JP2017048417A - High purity ferritic stainless steel sheet for deep draw forming excellent in secondary working brittleness resistance and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high purity ferritic stainless steel sheet for deep draw forming having remedied secondary working brittleness, excellent in resource saving and economic efficiency and suitable for electric pot use application, and a production method therefor.SOLUTION: The high purity ferritic stainless steel sheet for deep draw forming excellent in secondary working brittleness resistance is provided that contains, by mass%, C:0.03% or less, Si:1% or less, Mn:1% or less, P:0.035% or less, S:0.003% or less, Cr:13 to 23%, N:0.03% or less, Nb:0.5% or less, Ti:0.5% or less, Al:0.1% or less and further Ni:1% or less and/or Cu:1% or less and the balance Fe with inevitable impurities, the contents of Ni and Cu satisfying the following Formula (1).0.3<Ni+Cu≤1 Formula... (1).SELECTED DRAWING: None

Description

  The present invention relates to a high-purity ferritic stainless steel sheet having excellent secondary work brittleness resistance after deep drawing, and the use in which heat treatment is performed after deep drawing, such as brazing, corresponds to the heat treatment. Suitable for use.

  Ferritic stainless steel is used in a wide range of fields such as kitchen equipment, home appliances, and electronic equipment. Recent improvements in refining technology have enabled extremely low carbon, nitrogenization, reduction of impurity elements such as P and S, and addition of stabilizing elements such as Nb and Ti to improve ferritic stainless steel with improved weather resistance and workability Steel (hereinafter referred to as high-purity ferritic stainless steel) is being applied to a wide range of applications. This is because high-purity ferritic stainless steel is more economical than austenitic stainless steel containing a large amount of Ni whose raw material price fluctuates significantly.

  Various studies have been made to improve secondary work brittleness of high purity ferritic stainless steel sheets. For example, in Patent Document 1, C: 0.0030% or less, Cr: 9 to 25%, Ti: more than 0.07 to 0.15%, N: 0.0010 to 0.0070%, 5 × (C + N ) ≦ Ti ≦ 0.12 + {4 × C + (24/7) × N}, and B: 0.0003 to 0.0050% is added as necessary. In Patent Document 2, C: 0.01% or less, Cr: 11-23%, N: 0.04% or less, B: 0.0005-0.01%, 18 ≦ Nb / (C + N) + 2 × (Ti A ferritic stainless steel sheet excellent in deep drawability and secondary work brittleness resistance containing / (C + N)) ≦ 60 is disclosed. Patent Document 3 discloses that C: 0.010% or less, Cr: 9 to 25%, Ti + Nb: 0.6% or less and (Ti + Nb) / (C + N) ≧ 7, Mg: 0.0050% or less, and Mg ≧ It is a high-purity ferritic stainless steel sheet excellent in secondary work brittleness resistance after deep drawing, characterized by satisfying 0.05 × P.

  The steel described above is characterized by excessive reduction of C and N and control of B and Mg as Ti and Nb and trace elements. Reduction of C to 0.0050% or less significantly impairs steelmaking capacity in industrial production. B, like C and N, acts as an intrusion-type solid solution element, and has the detrimental effect of reducing workability while improving secondary work embrittlement resistance. Although Mg acts effectively as a deoxidizing element, the yield to steel is extremely small, and there is also a problem that the productivity related to the steel making process is remarkably impaired by addition for ensuring secondary work brittleness resistance.

  As a method for improving secondary workability without depending on the control of Ti, Nb, B and Mg, Patent Document 4 discloses that Ni: 0.2 to 3% and Cu: 1% in a Si-containing steel exceeding 1%. Hereinafter, ferritic stainless steel excellent in wear resistance and secondary workability to which Mo: 3% or less is added is disclosed. Patent Document 5 adds Co: 0.01 to 0.3%, V: 0.01 to 0.3%, and B: 0.0002 to 0.005% in a Mo-containing steel of 1% or more. Thus, a ferritic stainless steel excellent in secondary work resistance and high temperature fatigue properties that suppresses the occurrence of vertical cracks after cup molding is disclosed. These steels are high-strength steels in which Si and Mo exceeding 1% are highly alloyed, and are completely different in composition and characteristics of high-purity ferritic stainless steels that are soft and excellent in workability.

  Until now, the inventors have disclosed a high-purity ferritic stainless steel that has improved corrosion resistance and workability by adding a small amount of Sn, regardless of the use of a high alloy such as Cr or Mo, from the viewpoint of resource saving and economy. Yes. In Patent Documents 6 and 7, C, N, Si, Mn, and P are reduced with Cr: 13 to 22%, Sn: 0.001 to 1%, and Ti and Nb stabilizing elements are added as necessary. High purity ferritic stainless steel. None of these publications discusses the effect of the alloying elements on the secondary work brittleness resistance after deep drawing, which is the object of the present invention.

JP-A-8-296000 JP 2003-201547 A Japanese Patent No. 3477113 Japanese Examined Patent Publication No. 63-149358 JP 2002-80943 A Japanese Patent No. 4651682 Japanese Patent No. 4624473

As described above, control of Ti and Nb and B and Mg as trace elements is effective for improving secondary work embrittlement of high purity ferritic stainless steel sheets, but problems remain in workability and manufacturability. In addition, the control of such elements and the alloying of Si, Mo, etc. disclosed in high-strength steels are manifested when heat treatment such as brazing is performed after the deep drawing process intended by the present invention. It is also unclear whether it will work effectively against work brittleness.
Therefore, the object of the present invention is to provide deep drawing with excellent resource saving and economical efficiency that has improved secondary work brittleness that is manifested by heat treatment after deep drawing, regardless of alloying of trace elements, Si, Mo, etc. The object is to provide a high purity ferritic stainless steel sheet for forming.

  In order to solve the above-mentioned problems, the present inventors, in a high-purity ferritic stainless steel sheet, the effect of heat treatment on the secondary work brittleness after deep drawing and the effect of alloying elements to improve the secondary work brittleness As a result, the present inventors have made the present invention with the following new findings.

(A) Secondary work brittleness after deep drawing is manifested by heating to 600 to 650 ° C. and slow cooling equivalent to furnace cooling (3 ° C./min or less). Even in a high purity ferritic stainless steel sheet that has not occurred, brittle cracking may be induced. These brittle cracks are generated when a deep-drawn product is hit with a hammer or dropped, and propagates into the grains starting from the crystal grain boundaries to extend into macro vertical cracks. As a result of revealing and analyzing the grain boundary that is the starting point of cracking in an ultrahigh vacuum of an electron microscope, it was found that the grain boundary segregation of P became apparent. P grain boundary segregation is the bonding force of grain boundaries. It is thought that the starting point of cracking was given by lowering.

(B) In order to suppress the secondary work brittleness described above, it is effective to add 1% or less of Ni or Cu alone or in combination without depending on an excessive decrease of P of 0.01% or less. These elements are considered to have the effect of delaying the grain boundary segregation of P generated in the cooling process after heating at 600 to 650 ° C. Similar effects are also found in the addition of Mo or Sn, where Sn is preferably added as a second or third element combined with Ni, Cu or Mo.

(C) It has been found that the grain boundary segregation of P, which is the main cause of secondary work brittleness, can be remarkably suppressed by precipitating a fine phosphorus compound in advance, overlapping with the effects of the alloy elements described above. It has been newly found that such control of the phosphorus compound is effective to hold the metal structure after the finish annealing at a temperature of 650 to 750 ° C. for more than 5 minutes and for 3 hours or less.

(D) In order to improve the secondary work brittleness resistance and deep drawability described above, in addition to the reduction of P, S, and N, and the stabilization elements of Nb and Ti, trace elements of V, W, Zr, and Co are used. It is also effective to add.

  The gist of the present invention based on the above findings (a) to (d) is as follows.

(1) In mass%, C: 0.03% or less, Si: 1% or less, Mn: 1% or less, P: 0.035% or less, S: 0.003% or less, Cr: 13-23% N: 0.03% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.1% or less, Ni: 1% or less and / or Cu: 1% or less A high-purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance, characterized in that the balance consists of Fe and inevitable impurities and the contents of Ni and Cu satisfy the following formula (1) .
0.3 <Ni + Cu ≦ 1 Formula (1)

(2) Further, in mass%, Mo: 1% or less, Sn: 0.5% or less, and the contents of Ni, Cu, Mo and Sn satisfy the following formula (2) The high-purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance as described in (1).
0.3 <Ni + Cu + Mo + Sn ≦ 2 (2)

(3) Further, by mass%, Sb: 0.2% or less, V: 0.5% or less, W: 0.5% or less, Zr: 0.5% or less, Co: 0.5% or less, Mg : 0.005% or less, B: 0.005% or less, Ca: 0.005% or less, Ga: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0 .1% or less, REM: 0.1% or less, 1 type or 2 types or more, (1) or (2) for deep drawing excellent in secondary work brittleness resistance High purity ferritic stainless steel sheet.

(4) The amount of extraction residue of P is 0.01% by mass or more, and the size of the precipitated phosphorus compound in the longitudinal direction is 1 μm or less, any one of (1) to (3) A high-purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance described in 1.

(5) The crystal grain boundary surface exposed when the steel sheet is broken has a P concentration of 3% by mass or less with respect to 100% by mass of all elements, and any one of (1) to (4) A high-purity ferritic stainless steel sheet for deep drawing with excellent secondary work brittleness resistance.

(6) Crystal grains when cylindrical deep drawing with a total drawing ratio of 3.5 to 4.0 is performed, then heated to 600 to 650 ° C., and then cooled to 400 ° C. at an average cooling rate of 3 ° C./min or less. The high-concentration ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance according to any one of (1) to (4), wherein the P concentration of the boundary is 3% by mass or less.

(7) The high purity ferritic stainless steel sheet according to any one of (1) to (6), wherein the high purity ferritic stainless steel sheet is used for a structure to which heat treatment is applied after deep drawing.

(8) A steel sheet having high purity ferritic stainless steel having the component composition described in any one of (1) to (3) as a hot-rolled steel sheet by hot forging or hot rolling, and repeating cold rolling and annealing. In the manufacturing method, deep drawing forming excellent in secondary work brittleness resistance is characterized in that finish annealing is performed at a temperature higher than 800 ° C., and then the temperature is maintained at 650 to 750 ° C. for more than 5 minutes and not more than 3 hours. For producing high-purity ferritic stainless steel sheet for use in a vehicle.

  According to the present invention, resource saving and economic efficiency improved by secondary processing embrittlement that is manifested by heat treatment after deep drawing, regardless of excessive P reduction and alloying of trace elements, Si and Mo, etc. It is possible to obtain a remarkable effect that a high-purity ferritic stainless steel sheet for deep drawing can be obtained.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

The reason for limiting the component (I) will be described below.
Since C reduces workability and corrosion resistance, the lower the content, the better. Therefore, the upper limit is made 0.03%. On the other hand, C is an intrusion-type solid solution element, an element having a large segregation tendency to the grain boundary, and contributes to strengthening of the grain boundary. Therefore, C is effective for suppressing grain boundary segregation of P, which is the target of the present invention. In order to obtain the effects of C, the lower limit is preferably 0.001%. A preferable range in consideration of the refining cost is 0.003 to 0.015%.

  Si is effective as a deoxidizing element and improves oxidation resistance. On the other hand, it acts as a solid solution strengthening element and causes a decrease in workability and a reduction in secondary work brittleness resistance, which is a target of the present invention, so the upper limit is made 1%. In order to ensure deoxidation and oxidation resistance, the lower limit is preferably set to 0.01%. A preferable range is 0.05 to 0.5% in consideration of effects and manufacturability.

  Mn is a deoxidizing element and an element effective in fixing S. On the other hand, the upper limit is set to 1% in order to reduce the corrosion resistance and oxidation resistance. In order to ensure the action of deoxidation and S fixation, the lower limit is preferably set to 0.01%. A preferable range is 0.05 to 0.5% in consideration of the effect and the manufacturing cost.

  P is an element that hinders manufacturability and weldability, and is a main cause of lowering the secondary work embrittlement resistance targeted by the present invention. Therefore, the lower the content, the better. And However, excessive reduction leads to an increase in refining costs, so the lower limit is made 0.005%. A preferable range is 0.01 to 0.03% in consideration of manufacturing cost.

  S segregates at the grain boundaries and leads to a decrease in hot workability and secondary work embrittlement resistance as a target of the present invention. Therefore, the lower the content, the better. Therefore, the upper limit is made 0.003%. . However, excessive reduction leads to an increase in raw materials and refining costs, so the lower limit is set to 0.0001. A preferable range is 0.0002 to 0.0015% in consideration of suppression of embrittlement and manufacturing cost.

  Cr is a basic element of the high purity ferritic stainless steel of the present invention, and is an essential element for ensuring corrosion resistance and heat resistance. In order to ensure corrosion resistance and heat resistance assuming the use of the electric pot of the present invention, the lower limit is made 13%. The upper limit is made 23% from the viewpoint of workability and manufacturability. However, the preferred range is 15 to 19% in view of economic efficiency compared with SUS430J1L and SUS436J1L. In view of performance and alloy cost, a more preferable range is 16 to 18%.

  N, like C, reduces workability and corrosion resistance, so the lower the content, the better. Therefore, the upper limit is made 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably made 0.001%. N, like C, is an intrusion-type solid solution element, but its segregation tendency to the crystal grain boundary is small and contributes little to strengthening of the crystal grain boundary. Incurs a decline. Therefore, a preferable range is 0.005 to 0.015% in consideration of performance and manufacturing cost.

  Nb and Ti are effective elements for improving the secondary work embrittlement resistance, which is a target of the present invention, in addition to workability and corrosion resistance by the action of a stabilizing element for fixing C and N. When added, the content is set to 0.01% or more where the effect is exhibited. However, excessive addition leads to a decrease in manufacturability accompanying an increase in alloy costs and a recrystallization temperature, so the upper limit is made 0.5%. A preferable range is 0.05 to 0.5% for one or two of Nb and Ti in consideration of effects, alloy costs, and manufacturability. A more preferable range is 0.1 to 0.3%.

  Al is an extremely effective element as a deoxidizing element. On the other hand, the upper limit is made 0.1% in order to reduce the toughness and weldability of the steel. The lower limit is preferably 0.005% in consideration of the deoxidation effect. A preferable range is 0.01 to 0.07% in consideration of manufacturability and performance.

Ni and Cu are effective elements for corrosion resistance, and are indispensable elements for obtaining secondary processing embrittlement resistance which is a target of the present invention. In order to delay the P segregation after the heat treatment that is the subject of the present invention and obtain secondary work brittleness resistance, when not containing Mo and Sn described later, as shown by the following formula (1), Ni, Cu alone or combined addition exceeds 0.3%. On the other hand, excessive addition causes an increase in alloy costs and a decrease in workability due to an increase in material strength. Therefore, the upper limit is set to 1% individually or in combination. In consideration of performance and alloy cost, a preferable range for single or combined addition is Ni + Cu: 0.4 to 1.0%, and more preferably Ni + Cu: 0.5 to 1.0%.
0.3 <Ni + Cu ≦ 1 Formula (1)

Mo and Sn are effective elements for obtaining secondary processing embrittlement resistance, which is a target of the present invention, in addition to corrosion resistance, similarly to Ni and Cu, and are added as necessary. When adding, in order to express each effect, Mo is 0.1% or more, and Sn is 0.01% or more. However, excessive addition hinders increase in alloy costs and manufacturability of hot working and cold working, so the upper limit is preferably 1% for Mo and 0.5% for Sn, respectively, and the upper limit for Ni + Cu + Mo + Sn. Is 2%. In the present invention, when Mo and Sn are further added in addition to Ni and / or Cu, the Ni, Cu, Mo, and Sn are added in a range that satisfies the following formula (2).
0.3 <Ni + Cu + Mo + Sn ≦ 2 (2)

  In addition, considering the performance and manufacturability and the alloy cost, preferable ranges in the case of addition are 0.2 to 0.8% for Mo, 0.05 to 0.25%, and Ni + Cu + Mo + Sn: 0.0. 4 to 2.0%, more preferably Ni + Cu + Mo + Sn: 0.5 to 1.5%.

  Sb, V, W, Zr, and Co are effective elements for improving the corrosion resistance and the secondary work embrittlement resistance targeted by the present invention, and are added as necessary. When added, the content is set to 0.01% or more where the effect is exhibited. Excessive addition leads to an increase in alloy costs and a decrease in manufacturability, so the upper limit is made 0.5%. In addition, the preferable range in the case of adding is 0.01 to 0.1% for Sb and 0.02 to 0.3% for V, W, Zr, and Co in consideration of performance, manufacturability and alloy cost. is there.

  Mg forms Mg oxide with Al in molten steel and acts as a deoxidizer, and also acts as a crystallization nucleus of TiN. TiN becomes a solidification nucleus of the ferrite phase in the solidification process, and by facilitating crystallization of TiN, the ferrite phase can be finely formed during solidification. By making the solidified structure finer, it is possible to prevent surface defects caused by coarse solidified structures such as ridging and roping of the product, and to improve workability, it is added as necessary. When added, the content is 0.0001%. However, if it exceeds 0.005%, manufacturability deteriorates, so the upper limit is made 0.005%. Preferably, considering the manufacturability, the content is made 0.0003 to 0.002%.

  B is an element that improves hot workability and secondary work embrittlement characteristics, and addition to high purity ferritic stainless steel is effective. When adding, it is made 0.0003% or more to express these effects. However, excessive addition causes a decrease in elongation, so the upper limit is made 0.005%. Preferably, considering the material cost and workability, the content is made 0.0005 to 0.002%.

  Ca and Ga are elements that improve hot workability and steel cleanliness, and are added as necessary. When adding, it is made 0.0003% or more to express these effects. However, excessive addition leads to a decrease in manufacturability and a decrease in corrosion resistance due to water-soluble inclusions such as CaS, so the upper limit is made 0.005%. Preferably, considering the manufacturability and oxidation resistance, the content is made 0.0003 to 0.0015%.

  La, Y, Hf, and REM have the effects of improving hot workability and steel cleanliness and remarkably improving oxidation resistance and hot workability, and may be added as necessary. When added, the content is set to 0.001% or more where the effect is exhibited. However, excessive addition leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit is made 0.1%. Preferably, considering the effect, economic efficiency, and manufacturability, one or two or more is 0.001 to 0.05%.

(II) The reason for limitation regarding the metal structure will be described below.
In order to obtain the secondary work embrittlement resistance target of the present invention having the component described in the above item (I), the requirements for a suitable metal structure are defined.

  In order to improve the secondary work brittleness that is manifested by heat treatment after the deep drawing as the target of the present invention, it is necessary to reduce the solid solution P in the steel by precipitating P as a compound. This is effective for delaying the grain boundary segregation. Therefore, the residual amount of P extracted as a phosphorus compound such as FeTiP is preferably 0.01% by mass or more. More preferably, it is 0.015% or more, and the upper limit is not specified, but it is preferably 0.03% in consideration of the addition amount or heat treatment conditions required for precipitation of the phosphorus compound.

The residual amount of P can be measured by a constant current electrolysis method. For example, the electrolytic solution is 20% calcium salicylate-0.5% salicylic acid-1% lithium chloride-methanol solution, electrolyzed at a current density of 20 mA / cm ² , filtered through a filter, and then the remaining amount of extracted P Can be quantitatively determined by emission spectroscopy using high frequency inductively coupled plasma (ICP) as a light source.

  Along with the above-described residual amount of P, the size of the phosphorus compound is preferably 1 μm or less in the longitudinal direction (hereinafter referred to as “long side”). The phosphorus compound having a long side exceeding 1 μm acts as a starting point of cracking when precipitated at the grain boundary. Accordingly, the smaller the size of the phosphorus compound, the better. Therefore, it is preferably 0.5 μm or less, and more preferably 0.1 μm or less. Such a fine phosphorus compound reduces the solid solution P in the steel and has a larger effect of delaying the P segregation to the grain boundary than to act as a starting point of cracking even when it precipitates at the grain boundary. It is effective in avoiding cracks starting from the grain boundaries.

  In order to avoid cracks originating from the grain boundaries, the P element concentration on the exposed grain boundary surface when the total elements on the grain boundary surface exposed when the steel sheet is broken is 100% by mass The concentration in terms of mass% is 3 mass% or less. Such grain boundary segregation of P is easily manifested when the average cooling rate up to 400 ° C. is 3 ° C./min or less after heating to 600-650 ° C. after deep drawing. The grain boundary segregation of P is promoted as the deep drawing degree is increased, and tends to be saturated at a total drawing ratio of 3.5 to 4 close to a practical deep drawing limit. The drawing ratio is the diameter of the disk used for deep drawing ÷ the diameter of the punch for deep drawing. The total drawing ratio is the diameter of the disk ÷ diameter of the punch in the final process in multi-step deep drawing. Means. The degree of deep drawing increases as the total drawing ratio increases. Therefore, the target secondary work brittleness resistance of the present invention is the deep drawing with the total drawing ratio of 3.5 to 4, followed by heating to 600 to 650 ° C. and cooling at 3 ° C./min or less. When the obtained processed product is broken, grain boundary segregation of P at the grain boundaries exposed at the fracture surface is suppressed to 3% or less. Preferably it is 2.5% or less, More preferably, it is 2% or less.

(III) The reason for limitation regarding the manufacturing method will be described below.
It has the component described in the item (I) and defines a preferable production method in order to obtain a suitable metal structure described in the item (II).

  In the present invention, if the component described in the above item (I) is satisfied, it is possible to ensure the secondary work embrittlement resistance which is the target of the present invention even if manufactured under normal process conditions. In order to satisfy the requirements of the structure, after finishing annealing at a temperature higher than 800 ° C., hold the temperature for more than 5 minutes at 650-750 ° C. without lowering the temperature to less than 650 ° C. It is preferable to perform the temperature holding so that the temperature holding at 650 to 750 ° C. is more than 5 minutes and 3 hours or less in total.

  The reason why the final annealing is set to a temperature higher than 800 ° C. is to ensure workability by recrystallizing the steel after cold working. An excessive increase in the annealing temperature leads to a decrease in surface quality, such as roughening of the crystal grain size and rough skin due to processing. Preferably, the upper limit of the annealing temperature is 1000 ° C.

  After finishing annealing, adjust the cooling rate to make the temperature holding time longer than 5 minutes in the temperature range of 650 to 750 ° C, or reheat to 650 to 750 ° C and hold for more than 5 minutes and 3 hours or less. It doesn't matter. If the temperature exceeds 750 ° C., a coarse phosphorus compound having a long side exceeding 1 μm is likely to precipitate, or an increase in solid solution P may cause a decrease in secondary work brittleness resistance, so the upper limit is set to 750 ° C. When the temperature is lower than 650 ° C., the solid solution P increases as the temperature decreases, and the grain boundary segregation of P proceeds. Therefore, when the manufactured steel sheet is heat-treated after deep drawing, the grain boundary segregation of P is promoted and the secondary work brittleness resistance is lowered, so the lower limit is set to 650 ° C.

The temperature holding time at 650 to 750 ° C. is more than 5 minutes in order to obtain a suitable metal structure requirement. The upper limit of the temperature holding time is 1 h or less when the temperature is held without cooling to less than 650 ° C. after high-temperature finish annealing in order to suppress P grain boundary segregation and phosphorus compound coarsening, and 650 after high-temperature finish annealing. When the temperature is maintained by reheating after being once cooled to less than 0 ° C., the total time for maintaining all temperatures after the high-temperature finish annealing is 3 h or less.
In order to suppress the grain boundary segregation of P of the deep drawn steel sheet, which is the target of the present invention, it is important to precipitate fine phosphides in the steel sheet before deep drawing, and in the case of holding the temperature after annealing The temperature is maintained at 650 ° C. or more and 700 ° C. or less for more than 5 minutes to 1 hour or less. Further, when the temperature is once lowered to less than 650 ° C. and then re-heated, it is preferable that the temperature holding at 650 ° C. or more and 700 ° C. or less is in the range of more than 5 minutes to 3 hours or less in total. In a temperature range of 650 to 750 ° C., holding for more than 5 minutes at 650 to 700 ° C. on the low temperature side makes it possible to form a fine phosphide precipitation form that can most reduce the grain boundary segregation of P in the deep drawn steel sheet. be able to.

  Examples of the present invention will be described below.

  Ferritic stainless steel having the composition shown in Table 1 was melted and hot rolled at a heating temperature of 1150 to 1250 ° C. to produce hot rolled steel sheets A to R * having a thickness of 4 mm. Hot-rolled steel sheets A to R * are annealed under the conditions shown in Table 2, cold-rolled to a thickness of 0.6 mm after pickling, subjected to finish annealing and pickling at 900 to 980 ° C., and resistance to secondary work brittleness We used for evaluation. The components of the steel were also carried out in the range specified in the present invention and other cases. The temperature holding at 650 to 750 ° C. after the finish annealing was performed under the conditions specified in the present invention and other conditions.

  The secondary work brittleness resistance was evaluated according to the following procedure. First, a disc (blank) having a diameter of 80 mm is prepared from each of the above steel plates A to R *, and a punch diameter: 40 mm → 35 mm → 30 mm → 25 mm → 22 mm four-stage cylindrical deep drawing test (total drawing ratio: 80 ÷ 22) = 3.64) to produce a cup-shaped four-stage cylindrical deep drawn product having an inner diameter of 22 mm. Each of the four-stage cylindrical deep drawn products was produced from each of the steel plates A to R *. Next, four of the four-stage cylindrical deep drawn products prepared for each of the steel plates A to R * were heated at 600 ° C. for 30 minutes and then cooled in the furnace, and the remaining four were heated at 650 ° C. for 30 minutes and then the furnace. All were cooled to 400 ° C. at an average cooling rate of 0.5 to 1 ° C./min. Thereafter, the secondary work brittleness resistance was evaluated by a drop weight test. The drop weight test was performed by dropping a load having a weight of 100 g from a height of 1 m onto the upper edge of the opening side of the four-stage cylindrical deep drawn product. When N pieces (N = 4) of deep drawn products manufactured for each of the steel plates A to R * and heated at 600 ° C. or 650 ° C., even one vertical crack parallel to the deep drawing direction occurs. The case where no vertical cracks occurred was judged as “pass”. Evaluation is “◎” when both 600 ° C. and 650 ° C. were not cracked at all, “◯” when there was no crack at any of the heating temperatures, and both heating temperatures were The case where even one piece was broken was marked with “x”. Examples of the present invention are “◎” and “◯”, and comparative examples are “x”.

The residual amount of P is 20% calcium salicylate-0.5% under the conditions described in item 0042 after the test material for measuring the residual amount of P is prepared by subjecting it to a heat treatment under the same conditions as a cylindrical deep drawn product. Using a salicylic acid-1% lithium chloride-methanol solution, a constant current electrolysis method was carried out at a current density of 20 mA / cm ^{2 and} obtained by ICP analysis. The size of the phosphorus compound is measured by subjecting the extraction residue obtained by the constant current electrolysis method to SEM (scanning electron microscope) observation and confirming that P is detected by an EDS (energy dispersive X-ray spectrometer). did. For the P concentration at the grain boundary, an analytical sample with a V notch of 4 mm × 20 mm was taken from the cup on the opening side of a cup-shaped cylindrical deep-drawn product, and was cooled in liquid nitrogen in an AES (Auger Electron Spectrometer). The fracture surface was exposed by fracture, and the concentration elements at the grain boundaries exposed on the fracture surface were measured. Here, the surface breakage in the AES apparatus was performed after securing the degree of vacuum of 10 ⁻⁴ Pa or less and the analysis sample was cooled with liquid nitrogen (77 K), and then the concentrated element was quantitatively analyzed.

Table 2 summarizes the test results.
From Table 2, test numbers 1, 4, 5, 7, and 14 are high-purity ferritic stainless steel sheets that satisfy all of the preferable components (Ni + Cu amount or Ni + Cu + Mo + Sn amount) and the production method defined in the present invention. These steel sheets satisfy all the requirements of the preferred metal structure prescribed in the present invention, do not generate any cracks in the evaluation of deep drawn products, and obtained excellent secondary work brittleness resistance of evaluation `` ◎ ''. is there.

  Test Nos. 8 and 9 satisfy the components and the production method defined in the present invention, and the improvement of secondary work brittleness resistance is recognized by addition of trace elements V, W, Zr, and Co. These steel sheets satisfy all the requirements of the preferred metal structure prescribed in the present invention, do not generate any cracks in the evaluation of deep drawn products, and obtained excellent secondary work brittleness resistance of evaluation `` ◎ ''. is there.

  Test numbers 2, 11, 12, 15, and 16 are high-purity ferritic diameter stainless steel sheets that satisfy the components and manufacturing method defined in the present invention. These steel plates do not crack at all in deep drawn products under furnace heating after heating at 600 ° C. or 650 ° C. under one of the heating / furnace cooling conditions, resulting in good secondary work brittleness resistance with an evaluation of “◯”. It is what was done.

  Test Nos. 6 and 10 are high-purity ferritic stainless steel sheets to which preferable components (Ni + Cu + Mo + Sn amount) specified in the present invention and trace elements (Zr, Co) effective for secondary work brittleness resistance are added, and finish annealing. It was manufactured by omitting the subsequent temperature holding treatment in the temperature range of 650 to 750 ° C. Even if these steel sheets do not necessarily satisfy the preferred production method defined in the present invention, it is possible to obtain a suitable metal structure, and an evaluation “◯” of secondary work brittleness resistance is obtained.

  Test numbers 17 to 22 are not included in the components defined in the present invention. Even if the preferable manufacturing method prescribed | regulated by this invention was implemented for these steel plates, the secondary workability which was the target of this invention was not obtained, but it was evaluated as "x". Test Nos. 3 and 13 satisfy the components defined in the present invention, but deviate from the preferred components, and did not reach the target characteristics when the preferred production method defined in the present invention was not carried out. These steel sheets were evaluated as “x” because the secondary workability targeted by the present invention was not obtained.

  From this result, in order to obtain the secondary work resistance which is the target of the present invention, it is important to adjust the amount of “Ni + Cu” or “Ni + Cu + Mo + Sn”, which is a preferable component defined in the present invention. Furthermore, the addition of trace elements V, W, Zr and Co and the preferred production method defined in the present invention are effective for improving the secondary work brittleness resistance in the components defined in the present invention.

  According to the present invention, heat treatment is performed after deep drawing and becomes apparent, regardless of excessive P reduction and addition of B and Mg as trace elements and high alloying such as Si and Mo exceeding 1%. A high-purity ferritic stainless steel sheet for deep drawing excellent in resource-saving and economical efficiency with improved secondary work brittleness can be obtained. The steel sheet can be suitably used for a structure to which heat treatment is performed after deep drawing, for example, a structure in which brazing or the like corresponds to the heat treatment, particularly an electric pot or the like.

Claims (8)

In mass%, C: 0.03% or less, Si: 1% or less, Mn: 1% or less, P: 0.035% or less, S: 0.003% or less, Cr: 13-23%, N: 0.03% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.1% or less, further including Ni: 1% or less and / or Cu: 1% or less, A high-purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance, wherein the balance is Fe and inevitable impurities, and the contents of Ni and Cu satisfy the following formula (1).
0.3 <Ni + Cu ≦ 1 Formula (1)
Further, in terms of mass%, Mo: 1% or less, Sn: 0.5% or less, the balance consists of Fe and inevitable impurities, and the contents of Ni, Cu, Mo and Sn are the following formula (2) The high-purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance according to claim 1, characterized in that:
0.3 <Ni + Cu + Mo + Sn ≦ 2 (2)
Further, in terms of mass%, Sb: 0.2% or less, V: 0.5% or less, W: 0.5% or less, Zr: 0.5% or less, Co: 0.5% or less, Mg: 0. 005% or less, B: 0.005% or less, Ca: 0.005% or less, Ga: 0.005% or less, La: 0.1% or less, Y: 0.1% or less, Hf: 0.1% 3. High purity ferritic stainless steel for deep drawing excellent in secondary work brittleness resistance according to claim 1 or 2, wherein REM: 0.1% or less, 1 type or 2 types or more are contained steel sheet.
The amount of extraction residue of P is 0.01% by mass or more, and the size of the precipitated phosphorus compound in the longitudinal direction is 1 μm or less. High-purity ferritic stainless steel sheet for deep drawing with excellent secondary work brittleness resistance.
5. The P concentration is 3% by mass or less based on 100% by mass of all elements in the grain boundary surface exposed when the steel plate is broken. 5. High-purity ferritic stainless steel sheet for deep drawing with excellent secondary work brittleness resistance.
After performing cylindrical deep drawing with a total drawing ratio of 3.5 to 4.0, and then heating to 600 to 650 ° C., cooling to 400 ° C. at an average cooling rate of 3 ° C./min or less is performed at the grain boundary P. The high purity ferritic stainless steel sheet for deep drawing excellent in secondary work brittleness resistance according to any one of claims 1 to 4, wherein the concentration is 3% by mass or less.
The high-purity ferritic stainless steel sheet according to any one of claims 1 to 6, wherein the high-purity ferritic stainless steel sheet is used for structural applications in which heat treatment is applied after deep drawing.
  In the manufacturing method of the steel plate which repeats cold rolling and annealing, making the high purity ferritic stainless steel which has the ingredient composition according to any one of claims 1 to 3 by hot forging or hot rolling, 5. Deep-drawing with excellent secondary work brittleness resistance according to claim 4, characterized in that finish annealing is performed at a temperature higher than 800 ° C., and thereafter, holding at 650 to 750 ° C. for more than 5 minutes and not more than 3 hours. For producing high-purity ferritic stainless steel sheet for use in a vehicle.