TW201641718A - Ferritic-austenitic stainless steel sheet having excellent corrosion resistance in shear end face - Google Patents

Abstract

This ferritic-austenitic stainless steel sheet includes, in terms of percent by mass: C: 0.03% or less; Si: 0.1 to 1.0%; Mn: 0.5 to 5.0%; P: 0.04% or less; Al: 0.015 to 0.10%; Cr: 19.0 to 24.0%; Ni: 0.60 to 2.30%; Cu: 0.5 to 1.5%; Co: 0.05 to 0.25%; V: 0.01 to 0.15%; Ca: 0.002% or less; N: 0.06 to 0.20%; and S: 0.0002 to 0.0040%, with the remainder being Fe and inevitable impurities, wherein a value of Co + 0.25V is 0.10 to less than 0.25, a microstructure consists of ferrite phases and austenite phases, an average crystal grain size of the ferrite phases is 5 to 20 [mu]m, an average crystal grain size of the austenite phases is 2 to 10 [mu]m, and sulfides having major axes of 1 to 5 [mu]m are contained at an amount of 5 to 20 pieces per 5 mm2.

Description

Fertilizer iron-Worthian iron-based stainless steel plate with excellent corrosion resistance Technical field

The present invention relates to a ferrite-iron-wose iron-based (two-phase) stainless steel sheet excellent in corrosion resistance of a sheared end surface. In particular, the present invention relates to a ferrite-iron-Worstian iron-based stainless steel sheet which is suitable for use in a shear-processed state in an atmospheric environment and which is used for the corrosion-resistant treatment of a sheared end face.

The present application claims priority on Japanese Patent Application No. 2015-065028, filed on Jan.

Background technique

The ferrite-Worthian iron (two-phase) stainless steel is used in a wide range of applications due to its excellent strength and corrosion resistance. From the standpoints of solar cells, such as those that do not require processing, to the complicated processing of support members such as wiring outside the house, the uses are various.

In the manufacturing process of such a ferrite-iron-Worstian iron-based stainless steel sheet, the steel sheet is cut, formed, and punched by shearing because of its convenience. In addition, usually, the ferrite iron-Worthfield iron-based stainless steel plate is cut. The state of the cutting process is used without performing the corrosion resistance treatment of the cut end face.

When cutting and processing the ferrite-iron-Worstian iron-based stainless steel and using it without the corrosion treatment of the end face, the corrosion of the end face (end surface corrosion, end face rust) is serious compared to the smooth surface, and the end face is corroded. It is a cause of rust or rust stains, which causes a decrease in corrosion resistance of the entire steel sheet. The problem of rust on the end face is different from that of the plated steel sheet in which the end face is exposed to the matrix iron. Even if it is an end face, the ferrite-iron-Worstian iron-based stainless steel which retains a certain degree of corrosion resistance due to inactivation is not taken seriously.

However, as the market for the ferrite-Worthfield iron-based stainless steel expands, the use environment also expands, and the smooth surface and end face corrosion resistance causes problems. The ferrite-iron-Worstian iron-based stainless steel plate has a ferrite-grained iron phase and a Worthite iron phase at normal temperature, and the ferrite-grained iron phase existing in the sheared end face becomes a cause of rust.

The rust of the end face of the ferrite-iron stainless steel is called due to the corrosion of the micro-gap of the unevenness. A study on the corrosion of the slits has been conducted for a long time, and a ferrite-based iron-based stainless steel sheet excellent in the corrosion resistance of the slit is disclosed in Patent Document 1 or Patent Document 2, and the like.

These ferrite-based iron-based stainless steel sheets can effectively cope with local corrosion such as crack corrosion, but are not sufficient for suppressing rust of the shear end face (production of rust), and there is a case where end face corrosion occurs.

In view of the above, Patent Document 3 focusing on the burr characteristics of the end faces discloses a ferrite-based iron-based stainless steel sheet excellent in corrosion resistance of the sheared end surface. Further, Patent Document 4 discloses a processing method which can obtain a good shape of a cut end face.

As described above, various methods for improving the corrosion resistance of the end face of the ferrite-based iron-based stainless steel have been reviewed and developed.

However, the ferrite-Worthfield iron-based stainless steel has a higher strength than the ferrite-based stainless steel. Therefore, the characteristics of the shear surface of the ferrite-Worthfield iron-based stainless steel are significantly different from those of the ferrite-based stainless steel. Further, in addition to the shape of the shear plane, the difference in strength between the iron phase of the Instron and the ferrite phase tends to form a minute void shape, which greatly affects the corrosion resistance. Therefore, in order to improve the corrosion resistance of the sheared end surface of the ferrite-iron-Worstian iron-based stainless steel, only the conventional method described in the above-mentioned patent documents is insufficient, and the problem of rust generated in the sheared end surface remains.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-89828

Patent Document 2: Japanese Patent Laid-Open Publication No. 2006-257544

Patent Document 3: Japanese Patent No. 5375069

Patent Document 4: Japanese Patent Laid-Open Publication No. 2010-137344

Summary of invention

The present invention can effectively solve the aforementioned problems, namely, a ferrite-iron-Worstian iron-based stainless steel sheet which can be used in an atmospheric environment without performing corrosion resistance treatment, and an object of the present invention is to provide a shear end surface which can be improved. Corrosion-resistant ferrite iron - Vostian iron stainless steel plate.

The inventors conducted various reviews to improve the corrosion resistance of the sheared end faces of the ferrite-iron-Worstian iron-based stainless steel sheets. In particular, after carefully observing the corrosion state of the sheared end face, it is found that the starting point of the corrosion is on the fracture surface, and the fracture surface is reduced, and the surface roughness of the fracture surface is lowered to prevent corrosion.

Here, the fracture surface is one of the surface states called "depression", "shear surface", "fracture surface", and "burr" confirmed when the machined surface is observed after shearing the steel sheet.

Therefore, in order to improve the corrosion resistance, the inventors have found that by controlling the fracture surface, it is found that the crystal grain size of the iron phase of the ferrite and the iron phase of the Vostian is within an appropriate range, and appropriately The presence of a sulfide system is effective. Further, the inventors have found that by adding a small amount of Co and V as a component for improving corrosion resistance, the corrosion resistance of the Worthfield iron phase and the ferrite grain iron phase can be improved, and as a result, the corrosion resistance of the shear portion is also improved.

One aspect of the present invention is based on the knowledge gained from the foregoing observations, the requirements of which are as follows.

(1) A ferrite-iron-Worstian iron-based stainless steel sheet excellent in corrosion resistance of a sheared end surface, having the following chemical composition: C: 0.03% or less, Si: 0.1 to 1.0%, Mn by mass% : 0.5 to 5.0%, P: 0.04% or less, Al: 0.015 to 0.10%, Cr: 19.0 to 24.0%, Ni: 0.60 to 2.30%, Cu: 0.5 to 1.5%, Co: 0.05 to 0.25%, V: 0.01 ~0.15%, Ca: 0.002% or less, N: 0.06 to 0.20%, and S: 0.0002 to 0.0040%, and the remainder is Fe and unavoidable impurities; Co+0.25V is 0.10 or more and less than 0.25; The microstructure consists only of the ferrite phase and the Worthite iron phase; the average grain size of the ferrite phase is in the range of 5-20 μm, and the average crystal grain size of the aforementioned Worstian iron phase is in the range of 2-10 μm; The sulfide having a long diameter of 1 to 5 μm in the steel is present in an amount of 5 to 20 per 5 mm ² .

(2) The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to the above (1), which further contains one or more selected from the group consisting of the following.

The first group: one or more selected from the group consisting of Nb: 0.005 to 0.2%, Ti: 0.005 to 0.2%, W: 0.005 to 0.2%, and Mo: 0.01 to 1.0% by mass%.

Group 2: in terms of mass%, selected from Sn: 0.005 to 0.2%, Sb: 0.005 to 0.2%, Ga: 0.001 to 0.05%, Zr: 0.005 to 0.5%, Ta: 0.005 to 0.1%, and B : 1 or 2 or more types of 0.0002 to 0.0050%.

(3) The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to (1) or (2), wherein the value of Co+0.25V is 0.12 or more and less than 0.25.

(4) The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to any one of (1) to (3), wherein the Co, V, S, N, Cr, and Ni are The content of each of the above ones or more is in the following range in terms of mass %: Co: 0.05 to 0.12%, V: 0.08 to 0.12%, S: 0.0003 to 0.0010%, N: 0.08 to 0.17%, and Cr: 20.0 to 23.0%, Ni: 1.0 to 1.5%.

(5) The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to any one of (1) to (3), wherein the V content satisfies the following range in mass%: V : 0.01%~ less than 0.05%.

(6) The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to (5), wherein each of the above-mentioned Co, S, N, Cr, and Ni is contained in a content of at least one of The following ranges are satisfied in terms of mass %: Co: 0.05 to 0.12%, S: 0.0003 to 0.0010%, N: 0.08 to 0.17%, Cr: 20.0 to 23.0%, and Ni: 1.0 to 1.5%.

According to one aspect of the present invention, in the state in which the shear end face is not subjected to the corrosion resistance treatment, the cut end face can be raised in the ferrite-iron-Worstian iron-based stainless steel plate mainly used in the atmospheric environment. Corrosion resistance. Therefore, the overall corrosion resistance of the ferrite-iron-Worstian iron-based stainless steel plate can be improved. As a result, it is possible to suppress an aesthetic loss, a decrease in life, and the like due to corrosion of the steel sheet.

Fig. 1 is a graph showing the relationship between the ferrite iron phase which affects the corrosion resistance after shear processing and the average crystal grain size of the Worthfield iron phase.

Fig. 2 is a graph showing the relationship between the size and the number of sulfides which affect the corrosion resistance after shear processing.

Form for implementing the invention

Hereinafter, an embodiment of the ferrite-iron-wose iron-based stainless steel sheet (hereinafter, simply referred to as a steel sheet) of the present invention will be described.

First, the reason for limiting the chemical composition of the steel sheet according to the present embodiment will be described. Further, the % of the component of the steel is shown to be % by mass unless otherwise specified.

C: 0.03% or less

The C system inevitably mixes the elements in the steel. However, when the amount of C is more than 0.03%, Cr ₂₃ C _{6 is} precipitated in the iron phase of the Worth and the ferrite phase, and the grain boundary is sensitized to lower the corrosion resistance. Therefore, the amount of C is preferably as small as possible, but may be 0.03% or less. The lower limit of the amount of C is not particularly limited, but from the viewpoint of productivity and cost, it is preferably 0.002% or more, more preferably 0.008% or more. The upper limit of the amount of C is preferably 0.025% or less.

Si: 0.1~1.0%

The Si system can be used as an element of a deoxidizer. However, when the amount of Si (content ratio) is less than 0.1%, a sufficient deoxidation effect is not obtained, and the oxide is largely dispersed in the steel, and the origin of the crack at the time of extrusion processing increases. On the other hand, when Si is added in an amount of more than 1.0%, the ferrite iron phase is hardened to cause a decrease in workability. Therefore, the amount of Si is limited to the range of 0.1 to 1.0%. The amount of Si is preferably 0.3% or more, which is more It is preferable to suppress the decrease in workability, and it is preferably 0.7% or less.

Mn: 0.5~5.0%

Mn has a deoxidation effect. Further, in the present embodiment, it is understood that the effect of preventing the surface roughness of the fracture surface portion in the shear end surface from increasing is controlled by controlling the dispersion state of MnS. This mechanism is not yet clear, but can be inferred from the following.

In other words, the fine MnS particles which do not affect the corrosion resistance easily spread the crack of the fracture surface, and the shape of the fracture surface of the straight line easily occurs. However, in the steel sheet of the present embodiment in which the amount of S is small, the effect is not obtained when the amount of Mn is less than 0.5%. On the other hand, when the amount of Mn is more than 5.0%, generation of Mn oxide starts in the passivation film, which in turn causes deterioration in corrosion resistance. Therefore, the amount of Mn is limited to be in the range of 0.5 to 5.0%. From the viewpoint of preventing a decrease in surface roughness, the amount of Mn is preferably 1.0% or more. In order to suppress the formation of the Mn oxide in the passivation film, the amount of Mn is preferably 4.0% or less.

P: 0.04% or less

P is an element that reduces corrosion resistance. Further, since segregation P at the crystal grain boundary deteriorates hot workability, it is difficult to add an excessive amount of P. Thereby, the P content is preferably low, but it is allowed to be 0.04% or less, so the amount of P is limited to 0.04% or less. The amount of P is preferably set to 0.03% or less.

Al: 0.015~0.10%

Since Al is an active component for deoxidation, it is required to contain 0.015% or more of Al. On the other hand, when the amount of Al is more than 0.10%, surface enthalpy due to Al-based non-metallic inclusions increases, and Al-based non-metallic inclusions become cracks. Trace starting point. Therefore, the amount of Al is set to 0.015 to 0.10%. From the viewpoint of sufficiently exerting the deoxidation effect, the amount of Al is preferably 0.02% or more. In order to suppress the formation of Al-based non-metallic inclusions, the amount of Al is preferably 0.05% or less.

Cr: 19.0~24.0%

Cr is an important element that determines the corrosion resistance of stainless steel. In the present embodiment, a structure in which about 50% of the ferrite-grained iron phase and the Worthite iron phase are mixed, and when separated into two phases, Cr is concentrated in the ferrite-rich iron phase. On the other hand, the amount of Cr in the iron phase of Vostian decreased, but it was concentrated as the N-forming element of Worthite. In order to ensure the corrosion resistance of the Worthite iron phase, it contains 19.0% or more of Cr. The amount of Cr is preferably 20.0% or more.

On the other hand, when the amount of Cr is more than 24.0%, it is easy to generate a sigma phase which is converted into a ferrite grain iron phase, resulting in hardening of the material and deterioration of corrosion resistance. Therefore, the amount of Cr is set to 24.0% or less. The amount of Cr is preferably set to 23.0% or less.

Cu: 0.5% to 1.5%

Cu forms a film on the surface of the stainless steel after corrosion, and suppresses the effect of dissolution of matrix iron by the anode reaction. Therefore, it is also an element that contributes to the improvement of rust resistance and the improvement of the corrosion resistance of the gap.

This effect cannot be expected when the amount of Cu is less than 0.5%. On the other hand, when the amount of Cu is more than 1.5%, embrittlement is promoted at a high temperature, and hot workability is lowered. Therefore, the amount of Cu is limited to the range of 0.5% to 1.5%. From the viewpoint of improving rust resistance and seam corrosion resistance, the amount of Cu is preferably set to 0.7% or more. In order to further suppress the decrease in hot workability, the amount of Cu is preferably 1.2% or less.

Ni: 0.60~2.30%

Ni system can inhibit the anode reaction caused by acid, and can also be maintained at lower pH. Hold the passive element. In other words, Ni has a high effect on the corrosion resistance of the slit, and remarkably suppresses the progress of corrosion in the active dissolved state. When the amount of Ni is less than 0.60%, the effect of improving the corrosion resistance of the gap is not obtained, and the ratio of the iron phase of the Vostian is lowered, so that the workability is remarkably lowered. On the other hand, when the amount of Ni is more than 2.30%, the ratio of the iron phase of the Worthfield increases, and the hot workability is lowered. Therefore, the amount of Ni is limited to the range of 0.60 to 2.30%. Further, the lower limit of the amount of Ni is preferably 1.0% or more, more preferably 1.5% or more. The upper limit of the amount of Ni is preferably 1.5% or less.

N: 0.06~0.20%

The N series stabilizes the iron phase of the Worthfield and even enhances the important elements of corrosion resistance. When the amount of N is less than 0.06%, the ratio of the iron phase of the Worthfield is small, the workability is lowered, and the corrosion resistance of the iron phase of the Worthfield is lowered. On the other hand, when the amount of N is more than 0.20%, a large amount of the Worthite iron phase is formed, and the hot workability is remarkably lowered. Therefore, the amount of N is set to 0.06 to 0.20%. From the viewpoint of stabilizing the iron phase of Vostian, the amount of N is preferably set to 0.08% or more. In order to further suppress the decrease in hot workability, the amount of N is preferably set to 0.17% or less.

Co: 0.05~0.25%

The Co system shows the same action as Ni, making the iron phase stable in the Vostian. This effect was exhibited by the addition of a trace amount of Co by coexistence with Ni, but when the amount of Co was less than 0.05%, this effect was not exhibited. Further, Co stabilizes the precipitation of the iron phase of the Worthfield in the high temperature region, and promotes the concentration of N in the iron phase of the Wostian, and the amount of N in the ferrite phase is greatly reduced. Therefore, Co contributes to suppress precipitation of Cr carbonitrides (especially Cr nitrides). The main cause of the deterioration of the corrosion resistance of the steel sheet according to the present embodiment is the precipitation of Cr carbonitride, and the Cr carbonitride week. The concentration of Cr on the side decreases. Therefore, Co is particularly useful for suppressing the precipitation of Cr carbonitrides, and suppressing deterioration of corrosion resistance at the interface between the ferrite grain boundary or the ferrite phase and the Worthite iron phase. On the other hand, the addition of an excessive amount of Co causes the ratio of the iron phase of the Vostian to rise, resulting in a decrease in hot workability. In addition, since Co is a rare element and the price is high, adding a large amount of Co will result in an excessive cost increase. Therefore, the upper limit of the amount of Co is set to 0.25% or less. From the viewpoint of stabilizing Worth Tin, the amount of Co is preferably set to 0.08% or more. In order to further suppress the decrease in hot workability, the upper limit of the Co amount is preferably 0.20% or less, more preferably 0.12% or less.

V: 0.01~0.15%

V is a very strong carbonitride-forming element. By the presence of V in the ferrite phase, it is easy to form carbonitrides in the high temperature domain. The main cause of the deterioration of the corrosion resistance of the steel sheet according to the present embodiment is that the Cr concentration around the Cr carbonitride decreases as the Cr carbonitride precipitates. Therefore, precipitation of Cr carbonitride in a low temperature region can be suppressed by depositing V carbonitride in a high temperature region. This effect is exhibited when 0.01% or more of V is added, so the lower limit of the amount of V is made 0.01% or more. On the other hand, the addition of the excess amount of V causes hardening, so the upper limit of the amount of V is made 0.15% or less. From the viewpoint of promoting the formation of the V-based carbonitride and suppressing the precipitation of the Cr carbonitride, the amount of V is preferably 0.05% or more, and more preferably 0.08% or more. In order to further suppress the hardening, it is preferred to set the amount of V to 0.12% or less. When the aforementioned effect is exhibited by a small amount of V, the amount of V is preferably less than 0.05%.

Ca: 0.002% or less

Ca is a component that helps deoxidize. Also, Ca is also a member of sulfide formation. Element, Ca is an element that contributes to the stabilization of sulfides that impart good properties to the shear fracture surface. In order to obtain this effect, the amount of Ca is preferably 0.0003% or more. However, when the amount of Ca is more than 0.002%, coarse CaS is formed and becomes a rust starting point. Therefore, the amount of Ca is made 0.002% or less.

S: 0.0002~0.0040%

S is an important element in this embodiment. Conventionally, S forms a sulfide such as Mn or Ca in stainless steel, which is a main cause of a decrease in corrosion resistance. Therefore, it is required to reduce the amount of S. However, in the study by the inventors and the like, even if the MnS or CaS which is not favored in the related art is appropriately controlled, the particle size and the dispersed state can be appropriately controlled, and the surface characteristics of the sheared end face can be stably maintained at a high level, and it is possible to surely not Corrosion resistance is reduced.

In order to make the S content less than 0.0002%, the raw material needs to be strictly selected, and since the burden of the desulfurization step is increased, the lower limit of the S amount is made 0.0002% or more. On the other hand, when the amount of S is more than 0.0040%, it is found that the sulfide is coarsened and causes rust. Therefore, the amount of S is limited to the range of 0.0002 to 0.0040%. The lower limit of the amount of S is preferably 0.0003% or more, and the upper limit of the amount of S is preferably 0.0010% or less. Therefore, the preferred range of S amount is 0.0003 to 0.0010%.

Co+0.25V value: 0.10 or more and less than 0.25

The main cause of the deterioration of the corrosion resistance of the steel sheet according to the present embodiment is that the Cr concentration around the Cr carbonitride decreases as the Cr carbonitride precipitates. In the present embodiment, in order to suppress the formation of Cr carbonitrides, particularly Cr nitrides, a sufficient amount of the Worthfield iron phase is precipitated until the upper limit of the precipitation temperature of the Cr nitrides, so as to reduce the N amount in the ferrite phase. It is important. Therefore, by adding Co, it is possible to effectively promote the precipitation of the iron phase of the Vostian, and fix the fertilizer by V. N remaining in the iron phase. When the value of Co+0.25V is less than 0.10, there is no effect of reducing the amount of N in the ferrite grain iron phase. Therefore, in the ferrite iron/fat iron grain boundary, Cr nitride is formed to deteriorate the corrosion resistance. Therefore, the lower limit of the Co+0.25V value is set to 0.10 or more. By setting the value of Co + 0.25 V to 0.12 or more, the amount of formation of Cr nitride can be surely reduced. Therefore, the lower limit of the Co+0.25V value is preferably 0.12 or more. On the other hand, when the value of Co+0.25V is too large, the ratio of the iron phase of the Worthfield is excessively increased, and there is a concern that the hot workability is lowered. Therefore, the upper limit of the Co+0.25V value is set to be less than 0.25.

Further, in Co + 0.25 V, Co and V showed respective element contents (% by mass).

Although the basic components of the steel sheet according to the present embodiment have been described above, in the present embodiment, in order to improve other corrosion resistance, the following elements may be appropriately contained.

Nb: 0.005~0.2%

Nb is an element that fixes C and N to prevent sensitization due to Cr carbonitride and improve corrosion resistance. However, when the amount of Nb is less than 0.005%, the addition effect is poor. On the other hand, when the amount of Nb is more than 0.2%, the iron phase of the solid solution-strengthening fertilizer is hardened, resulting in a decrease in workability. Therefore, it is preferable to set the amount of Nb to be in the range of 0.005 to 0.2%.

Ti: 0.005~0.2%

Ti-based fixing of C and N prevents sensitization by Cr carbonitride and improves corrosion resistance. However, when the amount of Ti is less than 0.005%, the addition effect is poor. On the other hand, if the amount of Ti is more than 0.2%, the ferrite grain iron phase is hardened, resulting in a decrease in toughness. Further, the surface roughness is lowered by the Ti-based precipitates. therefore, It is preferable to set the amount of Ti to be in the range of 0.005 to 0.2%.

W: 0.005~0.2%

W also has the same fixed C and N as Ti to prevent sensitization due to Cr carbonitride. However, this effect was not exhibited when the amount of W was less than 0.005%. On the other hand, when the amount of W is more than 0.2%, hardening is caused, and workability is lowered. Therefore, it is preferable to set the amount of W to be in the range of 0.005 to 0.2%.

Mo: 0.01~1.0%

Mo is an element that enhances corrosion resistance. However, when the amount of Mo is less than 0.01%, the addition effect is poor. On the other hand, when the amount of Mo is more than 1.0%, hardening is caused, and workability is lowered. Therefore, it is preferable to set the amount of Mo to 0.01 to 1.0%.

In the present embodiment, the elements described below can be appropriately contained.

Sn, Sb: 0.005~0.2%

Sn and Sb are elements that enhance corrosion resistance, and are also elements of solid solution strengthening ferrite. Therefore, the upper limit of each of Sn and Sb is set to 0.2%. When the amount of either Sn or Sb is 0.005% or more, the effect of improving corrosion resistance is exhibited. Therefore, the amounts of Sn and Sb are set to 0.005 to 0.2%. The lower limit of the respective amounts of Sn and Sb is preferably 0.03% or more. The upper limit of the respective amounts of Sn and Sb is preferably 0.1% or less.

Ga: 0.001~0.05%

The Ga system helps to improve the corrosion resistance. When the amount of Ga is 0.001% or more, the effect will be exhibited. When the amount of Ga is more than 0.05%, the effect is saturated. Therefore, Ga may be contained in an amount ranging from 0.001 to 0.05%.

Zr: 0.005~0.5%

Zr is an element that contributes to the improvement of corrosion resistance. When the amount of Zr is 0.005% or more, the effect will be exhibited. When the amount of Zr is more than 0.5%, the effect is saturated. Therefore, Zr can be contained in an amount ranging from 0.005 to 0.5%.

Ta: 0.005~0.1%

Ta is an element that enhances corrosion resistance by modifying inclusions and may be contained as needed. Since Ta can be used in an amount of 0.005% or more, the lower limit of the amount of Ta is preferably 0.005% or more. However, when the amount of Ta is more than 0.1%, the ductility at normal temperature is lowered or the toughness is lowered. Therefore, the upper limit of the amount of Ta is preferably 0.1% or less, more preferably 0.050% or less. When the above effect is exhibited by a small amount of Ta, the amount of Ta is preferably set to 0.020% or less.

B: 0.0002~0.0050%

B is an element that effectively prevents secondary work embrittlement or deterioration of hot workability, and is an element that does not affect corrosion resistance. Therefore, B may be contained in an amount of 0.0002% or more as the lower limit of the amount of B. However, when the amount of B is more than 0.0050%, the hot workability is deteriorated. Therefore, the upper limit of the amount of B is preferably made 0.0050% or less. The upper limit of the B content is preferably 0.0020% or less.

In the steel sheet of the present embodiment, the remainder other than the above elements are Fe and unavoidable impurities, but other elements than the above elements may be contained within a range that does not impair the effects of the embodiment.

Although the component system has been described above, the steel sheet of the present embodiment is insufficient in that only the component composition is within the above range, and the average crystal grain size of the ferrite-grained iron phase and the Worthite iron phase and the precipitation state of MnS are set. The following ranges are important.

<Average crystal grain size of ferrite phase: 5~20μm>

<Warsfield iron phase average crystal grain size: 2~10μm>

The metal structure of the ferrite-Worthfield iron-based stainless steel plate consists only of the ferrite phase and the Worthite iron phase. The crystal grain size of the ferrite phase and the Worthite iron phase will have a great influence on the mechanical properties or the surface characteristics of the sheared end faces.

The ferrite phase is different from the recrystallization temperature of the Worthfield iron phase, and the ferrite phase produces grain growth in the recrystallization temperature range of the Worthfield iron phase. Therefore, the average crystal grain size of the ferrite grain iron phase is larger than the average crystal grain size of the Worthfield iron phase, but when the particle size difference between the ferrite grain iron phase and the Worthfield iron phase becomes larger, the intensity difference also increases (larger). . When the strength difference is large, the shearing process will cause cracks at the interface between the ferrite grain iron phase and the Worthfield iron phase, which becomes the starting point of the crack corrosion.

Therefore, the limit value of the average crystal grain size at which no crack occurred during shear processing was investigated. The results are shown in Figure 1.

Fig. 1 is a graph showing the relationship between the ferrite iron phase which affects the corrosion resistance after shear processing and the average crystal grain size of the Worthfield iron phase. It can be seen from Fig. 1 that there is an appropriate combination of the average grain size of the ferrite phase and the Worthfield iron phase. From the result of Fig. 1, the upper limit of the average crystal grain size of the ferrite iron phase was set to 20 μm.

Here, when the average crystal grain size of the ferrite-grained iron phase is less than 5 μm, the recrystallization of the Worthite iron phase is not completed, so that the strength is increased and burrs are less likely to form. However, the area of the fracture surface is greatly increased, and the corrosion resistance is lowered. When the average crystal grain size of the Worthite iron phase is less than 2 μm, the strength increase is remarkable, and the corrosion resistance is lowered for the same reason.

On the other hand, when the average crystal grain size of the iron phase of Vostian is more than 10 μm, the burrs increase due to the influence of softening, and the thickness of the fracture surface decreases, and the shape Into a tiny gap. In addition, coarse particles are formed in one part of the ferrite phase to promote interfacial cracks. With the above, the corrosion resistance is greatly reduced.

Therefore, the average crystal grain size of the iron phase of the ferrite is set to 5 to 20 μm, and the average crystal grain size of the iron phase of the Vostian is set to 2 to 10 μm.

<Sulphide: Particles (sulfide) having a long diameter of 1 to 5 μm exist in an amount of 5 to 20 per 5 mm ² >

Hereinafter, the reason why the precipitation state of the sulfide in the steel sheet is limited to the above range will be described.

The inventors of the present invention confirmed that the corrosion origin of the end surface to be sheared is the boundary portion between the shear surface and the fracture surface and the fracture surface. Since it is easy to form a void at the boundary portion between the sheared surface and the fracture surface, it is easy to deposit a corrosion factor. Further, the fine void shape formed by the irregularities caused by the fracture of the dimples promotes low pH and high salt differentiation of the adhesion solution (reducing the pH of the adhesion solution and enriching the salt in the adhesion solution). Therefore, the boundary portion between the sheared surface and the fracture surface and the fracture surface become an environment in which corrosion is likely to occur, and become a corrosion starting point. Therefore, by suppressing the formation of voids at the boundary between the sheared surface and the fracture surface, it is presumed that a sheared end surface which is less likely to cause corrosion can be formed. Here, the sulfide shows CaS, MnS, CrS, TiCS, CuS, and the like.

Here, the inventors of the present invention performed a corrosion resistance test using a test piece which was changed in manufacturing conditions. Further, several test pieces having good corrosion resistance and test pieces having poor corrosion resistance were taken out, and the microstructure of the test piece was analyzed. This result is shown in Figure 2. Fig. 2 is a graph showing the relationship between the size and the number of sulfides which affect the corrosion resistance after shear processing. Here, the size (size) of the sulfide in Fig. 2 is the maximum value of the long diameter of the sulfide after stretching. The number of sulfides in Fig. 2 is the number of sulfides having a long diameter of 1 to 5 μm (the number per 5 mm ² ). As shown in Fig. 2, the precipitation form of the sulfide is related to the characteristics of the sheared end surface, and it is understood that there are conditions in which corrosion is less. In other words, it is considered that it is important that the sulfide having a long diameter of 1 to 5 μm in the steel is present in an amount of 5 to 20 per 5 mm ² . Here, the effect of suppressing the crack generated at the time of fracture at the time of the sulfide having a long diameter of less than 1 μm (the size of the sulfide in the second embodiment (the maximum value of the long diameter) is less than 1 μm) is small. On the other hand, in the case of a sulfide having a major axis of more than 5 μm (the size of the sulfide in FIG. 2 (the maximum value of the major axis) is more than 5 μm), the sulfide present on the surface may fall off to form a large crack. Therefore, the long diameter of the sulfide of the object is set to be in the range of 1 to 5 μm. Therefore, in the present embodiment, the sulfide having a long diameter of 1 to 5 μm is used as a control target. Here, the long diameter of the sulfide to be controlled is the long diameter of each sulfide.

Further, in the present embodiment, the maximum value of the major axis of the sulfide is preferably 1 to 5 μm.

Next, after investigating the precipitation state of the sulfide, the following matters were known. Unit area: When the number of precipitates (sulfides) per 5 mm ² is less than 5, it is understood that the effect of suppressing the progress of cracks is poor. When the number of precipitates (sulfides) per 5 mm ² is more than 20, it is understood that voids are formed in a large amount, and corrosion resistance is lowered. Therefore, in the present embodiment, the sulfide having a major axis of 1 to 5 μm is present in an amount of 5 to 20 per 5 mm ² . Further, it is preferable that the sulfide having a long diameter of 1 to 5 μm is present in an amount of 6 or more and 15 or less per 5 mm ² .

Next, a method of producing the ferrite-iron-wose iron-based stainless steel sheet of the present embodiment will be described.

In the present embodiment, as described above, the average crystal grain size of the ferrite-grained iron phase and the Worthite iron phase and the state in which the sulfide is precipitated and dispersed are important. The manufacture of steel sheets under the following conditions is quite important.

In order to control the average crystal grain size of the ferrite grain iron phase and the Worthfield iron phase within the above range, the rolling rate of the hot rolling and cold rolling step is important. In the rough rolling step of the hot rolling, at least one pass is required to be a rolling reduction ratio of 30% or more, and in the rough rolling step, the processing is performed at a temperature of 1000 ° C or higher for 5 passes or more. Further, it is necessary to set the rolling reduction ratio of the cold rolling to 75% or more, and set the temperature of the cold rolling delay to 150 ° C or more at the end of the final pass.

The strain introduced during cold rolling becomes a nucleus of recrystallized grains. However, when work hardening is performed in the high-strength steel of the present embodiment, a large load is placed in the cold rolling step. Therefore, the burden can be reduced by increasing the temperature of the cold rolling delay. This effect not only reduces the burden of the cold rolling step, but also does not generate an excessively large recrystallized nucleus, so that the crystal grain size can be effectively controlled. In order to set the average crystal grain size of the ferrite grain iron phase to 5 to 20 μm, and the average crystal grain size of the Worthfield iron phase to be 2 to 10 μm, it is necessary to control the plate temperature after the final pass to be 150 ° C or more. Furthermore, the plate temperature after the final pass can be controlled by changing the rolling rate or the rolling speed per pass.

In order to control the size of the sulfide and the number of precipitates within the above range, the processing temperatures of the annealing of the hot rolled sheet and the annealing of the cold rolled sheet are important. The condition is set to be an annealing temperature of the hot-rolled sheet: 1000 to 1100 ° C, and an annealing temperature of the cold-rolled sheet: 950 to 1050 ° C.

Further, other steps are not particularly limited, and methods known in the art can be used. Further, representative manufacturing conditions are shown below.

First, the ferrite-Worthfield iron-based stainless steel with the above composition is heated to 1150~1250 °C, and then the finished temperature is above 950 °C. The hot rolling is performed to make the plate thickness 3.0 to 6 mm. At this time, the rolling reduction rate of at least one pass of the rough rolling step is set to 30% or more. When the finished rolling is cooled at the usual cooling rate, the precipitation of the iron phase of Vostian will become insufficient. Therefore, the temperature of the completion rolling is set to 950 ° C or higher, and then the hot rolled sheet is taken up without actively cooling. Slowly cool (slowly cool) to below 500 ° C, then place the hot rolled plate in a water bath for rapid cooling.

The cooling rate after coiling is not specifically specified. However, the toughness due to brittleness at 475 ° C occurs near 475 ° C, so the cooling rate in the range of 425 to 525 ° C is preferably 100 ° C / h or more.

The hot rolled steel strip thus produced is subjected to annealing at a temperature of 1000 to 1100 ° C, followed by pickling.

Then, the cold rolling delay of 75% or more is performed, and the reverse rolling is continuously performed so that the processing heat generated by the cold rolling is not cooled to room temperature, and the cold rolling is performed so that the temperature of the plate on the exit side of the final pass is 160. Above °C. The obtained cold-rolled sheet was annealed at a temperature of 950 to 1050 ° C, and then subjected to pickling to obtain a cold-rolled product.

The ferrite-iron-Worstian iron-based stainless steel of the present embodiment can be obtained by the above-described production method, but the present embodiment cannot be limited by the above respective steps and conditions.

Next, a method of shearing which can reduce the fracture surface when the ferrite-Worth iron-based stainless steel sheet of the present embodiment is sheared will be described. Further, the method of reducing the fracture surface is not particularly limited, and can be appropriately adjusted and set when the steel sheet is sheared. The following is an example of a method for reducing the fracture surface.

In order to achieve the above object, the inventors have changed various cuts and additions. Working conditions, after conducting numerous experiments, it was found that the control of clearance is particularly effective for reducing the fracture surface rate. Here, the ratio of the gap-based steel sheet to the gap x between the blade of the thickness d and the table is shown.

The gap during the shearing process will affect the area of the fracture surface in the shear end face and the height of the burr. As a result of reviewing the various gaps, it was found that in the case of the ferrite-Worthfield iron-based stainless steel of the present embodiment, as long as the gap is set to 5 to 20%, the area of the fracture surface and the height of the burr can be reduced, and the corrosion resistance can be improved. . The gap during the shearing process is preferably set to 10 to 15%.

Example

The examples of the present invention are described below, but the conditions in the examples are used to confirm the conditions and effects of the present invention, and the present invention is not limited by the conditions used in the following examples. The present invention can be used in various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Further, the bottom line in the table is shown to be outside the scope of the embodiment.

The ferrite-iron-Worstian iron-based stainless steel having the chemical composition shown in Tables 1 and 2 is melted. Subsequently, the film was heated at a temperature of 1200 ° C, and further rolled at a finishing temperature of 980 ° C to obtain a hot rolled sheet having a thickness of 4 mm. The rolling reduction ratio of at least one pass in the rough rolling step of the hot rolling is 30% or more. Further, after the hot-rolled sheet was taken up, it was slowly cooled to 500 ° C or lower, and then rapidly cooled.

Thereafter, the hot rolled sheet was annealed and pickled at the annealing temperatures described in Tables 3 and 5. Next, the thickness is set by cold rolling to be 0.6 to 1.2 mm. The cold rolling delay was such that the entry temperature of the initial pass was set to 60 ° C, and rolling was continuously performed without lowering the temperature of the plate. The cold rolling rate and the plate temperature after the final pass (the final pass temperature) are shown in Table 3. The value shown in 5. The obtained cold-rolled sheet was subjected to cold-rolled sheet annealing, and a test piece was formed by pickling a finished surface.

The size and number of sulfides of the test piece thus obtained were measured by an optical microscope and SEM-EDS method. The measurement method is shown below. First, the surface of the test piece was ground with #600, and then mirror-polished to be completed. Next, a square of 5 mm × 5 mm was drawn on the surface of the test piece. The inclusions were observed in the range after drawing using an optical microscope, and inclusions having a size of about 1 μm or more present in the range were marked. In this way, the size of the inclusions is grasped by observation and the measured inclusions are selected.

Next, when the total number of inclusions was more than 5, the composition of the inclusions was measured by the SEM-EDS method at two locations. When it is confirmed that there is one composition in which the S concentration is 50% or more, it is determined that the inclusion is a sulfide.

The long diameter of the inclusion determined to be sulfide was measured by the following method. Sulfides have softer properties. Therefore, most of the sulfides exist in a form extending in the rolling direction. Therefore, the length in the rolling direction is taken as the long diameter, and the length from the front end to the rear end (the maximum length) of the sulfide is measured as the long diameter. Further, the measured value of the long diameter of the sulfide is calculated by rounding off the first digit after the decimal point as an integer. The maximum of the measured values of the obtained long diameters is recorded in the "long diameter of sulfide" column of Tables 4 and 6.

Further, the number of sulfides having a measured value of the long diameter of 1 to 5 μm was measured, and the number of each of 5 mm ² was determined. The number of sulfides having a long diameter of 1 to 5 μm (the number per 5 mm ² ) is recorded in the "number of sulfides" in Tables 4 and 6.

Further, a field emission type scanning electron microscope JSM-7000F manufactured by JEOL Ltd. was used by electron backscatter diffraction (EBSD). The method separates the iron phase of the ferrite and the iron phase of the Vostian, and measures the crystal grain size of the iron phase of the ferrite and the iron phase of the Vostian. The acceleration voltage during the measurement was set to 25 kV, the step length was set to 0.5 μm, and the measurement position was set to the center portion of the thickness of the cross section of the width of the test piece at the center of the test piece. The azimuth analysis system uses the OIM software of TSL Solutions to measure the crystal grain size of the iron phase of the ferrite and the iron phase of the Worthfield by using the grain boundary of the adjacent crystal grain with a difference in orientation of 15° or more as the grain boundary. The ferrite iron phase and the Worthfield iron phase respectively calculate the average value of the measured crystal grain diameters to obtain an average crystal grain size.

The average crystal grain size of the ferrite grain iron phase is recorded in the column of "particle diameter of the ferrite grain iron phase" in Tables 4 and 6. The average crystal grain size of the Vostian iron phase is recorded in the "particle size of the Worthite iron phase" column of Tables 4 and 6.

The test piece of the ferrite-iron-Worstian iron-based stainless steel plate obtained under the above production conditions was cut into a size of 120 mm × 75 mm, and a silicone bag was attached to the cut surface to make the influence of the square end faces harmless. The gap (shear gap) between the male and female molds of the punching tool is adjusted by using a female mold having punching tools of various diameters. A circular shearing process was performed on the central portion of the sample at various shear gaps. Here, the shear gap (%) is a value calculated by the following formula.

{(The difference between the diameter of the male die of the punching tool and the diameter of the negative die) / The thickness of the test piece (steel plate)}×100

After cutting out (shearing), it is degreased with acetone. The sample was placed in a cyclic corrosion tester with the surface having the burrs as the upper slope: 75°. Further, a cyclic corrosion test based on JASO M 609-91 of 6 cycles was performed. After the test, the sample which was not corroded in the sheared end face was evaluated as "no rust generation", and it was found that the sample produced by the corrosion was "having rust."

The results obtained are shown in Tables 4 and 6.

From the results of Test Nos. 1, 2, 4, 5, 8 to 11, 13, 14, 16, 19 to 27, it is understood that the corrosion resistance of the sheared end face is preferably as long as the range of the present embodiment is satisfied.

From the results of Test Nos. 3, 6, 7, 17, and 22, it is understood that one or both of the average crystal grain size of the ferrite grain iron phase and the average crystal grain size of the Worstian iron phase are outside the range of the present embodiment. At this time, rust will occur on the cut end face. In particular, in Test No. 22, the plate temperature after the final pass of the cold rolling delay was lower than 160 °C. Therefore, the introduction of strain due to cold rolling is very large, and the strain can become a core of recrystallization, so that it becomes fine crystal grains and causes rust.

From the results of Test Nos. 12, 15, 17, and 18, it was found that when one or both of the number of sulfides and the long diameter of the sulfide exceeded the range of the present embodiment, rust was generated in the sheared end surface.

From the results of Test Nos. 28 to 46, it was found that when the chemical composition was outside the range of the present embodiment, rust was generated at the sheared end surface.

Industrial availability

According to the ferrite-iron-Worstian iron-based stainless steel sheet of the present embodiment, the corrosion resistance of the sheared end surface is excellent even when it is used in an atmospheric environment without performing the corrosion resistance treatment of the sheared end surface. Therefore, the ferrite-Worthfield iron-based stainless steel sheet of the present embodiment can be preferably used for a power conditioner, a PV (Photovoltaic) inverter, a duct cover, and a solar cell stand. , drainage ditch and its cover for various purposes.

Claims (6)

A ferrite-iron-Worstian iron-based stainless steel sheet excellent in corrosion resistance of a sheared end surface is characterized by having the following chemical composition: C: 0.03% or less, Si: 0.1 to 1.0%, and Mn: by mass%: 0.5~5.0%, P: 0.04% or less, Al: 0.015~0.10%, Cr: 19.0~24.0%, Ni: 0.60~2.30%, Cu: 0.5~1.5%, Co: 0.05~0.25%, V: 0.01~ 0.15%, Ca: 0.002% or less, N: 0.06 to 0.20%, and S: 0.0002 to 0.0040%, and the remainder is Fe and unavoidable impurities; Co+0.25V is 0.10 or more and less than 0.25; metal structure It consists only of the ferrite phase and the Worthite iron phase; the average grain size of the ferrite phase is in the range of 5 to 20 μm, and the average grain size of the aforementioned Worthfield iron phase is in the range of 2 to 10 μm; The sulfide having a long diameter of 1 to 5 μm is present in an amount of 5 to 20 per 5 mm ² . The ferrite-iron-Worsfield iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface of claim 1 further contains one or more selected from the group consisting of: Group 1 : by mass %, selected from Nb: 0.005 to 0.2%, Ti: 0.005 to 0.2%, W: 0.005 to 0.2%, and Mo: 0.01 to 1.0% of one or two or more; Group 2: by mass%, selected from Sn: 0.005 to 0.2%, Sb: 0.005 to 0.2%, Ga: 0.001 to 0.05%, Zr: 0.005 to 0.5%, Ta: 0.005 to 0.1%, and B: 0.0002 to 0.0050% of one or more. The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface of claim 1 or 2, wherein the value of Co+0.25V is 0.12 or more and less than 0.25. The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to any one of claims 1 to 3, wherein any one of Co, V, S, N, Cr, and Ni is used. The respective contents satisfy the following ranges in mass %: Co: 0.05 to 0.12%, V: 0.08 to 0.12%, S: 0.0003 to 0.0010%, N: 0.08 to 0.17%, Cr: 20.0 to 23.0%, and Ni: 1.0 to 1.5%. The ferrite-iron-Worstian iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface according to any one of claims 1 to 3, wherein the V content satisfies the following range in mass%: V: 0.01% to less than 0.05 %. The ferrite-iron-Worsfield iron-based stainless steel sheet having excellent corrosion resistance of the sheared end surface of claim 5, wherein each of the above-mentioned Co, S, N, Cr, and Ni content is satisfied by mass% The following ranges: Co: 0.05 to 0.12%, S: 0.0003 to 0.0010%, N: 0.08 to 0.17%, Cr: 20.0 to 23.0%, and Ni: 1.0 to 1.5%.