JPH01172525A - Production of complex phase structure chromium stainless steel strip having excellent grain boundary corrosion resistance and high ductility and strength - Google Patents

Abstract

PURPOSE:To produce a complex phase chromium stainless steel strip having excellent grain boundary corrosion resistance and high ductility and strength by executing the specific continuous heat treatment to the cold-rolled steel strip containing the specific composition of Cr, Ni, Mn, Cu, Ti, Nb, Zr, etc. CONSTITUTION:The hot-rolling and cold-rolling as conventional method is executed to the chromium stainless steel containing 10-20wt.% Cr, 0.1-4.0% one or more kinds among Ni, Mn and Cu and 0.05-0.50% one or more kinds among Ti, Nb and Zr as the essential components. The obtd. cold-rolled steel strip is heated at the temp., which becomes two phase zone of ferrite + austenite by using a continuous heat-treatment furnace. This temp. is suitable to be more than 700-1,100 deg.C and this heating time is desirable to be <=10min. Successively, the steel strip is cooled from this temp. to the prescribed temp. at 1-500 deg.C/sec cooling speed to make transformation from austenite to martensite. By this finished heat-treatment, the chromium stainless steel composing of hard martenstic phase and soft ferritic phase and excellent grain boundary corrosion resistance and good workability is obtd.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention provides a highly ductile, high-strength, multi-phase chromium stainless steel strip that has excellent intergranular corrosion resistance and is substantially composed of ferrite and martensite mixed &111 weave. The present invention provides a highly ductile, high-strength stainless steel strip that requires high strength and is used as a forming material that is subjected to processing such as press forming.

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

[Prior Art] Chromium stainless steel containing chromium as a main alloy component includes martensitic stainless steel and ferritic stainless steel.

Martensitic stainless steel has been well known as a high-strength chromium stainless steel strip. For example, JIS G 4307 specifies seven types of martensitic stainless steel.

These martensitic stainless steels have a C; 0.0
8% or less (SUS410S) to 0.60-0.75%
(SO5440A) contains a higher amount of C than ferritic stainless steel, and can be given high strength by quenching or quenching and tempering. In addition, in this JIS G 4307, 0.26 to 0.40
5US420J2 containing % C and 12.00-14.00% (DCr).

(from 980 to 1040℃) 3, after quenching by cold, 15
Hardness of HRC40 or higher can be obtained by air-cooling tempering at 0 to 400°C, and C of 0.60 to 0.75%.
and 5U containing 16.00-18.00% Cr
For S440^, it has been shown that after quenching by rapid cooling from 1010 to 1070°C, tempering by air cooling from 150 to 400°C gives the same hardness of <IIRC40 or higher. Although martensitic stainless steel can achieve high strength through heat treatment, in most cases when it is shipped from material manufacturers as stainless steel sheets or copper strips, it is shipped in an annealed state.
At that point, the strength and hardness are low as also shown in Table 16 of JTS G 4307. Hence, quenching.

Heat treatments such as quenching and tempering are usually performed by processing manufacturers.

Ferritic stainless steel strips, which are another type of chromium stainless steel, cannot be expected to be hardened very much by heat treatment, so one way to increase the strength is to perform further cold fA quality rolling after annealing to increase the strength through work hardening. It may peel off. However, the reality is that ferritic stainless steels are not often used in applications that inherently require high strength.

[Problem that the invention seeks to solve]

In martensitic stainless steel strips, the structure after quenching or quenching and tempering is basically a martensitic structure, as the name suggests.

Although very high strength and hardness are obtained, elongation is very low. Therefore, if a steel plate or steel strip is subjected to heat treatment before processing, subsequent processing becomes difficult. therefore,
Heat treatment is often applied after processing into a shape that approximates the final product. In particular, processing such as press molding is not possible after heat treatment. In any case, in order to obtain high strength with martensitic stainless steel, a heat treatment process at the processing manufacturer is essential, which increases the burden on the processing manufacturer, and this increases the cost of the final product. The problem was that I couldn't do it.

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 worsens, resulting in poor workability. The degree of increase in strength due to temper rolling is significantly higher in yield strength than in tensile strength. For this reason, when the rolling reduction becomes high, the difference between yield strength and tensile strength becomes smaller, and the yield ratio (-
When the yield strength (yield strength/tensile strength) approaches 1, the plastic working range of the material becomes very narrow, and when the yield strength is high, springback becomes large and shapeability after press working etc. becomes poor.

Furthermore, the temper-rolled material has very large in-plane anisotropy in strength and elongation, and even light press processing results in poor shape after processing. Furthermore, since the processing strain caused by rolling is larger closer to the surface of the plate, it is inevitable that the strain distribution in the thickness direction of the temper-rolled material will become non-uniform. This results in non-uniform distribution of residual stress in the thickness direction.

In particular, in extremely gJ steel plates, changes in shape such as warpage may occur after punching or photo-etching, which is a major problem in applications that require high precision, such as 1A child parts. In addition to the above-mentioned problems caused by skin-pass rolling, ferritic stainless steels also have the problem of ridging, which can be considered a wood-related defect. This is a big problem in applications where importance is placed on

[Means to solve problems]

To solve the above-mentioned problems, material manufacturers are trying to find a chromium stainless steel material that has moderately high strength, good ductility and workability that can be processed into the desired shape, has small anisotropy, and does not cause ridging. This could be solved if it could be provided in the form of steel plates or strips. In order to solve this problem, the present inventors have continued extensive research on chromium stainless steel from both the chemical composition and manufacturing conditions. As a result, instead of the conventional finish annealing at a temperature in the ferrite single-phase region, that is, the annealing treatment applied to steel sheets or copper strip products, the finish heat treatment consisting of heating to the ferrite + austenite two-phase region and subsequent rapid cooling treatment was applied to chromium stainless steel. Steel strip products (cold-rolled steel sheets or strips obtained by normal hot rolling or cold rolling)
If applied to.

We have achieved great results in solving virtually all of the problems mentioned above.

However, in this case, there is a new problem.

When heated and cooled to high temperatures during finishing heat treatment.

It has become clear that there are cases where sensitization occurs and corrosion resistance is significantly degraded. Sensitization here means that chromium carbides and chromium nitrides that had precipitated before finishing heat treatment become solid solutions during high-temperature heating, and then precipitate at the grain boundaries or phase boundaries of ferrite or martensite during cooling, resulting in a single grain. Cr-depleted layers are formed near grain boundaries and phase boundaries, and when continuous pickling is performed following finishing heat treatment, the nine grain boundaries are preferentially eroded, resulting in formation of grain units called "gold dust" or "killer stars". This refers to the occurrence of falling off or a significant decrease in intergranular corrosion resistance and rust resistance even after the product is made.

Therefore, 5 As a result of further investigation on this sensitization, it was found that the sensitization of dual-phase steel after finishing heat treatment occurs mainly at martensite grain boundaries or martensite phase boundaries, and the amount of carbon or nitrogen in austenite at high temperatures is important. 5 Also, as a means to avoid sensitization, carbon/nitride forming elements such as Ti, Nb, and Zr are added, and carbon and nitrogen in the steel are fixed as carbon/nitrides of these elements. We found that it is most effective to do so.

In addition, in order to avoid sensitization, the cooling rate after heating in the finishing heat treatment must be extremely high.

A method of suppressing the precipitation of chromium carbon and nitrides during cooling, and a method of suppressing the amount of carbon and nitrogen while simultaneously reducing the amount of Ni and Mn.

There is also a method of adding an austenite-forming element such as Cu to increase the amount of austenite and lowering the amount of carbon and nitrogen in the austenite phase. However, in any case, especially when the plate thickness is thick, sensitization may be unavoidable because there is a limit to the cooling rate in an industrial-scale continuous heat treatment furnace.

Thus, the present invention provides 10.0 to 20.0 as an essential component.
Cr in weight%, 0.1 to 4.0 weight% of one or more of Ni, Mn or Cu, 0.05 to 0
．． A cold-rolled steel obtained by manufacturing a cold-rolled steel strip of chromium stainless steel containing 50% by weight of one or more of Ti, Nb, or Zr through normal hot rolling and cold rolling. The strip is heated in a continuous heat treatment furnace to a temperature where it becomes a ferrite + austenite two-phase region.

A high-ductility, high-strength multi-phase structure with excellent intergranular corrosion resistance (substantially composed of ferrite and The present invention provides a method for manufacturing a chromium stainless steel strip having a structure consisting of martensite.

According to the method of the present invention, not only virtually all of the above-mentioned problems are solved, but also the strength can be freely and easily adjusted by controlling the composition or the heating temperature and cooling rate during finishing heat treatment. This technology provides an advantageous and new manufacturing technology for the industrial production of chromium stainless steel strip materials, and is unique to martensitic stainless steel sheets or copper strips and ferritic stainless steel sheets or steel strips that have been shipped to the market. The objective is to provide the market with a new chromium stainless steel material that has both excellent ductility and strength properties and has little in-plane anisotropy in ductility and strength.

Conventionally, for example, even in 5US430, which is a representative steel type of ferritic stainless steel, austenite is generated when heated to a temperature in the two-phase region, and this austenite is transformed into martensite by rapid cooling, resulting in a two-phase structure of ferrite + martensite. That itself was known. However, in the production of ferritic stainless steel sheets or steel strips that produce austenite at high temperatures, the heat treatment after cold rolling is essentially an annealing treatment at a temperature in the ferrite single phase range, and martensite may be produced after cooling. It is common knowledge to avoid high-temperature heat treatment because it causes deterioration in material quality such as a decrease in ductility, and has not been considered at all in the actual production of steel plates or steel strips. Therefore, detailed research has been conducted on the relationship between heating temperature, strength, and ductility, as well as the anisotropy of ductility and strength when finishing heat treatment as in the present invention is applied to steel strips after cold rolling of chromium stainless steel. There are no examples.

Cr1 in chromium stainless steel to which the method of the present invention is applied
Regarding L, it should be contained at least 10.0% as the minimum necessary amount to maintain the corrosion resistance of stainless steel, but if the Cr content is too high, it will generate a martensitic phase, which is necessary to obtain high strength. Ni, Mn, C
As the amount of austenite-forming elements such as u increases, the toughness decreases, so it is preferable to set the upper limit to 20.0%.

Regarding Ni, Mn, and Cul, in order to obtain a ferrite + austenite biphasic fiber at high temperatures, a certain amount or more is required depending on Crff1, and it is necessary to contain at least 0.1% or more. However, if it is too high, a large amount of martensite phase will be generated after finishing heat treatment, and in some cases, it will become 100% martensite, which may provide strength but reduce ductility, so it is better to set the upper limit to 4.0%. .

Ti, Nb, and Zr are necessary to fix C and N as carbon and nitride, prevent sensitization during finishing heat treatment, and suppress deterioration of intergranular corrosion resistance. For this purpose, it is necessary to add at least 0.05% or more of each of C and Nl, although it varies depending on C and Nl. , Zr is added alone to about 8 of (C0N)il.
It is better to double or more. In addition, when adding in combination,
It is sufficient that the total amount of Ti, Nb, and Zr is sufficient to fix C and N.

On the other hand, Ti, Nb, and Zr are also ferrite-forming elements, so if they are added in excess, a commensurate amount of N will be added.
It is necessary to further add austenite-forming elements such as i, Mn, and Cu, which makes the product expensive, so it is preferable to set the upper limit to 0.50% for each.

There is no particular restriction on the amounts of C and N, but in order to reduce the amounts of Ti, Nb, and Zr mentioned above, it is desirable that the amounts of C and N be lower, and C should be 0.03% or less. It can be said that N is preferably 0.015% or less.

It goes without saying that the chromium stainless steel to which the present invention is applied in addition to the above-mentioned restrictions on individual components must have its components adjusted so that it becomes a biphasic fiber of ferrite and austenite at high temperatures.

The heating temperature during the finishing heat treatment in the present invention is ferrite +
An absolute condition is that the temperature be in the austenite two-phase region.1 In chromium stainless steel, in which the present invention can be advantageously implemented, the lower limit temperature at which the ferrite + austenite two-phase IJ1m is achieved is approximately in the range of 700 to 900°C. It can also be said that the effects of the present invention are best manifested when the target is chromium stainless steel whose composition has been adjusted in this manner. Regarding the upper limit of the heating temperature during this finishing heat treatment, it is preferable to set the upper limit to 1100° C., since if the temperature is too high, the increase in strength will be saturated and it will also be disadvantageous in terms of manufacturing cost.

In the heating in the ferrite + austenite two-phase region during the finishing heat treatment in the method of the present invention, an approximately equilibrium amount of austenite phase is generated within a short time, so the heating time may be short, approximately 10 minutes or less. The fact that heating is sufficient for a short period of time enables continuous heat treatment in the actual operation of the method of the present invention, which is very advantageous in terms of production efficiency and manufacturing cost.

In the present invention, which targets Ti, Nb, and Zr-containing steels, there is no need to particularly regulate the cooling rate during finishing heat treatment from the viewpoint of sensitization. In order to obtain this, it is necessary to set the cooling rate to 1° C./sec or more.

It is virtually difficult to obtain cooling rates in excess of 500° C./sec. Therefore, in the present invention, cooling from two-phase temperature range heating is carried out at a cooling rate in the range of 1 to 500° C./sec. Although this cooling rate may be up to the final cooling temperature to room temperature, it is not necessarily necessary to employ this cooling rate in the cooling process after transformation to a low-temperature transformation phase, that is, a martensitic phase. As a cooling method, a forced cooling method in which a gas and/or liquid cooling medium is sprayed onto a steel plate or a copper strip, a roll cooling method using water-cooled rolls, etc. can be applied. The finishing heat treatment according to the present invention can be carried out by a continuous heat treatment method in which the 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 unwinding machine and the winding machine. can. Specifically, continuous bright annealing furnaces and continuous annealing and pickling furnaces for stainless steel.

Further, a continuous annealing furnace for ordinary steel can be applied.

Note that the copper strip to be subjected to this finishing continuous heat treatment can be manufactured through a normal hot rolling process and a cold rolling process. At that time, in the cold rolling process, the plate thickness may be reduced to the desired thickness by one cold rolling, or may be reduced to the desired plate thickness by two or more cold rollings with intermediate annealing in between. By performing the finishing continuous heat treatment according to the present invention, in-plane anisotropy substantially disappears in the dual-phase steel strip, but in the latter case, the in-plane anisotropy can be further reduced.

The strength of the multiphase steel obtained by the method of the present invention mainly depends on the amount (volume fraction) of the martensitic phase. Therefore, when carrying out the method of the present invention, from the chemical composition point of view, Nt,
Austenite forming elements such as Mn and Cu and Cr,
Si, Ti, Nb, Zr, A1. By adjusting the component balance of ferrite-forming elements such as Mo, the amount of austenite at high temperatures, that is, the volume fraction of the martensite phase after quenching, can be controlled, and thereby the strength after finishing heat treatment can be freely controlled. Also, from the viewpoint of manufacturing conditions, the volume fraction of the martensitic phase can be controlled by controlling the heating temperature during the finishing heat treatment.

In addition, in terms of chemical composition, in addition to controlling strength, Mo is added to improve corrosion resistance, and Y and REM are added to improve oxidation resistance. Various elements can be added or their contents can be regulated for the purpose of improving various properties of the 5J interlaced m steel. Further, as carbon/nitride forming elements, in addition to Tt, Nb, and Zr, elements such as V and Ta also have similar effects.

〔Example〕

Steel having the chemical composition shown in Table 1 is melted, hot-rolled to a thickness of 3.6 mm, annealed at 780°C for 6 hours, and pickled to a thickness of 0.7 mm. It was made into a cold rolled steel strip. These cold rolled steel strips were subjected to continuous finish heat treatment under the finish heat treatment conditions shown in Table 2. However, in Comparative Example Nα4, the finish heat treatment was performed in a punch-type box furnace rather than in a continuous heat treatment furnace, and the furnace cooling was performed at 930°C for 6 hours, and in Comparative Example Nα5, the hot rolled steel after pickling was After the strip was cold rolled to a thickness of 1.0 mm, it was heated at 1730℃
Intermediate annealing for I minutes and further cold rolling to 0.7 m
m is a temper-rolled material. These material properties are also listed in Table 2.

As is clear from Table 2, all nine samples obtained by the method of the present invention have high tensile strength, hardness, and good elongation. Further, in the method of the present invention, no occurrence of peeling was observed, and furthermore, no sensitization after finishing heat treatment was observed.

On the other hand, in Comparative Example Nα1.2, the finishing heat treatment conditions are within the range specified by the present invention and the mechanical properties and ridging properties are good, but in Comparative Example Nα1, the Ti addition amount is 0.
Comparative Example No. 2 is Ti.

Since carbon/nitride-forming elements such as Nb and Zr are not particularly added, both are sensitized and have poor intergranular corrosion resistance.

Comparative Example Nα3 has a low finishing heat treatment temperature of 730°C.

At this temperature, steel No. 4 does not reach the ferrite + austenite two-phase region, so the metal structure after finishing heat treatment is a ferrite single-phase structure without martensite, and although the elongation is relatively high.

The strength and hardness are low, and ridging occurs.

Comparative example Nα4 has a cooling rate of 0.03”C during finishing heat treatment.
/sec, so no martensite was generated after the heat treatment, and like Comparative Example Nα3, the elongation was high but the strength and hardness were low, and ridging occurred.

Comparative number 5 is a temper-rolled material, and compared to the inventive example, the elongation with respect to tensile strength (hardness) is significantly lower and the two-strength-elongation balance is inferior. Furthermore, 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 clear that the workability and shape properties after processing are inferior to the materials obtained by the second invention example.

As described above, (1) the method of the present invention has excellent intergranular corrosion resistance, high ductility and high strength, and (2) a complex phase with small in-plane anisotropy of strength and ductility, low proof stress, and low yield ratio. A textured steel strip is provided. In the field of chrome stainless steel sheets, there has never been an example of a high-strength material with such good workability being shipped to the market in the form of steel plates or steel strips. The purpose is to provide a new material steel plate or steel strip. The material according to the present invention is particularly useful as a high-strength material that requires processability into electronic parts, precision mechanical parts, etc., and can achieve great results in this field.

Claims (5)

[Claims] (1) 10.0 to 20.0% by weight of C as an essential component
r, 0.1 to 4.0% by weight of one or more of Ni, Mn or Cu, 0.05 to 0.50% by weight of one or two of Ti, Nb or Zr A cold-rolled steel strip of chromium stainless steel containing the above is produced through normal hot rolling and cold rolling, and the obtained cold-rolled steel strip is converted into a two-phase region of ferrite + austenite using a continuous heat treatment furnace. Heat to a temperature of 1°C/sec or more from this temperature 5
A method for producing a multi-phase chromium stainless steel strip with high ductility and high strength and excellent intergranular corrosion resistance, characterized by performing finishing heat treatment by cooling at a cooling rate of 00° C./sec or less. (2) The manufacturing method according to claim 1, wherein the chromium stainless steel is a steel whose composition is adjusted so that the temperature at which the ferrite + austenite two-phase region occurs exceeds 700°C. (3) The manufacturing method according to claim 1 or 2, wherein the heating temperature of the finishing heat treatment is 1100°C or less. (4) The manufacturing method according to claim 1, 2, or 3, wherein the heating time of the finishing heat treatment is 10 minutes or less. (5) The manufacturing method according to claim 1, 2, 3, or 4, in which cooling in the finishing heat treatment is performed at a cooling rate and cooling end point temperature sufficient to transform austenite into martensite.