JPH07100820B2 - 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. The demand for is becoming more and more severe. For this reason, in addition to the original corrosion resistance of stainless steel and the above-mentioned characteristics of chrome stainless steel, further strength, workability and precision are required in terms of the material surface of chrome stainless steel. Therefore, the advent of a chrome stainless steel sheet material that has various characteristics such as high strength and high ductility, which are contradictory characteristics, excellent shape and thickness accuracy at the time of material steel sheet, and excellent shape accuracy after processing. Is waiting.

[Conventional technology]

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

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 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 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 processed into a shape close to the final product by the processing manufacturer. After that, 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, unlike temper rolling with a light rolling rate of at most 2 to 3% for the purpose of shape correction called so-called skin pass rolling or temper rolling, temper rolling for the purpose of increasing strength requires rolling. Since it is a substantial cold rolling with a rate of several tens of percent, it is difficult to obtain a steel strip with excellent shape as cold rolled. 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, further hot strip annealing, two or more times with intermediate annealing in the ferrite single-phase region sandwiched. Cold rolling is carried out to produce a cold rolled steel strip with the product thickness, and this cold rolled steel strip is
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, in the present invention, in% by weight, C: 0.10% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0.60% or less, Cr: 10.0 or more 14.0. % Or less, N: 0.08% or less, O: 0.02% 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 slab containing one or more of the following Ys, the balance of which is Fe and unavoidable impurities, and which satisfies the relationship of 0.02% ≤ C + N ≤ 0.12%, is manufactured and heat-treated. Process of hot rolling to produce hot rolled steel strip, process of producing cold rolled steel sheet of product thickness by two or more cold rolling processes with intermediate annealing of ferrite single phase temperature heating,
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 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 finish heat treatment. It provides an advantageous and new manufacturing technology for the industrial structure of chrome stainless steel sheet or steel strip material, and has been conventionally marketed in the martensitic stainless steel sheet or steel strip or ferritic stainless steel sheet or steel strip. The present invention provides the market with a new chromium stainless steel material having both ductility and strength, which does not exist, 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.

Since C and N are strong austenite-forming elements and have a large martensite strengthening ability, they are elements effective for controlling the strength and increasing the strength after continuous finishing heat treatment. Therefore, in order to obtain a multiphase structure containing 20% or more of martensite after the continuous finishing heat treatment process and to obtain sufficient strength of Hv200 or more, the (C + N) content should be at least 0.02
Need more than%. 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, the amount of (C + N) should be 0.12% or less, and 0.02% ≦ C +
It is necessary to satisfy the relationship of N ≦ 0.12%, and the amount of C 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.
08% 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 and Ni are austenite forming elements, and are elements effective for controlling the amount of martensite and the strength after continuous finishing heat treatment. 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: 1.0% of the normally accepted limit,
Ni; 0.6% is the upper limit.

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.

In order to maintain the corrosion resistance as stainless steel, Cr should be contained as at least 10.0% as a necessary minimum amount, but if the Cr amount is too high, the austenite necessary for forming a martensite phase and obtaining high strength is obtained. Since the amount of produced elements increases and the product becomes expensive, the upper limit is 14.0%.

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, the hot rolled steel strip (hot rolled steel strip after annealing the hot rolled sheet if necessary) is sandwiched by intermediate annealing of ferrite single phase temperature heating at least twice. This is a process of rolling to a product plate thickness by cold rolling. 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. 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 described later). ).

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 2CR material in (a) was cold-rolled to a plate thickness of 1 mm, heated at 800 ° C for 1 minute, subjected to intermediate annealing of air cooling, and then cold-rolled to a thickness of 0.3 mm. It was a cold rolled plate. Then, this cold-rolled sheet was soaked at a temperature of 980 ° C for 1 minute and then subjected to a finish heat treatment of cooling from that temperature to 100 ° C at an average cooling rate of 20 ° C / sec.

The 1CR material in (b) was cold-rolled to a thickness of 0.3 mm without intermediate annealing, and after the cold-rolled sheet was soaked at 980 ° C for 1 minute, 1 from that temperature
Finishing heat treatment for cooling up to 00 ° C at an average cooling rate of 20 ° C / sec was performed.

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. That is, the hot rolled sheet of Steel B (annealed, pickled, 3.6 mm thick) is cold rolled to 1.2 mm,
After annealing at 800 ° C for 1 minute, it was temper-rolled to 0.3 mm (tempered rolling rate
75%).

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 internal surface anisotropy, the difference in tensile strength depending on the direction, that is, the in-plane anisotropy is small in the 2CR and 1CR materials in terms of tensile strength. The difference in tensile strength between the L direction with the lowest tensile strength and the T direction with the highest tensile strength is 10 kgf / mm ² or more, and the in-plane anisotropy is large. Regarding the elongation, the in-plane anisotropy of the multiphase 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. Recognize. 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 shown in Table 2, when cold rolling with intermediate annealing is performed and a finishing heat treatment to give a multi-phase structure is performed, the ductility is excellent and the strength and ductility are small in the in-plane anisotropy. It is clear that a high-strength chromium stainless steel sheet having a phase structure can be 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 producing a multi-phase steel strip with low ductility and in-plane anisotropy, cold rolling may be performed twice or more to reduce the sheet thickness to the product sheet thickness, and intermediate annealing may be performed in the meantime. 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. Therefore, the temperature is set to Ac ₁ point or higher. In carrying out the method of the present invention,
When the steel strip is passed through a continuous heat treatment furnace and heated from a low temperature, the amount of austenite varies greatly with temperature change near the temperature at which austenite begins to form (that is, the temperature at the Ac ₁ point), 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 preferable that the temperature is not less than the point, preferably Ac ₁ point + 100 ° C or more. More specifically, 900 ° C or higher, more preferably 950 ° C
The above is preferable. On the other hand, regarding the upper limit of the heating temperature, the strength increase may not only be saturated at too high a temperature but may be lowered in some cases, and it is disadvantageous in terms of manufacturing cost. Therefore, it is preferable to set the upper limit to 1100 ° C.

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, austenite necessary for the amount of martensite generated by subsequent cooling to be 20% by volume or more can be generated.

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 steel in Table 1 above, the hot rolled sheet manufactured by the method already described (hot rolled sheet after annealing and pickling) was cold rolled to a thickness of 1 mm, ℃ × 1 minute · After air-cooling intermediate annealing, after further soaking for 1 minute at each temperature between 800 ~ 1100 ℃, finish heat treatment to cool from that temperature to 100 ℃ at an average cooling rate of 20 ℃ / sec The martensite content (% by volume) and hardness (HV) of the finished heat-treated material obtained by applying the heat treatment are shown in relation to the heating temperature during the finish heat treatment (A, B, C in the figure are the 1 represents the steel composition.

As is clear from Fig. 1, when the heating temperature exceeds 800 ° C and reaches the ferrite + austenite two-phase region, martensite appears after the finishing heat treatment, and the amount of martensite rapidly increases with increasing heating temperature, but 900 When the temperature exceeds ℃, the degree of the increase becomes small and it tends to be 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. 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. Fig. 1 shows that if the heat treatment temperature range where the cooling rate is almost constant and the hardness variation is small is adopted, the steel strip with little variation in hardness, that is, strength, even if there is some temperature variation in the continuous heat treatment line. It can be manufactured. If the strength level is controlled by the component control as described above, 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 easily and inexpensively manufactured 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 more pronounced in the Hv200 or higher region. For this reason, as can be seen from Fig. 1 above, the martensite content of about 20% 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 A 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) of hard martensite and the ductility of soft ferrite were obtained, and the in-plane anisotropy of strength and ductility was improved 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 ℃
× Hot-rolled sheet annealing with heating and furnace cooling for 6 hours, after pickling,
A cold-rolled steel strip having a plate thickness of 0.3 mm was cold-rolled under the cold-rolling conditions shown in Table 4, 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 anisotropy of 0.2% proof stress, tensile strength and elongation. In each of the steel strips produced by the method of the present invention, 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 C and N contents of the steel are the conditions for the present invention steel (C
+ N) ≥ 0.02%, (C + N) = 0.012% steel (No. 8 steel in Table 3), the amount of martensite after continuous finishing heat treatment is small, and the tensile strength and hardness are sufficient. Is not obtained.

In Comparative Example No. 2, the manufacturing conditions are still within the range of the present invention, but the C content of steel is the C content defined by the present invention (C ≦ 0.10%).
It is a steel with a higher C = 0.124% (No. 9 steel in Table 3) and the (C + N) content also exceeds 0.12% specified in the present invention, so the martensite content after continuous finishing heat treatment Is 10
The strength is 0%, but the elongation is very low.

In Comparative Example No. 3, the heating temperature in the continuous finishing heat treatment was as low as 800 ° C, and at this heating temperature, the steel of Steel No. 2 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: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0.60% or less, Cr: 10.0 or more A slab of steel containing 14.0% or less, N: 0.08% or less, O: 0.02% or less, the balance being Fe and inevitable impurities, and satisfying the relationship of 0.02% ≦ C + N ≦ 0.12%. To produce hot-rolled steel strip by hot rolling, and cold-rolled steel strip with product thickness is produced by cold rolling two or more times with intermediate annealing of ferrite single-phase temperature heating 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. 2. The heating temperature in the continuous finishing heat treatment step is A
c ₁ point +100 or more and 1100 ° C or less, Claim 1
The manufacturing method described in the item. 3. The heating temperature in the continuous finishing heat treatment step is 90.
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: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Ni: 0.60% or less, Cr: 10.0 or more 14.0% or less, N: 0.08% or less, O: 0.02% or less, and 0.20% or less Al, 0.0050% or less B, 1.0% or less Mo, 0.10% or less REM, 0.20% or less Y or A steel slab containing two or more kinds, the balance of which is Fe and unavoidable impurities, and which satisfies the relationship of 0.02% ≤ C + N ≤ 0.12% is manufactured, and hot rolled by hot rolling. The process of manufacturing a steel strip, the process of manufacturing a cold-rolled steel strip having a product thickness by cold rolling two or more times with intermediate annealing of ferrite single-phase temperature heating.
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 90.
The production method according to claim 4, which is 0 ° C or higher and 1100 ° C or lower.