JPH04110419A - Production of high ni stainless steel plate - Google Patents

Abstract

PURPOSE:To produce a high Ni stainless steel plate minimal in cast-slab crack, edge crack, and inclusion defects by subjecting a cast slab which has a specific composition consisting of Si, Mn, Cr, Ni, Mo, Cu, Al, N, Mg, and Fe and in which impurities are limited to specific hot rolling. CONSTITUTION:A cast slab having a composition which consists of <=2% Si, <=2% Mn, 15-26% Cr, 15-45% Ni, <=5.0% Mo, <=2.0% Cu, 0.005-0.10% Al, <=0.25% N, 0.0005-0.0045% Mg, and the balance Fe with inevitable impurities and further contains, if necessary, one or more kinds among <=1% Ti, <=1% Nb, <=2% V, <=2% W, and <0.01% B and in which the contents of C, P, S, and O among the impurities are limited to <=0.04%, <=0.03%, <=0.001%, and <=0.006%, respectively, is subjected to hot rolling where rolling is finished at <=900 deg.C. By this method, the high Ni stainless steel reduced in cracking at the time of casting even if continuous casting method is applied, remarkably reduced in the occurrence of edge crack at the time of hot rolling and accordingly improved in yield, and further reduced in the occurrence of defects which are caused-by inclusions and become problems at the time of welding can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a high-Ni stainless steel plate, in particular, a continuously cast slab of stainless steel containing a large amount of Ni is formed into a steel plate by hot rolling. It is a method that reduces the occurrence of cast side deviation during continuous casting (and reduces L during hot rolling).
The present invention relates to a method for producing a high-Ni stainless steel sheet that has less occurrence of edge cracks, has a good yield, and has fewer inclusion defects.

(Prior art) Stainless steel containing a large amount of Ni is highly resistant to pitting corrosion, crevice corrosion, and stress corrosion in humid environments containing hydrogen sulfide and chlorides. High/! It has high resistance to 11 corrosion and υ+IM creep, and is widely used for oil well piping, chemical manufacturing equipment, heat-resistant +, I rice, etc.

On the other hand, stainless steel Igl containing a large amount of Ni such as this
In addition to the high cost of raw materials, compared to the continuous casting method (hereinafter referred to as the continuous casting method), ! ! Steel ingot method with many culms and low yield (method of casting as an ingon, then going through an intermediate process such as blooming rolling to form a slab, and then hot working this)
It is a very expensive material because it is often manufactured using the same method, and because it tends to generate flaws during hot processing, resulting in a decrease in yield.

The biggest problem in manufacturing these stainless steel products is steel ingots and continuous casting slabs (hereinafter referred to as
This is a hot working process for continuous casting slabs. In the case of stainless steel with poor hot workability, large cracks will occur if a large amount of processing is applied to the process, making subsequent processing and product collection impossible. To avoid this, the steel ingot method is usually adopted. First, the steel is cast into a steel ingot and then rolled into an ingot, which is then processed in stages to form products such as steel plates.

Another reason why the ingot method, which is less cost-effective than the continuous casting method, is often adopted is that the continuous casting method is prone to solidification cracking during continuous casting, and it is difficult to obtain a healthy slab. is difficult. In other words, stainless steel containing a large amount of Ni has an austenitic structure at room temperature, and during solidification, it often solidifies with austenite as the initial material, but during this solidification, low melting point eutectic compounds are formed. Elements such as P and S tend to solidify and segregate, and high 11i stainless steel I
This is one of the important causes of increasing the solidification cracking susceptibility of steel.

In the past, advanced methods have been proposed for improving the hot workability of such high Ni stainless steel sheet alloys. Among them, the following tl i % alloys or stainless steels have been proposed, and it has been reported that improved effects can be obtained.

(A) B, Zr, and Ce are added in combination, and the impurity elements ○ (oxygen) and N (nitrogen) are fixed by Zr and Ce, making effective use of the two effects of B's hot workability. Ni-based heat-resistant alloy with improved workability
-3], Publication No. 345).

(B) Stainless steel with improved hot workability and pitting corrosion resistance by adding i and Ca in combination.

(C) S, 0 was reduced, Ca and Ce were added, and each component was expressed in ppm [S-1-0-0,
A high-alloy stainless steel with excellent hot workability that makes it possible to apply the continuous casting method by lowering the 8Ca-0, 3Ca1 formula to 401 degrees and suppressing solidification segregation by utilizing delta ferrite. Steel (Special Publication No. 2-1441.9).

(Problem to be solved by the invention) Ni 4% heat-resistant alloy of (^) illustrated in the previous section and (B)
Although the hot workability of stainless steel has been improved,
L: ↓No improvement has been made to the extent that these steel plates can be manufactured by applying the continuous casting method.

On the other hand, regarding the high alloy stainless steel (C), a continuous casting method is applied to form a hot rolled steel plate, and then cold rolled 1. If welding is then performed to form a shape such as a vibrator, there is a problem in that welding defects may occur due to coarse inclusions.

” If the hot workability of two-part single-base alloys and stainless steels can be sufficiently improved and the occurrence of hot working defects in continuous casting methods can be reduced, it will be possible to adopt continuous casting methods, and high Ni austenitic stainless steel products can be obtained with a high yield.

Even when the continuous casting method is applied, the present invention reduces the occurrence of slab cracks during casting, reduces the occurrence of edge cracks during hot rolling, and has a good yield, and further eliminates weld defects in the subsequent welding process. It is an object of the present invention to provide a method for manufacturing a high-Ni stainless steel sheet with fewer inclusion defects that may cause the defects.

(Means for solving the problem) The hot workability of stainless steel is due to S, which exists in the steel.
It is thought that impurity elements such as O have an adverse effect on the hot workability of the steel. The general idea is that it can improve Elements that can fix impurity elements as sulfides and oxides include CaXCe, Mg, Zr, Ti,...
) In, Si, Aff, and other elements are known.

On the other hand, slab cracks during continuous casting of stainless steel include surface vertical cracks, minute breakouts, center segregation cracks, etc., which are caused by a decrease in ductility during slab solidification and an increase in the stress acting on the slab during casting. It is more likely to occur.

The ductility of steel during solidification is reduced by the presence of elements that lower the melting point. In the solid-liquid two-phase coexistence temperature range where solidification is progressing, elements that lower the melting point will segregate and concentrate in the liquid phase, lowering the melting point of the final solidified portion. When stress is applied to a slab that has not completely solidified, cracks are likely to occur along the final solidified part. Elements that have this kind of action include P, S, Ti, Nb, Cu, and C.
, , B, etc. are known.

The present inventors used such general knowledge as a guideline, and 3! ! Even if the casting method is applied, there is little occurrence of slab cracking during casting, and the hot workability is good, there is no occurrence of edge cracking during hot rolling, and the yield is good, and furthermore, it is possible to eliminate the cause of defects in the subsequent welding process. As a result of repeated research aimed at establishing a manufacturing method for high-rise stainless steel sheets with fewer inclusion defects, the following new findings were obtained.

(a) Inclusion defects that cause welding defects are
Inclusion defects tend to occur when strong deoxidizing and desulfurizing elements such as Ca and Ce are added, but almost no inclusion defects are observed when Ca and Ce are added.

(b) 0.01% by weight of each of S, O, and P
Below, solidification cracking susceptibility is significantly improved by containing 0.0005 to 0.0055% of 1 in addition to 0.006% or less and 0.03% or less. "%" means weight ■%).

Figure 1 shows S:O,001%, 0:0.003%, P
: Test results of hot cracking susceptibility by longitudinal Hallestrain test (strain], 5%) of 20Cr 2ONi stainless steel ingot (as solidified) containing 0.015%.
It can be seen that the susceptibility to cracking caused by the total solidification crack length is significantly reduced when the amount of M (containing T) is in the range of 5 to 55 ppm.

(C) Hot stress of high Ni stainless steel with solidified Mi weave [1 workability is 0.001% each with S10 content]
, to 0.006% or less, containing 5 to 45 ppm of xg, and further completing the hot rolling at a high temperature of 900°C or higher.

It has been known that when the content of S, 0 is large, the hot stress IIT properties of high Ni stainless steel containing a large amount of I[] deteriorate. However, S and 0 are each 0.0
No research has been found that examines in detail the influence of the Mg addition amount on high-Ni stainless steels with Ni content of 0.01% or less or 0.006% or less. The present inventors found that the S and ○ contents were 0001% and 0.006%, respectively (8ρ0, 006~
0.015%) or less in high-rise stainless steel with a small amount of 11F
, was added, a hot rolling test was carried out, and the hot ductility (evaluated by the reduction of area) was evaluated by a bow tension test for 8 rounds.

FIG. 2 is a graph showing the results. As seen in this figure, by adding Mg, the temperature
Although hot ductility is poor in the temperature range of 800°C to 900°C, it
The hot ductility begins to decrease in the temperature range of M[(5
It has been revealed that in steel containing up to 45 ppm, hot ductility is improved and edge cracking can be prevented by completing hot rolling at 900"C or higher.

(d) ,, ]: As a result of applying the results of (a) to (C) to actual equipment, it was found that solidification cracking in continuous casting and rubs and edge cracking during hot rolling were suppressed within the allowable range, and that no interference was observed. It was confirmed that it is possible to obtain high-rise stainless steel sheets with few defects.

The present invention has been made based on the above findings, and its gist lies in the following methods for producing high-Ni stainless steel sheets (1) and (2).

■ Si: 2% or less, Mn: 2% or less, Cr: 15
~26%, N port] 5-45%, ) lo: 5.0% or less, Cu: 2.0% or less, 8ff: o, 005-0.1
0%, N・0.25% or less, 'Kg: 0.0005
~0.0045%, the remainder consists of FG and unavoidable impurities, C in the impurities is 0.04% or less, P is 0.
,03% or less, S is 0.001% or less, 0 is 0.006
% or less, a method for producing a high Ni stainless steel sheet, characterized in that hot rolling is completed at 900° C. or higher.

(2) In addition to the ingredients described in (1) above, Ti is 11% or less, Nb: 1% or less, ■, 2% or less, W-2% or less, r
t: o, contains less than ot% of one or more of the following, the remainder consists of Fe and unavoidable impurities, and C in the impurities is 0.
．． 04% or less, P 0.03% or less, S 0.001%
Hereinafter, a cylinder N i characterized by subjecting a slab containing O to 0.006% or less to hot rolling that ends at 900°C or higher.
Method of manufacturing stainless steel plate.

(Function) First, the function and effect of each component contained in the high Ni stainless steel sheet produced by the method of the present invention and the reason for limiting their content will be described.

Both Si and Mn are elements used as deoxidizing agents during the production of steel, but excessive addition deteriorates the toughness of Steel 1. Therefore, their content was determined to be 2% or less.

Cr is an important element that improves the general corrosion resistance of steel, and must be contained in an amount of 15% or more to ensure the desired corrosion resistance required for stainless steel. However, if Cr content exceeds 26%, the strength, workability, and weldability deteriorate, so the Cr content was set at 15 to 26%.

Ni improves the mechanical properties, workability, and general corrosion resistance of steel, and is an essential component for making the steel structure austenitic. However, if its content is less than 15%, its effect is not sufficient. On the other hand, even if the content exceeds 45%, an improvement effect commensurate with -1 s[.

Since Mo has the effect of significantly improving the local corrosion resistance of steel, it is added as necessary. In this case, if the content exceeds 5%, not only will it not be possible to ensure an effect commensurate with the cost increase, but it will also promote the precipitation of sulfur at high temperatures and have a negative impact on corrosion resistance and toughness. Due to concerns, the content has been set at 315% or less.

Cu has the effect of improving general corrosion resistance and corrosion resistance (Improving corrosion resistance 11) of steel, and increases speaking force H when it is necessary to further improve the corrosion distribution property of stainless steel. However, 2%
If the content exceeds 2%, the susceptibility to solidification cracking increases, so the content was set at 2% or less.

A℃ is an element that must be added for deoxidation, but if its content is less than 0.005%, the 0 content will be reduced to 0.00
It is not possible to reduce the amount to 6% or less, and it is not possible to provide the excellent hot workability (') that is the objective of the present invention.

On the other hand, if the content exceeds 0.10%, it will cause a decrease in reverse Q2 toughness! The content is 0.005-0.
It was set at 10%.

N increases the speaking power when it is necessary to increase the strength and monotony. However, if the content exceeds 0.25%, the deformation resistance at both temperatures increases and the hot workability deteriorates, so that the method of the present invention is not applied. However, since the occurrence of cracks during hot rolling cannot be prevented, the content was set at 0.25% or less.

Ag is an important element in the method of the present invention, and is added to maintain good hot workability, castability during continuous casting, and inclusion morphology. By adding ugliness, Mg
Fine sulfides and oxides such as S, Mg (+-A f 20.) are generated in the steel, fixing the S and ○ in the steel and improving hot workability and solidification cracking susceptibility. If the content of hg is less than 5 ppm, hot workability and castability cannot be maintained within the good range targeted by the present invention, while if the content exceeds 5 ppm, the ℃
Even if hot rolling is completed in the above temperature range, the occurrence of edge cracking cannot be prevented. Furthermore, excessive talking power [1] of Mg significantly reduces hot ductility in the temperature range of 800 to 900°C. In addition, if the MP content is within the range of 5 to 45 ppm, the morphology of inclusions is in good condition and will not cause welding defects, so the I content should be 5 ppm or more and 45 ppm or less. It was determined that

The invention (2) above is a high-Ni stainless steel produced by hot rolling a cast slab consisting of the above-mentioned components and the remainder consisting of Fe and unavoidable impurities. Impurities include C and P.

It is important to suppress the two limits of S and 0.

It is preferable that C be as small as possible. C content is 0.0
When it exceeds 4%, carbides tend to precipitate at grain boundaries, resulting in deterioration of intergranular corrosion resistance.

P is an element that increases susceptibility to solidification cracking, and its content was set to 0.03% or less in order to prevent cracking of the continuously cast slab.

S and 0 are known to be elements that impair the hot workability of steel, but in order to sufficiently improve the workability when hot working steel with a solidified structure, their content should be adjusted to 0.
It is necessary to limit it to 0.001% or less and 0.006% or less.

Invention (2) above, in addition to the components of the high Ni stainless steel sheet produced by invention (2), further comprises:
■Contains one or more of W and B, with the remainder being Fe.
High-Ni steel is hot-rolled on slabs containing unavoidable impurities.
This is a method for manufacturing stainless steel sheets.

Ti and Ni1 form carbonitrides in steel and form solid solution C1N.
In addition to fixing the metal, it also improves corrosion resistance by solid solution in the steel. However, if added in large amounts, the susceptibility to solidification cracking increases, so the content was set to 1% or less, respectively.

(2) and W are added together with Cr and Mo because they have the effect of improving the local corrosion resistance of stainless steel.

However, excessive addition of (2) causes deterioration of toughness, and since W is expensive, the content thereof was set to 2% or less in both cases.

B is sometimes added to improve the hot workability and creep properties of steel, but on the other hand, it tends to increase susceptibility to solidification cracking, so its content was set to less than 0.01%.

Next, the hot rolling conditions used in the method of the present invention will be described.

The reason for hot rolling a slab of high Ni stainless steel containing each of the components listed in -1 and specifying the finishing temperature as 900°C or higher is because the method of the present invention This is because it is characterized by improved hot workability.

In other words, steel containing 5 to 45 ppm of h is 1000
This is because, although it has good hot workability in the temperature range of 900 to ROO<0>C, the hot ductility tends to decrease in the temperature range of 900 to ROO<0>C due to the addition of h. By keeping the content of molten g within the above range and completing hot rolling at 900°C or higher, edge cracking can be reduced to within the allowable range even when using slabs with coagulated thread [ ] weave. can.

(Example) In a test vacuum melting furnace, high Ni stainless steel 1 (No. 1 to 8) having the chemical composition shown in Table 1 was melted into slabs, and the solidification crackability, hot workability, and welding were evaluated. (presence or absence of welding defects). Note that No. 9 is high Ni stainless steel that was melted in the actual machine.

Regarding the slabs No. 1 to 8, the solidification cracking property was investigated by a vertical Harestrain test (strain 1.5%). The test piece used was a 9 m thick specimen taken from two cast slabs.
Immediately after the TlG lick welding was interrupted, a bending radius of 300 mm was applied to this test piece, and evaluation was made based on the total length of the solidification crack that occurred.

The investigation of hot separation properties was carried out by measuring the reduction ratio through a high-temperature tensile test and investigating the occurrence of edge cracking through a hot rolling test. A tensile test piece with a diameter of 10 mm taken from the above slab was used as the test piece for measuring the reduction ratio, and was heated at 900°C.
and the strain rate is 1. Deformation was given at /s.

The material for the hot rolling test was the above slab 44mm x 10
Machined to dimensions of 5mm x 135mm, 1
After heating to 250°C for 1 hour, rolling was started at 1]50°C and finished to a thickness of 6mm in 7 passes. The finishing temperature (hot rolling end temperature) at this time was 8 as shown in Table 2.
50~]050°C.

Weldability investigation (presence or absence of welding defects) was carried out by hot rolling the above material at a finishing temperature of 950°C to make a 6 mm thick steel plate, annealing this steel plate, and cold rolling it into a 0.5 mm thin plate. After that, a 1 m long bead was placed on the thin plate 2 by TIG welding, and the heat shape was observed to investigate the presence or absence of l/-tar-shaped welding defects caused by large inclusions.

The survey results are shown in Tables 1 and 2.

If the "reduction ratio" in Table 1 is 70% or more, it can be considered that the hot ductility is good. "Solidification cracks" are those with a total solidification crack length of less than 0.5 mm, ○, 0.5 mm.
The following two items are indicated as ×. In addition, for "welding defects", those without crater-shaped welding defects were rated as ○, and those with crater-like welding defects were rated as ×. For "edge cracking" in Table 2, the maximum depth of left and right edges of a hot-rolled steel plate was measured, and those where the sum of the left and right sides was less than 100 nm were marked as ○, and those exceeding 1 mm were marked as ×.

According to the investigation results shown in Tables 1 and 2, all the high Ni stainless steel slabs used in the method of the present invention have low susceptibility to solidification cracking and have a drawing ratio of 0% or less at 900°C. It is clear that edge cracking can be significantly reduced by performing IH at 90(]]°C or higher.

No. As for the 9 straight (4H stainless steel), a continuous cast slab with a thickness of 200 mm was inspected for defects after cleaning with a 2 mm deep grinder, and as a result, there were no cracks in the slab. A steel plate with a thickness of 6 mm was obtained by hot rolling at a finishing temperature of 935°C, and no edge cracking occurred.Also, a thin plate with a thickness of 0.5 mm obtained by cold rolling this steel plate was During welding, no welding defects caused by inclusions were observed.

The reduction ratio was measured in the same manner as in the case of the slab described above, and good results were obtained.

(Hereinafter, the margins) (mlJJ) ;'￥道い１い (Effects of the invention) According to the method of the present invention, even if a continuous casting method is applied, the occurrence of cracks during casting is small, and The yield rate is good with very little edge cracking during rolling, and there are also fewer defects caused by inclusions that cause problems during welding.
It is possible to obtain a 1i stainless steel plate. As a result, a high-Ni stainless steel plate having good properties can be supplied at a low cost.

[Brief explanation of drawings]

Figure 1 shows the solidification cracking susceptibility and mg of high Ni stainless steel.
It is a graph showing the relationship between contents. FIG. 2 is a graph showing the temperature dependence of the reduction ratio and the influence of the h content in a hot tensile test of high Ni stainless steel.

Claims (2)

[Claims] (1) In weight%, Si: 2% or less, Mn: 2% or less, C
r: 15-26%, Ni: 15-45%, Mo: 5.0
% or less, Cu: 2.0% or less, Al: 0.005 to 0.
10%, N: 0.25% or less, Mg: 0.0005-0
．． 0.045%, the remainder consists of Fe and unavoidable impurities, C in the impurities is 0.04% or less, P is 0.0
3% or less, S is 0.001% or less, and O is 0.006% or less. A method for producing a high-Ni stainless steel sheet, which comprises subjecting a slab containing S to 0.001% or less and O to 0.006% or less to hot rolling that ends at 900° C. or higher. (2) In addition to the components described in claim (1), furthermore, % by weight
and Ti: 1% or less, Nb: 1% or less, V: 2% or less,
Contains one or more of W: 2% or less, B: less than 0.01%, the remainder consists of Fe and unavoidable impurities, C in the impurities is 0.04% or less, P is 0.03% or less, S is 0
．． 0.001% or less, O is 0.006% or less,
A method for producing a high-Ni stainless steel sheet, comprising hot rolling that ends at 900°C or higher.