EP2578715B1

EP2578715B1 - Structural stainless steel sheet having excellent corrosion resistance in welded part, and method for producing same

Info

Publication number: EP2578715B1
Application number: EP11789877.5A
Authority: EP
Inventors: Hiroki Ota; Kenichi Fujita; Yasushi Kato
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2010-05-31
Filing date: 2011-05-26
Publication date: 2017-07-12
Anticipated expiration: 2031-05-26
Also published as: RU2012157554A; TW201207128A; KR101409291B1; US20130126052A1; CA2799696A1; CN102933732B; WO2011152475A1; JP4893866B2; BR112012030684B1; AU2011259992B2; AU2011259992A1; ES2643150T3; BR112012030684A2; TWI439555B; EP2578715A4; CA2799696C; KR20130034025A; EP2578715A1; JP2012012702A; CN102933732A

Description

[Technical Field]

The present invention relates to a structural stainless steel sheet having excellent welded part corrosion resistance which is suitably used as a material for a body of a railway wagon which carries coal or iron ore, for example, and a method of manufacturing the structural stainless steel sheet.

[Background Art]

As a material for a body of a railway wagon which carries coal or iron ore, stainless steel has been popularly used. Since mined coal contains large sulfur content, the material for the body of the railway wagon is required to possess sulfuric acid corrosion resistance, and particularly intergranular corrosion resistance of the welded part.
As the stainless steel which possesses both corrosion resistance and weldability, for example, patent document 1 discloses a Ti-containing ferritic stainless steel which exhibits excellent weld toughness thereof. However, in the technique disclosed in patent document 1, components are designed such that the structure of the welded part has a ferrite phase and hence, there exists a drawback that weld toughness and corrosion resistance of the welded part are not sufficient.
On the other hand, patent document 2 and patent document 3 disclose a technique where a proper quantity of martensitic phase is formed in a welded part by controlling a phase fraction at a high temperature thus improving workability and corrosion resistance of the welded part. Further, patent document 4 discloses stainless steel which is suitable for a welding method using a carbon dioxide gas. Further, one of inventors of the present invention has proposed previously a structural stainless steel sheet which improves corrosion resistance of a welded part by properly regulating the composition using parameters which can accurately predict the structure of the welded part (patent document 5). Furthermore patent document 6 provides Cr-containing steel with proper amounts of V that has satisfactory levels of toughness, strength (hardness) and corrosion resistance of the heat affected zone.

[Prior art literature]

[Patent document]

[Patent document 1] JP-A-3-249150
[Patent document 2] JP-A-2002-167653
[Patent document 3] JP-A-2009-13431
[Patent document 4] JP-A-2002-30391
[Patent document 5] JP-A-2009-280850
[Patent document 6] US 2003/0044305

[Summary of the Invention]

[Task to be solved by the Invention]

However, in the techniques disclosed in these patent documents 2 to 5, studies on an optimum component range have not been entirely sufficient. Particularly, manufacturability has been hardly taken into consideration in these techniques. Accordingly, the occurrence of cracks in a slab stage and the occurrence of a surface defect called as scabs are conspicuous and hence, it is difficult to obviate a cost rise caused by lowering of a yield ratio.
The present invention has been made under such circumstances, and it is an object of the present invention to provide a structural stainless steel sheet which can be manufactured at a low cost with high efficiency, and possesses excellent welded-part corrosion resistance.

[Means for solving the Task]

One of inventors of the present invention has made extensive studies to overcome the above-mentioned drawback, and has found that intergranular corrosion caused by depletion of Cr in the vicinity of a grain boundary can be suppressed and a welded heat affected zone can be formed into the structure which is mainly formed of martensite by adjusting chemical components, particularly, contents of Mn and Ti, and a balance between the respective components within proper ranges, and has proposed a parameter (F value) shown in patent document 5. Then, the inventors of the present invention have continued detailed studies particularly on the manufacturability based on the finding and, as a result of the studies, have found that slab cracks and scabs (surface defects) caused by inclusions can be remarkably reduced when a proper quantity of Al is added to the composition, contents of V, Ca, O are reduced to predetermined ranges or less, and an FFV value is set within a proper range as a new parameter indicative of whether or not manufacturability is favorable, and have completed the present invention.
That is, the present invention provides the structural stainless steel sheet having excellent welded part corrosion resistance, the structural stainless steel sheet having a composition which contains by mass% 0.01 to 0.03% C, 0.01 to 0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn, 0.04% or less P, 0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0% Ni, 4x (C+N) or more and 0.3% or less Ti (C, N indicating contents (mass%) of C and N), and Fe and unavoidable impurities as a balance, V, Ca and 0 in the unavoidable impurities being regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O, wherein the steel sheet further optionally contains 1.0% or less Cu by mass% and/or 1.0% or less Mo by mass%, and/or 0.005% or less B, wherein an F value and an FFV value expressed by following formulae satisfy a condition that F value≤11 and FFV value≤9.0. $F value = Cr + 2 \times Si + 4 \times Ti - 2 \times Ni - Mn - 30 \times (C + N)$
$FFV value = Cr + 3 \times Si + 16 \times Ti + Mo + 2 \times Al - 2 \times Mn - 4 \times (Ni + Cu) - 40 \times (C + N) + 20 \times V$
In the formulae, the respective element symbols are contents of the elements (mass%).
Further, the present invention provides a method of manufacturing a structural stainless steel sheet, wherein a steel slab having a composition which contains by mass% 0.01 to 0.03% C, 0.01 to 0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn, 0.04% or less P, 0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0% Ni, 4×(C+N) or more and 0.3% or less Ti (C, N indicating contents (mass%) of C and N), and Fe and unavoidable impurities as a balance, V, Ca and O in the unavoidable impurities being regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O, wherein the steel sheet further optionally contains 1.0% or less Cu by mass% and/or 1.0% or less Mo by mass%, and/or 0.005% or less B, wherein an F value and an FFV value expressed by following formulae satisfy a condition that F value≤11 and FFV value≤9.0 is heated at a temperature of 1100 to 1300°C and, thereafter, hot rolling which includes a rough hot rolling where rolling is performed for at least 1 pass or more at a reduction rate of 30% or more in a temperature range exceeding 1000°C, or the hot rolling is performed without annealing the hot-rolled sheet or after annealing the hot-rolled sheet at a temperature of 600 to 1000°C. And, thereafter, pickling is applied to a hot-rolled sheet or an annealed hot-rolled sheet. $F value = Cr + 2 \times Si + 4 \times Ti - 2 \times Ni - Mn - 30 \times (C + N)$
$FFV value = Cr + 3 \times Si + 16 \times Ti + Mo + 2 \times Al - 2 \times Mn - 4 \times (Ni + Cu) - 40 \times (C + N) + 20 \times V$
In the formulae, the respective element symbols are contents (mass%) of the elements.

[Advantage of the Invention]

According to the present invention, it is possible to provide the structural stainless steel sheet having excellent welded part corrosion resistance which is manufactured at a low cost and with high efficiency, and is suitably used as a material for a body of a railway wagon which carries coal or iron ore, for example.

[Brief description of the drawings]

Fig. 1 is a graph showing the relationship between an FFV value and a surface defect occurrence rate.
Fig. 2 is an optical micrograph showing an observation example when deep pit-shaped corrosion is recognized in a welded heat affected zone in cross section of a specimen after a sulfuric acid-copper sulfate corrosion test.

[Best Mode for carrying out the Invention]

The present invention is explained in detail hereinafter.
Firstly, the composition of the present invention is explained. In the explanation made hereinafter, the % indication is mass%.

C: 0.01 to 0.03%

N: 0.01 to 0.03%

It is necessary for a structural stainless steel sheet to contain both at least 0.01 or more C and 0.01 or more N for acquiring strength necessary for the structural stainless steel sheet. On the other hand, when the contents of C, N exceed 0.03%, Cr carbide or Cr carbonitride tends to precipitate so that corrosion resistance, and particularly, corrosion resistance of a welded heat affected zone is deteriorated. Further, the welded heat affected zone is hardened thus also deteriorating toughness. Accordingly, both contents of C and N are limited to values which fall within a range from 0.01 to 0.03%. The content of C is preferably limited to a value which falls within a range from 0.015 to 0.025%, and the content of N is preferably limited to a value which falls within a range from 0.012 to 0.02%.

Si: 0.10 to 0.40%

Si is an element which is used as a deoxidizer, and it is necessary to contain 0.10% or more Si to acquire such an advantage brought about by Si. On the other hand, when the content of Si exceeds 0.40%, toughness of a hot-rolled steel sheet is deteriorated. Accordingly, the content of Si is limited to a value which falls within a range from 0.10 to 0.40%. A lower limit of the Si content is preferably set to 0.20%, and an upper limit of the Si content is preferably set to 0.30%.

Mn: 1.5 to 2.5%

Mn is a useful element as a deoxidizer and also as a reinforcing element for securing strength necessary for a structural stainless steel sheet, and Mn is also an austenite stabilizing element at a high temperature. Further, in the present invention, Mn is an important element for controlling the microstructure of the welded heat affected zone to the martensitic structure having desired volume fraction. To allow Mn to exhibit such function, it is necessary to set the content of Mn to 1.5% or more. On the other hand, even when the content of Mn exceeds 2.5%, not only the advantage of Mn is saturated but also the excessive content of Mn deteriorats toughness of the steel sheet, adversely influences a surface property by deterioration descaling property during a manufacturing step, and pushes up an alloy cost. Accordingly, the content of Mn is limited to a value which falls within a range from 1. 5 to 2.5%. The content of Mn is preferably limited to a value which falls within a range from 1.8 to 2.5%. The content of Mn is more preferably limited to a value which falls within a range from 1.85 to 2.0%.

P: 0.04% or less

The content of P is preferably set small from a viewpoint of hot workability, and an allowable upper limit of the content of P is set to 0.04%. The upper limit of the content of P is more preferably set to 0.035% or less.

S: 0.02% or less

The content of S is preferably set small from a viewpoint of hot workability and corrosion resistance, and an allowable upper limit of the content of S is set to 0.02%. The upper limit of the content of S is more preferably set to 0.005% or less.

Al: 0. 05 to 0.15%

Although Al is an element which is added to the composition for deoxidization in general, according to the present invention, the inventors of the present invention have found that Al enhances manufacturability, and effectively functions to suppress the occurrence of cracks in a slab stage particularly, and a proper quantity of Al is added for allowing Al to exhibit such a function. To suppress the occurrence of cracks in a slab, in addition to the containing of Al, the reduction of V, Ca and O, and the optimization of an FFV value are necessary as described later. Although the mechanism where the occurrence of cracks in a slab is suppressed due to the containing of Al is not entirely clarified, it is estimated that such improvement is brought about by properly regulating a phase fraction and by controlling a morphology of inclusion. To acquire such an advantage, it is necessary to set the content of Al to 0.05% or more. On the other hand, when the content of Al exceeds 0.15%, large-sized Al-based inclusion is generated thus causing a surface defect. Accordingly, the content of Al is limited to a value which falls within a range from 0.05 to 0.15%. The content of Al is preferably limited to a value which falls within a range from 0.080 to 0.150%. The content of Al is more preferably limited to a value which falls within a range from 0.085 to 0.120%.

Cr: 10 to 13%

Cr is an element which forms a passive film, and is inevitable for securing corrosion resistance, particularly, corrosion resistance of a welded heat affected zone. It is necessary to set the content of Cr to 10% or more to acquire such an advantage. On the other hand, when the content of Cr exceeds 13%, not only a cost is pushed up but also it is difficult to secure a sufficient austenite phase at a high temperature in a welded part and hence, it is difficult to acquire the martensitic structure of a fraction necessary for a welded heat affected zone after welding. As a result, deterioration of intergranular corrosion resistance at the welded heat affected zone is brought about. Accordingly, the content of Cr is limited to a value which falls within a range from 10 to 13%. The content of Cr is preferably limited to a value which falls within a range from 10.5 to 12.5%.

Ni: 0.5 to 1.0%

The content of Ni is set to 0.5% or more to secure strength and toughness. On the other hand, Ni is an expensive element and hence, an upper limit of the content of Ni is set to 1.0% from an economical point of view. Ni is, in the same manner as Mn, an austenite stabilizing element at a high temperature and hence, Ni is useful in controlling the microstructure of a welded heat affected zone to the martensitic structure having desired volume fraction. However, this advantage can be sufficiently acquired due to the addition of Mn and hence, it is reasonable to limit the content of Ni to a value which falls within a range from 0.5 to 1.0%. The content of Ni is preferably limited to a value which falls within a range from 0. 60 to 1.0%. The content of Ni is more preferably limited to a value which falls within a range from 0.60 to 0.90%.

Ti: 4×(C+N) or more and 0.3% or less

Ti is an important element for acquiring excellent welded part corrosion resistance in the present invention, and is an element particularly inevitable for enhancing intergranular corrosion resistance of a welded heat affected zone. Ti has an advantage that Ti precipitates and fixes C, N in steel as carbide, nitride or carbonitride of Ti (hereinafter three kinds of compositions consisting of carbide, nitride and carbonitride being collectively referred to as carbonitride or the like) thus suppressing the generation of carbonitride or the like of Cr. In the present invention, in a welded heat affected zone of a steel sheet which has the structure formed of ferrite and martensite, from a viewpoint of corrosion resistance, deterioration of corrosion resistance of a ferrite phase part which causes the precipitation of carbonitride or the like during cooling becomes a problem. In the steel sheet according to the present invention, carbonitride or the like of Cr precipitates in the welded heat affected zone at the time of welding so that Cr depletion occurs in the vicinity of the grain boundary whereby, particularly, a drawback that intergranular corrosion resistance of the ferrite phase part is deteriorated can be overcome due to the containing of Ti. To allow Ti to exhibit such function, it is necessary to set the content of Ti to 4×(C+N) or more (C, N indicating contents (mass%) of C and N). On the other hand, even when the content of Ti exceeds 0.3%, not only the advantage of Ti is saturated but also a large quantity of carbonitride or the like of Ti precipitates in the steel thus bringing about the deterioration of toughness of the steel sheet. Accordingly, the content of Ti is limited to 4×(C+N) or more and 0.3% or less. The content of Ti is more preferably limited to a value which falls within a range from 0.180 to 0.230%. That is, it is effective for the steel sheet to reduce C, N such that the content of Ti simultaneously satisfies 4×(C+N) or more.
In the present invention, to increase productivity (yield rate) or manufacturability, and particularly to suppress the occurrence of scabs (surface defects) which occur due to cracks or inclusion in a slab stage, it is important to reduce V, Ca and O as described hereinafter.

V: 0.05% or less

It is often the case that V is added to a steel sheet as an impurity in a Cr raw material or the like, and there may be case where V is added to a steel sheet unintentionally. However, to suppress the occurrence of cracks particularly in a slab stage, it is necessary to strictly regulate the content of V. From such a viewpoint, it is necessary to limit the content of V to 0.05% or less. It is more preferable to limit to the content of V to 0.03% or less. It is still more preferable to limit to the content of V to less than 0.03%. Although a larger crack suppression effect can be obtained by limiting the content of V to 0.01% or less, the selection of a raw material or the like becomes necessary and hence, such limitation of the content of V becomes economically disadvantageous.

Ca: 0.0030% or less

Calcium forms an inclusion of a low melting point and hence, Ca becomes a cause of a surface defect particularly attributed to the inclusion. Accordingly, in the present invention, it is necessary to strictly restrict the content of Ca, and an upper limit of the content of Ca is limited to 0.0030%. It is preferable that the content of Ca is as small as possible, and the content of Ca may be preferably limited to 0.0010% and may be more preferably limited to 0.0002% or less. However, the selection of the raw material or the like becomes necessary and hence, such limitation of the content of Ca becomes economically disadvantageous.

O: 0.0080% or less

It is necessary to suppress the content of O so as to suppress the generation of an oxide-based inclusion thus securing high productivity and hence, an upper limit of the content O is set to 0.0080%. The upper limit of the content of O is more preferably set to 0.060% or less.
Further, in the present invention, corrosion resistance and productivity can be largely improved by setting an F value and an FFV value described hereinafter to within proper ranges.

F value≤11

The F value is expressed by Cr+2×Si+4×Ti-2×Ni-Mn-30×(C+N) (respective element symbols being contents of the elements (mass%)), and is a parameter for estimating the microstructure of a welded heat affected zone at the time of welding. To be more specific, the F value is a parameter for estimating a volume fraction of the martensitic structure (a residual rate of the ferrite structure). In a part of a steel sheet such as a welded heat affected zone which is exposed to a high temperature, a part of the zone is transformed into austenite (or a portion of the part is further transformed into δ ferrite (delta ferrite)), and these phase are transformed into martensite in a cooling step. The rate is influenced by a quantitative balance between ferrite stabilizing elements (ferrite formation elements) and austenite stabilizing elements (austenite formation elements). In the above-mentioned formula expressing the F value, elements having a positive coefficient (Cr, Si, Ti) are the ferrite stabilizing elements and elements having a negative coefficient (Ni, Mn, C, N) are the austenite stabilizing elements. That is, the larger the F value, the more the ferrite structure is likely to remain (the larger a volume fraction of the ferrite structure becomes, that is, the smaller a volume fraction of the martensitic structure becomes), while the smaller the F value, the more scarcely the ferrite structure remains (the smaller a volume fraction of the ferrite becomes, that is, the larger a volume fraction of the martensitic structure becomes).
In patent document 5, the optimization of content is attempted by investigating the relationship between the F value and a volume fraction of the martensitic structure of the welded heat affected zone and by evaluating corrosion resistance of an area in the vicinity of the welded heat affected zone by a sulfuric acid-copper sulfate corrosion test. Also in this embodiment, in the same manner as the above-mentioned patent document 5, to enhance the corrosion resistance of the welded heat affected zone, the above-mentioned F value is limited to 11 or less (martensite volume fraction: 40% or more). The above-mentioned F value is preferably limited to 10.5 or less (martensite volume fraction: 60% or more), and is more preferably limited to 10 or less. Here, from a viewpoint of corrosion resistance at the welded part, a lower limit of the F value is preferably set to 5. 0 or more, and is more preferably set to 6.0 or more.

FFV value≤9.0.

The FFV value is expressed by Cr+3×Si+16×Ti+Mo+2×Al-2×Mn-4×(Ni+Cu)-40×(C+N)+20×V (the respective element symbols being contents of the elements (mass%)). The FFV is newly introduced in the present invention as an index for indicating manufacturability. The FFV value is set by taking a phase balance during hot rolling into consideration. By adjusting the components as described above, particularly by regulating the content of Al and upper limits of V, Ca, O and, thereafter, by setting this FFV value smaller, the occurrence of surface defects caused by cracks in a slab stage or inclusions can be remarkably reduced. The significant technical feature of present invention lies in succeeding in largely suppressing the lowering of a yield rate caused by the occurrence of a surface defect by optimizing a new parameter which takes an Al quantity which was not taken into consideration at the time of inventing the F value into consideration. Although the mechanism of the improvement of the manufacturability by optimization of the FFV value is not entirely clarified, since the manufacturability is largely improved by limiting the FFV value to 9.0 or less, the FFV value is set to 9.0 or less. The FFV value is preferably set to 8. 5 or less. Although it is effective to decrease a Cr quantity or to increase C, N quantities to make the FFV value small, there is a possibility that the reduction of Cr quantity or the increase of C, N quantities deteriorats corrosion resistance. Accordingly, it is preferable to set the lower limit of the FFV value to 5.0 or more, and it is more preferable to set the lower limit of the FFV value to 6.0 or more.
For the steel sheet of the present invention which is used in a state of a hot-rolled sheet or a hot-rolled annealed sheet, the control of cracks in a slab stage and inclusions is important for reducing surface defects. It is because, with respect to the occurrence of surface defects, portions such as cracks or scabs which largely lower a yield rate not only deteriorate the appearance but also become a starting point of the occurrence of rust and hence, it is necessary to cut off the portions where cracks or scab occur at the time of shipping the steel sheet as a product. Although the above-mentioned formula on the FFV value includes Mo, V, Cu, there may be a case where these components are not added to the steel. When these contents are not added to the steel, the FFV value is calculated by setting the contents of the components not contained in the steel to 0%.
Fig. 1 shows the relationship between the FFV value and a surface defect occurrence rate. The surface defect occurrence rate was calculated based on a length of a portion where defects occur with respect to a total length of a coil. It is understood that by limiting the FFV value within a range of 9.0 or below, the occurrence of surface defects can be remarkably suppressed.
In the present invention, the steel may contain Cu within a following range when necessary in addition to the above-mentioned components.

Cu: 1.0% or less

Cu is an element which enhances corrosion resistance, and is an element which particularly reduces crevice corrosion. Accordingly, Cu can be added when the steel is requested to possess high corrosion resistance. However, when the content of Cu exceeds 1.0%, hot workability is deteriorated, and also a phase balance at a high temperature collapses and hence, it is difficult for a welded heat affected zone to acquire the desired microstructure. Accordingly, when Cu is added to the composition, an upper limit of the content of Cu is set to 1.0%. To allow Cu to exhibit a sufficient corrosion resistance enhancing effect, it is effective to set the content of Cu to 0.3% or more. The content of Cu is more preferably set to a value which falls within a range from 0.3 to 0.5%.

Mo: 1.0% or less

Mo is an element which enhances corrosion resistance, and can be added to the composition when a steel sheet is requested to possess high corrosion resistance particularly. However, when the content of Mo exceeds 1.0%, cold workability is deteriorated, and also a rough surface occurs in hot rolling so that surface quality is extremely deteriorated. Accordingly, when Mo is added to the composition, an upper limit of the content of Mo is set to 1.0%. To allow Mo to exhibit sufficient corrosion resistance, it is effective to set the content of Mo to 0.03% or more. The content of Mo is more preferably set to a value which falls within a range from 0.1 to 1.0%.
In the present invention, besides the improvement of corrosion resistance acquired by adding 1.0% or less of Cu or Mo described above, other elements may be added based on conventional finding for improving ductility or the like due to addition of 0.005% or less B. Also in this case, it is important to take a phase balance at a high temperature into consideration. Nb is a strong stabilizing element and largely collapses a phase balance by combining with C or N and hence, Nb is not added in the present invention. A balance other than the above-prescribed elements is constituted of Fe and unavoidable impurities.
In the steel sheet of the present invention, by setting the above-mentioned F value to 11 or less to enhance corrosion resistance of a welded heat affected zone, a martensite in volume fraction of the welded heat affected zone becomes 40% or more. By preferably setting the above-mentioned F value to 10.5 or less, the martensite fraction of the welded heat affected zone becomes 60% or more. By further preferably setting the above-mentioned F value to 10 or less, the martensite in volume fraction of the welded heat affected zone becomes 80% or more in this case. Also in the steel sheet according to the present invention, 50% or more of a matrix steel (base material) portion in volume fraction is formed of the ferrite structure. The remaining structure is formed of, particularly in a hot-rolled state, the structure where a martensite phase and a residual γ phase are present and partially contains carbonitride or the like. Particularly, with respect to the structure of a hot-rolled annealed sheet which is manufactured as described later such that contents of components are set to fall within a proper composition range and hot-rolled-sheet annealing is applied under a proper annealing condition, almost 100% of the structure has the ferrite-phase structure in volume fraction and hence, the structure possesses the excellent workability.
Next, a method of manufacturing a stainless steel sheet according to the present invention is explained.
The method of manufacturing a stainless steel sheet of the present invention may be performed in accordance with a given method and is not specifically limited. However, as a method which can manufacture a stainless steel sheet with high efficiency, a method where a molten steel having the above-mentioned composition is formed into a slab by continuous casting or the like, the slab is formed into a hot-rolled coil, the hot-rolled coil is annealed when necessary and, thereafter, descaling (shot blasting, pickling and the like) is performed thus manufacturing a stainless steel sheet according to the present invention is recommended.
Hereinafter, the method of the present invention is explained in detail.
Firstly, a molten steel adjusted to the composition of the present invention is produced by a known commonly used melting furnace such as a steel converter or an electric furnace and, thereafter, the molten steel is refined by a known refining method such as a vacuum degassing method (RH method), a VOD (Vacuum Oxygen Decarburization) method or an AOD (Argon Oxygen Decarburization) method, and the molten steel is formed into a steel slab (raw steel material) by a continuous casting or an ingot-making/blooming method. It is preferable to adopt continuous casting as a casting method from a viewpoint of productivity and quality. Further, a thickness of a slab may preferably be set to 100mm or more for securing a reduction ratio in hot coarse rolling described later. It is more preferable to set the thickness of the slab within a range of 200mm or more.
Next, the steel slab is heated up to a temperature of 1100 to 1300°C and, thereafter, is subjected to hot rolling whereby a hot-rolled steel sheet is formed. It is desirable to set the slab heating temperature high for enhancing surface roughness resistance of the hot-rolled sheet or anti-ridging property or ridging property after annealing in cold rolling. However, when the slab heating temperature exceeds 1300°C, slag sag becomes conspicuous, and crystal grains become coarse thus deteriorating toughness of the hot-rolled sheet. On the other hand, when the slab heating temperature is below 1100°C, a load in the hot rolling becomes high and hence, rough surface in hot rolling becomes conspicuous, and also the recrystallization during hot rolling becomes insufficient thus also deteriorating toughness of the hot-rolled sheet.
In a hot rough rolling step, it is preferable to perform rolling at a reduction rate of 30% or more in a temperature range exceeding 1000°C for at least 1 pass or more. Due to this rolling with a high reduction rate, the grain (crystal) structure of a steel sheet is made fine so that toughness of the steel sheet is enhanced. After hot rough rolling, hot finish rolling is performed in accordance with a given method (under a condition of usual hot finish rolling).
A hot-rolled sheet having a sheet thickness of approximately 2.0 to 8.0mm which is manufactured by hot rolling is used as a structural material directly or through pickling without annealing. Pickling may be applied to the hot-rolled sheet after the hot-rolled sheet is annealed at a temperature of 600 to 1000°C. When an annealing temperature of the hot-rolled sheet is below 600°C, there may be a case where a martensite phase or a residual γ phase which has a possibility of existing in a hot-rolled state remains and hence, the ferrite structure becomes 50% or less in terms of a volume fraction whereby the steel sheet cannot acquire the sufficient workability. On the other hand, when the annealing temperature exceeds 1000°C, the coarsening of grain size becomes conspicuous and hence, toughness of the hot-rolled sheet is deteriorated. Annealing of the hot-rolled sheet may preferably be performed such that the hot-rolled sheet is held at a predetermined temperature of 600 to 1000°C for 1 hour or more by so-called box annealing. Further, when the annealing temperature becomes excessively high, there is a case where the hot-rolled sheet enters a temperature at which the γ transformation occurs and hence, the excessively high temperature is not preferable. Accordingly, it is necessary to adjust the composition within a proper range and to select a proper temperature range corresponding to the composition. In the composition range of the steel of the present invention, when the annealing temperature is mainly set to a value which falls within 600 to 900°C, almost 100% of the hot-rolled sheet becomes a ferrite phase in terms of a volume fraction and hence, it is preferable to set the annealing temperature within this temperature range.
As welding a stainless steel sheet according to the present invention, all usual welding methods including arc welding such as TIG welding or MIG welding, seam welding, resistance welding such as spot welding, laser welding and the like are applicable to the steel of the present invention.

[Embodiment]

Stainless steel having the composition shown in Table 1 is formed into slabs having a thickness of 200mm through a steel converter, VOD and continuous casting. These slabs are heated at a temperature of 1180°C and, thereafter, the slab is formed into a coil-shaped hot-rolled sheet having a sheet thickness of 5.0mm by hot rolling. A hot rolling finish (delivering) temperature is set to 900°C, and a coiling temperature after hot rolling is set to 700°C. The obtained hot-rolled steel sheet is subjected to annealing at a temperature of 690°C for 10 hours and, thereafter, scales are removed from the hot-rolled steel sheet by shot blasting and pickling.
Flat plate samples are cut out from the steel sheet after removing scales, T-shaped specimens each of which is formed of a lower plate and a vertical plate are assembled, and both side one pass fillet welding (gas metal arc welding, shielding gas: 98 volume% Ar-2 volume%O₂, flow rate: 20 litter/min) is applied to the T-shaped specimens thus forming three fillet welding specimens. MGS-309LS made by Kobe steel limited is used as a welding rod, and a welding input heat is set to a value which falls within a range from 0.4 to 0.8kJ/mm.
Corrosion test specimens are sampled from these filled welded parts of these fillet welding specimens, and the corrosion specimens are subjected to a sulfuric acid-copper sulfate corrosion test (Modified Strauss test in accordance with ASTM A262 practice E and ASTM A763 practice Z, a test liquid: Cu/6%CuSO₄/0.5%H₂SO₄, a specimen with polished end surfaces being immersed in the boiling test liquid for 20 hours), and a corrosion state of an area in the vicinity of a welded heat affected zone is observed.
Fig. 2 is an optical micrograph showing an observation example of a cross section of the specimen after the sulfuric acid-copper sulfate corrosion test. The evaluation "C" is given to a case where intergranular corrosion is observed or pit-shaped corrosion far deeper than intergranular corrosion is observed in the welded heat affected zone as shown in the photograph. The evaluation "B" is given to a case where slight corrosion is observed in the welded heat affected zone. The evaluation "A" is given to a case where corrosion is not observed by the observation using an optical microscope. Further, a surface state of the hot-rolled annealed sheet after pickling is observed over the whole length of the sheet. Using a rate of a length of the hot-rolled annealed sheet along which a surface defect caused by cracks in a slab or inclusion is observed with respect to the whole length of the hot-rolled annealed sheet as an index, the evaluation is made by giving "a" to a case where the defect occurrence rate is 3% or less, "b" to a case where the defect occurrence rate is 3% or more and 30% or less, and "c" to a case where the defect occurrence rate is more than 30%. These results are shown in Table 2.
As a result, with respect to present invention examples No.1 to 5, 10 to 13 and 15 which fall within the scope of the present invention, these examples exhibit favorable welded part corrosion resistance and a surface state of the welded part is also extremely favorable. To the contrary, with respect to comparison examples No. 9 and 14 where the F value falls outside the scope of the present invention, a martensite generation quantity in the welded heat affected zone is small and hence, these examples exhibit the intergranular corrosion resistance clearly inferior to the intergranular corrosion resistance of the present invention examples. Further, with respect to a comparison example No. 6 where an Si content is higher than a range of Si content of the present invention and an A1 content is lower than a range of Al content of the present invention and comparison examples No. 7, 8, 9 and 14 where the FFV value falls outside a range of the FFV value of the present invention, in the surface observation carried out after hot rolling and annealing, many cracks attributed to slab and many scabs attributed to inclusions are observed.

Since the present invention steel is used in a state of a hot-rolled sheet or a hot-rolled annealed sheet, the occurrence of scabs largely lowers a yield rate. This is because the scab portions not only exhibit poor appearance but also become a starting point of the occurrence of rust and hence, it is necessary to cut off portions corresponding to the scab portions at the time of shipping the hot-rolled sheet or the hot-rolled annealed sheet as a product.

Table 1

No.	chemical composition (mass%)														F value	FFV value
No.	C	Si	Mn	P	S	Al	Cu	Ni	Cr	Ti	V	N	O	Ca	F value	FFV value
1	0.022	0.24	1.87	0.034	0.005	0.105		0.65	11.2	0.194	0.01	0.0150	0.0052	0.0010	8.2	7.6	present invention steel
2	0.025	0.30	1.53	0.029	0.001	0.120		0.85	12.6	0.180	0.01	0.0242	0.0050	0.0005	9.2	8.4	present invention steel
3	0.015	0.28	1.90	0.031	0.004	0.119		0.70	11.4	0.210	0.01	0.0195	0.0065	0.0001	8.5	8.1	present invention steel
4	0.020	0.21	1.64	0.034	0.003	0.082	0.40	0.80	12.0	0.192	0.01	0.0165	0.0055	0.0024	8.9	6.5	present invention steel
5	0.018	0.24	1.95	0.030	0.003	0.103		0.60	11.0	0.185	0.03	0.0145	0.0050	0.0001	8.1	7.9	present invention steel
6	0.018	0.45	1.70	0.030	0.010	0.013		0.91	11.2	0.240	0.01	0.0130	0.0062	0.0001	8.6	8.3	comparison steel
7	0.022	0.40	1.70	0.025	0.002	0.005		0.40	11.1	0.200	0.01	0.0140	0.0054	0.0010	9.1	9.3	comparison steel
8	0.020	0.40	1.9	0.030	0.006	0.014		0.91	11.2	0.251	0.10	0.0100	0.0055	0.0001	8.4	9.9	comparison steel
9	0.01	0.50	1.20	0.029	0.002	0.004		0.30	11.9	0.200	0.01	0.0120	0.0057	0.0002	11.2	12.3	comparison steel
10	0.020	0.29	1.91	0.026	0.002	0.113		0.86	11.5	0.221	0.008	0.0171	0.0054	0.0004	8.2	7.5	present invention steel
11	0.019	0.40	1.81	0.030	0.001	0.107	0.45	0.95	13.0	0.298	0.02	0.0171	0.0054	0.0004	10.2	8.9	present invention steel
12	0.025	0.19	1.95	0.031	0.002	0.150		0.95	10.1	0.194	0.04	0.0198	0.0054	0.0004	6.1	5.4	present invention steel
13	0.022	0.22	1.89	0.031	0.002	0.122		0.80	12.1	0.205	0.01	0.0178	0.0049	0.0002	8.7	7.9	present invention steel
14	0.025	0.38	1.12	0.034	0.003	0.250		0.60	13.0	0.297	0.03	0.0193	0.0056	0.0005	11.3	13.6	comparison steel
15	0.023	0.25	1.85	0.030	0.002	0.110		0.71	11.5	0.216	0.02	0.0154	0.0051	0.0005	8.4	8.4	present invention steel

Table 2

No.	sulfuric acid-copper sulfate test result	surface quality
1	A	a	present invention steel
2	A	a	present invention steel
3	A	a	present invention steel
4	A	a	present invention steel
5	A	a	present invention steel
6	A	b	comparison steel
7	A	b	comparison steel
8	B	b	comparison steel
9	C	c	comparison steel
10	A	a	present invention steel
11	A	a	present invention steel
12	A	a	present invention steel
13	A	a	present invention steel
14	C	b	comparison steel
15	A	a	present invention steel

A: no corrosion
B: slight corrosion
C: intergranular corrosion or deep pit-shaped corrosion
a: defect occurrence rate of 3% or less
b: defect occurrence rate of exceeding 3% and 30% or less
c: defect occurrence rate exceeding 30%

Claims

A structural stainless steel sheet having a composition which contains by mass% 0.01 to 0.03% C, 0.01 to 0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn, 0.04% or less P, 0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0% Ni, 4× (C+N) or more and 0.3% or less Ti (C, N indicating contents (mass%) of C and N), and Fe and unavoidable impurities as a balance, V, Ca and O in the unavoidable impurities being regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O, wherein the steel sheet further optionally contains 1.0% or less Cu by mass% and/or 1.0% or less Mo by mass%, and/or 0.005% or less B, wherein an F value and an FFV value expressed by following formulae satisfy a condition that F value≤11 and FFV value≤9.0. $F value = Cr + 2 \times Si + 4 \times Ti - 2 \times Ni - Mn - 30 \times (C + N)$
$FFV value = Cr + 3 \times Si + 16 \times Ti + Mo + 2 \times Al - 2 \times Mn - 4 \times (Ni + Cu) - 40 \times (C + N) + 20 \times V$
In the formulae, the respective element symbols are contents of the elements (mass%).
The method of manufacturing a structural stainless steel sheet, wherein a steel slab having a composition which contains by mass% 0.01 to 0.03% C, 0.01 to 0.03% N, 0.10 to 0.40% Si, 1.5 to 2.5% Mn, 0.04% or less P, 0.02% or less S, 0.05 to 0.15% Al, 10 to 13% Cr, 0.5 to 1.0% Ni, 4× (C+N) or more and 0.3% or less Ti (C, N indicating contents (mass%) of C and N), and Fe and unavoidable impurities as a balance, V, Ca and O in the unavoidable impurities being regulated to 0.05% or less V, 0.0030% or less Ca and 0.0080% or less O, wherein the steel sheet further optionally contains 1.0% or less Cu by mass% and/or 1.0% or less Mo by mass%, and/or 0.005% or less B, wherein an F value and an FFV value expressed by following formulae satisfy a condition that F value≤11 and FFV value≤9.0 is heated at a temperature of 1100°C to 1300°C and, thereafter, hot rolling which includes a rough hot rolling where rolling is performed for at least 1 pass or more at a reduction rate of 30% or more in a temperature range exceeding 1000°C, or the hot rolling is performed without annealing the hot-rolled sheet or after annealing the hot-rolled sheet at a temperature of 600 to 1000°C and, thereafter, pickling is applied to the hot-rolled sheet or the annealed hot-rolled sheet. $F value = Cr + 2 \times Si + 4 \times Ti - 2 \times Ni - Mn - 30 \times (C + N)$
$FFV value = Cr + 3 \times Si + 16 \times Ti + Mo + 2 \times Al - 2 \times Mn - 4 \times (Ni + Cu) - 40 \times (C + N) + 20 \times V$
In the formulae, the respective element symbols are contents of the elements (mass%).