JP2008127650A - Austenitic stainless steel sheet having excellent descaling property, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel sheet having excellent descaling properties upon pickling. <P>SOLUTION: The cold rolled steel sheet of steel having a componential composition regulated in such a manner that a martensitic transformation point Ms(°C) defined by formula (1) is controlled to ≤-100°C is subjected to finish annealing under the temperature condition of ≤1,100°C with no soaking in an air atmosphere so as to obtain an austenitic stainless steel sheet in which, regarding the crystal structure of base phase metal in the surface of the steel sheet after annealing-pickling, provided that the X-ray diffraction intensity of an austenitic phase is defined as Iγ(111) and the X-ray diffraction intensity of a martensitic phase is defined as Im(110), an X-ray diffraction intensity ratio expressed by Im(110)/Iγ(111) is ≤1.5: Ms=ä3000[0.068-(C+N)]+50(0.47-Si)+60(1.33-Mn)+110(8.9-Ni)+75(14.6-Cr)-32}×5/9 ...(1). Since the martensitic phase easy to appear on the surface is less, remaining of scale having no regularity can be evaded after the pickling, and a finely finished annealed-pickled austenitic stainless steel sheet can be obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an austenitic stainless steel sheet having excellent descalability during annealing pickling and a method for producing the same.

Austenitic stainless steel sheets are widely used in various industrial fields, taking advantage of various material functions not found in ordinary steel, such as corrosion resistance, acid resistance, heat resistance, high strength, and designability. In recent years, the trend of using high-functionality, long life, low cost, and reduced environmental impact has increased, and the use of stainless steel sheets is increasing in fields where high workability is required. For this reason, in addition to having the cost merit with respect to the current material, provision of a softer material is demanded.
In order to satisfy such a requirement, softening is achieved even if the technology for adding Mn or Cu instead of expensive Ni (see, for example, Patent Documents 1 and 2) or fine graining to suppress rough skin after polishing is used. A technique that can be achieved (see, for example, Patent Document 3) has been proposed. However, from the conventional technique described in Patent Document 3, it is understood that the coarsening of crystal grains is a conventional means of softening.
Japanese Patent Laid-Open No. 4-72038 JP 9-263905 A Japanese Patent Laid-Open No. 10-158792

However, when an austenitic stainless steel sheet with a reduced Ni content is annealed at a high temperature, a non-regular scale remains after pickling due to the martensite phase formed on the surface, and the finished appearance is beautiful. It may be missing.
Therefore, the present invention aims to provide an austenitic stainless steel sheet that reduces the martensite phase generated during annealing and avoids the remaining scale, which is a problem in manufacturing, and has excellent descaling properties during pickling. And

The austenitic stainless steel sheet of the present invention is a steel sheet having a component composition adjusted such that the martensitic transformation point Ms (° C.) defined by the following formula (1) is −100 ° C. or less, and is annealed pickled The crystal structure of the matrix metal on the surface of the later steel sheet is Im (110) / Iγ (when the X-ray diffraction intensity of the austenite phase is Iγ (111) and the X-ray diffraction intensity of the martensite phase is Im (110). 111), the X-ray diffraction intensity ratio is 1.5 or less.
Ms = {3000 [0.068- (C + N)] + 50 (0.47-Si) +60 (1.33-Mn)
+110 (8.9-Ni) +75 (14.6-Cr) -32} × 5/9 (1)

As such an austenitic stainless steel plate, C + N: 0.06 mass% or less, Si: 1.0 mass% or less, Mn: 5.0 mass% or less, Cr: 15-20 mass%, Ni: 5-15 It is preferable to have a component composition that includes mass%, Cu: 0.5 to 5.0 mass%, with the balance being Fe and inevitable impurities. Further, if necessary, Ti: 0.5 mass% or less, Nb: 0.5 mass% or less, Zr: 0.5 mass% or less, V: 0.5 mass% or less, Mo: 3.0 mass% or less , B: 0.03 mass% or less, REM (rare earth metal): 0.02 mass% or less, Ca: 0.03 mass% or less may be included.
Such an austenitic stainless steel sheet is manufactured by subjecting a stainless steel cold-rolled steel sheet having the above-described composition to a finish annealing at a temperature of 1100 ° C. or less in a soaking atmosphere for 0 seconds. The

  According to the present invention, an annealed austenitic stainless steel sheet having a low martensite phase on the surface is obtained, so that in the actual line production process, non-regular scale remains after pickling, and a beautiful finish annealed pickled austenitic stainless steel is obtained. It becomes possible to manufacture a steel plate stably.

The inventors of the present invention have made various studies on the cause of non-regular descaling defects that are likely to occur when manufacturing an austenitic stainless steel sheet on an actual line and countermeasures thereof.
First, when the occurrence state of descaling failure in the actual line manufacturing process is observed, the difference in descaling property between the portion where the rolling oil is attached and the portion where the rolling oil is not attached is remarkable.
Then, the property change of the steel plate surface in the site | part which the rolling oil has adhered and the site | part which has not adhered was examined.

In the portion where the rolling oil is adhered, the rolling oil burns when inserted into the annealing furnace, and C in the composition remains on the steel sheet surface. This C is carburized inward from the surface to produce Cr ₂₃ C ₆ . On the steel sheet surface where Cr ₂₃ C ₆ is generated, the Cr generation rate is increased while the heat absorption rate during annealing is increased due to the deterioration of Cr in the parent phase and the generated Cr ₂₃ C _6. The phenomenon is promoted. For this reason, the Ms point rises on the surface of the parent phase at the corresponding portion, and a phase transformation occurs in the cooling process after annealing to generate a martensite phase.
The produced martensite phase is inferior in corrosion resistance as compared with the austenite phase of the parent phase due to the difference in the amount of Cr contained, and as a result, the descalability is good.

On the other hand, the acid resistance is maintained in the portion where the rolling oil is not attached because the de-Cr phenomenon on the steel sheet surface is suppressed as compared with the portion where the rolling oil is attached. That is, the descalability becomes poor, and a difference occurs in the descalability between a portion where the rolling oil is attached and a portion where the rolling oil is not attached. As a result, a scale having no regularity remains in the actual line manufacturing process.
Thus, at the time of finish annealing in the actual line manufacturing process, the martensite phase is irregularly formed on the steel sheet surface, and the pickling property at the time of pickling at the part where the martensite phase remains and the part where the austenite phase remains A local difference arises, and it can be inferred that the difference in the pickling situation appeared as a descaling defect on the surface of the finished steel sheet.

Assuming as described above, the occurrence of descaling failure can be suppressed by reducing the remaining martensite phase. The inventors of the present invention further studied and searched for the remaining limit of the martensite phase on the descaling failure.
The abundance ratio of the martensite phase and the austenite phase on the stainless steel plate surface can be known by measuring the X-ray diffraction intensity on the steel plate surface after pickling. Therefore, the X-ray diffraction intensity Im (110) of the martensite (m) phase and the X-ray diffraction intensity Iγ (111) of the austenite (γ) phase on the surface of the annealed pickling plate are measured, and the ratio Im (110) of the two is measured. The relationship between / Iγ (111) and descalability was investigated.

X-ray diffraction intensity was measured using an X-ray diffractometer (RINT1500, manufactured by Rigaku Corporation). Co tube, tube voltage 40 KV, tube current 120 mA, scanning axis 2θ, θ fixed angle 5.000 °, scanning speed 1.000 ° / min, sample The measurement was performed by the X-ray diffraction thin film method under the condition of a width of 0.010 °. The integrated intensities of the γ phase (111) and m phase (110) planes were Iγ (111) and Im (110), respectively.
As a result of the investigation, the details will be shown in Examples, but it was found that if the X-ray diffraction intensity ratio Im (110) / Iγ (111) is 1.5 or less, the occurrence of descaling failure can be avoided.

By the way, the martensite phase is easily generated as the Ms point increases even in a portion where the rolling oil is not attached. If the Ms point is high and the martensite phase formation region is widened, the pickling property of the entire surface is improved and the descaling property becomes uniform, but at the same time the oxide scale increases and the weight loss due to annealing increases. This is not preferable in terms of manufacturing cost. Moreover, since the martensitic transformation of the whole base material progresses after annealing due to the rise of the Ms point and the whole base material becomes hard, it becomes unsuitable for use as an original austenitic stainless steel.
Since this tendency becomes remarkable when the Ms point (° C.) exceeds −100 ° C., the upper limit value of the Ms point is set to −100 ° C. Therefore, the austenitic stainless steel sheet of the present invention is premised on satisfying the requirements regarding the Ms point.
In the present specification, the Ms point is defined by the above formula (1).

Next, a description will be given of a measure for suppressing the occurrence of a descaling failure when an austenitic stainless steel sheet is produced on a normal line.
The austenitic stainless steel targeted by the present invention is preferably C + N: 0.06 mass% or less, Si: 1.0 mass% or less, Mn: 5.0 mass% or less, Cr: 15-20 mass% , Ni: 5 to 15% by mass, Cu: 0.5 to 5.0% by mass, with the balance being composed of Fe and inevitable impurities. Further, if necessary, Ti: 0.5 mass% or less, Nb: 0.5 mass% or less, Zr: 0.5 mass% or less, V: 0.5 mass% or less, Mo: 3.0 mass% or less , B: 0.03 mass% or less, REM (rare earth metal): 0.02 mass% or less, Ca: 0.03 mass% or less may be included.
Within the said range, it adjusts so that Ms point mentioned above may satisfy | fill Formula (1).

Here, the alloy component, content, etc. contained in the austenitic stainless steel sheet which this invention makes object are demonstrated.
C + N: 0.06% by mass or less If a large amount of C and N is contained, 0.2% yield strength and hardness increase due to solid solution strengthening, and the work-induced martensite phase hardens, so work hardening increases. As a result, the sharpness of the bending ridge line is lowered. For this reason, the (C + N) content is limited to 0.06% by mass or less.

Si: 1.0% by mass or less Si is an element that is effective as a deoxidizing agent during melting, but if it exceeds 1.0% by mass, it hardens and work hardening increases and the sharpness of the bending ridge line decreases. To do. Therefore, the Si content is limited to 1.0% by mass or less.

Mn: 5.0% by mass or less Mn is softened by decreasing 0.2% proof stress as the content increases, and work hardening decreases. However, if it is contained in a large amount, it not only causes damage to the refractory during steelmaking, but also increases the number of inclusions and easily causes work cracks. Therefore, the upper limit of the Mn content is 5.0 mass% at which the softening effect is saturated.

Cr: 15-20% by mass
A larger amount of Cr is preferable from the viewpoint of corrosion resistance. In the present invention, the Cr content is 15% by mass or more. However, when it contains excessively, hardness will increase and hardening will become remarkable. Therefore, in the present invention, the upper limit is 20% by mass.

Ni: 5 to 15% by mass
Ni is an indispensable element for austenitic stainless steel, and at least 5% is necessary. As the content increases, the material softens and work hardening is reduced. However, Ni is an expensive element, and considering the economical point, the upper limit of the Ni content is 15% by mass. Since the purpose of softening can be achieved with a content of 12.0% by mass or less, it is preferably 12% by mass or less.

Cu: 0.5-5.0 mass%
Cu contributes to suppression of work hardening and is an element useful for softening. The effect is manifested by inclusion of 0.5% by mass or more. The more it is, the more effective it is for softening. If the content exceeds 2.0% by mass, the degree of freedom of Ni content increases, and the Ni content can be reduced to its lower limit of nearly 5% by mass. It becomes easy and can contribute to cost reduction. For this reason, although the minimum of Cu content is 0.5 mass%, in order to obtain the softened stainless steel, it is preferable to make it contain 2.0 mass% or more. However, excessive content adversely affects hot workability, so the upper limit is made 5.0 mass%.

P: 0.045% by mass or less When the amount of P contained as an impurity is increased, not only is it hardened but also corrosion resistance is impaired. In order not to reveal the harmfulness, P as an impurity is preferably limited to 0.045% by mass or less.

S: 0.010 mass% or less Bending workability decreases with an increase in the S content, and in extreme cases, cracks occur in the bent portion. Moreover, since S reduces the hot workability during the production of the steel sheet, it is preferable to reduce it as much as possible. If the content is restricted to 0.010% by mass or less, the manifestation of the above-described adverse effects is suppressed, but when the purpose is to further improve the hot workability, the content is preferably 0.003% by mass or less.

Ti, Nb, Zr, V: 0.5% by mass or less, each of which is an alloy element added as necessary. In order to fix the solid solution strengthening element, the hardness of the steel material is reduced, resulting in improved workability. Has an effect. The effect of these elements is saturated at 0.5% by mass, and even if added more than this, an effect commensurate with the increase cannot be expected. Accordingly, when added, the upper limit is 0.5% by mass.

Mo: 3.0% by mass or less Mo is not an essential additive element in the present invention, but Mo is an element useful for improving corrosion resistance. However, if it exceeds 3.0% by mass, the hardness is increased, so when Mo is added, the upper limit is 3.0% by mass.

B: 0.03 mass% or less An alloying element added as necessary, improving hot workability and effective in preventing cracking during hot rolling. However, since hot workability deteriorates when contained in a large amount, when added, the upper limit is 0.03 mass%.

REM (rare earth metal): 0.02 mass% or less An alloying element added as necessary, and is effective for improving hot workability in the same manner as B. However, if it is added excessively, the effect is saturated, and it is hardened and the moldability is lowered. For this reason, when adding, upper limit is 0.02 mass%.

Ca: 0.03 mass% or less An alloying element added as necessary, and is effective in improving deoxidation and hot workability during steelmaking. However, even if Ca is added in excess of 0.03 mass%, an effect commensurate with the increase cannot be obtained. For this reason, when adding, upper limit is 0.03 mass%.

Next, the manufacturing method of the austenitic stainless steel plate of this invention is demonstrated.
When manufacturing the austenitic stainless steel sheet of the present invention, if the composition of the components is strictly adjusted so as to satisfy the Ms point, it is necessary to apply particularly fine conditions in the melting process, casting process, hot rolling process and cold rolling process. There is no. It is sufficient to perform each treatment under the conditions generally employed when producing an austenitic stainless steel sheet.
However, after the finish cold rolling, if the annealing is performed with the rolling oil adhered when performing the finish annealing, as described above, a martensite phase may partially appear and cause descaling failure during pickling. is there.

Therefore, the influence of the annealing conditions on the appearance of the martensite phase during finish annealing of the cold-rolled steel sheet was confirmed by performing annealing tests with various annealing temperatures.
As a result, although details are shown in the Examples, it has been found that if finish annealing is performed at a temperature of 1100 ° C. or lower, the appearance of the martensite phase can be suppressed regardless of the adhesion of the rolling oil.
The finish annealing of the finished cold-rolled steel sheet is usually performed by passing the cold-rolled steel sheet through an annealing furnace adjusted to a predetermined temperature in an air atmosphere.
For this reason, the austenitic stainless steel sheet of the present invention is manufactured by subjecting a finish cold-rolled sheet made by ordinary means to continuous finish annealing in an air atmosphere at a temperature of 1100 ° C. or lower. Become.
That is, if the finish cold-rolled sheet of an austenitic stainless steel sheet with an adjusted Ms point is subjected to a finish annealing of 0 seconds under a temperature condition of 1100 ° C. or less in an air atmosphere, descaling failure occurs during pickling. An austenitic stainless steel sheet that does not occur is obtained.

Example 1:
As test materials, steels A to H having the compositions shown in Table 1 were used. Various stainless steels were melted and, after continuous casting, hot rolled to a plate thickness of 3.0 mm at a finishing temperature of 1000 ° C. and a winding temperature of 900 ° C. Thereafter, the hot-rolled steel strip was annealed at 1100 ° C. × soaking for 1 minute and pickled, and then cold rolling, annealing, and pickling were repeated to produce a cold-rolled coil having a thickness of 0.7 mm and a width of 1200 mm. .
This cold-rolled coil was subjected to continuous annealing with a soaking time of 0 seconds in an air atmosphere in a continuous annealing furnace adjusted to a temperature of 1100 ° C.
At this time, a portion where the rolling oil was not attached before the annealing was marked to make a portion without the rolling oil. Moreover, the part which the rolling oil adhered was made into the rolling oil adhesion part. Hereinafter, they are referred to as “rolling oil-free portion” and “rolling oil adhering portion”.
Subsequently, continuous pickling was performed in a hydrofluoric acid (3% HF + 10% HNO ₃ + 87% H ₂ O) solution at a liquid temperature of 60 ° C. for an immersion time of 20 seconds.

X-ray diffraction intensity measurement and surface hardness measurement were performed on each steel plate after pickling.
X-ray diffraction intensity was measured using an X-ray diffractometer (RINT1500, manufactured by Rigaku Corporation). Co tube, tube voltage 40 KV, tube current 120 mA, scan axis 2θ, θ fixed angle 5.000 °, scan speed 1.000 ° / min, sample width 0.010 The diffraction intensity of the austenite phase (111) plane and martensite phase (110) plane was measured by the X-ray diffraction thin film method using the condition of ° C, and the integral intensity ratio Im (110) / Iγ (111) was calculated.
Further, the surface hardness of the steel sheet was measured with a Vickers hardness tester at a test load of 49.03 N by the method specified in JIS Z2244. Furthermore, after the pickling treatment described above, the descaling property is washed with water, dried, and immediately observed on the surface of the steel sheet between the cold rolled oil adhering part and the non-adhering part of the steel sheet before annealing. When the difference was recognized, the descalability was judged as poor, and when the difference was not recognized, the descalability was judged as good.
The results are shown in Table 2.

  Invented steels D to H annealed at 1100 ° C. exhibited good descaling properties even at the part where the rolling oil was adhered, and also exhibited a soft hardness of 121 to 181 HV in surface hardness. On the other hand, some scales remained in the comparative steels B and C. Comparative steel A also showed good descaling properties, but because this had a very high Ms point, the entire steel sheet had undergone martensitic transformation, resulting in a decrease in corrosion resistance. It just seemed like this. It can be seen from the fact that the surface hardness was 310 HV that the entire steel sheet had undergone martensitic transformation. By the way, the comparative steels B and C also have a martensitic transformation partly because the Ms point exceeds the specified value, and are accordingly hardened to 280HV and 242HV.

By the way, when the relationship between Im (110) / γ (111) and Ms point in the rolling oil adhering portion and the rolling oil-free portion shown in Table 2 is seen on the graph, it is as shown in FIG.
In comparative steels A to C having Ms points exceeding −100 ° C., the surface martensite phase tends to increase in the rolling oil adhering portion. On the other hand, in the steels C to H of the present invention having an Ms point of −100 ° C. or lower, the increase in the surface martensite phase of the rolling oil adhesion portion relative to the non-rolled portion is small, and the steel D having an Ms point of −102 ° C. , The X-ray diffraction intensity ratio of the martensite phase to the austenite phase is 1.5 or less.
From this result, it can be said that when the X-ray diffraction intensity ratio Im (110) / γ (111) is 1.5 or less, satisfactory descaling property is exhibited.

Example 2:
Steel E having a Ms point in Table 1 of −122 ° C. was used as a test material, and in the same manner as in Example 1, annealing was performed with various changes in the temperature range of 1000 to 1150 ° C.
Then, the same X-ray diffraction intensity measurement and descalability as in Example 1 were performed.
The results are shown in Table 3 and FIG.

As can be seen from the results shown in Table 3 and FIG. 2, even when the steel E has an Ms point of −122 ° C., when the annealing temperature reaches 1100 to 1150 ° C., the surface martensite phase of the rolling oil adhesion portion increases. Show a tendency to
If the annealing temperature is 1100 ° C. or less, the X-ray diffraction intensity ratio Im (110) / Iγ (111) of the martensite phase to the austenite phase is 1.5 or less even at the rolling oil adhering portion, and the desired descaling property is achieved. Presents.

From the results of Examples 1 and 2, if the finish cold-rolled sheet of austenitic stainless steel with an adjusted Ms point is subjected to finish annealing under predetermined conditions, even if the rolling oil at the time of cold rolling remains, it is formed on the surface of the steel sheet. An austenitic stainless steel sheet that does not cause descaling failure even in subsequent pickling is obtained without increasing the surface martensite phase.
Therefore, if the present invention is applied, scale residue with no regularity can be avoided after pickling even in the actual line manufacturing process, and austenitic stainless steel sheets with excellent descaling properties can be stably manufactured at low cost. become.

The graph which shows the relationship between Ms point and Im (110) / Iγ (111) in a rolling oil adhesion part and a rolling oil no part The graph which shows the relationship between the annealing temperature in a rolling oil adhesion part and a rolling oil no part, and Im (110) / Iγ (111)

Claims (4)

A steel plate having a component composition adjusted so that the martensite transformation point Ms (° C.) defined by the following formula (1) is −100 ° C. or less, and the matrix metal on the steel plate surface after annealing pickling X-ray diffraction expressed as Im (110) / Iγ (111) when the crystal structure is Iγ (111) for the X-ray diffraction intensity of the austenite phase and Im (110) for the X-ray diffraction intensity of the martensite phase An austenitic stainless steel sheet excellent in descaling characteristics, characterized in that the strength ratio is 1.5 or less.
Ms = {3000 [0.068- (C + N)] + 50 (0.47-Si) +60 (1.33-Mn)
+110 (8.9-Ni) +75 (14.6-Cr) -32} × 5/9 (1)   Component composition is C + N: 0.06 mass% or less, Si: 1.0 mass% or less, Mn: 5.0 mass% or less, Cr: 15-20 mass%, Ni: 5-15 mass%, Cu: 0 The austenitic stainless steel sheet excellent in descaling properties according to claim 1, comprising 0.5 to 5.0% by mass, the balance being made of Fe and inevitable impurities.   Component composition is further Ti: 0.5 mass% or less, Nb: 0.5 mass% or less, Zr: 0.5 mass% or less, V: 0.5 mass% or less, Mo: 3.0 mass% or less B: 0.03 mass% or less, REM (rare earth metal): 0.02 mass% or less, Ca: 0.03 mass% or less. Austenitic stainless steel plate with excellent resistance.   A stainless steel cold-rolled steel sheet having the component composition according to any one of claims 1 to 3 is subjected to a finish annealing at a temperature of 1100 ° C. or less in a soaking atmosphere for 0 seconds. A method for producing an austenitic stainless steel sheet having excellent scale properties.

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