JP2018016824A - Duplex stainless steel for thick electromagnetic cooker - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a duplex stainless steel which is suitably applied to a large-sized and thick electromagnetic cooker.SOLUTION: There is provided a duplex stainless steel for thick electromagnetic cooker consisting of two phases of 30 to 70% of a ferrite phase and 70 to 30% of an austenite phase, containing, by mass%, C:0.050% or less, Si:1.0% or less, Mn:6.0% or less, P:0.040% or less, S:0.030% or less, Ni:0.50 to 8.0%, Cr:19.0 to 27.0%, Mo:4.0% or less, Cu:3.0 or less, N:0.050 to 0.35% and the balance Fe with inevitable impurities, having sheet thickness of 5 mm or more and generating no crack at a flexure processed part when flexure process of over 90° is conducted.SELECTED DRAWING: None

Description

  The present invention relates to a duplex stainless steel for a thick electromagnetic cooker.

  The heating source for large-scale and large-scale food processing facilities used in lunch centers and the like is being switched from conventional gas and electric heating methods to electromagnetic heating methods (hereinafter also referred to as “IH methods”). The IH method is the reason that the heat environment is improved and the safety and operability are excellent as compared with the conventional gas and electric heating methods with less excess heat release.

  In the electromagnetic heating cooker made of the IH method, iron, ferritic stainless steel, or a clad material of iron and stainless steel has been used as a magnetic material. However, iron is inferior in corrosion resistance, and the clad material has a problem of high cost. Ferritic stainless steel has an intermediate cost between iron and cladding, but has a problem of poor workability and weldability. Moreover, although ferritic stainless steel has better corrosion resistance than iron, there is a problem that it is inferior in corrosion resistance to austenitic stainless steel. As a cooking utensil, aesthetics and corrosion resistance are also required, so the need for stainless steel is increasing.

  For this reason, duplex stainless steel (hereinafter referred to as “duplex steel”) composed of an austenite phase and a ferrite phase has characteristics of high strength and excellent corrosion resistance. In addition, austenitic stainless steel is non-magnetic and therefore cannot be applied to an electromagnetic cooker. However, by using austenite and ferrite dual phase steel, the advantages of ferrite with magnetism and austenite with corrosion resistance and workability can be utilized. be able to.

  Patent Documents 1 and 2 disclose an electromagnetic cooker using a duplex steel. Patent Document 1 proposes an electromagnetic cooker that simply uses the characteristics of an austenite phase and a ferrite phase. Patent Document 2 proposes to define the aspect ratio of austenite grains in order to improve anisotropy in magnetostriction and thermal expansion.

  However, Patent Document 1 does not specifically disclose the size and thickness of the electromagnetic cooker. Moreover, the plate | board thickness of the duplex stainless steel used for the electromagnetic cooker disclosed by patent document 2 is only 0.5 mm. In addition, neither of Patent Documents 1 and 2 specifically discloses workability. Thus, Patent Documents 1 and 2 do not have a problem recognition that a duplex stainless steel having a thickness of 5.0 mm or more is processed in order to manufacture a large-sized electromagnetic cooker used in a lunch center or the like.

Japanese Laid-Open Patent Publication No. 2-97069 JP 7-307195 A

  The size of the cooker used in the above-mentioned large-scale and large-scale food processing facilities such as a lunch center is, for example, an inner diameter of 600 to 1200 mm, a depth of 270 to 450 mm, and a product capacity of 60 to 400 L. It is much larger than the electromagnetic cooker for the kitchen. In addition, as the cooker size increases, thickening is essential to increase the strength of the container.

  Here, the plate thickness of 5.0 mm or more is referred to as “thick” in the present invention. However, in the case of a steam cooker, when it is necessary to conform to the regulations of the second type pressure vessel, a plate thickness of 7 to 8 mm or more is required.

  Moreover, since it is not heavy enough to be processed by humans due to the increase in size and thickness, the shape becomes complicated, such as being incorporated into a cooking machine. Therefore, a thickened electromagnetic cooker (thick electromagnetic cooker) is also required to be workable in order to cope with a complicated shape. Furthermore, of course, aesthetics and corrosion resistance are also required as cooking utensils.

  However, as described above, a practical material excellent in thick electromagnetic heating property and workability that can be used for a large and thick electromagnetic cooker has not been reported so far.

  In view of the above circumstances, an object of the present invention is to find a material that can be suitably applied to a thick electromagnetic cooker having a large and complex shape.

  In order to solve the above problems, the present inventor has conceived that duplex stainless steel is applied to a thick electromagnetic cooker, and has verified its applicability. That is, earnestly researched about various factors for preventing the bending portion from being cracked when the steel plate was bent over 90 ° so that the bending radius (R) was equal to or less than the plate thickness (t). As a result, the metal structure is composed of two phases of ferrite and austenite so that the ferrite phase is 30 to 70% and the austenite phase is 70 to 30%, and the chemical composition is reviewed and optimized, so that it is 5.0 mm or more. It has been found that the material can be used satisfying both electromagnetic characteristics and press workability as a material for a thick electromagnetic cooker.

The gist of the present invention is as follows.
(1) It consists of two phases of ferrite and austenite,
The ferrite phase is composed of 30 to 70%, the austenite phase is composed of 70 to 30%,
In mass%, C: 0.050% or less, Si: 1.0% or less, Mn: 6.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0.50 8.0%, Cr: 19.0 to 27.0%, Mo: 4.0% or less, Cu: 3.0 or less, N: 0.050 to 0.35%, the balance being Fe and inevitable Consisting of impurities,
A duplex stainless steel for a thick electromagnetic cooker, having a plate thickness of 5 mm or more and having no bending at the bending portion when it is bent over 90 °.
(2) By mass%, Ni: 1.0 to 6.0%, Cr: 20.5 to 24.5%, Mo: 1.9% or less, Cu: 2.0% or less, N: 0.050 The duplex stainless steel for thick electromagnetic cookers according to (1), characterized by being limited to ˜0.25%.
(3) By mass%, Mn: more than 2.0% to 6.0%, Ni: limited to 1.0 to less than 4.0%, described in (1) or (2) Duplex stainless steel for thick electromagnetic cookers.
(4) The duplex stainless steel for thick electromagnetic cooker according to any one of (1) to (3), wherein the following PRE value is 21 to 34.
PRE = Cr + 3.3 × Mo + 16 × N; provided that Cr, Mo and N in the formula indicate the respective contents (unit: mass%) of the elements Cr, Mo and N in the steel.

  ADVANTAGE OF THE INVENTION According to this invention, heating element apparatuses, such as a large-sized pan which respond | corresponds to IH heating and was excellent also in corrosion resistance and moldability, can be manufactured economically. Further, according to the present invention, the possibility of adopting IH is widened, and energy saving / cost saving (output control during cooking / automatic cooking possible) becomes possible. In addition, according to the present invention, since it is possible to reduce the residual heat emission during cooking in large-scale food cooking and processing facilities such as lunch centers, it is possible to introduce an IH heating method that can greatly improve the working environment (room temperature and humidity). Can be greatly expanded.

(A), (b) is an expanded sectional view of the bending process part for demonstrating a bending radius, (a) is an expanded sectional view of the bending process part when bent by bending angle (theta) ((theta)> = 90 degrees). And (b) is an enlarged cross-sectional view of the bent portion when bent at a bending angle of 180 °.

The composition range of steel in the present invention will be described below.
In the following description, “%” representing the content of each element means mass% unless otherwise specified.

(C: 0.050% or less)
C inhibits oxidation resistance by forming a solid solution or Cr carbide in the ferrite phase. On the other hand, C forms an carbide, is an element effective for strengthening steel and refining ferrite grains, and is effective for enhancing the stability of the austenite structure. However, when C is added in a large amount, the workability is deteriorated. From such a viewpoint, C may be contained with 0.050% as an upper limit. In addition, since excessively reducing C causes an increase in cost at the steelmaking stage, the lower limit may be set to 0.0005%. From the viewpoint of stable productivity, the content is preferably 0.0015% or more.

(Si: 1.0% or less)
Si may be used as a deoxidizing element during melting or may be positively added to improve oxidation resistance. It is also a ferrite stabilizing element. From the viewpoint of deoxidation, it is preferably 0.03% or more. Moreover, since addition of a large amount may cause deterioration in workability due to hardening of the material, the upper limit is made 1.0%. In view of workability and stable productivity, the content is preferably 0.30% or less, and more preferably 0.20% or less. Furthermore, it is desirable to make it 0.15% or less in order to ensure workability and stable manufacturability. However, since extremely low Si increases the cost, the lower limit is preferably 0.01%.

(Mn: 6.0% or less)
Mn is an element added as a deoxidizer during melting, and contributes to the stabilization of the structure by expanding the austenite single phase region. However, if added excessively, coarse inclusions are generated and workability deteriorates, so the upper limit is made 6.0%. The lower limit is not particularly defined, but is preferably more than 2.0% from the viewpoint of reliable stabilization of the austenite phase and workability.

(P: 0.040% or less)
P is an element contained as an impurity in the main raw material such as hot metal or ferrochrome. Since it is an element harmful to hot workability, it is set to 0.040% or less. In addition, Preferably it is 0.030% or less. Excessive reduction leads to an increase in cost, such as making it necessary to use a high-purity raw material, so 0.010% or more. Economically preferably 0.020% or more is desirable.

(S: 0.030% or less)
S forms sulfide inclusions and degrades the general corrosion resistance (entire corrosion and pitting corrosion) of steel materials. Therefore, the upper limit of the content is preferably as small as 0.030%. Further, the smaller the S content, the better the corrosion resistance. However, since the desulfurization load increases and the production cost increases for lowering the S content, the lower limit is preferably made 0.0001%. In addition, Preferably it is 0.0001-0.0010.

(Ni: 0.50% to 8.0%)
Ni is an element that stabilizes the austenite phase in the same way as Mn, and has an effect superior to that of Mn in terms of oxidation resistance. Since these effects are obtained at 0.50% or more, the lower limit is made 0.50% or more. The content of Ni is preferably 1.0% or more. On the other hand, excessive addition of Ni decreases the hot workability, so it is made 8.0% or less. From the viewpoint of improving workability, Ni is preferably 6.0% or less, and more preferably less than 4.0%.

(Cr: 19.0% to 27.0%)
Cr is a basic element of stainless steel, and is an essential element for ensuring oxidation resistance and corrosion resistance. If it is less than 19.0%, these effects do not appear, so the lower limit is made 19.0% or more. From the viewpoint of oxidation resistance, it is desirable to make it 20.5% or more. On the other hand, if it exceeds 27.0%, the austenite single-phase region is reduced, forming a compound with C and N to impair hot workability during production. Therefore, the upper limit is made 27.0%. Preferably, it is 24.5% or less.

(Mo: 4.0% or less)
Mo is an element that improves the corrosion resistance, and may be added as necessary. Since the effect is exhibited when Mo is added in an amount of 0.01% or more, it is preferable to set this as the lower limit. On the other hand, since Mo has a large atomic radius, its solid solution strengthening ability is strong, and as a result, steel is hardened and workability is deteriorated, so excessive addition is not preferable. Therefore, the upper limit of the addition amount of Mo is 4.0% or less. The upper limit of the amount of Mo added is preferably 1.9% or less.

(Cu: 3.0% or less)
Cu is an element that may be added as necessary, but is a relatively inexpensive element that substitutes for Ni as an austenite stabilizing element. Furthermore, it is effective in suppressing the progress of crevice corrosion and pitting corrosion. For that purpose, it is desirable to add 0.01% or more. However, if it exceeds 3.0%, the hot workability is lowered, so the content is made 3.0% or less. Preferably, it is 2.0% or less.

(N: 0.050% or more and 0.35% or less)
N is the most powerful austenite phase stabilizing element and is an effective interstitial solid solution strengthening element. In order to obtain the additive effect, the content is made 0.050% or more. However, excessive addition causes precipitation of nitrides, and neither the required strength nor the stability of the austenite phase can be obtained. Further, from the viewpoint of workability, the upper limit is made 0.35%. Preferably, it is 0.25% or less.

  The balance is basically composed of iron and inevitable impurities, but may contain elements other than the above elements as long as the effects of the present invention are not impaired.

(Pitting corrosion resistance)
As described above, excessive addition of Mo hardens the steel and deteriorates workability, so the upper limit of the amount of Mo added is preferably 1.9% or less. On the other hand, the pitting corrosion resistance of stainless steel is said to be better as the PRE value (Pitting Resistance Equivalent) defined by the following formula is larger.
PRE = Cr + 3.3 × Mo + 16 × N; provided that Cr, Mo and N in the formula indicate the respective contents (unit: mass%) of the elements Cr, Mo and N in the steel.
Therefore, it is preferable to adjust the Mo amount so that the PRE value is 21 to 34. A more preferred lower limit is 23, and a more preferred upper limit is 30.

(Ferrite phase 30-70%, austenite phase 70-30%)
In order to obtain good characteristics in the duplex stainless steel of the present invention, the austenite phase area ratio needs to be 30 to 70%. When the austenite phase area ratio is less than 30%, poor toughness occurs, and when it exceeds 70%, hot workability and stress corrosion cracking occur. In either case, the bending workability is poor, and cracking occurs in the bent portion when 90 ° bending is performed.

(Manufacturing process)
In the method for producing a duplex stainless steel for a thick electromagnetic cooker according to the present invention, the step of producing a steel plate to be subjected to finish rolling is not particularly limited. The steel melted by a known means (for example, an electric furnace) is cast into a slab having a thickness of 150 to 250 mm by a continuous casting machine, the surface is ground as required, and then heated to 1200 ° C. or higher to perform hot rolling. Hot-rolled steel strip by hot rolling. The hot-rolled steel strip is annealed at a temperature of about 1100 ° C. and pickled. Subsequently, cold rolling and annealing are repeated to obtain a steel sheet having a desired thickness. The finish annealing may be an annealed pickling finish (2B finish) or a BA (Bright Annealing) finish that is annealed in a non-oxidizing atmosphere. In addition, the process after finish rolling is not particularly limited, and a shape forcing or degreasing cleaning process may be applied.

  According to the present invention, even when the thickness (t) of the steel plate is 5.0 mm or more, as shown in FIGS. 1 (a) and 1 (b), the bending radius ( (Hereinafter referred to as “bending radius”) When the bending process is performed so that the size (R) is 90 ° or less, no cracks or cracks occur in the bent part by visual observation. Thus, according to the present invention, it is possible to provide a duplex stainless steel as a practical material excellent in thick electromagnetic heating property and workability that can be used for a thick electromagnetic cooker.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

  In this example, first, 50 kg of steels having the chemical components of Invention Examples 1 to 5 and Comparative Steels 1 to 4 shown in Table 1 were melted by vacuum melting and subjected to heat forging at 1200 ° C. to 60 mm thickness × 280 mm. A steel ingot having a width of about 360 mm was formed, followed by hot rolling to a thickness of 6.3 mm by heating at 1200 ° C. The hot-rolled steel sheets thus obtained were each annealed under the conditions shown in Table 2, and then pickled to obtain hot-rolled annealed steel sheets of Invention Examples 1-5 and Comparative Steels 1-4.

(Processability evaluation test)
Each workability of the hot-rolled annealed steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4 was evaluated in the following manner. First, from each of the hot-rolled annealed steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4, two sheets having a longitudinal direction of 180 mm × width of about 280 mm were sampled and flattened, and then the surface was # 120 in the 280 mm direction. The test piece of thick material (plate thickness 6 mm) was produced by finish polishing. Next, at 0 ° C., as shown in FIG. 1 (a), the test piece is bent to 135 ° in a direction perpendicular to the hot rolling direction so that the bending ridge line is parallel to the hot rolling direction, The crack generation state at that time was visually observed. The bending radius (R) was a plate thickness (t = 6.0 mm). The results are shown in the item “Evaluation of workability” in Table 4.

  In Comparative Examples 1 and 2, large cracks that penetrated the plate thickness up to a bending angle of 90 ° occurred, whereas inventive steels 1 to 3 and comparative steels 3 and 4 did not generate any cracks. As shown in b), bending by 180 ° was possible. Inventive steels 4 and 5 did not generate cracks up to a bending angle of 135 °, but at the bending angle of 180 °, minute cracks in the form of openings were observed on the edge portion and the bent outer surface in the vicinity thereof. From these results, the comparative steels 4 and 5 are inferior in workability (×), the inventive steels 4 and 5 are in good workability (◯), and the inventive steels 1 to 3 and the comparative steels 3 and 4 are excellent in workability. (◎).

(Electromagnetic heating characteristics test)
Except for the parts subjected to the workability evaluation test, the surfaces of the hot-rolled annealed steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4 were partially maintained with a # 60 grinder, and oxidized scales and defects The material for cold rolling of invention examples 1-5 and comparative steels 1-4 was produced by removing. Each of these cold-rolling materials was once cold-rolled to 2.4 mm, and then subjected to the first annealing under the conditions shown in Table 3, followed by pickling. Each surface of the cold-rolled materials of Invention Examples 1 to 5 and Comparative Steels 1 to 4 that were pickled was ground and removed with # 240 emery paper, and a 400 mm long plate was taken in the rolling direction, and these were respectively removed. Cold rolling was further performed to 1.0 mm.

  The cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4 having a thickness of 1.0 mm thus obtained were annealed for the second time under the conditions shown in Table 3, pickled, and Invention Examples 1 to 5 and Comparative Steels. 1-4 final cold-rolled steel sheets were obtained.

  Using each of the final cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4, the power consumption of the steel sheets themselves was compared in the following manner. That is, first, a circular flat plate having an outer diameter of 180 mm was collected from the final cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4, and was corrected to be as flat as possible to prepare a circular test piece. Next, the circular test piece is laid flat in the center of the work surface of a commercially available home electromagnetic cooker (rated 100 V, 1400 W), the output is set to “medium” (700 W), and measured with an infrared thermometer. The amount of power consumed until the surface temperature of the sample reached 200 ° C. was compared. As an evaluation standard for power consumption, the final cold-rolled steel sheet of comparative steel 4 corresponding to the ferritic representative steel type SUS430 is used as a reference, and 115% or less is good with respect to the power consumption of the steel sheet of the reference (◯), More than 115% was evaluated as inferior (x). This result is shown in the item “Evaluation of power consumption” in Table 4.

  Also, each of the final cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4 are press-molded into square tubes, and these are used as a simulated container. The electromagnetic heating performance of the material was evaluated by comparing the water temperature rise rate when heating was started in the cooker. That is, first, from each of the final cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4, a 230 mm square plate was press-formed with a 105 mm square punch and a depth of about 50 mm to produce a square tube. Next, the square tube was placed in the center of the work surface of a commercially available home electromagnetic cooker (rated 100 V, 1400 W), 500 ml of 25 ° C. tap water was placed therein, and the home electromagnetic cooker was heated at 1400 W. . The time until the measurement temperature of the thermocouple arranged at substantially the center in water reaches 95 ° C. by this heating was measured three times for each of the final cold-rolled steel sheets of Invention Examples 1 to 5 and Comparative Steels 1 to 4, and averaged. Time was compared. As an evaluation standard for electromagnetic heating performance, 105% of the average time until 25 ° C. water reaches 95 ° C. is based on the square tube of the final cold rolled steel sheet of comparative steel 4 corresponding to the ferritic representative steel type SUS430. The following was evaluated as good (◯), and over 105% was evaluated as inferior (×). The results are shown in the item “Evaluation of electromagnetic heating performance” in Table 4. In addition, the numerical value in the parenthesis of each column shows the average time of the said 3 times measurement.

  From the results of the workability evaluation in Table 4 and the results of the electromagnetic heating characteristic test, the applicability of the inventive steels 1 to 5 and the comparative steels 1 to 4 to the electromagnetic cooker was comprehensively evaluated. Among the results of the evaluation of workability and the electromagnetic heating characteristic test in Table 4, the steel type having an inferior (×) evaluation in any item was evaluated as having no applicability (×). On the other hand, among the results of the workability evaluation and electromagnetic heating characteristics test shown in Table 4, those in which all items were good were evaluated as acceptable (◯), and those with particularly excellent workability were evaluated as (◎).

  Thus, the steel plate which remove | deviates from the composition and / or metal structure of this invention is not suitable for use for an electromagnetic cooker. On the other hand, since the steel plate which has a composition and metal structure of this invention is excellent in workability and an electromagnetic heating characteristic, it can be applied suitably for an electromagnetic cooker.

  ADVANTAGE OF THE INVENTION According to this invention, the duplex stainless steel which can be applied suitably for a large sized and thick electromagnetic cooker can be provided.

Claims (4)

It consists of two phases of ferrite and austenite,
The ferrite phase is composed of 30 to 70%, the austenite phase is composed of 70 to 30%,
In mass%, C: 0.050% or less, Si: 1.0% or less, Mn: 6.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0.50 8.0%, Cr: 19.0 to 27.0%, Mo: 4.0% or less, Cu: 3.0 or less, N: 0.050 to 0.35%, the balance being Fe and inevitable Consisting of impurities,
A duplex stainless steel for a thick electromagnetic cooker, having a plate thickness of 5 mm or more and having no bending at the bending portion when it is bent over 90 °.
In mass%, Ni: 1.0-6.0%, Cr: 20.5-24.5%, Mo: 1.9% or less, Cu: 2.0% or less, N: 0.050-0. The duplex stainless steel for thick electromagnetic cooker according to claim 1, characterized by being limited to 25%.
The mass% is limited to Mn: more than 2.0% to 6.0% and Ni: 1.0 to less than 4.0%. Phase stainless steel.
The following PRE value is 21-34, The duplex stainless steel for thick electromagnetic cookers of any one of Claims 1-3 characterized by the above-mentioned.
PRE = Cr + 3.3 × Mo + 16 × N; provided that Cr, Mo and N in the formula indicate the respective contents (unit: mass%) of the elements Cr, Mo and N in the steel.

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