JPH11279706A - High strength stainless steel strip and steel sheet with double phase structure excellent in hydrophilic property and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive high strength stainless steel sheet with a double phase structure excellent in hydrophilic properties with less production processes. SOLUTION: A hot rolled steel strip, a cold rolled steel strip or an annealed steel strip of a low carbon martensitic stainless steel contg., by mass, 10 to 17% Cr, <=0.15% C and 1.0 to 6.0% Si is produced, this steel strip is heated to the temp. range of (As+30 deg.C) to the Af point (where to the range of <=900 deg.C) in a heat treating furnace to make a part of the martensitic phase the reversely transformed austenitic phase, the redistribution of the compsn. is allowed to occur between the martensitic phase and theaustenitic phase, thereafter, cooling is executed to form into a double phase structure of the fine austenitic phase and martensitic phase with 5 μm size or <=1 μm width, and subsequently, pickling is executed to preferentially corrode the austenitic phase, by which fine ruggedness of a depth of >=0.5 μm is imparted to the surface at the intervals of <=5 μm to obtain its hydrophilic properties.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength duplex stainless steel strip and a steel sheet having excellent hydrophilicity.

[0002]

2. Description of the Related Art Conventionally, stainless steel sheets have excellent corrosion resistance and strength, and thus have been used in a wide range of building materials, vehicles, appliances, kitchen appliances and the like. Especially in recent years,
In the field of construction and vehicles, in order to utilize the excellent corrosion resistance and strength of stainless steel, there are many cases where the steel is exposed to the atmosphere in an outdoor environment and used in a bare state. In this case, in an environment where the dry state continues, there is no particular problem of weather resistance such as rusting and corrosion. However, in the bad environment such as sea salt particles flying near the coast, high concentration of sulfurous acid gas in the industrial area, and in places with a lot of dust, even stainless steel often rusts depending on the use site. . In this case, even though the stainless steel having a high quality feel is mild, if rust is generated, the design is deteriorated and the image as the stainless steel is reduced, and the corrosion of the base material is promoted by rusting. In addition, when used as a structural member, there is a problem that pitting occurs and grows under the rusted part, causing a problem that the life of the structural body is shortened. Therefore, development of stainless steel with excellent weather resistance is desired. It is rare.

As methods for improving the weather resistance of stainless steel, there are methods of increasing the amount of Cr in stainless steel and adding Mo or N. However, the method of adding a large amount of expensive elements increases the price and reduces the competitiveness with other materials. In addition, in high-strength stainless steel, since the components are adjusted to obtain high strength, there is a problem in that addition and adjustment of these elements only for the purpose of rust resistance is restricted. is there.

There is also a method of improving the weather resistance by coating the surface of stainless steel with paint or plating.
Since this method requires the addition of a new surface treatment step to the stainless steel production step, it has problems such as an expensive material due to the cost required for these treatments and a decrease in productivity due to an increase in the number of steps. .

On the other hand, it is a general story that the weather resistance and rusting property outdoors depend greatly on the use environment. Recently, it has become clear that the weather resistance and the rusting property differ greatly depending on the use site. For example, when roofs and walls are constructed with stainless steel, parts that are sufficiently washed away by rainwater during rainfall, such as the roof top and under the wall, have less rust, and rainwater does not pour enough like under eaves, It became clear that there was much rust in the part where water droplets stayed for a long time. The reason is that corrosion products such as sea salt particles are washed away in the part that is sufficiently washed away with rainwater, so that these do not become concentrated. It is considered that crevice corrosion is unlikely to occur. Therefore, it is becoming clear that even if rainwater adheres, it does not become water droplets, and that the state of dripping becomes superior to rusting.

Even if rainwater adheres to the stainless steel, it is possible to obtain a state in which the rainwater does not become water droplets but is dripped off by increasing the hydrophilicity of the surface of the stainless steel. In other words, rainwater evaporates gradually when left for a long time in the state of water droplets, and the substance that inhibits weather resistance and rust resistance is washed away to the final evaporating part to render it harmless, or it is inhibited by hydrophilicity (wetting). The substance can be rendered harmless by lowering its concentration by diffusing the substance into a wide range. Therefore, stainless steel having a surface having excellent hydrophilicity has excellent weather resistance and rust resistance.

[0007]

SUMMARY OF THE INVENTION
An object of the present invention was to develop a high-strength stainless steel sheet which is inexpensive with few manufacturing steps and has excellent hydrophilicity.

[0008]

SUMMARY OF THE INVENTION The present invention has been completed based on the research results of the present inventors who have earnestly solved the above-mentioned problems, and contains 10 to 17% by mass of Cr. Producing a hot-rolled, cold-rolled or annealed steel strip and steel sheet of a low-carbon martensitic stainless steel having a C content of 0.15% by mass or less and a Si content of 1.0 to 6.0% by mass; The steel strip and the steel sheet are subjected to (As +
30 ° C or higher and below the Af point (900 ° C
(The following range) (reverse transformation treatment) to convert a part of the martensite phase into a reverse transformed austenite phase, and to cause a composition redistribution between the martensite phase and the austenite phase, followed by cooling to room temperature. A fine austenitic phase and a martensitic phase with a diameter of 5 μm or less and a width of 1 μm or less, and then pickling and corroding the austenite phase preferentially. There is a great feature in that fine irregularities are provided.

According to this method, the stainless steel sheet can be made hydrophilic even if it is not subjected to mechanical surface roughening or chemical surface pretreatment in order to improve the hydrophilicity in a lower step. An excellent high-strength stainless steel plate can be obtained. The As point is the temperature at which the transformation from the martensite phase to the austenite phase starts during the temperature rise process, and the Af point is the temperature at which the transformation from the martensite phase to the austenite phase ends during the temperature rise process.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION If heating is performed at a temperature not exceeding 900 ° C. and a temperature range of (As + 30 ° C.) or more and Af point or less, a part of the martensite phase can be reverse transformed into an austenite phase. The austenite phase formed by this reverse transformation is fine crystal grains having a diameter of 5 μm or a width of 1 μm or less. Further, composition redistribution occurs between the reverse transformed austenite phase and the untransformed martensite phase generated by this heat treatment. That is, the austenite stabilizing elements such as C, N, Si, and Ni move to the reverse transformed austenite phase to increase the concentration, and the stability of the austenite phase increases. On the other hand, in the untransformed martensitic phase, the concentration of Cr that controls corrosion resistance increases.

Therefore, when cooled to room temperature, the stable austenite phase remains as it is, and becomes a multiphase structure of a fine austenite phase of 5 μm or less and an untransformed martensite phase. In addition, when pickling is performed, the austenitic phase having a relatively low Cr concentration is preferentially corroded, so that a corrosion difference occurs in fine units of 5 μm or less. In order to increase the corrosion difference, if a peracid washing state is applied, which is not applied in a normal stainless steel manufacturing process, fine irregularities with a depth of 0.5 μm or more are formed at intervals of 5 μm or less on the surface.

Further, the fine austenite phase obtained by the reverse transformation treatment has high strength as inferred from the relationship of Cool-Petch, and the untransformed martensite phase originally has high strength. As a result, high strength can be maintained. Further, the austenite phase inherently has high ductility, and furthermore, a martensitic transformation is induced by strain due to processing to obtain high ductility by the TRIP effect, and a material having high strength and high ductility can be obtained.

Although the reason for the hydrophilicity is not clear at this time, the formation of fine irregularities on the surface reduces the contact angle with water and increases the hydrophilicity. Further, it was found that the acidity was applied after the final heat treatment in the production process, and the hydrophilicity after being left under sunlight for about one week was improved as the Si content increased.

Preferred Embodiment of the Invention

The heat treatment furnace for causing the reverse transformation according to the present invention can be a bell-type or box-type batch furnace, or a catenary-type or vertical continuous-annealing furnace. The atmosphere is preferably a reducing gas or an inert gas in consideration of preventing oxidation of the steel strip, but there is no particular problem even in an air atmosphere. In this case, the heating may be performed by a heating furnace of any method such as an electric heating element method, a combustion method of gas, oil, or the like.

An acid washing step is required to remove the surface oxide layer generated by the reverse transformation process and to form fine irregularities of 0.5 μm or more in depth at intervals of 5 μm or less according to the present invention. This pickling step may be any pickling equipment. In order to efficiently produce the steel of the present invention, it is preferable to continuously perform reverse transformation treatment in a continuous annealing and pickling facility and then continuously perform peracid washing in a pickling tank.

A feature of the present invention is that, when heat treatment is performed in this manner, a fine austenite phase is formed from a martensite phase by a reverse transformation to form a fine multiphase structure, and a composition is redistributed between the two phases. This fine duplex structure is maintained even after cooling at room temperature, and the surface is washed with acid so that the surface becomes uneven at a depth of 0.5 μm or more at intervals of 5 μm or less, thereby producing a high-strength duplex stainless steel plate with excellent hydrophilicity. To get.

Therefore, obtaining a stable and fine two-phase structure is the key. Since the amount of the reverse transformed austenite phase generated is too small in the heat treatment below (As point + 30 ° C.), and the amount of the reverse transformed austenite phase increases in the heat treatment at a temperature exceeding the Af point or a temperature range exceeding 900 ° C. The redistribution of the composition becomes insufficient and austenite does not remain. Therefore, the heat treatment is (As point +
30 ° C or higher and below the Af point (900 ° C
The following range).

As the stainless steel to which the method of the present invention is applied, a martensitic stainless steel exhibiting a martensitic structure in an annealed state is intended. The structure state of the steel sheet before performing the reverse transformation treatment needs to be substantially martensite structure, and this martensite structure is related to the components in the steel, but the annealed steel sheet has a martensite structure as it is annealed. A cold-rolled steel sheet obtained by cold-rolling this steel sheet (in some cases, a cold-rolled steel sheet in which work-induced martensite is generated by cold rolling), a hot-rolled steel sheet, and a steel strip thereof.

However, it is not always necessary to have a 100% martensite phase, and a ferrite phase or an austenite phase up to about 20% capacity may be present. In any case, redistribution of the composition occurs in the state of the fine two-phase structure obtained by the reverse transformation treatment, and the tensile strength is 1.
One of the pillars of the present invention is to obtain a high strength of 000 N / mm2 class or more, and the present invention includes steel components and composition ratios in a range satisfying this requirement.

Regarding the components of the steel, 10 to 17% by mass of Cr, 0.15% by mass or less of C, and 1.0 to 6.0% by mass
Of low carbon martensitic stainless steel containing Si. Ni can also be one of the main components, and it is of course possible to add a normal alloying element contained in this type of steel as long as the above requirements are satisfied.

The typical chemical components and their contents are as follows. C: 0.15% by mass or less, Si: 1.
0 to 6.0% by mass or less, Mn: 10.0% by mass or less, N
i: 8.0% by mass or less, Cr: 10.0 to 17.0% by mass, N: 0.3% by mass or less, Mo: 4.0% by mass or less,
Cu: 4.0% by mass or less, Co: 4.0% by mass or less, and can further contain Ti, Al, Nb, V, Zr, B, and rare earth elements in a total amount of 1.0% by mass or less.

However, Ti, Al, Nb, V, Zr,
B, when the composition does not include rare earth elements, Nieq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si + 20 (C + N) Ti, Al, Nb, V, Zr, B, rare earth In the case of a composition containing an element, the value of Nieq (Ni equivalent) defined by Nieq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si is 10.0 to 1
Each component is adjusted to be within the range of 7.5. The content range of these main elements is regulated in this way for the following reasons.

C is a powerful austenite-forming element, which effectively acts to stabilize the reverse-transformed austenite phase formed by heat treatment in a temperature range of (As point + 30 ° C.) or more and Af point or less. Effectively works to strengthen the austenite phase and martensite phase. However, when contained in a large amount, Cr carbide is generated during the reverse transformation treatment and the corrosion resistance is deteriorated, so the upper limit is 0.15% by mass.
And

Since Si widens the temperature range between the As and Af points, it effectively acts to obtain a stable two-phase structure of an austenite phase and a martensite phase, and reversely transformed austenite phase and martensite phase after heat treatment. It is also an effective element for strengthening steel. It is also an effective element for the purpose of improving hydrophilicity. However, if the content is less than 1.0% by mass, the effect of improving hydrophilicity is not remarkable, so the lower limit is set to 1.0% by mass.
Further, if contained in a large amount, a δ ferrite phase is formed and hot workability is impaired. Therefore, it is preferable that the upper limit is 6.0% by mass.

Mn, like Ni, is an austenite-forming element and effectively acts to stabilize the reverse transformed austenite phase formed by heat treatment in a temperature range from (As point + 30 ° C.) to Af point. However, if a large amount is contained, Mn fume is generated during melting and the productivity is lowered, so it is preferable to set the content to 10.0% by mass or less.

Cr is a basic component of stainless steel, and must be contained in an amount of 10% by mass or more in order to obtain good corrosion resistance. However, Cr is a ferrite-forming element, and when contained in a large amount, a large amount of δ-ferrite phase is formed, and after annealing, it becomes difficult to obtain a martensitic single-phase structure at room temperature. Therefore, the upper limit is set to 17.0 mass%.

Ni is an austenite forming element and effectively acts to stabilize the reverse transformed austenite phase generated by heat treatment in a temperature range from (As point + 30 ° C.) to Af point. However, if it is contained in a large amount, it becomes difficult to obtain a martensitic single phase structure at room temperature after annealing, so that it is 8.0 mass%.
It is preferred that the content be contained in the following range.

N, like C, is an austenite-forming element, and effectively acts to stabilize the reverse-transformed austenite phase formed by heat treatment at a temperature range of As point + 30 ° C. to Af point. Works effectively to strengthen the phase and martensite phase. Further, it is an element effective for strengthening the reverse transformed austenite phase and the martensite phase. However, if it is contained in a large amount, blowholes are generated at the time of smelting, and a sound steel ingot cannot be obtained. Therefore, the content is preferably set to 0.30% by mass or less.

Mo is an element that improves corrosion resistance and is effective in strengthening the reverse transformed austenite phase and martensite phase after heat treatment. However, Mo is a ferrite forming element, and when contained in a large amount, a large amount of the δ ferrite phase is formed, and it becomes difficult to obtain a structure of a martensite single phase at room temperature after annealing. Therefore, the content is preferably set to 4.0% by mass or less. .

Cu, like Ni, is an austenite-forming element and is effective in forming an austenite phase after heat treatment. However, if contained in a large amount, hot workability is reduced. Good.

Co, like Ni, is an austenite-forming element and is effective in forming an austenite phase after heat treatment.
It is preferred that the content be not more than mass%.

Each of Ti, Al, Nb, V, Zr, and B is effective for stabilizing the two-phase structure of austenite + martensite formed by the reverse transformation treatment and for maintaining a fine and uniform structure, and is effective for Cr. It is an element that is effective for suppressing the generation of carbides to maintain corrosion resistance and improving hydrophilicity. However, if contained in a large amount, the manufacturability is reduced. Therefore, the content of each is preferably 1.0% by mass or less, and the total amount thereof is preferably 1.0% by mass or less.

Nieq (Ni equivalent) is as follows. In the present invention, a fine austenite phase is generated by a reverse transformation from a martensite phase during a heat treatment to form a fine multiphase structure, and by maintaining this fine multiphase structure,
A high-strength duplex stainless steel is obtained. Further, the composition redistribution generated between the fine two-phase structures causes a difference in the corrosion rate between the respective structures, thereby obtaining fine irregularities and improving the hydrophilicity with the irregularities. Therefore, the key in the present invention is to obtain a stable and fine multiphase structure. Nieq is 10.0
If it is less than (As point + 30 ° C.) and the heat treatment is performed in a temperature range not more than the Af point, the amount of the reverse transformed austenite phase is reduced, and if the Nieq exceeds 17.5, the amount of the reverse transformed austenite phase is reduced. , And it is difficult to obtain a stable fine multi-phase structure. Therefore, Ni
It is preferable to adjust each component so that eq is 10.0 to 17.5.

[0034]

EXAMPLE A steel having the composition shown in Table 1 was melted in a conventional manner, forged and hot-rolled to a thickness of 6 mm, subjected to a solution treatment, subjected to pickling, cold-rolled and annealed, and then subjected to a predetermined process. Intermediate rolling and annealing are performed so that the rolling rate becomes 1 m.
m of cold-rolled material. Some cold rolled materials are 1030 ° C
The test material was annealed in the test. In addition, a hot-rolled material having a thickness of 6 mm was sampled as a sample of the hot-rolled material during the sample preparation process. In Table 1, As and Af points are also shown.
These transformation points were determined by using an electrical resistance measuring device to measure each specimen.
It was determined from the inflection point of the temperature-electric resistance relationship curve obtained by raising the temperature at a heating rate of ° C / min.

[0035]

[Table 1]

After subjecting these test materials to heat treatment under the various conditions shown in Table 2, they were heated at 60 ° C. (hydrofluoric acid: 3%
+ Nitric acid: 12% + Remainder: water) Pickling was performed with a pickling solution. Normal pickling ends when the oxide film during heat treatment can be removed, but in the present invention, in order to impart fine irregularities to the surface,
The pickling was usually three times the pickling time of 3 minutes, that is, 9 minutes.

After the pickling was completed, the material was allowed to stand under sunlight for one week so as to be in the same condition as the condition to which the building material was exposed. In the hydrophilicity test, 10 μl (0.010 ml) of distilled water was dropped on a sample in a room at a temperature of 20 ° C. and a humidity of 60%, and after 30 seconds, the droplet was enlarged and photographed from the lateral direction with a CCD camera, and the contact angle was measured. I asked. Further, a tensile test piece was sampled from the same material, and the mechanical properties were also investigated.

[0038]

[Table 2]

As is clear from the results in Table 2, according to the method of the present invention, each of the test materials has a proof strength of 800 N / mm.
2. The tensile strength is 1000 N / mm2 or more, and the austenite particle size after the treatment is 5 μm or less. Test No.
In Nos. 12 and 13, since the state before the heat treatment is an annealed state, the reverse transformed austenite becomes a lamellar shape because it nucleates and grows starting from the martensite lath.
０．４0.4 μm × 15 to 10 μm. Moreover, the depth of the surface concave portions after pickling is 0.5 μm or more, and has excellent hydrophilicity. Note that the reason why the depth of the surface concave portion is 0.5 μm or more is that it is impossible to measure the depth of 0.5 μm or more due to a stylus of an instrument for measuring the depth. Also, the result is that the higher the Si content, the better the hydrophilicity. On the other hand, Test No. 1,
No. 2 has low hydrophilicity and insufficient hydrophilicity despite high yield strength and tensile strength because of low Si. Test No. In Nos. 4, 9, and 16, the heat treatment temperature was out of the range of the present invention, so that a fine metal structure was not obtained, and the depth of the surface concave portion was also 1 μm. Therefore, compared with the method of the present invention, the yield strength is low and the hydrophilicity is low. Test No. In Nos. 8 and 14, although the pickling conditions are out of the range of the present application, the yield strength and tensile strength are high, but the depth of the surface concave portions after pickling is 0.3 μm, and the hydrophilicity is not sufficient. Test N
o. In Nos. 19 and 20, Nieq deviates from the present invention, so that the yield strength and tensile strength are low and the hydrophilicity is poor.

FIG. 1 shows the result of observing the surface of the test material with a scanning electron microscope. In the figure, a) is the result of the method of the present invention, and has a high yield strength and a high tensile strength and exhibits excellent hydrophilicity. 13 is the surface. Since this material was subjected to reverse transformation treatment on the annealed material, reverse transformation occurred based on fine lath martensite before the reverse transformation treatment, so that fine lamellar reverse transformation austenite was generated and 0.3 μm intervals A lamellar fine uneven surface is formed.
In the figure, b) is also a result of the method of the present invention, and has a high yield strength and tensile strength and exhibits excellent hydrophilicity. 15 surface. In the present material, since reverse transformation treatment was performed on the cold-rolled material, reverse transformation occurs when a large amount of rolling strain is applied to fine lath martensite before the reverse transformation treatment.
As a result, the inverse transformation generation nuclei become random, and consequently isotropic spherical inverse transformation austenite is generated. Since this portion is washed with peracid, fine uneven surfaces at 1 μm intervals are formed. On the other hand, c) in FIG. In the surface of No. 9, the composition did not redistribute between the crystal grains by the heat treatment deviating from the method of the present invention, and the difference in the degree of corrosion between the crystal grains was small even if the pickling time was lengthened. As a result, fine irregularities such as those obtained by the method of the present invention are not generated on the surface.

FIG. 2 shows part of the results of the hydrophilic test. In the figure, a) and b) are test Nos. Of the comparative method. 1, 14, but have a large contact angle and poor hydrophilicity. In the figure, c) is the test No. of the method of the present invention. 17, the contact angle was reduced and the hydrophilicity was greatly improved.

As can be seen from Table 2, FIG. 1 and FIG. 2, the excellent hydrophilicity of the method of the present invention indicates that the fine irregularities greatly contribute to the improvement.

[0043]

As described above, in the steel of the present invention, the mechanical surface roughening and the chemical surface pretreatment of the stainless steel plate for improving the hydrophilicity in the post-process are not performed as in the prior art. Even so, since it has excellent hydrophilicity, it is possible to provide a high-strength duplex stainless steel sheet that is inexpensive and has excellent hydrophilicity.

[Brief description of the drawings]

FIG. 1 is a photograph of the surfaces of the present invention and comparative steel observed with a scanning electron microscope.

FIG. 2 is a photograph showing the observation of the contact angle of the material obtained by the method of the present invention.

Claims (4)

[Claims] 1. A method according to claim 1, wherein the Cr content is 10 to 17% by mass, the C content is 0.15% by mass or less, and the Si content is 1.0 to 6.
Stainless steel in the range of 0% by mass,
A high-strength duplex stainless steel strip and steel sheet having excellent hydrophilicity, having irregularities of 0.5 μm or more at intervals of not more than μm. 2. In addition to the components described in claim 1, Mn: 10.0% by mass or less, Ni: 8.0% by mass or less, N: 0.3% by mass or less, Mo: 4.0% by mass % Or less, Cu: 4.0% by mass or less, Co: 4.0% by mass or less, and Nieq defined below is 1%.
A high-strength duplex stainless steel strip and steel sheet excellent in hydrophilicity, having a range of 0.0 to 17.5 and having irregularities with a depth of 0.5 μm or more at intervals of 5 μm or less on the surface. However, Nieq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si +
20 (C + N). 3. The composition according to claim 2, further comprising Ti,
Al, Nb, V, Zr, B, one or more of rare earth elements are contained in a total amount of 1% by mass, and Nieq defined below is 1%.
A high-strength duplex stainless steel strip and steel sheet excellent in hydrophilicity, having a range of 0.0 to 17.5 and having irregularities with a depth of 0.5 μm or more at intervals of 5 μm or less on the surface. However, Nieq = Ni + Mn + Cu + Mo + 0.2Co + 0.5Cr + 0.3Si
It is. 4. A temperature range of not less than (As + 30 ° C.) and not more than Af point of a hot-rolled, cold-rolled or annealed steel strip of stainless steel having the composition according to claim 1, 2 or 3 in a heat treatment furnace. (However, it is heated to 900 ° C. or less) to convert a part of the martensite phase into an inverse transformed austenite phase, to cause a composition redistribution between the martensite phase and the austenite phase, and then to cool to room temperature. A multi-phase structure of fine austenite phase and martensite phase with a diameter of 5 μm or less and a width of 1 μm or less, and then acid-washed to preferentially corrode the austenite phase. A method for producing a high-strength duplex stainless steel strip and a steel sheet having excellent hydrophilicity, characterized by providing irregularities.