JPH0814005B2 - Manufacturing method of high ductility and high strength dual phase chromium stainless steel strip with excellent intergranular corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a highly ductile and high-strength dual-phase structure chromium stainless steel strip having excellent intergranular corrosion resistance and consisting essentially of a mixed structure of ferrite and martensite. The present invention provides a highly ductile high-strength stainless steel strip as a forming material that requires high strength and is subjected to processing such as press forming in relation to a new industrial manufacturing method.

According to the method of the present invention, the product that has been subjected to the continuous finishing heat treatment step is industrially manufactured in the form of a steel strip, and when shipped to the market, the steel strip remains in a coil (coil) or a steel plate. It will be in the molded state.

[Conventional technology]

The chromium stainless steel containing chromium as a main alloy component includes martensitic stainless steel and ferritic stainless steel.

Conventionally, martensitic stainless steel is well known as a high-strength chromium stainless steel strip. For example, JIS G 4307 defines seven types of martensitic stainless steels. These martensitic stainless steels contain C: 0.08% or less (SUS410S) from 0.60 to
It contains 0.75% (SUS440A), which is higher than C in ferritic stainless steel, and can be given high strength by quenching or quenching and tempering. Also, this
According to JIS G 4307, 0.26 to 0.40% C and 12.00 to 1
For SUS420J2 containing 4.00% of Gr, after quenching by quenching from 980 to 1040 ℃, heat treatment by air tempering at 150 to 400 ℃
A hardness of RC 40 or more can be obtained, and 0.60 to 0.75
% Of C and 16.00-18.00% of Cr, SUS440A, after quenching by quenching from 1010-1070 ℃, 150-400 ℃
It has been shown that a hardness of HRC 40 or higher can be obtained by tempering with air cooling. In this way, although high strength is obtained by heat treatment in martensitic stainless steel, when it is shipped from a material manufacturer as a stainless steel plate or strip, it is shipped in an annealed state at that time. The strength and hardness are low as shown in Table 16 of JIS G 4307. Therefore, heat treatment such as quenching and quenching-tempering is usually performed by the processing manufacturer.

Since ferritic stainless steel strip, which is another type of chrome stainless steel, is unlikely to be hardened by heat treatment, the method for increasing strength is to increase strength by work hardening by temper tempering after annealing after annealing. May be measured. However, the fact is that ferritic stainless steel has not been used so much for applications that originally require high strength.

[Problems to be solved by the invention]

In the case of martensitic stainless steel strip, the structure after quenching or quenching-tempering is basically a martensitic structure as its name suggests, while very high strength and hardness are obtained, but elongation is very low. . for that reason,
If the steel plate or strip before processing is heat-treated, subsequent processing becomes difficult. Therefore, heat treatment is often applied after processing to a shape close to the final product. In particular, processing such as press molding is impossible after heat treatment. In any case, in the case of martensitic stainless steel, the heat treatment process at the processing maker is indispensable to obtain high strength, which increases the burden on the processing maker side, and for this reason, avoid increasing the cost of the final product. There was a problem that I could not.

On the other hand, when the strength of a ferritic stainless steel strip is increased by temper rolling, the elongation decreases significantly and the balance between strength and ductility deteriorates, resulting in poor workability. The degree of strength increase due to temper rolling is significantly higher in proof stress than in tensile strength. For this reason, when the rolling ratio is high, the difference between the yield strength and the tensile strength is small, and the yield ratio (= yield strength / tensile strength) is close to 1 and the plastic working area of the material is extremely narrow and the yield strength is high. The spring back becomes large, and the formability after pressing is deteriorated. Furthermore, the temper-rolled material has a very large in-plane anisotropy of strength and elongation, and the shape after processing deteriorates even with mild press working. In addition, since the processing strain due to rolling increases as it approaches the surface of the plate, it is unavoidable that strain distribution in the plate thickness direction becomes uneven in temper-rolled material. This causes a non-uniform distribution of residual stress in the plate thickness direction, and especially for ultra-thin steel plates, shape changes such as plate warpage may occur after punching or hole-punching by photo-etching, which makes high precision of electronic components and the like difficult. It is a big problem in the required application. In addition to the above problems caused by temper rolling, there is a problem of ridging that can be said to be an essential drawback in ferritic stainless steels. After temper rolling, ridging generally called ductile ridging occurs and surface roughness This is still a big problem in applications where is important.

[Means for solving problems]

As for the above-mentioned problems, the material manufacturer has a chrome stainless steel material that has moderately high strength, good ductility and workability that can be processed into a desired shape, and has little anisotropy and no ridging. It can be solved if it can be provided in the form of steel plate or strip. Therefore, the present inventors have continued extensive research on chromium stainless steel from the viewpoint of both chemical composition and manufacturing conditions for the purpose of solving this problem. As a result, instead of the conventional finish annealing at the ferrite single-phase region temperature, that is, the annealing treatment performed on the steel sheet or strip product, the finish heat treatment consisting of heating to the ferrite + austenite two-phase region and subsequent quenching treatment is performed on the chromium stainless steel. When applied to steel strip products of steel (normal cold-rolled steel sheets or strips obtained by hot-rolling or cold-rolling), we were able to obtain the excellent results that virtually all of the above problems could be solved. .

However, in this case, as a new problem, it became clear that when heating and cooling to a high temperature in the finish heat treatment, sensitization occurs and the corrosion resistance may be significantly deteriorated. Sensitization here means that the chromium carbides and chromium nitrides that had been precipitated before the finishing heat treatment once became a solid solution during high-temperature heating, and these precipitate at the grain boundaries and phase boundaries of ferrite or martensite during cooling. When a Cr-deficient layer is formed near the boundaries and phase boundaries, and the continuous pickling is carried out after the finishing heat treatment, the grain boundaries are preferentially eroded and the grain unit called “gold dust” or “Kira star” It means that the intergranular corrosion resistance and rusting resistance are significantly reduced even after the product has fallen off or becomes a product.

Therefore, as a result of further study on this sensitization, the sensitization of the dual phase steel after the finish heat treatment mainly occurs at the martensite grain boundary or the martensite phase boundary, and the amount of carbon or nitrogen in austenite at high temperature is important. It is presumed to be a factor, and as a means to avoid sensitization, carbon, nitrogen forming elements such as Ti, Nb, and Zr were added, and carbon and nitrogen in steel were fixed as carbon and nitride of these elements. We obtained the finding that it is most effective.

In addition, in order to avoid sensitization, the cooling rate after heating in the finishing heat treatment should be made extremely high to suppress the precipitation of chromium carbon / nitride during cooling, and the amount of carbon and nitrogen should be kept low at the same time. There is also a method of increasing the amount of austenite by adding austenite-forming elements such as Ni, Mn, and Cu to lower the amounts of carbon and nitrogen in the austenite phase. However, in both cases, especially when the plate thickness is large, sensitization may be unavoidable because the cooling rate is limited in an industrial scale continuous heat treatment furnace.

Thus, the present invention has an essential component of 10.0 to 20.0% by weight.
Cr, 0.1 to 4.0% by weight of Ni, Mn or Cu, one or more, and 0.05 to 0.50% by weight of Ti, Nb or Zr, one or more of chromium stainless steel. Cold-rolled steel strip is manufactured by ordinary hot rolling and cold rolling,
The obtained cold-rolled steel strip is ferrite + using a continuous heat treatment furnace.
Finishing heat treatment is performed by heating to a temperature in the austenite two-phase region and cooling from this temperature at a cooling rate of 1 ° C / sec to 500 ° C / sec. The present invention provides a method for producing a chromium stainless steel strip having a strong multi-phase structure (substantially composed of ferrite and martensite).

According to the method of the present invention, not only substantially all of the above-mentioned problems are solved, but also the strength can be freely and easily adjusted by controlling the steel composition or the heating temperature and the cooling rate during the finish heat treatment. It provides an advantageous and new manufacturing technology for the industrial manufacturing of chrome stainless steel strip materials, and is available in martensitic stainless steel sheets or steel strips or ferritic stainless steel sheets or steel strips that have hitherto been shipped to the market. The present invention provides a new chrome stainless steel material having both ductility and strength properties and a small in-plane anisotropy of ductility and strength to the market.

Conventionally, for example, even in SUS430, which is a representative steel type of ferritic stainless steel, austenite is generated when heated to the two-phase region temperature, and this austenite is transformed into martensite by quenching and becomes a two-phase structure of ferrite + martensite. The thing itself was known. However, in the production of ferritic stainless steel sheets or strips that produce austenite at high temperatures, the heat treatment after cold rolling is only an annealing treatment at the ferrite single phase region temperature, and it is likely that martensite will form after cooling. It is common sense to avoid high-temperature heat treatment as it causes deterioration of the material such as decreased ductility, and it was never considered in the actual manufacturing of steel sheets or strips. Therefore, detailed studies have been conducted on the relationship between the heating temperature and the strength and ductility, and the anisotropy of ductility and strength when the finishing heat treatment of the present invention is applied to the cold rolled steel strip of chrome stainless steel. There is no example.

Regarding the amount of Cr in the chromium stainless steel to which the present invention is applied, at least 10.0% should be contained as the necessary minimum amount in order to maintain the corrosion resistance as stainless steel, but if the amount of Cr is too high, a martensite phase is formed. As a result, the toughness decreases as the amount of austenite-forming elements such as Ni, Mn, and Cu required to obtain high strength increases, so 20.0% is preferable as the upper limit.

Regarding Ni, Mn, and Cu contents, a certain amount or more is necessary according to the Cr amount in order to obtain a ferrite + austenite two-phase structure at high temperature, and it is necessary to contain at least 0.1% or more. However, if it is too high, more martensite phase will be formed after the finish heat treatment, and in some cases 100%.
Although it becomes martensite and strength is obtained, ductility decreases, so it is better to set the upper limit to 4.0% for each.

Ti, Nb, and Zr are necessary to fix C and N as carbon and nitride, prevent sensitization during finish heat treatment, and suppress deterioration of intergranular corrosion resistance. To do this,
At least 0.05% for each, depending on the amount of C and N
The above additions are necessary, and more preferably about 4 times or more the amount of (C + N) in the case of adding Ti alone and about 8 times or more the amount of (C + N) in the case of adding each of Nb and Zr alone. In addition,
When they are added in combination, the total amount of Ti, Nb, and Zr should be sufficient to fix C and N.

On the other hand, Ti, Nb, and Zr are also ferrite-forming elements, so
If these are added excessively, the amount of Ni, Mn, Cu
It is necessary to further add austenite-forming elements such as, which makes the product expensive.
The upper limit is

Although the amount of C and N is not particularly limited, it is desirable that the amount of C and N be low in order to reduce the amount of addition of Ti, Nb, and Zr described above, with C of 0.03% or less and N of 0.015 or less. % Or less can be said to be preferable.

It is needless to say that the chromium stainless steel to which the present invention is applied together with the above-mentioned restrictions on the individual components need to have their components adjusted so as to have a ferrite + austenite two-phase structure at high temperatures.

It is an absolute condition that the heating temperature during the finish heat treatment in the present invention is the ferrite + austenite two-phase temperature. In the chromium stainless steel in which the present invention can be advantageously carried out, the lower limit temperature at which the ferrite + austenite two-phase structure is formed is generally in the range of 700 to 900 ° C. In other words, the chromium stainless steel whose composition is adjusted in this way is targeted. In this case, it can be said that the effects of the present invention are best exhibited. Regarding the upper limit of the heating temperature during this finishing heat treatment, if the temperature is too high, the strength increase saturates and there is a disadvantage in terms of manufacturing cost, so it is preferable to set the upper limit to 1100 ° C.

In the heating of the ferrite + austenite two-phase region during the finish heat treatment in the method of the present invention, the amount of the austenite phase in a nearly equilibrium state is generated in a short time, so the heating time may be short, generally other than 10 minutes. The fact that this short time heating is sufficient is very advantageous in terms of production efficiency and manufacturing cost because continuous heat treatment is possible in the actual operation of the method of the present invention.

Regarding the cooling rate during the finish heat treatment, in the present invention for Ti-, Nb-, and Zr-containing steels, there is no particular restriction from the viewpoint of sensitization, but the multiphase structure of the martensite phase and the soft ferrite phase It is necessary to set a cooling rate of 1 ° C / sec or more in order to obtain the above, but it is practically difficult to obtain a cooling rate of more than 500 ° C / sec. Therefore, in the present invention, the cooling from the two-phase temperature range heating is performed at a cooling rate in the range of 1 to 500 ° C / sec. This cooling rate may be up to the end point cooling temperature up to normal temperature, but it is not always necessary to adopt this cooling rate in the cooling process after transformation into the low temperature transformation phase, that is, the martensite phase. As a cooling method, a forced cooling method in which a gas and / or liquid cooling medium is sprayed on a steel plate or a steel strip, a roll cooling method using a water cooling roll, or the like can be applied. The finish heat treatment according to the present invention can be performed by a continuous heat treatment method in which a cold rolled strip of chrome stainless steel is passed through a continuous heat treatment furnace having a heating soaking zone and a quenching zone between the coil rewinding machine and the winding machine. it can. Specifically, a continuous bright annealing furnace for stainless steel, a continuous annealing pickling furnace, or a continuous annealing furnace for ordinary steel can be applied.

The steel strip to be subjected to this finishing continuous heat treatment can be manufactured through ordinary hot rolling and cold rolling steps. In that case, in the cold rolling step, the reduction may be performed once to the desired sheet thickness, or may be performed to the desired sheet thickness by two or more times of cold rolling with intermediate annealing. Even in the former case, the in-plane anisotropy does not substantially appear in the multiphase structure steel strip by performing the finishing continuous heat treatment according to the present invention, but in the latter case, the in-plane anisotropy should be further reduced. You can

The strength of the dual phase steel obtained by the method of the present invention mainly depends on the amount of martensite phase (volume fraction). Therefore, in carrying out the method of the present invention, Ni, Mn,
Austenite forming elements such as Cu and Cr, Si, Ti, Nb, Zr, Al,
The amount of austenite at high temperature, that is, the volume fraction of the martensite phase after rapid cooling can be controlled by adjusting the balance of the components of ferrite-forming elements such as Mo, and the strength after finishing heat treatment can be controlled freely. From the viewpoint of manufacturing conditions, the volume fraction of the martensite phase can be controlled by controlling the heating temperature during the finish heat treatment.

In terms of chemical composition, not only strength control but also addition of Mo for the purpose of improving corrosion resistance and addition of Y and REM from the viewpoint of improving oxidation resistance improve various properties of multi-phase steel. For this purpose, various elements can be added or their contents can be regulated. In addition to Ti, Nb, and Zr, elements such as V and Ta also have similar effects as carbon / nitride forming elements.

〔Example〕

Steel with the chemical composition shown in Table 1 was smelted, hot-rolled to a thickness of 3.6 mm, hot-rolled sheet was annealed at 780 ° C for 6 hours, pickled, and cooled to a thickness of 0.7 mm. A hot rolled steel strip was used. These cold-rolled steel strips were subjected to continuous finishing heat treatment under the finishing heat treatment conditions shown in Table 2. However, in Comparative Example No. 4, the finishing heat treatment is not performed in the continuous heat treatment furnace but in a batch type box furnace.
It was heat-treated by furnace cooling at 930 ° C for 6 hours, and in Comparative Example No. 5, the hot-rolled steel strip after pickling was cold-rolled to a plate thickness of 1.0 mm, then 730 ° C x 1 This is a temper-rolled material that has been subjected to intermediate annealing for a minute and has been further cold-rolled to 0.7 mm. These material properties are also shown in Table 2.

As is clear from Table 2, according to the method of the present invention, all have high tensile strength, hardness and good elongation.
Further, in the method of the present invention, no ridging was observed, and further no sensitization after finishing heat treatment was observed.

On the other hand, in Comparative Examples Nos. 1 and 2, the finish heat treatment conditions are within the range specified in the present invention and the mechanical properties and ridging characteristics are good, but in Comparative Example No. 1, the Ti addition amount is as low as 0.03%. In Comparative Example No. 2, since carbon, nitride forming elements such as Ti, Nb, and Zr were not added in particular, sensitization occurred and intergranular corrosion resistance was poor.

In Comparative Example No. 3, the finishing heat treatment temperature was as low as 730 ° C, and at this temperature, the steel of No. 4 did not enter into the ferrite + austenite two-phase region, so the metal structure after finishing heat treatment was a ferrite single ferrite without martensite. Although it has a phase structure and has relatively high elongation, it has low strength and hardness, and ridging occurs.

Comparative Example No. 4 has a very low cooling rate of 0.03 ° C / sec in the finishing heat treatment, so no martensite is formed after the heat treatment, and as with Comparative Example No. 3, it has high elongation but high strength and hardness. Is low and ridging is occurring.

Comparative example No. 5 is a temper-rolled material, and the elongation with respect to tensile strength (hardness) is significantly lower than that of the example of the present invention, and the strength-elongation balance is poor. The ratio of 0.2% proof stress to tensile strength, that is, the yield ratio is high, and the in-plane anisotropy of 0.2% proof stress and tensile strength is very large. Therefore, it is apparent that the workability and the shape property after processing are inferior to those of the materials obtained by the examples of the present invention.

As described above, according to the method of the present invention, a double phase having excellent intergranular corrosion resistance, high ductility and high strength, small in-plane anisotropy of strength and ductility, low yield strength and low yield ratio is obtained. A tissue strip is provided. In the field of chrome stainless steel sheets, there are no examples of high-strength materials with good workability that have been shipped to the market in the form of steel sheets or strips. Therefore, the present invention provides a new material steel plate or steel strip in the conventional chromium stainless steel plate field. The material according to the present invention is particularly useful as a high-strength material required to be processed into electronic parts, precision machine parts, etc., and can exert great results in this field.

Claims (5)

[Claims] 1. As an essential component, 10.0 to 20.0% by weight of Cr,
0.1 to 4.0% by weight of Ni, Mn or Cu, one or two
1 or more of 0.05 to 0.50% by weight of Ti, Nb or Zr.
Cold rolled steel strips of chromium stainless steel containing one or two or more types are manufactured through ordinary hot rolling and cold rolling, and the obtained cold rolled steel strips are processed into a ferrite + austenite alloy using a continuous heat treatment furnace. A finish heat treatment is performed by heating to a temperature in the phase region and cooling from this temperature at a cooling rate of 1 ° C / sec or more and 500 ° C / sec or less. A method for producing a high-ductility, high-strength, dual-phase chromium stainless steel strip with excellent intergranular corrosion. 2. The method according to claim 1, wherein the chromium stainless steel is a steel whose components are adjusted so that the temperature of the two-phase region of ferrite + austenite exceeds 700 ° C. 3. The manufacturing method according to claim 1, wherein the heating temperature of the finish heat treatment is 1100 ° C. or lower. 4. The method according to claim 1, 2 or 3, wherein the heating time of the finish heat treatment is within 10 minutes. 5. The production according to claim 1, claim 2, claim 3 or claim 4, wherein the cooling in the finish heat treatment is carried out at a cooling rate and a cooling end point temperature sufficient to transform austenite into martensite. Law.