JPS59166655A - High purity and high cleanliness stainless steel excellent in gap corrosion resistance and anti-rust property and preparation thereof - Google Patents

Abstract

PURPOSE:To prepare the titled stainless steel excellent in processability and low in cost, by casting or heat treating molten steel having a specific composition consisting of C, Si, Mn, Cr, N, P, S, Al, O and Fe and prescribed in cleanliness at a specific temp. CONSTITUTION:Molten steel which contains 0.01-0.1% C, 3% or less Si, 2% or less Mn, 14-26% Cr, 0.005-0.2% N, 0.02% or less P, below 0.001% S, 0.02- 0.2% Al, below 0.003% O and, if necessary, further contains one or more of elements selected from 3% or less Mo, 2% or less Cu, 2% or less Ni, 0.6% or less Ti, 0.02-0.5% V, 0.02-0.2% Nb and 0.01% or less B and comprises the remainder of substantially Fe and has cleanliness comprising the sum of oxide type impurities and sulfide type impurities of 0.02 or less is continuously cast at a temp. of DELTAT deg.C<=45 deg.C [DELTAT deg.C= (the temp. deg.C of molten steel in a tandish during continuous casting) - (the solidification temp. deg.C of molten steel)] to obtain a cast piece which is, in turn, heated to 1,230 deg.C or less or held thereto and the heated cast piece is hot rolled to obtain high purity and high cleanliness steel, excellent in gap corrosion resistance and anti-rust property.

Description

[Detailed Description of the Invention] The object of the present invention is to provide an inexpensive ferritic stainless steel with excellent corrosion resistance and product workability by utilizing high-purity and high-cleanliness refining technology for stainless steel. It concerns the composition and manufacturing method of ferritic stainless steel.

17 Cr-based ferritic stainless steel has traditionally been widely used mainly as thin plates due to its low cost, but compared to 18Cr-8Ni-based austenitic stainless steel, it has poor corrosion resistance and workability. It is considerably inferior. In particular, in terms of corrosion resistance, it is used under relatively mild conditions such as in the atmosphere, naturally occurring water, tap water, or hot water, but it easily rusts in welded or processed parts. Corrosion resistance is also poor in the base metal. Significant improvements in corrosion resistance are strongly desired for future expansion of applications. In addition, in terms of processability, it is at the current level in terms of drawing properties and stretchability. Of course, as a result of extensive research into improving corrosion resistance and workability,
Improvements have been made mainly by adding alloys. Concerning corrosion resistance, there is no fixed standard that requires different degrees of corrosion resistance depending on the environment of use. Therefore, Mo, C
The selective addition of u, Ni, 'rt, Nb, etc. is well known,
It has been put into practical use. On the other hand, for improving workability, Ti, B
, addition of At, reduction of C and N, combinations with hot rolling conditions, heat treatment conditions, etc. have been studied. However, if emphasis is placed on alloy addition in this way, costs will be high and process simplification will be hindered, and further progress is desired in terms of both quality and cost.

In response to this current situation, the present inventors focused on refining technology, especially refining technology for high purification of S, P, O, etc., to minimize the amount of alloying, improve corrosion resistance, improve workability, Many studies have been conducted with the aim of simplifying the process. As a result, we discovered that high-purity refining technology that reduces S, O, and P in stainless steel and further reduces sulfide and oxide inclusions as much as possible meets this aim, and completed the present invention. It is something.

C is an impurity in ferritic high-grade stainless steel.
, technology to reduce N'i is progressing, 9, C+N amount is 0.
．． 0.1% stainless steel has been put into practical use, but the inventors of the present invention fully clarified the roles of C and N and considered how to utilize them effectively. This is a feature of the invention.

The present invention, which was completed in this manner, relates to a composition of high-purity and high-cleanliness ferritic stainless steel and a method for producing the same, and its gist is as follows.

(1) Weight: C0.01~0.1%, Si3% or less,
Mn 2% or less, Cr 14% to 26%, NO, 005 to
0.2 section, Po, 02% or less, less than 80.001%, A
t0102 to 0.2, less than 00.003, and if necessary, Mo3. % or less, Cu2% or less, Ni2%
Below, T106% or less, Vo, 02-0.5%, Nb0
．． 02 to 0.2%, Bo, 01% or less of one or more additive elements, and the cleanliness consisting of the sum of oxide inclusions and sulfide inclusions is 0.02 or less, with the balance Substantially Fe
High-purity, high-cleanliness stainless steel with excellent usability such as crevice corrosion resistance and induction resistance.

(2) After melting the above alloy steel, continuous casting is performed at a casting temperature of ΔT and 45°C, and the resulting slab is heated or kept at a temperature not exceeding 1230°C, and then hot rolled. A manufacturing method characterized by

Hereinafter, the present invention will be described in detail.

It is clear that high-purity refining technology can reduce C4'N to less than 810 ppm and P200 ppm at low cost even in cystine-less steel by injecting CaC2+CaF2-based blacks, etc. Furthermore, reduction of C4'N has already been achieved industrially. It has been realized on a scale of

The present inventors focused on these high-purity refining technologies and took into consideration the manufacturing process.
As a result of an electrochemical study of the corrosion resistance, particularly the rusting property, of ferritic stainless steels, it was found that reducing the P content is extremely effective against the passive breakdown characteristics caused by Ct. Furthermore, it was found that reducing S greatly improved the passivation properties of the 17Cr system, and further, due to the synergistic effect with the above-mentioned low P, the passivity destruction properties due to C2- were greatly improved. .

Furthermore, in the case of low-S steel, deoxidizing the steel with At or T1 makes it easy for sulfides and oxide inclusions to float, and the steel can be made into an extremely clean steel. It was revealed that general corrosion resistance, crevice corrosion resistance, and even bendability were improved.

The experimental facts that led to the above findings are described below.・17C
Mainly r-type alloys are melted in a vacuum melting furnace with a focus on low 0, low P1, low S, and the heating temperature of hot rolling, hot rolling conditions, hot rolled plate annealing conditions, cold rolling conditions, final Corrosion resistance and workability were investigated by taking into consideration annealing conditions, etc. Product is 0
．． The thickness was 7rm.

Regarding corrosion resistance, these products were subjected to various immersion tests as well as electrochemical measurements. The effect of process conditions on corrosion resistance was not significant, and the effect of alloy composition was significant. In particular, the passivation properties and Ct-
It has been found that the passive breakdown resistance can be greatly improved due to the above (Fig. 1).

In Fig. 1, Fig. 1(a): Curve ■ in steel, P50ppm, 85
In steel with ppm curve ■, P 50 ppm, 89 ppm in steel with curve ■, P 50 ppm, 860 ppm In steel with curve ■, P 50 ppm, 8140 ppm
b): P50ppm in steel with curve ■, Plooppm in steel with 38ppm curve ■, Plooppm in steel with 88ppm curve ■, P150ppm, P in steel with 88ppm curve ■
250ppm, 88ppm curve■ in steel, P340
ppm, 88 ppm, and S is 10 ppm or more in Figure 1 (, ), curve ■.

■, P is 200 ppm or more, Figure 1 (b), curve ■,
It can be seen that the passivation breakdown potential (g) due to the formation force \raCt- of (2) is on the negative side, and the passivation properties are poor. These results are noticeable in the crevice corrosion test, and as shown in FIG. 2, a remarkable effect is shown at less than 810 ppm and less than P 200 ppm.

Figure 2 shows the effect of low S and low P on the crevice corrosion test that occurs between 17Cr stainless steel plates.The test conditions were 600 ppmct-, 10ppmCu.
+ 80℃ x 14 days'i air blowing, and the average depth of the five deep points in the crevice was plotted as the maximum crevice corrosion depth + II + I +).P is 3 o o p
pm, it can be seen that crevice corrosion is deep even when S is 10 ppm or less.

As the S content decreases, nonmetallic inclusions in the steel decrease significantly, and when the S content reaches 10 ppm, A
B-type inclusions (sulfide-type, sulfide + oxide-type inclusions) are no longer recognized, and B-type and C-type inclusions (both oxide System inclusions) also float easily, resulting in a steel material with low O content, significantly less nonmetallic inclusions, and a cleanliness level of 0.02 or less. (Cleanliness measurement is JIS method) Corresponding to this behavior, 3.5
The pitting potential in the %NthCL solution also becomes significantly nobler.

The above phenomenon is illustrated in FIG. 3, which shows changes in inclusion cleanliness (bottom diagram) and pitting corrosion potential (top diagram) due to low S content in 17Cr steel. The figure below shows the relationship between S and inclusion cleanliness of a 50Y ingot of 17Cr steel. It is represented by a dotted line. Furthermore, the above diagram shows the relationship between the pitting corrosion potential (V) and S measured on the ≠600 polished surface of a 17Cr product plate, and it can be seen that it becomes significantly nobler at 810 ppm or less.

Figure 4 shows the effects of S, P, and 0 on the rusting resistance of 17Cr stainless steel.
If it is less than ppm, P 200 ppm or less, S 1
It can be seen that when the distance is less than 0.092 m and the cleanliness is set to 0.02 or less, the rusting rank increases rapidly.

In other words, the high-purity, high-cleanliness stainless steel has improved basic corrosion resistance such as active dissolution behavior, pitting corrosion resistance, and crevice corrosion resistance, and has improved salt water test results that simulate rusting in the atmosphere. improve.

In addition, Fig. 4 shows o, 5% Na CZ + O-2%
Figure 2 shows the results of a modified salt spray test with a 30 °C solution of H202.

A drastic improvement in corrosion resistance can be further ensured by the above-mentioned high purity and high purity and by adding small amounts of various elements effective for improving corrosion resistance.

Cr, Ni, Mo, Cu for various uses.

Examining the effects of adding Ti, AA, Nb, St, V, etc.,
Furthermore, C and N were studied with a view to their effective use as additive elements.

4% NaC as an accelerated test in a near-neutral corrosive environment
A + 0.2% H20□, 60°C immersion test was conducted.

From these results, the amount of Cr (Fig. 5), Mo, Cu, Ni
,V.

It has been found that the effect of addition of Ti, etc. (Fig. 6) is more pronounced in low S, low P, and low O alloys. Thus low P1
High purification such as low S and low O has an effect on corrosion resistance in itself, but it makes the effects of alloying elements such as Mo, Cu, and Ni even more pronounced, and can reduce the amount of these expensive alloying elements added. The name was revealed for the first time.

Regarding the workability requirements of ferritic stainless steel products, we investigated bendability and bendability after cold working, as well as deep drawing properties and writhing characteristics during drawing, depending on the application.

First, regarding bendability, the influence of the process is small, and the influence of the alloy composition is large. In particular, cracks occurred due to the alloy in a processing zero bending test in which the product sheet was subjected to about 30 inches of cold working and then closely bent in a direction perpendicular to the rolling direction. Clearly less than 5O8001q6, less than 00.003% and P
No cracking occurred at all during close bending in alloys containing 0.02% or less.

The deep drawing characteristics were evaluated by determining the depth. Tensile test specimens were taken from the product sheet in the rolling direction, in the direction perpendicular to the rolling direction, and in the four directions, and the r value was measured. Further, after applying a tensile strain of 20 inches to a tensile test piece taken from the rolling direction, the height of jings generated was measured using a roughness meter.

It is well known that the gluability and r value of ferritic stainless steel are greatly influenced by not only the alloy composition but also hot rolling conditions and subsequent heat treatment. For high-purity, high-cleanliness steel, the influence of hot-rolled plate annealing is especially large, and there are two methods: continuous annealing, which rapidly heats the steel to a temperature range of 850 to 1050°C, and conventional bell-type annealing, which omit hot-rolled plate annealing. We considered the case. As a result, the results are basically the same as the conventional knowledge, and it is effective to use C and N in appropriate amounts.

In this way, it was found that even high-purity, high-cleanliness steel has the previously known effects of alloying elements, hot-rolling methods, and hot-rolled sheet annealing on ridging and r-value, and it has been found that high-purity, high-cleanliness steel has the effects of conventionally known alloying elements, hot-rolling methods, and hot-rolled plate annealing. Regarding the request, the addition of T1 and At and the effects of hot-rolled sheet annealing should be utilized.

However, for high-purity alloys, it has been found that important control points are particularly regarding grain refinement during casting, appropriate hot rolling heating temperature, and r value. This is because high-purity alloys tend to grow grains easily and become coarse.

That is, for high-purity, high-cleanliness steel, in order to refine the casting structure, the temperature of 31°C (molten steel temperature in tundish - solidification temperature of molten steel (calculated value)) during casting is reduced, and ΔTt1. <45
°C is required. In addition, the heating temperature before hot rolling must be kept at 1230°C or lower to prevent coarsening.As mentioned above, advances in refining technology have made it possible to eliminate P and S, which are particularly harmful impurities, by using CaC2-based 7 lux. In ferritic stainless steel, which has been made highly clean by further reducing O at the same pace as the method of reducing O to the extent possible, the passivation ability has been improved, corrosion resistance has been improved, and addition of Mo, Cu, Ni, etc. has been improved. It has even greater effectiveness and produces a greater effect with a smaller amount than conventional products. In addition, severe bending workability is reduced to low S.
and lower O.

Furthermore, regarding the workability of thin plates, the effects of Ti, At, etc. are remarkable in low P1, low S, and low O alloys, and it is possible to obtain a large improvement effect by adding a small amount together with the addition of an appropriate amount of CN. found.

The above findings are extremely innovative and have a significant effect on the purpose of supplying ferritic stainless steel of excellent quality at low cost.

The reasons for limiting the components of the present invention will be described below.

C: C is effective for corrosion resistance and workability in low S1 low P1 low 0 steel, and is added in a range of 0.01 to 0.1%.

If it exceeds 0.1%, corrosion resistance will deteriorate, and if it is less than 0.01%, workability will deteriorate.

st:si slightly improves corrosion resistance in low S1 low P1 low 0 steel, but there is no change in workability. Therefore, it was set at 3% or less. If it exceeds 3%, it will harden.

Mn: It is desirable for Mn to be low in terms of corrosion resistance, and 2
, O or less.

P: P impairs the passive properties of ferritic stainless steel, particularly the passive breakdown resistance due to Ct-, and is preferably as low as 0.02% or less.

S: S impairs the passive properties of ferritic stainless steel, and the lower it is, the more desirable it is.
- increases passive breakdown resistance, improves induction resistance, and
Improve bending properties.

O:0 is SO, less than 1% is oxide inclusion,
A lower value is preferable since it causes deterioration of induction resistance and pitting corrosion resistance. If it is less than 80.001%, there will be no sulfide;
The floating property of the oxide was set to be less than 0.003 inches.

Cr: Cr is essential for ferritic stainless steel and greatly improves corrosion resistance from 14% to 26%.

If it is less than 14%, corrosion resistance is insufficient, and if it exceeds 26%, workability deteriorates.

At: At is a low S1, low P1, low 0 series ferrite stainless steel, and at 0.02 to 0.2%, it greatly improves the sharpness.
and improve cleanliness. If it is less than 0.02%, the effect will be small, and if it exceeds 0.2%, the properties will deteriorate.

v:v improves the corrosion resistance properties in low S1 low P1 low 0 series ferrite stainless steel at 0.02% to 0.50%.

If it is less than 0.02%, the effect is small, and if it exceeds 0.5%, the effect is saturated, so it should be added selectively depending on the application.

N: N improves the corrosion resistance of high-Cr, high-purity, high-cleanliness steel, but is preferably 0.2 inches or less for workability. Therefore 0.
0.005% to 0.2 inch.

Mo: Mo can significantly improve corrosion resistance when added in small amounts, especially at low P1, low S, and low 0 paces, and is selectively added at 3% or less depending on the application. This is because if the number exceeds 3, the cost will increase.

Cu: CLI significantly improves corrosion resistance when added in a small amount, especially at a low P1, low S1, low O pace, and is selectively added at 2% or less depending on the application. When it exceeds 2%, the effect is saturated.

Ni: Ni improves corrosion resistance by adding a small amount at a low P1 low S1 low O pace, and is selectively added at 2% or less depending on the application. 2
% of the seven effects are saturated.

Ti: Adding a small amount of Ti at a low P, low S1, low 0 pace improves corrosion resistance, workability, and further improves cleanliness. Therefore, it should be added selectively within 0.6% depending on the application. 0.
The effect saturates when it exceeds 6 inches.

B: B improves processability by adding a small amount at a low P, low S1, low O pace, and is selectively added within 0.01% depending on the application.

If it exceeds 0.0196e, corrosion resistance will deteriorate.

Nb: Adding 0.02% to 0.2% of Nb to low S1 low P1 low O ferritic stainless steel improves resonating properties and corrosion resistance. If it is less than 0.02%, the effect is small, and if it exceeds 0.2%, the effect is saturated, so it should be added selectively depending on the application.

Cleanliness: Nonmetallic inclusions such as sulfides and oxides accelerate rusting, which is the starting point of pitting corrosion. Furthermore, since it completely deteriorates the bendability, it is desirable that it be as low as possible. After melting low-S ferritic stainless steel, it is desirable to deoxidize At, Tt, etc., and to take time for floating, so that the cleanliness of the hot-rolled sheet is e0.02 or less.

ΔT: Casting temperature is low S, ΔT≦4 at low S1 low 0 pace
A temperature of 5°C is desirable. When ΔT exceeds ・45°C, it tends to become coarse and the desired workability cannot be obtained. Similarly, unless the hot rolling heating temperature or heat retention temperature is 1230° C. or lower, it tends to become coarse and the desired workability cannot be obtained.

Examples of the present invention will be described below.

High-purity stainless steel alloys are melted using hot metal that has been pretreated, Fe-Cr alloy is added, melted in a 150T converter, tapped at a C level of about 0.2%, and ladled. At C
a C2-based flux was injected to reduce P to less than 0.015% and Se to less than 0.001%, and then final decarburization was performed in a VOD furnace. After that, after further desulfurization with desulfurization flux, A
After blowing in 7 or Ti to deoxidize and float the inclusions, continuous casting was performed to obtain a 200-piece thick CC slat, and a portion was made into an ingot. In the case of continuous casting, the casting conditions are ΔT≦4
It was poured to a temperature of 5°C to form a slab. The ingot was bloomed into a slab. The hot rolling heating temperature of this slab was 1100°C, and the hot rolling conditions were as follows: the finish rolling start temperature was 900°C.
A hot coil having a thickness of 3wIn was obtained by low-temperature rolling controlled at a temperature of 0.degree. C. or below. Thereafter, hot-rolled plate annealing consisting of rapid heating to 1000° C. was performed by continuous annealing, followed by continuous pickling. All cold rolling is done once to 0.7mm, and the temperature is 850℃.
A final annealing process was carried out, followed by pickling to obtain a product plate. As a comparison material, a thin stainless steel plate manufactured under normal conditions was used.

The results of the obtained products are shown in Table 1. The steel of the present invention is C
a Due to CZ-based high purification treatment, all S<0.001
%, P≦0.02%, O (0,003%).Furthermore, the inclusion cleanliness measured with hot rolled sheets is also extremely excellent.The characteristic test results of these products are shown in Table 2. It provides excellent usability, especially in terms of corrosion resistance and processability.
The effects of the present invention were confirmed.

As described above, the steel of the present invention was obtained by clarifying the effects of high purification and high cleanliness of the alloy on the basic characteristics of the steel, mainly corrosion resistance, and its usage characteristics, and by combining it with a small amount of effective additive elements. Furthermore, the manufacturing method requires regulating the casting conditions and heating temperature conditions of the slab during continuous casting, but manufacturing conditions other than those of the present invention, such as continuous casting and hot CC-D that directly connects rolling
It is clear that the basic properties of the steel of the present invention will still exhibit the desired properties whether produced by the Rf process or the CC-hot charge process. It also shows excellent properties in products such as bright annealing.

[Brief explanation of the drawing]

Figures 1a and b are diagrams showing the influence of the amount of p1s on the anodic polarization curve of 17Cr stainless steel in a solution containing ct-2 (3% NaCl + 5% H2SO4, 30°C, Ar degassing). Figure 3 shows the effect of low S and low P on the crevice corrosion test that occurs between 17Cr stainless steel plates. Figure 3 shows the changes in inclusion cleanliness and pitting potential due to low S in 17Cr stainless steel. Figure 4 shows 17 Cr
Figure 5 shows the effects of s, p, and o on the rusting resistance of stainless steels.
．． Figure 6 shows the effect of Cr content and high purity alloy on corrosion resistance at 2% H2O2 at 60°C. Figure 6 shows the effect of various additive elements on 17Cr stainless steel (C0.03%, NO.01%). Corrosion rate of practical alloys and high purity alloys (4%
FIG. In Figure 6, Ro-co practical alloys (PO,03%, SO,005CH, OO,00
5%) Concave] High purity alloy (PO, 014%, SO, 0007%, 00.0
Section 018) Frosting V (S, C, E) e
AQ V (S, C, E) 5 PP” ppm Figure 5

Claims (4)

[Claims] (1) C: 0.01 to 0.1% by weight, Si: 3 or less, Mn: 2% or less, Cr: 14 to 26, N: 0
．． 005-0.2 ratio, P: 0.02% or less, S: 0. O
O less than 1 inch, At: 0.02 to 0.2, O: 0.0
Excellent crevice corrosion resistance and induction resistance characterized by less than 0.03%, the sum of cleanliness of oxide and sulfide inclusions being 0.02 or less, and the remainder essentially consisting of Fe. Made of high purity, high cleanliness stainless steel. (2) C: o, oi to o, i% in weight%, Si: 3% or less, Mn: 2% or less, Cr: 14 to 26%, N:
0.005 to 0.2 inch, P: 0.02 inch or less, S: O,
OO less than 1 inch, At: 0.02 to 0.2, O: 0.0
Further, MO: 3% or less, Cu: 2% or less, Ni: 2% or less, Ti: 0.6% or less, V: 0
．． 02-0.5%, Nb: 0.02-02%, B: 0
．． Contains one or more additive elements of 01 or lower, and has a cleanliness of 0, which is the sum of oxide inclusions and sulfide inclusions.
A high-purity, high-cleanliness stainless steel with excellent crevice corrosion resistance and induction resistance, characterized by comprising 02 or less of sb and the remainder substantially of Fe. (3) C: o, oi ~ 0.1% by weight, Si: 3% or less, Mn: 2% or less, Cr: 14-26%, N
: 0.005 to 0.2 inch, P: 0.02 inch or less, S
: O, less than 001%, At: 0.02-0.2%, O:
Molten steel with a cleanliness of less than 0.003%, the sum of oxide inclusions and sulfide inclusions being 0.02 or less, and the remainder substantially consisting of Fe, is cast under a casting temperature condition of ΔT°C < 45°C. High-purity, high-clean stainless steel with excellent crevice corrosion resistance and induction resistance, characterized by continuous casting, heating or holding the resulting slab at a temperature not exceeding 1230°C, and then hot rolling. Method of manufacturing steel. Here, ΔT℃ - (molten steel temperature at tongue push during continuous casting (℃) - C solidification temperature of molten steel (℃)) (4) C: 0.01 to 0.1% by weight, Si: 3% or less, Mn: 2% or less, Cr: 14 to 26%,
N:0. O05~0.2%, P: 0.02% or less, S:
O, OO less than 1 inch, At: 0.02 to 0.2 inch, O: 0
．． Less than 0.003%, furthermore, Mo: 3% or less, Cu: 2
% or less, Ni: 2% or less, Ti: 0.6% or less, V:
0.02-0.5%, Nb: 0.02-0.2%,
B: Contains one or more additive elements of 0.01% or less, and the cleanliness of the sum of oxide inclusions and sulfide inclusions is 0.02 or less, and the remainder is substantially Fe. Continuous casting of molten steel consisting of Manufacturing method of high-purity, high-cleanliness stainless steel with excellent crevice corrosion and induction resistance Here, ΔT°C = (molten steel temperature at tump push during continuous casting in °C) - (1 longitudinal temperature of molten steel in °C)