JP2010059541A - Ferritic-austenitic stainless steel having excellent ingot crack resistance and workability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic-austenitic stainless steel having excellent ingot crack resistance and workability with which cracks in the surface of an ingot is solved by regulating the composition of the steel and performing controlled cooling in a solidification-cooling process, and further, the cold rolled steel sheet to be obtained has workability, and to provide a method for producing the same. <P>SOLUTION: The ferritic-austenitic stainless steel having excellent ingot crack resistance and workability has a composition comprising, by mass, ≤0.10% C, ≤2.0% Si, ≤4.0% Mn, <0.050% P, <0.010% S, 17 to 25% Cr, 0.6 to 5.0% Ni and 0.01 to <0.15% N, and the balance iron with inevitable impurities, and in which DF value is controlled to ≤70. The method for producing the steel is characterized in that the total time in the range of 1,100 to 900°C upon cooling of an ingot is controlled to ≥40 min by continuous casting so as to control the ratio of the ferritic phase at ordinary temperature to 35 to 70%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  Stainless steel can be broadly classified into austenitic stainless steel, ferritic stainless steel, and two-phase (ferrite / austenite) stainless steel. Austenitic stainless steel contains 7% or more of Ni, and many steel types are excellent in formability. Ferritic stainless steel contains little Ni and generally has a much lower formability than austenitic stainless steel. On the other hand, two-phase (ferrite / austenite) stainless steels have so far many steel types that have an intermediate position between austenitic stainless steels and ferritic stainless steels in terms of formability and corrosion resistance. However, in recent years, a technology having formability close to that of austenitic stainless steel has also been developed in ferrite-austenitic stainless steel by utilizing the work-induced martensitic transformation (hereinafter referred to as TRIP phenomenon) of the austenitic phase during plastic working. Patent Document 1 describes a technique in which the tensile elongation at break is increased by the TRIP phenomenon using a stainless steel containing a retained austenite phase whose main constituent phase is a ferrite phase. Patent Document 2 describes a method of increasing the tensile elongation by defining the stability of the austenite phase. Patent Document 3 discloses a technique for increasing the total elongation in a tensile test by defining the austenite phase fraction and the amounts of C and N in the austenite phase. However, in any case, there is knowledge to improve the ductility, but there is no description of ingot placement cracks and low toughness, which is a major issue in the manufacture of ferritic / austenitic stainless steels. Even if a high ferritic / austenitic stainless steel is obtained, a high yield cannot be expected.

  In ferrite and austenitic stainless steels, slab cracking and low toughness are problematic because of the formation of the structure. In general, ferrite-austenitic stainless steel is known to exhibit a tendency that a large amount of ferrite-forming elements such as Cr are contained in the ferrite phase because the elements are distributed in each of the ferrite phase and the austenite phase. In particular, ferritic stainless steel containing a large amount of Cr is recognized as a problem at the time of production due to poor toughness, and the same phenomenon occurs when the ferrite phase of ferrite-austenitic stainless steel contains a large amount of Cr. It can be estimated easily.

  In the case of ferritic stainless steel with a Cr content of 14% or less, the transition temperature is low, so the slab temperature transitions even when the slab is piled up and the insulation cover is applied with a heat insulating material, even if the time from cutting to insertion of the furnace is long Since the temperature does not become lower than the temperature, the ingot is not cracked by residual stress.

  As described above, the high Cr ferritic stainless steel exceeding 20% has low toughness and a transition temperature of about 400 to 500 ° C. When the slab temperature of high Cr ferritic stainless steel falls below the transition temperature between slab cutting and heating furnace insertion, there is a problem that transverse cracks occur in the slab.

  The cause of transverse cracking is internal stress generated in the slab during the cooling process. That is, the continuous cast slab is sprayed with spray water until the solidification is completed from the lower part of the mold to promote the solidification, but a large internal stress is generated by this cooling. Further, since the wall thickness is as thin as about 200 mm, the temperature is likely to decrease, and steel with poor toughness is very disadvantageous for cracking of the slab. Large transverse cracks are thought to be generated by thermal stress starting from some defects when the toughness of the slab is poor. Once transverse cracking occurs, the slab may fall out of the conveying device in the heating furnace, making extraction impossible, and the effect on the heating furnace operation is significant.

  Therefore, as a method for suppressing the temperature decrease, Patent Document 4 discloses a method in which a continuous casting slab is heated by hot ingot direct feeding and hot-rolled for high-purity ferritic stainless steel containing 10 to 30% Cr. ing. The transition temperature of this ferritic stainless steel is described to be about 300 ° C., but when it contains 25% or more of high Cr, it is estimated that the transition temperature is 300 ° C. or more. In order to prevent the temperature drop of the slab after cutting, it must be inserted into a large-capacity heat-retaining furnace, and expensive equipment is required.

  Therefore, Patent Document 5 describes a method for reducing the transverse cracking sensitivity by setting the transition temperature to 300 ° C. or less by setting the Nb content and the C + N amount to 0.015% or less so as not to require a heat insulation furnace. This method is effective for ferritic stainless steels whose composition can be accurately grasped, but in ferritic and austenitic stainless steels, the composition of the ferrite phase changes with changes in temperature during manufacturing, so it completely prevents transverse cracking. I couldn't.

JP-A-10-219407 JP-A-11-71643 JP 2006-169622 A Japanese Examined Patent Publication No. 61-44121 JP-A-8-253813

  In view of the above problems, in the present invention, ferritic and austenitic stainless steels with excellent ingot crack resistance and workability that prevent the ingot cracking that occurs in the process of uniform elongation and solidification cooling, which is important for actual formability. It aims at providing steel and its manufacturing method.

In order to investigate the factors that achieve both uniform elongation and ingot cracking resistance, the present inventors have melted various ferritic and austenitic stainless steels in the laboratory to change the solidification rate. Casted and hot rolled. Thereafter, annealing, cold rolling and annealing were performed to manufacture a thin steel plate. The number of cracks on the surface of the obtained slab was measured and the structure of the cracks was observed. Moreover, as a result of implementing a tensile test using the manufactured thin steel piece and measuring uniform elongation, the following knowledge (a) to (c) was obtained.
(A) The crack of the slab breaks up the ferrite phase or progresses along the interface between the ferrite phase and the austenite phase.
(B) There is a range in which cracks do not occur depending on the combination of compositions.
(C) A uniform elongation is 30% or more in a composition in which no cracks occur.

Details are described below. Several types of ferrite and austenitic stainless steels with different chemical compositions were melted in the laboratory. At that time, the cooling rate was changed by changing the material of the mold or putting it in a heat-retaining furnace. The relationship between temperature and time was recorded. The surface scale of the obtained slab was dissolved and removed with an HF / HNO ₃ solution, the number of cracks observed on the surface was measured, and the amount of ferrite was measured using a ferrite meter. Then, it grind | polished until there was no crack and implemented hot rolling, and produced the thin steel piece of thickness 1mm by annealing, cold rolling, and annealing. The obtained thin steel plate was processed into a JIS No. 13 B test piece, a tensile test was performed, and uniform elongation was measured.

As a result, it has been found that there is a combination of components in which cracks are not generated on the surface of the slab, and the DF values constituting these relationships have been reached. The DF value is composed of a relationship between an element that stabilizes the ferrite phase and an element that stabilizes the austenite phase, and the effect of each element is influenced by the mutual relationship between the elements, and therefore the coefficients are different. As a result of examining the relationship between the DF value and the uniform elongation of the thin steel sheet, it was found that the DF value was 70 or less and the uniform elongation was 30% or more.
The present invention has been made based on this finding, and the gist of the invention is as follows.

(1) In mass%,
C: 0.10% or less,
Si: 2.0% or less,
Mn: 4.0% or less,
P: 0.050% or less,
S: 0.010% or less,
Cr: 17 to 25%,
Ni: 0.6 to 5.0%,
N: 0.01 to less than 0.15%
The balance consists of iron and inevitable impurities, the DF value represented by the following formula (1) is 70 or less, and the ratio of the ferrite phase at room temperature is 35% to 70%. Ferritic / austenitic stainless steel with excellent ingot cracking resistance and workability.
DF value = 7.2 × (Cr + 0.88 × Mo + 0.78 × Si)
−8.9 × (Ni + 0.03 × Mn + 0.72 × Cu + 22 × C + 21 × N) −44.9
(1) Formula

(2) By mass%
Cu: 2.0% or less,
Mo: 2.0% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability as described in the above item (1), comprising one or two of the following:

(3) In mass%,
Nb: 0.50% or less,
Ti: 0.50% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability as described in the above item (1) or (2), characterized by containing one or two of the above.

(4) By mass%
Ca: 0.003% or less,
Mg: 0.003% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability according to any one of the above items (1) to (3), characterized by containing one or two of the following.

  (5) Ferrite-austenitic stainless steel excellent in ingot crack resistance and workability according to any one of (1) to (4) above, wherein uniform elongation in a tensile test is 30% or more steel.

  (6) When manufacturing the steel according to any one of (1) to (5), the total time between 1100 ° C. and 900 ° C. during solidification is 40 minutes or more. A method for producing ferritic / austenitic stainless steel with excellent lump cracking and workability.

  According to the present invention, without containing a large amount of expensive and rare element Ni, it is possible to obtain a ferrite-austenitic stainless steel sheet that can prevent slab breakage and is excellent in uniform elongation. Since it can be applied to parts where austenitic stainless steel plates containing a large amount of Ni have been used, it contributes greatly to the global environment in terms of saving resources called Ni.

It is the figure which showed the relationship between DF value and the number of cracks of the ingot surface. It is the figure which showed the relationship between DF value and uniform elongation.

  The present invention is described in detail below.

First, the reason for limiting the DF value expressed by the following formula (1) that defines the relationship between the element that stabilizes the ferrite phase and the element that stabilizes the austenite phase, which is an index of the alloy element that is an important element of the present invention will be described. To do.
DF value = 7.2 × (Cr + 0.88 × Mo + 0.78 × Si)
−8.9 × (Ni + 0.03 × Mn + 0.72 × Cu + 22 × C + 21 × N) −44.9
(1) Formula

  DF value of 70 or less, ferrite phase ratio of 35% to 70%: The metal structure of ferrite-austenitic stainless steel is composed of two phases of ferrite phase and austenite phase. The reason for being two phases at room temperature is that the composition contained in each phase (especially the element having a large diffusion rate such as N) is greatly influenced, and the composition tends to approach the equilibrium state in each temperature range during the cooling process from solidification to room temperature. This is because of the continuous distribution and organization. Therefore, the phase ratio of the two phases formed by the initial composition can be controlled by combining the temperature and the holding time. When the temperature is higher than 1100 ° C., the ferrite phase is stable and the phase ratio is high, so the tensile properties of the ferrite phase are strongly reflected and the uniform elongation is less than 30%, so the ratio of the ferrite phase (ferrite phase ratio) If the austenite phase-forming element such as N is contained in a large amount, the ferrite phase ratio will decrease, and if it is too small, stress concentration during deformation will occur in the ferrite phase, so the ferrite phase ratio will be 35% or more. It was. The ferrite phase ratio is preferably 40 to 60%.

FIG. 1 shows the relationship between the DF value and the number of cracks on the slab surface. When the DF value is 70 or less, the number of cracks becomes very small. When the DF value exceeds 70, cracks can be observed on the surface of the slab, and the length that can be observed on the surface becomes large. From the cross-sectional observation, it was confirmed that the crack propagated through the interface between the ferrite phase and the austenite phase or the ferrite phase. The DF value indicates the relationship between the element that stabilizes the ferrite phase and the element that stabilizes the austenite phase. When the DF value is 70 or less, it can be estimated that the toughness of the ferrite phase is improved because the ratio of the austenite-forming element slightly contained in the ferrite phase increases. Therefore, in the present invention, the DF value is limited to 70 or less. Here, as a result of investigating the relationship between the size, depth, and number of cracks, when the number of cracks with a crack length of 1 cm or more when observed from the surface is less than 2/100 cm ² , the crack depth is 2 mm. Since no cracks could be observed after hot rolling, a threshold value of less than 2 pieces / 100 cm ² was set. Here, the number of cracks was measured from two surfaces of a flat ingot melted in the laboratory and having a large surface area. However, since the number of cracks is greatly different from the actually produced slab size, it is converted to a number of 10 cm ² .

  Uniform elongation of 30% or more: Uniform elongation is an important index indicating that the metal material is uniformly deformed. That is, since the deformation resistance and hardness difference between the ferrite phase and the austenite phase are small and represent how uniformly the ferrite phase and the austenite phase can be deformed, if the DF value is 70 or less as shown in FIG. Since the difference in strength is small, the uniform elongation is 30% or more.

  The reasons for limiting the steel components are described below. In addition, "%" shown below represents mass%.

  C: C is an element that greatly affects the stability of the austenite phase. If the content exceeds 0.10%, the hardness of the austenite phase may remarkably increase. Further, in order to promote the precipitation of Cr carbide, it causes the occurrence of intergranular corrosion, so the upper limit was made 0.10%. Moreover, although it is more preferable to make C low from a corrosion-resistant point, in order to reduce, it causes the cost increase at the time of refining. Preferably, it is 0.01 to 0.04% of range, and more preferably 0.025 to 0.04% of range.

  Si: Si may be used as a deoxidizing element or may be contained for improving oxidation resistance. However, if the content exceeds 2.0%, the material is hardened and the uniform elongation is lowered, so this was made the upper limit. Moreover, in order to reduce Si extremely, the cost at the time of refining is increased. Preferably, it is in the range of 0.05 to 1.0%, and more preferably in the range of 0.4 to 1.0%.

  Mn: Mn concentrates in the austenite phase and plays an important role in changing the stability of the austenite phase. However, a large amount causes a decrease in corrosion resistance and hot workability, so the upper limit was made 4.0%. In order to make it less than 0.05%, the cost in the smelting process is increased. From the viewpoint of corrosion resistance, a lower value is preferable, and the upper limit is preferably set to 3.0%.

  P: P is an element inevitably mixed in, and since it is contained in raw materials such as Cr, it is difficult to reduce it. However, if it is contained in a large amount, the formability is lowered, so the upper limit is set. It was less than 0.050%.

  S: S binds to Mn to form inclusions, which may serve as a starting point for glazing, so the upper limit was made less than 0.010%. Since it is preferable from the viewpoint of corrosion resistance, the lower the content, the lower the content is preferably 0.003% or less.

  Cr: Cr is an element necessary for ensuring corrosion resistance, and the content of 17% or more is necessary. However, a large amount causes hot working cracks and leads to an increase in the cost of the refining process, so the upper limit was made 25%. Preferably, it is 20 to 23% of range.

  Ni: Ni is an austenite stabilizing element and is an important element for adjusting the stability of the austenite phase. Moreover, in order to have an effect which suppresses a hot work crack, it is made to contain 0.6% or more. If the content exceeds 5.0%, the raw material cost increases, and it may be difficult to obtain a two-phase structure of austenite and ferrite, so this was made the upper limit. Preferably, it is 0.6 to 2.4% of range. More preferably, it is 0.6 to 2.2% of range.

  N: N, like C, is an element that greatly affects the stability of the austenite phase. Moreover, since it has the effect of improving corrosion resistance when it exists in solid solution, it shall contain 0.01 or more. However, when the content is 0.15% or more, the uniform elongation may be reduced, and Cr nitride is liable to precipitate, and conversely, the corrosion resistance is lowered. In order to acquire an effect stably, 0.06% or more is preferred.

  Moreover, the following elements can be selectively contained.

  Cu: Cu, like Ni, is an austenite stabilizing element and is an important element for adjusting the stability of the austenite phase. However, if the content exceeds 2.0%, cracking during hot working is promoted and the strength is increased, so this was made the upper limit. In order to obtain an effect stably, 0.1% or more is preferable.

  Mo: Since Mo is an element that improves corrosion resistance, it may be selectively contained. By containing 0.1% or more, the corrosion resistance improving effect is exhibited. In order to obtain an effect stably, 0.5% or more is preferable. However, when it exceeds 2.0%, the uniform elongation is lowered and the raw material cost is greatly increased.

  Nb: Nb has the effect of preventing the weld heat-affected zone from becoming coarse, but the content exceeding 0.50% lowers the uniform elongation, so this was made the upper limit. In order to obtain an effect stably, 0.03% or more is desirable.

  Ti: Ti, as well as Nb, has the effect of preventing the weld heat affected zone from becoming coarse. Furthermore, the content is preferably 0.03% or more in order to finely equiax the solidified structure. However, the content exceeding 0.50% lowers the uniform elongation, so this was made the upper limit.

  Ca: Ca may be slightly contained for desulfurization and deoxidation. However, the content of over 0.003% tends to cause hot working cracks and lowers the corrosion resistance, so this was made the upper limit. In order to obtain the effect stably, 0.0005% or more is desirable.

  Mg: Mg has not only deoxidation but also an effect of refining the solidified structure. In order to stably exhibit these effects, the content is preferably 0.0005% or more. Moreover, since content exceeding 0.003% brings about the cost increase in a steelmaking process, this was made into the upper limit.

Next, the reason for limiting the manufacturing method will be described.
As described above, in order to obtain good ingot cracking resistance, it is necessary to control the composition, but it is necessary to control not only the chemical composition but also the ratio of the ferrite phase. For example, it is obtained by satisfying the production conditions as described below. Since the phase ratio of the austenite phase obtained by computational thermodynamics shows a maximum value between 1100 and 900 ° C., the total time passing through this temperature is important. When the total time was less than 40 minutes, the ferrite phase ratio was changed, and a tendency to converge with time could be confirmed. Since the change in the ferrite phase ratio by the ferrite meter was not observed after the total time of 40 minutes or more, it was judged that the element distribution was close to equilibrium. If the distribution close to the equilibrium state occurs, the redistribution of elements can be suppressed to a minimum reduction even in the subsequent heat treatment. In the cooling process below 900 ° C., the movement of elements hardly changes except for C and N. In addition, since the austenite phase increases with a decrease in temperature in the temperature range exceeding 1100 ° C., the composition of each phase changes significantly in the subsequent cooling process, which is unsuitable for structure control.

  Hereinafter, the present invention will be described in detail based on examples.

  Steel No. shown in Table 1 Using 20 steel types 1 to 20, a 1.0 mm thick thin steel sheet was produced through the steps of laboratory melting, hot rolling, hot rolled sheet annealing, cold rolling, and final annealing. In producing the steel sheet, only the temperature range of 1100 to 900 ° C. was controlled and cooled in the heat treatment furnace without cooling the ingot after melting in the laboratory. After cooling to room temperature, the cracks on the ingot surface were measured. Then, after grinding until the ingot crack is eliminated and holding at a heating temperature of 1200 ° C. for 60 minutes, a hot-rolled sheet having a thickness of 4 mm is produced by hot rolling of 10 passes, and 1050 ° C. × 1 minute of hot-rolled sheet annealing, After pickling, it was extended to 1 mm by cold rolling. The final annealing temperature and time were set to 1050 ° C. × 1 minute, a tensile test using a JIS No. 13 B specimen taken from a direction parallel to the rolling direction was performed, and uniform elongation was measured. In the steel of the present invention example having a DF value of 70 or less, the uniform elongation was 30% or more. In the steel of the comparative example, the DF value exceeds 70 and the uniform elongation is less than 30%. The controlled cooling conditions in the temperature range of 1100 to 900 ° C. for these steels are all implemented with a total time of 40 minutes or more, and the element distribution of δ / γ is estimated to be close to equilibrium, and the effect of the DF value Is reflected in the uniform elongation.

  Table 2 shows the total time at 1100 to 900 ° C, the ferrite phase ratio, the number of cracks on the ingot surface, and the uniform elongation.

  As shown in Table 2, when the ferrite-austenitic stainless steel with a DF value of 70 or less regulated in the present invention satisfies the invention conditions (composition, total time 40 minutes or more, ferrite phase ratio 35 to 70%), casting No cracks occur on the lump surface, and the uniform elongation shows a value of 30% or more. However, no. In the case of 13, the uniform elongation is 30% or more, but cracks are generated on the ingot surface due to the DF value being off.

  Further, under the condition where the total time at 1100 to 900 ° C. is less than 40 minutes, the elemental distribution of the ferrite phase and the austenite phase is insufficient, so the uniform elongation is less than 30%.

  The present invention relates to a ferritic / austenitic stainless steel excellent in ingot crack resistance and workability and a method for producing the same. According to the present invention, ferritic and austenitic stainless steels excellent in workability can be produced without containing a large amount of expensive and rare element Ni, which can contribute to resource protection and environmental conservation. it is conceivable that.

Claims (6)

% By mass
C: 0.10% or less,
Si: 2.0% or less,
Mn: 4.0% or less,
P: 0.050% or less,
S: 0.010% or less,
Cr: 17 to 25%,
Ni: 0.6 to 5.0%,
N: 0.01 to less than 0.15%
The balance is made of iron and inevitable impurities, and the DF value represented by the following formula (1) is 70.
A ferritic / austenitic stainless steel excellent in ingot crack resistance and workability, characterized in that the ratio of the ferrite phase at normal temperature is 35% to 70%.
DF value = 7.2 × (Cr + 0.88 × Mo + 0.78 × Si)
−8.9 × (Ni + 0.03 × Mn + 0.72 × Cu + 22 × C + 21 × N) −44.9
... (1) Formula % By mass
Cu: 2.0% or less,
Mo: 2.0% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability according to claim 1, characterized by containing at least one of the following. % By mass
Nb: 0.50% or less,
Ti: 0.50% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability according to claim 1 or 2, characterized by containing at least one of the following. % By mass
Ca: 0.003% or less,
Mg: 0.003% or less,
The ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability according to any one of claims 1 to 3, characterized by containing at least one of the following. The ferrite-austenitic stainless steel excellent in ingot crack resistance and workability according to any one of claims 1 to 4, wherein the uniform elongation in a tensile test is 30% or more. In producing the steel according to any one of claims 1 to 5, from 1100 ° C to 900 at the time of solidification.
A method for producing a ferritic / austenitic stainless steel excellent in ingot cracking resistance and workability, characterized in that the total time between temperatures is 40 minutes or more.

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