JP4458363B2 - Method for producing Cr-Ni stainless steel strip - Google Patents

Description

  The present invention relates to a method for producing a Cr—Ni-based stainless steel strip having a good surface property by preventing the occurrence of lashes that are likely to occur on the surface of a steel strip during hot rolling.

  The Cr—Ni-based stainless steel strip is a steel type in which surface defects called “hege wrinkles” tend to occur on the surface of the steel strip. Hege not only detracts from the appearance of the steel strip surface, but also adversely affects product properties such as mechanical properties, fatigue properties, and corrosion resistance. For this reason, a process such as surface grinding for removing lashes is incorporated, but it often remains even by grinding using a grinder and cannot be removed by barrel polishing after product processing. As a result, it is obliged to discriminate parts having good surface properties without shaving and ship them to customers. The selection of non-defective parts causes the process load to increase remarkably, resulting in a decrease in product yield and an increase in product cost.

The baldness of the Cr-Ni stainless steel strip occurs during hot rolling and is often promoted by cold rolling in the subsequent process. Even in a hot-rolled steel strip where no lashes are detected, significant surface defects may appear after cold rolling. Therefore, in order to produce a Cr-Ni stainless steel strip with good surface properties over the entire length, it is important not to remove the lashes by polishing and grinding, but to suppress the occurrence of lashes during hot rolling. It is.
Up to now, the methods shown in the following Patent Documents 1, 2, 3 and the like have been proposed as measures for preventing the occurrence of baldness of Cr—Ni stainless steel during hot rolling.

  That is, in Patent Document 1, the slab having the width edge portion thicker than the width center portion is hot-rolled to form a complete rectangular shape having the same thickness in the width direction, thereby causing the occurrence of baldness at the time of hot rolling. It is preventing. Further, in Patent Document 2, δ ferrite present in the slab is considered to be the cause of whipping during hot rolling, and the δ ferrite in the slab is controlled by controlling the component design of the steel material and the heating conditions of the slab. It has been proposed to eliminate the occurrence of baldness by eliminating it.

Furthermore, in patent document 3, the rolling direction stress state which generate | occur | produces inside a slab is adjusted by rolling so that the average rolling reduction of an edge part may become larger than the average rolling reduction of a width center part, and it is a cause of crack generation. A method for preventing microcracking by suppressing generation of tensile stress in the vicinity of the edge has been proposed. Specifically, the width direction distribution of the stress in the rolling direction is calculated by a three-dimensional rigid-plastic finite element method, and the edge region is determined based on the analysis result that tensile stress is generated in the 1/6 width region from the plate end. It is defined as a 1/6 width region from the end, and the reduction rate of the edge portion increases toward the edge portion end, and the reduction rate in the section from the width center to the edge portion (width center portion) is constant in the width direction. It is prescribed to roll as follows.
JP-A-6-304606 JP-A-6-279856 Japanese Patent No. 3221790

However, even if a slab having a completely rectangular shape is rolled, scabs may occur, and the method proposed in Patent Document 1 is also insufficient as a measure for preventing scabs. In addition, the austenite (γ) single-phase Cr—Ni stainless steel from which δ ferrite has disappeared also has hege soot. Is insufficient.
Further, according to Patent Document 3, the region of the slab edge portion is defined as a 1/6 width region from the plate end, and the rolling reduction of the edge portion increases toward the edge portion end, and the interval from the edge portion to the width center. Even if rolling was performed so that the rolling reduction of the (width center portion) was constant in the width direction, the occurrence of scabs could be suppressed at the end of the slab, but surface flaws sometimes occurred at the center of the slab. .

  The present invention is based on the knowledge that the proper dispersion of δ ferrite is effective in preventing the occurrence of whipping, as opposed to the conventional one, and by enabling the component design that enables the proper dispersion of δ ferrite, The object is to produce a Cr-Ni stainless steel strip with good surface properties that can be hot-rolled under conditions in which the occurrence and growth of cracks leading to the baldness are suppressed.

  The Cr—Ni stainless steel used in the present invention is C: 0.080 mass% or less, Si: 0.01-1.5 mass%, Mn: 0.01-4.0 mass%, P: 0.00. 05 mass% or less, S: 0.03 mass% or less, Ni: 16.0 to 25.0 mass%, Cr: 20.0 to 30.0 mass%, N: 0.05 mass% or less, Mo: 0 0.01 to 1.0% by mass, Cu: 0.001 to 1.0% by mass, with the balance being composed of Fe and inevitable impurities. In this component system, F value defined as F (mass%) = Cr + Mo + Si, A (mass%) = Ni + 35C + 20N + 0.2Mn + 0.25 Cu, A value is F = 22.0-33.0, A = 17.0-28 The components are adjusted to satisfy 0.0, F−A = 3.5 to 11.5. As needed, you may include 1 type or 2 types of Ca: 0.010 mass% or less and B: 0.010 mass% or less.

  After melting a Cr-Ni stainless steel having a predetermined composition, it is continuously cast into a slab, and the amount of δ ferrite of the slab is adjusted to a range of 0.5 to 5.0% by volume by heat treatment at 1000 to 1250 ° C. After that, in the first pass of rough rolling, the average rolling reduction of the section (hereinafter referred to as edge portion) within 1/9 of the slab width from both ends in the width direction of the slab is set to the section closer to the width center (hereinafter referred to as the width center). The Cr-Ni-based stainless steel strip having no lashes is produced by hot rolling at an average rolling reduction greater than the average reduction ratio.

In the present invention, F value and A value are defined as F (mass%) = Cr + Mo + Si, A = Ni + 35C + 20N + 0.2Mn + 0.25Cu, and F = 22.0-33.0, A = 17.0-28.0, F By adopting a component design that satisfies -A = 3.5 to 11.5, the occurrence of lashes that tend to occur during hot rolling of Cr-Ni stainless steel is suppressed. In addition, instead of averaging in the slab thickness direction, the distribution of the rolling reduction in the width direction is more optimized by evaluating the presence or absence of surface flaws based on the analysis result of the stress in the rolling direction of the slab surface layer. Surface flaws can be significantly reduced over the entire width of the slab including the center.
Therefore, according to the present invention, a Cr—Ni stainless steel strip having a good surface property can be manufactured at low cost.

As a result of a detailed investigation of the Cr—Ni-based stainless steel strip in which hege wrinkles were generated after hot rolling, the present inventors found that hege wrinkles occurred along the γ / γ grain boundaries. It was presumed that grain boundary oxidation in the surface layer of the slab before hot rolling was the cause of hege soot. As a result of further detailed investigation, (1) it is possible to suppress grain boundary oxidation on the slab surface by dispersing an appropriate amount of δ ferrite in the heated slab before hot rolling, and (2) a cast structure that is brittle in material. It has been found that the occurrence of lashes can be suppressed by optimizing the distribution of the rolling reduction in the width direction of the rough rolling first pass performed on the slab having the slab and reducing the tensile stress at the end of the slab.
Hereinafter, the experiment will be described in detail from the inventors.

FIG. 1 shows an external appearance and a cross-sectional structure of a hot-rolled sheet after pickling a Cr—Ni-based stainless steel in which whipping has occurred. The steel shown in FIG. 1 is a steel type B shown in Table 1 and is a Cr—Ni stainless steel that does not satisfy the component conditions defined in the present invention. The hot rolling was performed using a laboratory hot rolling mill after the block cut out from the slab was held at 1230 ° C. for 2 hours in an air atmosphere.
When the appearance is observed, it can be seen that a large number of scabs are generated on the surface of the hot-rolled sheet, and that scabs are generated along the γ / γ grain boundaries from the cross-sectional structure.

In FIG. 2, the cross-sectional structure | tissue after the slab heating of the steel type B outside the component range of this invention and the steel type in the component range of this invention is shown. These are obtained by observing the cross sections of the samples cut from the steel type A and steel type B slabs at 1230 ° C. for 2 hours in air and then cooled with water.
From the observation results, in steel type B, grain boundary oxidation proceeds from the slab surface to the inside, whereas in steel type A, grain boundary oxidation is not confirmed and δ ferrite is dispersed in a form along the γ grain boundary. You can see that

From the above results, (1) the occurrence of whipping in Cr-Ni stainless steel is due to oxidation of γ / γ grain boundaries in the slab surface layer, and (2) intergranular oxidation due to dispersion of δ ferrite. It became clear that it could be prevented.
Therefore, the relationship between the amount of δ ferrite and surface cracks on the slab surface was investigated in detail through laboratory studies.

FIG. 3 shows the relationship between the amount of δ ferrite and the amount of surface crack initiation limit strain. The method for measuring the surface crack initiation limit strain is as follows.
A round bar tensile test piece was collected from the vicinity of the slab surface of a steel having various components, and the test piece was heat-treated in air at 1230 ° C for 2 hours, and the amount of δ ferrite was measured with a ferrite scope. did.
Thereafter, a tensile test was performed at a temperature of 1100 ° C. with various amounts of strain to confirm the presence or absence of cracks on the surface of the test piece. And the minimum amount of strain which can confirm the surface crack was made into the surface crack generation | occurrence | production limit strain amount.

The amount of surface crack initiation limit strain increased with increasing amount of δ ferrite. As shown in FIG. 3, the surface crack initiation limit strain amount is stable at a high value of 70% or more when the amount of δ ferrite is 0.5% by volume or more. However, when the amount of δ ferrite is 5.0% by volume, the surface crack generation limit strain amount is reduced. This means that when an excessive amount of δ ferrite exceeding 5.0% by volume is dispersed, the surface crack initiation limit strain amount is reduced.
From the above results, it was found that surface cracks can be suppressed by the dispersion of δ ferrite, and that the effect of suppressing surface cracks is large when the amount of δ ferrite is in the range of 0.5 to 5.0% by volume. In other words, it is predicted that the effect of suppressing the occurrence of lashes is large when the amount of δ ferrite is in the range of 0.5 to 5.0% by volume.

Next, the relationship between the amount of δ ferrite and the amount of component elements during slab heating was investigated.
The amount of δ ferrite is generally determined by the balance between ferrite-forming elements and austenite-forming elements. Therefore, in the present invention, the contribution of ferrite-forming elements to the formation of δ-ferrite is F value (= Cr + Mo + Si), and the contribution of austenite-forming elements is A value (A = Ni + 35C + 20N + 0.2Mn + 0.25Cu). The relationship was investigated.
FIG. 4 shows the relationship between the amount of δ ferrite, the F value, and the A value when a slab having various component compositions is heated to a temperature range of 1000 to 1250 ° C.

  From FIG. 4 showing the relationship between the amount of δ ferrite and the F value and the A value, a range of δ ferrite amount: 0.5 to 5.0% by volume effective in suppressing the occurrence of surface cracks, that is, haze wrinkles, F = 22.0-33.0, A = 17.0-28.0, F-A = 3.5-11.5 are obtained. F = 22.0 to 33.0, A = 17.0 to 28.0, and F−A = 3.5 to 11.5 are hatched regions in FIG. 5, and Cr does not generate baldness during hot rolling. -In order to manufacture a Ni-type stainless steel strip, it is necessary to adjust each component amount so that these conditions may be satisfied.

The inventors of the present invention have also investigated in detail the site of the occurrence of lashes that occur during hot rolling. As a result, it was found that the occurrence of lashes was concentrated in the section of 1/9 of the slab width from both ends of the slab and increased as it became closer to the slab edge.
Then, considering that the cause of hege wrinkles is the tensile stress in the rolling direction generated in the slab surface layer, the stress in the rolling direction of the slab surface layer is analyzed by the three-dimensional rigid plastic finite element method, and the following knowledge Got.

As shown in FIG. 6, the tensile stress in the rolling direction acts on the slab surface layer over the entire width, but the tensile stress increases in the 1/9 section of the slab width from both ends of the slab. It corresponds well to the generation area. This means that, in the slab surface layer portion, heels are concentrated and generated at a site where a large rolling direction tensile stress is applied. Therefore, if the tensile stress at the end of the slab can be reduced, it also means that it is possible to suppress the occurrence of baldness.
In addition, all description of drawing attached to this specification including FIG. 6 was created based on the data obtained when the slab of width 1050mm was rough-rolled.

The distribution in the width direction of the stress in the rolling direction in the slab surface layer portion is due to the difference in stretching between the end portion of the slab and the center portion of the slab accompanying the metal flow in the width direction. Therefore, if a width direction distribution of the rolling reduction that increases the rolling reduction of the slab end portion is given, it is considered that the large tensile stress of the surface layer portion generated at the slab end portion can be reduced.
In the case where the rolling reduction is uniform, the section of 1/9 of the slab width from both ends, that is, the surface layer tensile stress of the edge defined in the present invention increases, so the rolling reduction of the edge as shown in FIG. The stress distribution when increased was analyzed by the three-dimensional rigid-plastic finite element method described above.
As a result, as shown in FIG. 8, it was found that the tensile stress at the edge portion decreased.

  As a method of changing the rolling reduction distribution in the slab width direction in the first pass of rough rolling, a method of grinding so that the edge portion of the slab becomes thicker than the thickness of the central portion of the slab, or a method of forming dog bones by edging rolling Etc. It is also possible to combine grinding and dogbone.

From the above, the baldness prevention mechanism in the present invention can be organized as follows.
That is, (1) by adjusting the component elements to appropriate amounts, δ ferrite is dispersed on the surface of the slab during slab heating, and grain boundary oxidation, which is the starting point of the baldness, is suppressed. Further, (2) generation of lashes can be prevented by optimizing the distribution of the rolling reduction in the width direction of the first rough rolling and reducing the tensile stress at the end of the slab.

Hereinafter, alloy components, content, production conditions, and the like of the Cr—Ni stainless steel will be described.
C: 0.080% by mass or less C is an austenite-forming element necessary for adjusting the A value, and contributes to strength improvement. The effect of improving the strength by C is observed at a C content of 0.01% by mass or more. However, since corrosion resistance falls when an excessive amount of C is contained, the upper limit was set to 0.080 mass%.

Si: 0.01-1.5 mass%
It is a component necessary for deoxidation of molten steel, and an effect is seen at 0.01 mass% or more. However, excessive addition not only saturates the effect but also increases the manufacturing cost, so the upper limit was set to 1.5% by mass. Further, since it is involved in the F value that affects the amount of δ ferrite produced, it is preferable to determine the Si content in the range of 0.1 to 1.5 mass%.

Mn: 0.01 to 4.0% by mass
Like Si, it is a necessary component for deoxidation of molten steel, and an effect is seen at 0.01 mass% or more. However, excessive addition causes a decrease in corrosion resistance, so the upper limit was set to 4.0% by mass. In addition, since Mn is an element that affects the A value and affects the amount of δ ferrite produced, in the case of the Cr—Ni-based stainless steel as a control of the present invention, 0.1 to 2.0 mass%. It is preferable to determine the Mn content within a range.

P: 0.05% by mass or less P is a component that deteriorates the corrosion resistance, but by reducing it to 0.05% by mass or less (preferably 0.03% by mass or less), the adverse effect of P can be suppressed.
S: 0.03 mass% or less Since it segregates at the γ / γ grain boundary and the hot workability is remarkably lowered, it is preferably reduced as much as possible. In the present invention, defects caused by S or sulfide are suppressed by reducing the S content to 0.03 mass% (preferably 0.01 mass%) or less.

Ni: 16.0 to 25.0 mass%
Along with Cr, it is a basic component of Cr-Ni stainless steel and is an austenite-forming element necessary for adjusting the A value. From the viewpoint of corrosion resistance, 16.0% by mass or more of Ni is necessary. However, an excessive amount of Ni exceeding 25.0% by mass stabilizes the γ phase and does not produce the necessary amount of δ ferrite. Moreover, since excessive addition of Ni leads to strength reduction, it is preferable to determine the Ni content in the range of 18.0 to 22.0% by mass in applications where strength and corrosion resistance are required.

Cr: 20.0-30.0 mass%
It is a basic component of stainless steel and contributes to improvement of corrosion resistance. 20.0% by mass or more of Cr is necessary to enable generation of an appropriate amount of δ ferrite without stabilizing the γ phase, but when an excessive amount of Cr exceeding 30.0% by mass is included. δ ferrite becomes excessive, and it becomes easy to generate a bevel. Moreover, since excessive addition of Cr tends to cause a decrease in toughness, it is preferable to determine the Cr content in the range of 23.0 to 27.0% by mass in applications where toughness and corrosion resistance are required.

N: 0.05 mass% or less Like C, it is an austenite-forming element and is necessary for adjusting the A value. However, since excessive N content reduces corrosion resistance, the upper limit was set to 0.05% by mass (preferably 0.03% by mass) or less.

Mo: 0.01-1.0 mass%
It is an effective component for improving the corrosion resistance, and the effect of adding Mo is seen at 0.01 mass% or more. However, since it is an expensive element and excessive addition causes an increase in steel material cost, the upper limit was set to 1.0% by mass. In applications where corrosion resistance is required, it is preferable to determine the Mo content in the range of 0.5 to 1.0 mass%.

Cu: 0.001 to 1.0 mass%
It is an effective component for improving corrosion resistance, and the effect of addition of Cu is observed at 0.001% by mass or more. However, excessive addition causes a decrease in hot workability, so the upper limit was set to 1.0% by mass. In applications where corrosion resistance is required, the Cu content is preferably determined in the range of 0.05 to 1.0 mass%.

Ca: 0.010% by mass or less Ca is a component added as necessary, and exhibits an effect of improving hot workability. Addition of 0.001% by mass or more shows an effect of improving hot workability by Ca. However, excessive addition reduces the cleanliness of the steel material, so the upper limit was set to 0.010% by mass.

B: 0.010% by mass or less B is a component added as necessary, and exhibits the effect of improving hot workability like Ca. The effect of addition of B is observed at 0.001% by mass or more. However, excessive addition generates Cr boride which is harmful to corrosion resistance, so the upper limit was set to 0.010% by mass.

Slab heating temperature: 1000-1250 ° C
In order to prevent lashing, it is necessary to heat the slab to a temperature range of 1000 to 1250 ° C. and disperse an appropriate amount of δ ferrite prior to hot rolling. When the heating temperature does not reach 1000 ° C., the amount of δ ferrite decreases and the effect of suppressing the occurrence of whipping is insufficient, and the deformation resistance during hot rolling increases remarkably and the rolling load increases. On the other hand, when the heating temperature exceeds 1250 ° C., the amount of δ ferrite decreases as the soaking time elapses depending on the components of the stainless steel, and the effect of suppressing the occurrence of whipping is lost.

The following experiments were conducted using the two types of stainless steel shown in Table 1 as test materials. Steel type A is a steel having a component composition in the range specified in the present invention, and steel type B is a comparative steel.
In either case, a slab having a thickness of 200 mm and a width of 1050 mm was obtained by continuous casting after melting. Then, after grinding the slab and heating the slab under various conditions, hot rolling with various changes in the rolling reduction in the first pass of the rough rolling was performed at the slab edge part and the slab center part, A rolled sheet was obtained.

The obtained hot-rolled coil was pickled to remove the oxide scale, and then the appearance of the hot-rolled coil was visually observed to investigate the presence or absence of lashes over the entire length. The case where no lashes were observed over the entire length of the coil was evaluated as ◯, the case where ridges were observed only at the end in the width direction was evaluated as △, and the case where lashes were observed over the entire coil was evaluated as ×.
Separately from the hot-rolled slab, a block cut out from the slab of the same component was heated and held under the same conditions and then water-cooled. The sample after water cooling was observed with an optical microscope, and the amount of δ ferrite was calculated.

Table 2 shows the survey results.
In Invention Example Test No. A-1, the component composition and the amount of δ ferrite after slab heating are in the specified range, and the average reduction rate of the slab edge portion in the first pass of rough rolling is the average reduction rate of the slab center portion. As a result, the baldness did not occur.
On the other hand, even if the component composition and the amount of δ ferrite after slab heating are within the specified range, the average rolling reduction of the slab edge portion in the first pass of the rough rolling is made smaller than the average rolling reduction of the slab center portion. In Comparative Example Test No. A-2, baldness was generated. Although the component composition is within the specified range, the heating temperature is high and long, so the slab in which the amount of δ ferrite is 0% by volume, the rolling reduction of the slab edge part is made larger than the central part of the slab, and the first pass of rough rolling In Comparative Example Test No. A-3, which was passed, bald wrinkles were generated on the surface of the hot-rolled sheet. Furthermore, in Comparative Example Test Nos. B-1 and B-2 using a steel type B whose component composition is outside the specified range, δ ferrite does not occur even when a predetermined heat treatment is performed. Even when the first rough rolling pass was performed, scabs were generated.

Figure showing the appearance and cross-sectional structure of hot-rolled sheet after pickling of Cr-Ni stainless steel with baldness Diagram comparing grain boundary oxidation and δ-ferrite observed in slab cross-section subjected to heat treatment before hot rolling Diagram showing the relationship between the amount of δ ferrite and the amount of surface crack initiation limit strain The figure which arranged the amount of δ ferrite obtained when the slab was heated in the temperature range of 1000 to 1250 ° C in relation to the F value and the A value. The figure which showed the condition of F value and A value which can obtain the amount of δ ferrite effective for the prevention of the occurrence of baldness Analysis result of widthwise distribution of stress in rolling direction in slab surface layer. Width distribution of rolling reduction when rolling reduction of slab part is increased Analysis result of width direction distribution of stress in rolling direction in slab surface layer corresponding to rolling reduction distribution in FIG.

Claims (2)

  C: 0.080 mass% or less, Si: 0.01-1.5 mass%, Mn: 0.01-4.0 mass%, P: 0.05 mass% or less, S: 0.03 mass% or less , Ni: 16.0 to 25.0 mass%, Cr: 20.0 to 30.0 mass%, N: 0.05 mass% or less, Mo: 0.01 to 1.0 mass%, Cu: 0.0 The F value and A value defined as F (mass%) = Cr + Mo + Si, A (mass%) = Ni + 35C + 20N + 0.2Mn + 0.25Cu, including 001 to 1.0% by mass, the balance being Fe and inevitable impurities. Fr. 22.0-33.0, A = 17.0-28.0, F-A = 3.5-11.5 The Cr—Ni stainless steel is continuously cast on a slab, and the amount of δ ferrite of the slab is reduced to 0.5 to 5.0 by heat treatment at 1000 to 1250 ° C. After adjusting the volume% range, the average reduction ratio of the slab after that in the first pass of rough rolling in the width direction of the slab from within 1/9 of the width of the slab (hereinafter referred to as an edge portion). A method for producing a Cr—Ni-based stainless steel strip, which is hot-rolled with an average rolling reduction greater than an average reduction ratio in a section closer to the width center (hereinafter referred to as a width center portion).   The manufacturing method according to claim 1, wherein the Cr-Ni-based stainless steel further contains one or two of Ca: 0.010 mass% or less and B: 0.010 mass% or less.