JP5349015B2 - Method for producing Ni-saving austenitic stainless hot-rolled steel sheet, slab and hot-rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress occurrence of edge-cracking in a hot-rolled Ni-saving type austenitic stainless steel with reduced Mn content. <P>SOLUTION: The method for manufacturing the hot-rolled Ni-saving type austenitic stainless steel sheet comprises: a slab composed, by mass, of &gt;0.05% C, satisfying a formula: 0.10%&le;C+0.5N&le;0.25%, and &le;4% Si, 0.5 to &lt;3% Mn, &le;0.06% P, &le;0.005% S, 0.5 to &lt;5% Ni, &gt;16 to 19% Cr, &gt;0.05% N in the above formula, 0.8-3.5% Cu and the balance Fe with inevitable impurities, is held in the temperature range of the austenitic-single phase at 1100-1250&deg;C; the slab is made to have structural-state having &le;4% &delta;-ferritic phase area ratio in the range within 100 &mu;m from the slab edge part and &le;30 &mu;m average value of the upper class 20% in the &delta;-ferritic phase long diameter; and hot-rolling is applied to the slab. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for manufacturing a Ni-saving austenitic stainless hot-rolled steel sheet that suppresses the occurrence of ear cracks during hot rolling, a cast slab and a heated slab for use in the manufacturing method, and a hot-rolled steel sheet.

  Work hardening type metastable austenitic stainless steel (so-called 300 series stainless steel) represented by SUS301, SUS304 and the like is used for various applications by taking advantage of its excellent manufacturability, corrosion resistance, workability and the like. Austenitic stainless steel is expensive because it generally contains a large amount of Ni.

  In applications that require cheaper austenitic steels, Ni-reducing austenitic stainless steels such as SUS201 and SUS202 (so-called 200 series stainless steels) that contain a large amount of austenite-forming elements such as Mn instead are reduced. Alternatively, high Mn austenitic steel types based on them may be applied. Patent Documents 1 to 4 describe various high Mn austenitic stainless steels.

  High Mn austenitic steel is generally inferior in corrosion resistance, hot workability and formability compared to 300 stainless steel. Further, since a large amount of Mn is contained, harmful Mn oxide fine particles (Mn fume) are generated in the steelmaking process, and environmental measures are required. In the lower processes such as cold rolling, annealing, pickling, etc., the surface quality of the product is likely to deteriorate due to the high Mn content. Therefore, there are many problems in terms of manufacturability and material characteristics when applying high Mn austenitic steel as an alternative to 300 stainless steel.

  Patent Documents 5 and 6 show Ni-saving austenitic stainless steel with a reduced amount of Mn. However, these materials have low strength or workability, and have not obtained material properties that can replace 300 series stainless steel. Patent Document 7 discloses Ni-saving austenitic stainless steel in which the amount of Mn, which has hot workability and corrosion resistance equivalent to SUS304, is relatively reduced. However, the Mn content is 3% by mass or more, and further reduction of the Mn content is desired in order to solve the problems of environmental degradation at the production site and the deterioration of the surface properties of the product.

JP 2006-111932 A JP 2007-197806 A JP-A-11-241145 JP-A-7-70700 JP 56-15951 A JP 2006-22369 A JP 2007-63632 A

  According to the investigation by the inventors, in Ni-saving austenitic stainless steel, when the Mn content is reduced to a level of less than 3%, hot workability is lowered, and a healthy hot-rolled steel sheet without ear cracks is obtained. Was found to be very difficult. Occurrence of ear cracks can be a problem in the safe production of high-quality steel sheets, and if the degree of ear cracks is large, it can cause a decrease in yield due to trimming and increase production costs. .

  The present invention provides a technology for suppressing the occurrence of ear cracks in hot rolling in Ni-saving austenitic stainless steel with reduced Mn content, and can be applied in many applications as an alternative to 300-based stainless steel. It is an object to provide a material having excellent corrosion resistance and material characteristics.

The above-mentioned purpose is in mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8 % To 3.5%, the balance being Fe and inevitable impurities, and if necessary, the austenite stability index Md ₃₀ defined by the following formula (2) is 0 to 80 and the following formula (3) A slab having a chemical composition satisfying the stacking fault energy index SFE defined by ≦ 0 and less than 40 is charged into a heating furnace and maintained in a temperature range of 1100 to 1250 ° C. and an austenite single-phase temperature range (A) below. The process of
Taking out the slab in the textured state from the heating furnace, and subjecting the slab to hot rolling,
This is achieved by a method for producing a Ni-saving austenitic stainless hot-rolled steel sheet having:
0.10 ≦ C + 0.5N ≦ 0.25 (1)
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr ... (2)
SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (3)
Here, the content value of the element expressed by mass% is substituted for the element symbol in the formulas (1) to (3).
(A) In the region within the depth of 100 μm in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is 30 μm or less on average. A certain tissue state The central portion of the slab thickness is a central region in the thickness direction corresponding to 15% of the slab thickness.

As the slab before charging into the heating furnace, it is more preferable to apply a cast slab having the following structural state (B).
(B) In the region within 100 μm depth in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 15% or less, and the major axis of the δ ferrite phase is 60 μm or less on average. A certain structure state When such a cast slab is used, it becomes possible to make the time to hold | maintain in 1100-1250 degreeC and an austenite single phase temperature range about 1-5h, and the heating furnace in a general continuous hot rolling line Can be used. That is, it is not necessary to perform heating and holding for a very long time.

  The cast slab having the above-described structure state is obtained by injecting molten steel whose components are adjusted to a predetermined composition into a mold for manufacturing a slab, and an average cooling rate from the solidification start temperature to 1250 ° C. at the surface of the slab edge at the center of the slab thickness. Can be obtained by a casting process so that the temperature is 50 ° C./min or more.

  Moreover, in this invention, it is a "cast slab" for charging in the heating furnace of a hot rolling process as a slab which has the said chemical composition, Comprising: The thing which is the structure state of the said (B) is provided. In addition, there is provided a “heated slab” held in the heating furnace at 1100 to 1250 ° C. and in the austenite single-phase temperature range, which is in the textured state of (A).

Furthermore, in the present invention, it is a hot rolled steel sheet with no ears, which is obtained by hot-rolling a heated slab having the above chemical composition and adjusted to the above structural state (A), and no ear cracks are observed. A Ni-saving austenitic stainless hot-rolled steel sheet is provided. The steel sheet includes a steel strip. Ear cracks are cracks that occur at the edges (widthwise ends; called “ears”) of the plate during rolling. When an ear crack occurs in a certain rolling pass, the crack width increases as the plate extends in the subsequent rolling pass, and in some cases, operational troubles may be caused. The ear crack depth is the distance in the width direction (perpendicular to the rolling direction) from the edge of the steel sheet to the crack tip. In this specification, an edge defect having a depth of 1 mm or less is not treated as an ear crack. Such minute edge defects do not adversely affect the processes after hot rolling in the steel of the present invention. That is, the term “ear crack” as used herein means that the depth exceeds 1 mm.

  According to the present invention, in Ni-saving austenitic stainless steel, it is possible to achieve both “reduction of Mn content” and “inhibition of ear cracking during hot rolling”, and a healthy hot-rolled steel sheet (steel strip) ) Can be obtained stably. By reducing the Mn content, the environment at the manufacturing site is improved, and the surface quality of the steel sheet is also prevented from lowering. The obtained steel plate has good corrosion resistance and mechanical properties, and can be used as a substitute for 300 series stainless steel in many applications including springs, mechanical parts, and gaskets, and can contribute to cost reduction of products. .

  The inventors have studied a technique for improving hot workability in Ni-saving austenitic stainless steel with a reduced Mn content. As a result, it was confirmed that in such a component system, a δ ferrite was easily distributed in the austenite substrate, and the interface between the austenite substrate and δ ferrite functioned as a starting point or propagation path of hot-ear cracks. Therefore, it is advantageous for preventing ear cracks that the amount of δ ferrite phase is as small as possible during hot rolling. However, reducing the amount of δ ferrite only by adjusting the chemical composition is limited to a limited composition range and is not always a good idea.

  As a result of various studies, it is not necessary to reduce the amount of δ ferrite for the entire inside of the heated slab to be subjected to hot rolling, and the amount of δ ferrite phase present only in the surface layer portion near the slab edge where strain is likely to act during hot rolling. It was found that by reducing the size, the ear cracks in hot rolling can be stably suppressed.

  FIG. 1 illustrates a metal structure of a cross section of a slab thickness central portion parallel to a slab longitudinal direction (casting direction) and a slab width direction of a cast slab and a heated slab equivalent material corresponding to the present invention. The “slab edge center” means the center of the slab edge in the thickness direction. The chemical composition of the steel illustrated in FIG. 1 is as follows: C: 0.12%, Si: 0.5%, Mn: 2.8%, Ni: 2.3%, Cr: 16.3%. Cu: 2.8%, N: 0.12%, balance Fe and inevitable impurities. Here, the material corresponding to the heated slab shown in FIG. 1B is obtained by heating a sample collected from the cast slab at 1230 ° C., which is an austenite single phase region, for 2 hours and then rapidly cooling it in water. In either case, the sample was embedded in a resin so that the cross section of the slab thickness central part parallel to the slab longitudinal direction and the slab width direction (that is, the cross section of the slab thickness central part parallel to the slab wide surface) could be observed, polished and by NaOH aqueous solution Electrolytic etching was performed and observed with an optical microscope.

  In the cast slab (FIG. 1A), there is a difference in the distribution form of the δ ferrite phase between the vicinity of the slab edge and the inside in the width direction. In the vicinity of the slab edge, the cooling rate at the time of casting is high, so the amount of δ ferrite phase is smaller than the inside and the size is also small. In such a heated slab (FIG. 1 (b)) in which the cast slab is heated in the austenite single phase region, the amount and size of the δ ferrite phase are considerably reduced in the vicinity of the slab edge.

As a result of detailed examination of the relationship between the structure of the heated slab and the hot workability, the inventors have found that the ear cracks in the hot rolling of the steel types targeted by the present invention are austenite and δ ferrite. The interface between the austenite substrate and the δ ferrite is a crack propagation path. The smaller the amount of δ ferrite and the smaller the interfacial area between the austenite phase and δ ferrite phase, the lower the ear cracking sensitivity in hot rolling. The amount of δ ferrite can be expressed as the volume ratio of the δ ferrite phase (area ratio in cross-sectional observation), and the interface area between the austenite phase and the δ ferrite phase greatly depends on the major axis of the δ ferrite phase. According to the study by the inventors, when the following (A) structure is obtained in the heating furnace before hot rolling, it becomes clear that the ear cracks in hot rolling can be stably suppressed remarkably. It was.
(A) In the region within the depth of 100 μm in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is 30 μm or less on average. A certain structural state In the steel of the chemical composition (described later) targeted by the present invention, the internal δ ferrite form exceeding 100 μm from the slab edge has little influence on the ear cracks in hot rolling.

  The mechanism of hot cracking at the interface between the austenite phase and the δ ferrite phase is not clear, but the ferrite-forming elements P and S are concentrated in the δ ferrite phase and are heated before and after hot rolling. In the process of transformation of the δ ferrite phase to the austenite phase during rolling, P and S are segregated at the interface between the two, and the interfacial bond strength is reduced, and hot rolling strain is accumulated at the interface, leading to cracking. .

As for the area ratio of the δ ferrite phase, the area ratio of the δ ferrite phase is measured in the metal structure of the cross section of the slab thickness central portion parallel to the slab longitudinal direction and the slab width direction as illustrated in FIG. Can be determined by The major axis of the δ ferrite phase refers to the diameter of the longest part of each δ ferrite phase that appears in the form of islands or particles appearing in the cross section. In cross-sectional observation, a region having a depth of 100 μm from the surface of the slab edge is observed over a distance of 15 mm or more in the longitudinal direction of the slab, and data of the δ ferrite phase observed in these regions is adopted. For the δ ferrite phase existing across the inside and outside of the region, when calculating the area ratio, only the area of the portion of the δ ferrite phase existing in the region is adopted, and the major axis is the entire δ ferrite phase. Determine from the image. The top 20% of the major axis of the δ ferrite phase means that the total number of δ ferrite phases to be measured is n, and the n major axis data are arranged in descending order from the largest, n × 0.2 from the first. It is the average value for the data up to (rounded down).
This measurement method can be applied to the determination of the tissue state of (A) and the tissue states of (B) and (C) described later. However, in the determination of the structure state of (C), “slab” may be replaced with “steel plate” in the above description.

  In order to obtain a heated slab having the above-described (A) structure, it is important to maintain the cast slab at 1100 to 1250 ° C. and in the austenite single-phase temperature range. When the holding temperature is lower than 1100 ° C., the finishing temperature in hot rolling is lowered and the deformation resistance is likely to increase. In addition, it may be difficult to control the amount and size of the δ ferrite phase in the vicinity of the slab edge within the predetermined range. If it exceeds 1250 ° C., it may approach the solidus temperature, which is not preferable.

  In FIG. 2, C: 0.12% by mass, Si: 0.5%, Mn: 2.8%, Ni: 2.3%, Cr: 15-19%, Cu: 2.8%, N : An example of an equilibrium diagram obtained by calculation when the content of Cr is taken on the horizontal axis in the composition of 0.12% and the balance Fe. In this case, for example, when heated and held in the region indicated by b, a δ ferrite phase is generated. Therefore, it becomes difficult to reduce the amount and size of the δ ferrite phase in the vicinity of the slab edge to the predetermined range. Therefore, for example, it is necessary to heat and hold in the austenite single phase region as indicated by a in FIG. By controlling the heating and holding time according to the form of δ ferrite in the cast slab, the above-described desired structure state can be obtained. Even if the austenite single phase region is heated for a long time, it is generally difficult to completely eliminate the δ ferrite phase generated during casting by diffusion, and a certain amount of δ ferrite phase usually remains.

When a cast slab having a large amount of δ ferrite phase is used, heating for a long time is required in order to obtain the above-described microstructure (A). Therefore, in order not to significantly increase the heating time during hot rolling, it is advantageous to apply a cast slab whose structure is suitably adjusted. Specifically, it has been found desirable to employ a cast slab having the following structural state (B).
(B) In the region within 100 μm depth in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 15% or less, and the major axis of the δ ferrite phase is 60 μm or less on average. If the amount and size of the δ ferrite phase near the slab edge is reduced to this extent in a cast slab, the heating time at 1100 to 1250 ° C. and the austenite single phase temperature range can be set to 1 to 5 hours. Thus, a heating furnace of a continuous hot rolling line can be used.

  The cast slab having the above-described structure (B) can be obtained by controlling the cooling rate at the time of casting according to the chemical composition of the steel (ease of formation of δ ferrite phase). Specifically, molten steel having the chemical composition described below is poured into a mold for slab manufacturing, and the average cooling rate from the solidification start temperature to 1250 ° C. on the slab edge surface in the center of the slab thickness becomes a predetermined value or more. As long as it is cast. The predetermined value (lower limit of the average cooling rate) varies depending on the chemical composition of the steel (ease of formation of δ ferrite phase), but the structure of (B) above is stable over the entire composition range defined in the present invention. In order to obtain a cast slab having a state, the average cooling rate is preferably 50 ° C./min. The temperature range from the solidification start temperature to 1250 ° C. is a range including a temperature range where the δ ferrite phase crystallizes, as shown in FIG. 2, for example. By shortening the time for passing through this temperature range, the amount of δ ferrite phase produced can be suppressed. It is possible to control the cooling rate during casting by adjusting the water cooling conditions of the mold according to the casting rate. Suitable casting methods include continuous casting and batch casting using a flat mold.

  Hot rolling can be performed by a conventional method. Specifically, the heated slab adjusted to the above-described textured state (A) is taken out of the furnace and subjected to a plurality of passes of hot rolling. For example, the finish rolling temperature is 900 to 1050 ° C., and the winding temperature is 400 to 900. It may be set to ° C. According to detailed studies by the inventors, in the case of the steel type targeted by the present invention, the ear cracks occur in a hot rolling pass with a hot rolling rate of 60% or more. The cause is considered to be the balance between the accumulated hot-rolling strain and the form of the δ ferrite phase that changes due to hot rolling. That is, as the hot rolling rate increases, the hot rolling strain increases and the δ ferrite phase also increases the major axis. For this reason, as the hot rolling rate increases, the hot-rolling cracking sensitivity also increases. However, when the hot rolling rate is further increased, the δ ferrite phase is stretched in layers, and the hot cracking susceptibility due to the form of the δ ferrite phase starts to decrease. As a result of various studies, when the hot rolling rate is 60 to 70%, the hot-rolling cracking susceptibility caused by the form of the δ ferrite phase is maximized, and the hot-rolling susceptibility caused by hot-rolling strain is increased by the subsequent hot-rolling pass. Even if increases, the increase rate of the total hot-rolling susceptibility expressed as “hot-rolling cracking susceptibility due to hot-rolling strain” + “hot-rolling cracking susceptibility due to the form of δ ferrite phase” decreases, In some cases, it may be reduced. Accordingly, if the form of the δ ferrite phase immediately after receiving the first hot rolling pass corresponding to a hot rolling rate of 60% or more (for example, 66% rolling rate) is sufficiently low in hot rolling cracking, Furthermore, even if hot rolling is continued, it becomes possible to suppress the occurrence of edge cracks up to a hot rolling rate of about 95%.

When examined in detail, immediately after receiving the first hot rolling pass corresponding to a hot rolling rate of 60% or more, if the following (C) microstructure is obtained, the total hot rolling rate in the subsequent rolling pass For example, it was found that even if hot rolling was performed until 95% reached 95%, no ear cracks occurred.
(C) In the region within the depth of 100 μm from the surface of the central part of the thickness of the steel sheet edge (ear), the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is the average value of the top 20%. Structure state of 50 μm or less In addition, if the edge defect is 1 mm or less in depth, there is no problem in operation in a hot rolling line or a cold rolling / annealing line.
When it has the structure state of (C) immediately after receiving the first hot rolling pass corresponding to a hot rolling rate of 60% or more, the hot rolled steel sheet obtained by continuing hot rolling thereafter Has an organizational state satisfying the above (C).

  After hot rolling, according to a general method for producing austenitic stainless cold-rolled steel sheet, for example, a sheet thickness of about 0.1 to 3 mm is achieved by a process of “cold rolling → intermediate annealing and cold rolling → finish annealing as necessary”, for example. Annealed steel sheet can be used. Thereafter, shape correction and temper rolling can be appropriately performed.

Hereinafter, the chemical composition of the steel targeted in the present invention will be described. Unless otherwise specified, “%” for steel component elements means “mass%”.
C and N are useful elements for strengthening the solution-induced martensite phase (α ′ phase) by solid solution. In the steel targeted by the present invention, the contribution of N to solid solution strengthening is about half of C. As a result of various studies, in order to obtain an excellent strength comparable to that of SUS301 by solid solution strengthening of the α ′ phase generated by molding, after securing a content of 0.05% or more for both C and N, It is very effective to contain C and N so that C + 0.5N is 0.1 or more. However, it has been found that when the contents of C and N increase, the steel material hardens, and when C + 0.5N exceeds 0.25, workability may be hindered. Therefore, in this invention, C and N are contained in the range with which following (1) Formula is satisfy | filled. The C content is more preferably 0.12% or less, and the N content is more preferably 0.18% or less.
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).

  Si is an element effective for deoxidation in steel making and contributes to solid solution strengthening. In order to obtain the effect sufficiently, it is more effective to secure a Si content of 0.3% or more. However, if excessively contained, the steel is hardened and the workability is reduced. Si is a ferrite-forming element, and its action is smaller than that of Cr. However, if it is excessively contained, a δ ferrite phase is likely to be generated in a high temperature range, and it is difficult to sufficiently improve hot workability. These adverse effects are prominent when the Si content exceeds 4%. Therefore, the Si content is in the range of 4% or less.

  Mn is a useful austenite forming element that is less expensive than Ni and can replace the function of Ni. In the case where Ni is reduced within the range described later, the Mn content must be 0.5% or more. It is more effective to secure 1% or more, and it is more effective to set it to 1.5% or more. On the other hand, when the Mn content is increased, environmental problems in the steelmaking process, surface quality deterioration problems of the steel sheet, workability deterioration due to inclusions, corrosion resistance deterioration problems, and the like are likely to occur. For this reason, in the present invention, the Mn content is limited to less than 3%.

  P and S are mixed from the raw material, but the lower the content, the better. As a range that does not have a great adverse effect on manufacturability, workability and other material properties, P is allowed to be contained in a range of 0.06% or less and S in a range of 0.005% or less.

  Ni is an essential alloying element for austenitic stainless steel, but in the present invention, a component design is performed to reduce the Ni content as much as possible from the viewpoint of cost reduction, and the Ni content is set to a range of less than 5%. However, in order to obtain good hot workability when the content of C, N, and Mn is in the above range, it is necessary to secure Ni content of 0.5% or more, and 1% or more. Is more effective.

  Cr is an essential element for forming a passive film that ensures the corrosion resistance of stainless steel. In the present invention, in order to obtain the corrosion resistance required for the conventional 300 series austenitic stainless steel, which is an alternative object, a Cr content exceeding 16% is secured. However, Cr is a ferrite-forming element. As shown in FIG. 2, when the Cr content increases, the coexistence region of δ ferrite phase + austenite phase (γ) tends to expand at high temperatures. According to the study by the inventors, in order to sufficiently secure the γ single-phase region in the heating temperature range during hot rolling, it is advantageous to make the Cr content 19% or less. Accordingly, in the present invention, the Cr content is in the range of more than 16% and not more than 19%.

  Since Cu is an austenite-generating element, inclusion of Cu facilitates component design for reducing Ni and reducing Mn. As a result of detailed studies, it is extremely advantageous to contain Cu in an amount of 0.8% or more in order to achieve low Ni and low Mn in the range of the Ni content and the Mn content. However, if Cu is contained in a large amount exceeding 3.5%, the hot workability tends to be lowered. Therefore, the Cu content is set to 0.8 to 3.5%.

  As elements other than the above, V: 0.3% or less, Zr: 0.3% or less, Mo: 0.5% or less, B, Ca, Mg, Co, and REM (rare earth elements): total 0.1% or less Such elements are allowed to be mixed. These may be inevitably mixed from raw materials such as scrap, but if mixed within the above range, the effect of the present invention is not hindered.

In the resulting steel sheet, the strength, ductility, bendability, the material properties such as sag resistance to the SUS301 equivalent level, austenite stability index Md _30, which is defined by the following equation (2) is 0 or more 80 It is desirable to adjust the component composition so that the stacking fault energy index SFE defined by the following equation (3) satisfies 0 or more and less than 40.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr ... (2)
SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (3)

When Md ₃₀ is large, transformation from the austenite phase to the work-induced martensite phase is likely to occur. Therefore, high strength can be obtained by applying mild cold-rolling strain, and the cold rolling rate is set low to improve ductility. Is particularly advantageous. Further, even in the forming process to the spring, the portion to which the processing strain is applied at the bent portion or the like is further strengthened by the TRIP phenomenon, and excellent sag resistance is obtained. These effects are remarkably exhibited by setting Md ₃₀ to 0 or more. However, if Md ₃₀ exceeds 80, the amount of work-induced martensite generation in the processed part becomes too large, and cracking is likely to occur particularly in bending.

  As the SEF increases, the work hardening of the austenite phase decreases, so the difference in hardness between the work-induced martensite phase generated during processing and the austenite phase that is the parent phase increases. Especially when the SFE is 40 or more, bending work Cracks are likely to occur near the interface between both phases. On the other hand, if the SFE is smaller than 0, the work hardening of the austenite phase becomes excessive, and a decrease in ductility tends to be a problem.

  Steels having chemical compositions shown in Table 1 were melted. In Table 1, C1 is SUS301 and C2 is SUS304. In either case, 270 kg of molten steel was cast into a water-cooled copper mold to obtain a cast slab.

  For steels other than C1 and C2, the temperature change during solidification is monitored by several thermocouples installed in the mold during casting, and the cooling rate distribution from the center surface of the slab edge thickness to the inside of the slab width direction is measured. It was measured. A cast slab sample having a thickness of 60 mm, a width of 80 mm, and a length of 120 mm was collected from the obtained cast slab. However, by changing the sampling position (distance from the slab edge to the edge of the cast slab sample) on the basis of the above cooling rate distribution data, the cast slab sample becomes “from the solidification start temperature on the center surface of the edge thickness. The “average cooling rate to 1250 ° C.” in various stages was prepared.

  About each casting slab sample, the sample which can observe the cross section of the thickness center part of the edge vicinity was prepared, and the cross section which gave the grinding | polishing and the electrolytic etching by NaOH aqueous solution was observed with the optical microscope. Cross-sectional observation is performed in a region of 100 μm depth from the slab sample edge surface over a distance of 15 mm in the longitudinal direction of the slab sample, and in the manner described above, the area ratio of the δ ferrite phase and the top 20% of the major axis of the δ ferrite phase in that region. The average value was obtained. The area ratio of the δ ferrite phase was determined using image processing of an optical microscope image.

  Each cast slab sample was heated and held at a predetermined holding temperature in the range of 1150 to 1230 ° C., then taken out of the furnace and subjected to hot rolling. The holding temperature and holding time are described in Table 3 below. Except for test number P15 in Table 3, heating was held in the austenite single-phase region, and P15 in the δ ferrite phase + austenite phase coexistence region. The hot rolling pass schedule was set as shown in Table 2. The delay of each pass was about 7 seconds, the rolling speed was about 30 m / min, and hot rolling was performed in the atmosphere under the condition that water was not sprayed. During hot rolling, the presence or absence of ear cracks was observed on the exit side of each pass. Even when the total hot rolling rate was 95%, the case where no cracking occurred was determined to be acceptable (hot workability; good).

  In order to know the structure of the heated slab (hot slab) before hot rolling, the test piece corresponding to the position near the edge of each cast slab sample was heated and held under the same conditions as the hot rolling sample. The sample for observation was produced by taking out from the furnace at the time when the holding time had elapsed, and immediately quenching in water. Using this, the area ratio of the δ ferrite phase and the average value of the upper 20% of the major axis of the δ ferrite phase in the region within 100 μm in the width direction depth from the surface of the central portion of the slab sample edge are obtained by the observation method described above. It was. In addition, for the sample that was quenched into water within 5 seconds after finishing the third pass of the hot rolling (total hot rolling ratio: 66%), the central portion of the thickness of the steel sheet edge (ear) was measured by the aforementioned observation method. The area ratio of the δ ferrite phase in the region within 100 μm in the width direction from the surface and the average value of the top 20% of the major axis of the δ ferrite phase were determined.

  Table 3 shows these results. For the A1 to A5 steels in FIG. 3, the area ratio of the δ ferrite phase in the region within a depth of 100 μm from the surface in the thickness direction of the heated slab edge (area ratio of the δ ferrite phase in the slab edge surface layer portion of 100 μm) and The relationship between the average value of the upper 20% of the major axis of the δ ferrite phase (the major axis of the δ ferrite phase in the surface layer portion of the slab edge of 100 μm) and the presence or absence of ear cracks is shown.

  As can be seen from Table 2 and FIG. 3, all of the examples of the present invention having the chemical composition defined in the present invention and satisfying the above-mentioned (A) structure in the heating slab have an initial hot rolling of 60% or more. Even when the structure of the plate immediately after the pass satisfies the above (C), no cracking occurred even at a total hot rolling rate of 95%. Among them, P12 has a slow cooling rate at a high temperature at the time of casting, but the cast slab exhibits the above-mentioned microstructure (B) because it has a chemical composition that is relatively difficult to produce a δ ferrite phase. As a result, the hot workability was good. P15 is a case where a cast slab that does not satisfy the structural state of (B) is applied, but the structural state of (A) is obtained by keeping the heating in the austenite single-phase temperature range during hot rolling for a long time. A heated slab satisfying the above conditions was obtained, and the hot workability was good.

  On the other hand, Comparative Examples P1, P2, P7, P10, P13, and P14 had a cooling rate at a high temperature during casting that was too slow for each chemical composition (easiness of formation of δ ferrite phase). A cast slab satisfying the structural state of (B) was not obtained, and because of this, a heated slab satisfying the structural state of (A) was not obtained, and thus hot workability was poor. Among these, P13 had a heating holding temperature before hot rolling in the austenite phase + δ ferrite phase coexistence region, so the amount of δ ferrite in the heated slab increased considerably, and ear cracks occurred earlier than P14. In P16, the hot workability deteriorated due to the Cu content being too high. As for P17, hot workability fell because S content was too high.

Using each steel shown in Table 1, the material properties of the cold rolled steel sheet were examined. For each steel, a hot-rolled sheet having a thickness of 3 mm was used as a starting material, and a finish annealed material and a 25% tempered rolled material were obtained in the following steps.
Finish annealing material; Solution treatment at 1060 ° C. and water cooling → Pickling → Cold rolling to 1 mm → Finish annealing and water cooling at 1080 ° C. 25% temper rolled material; Solution treatment at 1060 ° C. and water cooling → Pickling → Cold rolling to 1.33 mm → Finish annealing at 1080 ° C. and water cooling → Temper rolling to 1 mm (25%)

The finish annealed material was subjected to a tensile test in the rolling direction (crosshead speed 3 mm / min, distance between gauge points 50 mm, room temperature) using a No. 13B test piece based on JIS Z2201. Moreover, the pitting corrosion potential in a 3.5% sodium chloride aqueous solution at 30 ° C. was measured by a method according to JIS G0577 using a sample whose surface was # 600 polished.
About the 25% tempered rolling material, the Vickers hardness in 10 kg of loads was measured by the method based on JISZ2244. Further, using a test piece having a width direction of 10 mm and a length of 200 mm as a longitudinal direction, a “repetitive deflection test” was performed by a method based on JIS H3130, and a spring limit value was obtained.
The tensile test value is an average value of n = 3, the pitting potential is n = 3, the Vickers hardness is n = 5, and the spring limit value is n = 5.
The results are shown in Table 4.

  As can be seen from Table 4, the examples of the present invention were confirmed to have the same level of strength, ductility, corrosion resistance and springiness as SUS301 and SUS304.

An example of the metal structure photograph about the casting slab equivalent to this invention, and a heating slab equivalent material. An example of an equilibrium diagram obtained by calculation. Relationship between “area ratio of δ ferrite phase” and “average value of upper 20% of major axis of δ ferrite phase” in the region within 100 μm in the width direction from the surface of the central part of the thickness of the heated slab edge, and the presence or absence of the occurrence of ear cracks Graph showing.

Claims (8)

In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. 5% or less, the remaining Fe and a slab having a chemical composition that is an inevitable impurity are charged into a heating furnace, and maintained in a 1100 to 1250 ° C. and austenite single-phase temperature range to obtain the following (A) texture state;
Taking out the slab in the textured state from the heating furnace, and subjecting the slab to hot rolling,
A method for producing a Ni-saving austenitic stainless hot-rolled steel sheet having the following:
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(A) In the region within the depth of 100 μm in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is 30 μm or less on average. An organizational state In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. A cast slab having a chemical composition that is 5% or less, the balance Fe and unavoidable impurities and having the following (B) structure is charged in a heating furnace, 1-100 to 1250 ° C. and 1 to 1 austenite single-phase temperature range. Holding for 5 hours,
Removing the slab after the holding from the heating furnace and subjecting the slab to hot rolling,
A method for producing a Ni-saving austenitic stainless hot-rolled steel sheet having the following:
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(B) In the region within 100 μm depth in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 15% or less, and the major axis of the δ ferrite phase is 60 μm or less on average. An organizational state In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. Molten steel having a chemical composition of 5% or less, the balance Fe and unavoidable impurities is injected into a mold for slab production, and the average cooling rate from the solidification start temperature to 1250 ° C. at the slab edge surface in the center of the slab thickness is A process of obtaining a cast slab which is cast so as to be 50 ° C./min or more and which is in the following structural state (B)
Charging the obtained cast slab into a heating furnace and maintaining the temperature in the austenite single-phase temperature range from 1100 to 1250 ° C. for 1 to 5 hours;
Removing the slab after the holding from the heating furnace and subjecting the slab to hot rolling,
A method for producing a Ni-saving austenitic stainless hot-rolled steel sheet having the following:
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(B) In the region within 100 μm depth in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 15% or less, and the major axis of the δ ferrite phase is 60 μm or less on average. An organizational state Chemical composition of the steel further below (2) an austenite stability index Md _30, which is defined 0 to 80. In formula, and (3) below the stacking fault energy index SFE defined by the equation satisfies 0 or more and less than 40 The method for producing a Ni-saving austenitic stainless hot-rolled steel sheet according to any one of claims 1 to 3.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr ... (2)
SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (3)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (2) and formula (3). In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. A Ni-saving austenitic stainless steel slab having a chemical composition of 5% or less, the balance Fe and unavoidable impurities and having the following structural state (B).
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(B) In the region within 100 μm depth in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 15% or less, and the major axis of the δ ferrite phase is 60 μm or less on average. An organizational state In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. A hot slab having a chemical composition of 5% or less, the balance Fe and inevitable impurities, maintained at 1100 to 1250 ° C. and in the austenite single-phase temperature range, and having the following (A) microstructure Ni-saving austenitic stainless steel slab for rolling process.
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(A) In the region within the depth of 100 μm in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is 30 μm or less on average. An organizational state In the range of mass%, C: more than 0.05% and satisfying the following formula (1), Si: 4% or less, Mn: 0.5% or more and less than 3%, P: 0.06% or less, S: 0.0. 005% or less, Ni: 0.5% or more and less than 5%, Cr: more than 16% and less than 19%, N: more than 0.05% and satisfying the following formula (1), Cu: 0.8% or more and 3. 5% or less, the balance Fe and a chemical composition which is an unavoidable impurity, and is a hot rolled steel sheet with no ears that is hot-rolled to a heated slab adjusted to the following (A) textured state. Ni-saving austenitic stainless hot-rolled steel sheet with no cracks observed.
0.10 ≦ C + 0.5N ≦ 0.25 (1)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (1).
(A) In the region within the depth of 100 μm in the slab width direction from the surface of the central part of the slab edge, the area ratio of the δ ferrite phase is 4% or less, and the major axis of the δ ferrite phase is 30 μm or less on average. An organizational state Chemical composition of the steel further below (2) an austenite stability index Md _30, which is defined 0 to 80. In formula, and (3) below the stacking fault energy index SFE defined by the equation satisfies 0 or more and less than 40 The Ni-saving austenitic stainless steel slab according to any one of claims 5 and 6, or the Ni-saving austenitic stainless hot-rolled steel sheet according to claim 7.
_{Md 30 = 551-462 (C + N} ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr ... (2)
SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (3)
Here, the content value of the element expressed in mass% is substituted for the element symbol in the formula (2) and formula (3).