. 1, MnS at the time of heating can be changed to Ti-based precipitates, and the precipitation of MnS that is the starting point of rusting can be prevented.

That is, when Mn and Ti and the heating conditions are made to satisfy the following expressions, sulfides mainly composed of Mn are not formed until the final product after hot rolling. However, the following formula is S
The amount is less than 100ppm, and from the relationship in this figure, Ti
It is necessary to satisfy the following formula as conditions for depositing the system deposits. Here, HT is a heating temperature (° C.). Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn (%) +0.
06 In addition, 26% Cr-2% Mo-30ppm C-60ppm N
After heating a lab steel ingot containing as a basic component at 1100 ° C.,
Hot rolling was performed up to mm to evaluate the toughness of the hot rolled sheet. The result is shown in FIG. From this figure, the toughness of hot-rolled sheet varies greatly depending on the amount of Ti. By setting Ti to 0.25% or less, the impact value at room temperature (25 ° C) can be secured at ² kg · m / cm ² or more, and The post-development process will be at a level that can be implemented.
From the above, the upper limit of the Ti amount is 0.25%, and in the present invention, the S amount is 100 ppm or less, and it is necessary that the relationship between Mn, Ti and the heating temperature satisfies the following equation. 0.25 ≧ Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn
(%) +0.06

When the heating temperature is less than 1100 ° C., scale defects occur, and when it exceeds 1300 ° C., the slab is deformed by its own weight during heating, which makes rolling difficult.
It is desirable to set the temperature to 0 ° C to 1300 ° C. Regarding the hot rolled sheet annealing and other heat treatments in the steps after hot rolling,
By controlling the heating temperature HT or less, which is derived from the following formula using the amounts of Ti and Mn in the steel, the precipitates are converted to Ti until the final product.
The system can be the main component, and the corrosion resistance can be further stabilized. Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn (%) +0.
06

Next, the reasons for limiting the components and the like in the present invention will be described. C: C is harmful from the viewpoint of corrosion resistance, and particularly adversely affects the corrosion resistance of the welded portion, but it is necessary to some extent from the viewpoint of strength. At present, if it is less than 0.001%, the manufacturing cost becomes high, and if it is added in excess of 0.08%, the workability and toughness deteriorate, so C is 0.001 to 0.0.
8%.

Mn: Mn is added as a deoxidizing element,
If it is less than 0.01%, the effect is not sufficient, and if it is added in excess of 1%, the effect is saturated, and the precipitation of MnS is promoted to deteriorate the corrosion resistance. . Si: Si is used as a deoxidizer, but if it is less than 0.01%, it has no sufficient effect, and if it is added in excess of 1%, embrittlement is significantly promoted and ductility and toughness are deteriorated.
Add at 01-1.0%.

S: S deteriorates ductility, toughness, etc., and is harmful from the viewpoint of corrosion resistance. In the present invention, Mn,
From the viewpoint of securing the toughness of the hot-rolled sheet in relation to Ti and securing the corrosion resistance by modifying the non-metallic inclusions, it is 0.010% or less. P: P is harmful in terms of workability, toughness and corrosion resistance,
The smaller the content, the more desirable and 0.040% or less.

Cr: Cr is a main element of the ferritic stainless steel of the present invention, and must be added in an amount of 15% or more in order to suppress rust generation outdoors. But 35
%, The corrosion resistance is improved, but the workability and toughness are deteriorated. Therefore, the upper limit of Cr is 35%.

Ti: In the present invention, Ti is an essential element from the viewpoint of preventing precipitation of MnS in addition to fixing C and N. Mn and heating temperature during hot rolling: HT
From the viewpoint of ensuring the toughness and the relationship with (° C), 0.06 to
It is necessary to satisfy the following expression within the range of 0.25%. 0.25 ≧ Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn
(%) +0.06

Al: Al is used as a deoxidizing element. In the present invention, since the sulfide is a Ti-based material, it is important that the oxide is an Al-based oxide rather than a Ti-based oxide. Therefore, Al is Ti used as a deoxidizing element.
0.15 times or more is required. The upper limit was set to 0.05% because the degree of deoxidation is saturated at 0.05% or more.

N: Like C, the smaller the content of N, the more preferable the corrosion resistance and workability are, but it is industrially difficult to make it less than 0.001%, and N is added in excess of 0.05%. Then, the workability and toughness deteriorate, so N is 0.00
Add in the range of 1-0.05%. O: O deteriorates the toughness of the hot rolled sheet, causes nozzle clogging and scratches during casting, and deteriorates the toughness of the hot rolled sheet, so in the present invention, it is set to 0.01% or less.

In the present invention, if necessary, Mo, Ni, Cu
Any one or more of the above can be contained. Mo: Mo is a preferable element in terms of corrosion resistance, and can be added as a selective element if necessary. If it is less than 0.5, the effect is not sufficient, and even if it exceeds 5.0%, the effect is saturated and the embrittlement is remarkable, so the content was made 0.5 to 5.0%.

Ni: Ni has the effect of improving the corrosion resistance of ferritic stainless steel and can be added as a selective element, but if it is less than 0.1% it has no effect, and if it exceeds 5.0% it forms a ferrite phase. Since it becomes unstable and easily causes embrittlement during hot, it is set to 0.1 to 5.0%. Cu: Cu is a preferable element in terms of corrosion resistance and can be added as a selective element, but if it is less than 0.1, its effect is not sufficient, and if it is added in excess of 3.0%, its effect is saturated. Add at 0.1-3.0%.

In the present invention, in order to further improve the corrosion resistance, one or more kinds of Nb, Zr and V can be contained. Nb: Nb fixes C and N, so it is especially C at the weld.
In order to suppress the precipitation of r carbonitride and improve the corrosion resistance,
As a selective element, 0.01% or more can be added. Also 0.
Even if added in an amount of 5% or more, the toughness and ductility are reduced, so the content was made 0.01 to 0.5%.

Zr: Zr can be added as a selective element in an amount of 0.01% or more in order to fix C and N, and particularly to suppress the precipitation of Cr carbonitride in the welded portion and improve the corrosion resistance. Even if added in an amount of 0.5% or more, the toughness and ductility are reduced, so the content was made 0.01 to 0.5%. V: V fixes C and N, and thus can be added in an amount of 0.01% or more as a selective element in order to suppress the precipitation of Cr carbonitride particularly in the welded portion and improve the corrosion resistance. Also 0.5%
Even if added above, the toughness and ductility are reduced, so 0.0
It was set to 1 to 0.5%.

[0023]

Example A ferritic stainless steel having the components shown in Table 1 was melted by vacuum melting in a lab to manufacture a 50 kg steel ingot. Then, after soaking at the temperature shown in Table 1 for 60 minutes, hot rolling was performed to 3 mm, annealing was performed at 1000 ° C. for 1 minute, pickling, cold rolling to 1 mm, and annealing at 1050 ° C. for 30 seconds, A sample for corrosion resistance evaluation was prepared. Corrosion resistance was evaluated by a combined cycle corrosion test using artificial seawater at 35 ° C. for 10 minutes → drying 6
0 ℃ -60 minutes → Wetness: 80% humidity: 50 ℃ -60th
100 cycles were carried out to evaluate the degree of rusting.

As a result, as shown in Table 1, the steel produced by the method of the present invention showed no corrosion and showed excellent corrosion resistance.

[0025]

[Table 1]

[0026]

As described above, according to the method of the present invention,
By controlling the composition and the heating temperature during hot rolling, the non-metallic inclusions that are the starting point of rusting of ferritic stainless steel are modified so that they do not become the starting point of rusting, producing ferritic stainless steel with excellent corrosion resistance. it can.

[Brief description of drawings]

FIG. 1 is a diagram showing changes in sulfides depending on the amounts of Mn and Ti and the heating temperature during hot rolling when the amount of S is 100 ppm or less.

FIG. 2 is a diagram showing the effect of Ti content on the toughness of a hot rolled sheet.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Kihira 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technology Development Division

Claims (3)

[Claims] 1. By weight%, C: 0.001 to 0.080%, Si: 0.01 to 1.0%, Mn: 0.01 to 1.0%, S: 0.010% or less, P: 0.04% or less, Cr: 15.0 to 35.0%, Ti: 0.06 to 0.25%, N: 0.001 to 0.050%, Al: 0.01 to 0.050 % And 0.15 × Ti or more and O: 0.010% or less, the balance of which is Fe and unavoidable impurities, is to be heated at a heating temperature satisfying the following formula before hot rolling. A method for producing a ferritic stainless steel having excellent corrosion resistance. 0.25 ≧ Ti (%) ≧ (1.28 × 10 ⁻³ · HT-1.37) × Mn
(%) +0.06 where HT: Heating temperature (° C) 2. The ferritic stainless steel component according to claim 1, further comprising, by weight%, Mo: 0.5 to 5.0%, Ni: 0.1 to 5.0%, Cu: 0.1: 3. The method for producing a ferritic stainless steel having excellent corrosion resistance according to claim 1, wherein the ferritic stainless steel contains at least one of 3.0%. 3. The ferritic stainless steel component according to claim 1 or 2, further comprising, by weight%, Nb: 0.01 to 0.5%, Zr: 0.01 to 0.5%, V: 0. The method for producing a ferritic stainless steel having excellent corrosion resistance according to claim 1 or 2, characterized in that it contains one or more of 01: 0.5%.