EP1484424B1

EP1484424B1 - Ferritic stainless steel sheet excellent in press formability and secondary formability and its manufacturing method

Info

Publication number: EP1484424B1
Application number: EP04012345A
Authority: EP
Inventors: Yasutoshi Steel & Tech. Dev. Labs. Hideshima; Naoto Steel & Tech. Dev. Labs. Hiramatsu; Kouki Steel & Tech. Dev. Labs. Tomimura
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2003-06-04
Filing date: 2004-05-25
Publication date: 2010-08-25
Anticipated expiration: 2024-05-25
Also published as: KR100595383B1; US20040244884A1; US20090165905A1; JP2004360003A; CN100363523C; JP3886933B2; ES2357303T3; EP1484424A1; CN1572895A; DE602004028780D1; KR20040104939A

Description

The present invention relates to a ferritic stainless steel sheet, which can be press-formed to a predetermined profile without defects such as poor circularity and torsion and then secondary-formed to a final profile with good hot-extruding, and also relates to a method of manufacturing thereof.
Ferritic stainless steels, represented by SUS430 or SUS430LX, have been used so far in various fields, e.g. durable consumers' goods, due to good corrosion-resistance and cheapness compared with austenitic stainless steels, which contain Ni as an expensive element. Conditions for press-forming a ferritic stainless steel sheet to a product profile become severe and severe as development of its application. A press-formed steel sheet is often secondarily formed for extrusion of a hole, for instance. In response to development of the application, there is a strong demand for provision of a new ferritic stainless steel sheet, which is fairly excellent in formability compared with conventional ferritic stainless steel sheets and so formed to a product profile without defects even under severe conditions.
There are many reports on formability of ferritic stainless steel sheets. A representative improvement is addition both of Ti and Nb for stabilization of dissolved C and N as carbonitrides. Furthermore, JP2000-192199A teaches distribution of magnesium inclusions, which are effective on anti-ridging property, in a ferritic stainless steel containing both of Ti and Nb. JP8-26436B ( JP-A-51086015 ) teaches combination of hot-rolling conditions, which are designed for improvement of Lankford value (r) as an index of formability, with addition of Ti and Nb.
Shape-freezability and secondary formability of a primary-formed steel sheet, which will be formed to a final profile, are also important factors as well as to Lankford value (r) and anti-ridging property.
A ferritic stainless steel sheet is generally inferior in formability to an austenitic stainless steel sheet. Especially, it significantly reduces thickness in a primary-formed state, and thickness reduction is anisotropic. Consequently, dimensional accuracy such as circularity becomes worse as severer forming conditions, when the ferritic stainless steel sheet is press-formed to a cylindrical profile. Thickness deviation in the primary-formed state leads to serious degradation of secondary formability such as hole-extruding.
In the case where a ferritic stainless steel sheet maintains high dimensional accuracy (e.g. circularity, straightness and anti-torsion) as well as secondary formability in a press-formed state, a cheap ferritic stainless steel sheet can be used as parts or members, instead of an expensive austenitic stainless steel sheet, which have been unavoidably used in view of severe forming conditions.
EP-A2-1308532 discloses a ferritic steel sheet manufactured by hot rolling, annealing, optionally double cold rolling and intermediate annealing and finish annealing suitable to be deep-drawn and having brittle resistance to secondary processing, the steel possesses a controlled grain size and carbonitrides of Nb and Ti.
The present invention aims at provision of a ferritic stainless steel sheet improved in dimensional accuracy and secondary formability in a press-formed state by controlling particle size and distribution of precipitates, which are dispersed in a steel matrix.
The present invention as defined in claim 1, proposes a new ferritic stainless steel sheet, which consists of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (%Ti x %N) less than 0.005. Its metallurgical structure is defined by distribution of precipitates of 0.15 µm or more in particle size except TiN at a rate of 5000-50000/mm².
The ferritic stainless steel sheet is manufactured as disclosed in claim 2 and in particular, as follows:
A molten steel with a predetermined composition is cast to a slab. The slab is hot-rolled to a steel sheet at a finish-temperature of 800°C or lower and annealed at 450-1080°C. The annealed hot-rolled steel sheet is pickled and cold-rolled in accompaniment with at least one intermediate-annealing within a temperature range of from (a recrystallization-finishing temperature - 100°C) to (a recrystallization-finishing temperature). The cold-rolled steel sheet is finally subjected to finish-annealing at a temperature of 1080°C or lower.
The hot-rolled steel sheet may be box-annealed for a predetermined time period of one hour or shorter. The inter-mediate annealing and the finish-annealing may be performed in a continuous annealing furnace for one minute or shorter.
Fig. 1 is a view for explaining circularity of a steel sheet, which is cylindrically formed by a multiplaten press.
The inventors have researched and examined on manufacturing conditions of ferritic stainless steel sheets for improvement of dimensional accuracy (e.g. circularity, straightness and torsion) from various aspects, and discovered that circularity and secondary formability of a press-formed steel sheet are fairly affected by shape and distribution of TiN and other precipitates in an annealed state. The inventors presume on the basis of the discovery that objective properties are imparted to a ferritic stainless steel sheet by properly controlling shape and distribution of the precipitates. Formation of the precipitates to the shape and distribution suitable for the purpose is realized by adding both of Ti and Nb to a ferritic stainless steel at a value more than a stoichiometric ratio for stabilization of C and N as carbonitrides and subjecting the ferritic stainless steel to optimum thermo-mechanical treatment.
The effects of shape and distribution of the precipitates on press formability and dimensional accuracy may be explained as follows:
C and N in a ferritic stainless steel are mostly precipitated as carbonitrides by addition of Ti and Nb. The precipitated carbonitrides except TiN are substantially reformed to very fine particles in the manufacturing process of from annealing a hot-rolled steel sheet through cold-rolling to finish-annealing. The fine particles allow predominant growth of recrystallized grains with a certain orientation without pinning action, when the manufactured steel sheet is annealed for recrystallization, resulting in formation of an anisotropic grain-mixed structure. The anisotropy causes concentration of strains along a certain direction during primary-forming a steel sheet and so worsens press formability and dimensional accuracy of the steel sheet.
The pinning action during recrystallization annealing is expected by distribution of precipitates having a particle size bigger than a certain value. The pinning action suppresses orientative grain growth or growth to coarse grains, so as to improve isotropy and dimensional accuracy of a press-formed steel sheet. The effects of the pinning action on press formability and dimensional accuracy are typically noted by distribution of precipitates of 0.15 µm or more in particle size except TiN at a rate of 5000-50000/mm², as recognized in the later examples.
Among the precipitates, TiN is disadvantageous for press formability and dimensional accuracy. In fact, a steel sheet, which has a product of (%Ti × %N) above 0.005, is cracked in a press-formed state. Coarse TiN particles, which have grown to a cubic shape, are observed at starting points of cracks. The observation result means that strains concentrate at cubic apices during press-forming and induce micro-cracks. Concentration of strains and formation of micro-cracks around the TiN particles are also unfavorable for hole-extruding in a secondary-forming step.
The inventive ferritic stainless steel sheet contains alloying components at predetermined ratios, as follows:

[0.02 mass % or less of C]

C is converted to carbides effective for random growth of recrystallized ferritic grains in a finish-annealing step, but degrades formability of a steel sheet due to its hardening effect. Precipitation of carbides also cause inferior corrosion-resistance. In this regard, a C content is controlled at a lowest possible level, i.e. 0.02 mass % or less for formability and corrosion-resistance. The C content is preferably reduced to 0.015 mass % or less for improvement of secondary formability. However, reduction of the C content to an extremely lower level necessitates a long-term refining operation and raises a steel-manufacturing cost. Therefore, a lower limit of the C content is preferably determined at 0.001 mass %. Definition of the lower limit also assures the effect of carbides on random growth of recrystallized ferritic grains in a finish-annealing step.

[0.8 mass % or less of Si]

Si is an alloying component, which is added as a deoxidizing agent during a steel-making process, but has a strong solid-solution hardening effect. Excess Si above 0.8 mass % unfavorably hardens a steel sheet, resulting in poor ductility. An upper limit of a Si content is preferably determined at 0.5 mass % for ductility and secondary formability.

[1.5 mass % or less of Mn]

Mn does not harden a steel sheet so much due to its solid-solution hardening effect weaker than Si. However, excess Mn above 1.5 mass % causes discharge of manganese fumes during a steel-making process, resulting in poor productivity.

[0.050 mass % or less of P]

P is harmful on hot-workability, so that its upper limit is determined at 0.050 mass %.

[0.01 mass % or less of S]

S is a harmful element, which segregates at grain boundaries and embrittles the grain boundaries. Such defects can be suppressed by controlling a S content to 0.01 mass % or less.

[8.0-35.0 mass % of Cr]

A Cr content is controlled to 8.0 mass % or more for assuring corrosion-resistance of a stainless steel. However, toughness and formability of the stainless steel becomes worse as an increase of the Cr content, so that an upper limit of the Cr content is determined at 35.0 mass %. The Cr content is preferably controlled to 20.0 mass % or less for further improvement of ductility and secondary formability.

[0.05 mass % or less of N]

N is converted to nitrides effective for random growth of recrystallized ferritic grains in a finish-annealing step, but has a hardening effect. Since excess N degrades ductility of a steel sheet, a N content is controlled to a lowest possible level, i.e. 0.05 mass % or less. The N content is preferably controlled to 0.02 mass % or less for further improvement of ductility and secondary formability. However, reduction of the N content to an extremely lower level necessitates a long-term refining operation and raises a steel-manufacturing cost. Therefore, a lower limit of the N content is preferably determined at 0.001 mass %. Definition of the lower limit also assures the effect of nitrides on random growth of recrystallized ferritic grains in a finish-annealing step.

[0.05-0.40 mass % of Ti]

Ti is an alloying component, which stabilizes C and N as carbonitrides for formability and corrosion-resistance. Such effect is apparently noted at a Ti content of 0.05 mass % or more. However, excess Ti above 0.40 mass % leads to rising of a steel cost and induces surface defects originated in titanium inclusions.

[0.10-0.50 mass % of Nb]

Nb, which has the same effect as Ti on stabilization of C and N, is an essential component for precipitation of niobium inclusions of 0.15 µm or more in particle size except TiN. The niobium inclusions are probably composed of carbides and Fe₂Nb. A Nb content of 0.10 mass % or more is necessary for precipitation of such niobium inclusions. However, excess Nb above 0.50 mass % causes exaggerated precipitation and unfavorably raises a recrystallizing temperature of a ferritic stainless steel.

[0.50 mass % or less of Ni]

Ni is an optional element for toughness of a hot-rolled steel sheet and corrosion-resistance. But excess addition of Ni raises a material cost and hardens a steel sheet, so that an upper limit of a Ni content is determined at 0.5 mass %.

[3.0 mass % or less of Mo]

Mo is an optional element for corrosion-resistance, but excess Mo above 3.0 mass % is unfavorable for hot-workability.

[2.0 mass % or less of Cu]

Cu is an optional element, which is often included in a stainless steel from scraps during a steel-making process. Since excess Cu causes poor toughness and degradation of hot-workability, a Cu content is controlled to 2.0 mass % at most.

[0.3 mass % or less each of V and Zr]

V and Zr are optional elements. V fixes free C as carbide in a steel matrix for formability, while Zr captures free O for formability and toughness. However, excess addition of V or Zr is necessarily avoided for productivity. In this sense, an upper limit each of V and Zr is determined at 0.3 mass %.

[0.3 mass % or less of Al]

Al is an optional element, which is added as a deoxidizing agent in a steel-making process. However, excess Al above 0.3 mass % causes an increase of nonmetallic inclusions, resulting in poor toughness and surface defects.

[0.0100 mass % or less of B]

B is an optional element, which stabilizes N and improves corrosion-resistance and formability of a stainless steel. The effects of B are apparently noted at 0.0010 mass % or more, but excess B above 0.0100 mass % is disadvantageous for hot-workability and weldability.
Ca, Mg, Co, REM (rare earth metals), etc. other than the above elements may be included from scraps during steel-making. Such elements do not put significant effects on circularity of a deeply-drawn steel sheet or dimensional accuracy of a press-formed steel sheet, unless they are included at extraordinary ratios.

[(%Ti×%N)<0.005]

TiN grows to coarse particles or forms clusters as an increase of (%Ti × %N). The coarse TiN particles or cluster promotes accumulation of strains during primary-forming, resulting in formation of micro-cracks at an early stage of a drawing step. Such harmful effects of the coarse TiN particles or cluster are eliminated by control of (%Ti×%N) to a value less than 0.005, as recognized in the later examples.

[A distribution rate of precipitates of 0.15 µm or more in particle size except TiN at 5000-50000/mm²]

Carbide and nitride precipitates of 0.15 µm or more in particle size have a pinning action to suppress orientative grain growth and also growth to coarse grains, so as to improve isotropy of a stainless steel sheet, circularity in a cylindrically-drawn state and dimensional accuracy in a press-formed state.
Precipitates are carbides and nitrides of Ti and Nb, Laves phase and mixtures thereof. TiN particles, which are precipitated in a cubic shape, are excluded from the precipitates effective for press formability and dimensional accuracy, since the cubic TiN particles are likely to concentrate strains at their apices and act as a starting point of micro-cracks. Distribution of the precipitates of 0.15 µm or more in particle size except TiN at a rate of 5000-50000/mm² assures a pinning action effective for press formability and dimensional accuracy of a press-formed steel piece, as noted in the later examples.
The effects of the precipitates on press formability and dimensional accuracy of a press-formed steel piece are noted at a particle size of 0.15 µm or more, and become bigger as an increase of the particle size. However, coarse precipitates above 1.0 µm in particle size are unfavorable, since the coarse particles promote accumulation of strains and formation of micro-cracks during press-forming, resulting in poor shape-freezability The pinning action of the precipitates is apparently noted at a distribution rate of 5000/mm² or more, but excess distribution of the precipitates above 50000/mm² rather degrades ductility and deep-drawability of a steel sheet. The excess distribution unfavorably raises a recrystallizing temperature of the steel sheet, so that the steel sheet is hardly annealed to a recrystallized state.
Manufacturing conditions, which are necessary to control shape and distribution of precipitates, will be understood from the following explanation.

[Hot-rolling at a finish-temperature of 800°C or lower]

A ferritic stainless steel sheet is hot-rolled at a relatively lower finish-temperature in order to induce nuclear site for precipitates, which will be distributed in a finish-annealed steel sheet. Boundaries of ferritic grains and internal strains in a hot-rolled state serve as the nuclear site. A finish-temperature of hot-rolling is determined at 800°C or lower in order to induce the nuclear sites as many as possible.

[Annealing a hot-rolled steel sheet at 450-1080°C]

Precipitates in a hot-rolled steel sheet are moderated to a shape suitable for controlling the precipitates, which will be distributed in a finish-annealed steel sheet, to 0.15 µm or more in particle size, by annealing the hot-rolled steel sheet at 450-1080°C. If the annealing temperature is lower than 450°C, effective precipitates are scarcely formed. If the hot-rolled steel sheet is heated at a temperature above 1080°C on the contrary, the precipitates except TiN are unpreferably re-dissolved in a steel matrix.
The annealing is completed in one hour for properly controlling distribution number of precipitates without growth to coarse particles.

[Intermediate-annealing at a temperature within a range of from (a recrystallizing temperature - 100°C) to (a recrystallizing temperature)]

During cold-rolling, a steel sheet is annealed at a relatively lower temperature in order to inhibit re-dissolution of the precipitates, which have been formed by annealing the hot-rolled steel sheet. A temperature for intermediate-annealing just below a recrystallization-finishing temperature is preferable for relief of stress, which is introduced into the steel sheet by cold-rolling. The steel sheet can be softened without re-dissolution of precipitates regardless somewhat remains of rolling texture, which is not recrystallized yet, as far as the annealing temperature is held within a range of from (a recrystallizing temperature - 100°C) to (a recrystallizing temperature).
The intermediate annealing period is completed in one minute in order to avoid re-dissolution of the precipitates, accounting faculty of a conventional continuous annealing furnace.

[Finish-annealing at a temperature of 1080°C or lower]

A rolling texture is eliminated by finish-annealing. But, a heating temperature above 1080°C is not only disadvantageous for mass-productivity but also promotes re-dissolution of the precipitates and growth to coarse grains, resulting in poor toughness.
The finish-annealing is completed in one minute, accounting faculty of a conventional continuous annealing furnace.
The other features of the present invention will be apparently understood from the following examples, although the scope of the present invention is not restricted by these examples.

Example 1 (Fundamental Experiments)

The inventors have investigated effects of TiN, which is often precipitated in a ferritic stainless steel matrix, as well as shape effects of precipitates on dimensional accuracy and secondary formability of a press-formed steel piece under the following conditions.
Several steels were melted in an experimental furnace and cast to slabs, wherein each steel was adjusted to a composition of 0.007 mass % C, 0.40 mass % Si, 0.25 mass % Mn, 0.030 mass % P, 0.0005 mass % S, 0.05 mass % Cu, 16.50 mass % Cr, 0.04 mass % Al except Fe and inevitable impurities with the provision that Nb, Ti and N contents were varied within ranges of 0.02-0.30 mass %, 0.05-0.30 mass % and 0.005-0.035 mass %, respectively.

Table 1 shows the Nb, Ti and N contents together with a product of (%Ti × %N) and a recrystallization-finishing temperature.

Table 1: Nb, and N contents (mass %) together with (%Ti × %N) and a recrystallization-finishing temperature T_rf (°C)

Steel No.	Nb	Ti	N	%Ti × %N	T_rf
1		0.06	0.005	0.0003	910
2		0.06	0.035	0.0021	900
3		0.2	0.01	0.0020	930
4	0.2	0.2	0.02	0.0040	940
5		0.3	0.01	0.0030	955
6		0.3	0.02	0.0060	950
7		0.3	0.035	0.0105	940
8	0.02	0.2	0.01	0.0020	910
9	0.3	0.2	0.01	0.0020	960

The underlined figures are values out of the present invention.

Each slab was hot-rolled to thickness of 4 mm at a finish-temperature of 750°C.
The hot-rolled steel sheets Nos. 1-7 were annealed at 800°C for 60 seconds, pickled and then cold-rolled to thickness of 2 mm. The steel sheets were further cold-rolled to final thickness of 0.5 mm, in accompaniment with intermediate-annealing at a temperature of (a recrystallization-finishing temperature -50°C) for 60 seconds. The cold-rolled steel sheets were finish-annealed at 1000°C for 60 seconds.

The hot-rolled steel sheets Nos. 8 and 9 were annealed, pickled and then cold-rolled to thickness of 2 mm. The steel sheets were intermediately annealed and further cold-rolled to final thickness of 0.5 mm. The cold-rolled steel sheets were subjected to finish-annealing. Table 2 shows conditions of annealing the hot-rolled steel sheets, intermediate-annealing and finish-annealing.

Table 2: Conditions of annealing hot-rolled steel sheets, intermediate-annealing and finish-annealing

Steel No.	Annealing No.	annealing hot-rolled steel sheets		intermediate-annealing		finish-annealing
		(°C)	(seconds)	(°C)	(seconds)	(°C)	(seconds)
8	Y1	1090	60	950	60	950	60
	Y2	700	60	850	60	950	60
	Y3	700	28800	850	60	950	60
9	Y4	1100	60	950	60	1000	60
	Y5	700	60	1000	60	1000	60
	Y6	700	60	900	60	1000	60
	Y7	750	1500	900	60	1000	60
	Y8	700	3000	900	60	1000	60
	Y9	700	28800	900	60	1000	60

[Distribution rate and shape of precipitates]

A test piece sampled from each annealed steel sheet was etched in a nonaqueous electrolyte of 10% acetylacetone-1% tetramethyl ammonium chloride-methyl alcohol under a potentiostatic condition and then observed by a scanning electron microscope to investigate distribution of precipitates. A cross section in parallel to a rolling direction was inspected at arbitrary 50 points, and maximum length of each precipitate was measured and evaluated as particle size.

[Dimensional accuracy of a press-formed steel piece]

A blank sampled from each annealed steel sheet was press-formed to a cylindrical profile (shown in Fig. 1) by a multiplaten press. Maximum and minimum radii of a cylindrical part C at a position apart 5mm from a flanged part F were measured by a laser displacement meter. A ratio of (the maximum diameter - the minimum diameter)/(the minimum diameter) was calculated and regarded as circularity to evaluate dimensional accuracy of a press-formed steel sheet.

[Secondary formability]

Another test piece was bulged to a height of 10 mm, using a punch of 103 mm in diameter with a curvature radius of shoulder being 10 mm and a die of 105 mm in diameter with a curvature radius of shoulder being 8 mm under the condition a flange of the test piece was fixed with a bead. A blank of 92 mm in diameter was sampled from a bottom of the bulged test piece, and a hole of 10 mm in diameter was formed at a center of the blank with a clearance of 10%. The blank was then subjected to a secondary formability test as follows:
The blank was held between a flathead punch of 40 mm in diameter with a curvature radius of shoulder being 3 mm and a die of 42 mm in diameter with a curvature radius of shoulder being 3 mm, in the manner that burrs around the hole faced to the die. The hole was extruded by the punch until occurrence of cracks at its edge, while a flange of the blank was fixed with a bead. A diameter of the hole was measured at the crack initiation. A secondary-extruding ratio was calculated according to the equation of a secondary-extruding ratio (%) = (the diameter of the extruded hole - the diameter of the un-extruded hole)/(the diameter of the un-extruded hole) × 100.
Results are shown in Table 3. It is noted that steel sheets were cracked during press-forming at a product of (%Ti × %N) above 0.005. Steel sheets with a Nb content less than 0.02 mass % were formed with poor circularity, regardless manufacturing conditions. Observation results on any of cracked steel sheets and formed steel sheets with poor circularity prove that precipitates of 0.15 µm in particle size except TiN were distributed in a steel matrix only a few.
On the other hand, circularity was improved as an increase of number of the precipitates, which were distributed in a steel matrix with a Nb content of 0.3 mass % or more, in combination with conditions of thermo-mechanical treatment. However, excess distribution of the precipitates was improper for the circularity.
Steel sheets with a product of (%Ti × %N) more than 0.005 were extremely inferior in secondary formability. Poor secondary formability was also noted as for steel sheets with 0.02 mass % of Nb.
Improvement of secondary formability (i.e. hole-extruding) was recognized as an increase of number of the precipitates, which were distributed in a steel matrix with a Nb content of 0.3 mass % or more. However, excess distribution of the precipitates was improper for the secondary formability.

The above results prove that dimensional accuracy and secondary formability of a press-formed steel sheet depend on distribution of precipitates of 0.15 µm or more in particle size except TiN. That is, the optimum thermo-mechanical treatment for controlled distribution of such precipitates at a rate of 5000-50000/mm² is effective for dimensional accuracy and secondary formability.

Table 3: Circularity and secondary formability in relation with distribution of precipitates except TiN

Sample No.	Steel	Annealing No.	Number of precipitates* (/mm²)	Circularity	Secondary formability
1	1	-	12000	0.8	51
2	2	-	11000	1.6	52
3	3	-	12700	1.7	59
4	4	-	14200	2.2	53
5	5	-	13500	1.9	52
6	6	-	12500	cracked	23
7	7	-	12300	cracked	20
8	8	Y1	50	3.9	43
9	8	Y2	50	4.2	48
10	8	Y3	150	3.1	46
11	9	Y4	1000	3.2	42
12	9	Y5	1500	3.7	40
13	9	Y6	7000	2.2	62
14	9	Y7	32000	1.8	58
15	9	Y8	42000	1.9	52
16	9	Y9	80000	4.2	38

Precipitates* are of 0.15 µm or more in particle size except TiN.

Example 2

Several stainless steels with compositions shown in Table 4 were melted in a vacuum furnace and cast to slabs. Steels A-H belong to the present invention, while Steels I-L do not satisfy the compositional definitions of the present invention.

Each slab was hot-rolled to thickness of 4.0 mm, annealed, pickled and cold-rolled to thickness of 2 mm. The cold-rolled steel sheet was intermediately annealed, further cold-rolled to final thickness of 0.5 mm and then finish-annealed. Table 5 shows conditions of a finish-temperature of hot-rolling, annealing hot-rolled steel sheets, intermediate-annealing and finish-annealing.

Table 4: Chemical compositions (mass %) of ferritic stainless steels

Steel Kind	C	Si	Mn	P	S	Cr	N	Ti	Nb	Others	Ti × N	Trf
A	0.013	0.06	0.18	0.032	0.0002	13.93	0.007	0.18	0.13	Zr : 0.21
A	0.013	0.06	0.18	0.032	0.0002	13.93	0.007	0.18	0.13	Al : 0.08
B	0.003	0.5	0.26	0.013	0.003	15.63	0.005	0.22	0.32	B : 0.0072	0.0011	970
B	0.003	0.5	0.26	0.013	0.003	15.63	0.005	0.22	0.32	Cu:1.22	0.0011	970
C	0.006	0.08	0.26	0.023	0.002	16.55	0.008	0.18	0.25	Al : 0.08	0.0014	960
C	0.006	0.08	0.26	0.023	0.002	16.55	0.008	0.18	0.25	B : 0.0015	0.0014	960
D	0.008	0.08	0.36	0.022	0.001	17.3	0.009	0.14	0.25	Mo : 1.02	0.0013	960
E	0.01	0.2	0.52	0.024.	0.003	22	0.011	0.31	0.24	Mo : 0.5	0.0034	990
F	0.008	0.16	0.36	0.009	0.004	9.8	0.009	0.2	0.22		0.0018	870
G	0.003	0.17	0.21	0.01	0.0005	32	0.005	0.12	0.2	V : 0.08	0.0006	1020
H	0.006	0.1	0.17	0.027	0.001	16.52	0.011	0.15	0.25	B : 0.0014	0.0017	950
										Al : 0.016
										Ni : 0.1
I	0.04	0.26	0.31	0.03	0.003	16.8	0.007	0.12	0.23		0.0008	950
J	0.008	0.2	0.31	0.02	0.002	18.3	0,008	0.1	0.04		0.0008	920
K	0.013	0.19	0.21	0.012	0.003	17.3	0.012	0.12	0.8		0.0014	1030
L	0.01	0.18	0.23	0.011	0.002	16.5	0.02	0.29	0.48		0.0058	1000

T_rf is a recrystallization-finishing temperature (°C).

Table 5: Finish-temperature of hot-rolling and heat-treatment of steel sheets

Steel Kind	Example No.	A finish-temperature (°C)	Annealing*¹		Annealing*²		Annealing*³		Note
Steel Kind	Example No.	A finish-temperature (°C)	(°C)	(sec.)	(°C)	(sec.)	(°C)	(sec.)	Note
A	A1	780	950	60	870	10	950	30	Inventive Example
A	A2	760	890	60	950	600	980	30	Comparative Example
B	B1	750	760	60	950	20	1000	5	Inventive Example
B	B2	830	760	60	950	20	1020	5	Comparative Example
C	C1	750	490	3600	950	30	1000	60	Inventive Example
C	C2	760	490	7200	950	30	1000	60	Comparative Example
D	D1	790	550	600	900	60	1000	20	Inventive Example
D	D2	740	550	600	900	60	1110	60	Comparative Example
E	E1	780	850	600	900	60	1040	60	Inventive Example
F	F1	790	700	600	800	60	950	60	"
G	G1	780	750	3600	1000	60	1000	60	"
H	H1	760	700	60	900	60	1000	60	"
	H2	760	700	60	1000	60	1000	60	Comparative Example
	H3	760	1100	60	900	60	1000	60	"
	H4	760	700	60	900	60	1100	70	3
I	I1	780	890	20	920	30	980	60	"
J	J1	770	700	3600	880	60	980	60	"
K	K1	760	800	600	950	60	1050	60	"
L	L1	770	760	60	950	60	1050	60	"

Annealing*¹ is heat-treatment to a hot-rolled steel sheet.
Annealing*² is intermediate heat-treatment during cold-rolling.
Annealing*³ is final heat-treatment to a cold-rolled steel sheet.

Each steel sheet was examined by the same way as Example 1, to investigate shape and distribution of precipitates as well as dimensional accuracy and secondary formability of a press-formed steel sheet.
Results shown in Table 6 prove that ferritic stainless steel sheets, wherein precipitates of 0.15 µm or more in particle size except TiN were distributed in steel matrix at a rate of 5000-50000/mm², were press-formed to a good profile with circularity of 2.5 % or less.
On the other hand, comparative steel sheets (Example Nos. A2, B2, C2 and D2), which satisfied compositional conditions of the present invention but were manufactured under improper conditions, had poor dimensional accuracy and secondary formability in a press-formed state due to the metallurgical structure that distribution number of the precipitates except TiN were out of 5000 - 50000/mm².
The steel sheet I was too hard due to excess C and cracked during press-forming. The steel sheet K was too strong due to excess Nb and cracked during press-forming. The steel sheet L with a product of (%Ti × %N) above 0.005 was also cracked during press-forming, wherein the cracks were initiated near coarse TiN particles. The steel sheet J with shortage of Nb was press-formed with poor circularity.

It is understood from the above comparison that ferritic stainless steel sheets can be press-formed to objective profiles with high dimensional accuracy and excellent secondary formability, by controlled distribution of precipitates of 0.15 µm or more in particle size except TiN.

Table 6: Circularity and secondary formability in relation with distribution of precipitates

Steel kind	Example No.	Distribution number of precipitates* (/mm²)	Circularity (%)	Secondary formability (%)	Note
A	A1	7200	1.3	51	Inventive Example
A	A2	4500	2.7	48	Comparative Example
B	B1	12000	2.3	53	Inventive Example
B	B2	4600	3.6	49	Comparative Example
C	C1	23000	2	53	Inventive Example
C	C2	80000	2.9	44	Comparative Example
D	D1	16000	2.2	60	Inventive Example
D	D2	320	3.6	32	Comparative Example
E	E1	8000	0.9	50	Inventive Example
F	F1	12000	0.8	51	"
G	G1	23000	2.2	50	"
H	H1	13000	0.8	68	"
	H2	100	2.9	48	Comparative Example
	H3	50	3.2	44	"
	H4	50	4	32	"
I	I1	12000	cracked	12	"
J	J1	130	3.3	34	"
K	K1	78000	cracked	22	"
L	L1	13000	cracked	22	"

*Precipitates are of 0.15 µm or more in particle size except TiN.

According to the present invention as the above, ferritic stainless steel sheets, which can be press-formed with high dimensional accuracy and excellent secondary formability, are provided by distribution of precipitates of 0.15 µm or more in particle size except TiN at a rate of 5000 - 50000/mm² in a steel matrix with controlled composition. Shape and distribution of such precipitates suitable for the purpose are realized by properly controlling a finish-temperature of hot-rolling and conditions of heat-treatment for annealing a hot-rolled steel sheet, intermediate-annealing during cold-rolling and finish-annealing a cold-rolled steel sheet. The ferritic stainless steel sheets manufactured in this way are useful as members or parts, which demand for strict dimensional precision, in various fields, e.g. sealing members for organic electro-luminescent devices, precision pressed parts, sinks, utensils, burners of stoves, oil filler tubes of fuel tanks, motor casings, covers, caps of sensors, injector tubes, thermostat valves, bearing seals, flanges and so on, instead of expensive austenitic stainless steel sheets.

Claims

A ferritic stainless steel sheet excellent in press formability and secondary formability, which has
a composition consisting of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (%Ti × %N) less than 0.005, and
the metallurgical structure that precipitates of <0.15-1.0 µm in particle size selected from the group consisting of TiC, NbC, NbN, laves phase and mixtures thereof but excluding TiN are distributed at a rate of 5000-50000/mm² in a steel matrix.
A method of manufacturing a ferritic stainless steel sheet excellent in press formability and secondary formability, which comprises the steps of:
providing a slab of a ferritic stainless steel having the composition consisting of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (%Ti × %N) less than 0.005;

hot-rolling the slab at a finish-temperature of 800°C or lower;

annealing the hot-rolled steel sheet at a temperature within a range of 450-1080°C for a time period avoiding the redissolution of precipitates;

cold-rolling the annealed steel sheet in accompaniment with at least one intermediate-annealing at a temperature within a range of from (a recrystallization-finishing temperature - 100°C) to (a recrystallization-finishing temperature for a time period avoiding the redissolution of precipitates ; and then

finish-annealing the cold-rolled steel sheet at a temperature of 1080°C or lower, for a time period avoiding the redissolution of precipitates .