JPH01100248A - Two-phase stainless steel and its production - Google Patents

Abstract

PURPOSE:To manufacture a two-phase stainless steel minimal in the occurrence of rolling defects by incorporating specific percentages of C, Si, Mn, P, S, Cr, Ni, Mo, W, Ti, B, Cu, Al, Ca, N, and O to Fe. CONSTITUTION:A two-phase stainless steel which has a composition consisting of, by weight, <=0.03% C, 0.1-1.0% Si, 0.3-2.0% Mn, <=0.035% P, <=0.0020% S, 23.5-26.5% Cr, 6.0-8.0% Ni, 2.5-3.5% Mo, 0.1-1.0% W, 0.005-0.10% Ti, 0.0005-0.0100% B, 0.2-2.0% Cu, 0.01-0.05% Al, 0.0005-0.0100% Ca, 0.08-0.2% N, <=0.0060% O, and the balance essentially Fe and further containing, if necessary, <=0.5% V and/or <=0.5% Nb is prepared. By this method, a sound high-Cr two-phase stainless steel strip practically free from rolling defects can be stably manufactured by means of hot coil rolling with high efficiency.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention provides a 25Cr duplex stainless steel that rarely generates rolling defects (surface defects and edge cracks) even during hot coil rolling from continuously cast slabs. The present invention also relates to a method for producing a 25Cr duplex stainless steel material with fewer rolling defects.

<Prior art and its problems> Duplex stainless steel containing a large amount of Cr has strong resistance to pitting corrosion, crevice corrosion, stress corrosion, etc. caused by chlorides, so it cannot be used for heat exchange using seawater. Duplex stainless steel has a wide range of uses, including containers, chemical manufacturing equipment, and oil well piping materials. However, on the other hand, duplex stainless steel has the disadvantage of generally inferior high-temperature deformability, and when hot-worked, it It was also a material that was extremely difficult to obtain into sound hot-processed products because it was prone to cracking.

Therefore, the following measures have been proposed to improve the high-temperature deformability of this duplex stainless steel, and it has been reported that a corresponding effect can be obtained.

(al) In addition to regulating the ratio of γ phase to 15% to 55% and designing the composition to ensure a sufficient amount of α phase, the strength of the γ phase and α phase at high temperatures can be improved by balancing the steel ingot components. A method of strengthening grain boundaries by increasing the grain boundaries and adding B (Japanese Patent Publication No. 59-14099).

(bl) A method of reducing S and 0 in steel and adding rare earth elements as necessary to strengthen the removal of 0 and S during steel melting (Japanese Patent Publication No. 15660/1983).

(A method of changing the form of inclusions by adding CI A1 and Ca in combination (Japanese Patent Publication No. 54-24364).

However, even if these methods are applied as they are, when hot coil rolling is performed using continuously cast slabs as raw materials, surface flaws and edge cracks (hereinafter collectively referred to as rolling flaws) can still be avoided. The current situation is that the technology for efficiently manufacturing a healthy high-Cr content hot-rolled steel strip with no defects has yet been established.

That is, the above-mentioned hot coil rolling can produce a slab with a thickness of about 200 mm, for example, as shown in FIG.
By applying a reduction comparable to that of a bath, severe processing is applied to produce a steel strip with a minimum thickness of about 31 nm. Therefore, even with the above-mentioned proposed measures, it is not possible to impart sufficient deformability to withstand hot coil rolling in a high Cr-containing duplex stainless steel material with a cast structure, such as a continuously cast slab. Rolling defects were unavoidable.

Therefore, until now, hot-rolled steel strips of high Cr-containing duplex stainless steels such as 25Cr-based steel strips have not been able to be manufactured without using rolled materials by the steel ingot method.

However, when using a material rolled by the steel ingot method, it is essential to perform blooming before hot coil rolling. Moreover, the problem is that the sigma phase is likely to precipitate in the slab after blooming, and once the sigma phase precipitates, the toughness decreases and cracks occur during slab maintenance, resulting in a significant decrease in yield. There was a problem with causing this. In order to avoid the occurrence of this sigma phase, it is necessary to perform water cooling immediately after blooming, or to carry out further solution treatment followed by slab cleaning, which is an inefficient process.

<Means for Solving the Problems> Based on the above, the present inventors have developed a duplex stainless steel that exhibits excellent high-temperature deformability and less occurrence of rolling defects even in the form of a continuously cast slab with a cast structure. We have found a stable duplex stainless steel hot-rolled steel strip that is stable and free from rolling defects even through hot coil rolling of continuously cast slabs, which is extremely advantageous in terms of work efficiency and manufacturing costs. They began research from various perspectives with the aim of establishing a means of manufacturing.

After repeating hot tensile tests, hot rolling tests, and actual prototype production, the following findings were obtained.

A) When a trace amount of Ca and B is added in combination to high Cr-containing duplex stainless steel and the components are finely adjusted, Ca
The effect of improving deformability from high temperature range to low temperature range due to the ingredients,
Coupled with the deformability improving effect in the medium temperature range due to component B,
A remarkable increase in the hot deformability of steel without any significant deterioration in corrosion resistance.

Figure 2 shows C: 0.015% (hereinafter, % representing component proportions is expressed as weight %), Si: 0.40%, M
n: 0.89%.

P: 0.023%, S: 0.0010%,
Cr: 25.2%, Ni near, 0%, Mo:
25.0%, W: 0.29%, Cu: 0.50
%.

Al1: 0.013%, N: 0.14% and O:
A steel containing 0.003% and the balance substantially consisting of Fe, and 0.0029% B and 0.0034% B.
of Ca, or 0.0030% B and 0.0036% C
A 17 kg flat steel ingot (48D thickness x 160m width x 195mm
This figure shows the hot tensile test results of a hot tensile (Greeple) test piece of 10 cranes φ diameter x 130 n length obtained by machining the crane length. Note that the hot tensile test was performed using Ar
First, the test piece was heated to 1200°C in an atmosphere, and the temperature was controlled by a thermocouple that was welded to the surface of the test piece. During cooling, the strain rate was approximately 1.
The hot deformability was evaluated by the cross-sectional reduction ratio after the fracture. The results shown in FIG. 2 also confirm that the hot ductility is significantly improved by the combined addition of trace amounts of B and Ca.

B) Furthermore, when a small amount of Ti is added in combination to the duplex stainless steel to which Ca and B have been added, the solidified structure due to the Ti component is refined, improving hot workability, toughness, and workability in a low temperature range. The functions are intertwined to improve hot rollability to such an extent that almost no rolling defects occur even when severe hot rolling is performed.

Figure 3 shows the tlm (B and Ca) where the results in Figure 1 were obtained.
46 mm thickness
After taking a hot-rolled test piece with a width of 00n and a length of 130vrx, it was soaked in an electric furnace at 1250°C for 1 hour and taken out of the furnace, and then rolling was started at 1150°C with the surface temperature of the test piece for 9 passes. (4611→40n+-3:3+m-=2
7m →22mm →17m+* →13mm-10u
+ → 8*m → 61M) to show the hot workability when finished to a thickness of 611 m. Note that the finishing bath was carried out at 850°C, and hot workability was evaluated by the "depth of edge cracking" after the test. The results shown in FIG. 3 also show that the combined addition of Ti+B and Ca significantly improves hot workability.

C) By carefully examining the behavior of the specific rolled material in the hot coil rolling process, we found that the strong reduction in the rough rolling roll group mainly induces surface defects, and the strong reduction in the finishing roll group mainly causes surface defects. It has become clear that this induces edge cracking, but in addition to using duplex stainless steel whose composition has been adjusted as shown in section B) above as the rolling material, the rough rolling roll group and finishing rolling roll group If the rolling reduction amount and rolling reduction rate are adjusted to specific ranges, and finish rolling is finished in a specific temperature range where the temperature does not decrease, even if a continuously cast slab with the cast structure is used as the rolled material, the surface To stably obtain a sound duplex stainless steel hot-rolled copper strip with almost no occurrence of defects.

This invention has been made based on the above findings, [duplex stainless steel, C: 0.03% or less, Si: 0.1 to 1.0]
%.

Mn: 0.3 to 2.0%, P: 0.035% or less.

S: 0.0020% or less, Cr: 23.5
~26.5%.

Ni: 6.0-8.0%, Mo: 2.5
~3.5%.

W: Q, 1-1.0%, Ti: 0.005-
0.10%.

B: 0.0005-0.0100%, Cu
: 0.2-2.0%.

Eight Il: 0.01-0.05%, C
a: 0.0005-0.0100%.

N: 0.08-0.2%, O: 0.006
Contains 0% or less, and further contains V:Q of 5% or less if necessary.

Nb: 0.5% or less, by configuring the composition to be composed of one or more of 0.5% or less, and the remainder being substantially Fe, the rolling bottom generation is reduced as much as possible. rc: 0.03% or less, Si: 0.1-1.0
%.

Mn: 0.3-2.0%, P: 0.035
%below.

S: 0.0020% or less, Cr: 23.5
~26.5%.

Ni: 6.0~8.0%, Mo: 2.5~
3.5%.

W: 0.1-1.0%, Ti: 0.005-0
．． 10%.

B: 0.0005-0.0100%, Cu:
0.2-2.0%.

81 7 0.01-0.05%, Ca
: 0.0005-0.0100%.

N: 0.08-0.2%, O: 0.006
Contains 0% or less, and further contains V:Q of 5% or less if necessary.

After soaking a continuous casting slab containing one or more of Nb: 0.5% or less and the remainder substantially consisting of Fe at 1200 to 1330°C, rolling with a group of rough rolling rolls per pass is performed. Reduction amount: 60
+n or less.

Rolling rate per pass: 40% or less, and rolling with the finishing roll group: rolling amount per pass: 121 m or less.

Reduction rate per pass = 35% or less.

It is also characterized by the fact that it is possible to stably produce duplex stainless steel with extremely few rolling bottoms by carrying out the rolling at a total rolling reduction of 90% or less and finishing the rolling at a temperature of 850°C or higher. It is something.

Here, the reason why the composition of the duplex stainless steel and the hot coil rolling conditions are limited as described above is as follows.

(A) Component composition (al C If the C content exceeds 0.03%, carbides will precipitate at grain boundaries, and corrosion resistance and toughness will tend to deteriorate. Therefore, the C content should be 0.03% or less. Established.

fb) 5t Si is an essential component for deoxidizing steel, and to ensure a sufficient deoxidizing effect, it must be contained at 0.1% or more, but if it is contained in excess of 1.0%, The S+ content was set at 0.1 to 1.0% since this would lead to deterioration of toughness.

(C) Mn Although the Mn component also has a deoxidizing effect like Si, if its content is less than 0.3%, the desired effect due to this effect cannot be obtained. On the other hand, Mn content up to 2.0% does not have any particular adverse effect on steel, so Mn content is 0.0%.
It was set at 3 to 2.0%.

+d) P P is an impurity element that deteriorates the hot workability and corrosion resistance of steel, but up to 0.035% is allowed. Based on the above, the P content was determined to be 0.035% or less.

(el SS S is an impurity element that significantly impairs the hot workability of steel,
If the content exceeds 0.0020%, desired hot rolling properties cannot be ensured. Therefore, the S content is 0
．． It is necessary to regulate it to 0.020% or less, but if possible, 0.
．． It is preferable to limit it to 0.010% or less.

(f) Cr Cr is an important component that improves the general corrosion resistance of steel,
In order to ensure the desired corrosion resistance and properties required for duplex stainless steel, it is necessary to contain 23.5% or more. On the other hand, if Cr content exceeds 26.5%, workability will be adversely affected, so the Cr content should be 2.
It was set at 3.5% to 26.5%.

(g) Ni The Ni component is essential for improving the mechanical properties, workability, and general corrosion resistance of steel, and for making the steel structure into an austenite-ferrite two-phase structure. If it is less than .0%, the above effects cannot be obtained;
Even if the Ni content exceeds 8.0%, the property improvement effect commensurate with the cost cannot be obtained, so the Ni content should be 6.0 to 8.0%.
It was set as 0%.

(h) M.

Since the Mo component has the effect of significantly improving the local corrosion resistance of steel, it is added together with Cr in corrosion-resistant alloys to further improve the corrosion resistance, but if the Mo content is less than 2.5%, the above effect will occur. On the other hand, 3
．． If the MO content exceeds 5%, not only will it not be possible to ensure an effect commensurate with the cost increase, but there is also concern that it will have a negative impact on processability, so the MO content should be 2.5 to 3%.
．． It was set at 5%.

(1) W The W component also has the effect of improving the local corrosion resistance of steel, but if its content is less than 0.1%, the desired effect due to the above effect cannot be obtained; on the other hand, if it exceeds 1.0%, Since it is difficult to obtain further corrosion resistance improvement effect even if the content is increased,
The W content was determined to be 0.1 to 1.0%.

(J) Ti Adding Ti to N-containing steel may cause TiN to precipitate in the molten steel and cause "submerged nozzle clogging" or "underground", so Ti is normally not added to N-containing steel. There was no. However, by adding a small amount of Ti to the steel according to the present invention, it does not cause "underground" or "submerged nozzle clogging", it refines the grains, improves hot workability, and improves corrosion resistance. This results in an improving effect. However, the Ti content is 0.005
If the content exceeds 0.10%, TiN will precipitate in the molten steel as described above, and if the content exceeds 0.10%, TiN will precipitate in the molten steel. The Ti content was determined to be 0.00% to 0.10%, since this would result in the formation of "underground".

(k) B The B component contains Ca and Ti to improve the hot workability of steel.
However, if the content is less than o, oos%, the effect of improving hot workability is insufficient;
If S content exceeds 0.00%, melt embrittlement will be promoted and hot workability will be adversely affected.
0005 to 0.0100%.

(1) Cu Although the Cu component has the effect of improving the corrosion resistance of steel, if the content is less than 0.2%, the desired effect due to the above effect cannot be obtained; on the other hand, if the content exceeds 2.0% Since no further corrosion resistance improvement effect can be obtained even if the Cu content is 0.
．． It was set at 2 to 2.0%.

(m) Al Al is also essential as a deoxidizing component, and in order to ensure a sufficient deoxidizing effect, it must be contained at 0.01% or more, but if it is contained in excess of 0.05%, the toughness The AN content should be 0.01 to 0.0.
It was set at 0.5%.

(n) Ca Ca component has the effect of fixing S in steel as sulfide and improving hot workability, and when added together with B and Ti, the effect of improving hot workability becomes even more pronounced. However, if the content is less than 0.0005%, the desired effect due to the above action cannot be obtained, while if the content exceeds o, oiooo%, hot workability and corrosion resistance will deteriorate. Therefore, the Ca content is 0.0005 to 0.0
It was set as 100%.

(0)N The N component has the effect of improving the pitting corrosion resistance and crevice corrosion resistance not only of the base metal but also of the welded part, but its content is 0.0
If it is less than 8%, the desired effect cannot be obtained in the above action;
If the N content exceeds 0.2%, nitrides tend to precipitate, which adversely affects corrosion resistance and toughness, so the N content was set at 0.08 to 0.2%.

(p) 0 0 is an impurity element that adversely affects the hot workability and toughness of steel, but if its content is less than 0.006%, the above-mentioned adverse effects will not be so pronounced. 0
．． It was limited to less than 0.006%.

(ql V, and Nb) These components both have the effect of improving hot workability by refining crystal grains and improving corrosion resistance, so either one or both of them can be used as necessary. Although it is added as a composite, in any case, if the content exceeds 0.5%, it will have a negative effect on toughness, so the content and Nb content are both set at 0.5% or less. Ta.

(B) Hot coil rolling conditions (a) Heating temperature If the slab heating temperature during rolling is less than 1200°C, hot workability will deteriorate, while if heated above 1330°C, "slab sag" in the furnace will occur. The heating temperature is set at 120° C. to reduce manufacturing efficiency.
It was set as 0 to 1330℃, but preferably 1230 to 128℃.
It is preferable to adjust the temperature to 0°C.

(b) Rolling conditions for the rough rolling roll group i) Reduction amount per bath Since applying a rolling reduction larger than 6011 per bath mainly tends to cause surface defects, the rolling conditions for the rough rolling roll group per bus The reduction amount was limited to 60 cranes or less.

1i) Reduction rate per bath ■ If a reduction greater than 40% per bath is applied, surface defects are likely to occur, so the reduction rate per bath in the rough rolling roll group is limited to 40 mm or less. did.

(C) Rolling conditions in the finishing roll group i) Reduction amount per bath The material to be rolled in the finishing rolling roll group has become thinner and the temperature at the edge has decreased to 950°C or less. . Therefore, 1
Since applying a rolling reduction greater than 120 per bus tends to cause edge cracks and surface flaws, the rolling reduction per bus in the finishing roll group was limited to 12w1 or less.

1i) Rolling reduction rate per bath ■ If a larger reduction than 35% is applied per bath, edge cracking is likely to occur, so the rolling reduction rate per bath is set to 35%.
Limited to the following.

iii) Total Reduction Ratio During rolling with the finishing roll group, 6 to 7 baths of reduction are applied continuously, and the time required for the part of the steel plate to enter the finishing roll group and come out is at least about It is 10 seconds. However, in the case of duplex stainless steel, the conditions are severe in that the recovery and recrystallization of the austenite phase hardly progresses during this rolling, so the deformability has a value comparable to the "total reduction in finish rolling". It will be requested as is. Of course, in the present invention, extremely high deformability and workability can be ensured by appropriately setting the steel composition, heating temperature, etc., but under a total reduction rate of over 90%, edge cracks and surface flaws occur. Since this cannot be prevented, the total rolling reduction rate was limited to 90% or less.

(dl Finish rolling temperature If the finish rolling temperature is lower than 850℃, the precipitation of carbonitrides or intermetallic compounds will be promoted and the toughness will be impaired, and edge cracks and surface defects will become noticeable. Therefore, the finish rolling temperature should be set to 850℃.
It was limited to temperatures above ℃.

Next, the present invention will be described in more detail using Examples and in comparison with Comparative Examples.

<Example> First, various duplex stainless steels having the compositions shown in Table 1 were melted using a conventional method, and slabs having the thickness shown in Table 2 were obtained by continuous casting.

Next, after heating these slabs, they were subjected to rough rolling as they were, or some of them were subjected to rough rolling, and then subjected to true coil rolling under the roll arrangement shown in FIG. 1 to produce hot rolled steel strips. The hot coil rolling conditions were as shown in Table 2.

The hot-rolled steel strip thus obtained was examined for surface flaws, and the maximum value of edge cracks that occurred was measured, and the results are also shown in Table 2.

The condition of surface flaws was investigated both at the center and at the edges of the copper strip.○...No cracks, △...The maximum crack depth was 0.0 m or more and less than 0.1 m.

×: The maximum crack depth was divided into 0.1 m and 1 m or more and displayed.

As is clear from the results shown in Table 2, the hot-rolled duplex stainless steel sheets manufactured according to the conditions specified in the present invention do not generate surface defects even when made from continuous cast slabs. While the edge cracking never exceeds 101, products that do not meet the conditions stipulated in the present invention show surface flaws and a large degree of edge cracking, making the product practically unsatisfactory. I can see that there isn't.

<Summary of Effects> As explained above (according to the present invention, even when continuous casting slabs are used as raw materials, a sound steel strip of high Cr-containing duplex stainless steel with almost no rolling defects is hot coiled. It brings industrially useful effects, such as the ability to produce stable, highly efficient rolling products, and the ability to provide duplex stainless steel sheets of stable quality, which are essential for corrosion-resistant components, at low cost. .

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram showing an example of roll arrangement in an in-shell hot coil rolling facility. FIG. 2 is a graph showing the hot tensile test results of 25Cr duplex stainless steel, comparing those without B or Ca additives and those with these added. FIG. 3 is a graph showing the influence of the amount of Ti added on the hot rolling properties of 25Cr duplex stainless steel. In the drawings, R7-Rb...Rough rolling roll group. F, ~F, ... finishing roll group. VSB...Scale breaker-1 DC...Down coiler.

Claims (4)

[Claims] (1) C: 0.03% or less, Si: 0.1-1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Cr: 23.5-26.5%, Ni: 6
．． 0-8.0%, Mo: 2.5-3.5%, W: 0.1
~1.0%, Ti:0.005~0.10%, B:0.
0005-0.0100%, Cu: 0.2-2.0%,
Al: 0.01-0.05%, Ca: 0.0005-0
．． 0100%, N: 0.08-0.2%, O: 0.00
A duplex stainless steel with less occurrence of rolling defects, characterized in that the stainless steel contains 60% or less of Fe, and the remainder is substantially made of Fe. (2) C: 0.03% or less, Si: 0.1 to 1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Cr: 23.5-26.5%, Ni: 6
．． 0-8.0%, Mo: 2.5-3.5%, W: 0.1
~1.0%, Ti:0.005~0.10%, B:0.
0005-0.0100%, Cu: 0.2-2.0%,
Al: 0.01-0.05%, Ca: 0.0005-0
．． 0100%, N: 0.08-0.2%, O: 0.00
60% or less, and further contains one or more of V: 0.5% or less and Nb: 0.5% or less, and the remainder is substantially composed of Fe. Duplex stainless steel with low generation. (3) C: 0.03% or less, Si: 0.1-1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Cr: 23.5-26.5%, Ni: 6
．． 0-8.0%, Mo: 2.5-3.5%, W: 0.1
~1.0%, Ti:0.005~0.10%, B:0.
0005-0.0100%, Cu: 0.2-2.0%,
Al: 0.01-0.05%, Ca: 0.0005-0
．． 0100%, N: 0.08-0.2%, O: 0.00
After soaking a continuous casting slab containing 60% or less and the remainder substantially consisting of Fe to 1200 to 1330°C,
Rolling with the rough rolling roll group is carried out under the following conditions: Reduction amount per pass: 60 mm or less, Reduction ratio per pass: 40% or less, and rolling with the finishing roll group is performed under the conditions of Reduction amount per pass: 12 mm. The following describes a duplex stainless steel with less occurrence of rolling defects, which is carried out under the conditions of a rolling reduction rate of 35% or less per pass, a total rolling reduction rate of 90% or less, and finishing the rolling at 850°C or higher. Production method. (4) C: 0.03% or less, Si: 0.1 to 1.0%, Mn:
0.3-2.0%, P: 0.035% or less, S: 0.0
020% or less, Cr: 23.5-26.5%, Ni: 6
．． 0-8.0%, Mo: 2.5-3.5%, W: 0.1
~1.0%, Ti:0.005~0.10%, B:0.
0005-0.0100%, Cu: 0.2-2.0%,
Al: 0.01-0.05%, Ca: 0.0005-0
．． 0100%, N: 0.08-0.2%, O: 0.00
A continuously cast slab containing 60% or less and also one or more of V: 0.5% or less and Nb: 0.5% or less, with the balance substantially consisting of Fe, is heated at 1200 to 1330°C. After soaking, rolling with a group of rough rolling rolls is carried out under the following conditions: rolling reduction amount per pass: 60 mm or less, rolling ratio per pass: 40% or less, and rolling with a group of finishing rolling rolls is carried out in 1 pass. The rolling reduction method is carried out under the following conditions: rolling reduction per pass: 12 mm or less, rolling reduction per pass: 35% or less, total rolling reduction: 90% or less, and finishing the rolling at 850°C or higher. A method of manufacturing less duplex stainless steel.