JPH07100823B2 - Manufacturing method of high ductility and high strength dual phase structure chromium stainless steel strip with small in-plane anisotropy. - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel industrial production method of a high-strength dual-phase chromium stainless steel strip having excellent ductility and strength and ductility with small in-plane anisotropy, It is intended to provide a method for producing a high-strength and high-ductility stainless steel strip as a forming material which requires high strength and is subjected to processing such as press forming.

[Background of this field]

The chromium stainless steel containing chromium as a main alloy component includes martensitic stainless steel and ferrite stainless steel. All of them are less expensive than austenitic stainless steels containing chromium and nickel as main alloying components, and have characteristics not found in austenitic stainless steels in terms of physical properties such as ferromagnetism and small thermal expansion coefficient. As a result, there are many applications that are limited to chrome stainless steel not only for economic reasons but also for their characteristics. Especially in the fields of electronic devices and precision machine parts in recent years, the demand for chrome stainless steel sheets has increased, and the processing products have become highly functional, compact, integrated, highly accurate, and simplified. It is well known that the demands for the are becoming more and more severe. For this reason, in addition to the original corrosion resistance of stainless steel and the above-mentioned properties of chrome stainless steel, further strength, workability and accuracy are required in terms of the material surface of chrome stainless steel. Therefore, it is a steel sheet material that has the contradictory characteristics of high strength and high ductility, and has excellent characteristics such as the shape and thickness accuracy at the time of the material steel sheet, and the shape accuracy after processing. The emergence of materials is awaited strongly.

[Conventional technology]

Regarding the conventional chromium stainless steel sheet material, from the viewpoint of strength, first, martensitic stainless steel is well known as a chromium stainless steel sheet having high strength. For example, JIS G 4305 cold-rolled stainless steel sheet defines seven types of steel as martensitic stainless steel. These martensitic stainless steel sheets are C
Is 0.08% or less (SUS410S) to 0.60 to 0.75% (SUS440A)
Therefore, when viewed at the same Cr content level as compared to ferritic stainless steel, it contains high C and can be given high strength by quenching treatment or quenching and tempering treatment. For example, in JIS G 4305, SUS420J2 containing 0.26 to 0.40% C and 12.00 to 14.00% Cr,
After quenching by quenching from 980 to 1040 ℃, tempering with air cooling from 150 to 400 ℃ can obtain hardness of HRC 40 or higher.
For SUS440A containing 0.60 to 0.75% C and 16.00 to 18.00% Cr, after quenching by quenching from 1010 to 1070 ° C and then tempering at 150 to 400 ° C in air, the same HRC40
It has been shown that the above hardness can be obtained.

On the other hand, in ferritic stainless steel sheets, which are chrome stainless steels, hardening due to heat treatment cannot be expected so much. Therefore, as a method of increasing strength, after annealing, temper rolling is further performed to increase strength by work hardening. Is being done. 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 martensitic stainless steel sheet, the structure after quenching or quenching-tempering is basically a martensitic structure as its name suggests, and although very high strength and hardness can be obtained, the elongation is very low. Therefore, the subsequent processing becomes difficult if the quenching or quenching and tempering treatment is performed. In particular, processing such as press molding is impossible after quenching or quenching and tempering. Therefore, when it is processed, it is applied before quenching or quenching and tempering. That is, the material manufacturer shipped the product in an annealed state, that is, in a soft state with low strength and hardness as shown in Table 16 of JIS G 4305, and the shape close to the final product desired by the processing manufacturer. After being processed, it is usual to carry out quenching or quenching and tempering. The surface oxide film (scale) generated by this quenching or quenching and tempering treatment is often not preferable in stainless steel where surface beauty is important, and as a countermeasure, heat treatment in a vacuum or an inert gas atmosphere It is necessary to perform steps such as applying heat treatment and removing scale by polishing after heat treatment. In any case, in order to obtain high strength with martensitic stainless steel sheets, the heat treatment process at the processing manufacturer is indispensable, which increases the burden on the processing manufacturer. Therefore, the cost increase of the final product is avoided. There was a problem that I could not.

On the other hand, when the strength of a ferritic stainless steel sheet 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 becomes small, the yield ratio (= yield strength / tensile strength) approaches 1, and the plastic working area of the material becomes extremely narrow and the yield strength is high. And the spring back becomes large and the formability after pressing etc. deteriorates.
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 in terms of material properties, temper rolled materials have many problems in terms of manufacturability. Looking first at the strength control, temper rolling is the most important factor that determines strength, because it uses work hardening by cold rolling. Therefore, strict control of the rolling rate, specifically, strict control of the initial plate thickness before temper rolling is extremely important for obtaining a product with excellent plate thickness accuracy and stable and accurate target strength level. It is important to control the strength level of the material before temper rolling. In terms of shape control, at most 2-3 for the purpose of shape correction called so-called skin pass rolling or temper rolling.
Unlike temper rolling with a light rolling ratio of 10%, temper rolling for the purpose of high strength is a substantial cold rolling with a rolling ratio of several tens of percent, so that the shape property can be maintained as cold rolled. It is difficult to obtain a good steel strip. Therefore, in order to correct the shape, the stress relieving process may be required by heating the material to a temperature range lower than the material recovery / recrystallization temperature range and not softening. As described above, the temper rolled material has various problems in terms of manufacturability.

In addition to the above problems caused by temper rolling, ferritic stainless steel sheets have a problem of ridging, which can be said to be an essential defect. Ridging is usually a type of surface defect that occurs when cold-rolled and annealed ferritic stainless steel plates are subjected to processing such as press forming. However, when ridging that is commonly called cold-rolled ridging occurs even after cold rolling However, this is still a major problem in applications where surface roughness 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 conducted extensive research on chromium stainless steel from both sides in terms of chemical composition and manufacturing conditions for the purpose of solving this problem. As a result, the steel components were properly controlled, and as a manufacturing condition, after hot rolling, and if necessary, hot-rolled sheet annealing was performed, and then intermediate annealing in the ferrite single-phase region was performed twice or more. Cold-rolled steel strip of the product thickness is manufactured by cold rolling of
Rather than the conventional finish annealing at the temperature of the ferrite single-phase region, that is, the annealing treatment applied to the steel plate or strip product, continuous heating under the appropriate conditions including heating to the appropriate ferrite + austenite two-phase region and subsequent quenching. If finishing heat treatment is applied, it is possible to form a multi-phase structure in which a substantially soft ferrite phase and a hard martensite phase are uniformly mixed, and it is possible to solve virtually all of the above-mentioned problems, which is an excellent result. I was able to.

Thus, the present invention, in wt%, C: 0.10% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 4.0% or less, Cr: 14.0 or more 20.0 % Or less, N: 0.12% or less, O: 0.02% or less, Cu: 4.0% or less, and in some cases, 0.20% or less of Al, 0.0
B less than 050%, Mo less than 1.0%, REM less than 0.10%, 0.20%
A steel containing one or more of the following Ys and the balance being Fe and inevitable impurities, and having a relationship of 0.01% ≦ C + N ≦ 0.20% and 0.5% ≦ Ni + (Mn + Cu) /3≦5.0. The process of producing a slab of satisfying steel and hot rolling it to produce a hot rolled steel strip. Cold rolling of the product sheet thickness by two or more cold rolling processes with intermediate annealing of ferrite single phase temperature heating. Steel strip manufacturing process,
Then, the obtained cold-rolled steel strip is passed through a continuous heat treatment furnace, and the two-phase region temperature of ferrite + austenite of Ac ₁ point or more and 1100 ° C or less is held for 10 minutes or less, and then 100 ° C from the maximum heating temperature. It has a continuous finishing heat treatment step of performing finishing heat treatment of cooling at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less, and has a hardness of HV200 or more and a small in-plane anisotropy and high ductility and high strength. It is intended to provide a method for producing a chromium stainless steel strip having a multiphase structure (structure substantially consisting of ferrite and martensite).

According to the method of the present invention, not only substantially all of the above 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 finishing heat treatment within the above range. It provides an advantageous and new manufacturing technology for the industrial structure of chrome stainless steel plate or steel strip material in that it can be manufactured, and it is a martensitic stainless steel plate or steel strip or ferritic stainless steel plate that has been shipped to the market. Alternatively, the present invention provides the market with a novel chromium stainless steel material having both ductility and strength, which steel strips do not have, and having low in-plane anisotropy of ductility and strength. According to the method of the present invention, the product which has undergone the final continuous finishing heat treatment step is industrially manufactured in the form of a steel strip, and when it is shipped to the market, the steel strip remains a coil (coil). Alternatively, it is in a state of being shaped into a steel plate.

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 and martensite. The thing itself was known. However, in the production of ferritic stainless steel strips that produce austenite at high temperatures, the heat treatment after cold rolling is merely an annealing treatment at the ferrite single-phase region temperature, and high-temperature heat treatment that produces martensite is not possible. It is common sense to avoid this as causing deterioration of the material such as deterioration of ductility, and it was never neglected in the actual manufacturing of industrial steel strip.

Therefore, the relationship between the heating temperature and the strength and the ductility, the ductility and the strength of the ductility and the strength in the case where the continuous heat treatment as in the present invention is assumed after the cold rolling process of the chromium stainless steel and the finishing heat treatment for heating to the ferrite + austenite two-phase region is performed. There are no examples of detailed research on anisotropy. INDUSTRIAL APPLICABILITY The present invention provides a completely new manufacturing method which has never been neglected as an industrial manufacturing method of high strength chromium stainless steel strip, and as a result, does not have the conventional chromium stainless steel sheet or steel strip. It is intended to provide a novel chrome stainless steel plate material having excellent properties.

[Detailed Description of the Invention]

The reason for limiting the range of the chemical composition value of steel controlled by the present invention and the content of each manufacturing process adopted in the method of the present invention will be specifically described below.

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

C and N are stronger and cheaper austenite-forming elements than Ni, Mn, Cu, etc., and have a large martensite strengthening ability, so they are effective in controlling strength and increasing strength after continuous finishing heat treatment. It is an element. Therefore,
After the continuous finishing heat treatment process, in order to obtain a multiphase structure containing 20% or more of martensite and a sufficient strength of Hv200 or more, Ni, M
Even if an austenite-forming element such as n or Cu is added, the (C + N) content must be at least 0.01% or more. However, if the C and N contents are too high, the amount of martensite formed after the continuous finishing heat treatment step increases, and in some cases, the martensite phase becomes 100% and the hardness of the martensite phase itself becomes very high. However, the ductility decreases. Therefore, it is necessary to set the amount of (C + N) to 0.20% or less and satisfy the relationship of 0.01% ≦ C + N ≦ 0.20%.

Further, if a large amount of C is added, Cr carbide may be precipitated in the grain boundaries during cooling in the continuous heat treatment for finishing, and the corrosion resistance may be deteriorated. Therefore, the C content should be 0.10% or less.

Further, it is difficult to add a large amount of N due to its solubility, and addition of a large amount causes an increase in surface defects.
12% or less.

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

Mn, Ni, and Cu are austenite-forming elements, and are elements effective in controlling the amount of martensite and the strength after continuous finishing heat treatment. Further, the addition amount of Mn, Ni, Cu can reduce the amount of C added, which can improve ductility as soft martensite and suppress the precipitation of Cr carbides at grain boundaries to prevent deterioration of corrosion resistance. it can. Furthermore, the important effect of Mn, Ni, Cu is the test results described later (for example, the relationship in Fig. 1).
In the continuous finishing heat treatment process according to the present invention, it is possible to obtain a stable region with small hardness variation over a wide temperature range from a lower temperature side in the continuous finishing heat treatment process according to the present invention. In terms of high temperature strength required for heat treatment and energy saving, great advantages are brought about in actual operation. Therefore M
The addition of n, Ni, and Cu not only contributes to the production of multi-phase steel strips with stable strength characteristics, but also enables heat treatment at lower temperatures with higher high-temperature strength, which allows the furnace to undergo continuous finishing heat treatment. It is possible to avoid troubles due to low temperature strength such as coil breakage, and to bring great effects from the viewpoint of energy saving. To obtain such effects, the total amount of Mn, Ni, and Cu must be at least 0.5% or more, but Ni has the greatest effect on the increase in hardness of the multiphase structure material after continuous finishing heat treatment. , Mn and Cu are about 1/3 of Ni. Therefore, when determining the addition amount of Mn, Ni, Cu, it is regulated by using the relational expression of Ni + (Mn + Cu) / 3, and at least 0.5% or more is added as Ni + (Mn + Cu) / 3. However, if added in a large amount, the product becomes expensive, which affects the economical efficiency, which is one of the features of the steel strip of the present invention. Therefore, Mn, Ni, and Cu alone should be 4.0% or less, respectively, and Ni + (Mn + Cu) / 3 should be 5.0%.
Below.

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

P is an element having a large solid solution strengthening ability, but addition of a large amount may lead to a decrease in toughness, so the content of P is set to 0.040% or less, which is the generally accepted level.

Cr is an element that has the most important effect on the corrosion resistance of stainless steel, and it should be contained in excess of 14.0% to obtain sufficient corrosion resistance. However, if the Cr content is too high, martensite phase is formed. As a result, the amount of austenite-forming elements required to obtain high strength increases and the product becomes expensive, so the upper limit is 20.0%.

O forms an oxide-based non-metallic inclusion and reduces the cleanliness of steel, so O is preferably low, and is made 0.02% or less.

Al is an effective element for deoxidation and has the effect of significantly reducing A ₂ -based inclusions that adversely affect press workability. However, if the content exceeds 0.20%, not only the effect is saturated but also adverse effects such as increase in surface defects are caused, so the upper limit is made 0.20%.

B is a component effective for improving toughness, but its effect is brought about by a very small amount, and when the amount exceeds 0.0050%, the effect is saturated, so the upper limit is made 0.0050%.

Mo is an element effective in improving the corrosion resistance, but if added in a large amount, the product becomes expensive, so the upper limit is 1.0%.

REM and Y are effective elements for improving hot workability. It is also an effective element for improving oxidation resistance. In the method of the present invention in which continuous finishing heat treatment is performed at a high temperature, generation of oxide scale is suppressed and it effectively acts to obtain a good surface texture after descaling. However, these effects saturate when the REM exceeds 0.10% and when the Y exceeds 0.20%, so the upper limits are 0.10% for REM and 0.20% for Y.

Next, the content of each manufacturing process of the multiphase structure steel strip according to the present invention will be described.

In the method of the present invention, a slab of chromium stainless steel whose composition range has been adjusted as described above is manufactured by a normal steelmaking casting technique, and this slab is hot rolled to manufacture a hot rolled steel strip. After hot rolling, it is preferable to anneal the hot rolled sheet and descale. Although it is not always necessary to perform hot-rolled sheet annealing,
By this annealing, the hot rolled steel strip can be softened to improve the cold rolling property, and the transformation phase (the portion that was austenite phase at high temperature) remaining in the hot rolled steel strip can be transformed and decomposed into ferrite + carbide. Therefore, it is desirable to obtain a steel strip with a uniform multiphase structure after cold rolling and continuous finishing heat treatment. This hot-rolled sheet annealing may be either continuous annealing or box annealing. In the descaling step, ordinary pickling may be performed. For the slab manufacturing process, hot rolling process, hot rolled sheet annealing process and descaling process up to this point, the conventional chrome stainless steel strip manufacturing technology can be applied to the method of the present invention as it is.

Next, a cold rolling process and a continuous finishing heat treatment process are carried out to produce a multi-phase steel strip, and these processes are characteristic steps in the method of the present invention and will be described in detail.

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

Steels A, B and C having the chemical composition shown in Table 1 were smelted and hot-rolled under normal conditions to form hot-rolled sheets with a thickness of 3.6 mm, heated at 780 ° C for 6 hours, and cooled at the furnace. After being annealed, it was pickled. Steels B and C are steels having the steel components regulated by the present invention, but steel A has a low Mn, Ni, Cu content and does not satisfy Ni + (Mn + Cu) /3≧0.5% prescribed by the present invention. That is, the steel is not covered by the present invention.

Tests were carried out using this hot-rolled sheet under different cold rolling conditions and finish heat treatment conditions (the data in FIGS. 1 and 2 also show the test results, the contents of which will be explained later). To).

Table 2 below shows (a) for Steel B in Table 1. A dual-phase structure material (hereinafter referred to as 2CR material) that has been subjected to finishing heat treatment by performing twice cold rolling with intermediate annealing during cold rolling, (b). A multi-phase structure material (hereinafter referred to as 1CR material), which has been subjected to finishing heat treatment by performing only one cold rolling without intermediate annealing, (c), has the same strength as 1CR material and 2CR material. Tensile strength (kgf / mm) of each steel sheet manufactured by three methods: temper-rolled material applied by rolling
² ) and elongation (%) are shown in the rolling direction (L), in the rolling direction 45 ° (D) and in the rolling direction 90 ° (T).

The 2RC material in (a) was cold-rolled from the above hot-rolled sheet to a thickness of 1 mm, heated at 750 ° C for 1 minute, air-cooled to an intermediate annealing, and then cold-rolled to a thickness of 0.3 mm. It was a cold rolled plate. After soaking this cold-rolled sheet at 1000 ℃ for 1 minute, the average cooling rate from that temperature to 100 ℃ was 20 ℃ / se.
A finishing heat treatment of cooling with c was performed.

The 1RC material in (b) was cold-rolled from the above hot-rolled sheet to a thickness of 0.3 mm without intermediate annealing, and this cold-rolled sheet was soaked at 1000 ° C for 1 minute From 100 ℃
Was subjected to a finishing heat treatment for cooling at an average cooling rate of 20 ° C / sec.

For the temper-rolled material of (c), the annealed hot-rolled sheet was cold-rolled to a predetermined sheet thickness so that the same strength as the 1CR and 2CR sheets could be obtained with a sheet thickness of 0.3 mm. After annealing, temper rolling was performed at a predetermined rolling rate.

As is clear from Table 2, the elongation of the multi-phase microstructured materials for both 2CR and 1CR is significantly superior to that of temper-rolled materials of equivalent hardness and strength level, and it has an excellent strength-elongation balance. You can see that Regarding the in-plane anisotropy, the difference in tensile strength depending on the direction, that is, the in-plane anisotropy is small in the 2CR and 1CR materials for the tensile strength. Indicates that the difference in tensile strength between the L direction with the lowest tensile strength and the T direction with the highest tensile strength is 12 kgf / mm ² or more, and the in-plane anisotropy is large. Regarding the elongation, the in-plane anisotropy of the multi-phase structure material with high elongation is relatively smaller than that of the temper-rolled material with low elongation, and in particular, 2CR material has even smaller in-plane anisotropy than 1CR material. I understand. That is, it can be said that the intermediate annealing is very important in reducing the in-plane anisotropy of elongation of the multiphase structure material. Therefore, from the results in Table 2, hot rolling,
When a finishing heat treatment is applied to obtain a multi-phase structure after cold rolling including hot-rolled sheet annealing and intermediate annealing, it is possible to obtain a high multi-phase structure with excellent ductility and small in-plane anisotropy of strength and ductility. It is clear that a high strength chrome stainless steel plate is obtained.

As can be seen from the test results and as shown in the examples described later, the in-plane anisotropy of the elongation of the multiphase structure material is determined by performing the cold rolling process in two or more cold rolling steps with intermediate annealing. It can be reduced by implementing. Therefore, in the production of multi-phase steel strips with low ductility in-plane anisotropy, it is possible to reduce the sheet thickness up to the product sheet thickness by cold rolling more than once and perform intermediate annealing during that period. It is important in the method of the present invention. The heating temperature of this intermediate annealing is the ferrite single-phase region temperature, that is, the temperature below the Ac ₁ point. The cold rolling ratio of cold rolling before and after the intermediate annealing is at least
30% or more is recommended.

"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, and the two-phase region of ferrite + austenite with Ac ₁ point or more and 1100 ° C or less After keeping the temperature within 10 minutes, the average cooling rate from the maximum heating temperature to 100 ℃ is 1 ℃ /
A continuous finishing heat treatment of cooling at a temperature of not less than sec and not more than 500 ° C / sec is performed. This continuous finishing heat treatment step is the most characteristic step of the method of the present invention, and this continuous finishing heat treatment condition is also shown in Examples described later. As described above, it has important significance in the present invention. The outline of the reason why the heating condition and the cooling condition in this continuous finishing heat treatment step are regulated is as follows.

It is an absolute condition that the heating temperature during the continuous finishing heat treatment is a ferrite + austenite two-phase region temperature. In the practice of the method of the present invention, in the vicinity of the temperature at which austenite begins to be generated when heated from a low temperature in a continuous heat treatment furnace (that is, the temperature at the Ac ₁ point), the amount of austenite varies greatly with temperature change, and the hardness is stable after rapid cooling. May not be obtained. However, in the steel composition range targeted by the present invention, it was found that such hardness variation does not substantially occur when heated to a high temperature range of 100 ° C. or higher from the Ac ₁ point. Therefore, the heating temperature for continuous finishing heat treatment is Ac ₁
It is recommended that the temperature be + 100 ° C or higher. More specifically, the temperature is preferably 850 ° C or higher, and more preferably 900 ° C or higher. On the other hand, regarding the upper limit of the heating temperature, the strength increase may not only saturate at too high a temperature, but may decrease in some cases, and it is disadvantageous in terms of manufacturing cost.
The upper limit is

The metallurgical significance of the two-phase heating of ferrite + austenite during the continuous finishing heat treatment in the method of the present invention is as follows: solid solution of Cr carbide and nitride, formation of austenite phase, and concentration of C and N in the formed austenite. I can point. In the case of the chromium stainless steel strip targeted by the method of the present invention, all of these objects reach an almost equilibrium state in a short time, so that heating in the above two-phase temperature range during the continuous finishing heat treatment in the present invention is performed. Time is short,
Heating for about 10 minutes is enough. The fact that this short-time heating is sufficient is very advantageous in terms of production efficiency and manufacturing cost in the actual operation of the method of the present invention. By the above heating conditions and holding time, necessary austenite can be generated in which the amount of martensite generated by subsequent cooling is 20% by volume or more.

The cooling rate during the finishing heat treatment is 1 ℃ / s in order to obtain a multi-phase structure of the martensite phase and the soft ferrite phase.
It is necessary to set the cooling rate to ec or more, but it is practically difficult to obtain the cooling rate to exceed 500 ° C / sec. Therefore,
In the present invention, the cooling from the two-phase temperature range heating is 1 to 500 ° C /
Perform at a cooling rate within the range of sec. This cooling rate is the average cooling rate from the maximum heating temperature to 100 ° C, but it is not always necessary to adopt this cooling rate in the cooling process after the transformation of austenite into martensite. The cooling rate and the cooling end temperature are sufficient to transform the austenite produced at high temperature under the above heating conditions into martensite. As a cooling method, a forced cooling method in which a gas and / or liquid cooling medium is sprayed on a steel strip, a roll cooling method using a water cooling roll, or the like 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 rewinding machine and the winding machine.

FIG. 1 shows that for each of the steels in Table 1 above, the hot-rolled sheet (hot-rolled sheet after hot-rolled sheet annealing and pickling) manufactured by the method already described was cold-rolled to a sheet thickness of 1 mm, 750 After heating at ℃ for 1 minute and performing intermediate annealing with air cooling, cold rolling is performed to make a cold rolled plate with a thickness of 0.3 mm, and this cold rolled plate is subjected to 1 at each temperature between 800 and 1100 ° C. The amount of martensite (volume%) and hardness (HV) of the finished heat-treated material obtained by performing soaking for a minute and then performing a finishing heat treatment that cools from that temperature to 100 ° C at an average cooling rate of 20 ° C / sec. Is shown by the relationship of each heating temperature during the finish heat treatment (A, B, and
C represents each steel in Table 1).

As is clear from Fig. 1, when the heating temperature is in the ferrite + austenite two-phase region, martensite appears after the finishing heat treatment, and the amount of martensite increases as the heating temperature rises. ~ 900 ℃
When it exceeds, the degree of increase becomes small and it tends to be gradually saturated. The behavior of hardness also shows the same tendency corresponding to the change of the amount of martensite, and the hardness increases as the amount of martensite increases. On the other hand, in steel A in which the amounts of Mn, Ni, and Cu are below the specifications of the present invention, the temperature range in which the amount of martensite and hardness are saturated is on the high temperature side and the range is narrow.
The results shown in FIG. 1 have important significance in performing finishing heat treatment on a continuous heat treatment line. In other words, in the continuous heat treatment line, some temperature fluctuations are unavoidable. In particular, fluctuations in the length direction of the steel strip, and even if the target temperature is the same, the difference in heat treatment temperature due to the difference in strip passing chance is In operation, it is necessary to expect a fluctuation of about ± 20 ° C with respect to the target temperature. Figure 1 shows that if the heat treatment temperature range where the cooling rate is almost constant and the hardness variation is small is adopted,
This indicates that even if there is some temperature fluctuation in the continuous heat treatment line, it is possible to manufacture steel strips with small fluctuations in hardness, that is, strength. Further, in particular, by adding an appropriate amount of Mn, Ni, and Cu, it is possible to obtain a wide range of temperature range for finishing heat treatment with small hardness fluctuation, which is more advantageous. When the strength level is controlled by the above-mentioned component control, the target strength can be stably obtained, and the strength variation is small over the entire length of the steel strip and the strength difference between the steel strips is small. The strength material can be manufactured easily and inexpensively using the existing continuous heat treatment line.

FIG. 2 is a graph showing the correlation between hardness and elongation (weighted average value in three directions) of several multiphase structural materials having different martensite contents within the steel composition and manufacturing conditions controlled by the present invention. This is shown in comparison with the correlation of the temper-rolled material. The structure of the multi-phase structure material is the same as that described in FIG. 1, and the heating temperature of the finish heat treatment is 900 ° C. or higher. Further, the temper-rolled material has its hardness changed by annealing after cold rolling and then changing the temper-rolling rate indicated by the suffix in the figure.

As is clear from FIG. 2, in the temper-rolled material, the elongation sharply decreases as the hardness increases as the temper-rolling rate increases.
On the other hand, in the multiphase structure material, the decrease in elongation is slow even if the hardness increases. In particular, the elongation of the multi-phase structure material is superior to that of the temper-rolled material in the high hardness region, specifically in the region of Hv200 or higher. In other words, the high ductility achieved by using a multi-phase structure material is even more pronounced in the Hv200 and above regions. To this end, as can be seen from Fig. 1 above, the martensite content of about 10% by volume or more is used. Is where Thus, there is a feature of the multi-phase structure material according to the method of the present invention that cannot be achieved with the temper-rolled material in that high ductility at hardness of Hv200 or more can be achieved. The obtained multiphase structure steel strip has properties such as press formability that cannot be obtained by temper rolling.

FIG. 3 is a photograph of the metallographic structure when Steel B in Table 1 was manufactured by the method (a) in Table 2. Areas that appear white in the photo are ferrite and areas that appear black or gray are martensite. As can be seen from this photograph, this material has a multiphase structure in which fine ferrite and martensite are uniformly mixed.

As explained above, high-strength and high-strength steel strip materials with small anisotropy of strength and ductility were obtained by hot rolling,
Martensite produced by transformation from austenite by fine ferrite and quenching by finishing heat treatment by heating to the two-phase region of ferrite and austenite after two or more cold rollings including hot-rolled sheet annealing and intermediate annealing. This can be achieved by forming a multiphase structure in which and are uniformly mixed. That is, the strength (hardness) was obtained by hard martensite, and the ductility was obtained by soft ferrite, and the in-plane anisotropy of strength and ductility was obtained by finely and uniformly mixing both phases. It can be made smaller. In the structure after the finish heat treatment, a trace amount of retained austenite may be detected by X-ray examination.

Hereinafter, the characteristics of the multi-phase steel strip obtained by the method of the present invention will be specifically described by giving examples of carrying out the method of the present invention without comparison with comparative examples.

Example A slab was manufactured by melting steel having the chemical composition shown in Table 3. And in each case, after hot rolling to a plate thickness of 3.6 mm, 780 ℃
× 6 hours, furnace-cooled hot-rolled sheet annealing, pickling, and 4th
Cold-rolled steel strips with a thickness of 0.3 mm were cold-rolled under the cold-rolling conditions shown in the table, and subjected to continuous finishing heat treatment in a continuous heat-treating furnace under the finishing heat treatment conditions shown in Table 4. The soaking time of the intermediate annealing in the cold rolling process is 1 minute in all, and the soaking time in the continuous finishing heat treatment process is 1 minute in all. The material properties of the steel strip after the finish heat treatment are also shown in Table 4.

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

On the other hand, in Comparative Example No. 1, the manufacturing conditions are within the range specified by the present invention, but the amounts of Mn, Ni, and Cu of the steel are Ni specified by the present invention.
Since it is No. 8 steel in Table 3 which is as low as 0.24%, which is out of the requirement of + (Mn + Cu) /3≧0.5%, martensite is not formed after continuous finishing heat treatment and its hardness is low.

In Comparative Example No. 2, the manufacturing conditions are still within the range of the present invention, but the C content and the Ni content of the steel are 0.405% and 0.105% or higher, which are higher than the values specified in the present invention, respectively.
Since it is steel No. 9 containing 5.07% of C and Ni, the amount of martensite after continuous finishing heat treatment is 100%, and although the strength is high, the elongation is low.

In Comparative Example No. 3, the heating temperature in the continuous finishing heat treatment was as low as 750 ° C, and at this heating temperature, the steel of Steel No. 1 was not in the ferrite + austenite two-phase region, so the metal structure after the finishing heat treatment was martensite. It is a ferrite single-phase microstructure in which no elongation exists, and although the elongation is high, the strength and hardness are low.

In Comparative Example No. 4, the finishing heat treatment was performed in a box furnace, and the cooling rate was very low at 0.03 ° C / sec because the furnace cooling was also used, so martensite did not form after the heat treatment.
Similar to No.3, it has high elongation but low strength and hardness.

Comparative Example No. 5 is a temper-rolled material, and the elongation is significantly lower than that of the present invention. The ratio of 0.2% proof stress to tensile strength, that is, the yield ratio is high, and the anisotropy of 0.2% proof stress, tensile strength, and elongation is large. Therefore, it is clear that the workability and the formability after working are inferior to those of the steel strip obtained by the method of the present invention.

In Comparative Example No. 6, since the intermediate annealing was not performed in the cold rolling before the continuous finishing heat treatment, the strength was high and the elongation was excellent, but the in-plane anisotropy of elongation was subjected to the intermediate annealing. It is bigger than the ones.

Regarding the steel strips of Comparative Examples Nos. 1, 3, 4 and 5, the occurrence of ridging was observed in the tensile test pieces after fracture, whereas the multiphase structure steel strips of the present invention example showed ridging. No occurrence is observed, which means that press forming and other processing can be performed well.

As described above, according to the method of the present invention, a multi-phase steel strip having both high ductility and high strength, small in-plane anisotropy of strength and ductility, low yield strength, and low yield ratio is provided. In the field of chrome stainless steel sheets, there are no examples of Hv200 or higher 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 invention is an electronic component,
It is especially useful as a high-strength material that is required to be processed into precision machine parts, and can exert great results in this field.

[Brief description of drawings]

FIG. 1 shows the relationship between the heating temperature, the amount of martensite and the hardness of the finish heat treatment according to the present invention, and FIG. 2 shows the hardness-elongation correlation for the finish heat treatment material and the temper rolled material according to the present invention. And FIG. 3 are micrographs showing the metallographic structure of the chromium stainless steel strip subjected to the heat treatment for continuous finishing according to the present invention.

Claims (6)

[Claims] 1. In% by weight, C: 0.10% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 4.0% or less, Cr: 14.0 or more 20.0 % Or less, N: 0.12% or less, O: 0.02% or less, Cu; 4.0% or less, the balance being Fe and inevitable impurities, and 0.01% ≤ C + N ≤ 0.20% and 0.5% ≤ Manufacture of a steel slab satisfying the relationship of Ni + (Mn + Cu) / 3 ≤ 5.0, hot rolling this to manufacture a hot-rolled steel strip, two or more times with intermediate annealing of ferrite single-phase temperature heating. Of cold-rolled steel strip of product thickness by cold rolling of
Then, the obtained cold-rolled steel strip is passed through a continuous heat treatment furnace, and the two-phase region temperature of ferrite + austenite of Ac ₁ point or more and 1100 ° C or less is held for 10 minutes or less, and then 100 ° C from the maximum heating temperature. A continuous finishing heat treatment process in which the finishing heat treatment is performed at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less, which has a hardness of HV200 or more, a small in-plane anisotropy, a high ductility and a high strength composite. Method for producing phase-structured chromium stainless steel strip. 2. The heating temperature in the continuous finishing heat treatment step is A
c The manufacturing method according to claim _{1, wherein 1} point + 100 ° C or higher and 1100 ° C or lower. 3. The heating temperature in the continuous finishing heat treatment step is 85.
The production method according to claim 1, which is 0 ° C or higher and 1100 ° C or lower. 4. In% by weight, C: 0.10% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 4.0% or less, Cr: 14.0 or more 20.0 % Or less, N: 0.12% or less, O: 0.02% or less, Cu; 4.0% or less, and Al of 0.20% or less, B of 0.0050% or less, Mo of 1.0% or less, REM of 0.10% or less, 0.20% or less Steel containing at least one of Y and the balance of Fe and inevitable impurities, and satisfying the relations of 0.01% ≦ C + N ≦ 0.20% and 0.5% ≦ Ni + (Mn + Cu) /3≦5.0. To produce a slab of steel for hot rolling, and to hot-roll it to produce hot-rolled steel strip, cold-rolled steel with a product thickness by two or more cold-rollings with intermediate annealing of ferrite single-phase temperature heating Band manufacturing process,
Then, the obtained cold-rolled steel strip is passed through a continuous heat treatment furnace, and the two-phase region temperature of ferrite + austenite of Ac ₁ point or more and 1100 ° C or less is held for 10 minutes or less, and then 100 ° C from the maximum heating temperature. A continuous finishing heat treatment process in which the finishing heat treatment is performed at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less, which has a hardness of HV200 or more, a small in-plane anisotropy, a high ductility and a high strength composite. Method for producing phase-structured chromium stainless steel strip. 5. The heating temperature in the continuous finishing heat treatment step is A
c The manufacturing method according to claim 4, wherein ₁ point + 100 ° C or more and 1100 ° C or less. 6. The heating temperature in the continuous finishing heat treatment step is 85.
The production method according to claim 4, which is 0 ° C or higher and 1100 ° C or lower.