JPS63169334A - Production of chromium stainless steel strip of double phase structure having small intra-surface anisotropy and high ductility and high strength - Google Patents

Abstract

PURPOSE:To obtain a stainless steel strip of double phase structure having small intra-surface anisotropy and having high ductility and high strength by hot rolling a steel slab having a specific compsn. essentially consisting of Cr, then subjecting the slab to cold rolling including intermediate annealing and holding the rolled sheet in a specific temp. region in a continuous heat treatment furnace then subjecting the sheet to controlled cooling. CONSTITUTION:The slab of the steel consisting of <=0.08wt.% C, <=2.0% Si, <=3.0% Mn, <=0.040% P, <=0.030% S, <=3.0% Ni, 10.0-14.0% Cr, <=0.08% N, <=0.02% O, <=3.0% Cu and the balance Fe and unavoidable impurities and satisfying the relations expressed by the formulas I, II is produced. Said slab is then subjected to hot rolling and >=2 passes of cold rolling including intermediate annealing under heating to the single phase region temp. of ferrite to the product sheet thickness. The rolled sheet is passed through the continuous heat treatment furnace where the sheet is held at the two-phase region temp. of ferrite + austenite of Ac1 point or above and <=1,100 deg.C within 10min and is then cooled at 1-500 deg.C/sec average cooling rate from the max. heating temp. down to 100 deg.C. The chromium stainless steel strip of double phase structure having >=200HV hardness, the small intra-surface anisotroy and the high ductility and high strength is obtd. by the above-mentioned finishing heat treat ment.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a new industrial manufacturing method for a high-strength, multi-phase Mi-woven chromium stainless steel strip with excellent ductility and low in-plane anisotropy of strength and ductility. The present invention provides a method for manufacturing a high-strength, high-ductility stainless steel strip that is required to have high strength and is used as a forming material that is subjected to processing such as press forming.

[Background of this field]

Chromium stainless steel containing chromium as a main alloy component includes martensitic stainless steel and ferritic stainless steel. Both are cheaper than austenitic stainless steels, which contain chromium and nickel as the main alloy components, and have physical properties that are not found in austenitic stainless steels, such as ferromagnetism and a small coefficient of thermal expansion. Therefore, there are many uses that are limited to chromium stainless steel not only for economic reasons but also for characteristics. Particularly in the fields of electronic equipment and precision mechanical parts in recent years, demand for chrome stainless steel sheets has increased, resulting in higher functionality, miniaturization, integration, higher precision, and simplification of processing processes for processed products in applications that use chrome stainless steel sheets. It is well known that the requirements for For this reason, in addition to the inherent corrosion resistance of No. 2 stainless steel and the above-mentioned characteristics of chrome stainless steel, the material of the chrome stainless steel plate requires even greater strength, workability, and precision. Therefore, it is a steel sheet material that has the contradictory characteristics of high strength and high ductility, and has excellent shape and thickness accuracy at the time of raw steel sheet.

The emergence of a chromium stainless steel sheet material that also has the characteristics of excellent shape accuracy after processing is strongly awaited.

[Conventional technology]

When looking at conventional chrome stainless steel plate materials from the viewpoint of strength, martensitic stainless steel is well known as a chrome stainless steel plate with high strength. For example, JIS G 4305 stipulates seven types of steel as martensitic stainless steel for cold rolled stainless steel sheets. These martensitic stainless steel sheets have a carbon content ranging from 0.08% or less (SUS410S) to 0.60 to 0.75% (SUS440A), and compared to ferritic stainless steels at the same -Cr content level, It contains high C and can be given high strength by hardening treatment or hardening and tempering treatment. for example,
In this JIS G 4305, 0.26 to 0.
980-1040℃ for 5US420J2 containing 40% C and 12.00-14.00% Cr
Hardness of HRC40 or higher can be obtained by quenching by quenching from 150 to 400℃ and then tempering by air cooling at 150 to 400℃.

and 0.60-0.75% C and 16.00
For 5O5440A containing ~18.00% Cr,
After quenching by rapid cooling from 1010 to 1070℃, 1
By air cooling tempering at 50-400℃, the same <HRC4
It has been shown that hardness of 0 or more can be obtained.

On the other hand, ferritic stainless steel sheets, which are chromium stainless steels, cannot be expected to harden very well through heat treatment, so the only way to increase their strength is by annealing and then cold treatment! J quality rolling is performed to increase strength through work hardening.

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 sheets, the structure after quenching or quenching and tempering is basically martensitic, as the name suggests.

Although it has very high strength and hardness, it has very low elongation, so it is difficult to process it after quenching or quenching and tempering. In particular, processing such as press forming is not possible after quenching or quench-tempering. Therefore, when processing is performed, it is performed before quenching or quenching and tempering. In other words, the material manufacturer says that it is in an annealed state, that is, JIS G 4.
As shown in Table 16 of 305, the product is shipped in a soft state with low strength and hardness, and after being processed by a processing manufacturer into a shape almost similar to the desired final product, it is subjected to quenching or quenching and tempering treatment. Normal. The surface oxide film (scale) produced by this quenching or quenching and tempering treatment is often undesirable for stainless steel, where surface beauty is important, and as a countermeasure, heat treatment in a vacuum or inert gas atmosphere is recommended. It is necessary to perform steps such as applying heat treatment or removing scale by polishing or the like after one heat treatment. In any case, in order to obtain high strength with martensitic stainless steel sheets, 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 sheet is increased by temper rolling, the elongation decreases significantly and the strength-ductility balance deteriorates.

This results in poor workability. The degree of increase in strength due to temper rolling is significantly higher in yield strength than in tensile strength.As the rolling rate increases, the difference between yield strength and tensile strength becomes smaller, resulting in a yield ratio (-proof strength/tensile strength). When the value of the material becomes close to 1, the plastic working range of the material becomes very narrow, and if the yield strength is high, the springback becomes large and the 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. In addition, since processing distortion due to rolling is larger closer to the surface of the plate, 1
In m-rolled materials, it is inevitable that the strain distribution in the thickness direction becomes non-uniform. This results in non-uniform distribution of residual stress in the plate thickness direction, which may cause changes in shape such as warping of the plate after punching or photo-etching, especially in ultra-thin steel plates. This is a big problem in the applications where it is needed.

In addition to the above-mentioned problems in terms of material properties, skin-pass rolled materials also have many problems in their manufacturability. First, regarding the control of strength, 1. Since temper rolling utilizes work hardening due to cold rolling, the rolling rate is the most important factor determining strength. Therefore, in order to obtain excellent plate thickness accuracy and the target strength level accurately and stably as a finished product, strict control of the rolling rate, specifically, strict control of the plate thickness during IIL rolling and grinding is required. In addition to being very important, it is necessary to control the strength level of the material before temper rolling.

In addition, in terms of shape control, there are at most two
Unlike skin pass rolling, which has a light rolling reduction of ~3%, temper rolling, which aims to increase strength, is essentially cold rolling with a rolling reduction of several tens of percent. It is difficult to obtain a tone belt with excellent properties. Therefore, for the purpose of shape correction, it may be necessary to heat the material to a temperature range lower than the recovery/recrystallization temperature range at which it does not soften, and to perform stress relief treatment. Like this 1! There are also many problems with manufacturability of rolled materials.

In addition to the problems caused by the above-mentioned temper rolling, ferritic stainless steel sheets also have the problem of ridging, which can be said to be an essential drawback. Rigging is normal.

It is a type of surface defect that occurs when a cold rolled annealed ferritic stainless steel plate is subjected to processing such as press forming.
Even after cold rolling, ridging, generally called cold rolling ridging, may occur.

This is still a big problem in applications where surface roughness is important.

[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 can be solved if it can be provided in the form of steel plates or steel 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, the steel composition was appropriately controlled, and the manufacturing conditions were as follows: after hot rolling and, if necessary, hot-rolled plate annealing, a plate rolling process or more with intermediate annealing in the ferrite single phase region. Cold rolling is performed to produce a cold-rolled steel strip of product thickness, and this cold-rolled steel strip is finished annealed at a temperature in the conventional ferrite single-phase range temperature, rather than the annealing treatment applied to steel plates or steel strip products. , if continuous finishing heat treatment is performed under specific conditions consisting of heating to the proper ferrite + austenite dual phase region and subsequent rapid cooling treatment,
It was possible to obtain a multi-phase structure in which a substantially soft ferrite phase and a hard martensitic phase were uniformly mixed, and to obtain an excellent result in which substantially all of the above-mentioned problems could be solved. However, the present invention is

In weight%.

c: o, os% or less.

Si: 2.0% or less.

Mn: 3.0% or less.

p: o, o4o% or less.

S: 0.030% or less.

Ni: 3.0% or less.

Cr: 10.0% or more and 14.0% or less.

N: 0.08% or less.

0: 0.02% or less.

Cu: 3.0% or less.

and, in some cases, further contain 0.20% or less of A.
f, B of 0.0050% or less, Mo of 1.0% or less
, 0, REM below 10%, Y below 0.20%
A steel containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities, and 0.01%≦C+N≦0.12% and 0.5%≦N
A process of manufacturing a steel slab that satisfies the relationship of i10 (Mn+Cu)/3≦3.0, and hot rolling it to manufacture a hot rolled steel strip.

A process of producing a cold-rolled steel strip of product thickness by two or more cold rollings sandwiching intermediate annealing in the ferrite single-phase region temperature heating;
and.

The obtained cold rolled steel strip was passed through a continuous VT heat treatment furnace.

A c + After holding within 10 minutes at the two-phase region temperature of ferrite ten austenite, which is intentionally below 1100°C, the average cooling rate is 1°C/sec from the maximum heating temperature to 100°C.
A continuous finishing heat treatment process in which finishing heat treatment is performed by cooling at a temperature above 500°C/see.

To provide a method for producing a chromium stainless steel band consisting of a highly ductile and high-strength double braided cord (structure substantially consisting of ferrite and martensite) with small in-plane anisotropy and a hardness of HV 200 or more. It is something.

According to the method of the present invention, not only can virtually all of the above-mentioned problems be solved, but also the strength can be freely and easily adjusted by controlling the 2w4 composition or the heating temperature and cooling rate during finishing heat treatment within the above ranges. In this sense, it provides an advantageous and new manufacturing technology for the industrial production of chrome stainless steel sheets or steel strip materials, and it is a method that can be used to manufacture martensitic stainless steel sheets, strips, and ferritic stainless steel sheets that have been conventionally shipped on the market. Another object of the present invention is to provide the market with a new chromium stainless steel material that has both ductility and strength properties that steel strips do not have, and has less in-plane anisotropy in ductility and strength. In addition.

According to the method of the present invention, the product that has undergone the final continuous finishing heat treatment process is industrially manufactured in the form of a steel strip, and when it is shipped to the market, it is either left as a steel strip (coil) or as a coil. It is shaped into a steel plate.

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. It was known that it would happen. However, in the production of ferritic stainless steel strips that produce austenite at Name (et al., Gaonchang), the heat treatment after cold rolling is strictly annealing treatment at a temperature in the ferrite single phase region, and it is difficult to produce martensite. It is common sense to avoid such high-temperature heat treatment as it causes deterioration in material properties such as a decrease in ductility, and has not been considered at all in the actual manufacture of industrial steel strips.

Therefore, assuming continuous heat treatment as in the present invention after the cold rolling process of chrome stainless steel, and performing finishing heat treatment to heat to the ferrite + austenite dual phase region, the relationship between heating temperature, strength and ductility, and the relationship between ductility and strength. There have been no detailed studies on anisotropy, etc. The present invention provides a completely new manufacturing method that has not been considered as an industrial manufacturing method for high-strength chromium stainless steel strips. As a result, a new chrome stainless steel sheet material with excellent properties not possessed by conventional chrome stainless steel sheets or belts is provided.

[Details of the invention]

Below, the reasons for limiting the range of chemical composition values of steel regulated by the present invention and the content of each manufacturing process carried out by the method of the present invention will be specifically explained in detail.

First, the reasons for limiting the content range of the components of chromium stainless steel (41%) to which the method of the present invention is applied are as follows.

This is because C and N are stronger and cheaper austenite-forming elements than N i + M n + Cu and the like, and are also elements with a large ability to strengthen martensite.

It is an effective element for controlling strength and enriching carbonization after continuous finishing heat treatment. Therefore, after the continuous finishing heat treatment process, 20
Multi-phase structure containing % or more of martensite Hv200
To obtain sufficient strength, use Ni.

Even if austenite forming elements such as Mn and Cu are added, at least 0.01% or more of (C+N)l is required. However, if the amounts of C and N are too high, a large amount of martensite fit will be generated after the continuous finishing heat treatment process, and in some cases it will become 100% martensite, and the hardness of the martensite phase itself will also become very high, making it difficult to achieve high strength. However, the ductility decreases. Therefore, the amount of (C+N) should be 0.12% or less, and 0.0
It is necessary to satisfy the relationship 1%≦C+N≦0.12%.

Furthermore, if a large amount of C is added, Cr carbides may precipitate at grain boundaries during cooling during continuous finishing heat treatment, resulting in a decrease in corrosion resistance. Therefore, the amount of C is set to be less than O,OS%.

Furthermore, it is difficult to add a large amount of N due to its solubility, and addition of a large amount leads to an increase in surface defects (therefore, it is limited to 0.08% or less.

Si is a ferrite-forming element and has a strong solid solution strengthening ability for both ferrite and martensite phases. Therefore, it is an effective element for controlling the f111 amount of martensite and the strength level. However, addition of a large amount leads to deterioration of hot workability and cold workability, so the upper limit is set at 2.0%.

Mn, Ni, and Cu are austenite-forming elements.

It is an effective element for controlling the amount of martensite and strength after continuous finishing heat treatment. Also, M n + N i + C
By adding u, the amount of C added can be reduced, improving ductility as soft martensite, and adding C to grain boundaries.
"It is possible to suppress the precipitation of carbides and prevent deterioration of corrosion resistance. Furthermore, the important effects of M n + N i + Cu are shown in the test results (for example, the relationship in Figure 1) and examples below. One is that in the continuous finishing heat treatment process according to the present invention, by adding Mn, Ni+Cu, a stable region with small hardness fluctuations can be obtained from a lower temperature side and over a wide temperature range.

This provides great advantages in actual operation, both in terms of high-temperature strength necessary for continuous finishing heat treatment and in terms of energy savings. Therefore, the addition of Mn, Ni+Cu not only contributes to the production of multi-phase steel strips with stable strength properties, but also enables heat treatment at low temperatures with higher high-temperature strength, which improves in-furnace heat treatment during continuous finishing heat treatment. In order to achieve this effect, M
n, N i + Cu in its total amount at least 0.5%
However, Ni has the greatest influence on the increase in hardness of the multiphase structure material after continuous finishing heat treatment, and Mn and C
u is approximately the process degree of 3 minutes of Ni. therefore,
When determining the amounts of Mn, Ni, and Cu added, Ni
+ (Mn +Cu) / 3 is regulated using the relational expression,
At least 0.0 as Ni+ (Mn +Cu)/3.
Add 5% or more. However, if a large amount is added, the product becomes expensive, which affects the economic efficiency, which is one of the characteristics of the steel strip of the present invention. Therefore, each of Mn+Ni+Cu alone should be 3.0% or less, and Ni+ (Mn+Cu
)/3 is also 3.0% or less.

If S is too high, it will adversely affect corrosion resistance and hot workability, so it is preferably lower, and the upper limit is 0.030%.

P is an element that has a large solid solution strengthening ability, but since adding a large amount may lead to a decrease in toughness, it is limited to 0.040% or less, which is the normally allowed level.

Although at least 10.0% of Cr should be contained as the minimum necessary amount in order to maintain the corrosion resistance of stainless steel, if the Cr content is too high.

The upper limit is set at 14.0% because the amount of austenite-forming elements required to generate martensitic phase and obtain high strength increases and the product becomes expensive.

◯ forms oxide-based nonmetallic inclusions and reduces the cleanliness of the steel, so a lower value is desirable, and the content should be 0.02% or less.

AA is an effective element for deoxidizing and has the effect of significantly reducing ^2 inclusions that adversely affect press workability. However, if the content exceeds 0.20%, the effect not only becomes saturated, but also brings about adverse effects such as an increase in surface defects, so the upper limit is set at 0.20%.

B is an effective component for improving toughness, but its effect is brought about in a very small amount, and its effect is saturated when it exceeds 0.0050%, so its upper limit is set to o, ooso%.

Mo is an element effective in improving corrosion resistance.

If added in large amounts, the product becomes expensive, so the upper limit is set at 1.0%.

REM and Y are elements effective in improving hot workability. It is also an effective element for improving oxidation resistance. The method of the present invention, which performs continuous finishing heat treatment at high temperatures, is effective in suppressing the generation of oxidized scale and obtaining a good surface texture after descaling. However, these effects
M saturates when it exceeds 0.10% and Y exceeds 0.20%, so the upper limit is 0.10% for REV and 0.10% for Y.
is 0.20%.

Next, the contents of each manufacturing process of the multi-phase & Il texture belt employed in the present invention will be explained.

In the method of the present invention, a slab of chromium stainless steel whose composition range has been adjusted as described above is produced by ordinary steel casting technology, and this slab is hot rolled to produce a hot rolled steel strip. After hot rolling, it is preferable to perform hot rolled sheet annealing and descaling. Although it is not necessary to carry out hot-rolled sheet annealing, this annealing softens the hot-rolled steel strip and improves its cold-rollability, and also removes the transformed phase remaining in the hot-rolled steel strip (austenitic phase at high temperatures). part) into ferrite + carbide.
Since it can be decomposed, it is desirable to obtain a toning zone with a uniform multi-phase texture after cold rolling and continuous finishing heat treatment.This hot rolled sheet annealing can be either continuous annealing or box annealing. The descaling process can be carried out by ordinary pickling.The conventional chrome stainless steel strip manufacturing technology is applied to the method of the present invention as is for the slab manufacturing process, 28-hour rolling process, hot-rolled plate annealing process, and descaling process. can do.

Next, a dual-phase steel strip is manufactured through a cold rolling process and a continuous finishing heat treatment process, and these processes are characteristic of the method of the present invention and will be explained in detail.

"Cold rolling process" In the cold rolling process, a hot rolled steel strip (hot rolled steel strip after hot rolled sheet tA annealing) is cold rolled two or more times with intermediate annealing of ferrite single phase region temperature heating. This is the process of rolling the material to the thickness of the plate. This intermediate annealing sandwiched between cold rolling plays an important role in reducing the in-plane anisotropy of the ductility of the multiphase steel strip after the continuous finishing heat treatment process, based on representative test results. I will explain.

111A, B and C steels having the chemical composition shown in Table 1 were melted and hot-rolled under normal conditions to form a hot-rolled plate with a thickness of 3.61 mm, heated at 780°C for 6 hours, and then cooled in the furnace. After annealing, pickling was performed. Steels B and C are steels targeted by the present invention, but steel A has M'n +Nj, Co11
is defined in the present invention as Nj+(Mn+Cu)/3≧0.5
This steel is outside the scope of the present invention in that it does not satisfy the above requirements.

Tests were conducted using this hot-rolled sheet under different cold rolling conditions and finishing heat treatment conditions (the data in Figures 1 and 2 also show the results of this test, the details of which will be explained later). do).

Table 2 below is for 1m1B in Table 1.

(a), a multi-phase structure material (hereinafter referred to as ZC
(referred to as R material).

(b), a multi-phase structure material (hereinafter referred to as IC
(referred to as R material).

A temper-rolled material that has the same strength as ic+, xcR and ZCR materials through cold rolling.

The tensile strength of each steel plate manufactured by the following three methods is kgf.
/mm") and elongation (χ) in the rolling direction (L), the value in the 45' direction (D), and the value in the 90' direction (T) with respect to the rolling direction. .

The 2CR material in (a) is obtained by cold rolling the above-mentioned hot-rolled plate to a thickness of III*, then performing intermediate annealing at 750°C for 1 minute and air cooling, and then cold rolling to a thickness of 0. .3
It was made into a cold rolled plate of mm. This cold-rolled plate was soaked for 1 minute at a temperature of 960°C, and then subjected to finishing heat treatment in which it was cooled from that temperature to 100°C at an average cooling rate of 20°C/sec.

In addition, TBL's ICR material is produced by cold-rolling the hot-rolled sheet described above to a thickness of 0.3 mm without performing intermediate annealing, and soaking this cold-rolled sheet at a temperature of 960°C for 1 minute. Finishing heat treatment was performed by cooling from temperature to 100°C at an average cooling rate of 20°C/sec.

(For cl temper rolled materials, ICR materials and zCR materials
The annealed hot rolled plate was cold rolled to a predetermined thickness, annealed, and then temper rolled at a predetermined rolling rate so that the same strength as that of the steel was obtained at a plate thickness of 0.3 mm.

Table 1, Table 2 (C), Adjusted page rolling, rolling rate: 73% As is clear from Table 2, the elongation of the multi-phase structure material for both ZCR and ICR materials is equivalent to hardness and strength. It can be seen that this is significantly superior to the level temper rolled material, and has an excellent two-strength-elongation balance. In addition, looking at the two-plane anisotropy, in terms of tensile strength, both the ZCR material and the ICR material have a small difference in tensile strength depending on the direction, that is, the in-plane anisotropy, whereas the The difference between the tensile strength in the T direction, which has the lowest tensile strength, and the T direction, which has the highest tensile strength, is 10 kg.
It has a large in-plane anisotropy.

Regarding elongation, multi-phase mm material with high elongation also has relatively smaller in-plane anisotropy than temper rolled material with low elongation.

In particular, it can be seen that the ZCR material has even smaller in-plane anisotropy than the ICR material. That is, it can be said that intermediate annealing is very important in reducing the in-plane anisotropy of elongation of the multi-phase &Il woven material. Therefore, from the results in Table 2, when finishing heat treatment is applied to form a multi-phase Ml weave through hot rolling, hot-rolled sheet annealing, and cold rolling with intermediate annealing, the weave has excellent ductility and strength. It is clear that a high-strength chromium stainless steel sheet with a dual-phase structure with small ductile in-plane anisotropy can be obtained.

As seen in this test result, and as shown in the examples below, the in-plane anisotropy of elongation of the multiphase structure material can be improved by cold rolling two or more times with intermediate annealing in between. It can be made smaller by implementing Therefore, in manufacturing a dual-phase steel strip with small in-plane ductility anisotropy, it is necessary to reduce the thickness to the product thickness by cold rolling two or more times, and perform intermediate annealing in between. This is important in the method of the present invention. The heating temperature for this intermediate annealing is the ferrite single phase region temperature, that is, the temperature below the Ac+ point. Further, the cold rolling ratios of the cold rolling before and after the intermediate annealing are preferably at least 30% or more.

"Continuous finishing heat treatment process" The cold rolled steel strip with the product thickness obtained in the cold rolling process is then passed through a continuous heat treatment furnace to achieve +A c 1 or more points of 1100.
1 for the two-phase region temperature of ferrite + austenite below ℃
After holding for less than 0 minutes, the average cooling rate from the maximum heating temperature to 100℃ is 1'C/see or more, 500℃/se
Continuous finishing heat treatment is performed by cooling at temperatures below c.

This continuous finishing heat treatment step is the most characteristic step of the method of the present invention, and the conditions for this continuous finishing heat treatment have an important meaning in the present invention, as will be shown in Examples below. An overview of the reasons for regulating the heating conditions and cooling conditions in this continuous finishing heat treatment step is as follows.

It is an absolute condition that the heating temperature during continuous finishing heat treatment is within the ferrite + austenite two-phase region temperature.In carrying out the method of the present invention, it is necessary to Ac
In the vicinity of + point temperature), austenite I fluctuates greatly with respect to temperature changes, and stable hardness may not be obtained after rapid cooling. However, in the steel composition range targeted by the present invention, it has been found that such hardness fluctuations do not substantially stop when heated to a high temperature range of 100'C or higher from the Ac+ point. Therefore, the heating temperature during the continuous finishing heat treatment is preferably set to the AcI point +100° C. or higher. More specifically, 850°C or higher, more preferably 900°C
It is better to set it to the above. On the other hand, the upper limit of the heating temperature is preferably set at 1100° C., since if the temperature is too high, the increase in strength will not only be saturated, but also decrease in some cases, and also be disadvantageous in terms of manufacturing cost.

The metallurgical significance of heating in the ferrite + austenite two-phase region during continuous finishing heat treatment in the method of the present invention is as follows: ■CrCr
Three points can be mentioned: carbide-nitride dissolution, (1) generation of austenite phase, and (2) concentration of C and N in the generated austenite.

In the case of the chromium stainless steel strip targeted by the method of the present invention, both of these IMs reach an almost equilibrium state within a short period of time, so 1. The heating time may be short, about 10 minutes or less. The fact that this short heating time is sufficient is very advantageous in terms of production efficiency and manufacturing cost in terms of actual operation of the method of the present invention. Due to the above heating conditions and holding time, the amount of martensite produced by subsequent cooling is 2
It is possible to generate the austenite necessary to achieve 0% by volume or more.

Regarding the cooling rate during finishing heat treatment, in order to obtain a multi-phase structure of martensitic phase and soft ferrite phase, it is necessary to set the cooling rate to 1°C 8 to 0 or more, but it is 500°C/s.
It is virtually difficult to obtain cooling rates exceeding ee. Therefore, in one aspect of 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. This cooling rate is the average cooling rate from the maximum heating temperature to 100℃, but it is not necessary to use this cooling rate in the cooling process after austenite has transformed into martensite. The end point temperature is sufficient for the austenite produced at high temperature under the above-mentioned heating conditions to transform into martensite. As a cooling method, a forced cooling method in which a gas and/or liquid cooling medium is sprayed onto the belt, a roll cooling method using water-cooled rolls, etc. can be applied. Continuous heating and cooling under such conditions can be carried out using a continuous heat treatment furnace having a heating soaking zone and a quenching zone between the coil unwinding machine and the winding machine.

FIG. 1 shows hot-rolled sheets (hot-rolled sheets after hot-rolled sheet annealing and pickling) manufactured by the method already described for each steel in Table 1 above, which are cold-rolled to a thickness of 1+w+w, with a thickness of 750 mm. ℃×
After performing intermediate annealing by heating and air cooling for 1 minute, the cold rolled plate was further cold rolled to a thickness of 0.3 mm, and this cold rolled plate was heated at each temperature between 800 and 1100°C for 1 minute. Amount of martensite (volume %) in finished heat-treated material obtained when finishing heat treatment is performed by soaking and then cooling from that temperature to 100 °C at an average cooling rate of 20 °C 7 seconds
and hardness IV).

This is shown in terms of the heating temperature during finishing heat treatment (A, B, and C in the figure represent each steel in Table 1).

As is clear from Fig. 1, when the heating temperature reaches the ferrite + austenite dual phase region, martensite appears after the final heat treatment, and the amount of martensite increases as the heating temperature increases, but for Steels B and C, 850 ~9
When the temperature exceeds 00°C, the degree of increase decreases and the temperature tends to gradually become saturated. The behavior of hardness also shows a similar tendency in response to changes in the amount of martensite, and the more martensite i, the higher the hardness, whereas + Pvl n
+N i+ Steel A, in which the amount of Cu is less than the specification of the present invention, has a temperature range in which the amount of martensite and hardness saturate is on the high temperature side and is narrow. It has an important meaning when performing on the line. In other words, a certain degree of temperature fluctuation is unavoidable in a continuous heat treatment line, and in particular, fluctuations in the length direction of the strip, and differences in heat treatment temperature due to differences in threading opportunities even if the target temperature is the same, will occur on the actual line. During operation, it is necessary to allow for fluctuations of about ±20°C with respect to the target temperature.

Figure 1 is.

It was shown that if the cooling rate is kept almost constant and a heat treatment temperature range with small hardness fluctuations is adopted, it is possible to produce copper strips with small fluctuations in hardness, that is, strength, even if there are slight temperature fluctuations in the continuous heat treatment line. ing. and,
In particular, by adding appropriate amounts of Mn, Ni, and Cu, a finishing heat treatment temperature range with small hardness fluctuations can be obtained at a low temperature side and over a wide range, which is even more advantageous. By controlling the intensity level by controlling the components as described above, the target intensity can be stably obtained.
Small strength variation over the entire length of the steel strip.

Furthermore, a high-strength material with small strength differences between steel strips can be easily and inexpensively produced using an existing continuous heat treatment line.

Figure 2 shows the correlation between hardness and elongation (weighted average value in three directions) of several multiphase materials with different amounts of martensite made within the range of steel composition and manufacturing conditions regulated by the present invention. this! It is shown in comparison with Pi-based rolled material. Note that the production of the multi-phase structure material is the same as that explained in FIG. 1, and the heating temperature in the finishing heat treatment is 900° C. or higher.

Further, the temper-rolled material is obtained by annealing after cold rolling, and then changing the hardness by changing the temper rolling rate indicated by the subscript in the figure.

As is clear from Figure 2,! J1 quality rolled material! As the hardness increases with the increase in quality rolling rate, the elongation decreases rapidly. On the other hand, even if the hardness of the multiphase structure material increases, the elongation decreases slowly. In particular, the elongation of the multi-phase structure material is superior to that of the temper-rolled material, in areas with high shear hardness, specifically H
This becomes noticeable in the region of ν200 or more. In other words, the increase in ductility achieved by forming a material with a multi-phase structure is even more pronounced in the Hv 200 or higher region, and for this purpose, as can be seen from Figure 1 above, martensite with a content of about 20% by volume or more is required. It's about quantity. In this way, the hardness is H
A feature of the multi-phase structure material produced by the present invention method is that it can achieve high ductility at v200 or higher, which cannot be achieved with 11 quality rolled material.
Due to this good strength-elongation balance, the multi-phase mm steel strip obtained by the method of the present invention also has a good workability such as press formability. It has characteristics that cannot be obtained by rolling.

FIG. 3 is a photograph of the metallographic structure of IIB shown in Table 1 produced by the method ta+ shown in Table 2. The white area in the photo is ferrite, and the black or gray area is martensite. As can be seen from this photo, this material has a multi-phase structure in which fine ferrite and martensite are evenly mixed.

As explained above, the high ductility and high strength belt material with low strength and ductility anisotropy was obtained by hot rolling.
After hot-rolled plate annealing and two or more cold rollings with intermediate annealing in between, finishing heat treatment involves heating to a two-phase region of ferrite-on-austenite and rapidly cooling, resulting in fine ferrite and martensite that is transformed from austenite by rapid cooling. This was achieved by creating a multi-phase structure with a uniform mixture of In other words, the hard martensite provides strength (hardness), the soft ferrite provides ductility, and the in-plane anisotropy of strength and ductility is achieved by finely and uniformly mixing both phases. It could have been made smaller. Note that a trace amount of retained austenite may be detected in X-ray examination of the structure after finishing heat treatment.

Two examples in which the method of the present invention was implemented are listed below.

The characteristics of a multi-phase steel strip obtained by the method of the present invention will be specifically shown in comparison with a comparative example.

Example Slabs were manufactured by melting steel having the chemical components shown in Table 3. After hot rolling to a thickness of 3.611+1, hot rolled sheets were annealed at 780°C for 6 hours in a furnace, and after pickling, the sheets were cold rolled under the cold rolling conditions shown in Table 4. Thickness 0.
A cold-rolled steel strip of 3+am+ was subjected to continuous finish heat treatment in a continuous heat treatment furnace under the finish heat treatment conditions shown in Table 4. The soaking time for both cold rolling and intermediate annealing in the process is 1 minute, and the soaking time for the continuous finishing heat treatment step is also 1 minute. The material properties of the steel strip after finishing heat treatment are also listed in Table 4.

As is clear from Table 4, according to the method of the present invention, multiphase steel strips having high tensile strength, hardness, and good elongation were obtained. Furthermore, it is clear that the steel strip produced by the method of the present invention has small anisotropy in 0.2% proof stress, tensile strength, and elongation, and no ridging was observed in the tensile test piece after fracture.

On the other hand, in Comparative Example 1, the manufacturing conditions are within the range specified by the present invention, but the M n + N i + Cu il of the steel
l is defined in the present invention as N++(Mn+Cu)/3≧0.
Since it is a steel in column 8 of Table 3, which is low at 0.19%, which is outside the requirement of 5%, no martensite is generated after continuous finishing heat treatment, and the hardness is low.

In comparison No. 2, the manufacturing conditions are still within the scope of the present invention, but the C content and Ni content of the steel are 0.3% or less and higher than the 0.08% or less and 3.0% or less stipulated in the present invention, respectively.
Since this steel contains 10% and 3.20% of C and Ni, the amount of martensite after continuous finishing heat treatment is 100% and the strength is high, but the elongation is low.

For comparison, the heating temperature for continuous finishing heat treatment on floor 3 is 780℃.
At this heating temperature, the steel of Steel 1 does not enter the ferrite + austenite dual phase region, so the metal structure after finishing heat treatment is a ferrite single phase structure without martensite, and although the elongation is high, the strength and hardness are low. is low.

In Comparison Level 4, the final heat treatment is performed in a box furnace.

Since the cooling is also by furnace cooling, the cooling rate is 0.03℃/se
Since the c is very low, no martensite is generated after heat treatment, and like Comparative Example 3, the elongation is high but the strength and hardness are low.

Comparative example Fh5 is a temper-rolled material and has significantly lower elongation than that of the present invention, and has a tensile strength of 0 and 2.
The ratio of % proof stress, that is, the yield ratio is high, and the anisotropy of 0.2% proof stress, tensile strength, and elongation is large. Therefore, the workability and shape after processing are higher than that of the steel strip obtained by the method of the present invention. It is clear that they are inferior in gender.

Comparative No. 6 has high strength and excellent elongation because no intermediate annealing was performed during cold rolling before continuous finishing heat treatment, but the in-plane anisotropy of elongation is lower than that of the present invention example subjected to intermediate annealing. It's bigger compared to the others.

Regarding the copper strips of Comparative Examples 11hl, 3.4, and 5, the occurrence of ridging was observed in the tensile test specimens after fracture, whereas the occurrence of ridging was observed in the two inventive steel strips. It can be seen that processing such as press molding can be performed satisfactorily.

As described above, according to the method of the present invention, a dual-phase steel strip having both high ductility and high strength, small in-plane anisotropy of strength and ductility, low proof stress, and low yield ratio is provided. In the field of chrome stainless steel sheets, there has never been an example of such a high strength material with Hv 200 or more combined with good workability being shipped to the market in the form of a steel plate or steel strip. Therefore, the present invention provides a new material steel sheet or copper strip for the conventional chrome stainless steel sheet field. The material according to the present invention is particularly useful as a high-strength material that requires workability into electronic parts, precision mechanical parts, etc. are currently in use, and much work could be done in this field.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between heating temperature, amount of martensite, and hardness in finishing heat treatment according to the present invention. FIG. 2 is a diagram showing the correlation between hardness and elongation of the finish heat-treated material according to the present invention and the tPI award-rolled material. FIG. 3 is a micrograph showing the metallographic structure of a chromium stainless steel strip subjected to continuous finishing heat treatment according to the present invention.

Claims (1)

[Claims] (1) In weight%, C: 0.08% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.030. % or less, Ni: 3.0% or less, Cr: 10.0 or more and 14.0% or less, N: 0.08% or less, O: 0.02% or less, Cu: 3.0% or less. , the balance is Fe and unavoidable impurities, and the steel satisfies the relationships of 0.01%≦C+N≦0.12% and 0.5%≦Ni+(Mn+Cu)/3≦3.0. A process of producing a slab and hot rolling it to produce a hot-rolled steel strip. A process of producing a cold-rolled steel strip of the product thickness by cold rolling two or more times with intermediate annealing in the ferrite single-phase region temperature heating. The process of
Then, the obtained cold rolled steel strip was passed through a continuous heat treatment furnace to obtain Ac_1
After maintaining the ferrite + austenite two-phase region temperature of 1100°C or higher for less than 10 minutes, the average cooling rate from the maximum heating temperature to 100°C is 1°C/sec or more and 500°C.
A method for producing a highly ductile, high-strength, multi-phase structure chromium stainless steel strip with a hardness of HV200 or more and a small in-plane anisotropy, the method comprising: a continuous finishing heat treatment process in which finishing heat treatment is performed by cooling at a temperature of 0.degree. C./sec or less. (2) The heating temperature in the continuous finishing heat treatment process is Ac_1
The manufacturing method according to claim 1, wherein the temperature is +100°C or more and 1100°C or less. (3) The heating temperature in the continuous finishing heat treatment process is 850℃
The manufacturing method according to claim 1, wherein the temperature is above 1100°C. (4) In weight%, C: 0.08% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 3 .00% or less, Cr: 10.0 or more and 14.0% or less, N: 0.08% or less, O: 0.02% or less, Cu: 3.0% or less, and 0.20% or less Al, 0.0050% or less B, 1.0% or less Mo, 0.10% or less REM, 0
．． A steel containing 20% or less of one or more types of Y, with the remainder consisting of Fe and unavoidable impurities, and 0.01%≦C+N≦0.12% and 0.5%≦Ni+(Mn+Cu )/3≦3.0, and hot rolling the steel slab to produce a hot rolled steel strip. A process of manufacturing cold rolled steel strip of product thickness by cold rolling,
Then, the obtained cold rolled steel strip was passed through a continuous heat treatment furnace to obtain Ac_1
After maintaining the ferrite + austenite two-phase region temperature of 1100°C or higher for less than 10 minutes, the average cooling rate from the maximum heating temperature to 100°C is 1°C/sec or more and 500°C.
A method for producing a highly ductile, high-strength, multi-phase structure chromium stainless steel strip with a hardness of HV200 or more and a small in-plane anisotropy, the method comprising: a continuous finishing heat treatment process in which finishing heat treatment is performed by cooling at a temperature of 0.degree. C./sec or less. (5) The heating temperature in the continuous finishing heat treatment process is Ac_1
The manufacturing method according to claim 4, wherein the temperature is +100°C or more and 1100°C or less. (6) The heating temperature in the continuous finishing heat treatment process is 850℃
The manufacturing method according to claim 4, wherein the temperature is above 1100°C.