JPH07107178B2 - Method for producing high strength dual phase chromium stainless steel strip with excellent ductility - 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
0.08% or less (SUS410S) to 0.60 to 0.75% (SUS440A). Compared to ferritic stainless steel, when viewed at the same Cr content level, it contains high C and imparts high strength by quenching or tempering. can do. For example, in JIS G 4305, SUS420J2 containing 0.26 to 0.40% C and 12.00 to 14.00% Cr has 980 to 10
After quenching by quenching from 40 ℃, tempering of 150-400 ℃ air cooling can obtain hardness of HRC 40 or more, and
Contains 0.60 to 0.75% C and 16.00 to 18.00% Cr
For SUS440A, after quenching by quenching from 1010 to 1070 ℃,
It has been shown that a hardness of HRC 40 or higher can also be obtained by tempering at 50 to 400 ° C air cooling.

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 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 inventors of the present invention have conducted extensive research on chromium stainless steel from the viewpoint of both chemical composition and manufacturing conditions for the purpose of solving this problem. As a result, the steel components were properly controlled, and as a manufacturing condition, after hot rolling, if necessary, hot rolling annealing was performed, and then cold rolling was performed to perform cold rolling of the product strip thickness. This cold-rolled steel strip is not subjected to conventional finish annealing at the ferrite single-phase region temperature, that is, annealing treatment applied to a steel sheet or a steel strip product, and is appropriately heated to the ferrite-austenite two-phase region and then If a continuous finishing heat treatment under specific conditions consisting of the rapid cooling treatment of is carried out, it is possible to obtain a multiphase structure in which a substantially soft ferrite phase and a hard martensite phase are mixed uniformly. We were able to get the wonderful result that virtually everything can be solved.

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: 10.0% 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
050% or less B, 1.0% or less Mo, 0.10% or less REM, 0.20%
A steel containing one or more of the following Y and the balance being Fe and inevitable impurities, and having a relationship of 0.01% ≤ C + N ≤ 0.20% 0.5% ≤ Ni + (Mn + Cu) / 3 ≤ 5.0%. Manufacture a slab of satisfying steel, and hot-roll it to produce hot-rolled steel strip, cold-rolled steel strip by cold-rolling to product thickness by single cold rolling without intermediate annealing cold rolling step to, then, Tsuban the resulting cold-rolled steel strip in a continuous heat treatment furnace, Ac ₁ or more points
For two-phase temperature of 1100 ℃ or less ferrite + ausnite
After holding for less than 10 minutes, a continuous finishing heat treatment process is performed in which the maximum heating temperature to 100 ° C is cooled at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less. The present invention provides a method for producing a high-strength dual-phase chromium stainless steel strip having excellent ductility.

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 finish heat treatment. It provides an advantageous and new manufacturing technology for the industrial production of chrome stainless steel sheet or steel strip material, and has been hitherto 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, SUS430, which is a representative ferritic stainless steel, also produces austenite when heated to the two-phase region temperature. This austenite is transformed into martensite by quenching and becomes a double-layer 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 it as a material inferior such as a decrease in ductility, and it was never considered in the actual manufacturing of industrial steel strips.

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,
In order to obtain a multiphase structure containing 10% or more of martensite after the continuous finishing heat treatment process and to obtain sufficient strength of Hv 200 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%.

Also, if a large amount of C is added, it will be cooled during the continuous finishing heat treatment.
Corrosion resistance may deteriorate due to precipitation of Cr carbides at grain boundaries. 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 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 effective elements for controlling the amount and strength of martensite after continuous finishing heat treatment. In addition, the addition of Mn, Ni, Cu can reduce the amount of C added, improve ductility as soft martensite, and suppress precipitation of Cr carbide at grain boundaries to prevent deterioration of corrosion resistance. You can Further, the important effects of Mn, Ni, Cu are shown in the test results described later (for example, the relationship of FIG. 1) and Examples, but in the continuous finishing heat treatment step according to the present invention, it is more effective by adding Mn, Ni, Cu. It is possible to obtain a stable region where the hardness variation is small from the low temperature side over a wide temperature range, and to bring great advantages in actual operation in terms of high temperature strength required for continuous finishing heat treatment and energy saving. Is. Therefore, Mn, Ni,
The addition of Cu not only contributes to the production of multi-phase steel strips with stable strength properties, but also enables heat treatment at lower temperatures with higher high-temperature strength, which leads to coil rupture in the furnace due to continuous finishing heat treatment. It is possible to avoid the occurrence of troubles due to the decrease in high-temperature strength, and to bring about a great effect from the viewpoint of energy saving. To obtain this effect, the total amount of Mn, Ni, and Cu must be at least 0.5%, but Ni has the greatest effect on the increase in hardness of the multi-phase structure material after continuous finishing heat treatment. , Mn and Cu are about 3 of Ni
It is about one-third. 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, adding a large amount makes the product expensive,
It 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 and Ni + (Mn + Cu) / 3 should be 5.0% or less.

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.

At least 10.0% of Cr should be contained as a minimum necessary amount for maintaining the corrosion resistance as stainless steel. However, if the Cr amount is too high, the austenite necessary for forming the martensite phase and obtaining high strength is obtained. Since the amount of produced elements increases and the product becomes expensive, 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 it is 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 adjusted to the above steel composition range is manufactured by a normal steel casting technique, and this slab is manufactured by a normal hot rolling to produce 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 anneal the hot-rolled sheet, this annealing softens the hot-rolled steel strip to improve the cold-rolling property, and the transformation phase remaining in the hot-rolled steel strip (it was an austenite phase at high temperature). Since it is possible to transform and decompose (part) into ferrite + carbide, it is desirable to make 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. Slab manufacturing process up to this point, hot rolling process,
For the hot-rolled sheet annealing step and the descaling step, the conventional chrome stainless steel strip manufacturing technique can be directly applied to the method of the present invention.

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 hot rolled sheet annealing) is cold rolled to the product sheet thickness by single cold rolling without intermediate annealing. Manufacture obi. Here, “single cold rolling without intermediate annealing” means that cold rolling is not performed twice or more with intermediate annealing. More specifically, it means that cold rolling is performed by one-pass cold rolling or multi-pass cold rolling without intermediate annealing to cold-roll to a product sheet thickness to produce a cold-rolled steel strip. Therefore, regardless of the number of times the strip is passed through the rolling rolls, from the thickness of the hot-rolled steel strip to the product thickness of the cold-rolled steel strip, the strip thickness reduction is performed cold without intermediate annealing.

In the case of the method of the present invention, since there is a continuous finishing heat treatment step described later after the cold rolling step, there is an in-plane difference in strength and elongation due to the directional rolling structure generated in the cold rolled steel strip. It was found that the hysteresis of the anisotropy was virtually eliminated in the multiphase steel strip obtained by the subsequent continuous finishing heat treatment.
Therefore, in the production of the cold-rolled steel strip according to the present invention, even if the sheet thickness is reduced to the product sheet thickness without performing the intermediate annealing, the final multi-phase structure is a steel strip having a small in-plane anisotropy of strength and elongation. can do. However, it was found that the in-plane anisotropy of elongation of the multi-phase microstructured steel strip became even smaller when the intermediate annealing was performed and cold rolling was performed multiple times. In this case, the intermediate annealing is performed. Due to this, an increase in man-hours and an increase in manufacturing cost due to heat energy consumption cannot be avoided. Therefore, according to the method of the present invention in which intermediate annealing is not performed, it is possible to economically manufacture a dual-phase steel strip. For this reason, in the present invention, the hot-rolled steel strip is rolled down to the product sheet thickness in one pass, or cold rolling is performed by adopting single cold rolling in which the hot-rolled steel strip is rolled down to the product sheet thickness in multiple passes without performing intermediate annealing. Manufacture steel strip. At that time, the rolling reduction is preferably 30% or more and 95% or less.

Below, it will be explained based on typical test results that a multiphase steel strip having a small in-plane anisotropy can be obtained by cold rolling without intermediate annealing.

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. This hot-rolled sheet was cold-rolled without intermediate annealing, and then tested under different finishing heat treatment conditions (the data in FIGS. 1 and 2 also show the test results. The contents will be described later).

Table 2 below shows (a) cold-rolled steel without hot-rolled steel (hot-rolled annealed and hot-rolled after pickling) in Table 1 without intermediate annealing to a thickness of 0.7 mm. After rolling (cold rolling rate 80.6%), this cold rolled sheet was heated to 1000 ° C
After soaking for a minute, a multiphase structure material obtained by finishing heat treatment from this temperature to 100 ° C at an average cooling rate of 20 ° C / sec, (b). Tensile strength (kgf / mm ² ) and elongation (%) of each tempered rolled sheet with the same strength as the multi-phase structure material of (a) given by cold rolling at a sheet thickness of 0.7 mm. ) Is the value in the rolling direction (L), and the value in the 45 ° direction with respect to the rolling direction (D)
And the value (T) in the 90 ° direction with respect to the rolling direction.

As is clear from Table 2, the elongation of the multiphase structure material is remarkably superior to that of the temper-rolled material having the same hardness and strength level, and the strength-elongation balance is excellent. Regarding the in-plane anisotropy, in the tensile strength, the difference in tensile strength depending on the direction in the multiphase structure material, that is, the in-plane anisotropy is small, whereas the temper-rolled material has the highest tensile strength. Low L
Direction and the highest tensile strength in the T direction are 14 kgf / mm ² or more, and the in-plane anisotropy is large. Regarding the elongation, it can be seen that 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. In other words, it is clear that even if cold rolling is performed without intermediate annealing, it is possible to obtain a high-strength chromium stainless steel sheet with excellent ductility and low ductility in-plane anisotropy by forming a multi-phase structure.

"Continuous finishing heat treatment process" The cold-rolled steel strip having 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 at 1100 ° C or more above Ac ₁ point. After keeping the temperature within 10 minutes, the average cooling rate is 1 ℃ / se from the maximum heating temperature to 100 ℃.
The continuous finishing heat treatment is performed at a temperature of not less than c and not more than 500 ° C / sec. This continuous finishing heat treatment step is the most characteristic step of the method of the present invention, and the continuous finishing heat treatment conditions are also shown in the 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 carrying out the method of the present invention, the temperature at which austenite begins to form when the steel strip is heated from a low temperature in a continuous heat treatment furnace (that is, Ac
(Temperature of ₁ point), the amount of austenite varies greatly with temperature change, and stable hardness may not be obtained after rapid cooling. However, it has been found that in the steel composition range targeted by the present invention, such hardness fluctuations do not substantially occur when heated to a high temperature range of 100 ° C. or higher from the Ac ₁ point. Therefore, the heating temperature during continuous heat treatment is Ac
₁ point + 100 ℃ or higher is recommended. More specifically, 850 ℃
More preferably, it is 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. Can be mentioned. In the case of the chromium stainless steel strip targeted by the method of the present invention, all of these phenomena reach an almost equilibrium state in a short time, so heating in the above two-phase temperature range during 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. Under the above heating conditions and holding time, austenite necessary for the amount of martensite generated by subsequent cooling to be 10% 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 the thickness of the hot-rolled sheet (hot-rolled sheet after hot-rolled sheet annealing and pickling) manufactured by the method already described for each steel in Table 1 by cold rolling without intermediate annealing. A cold rolled sheet of 0.7 mm (cold rolling rate; 80.6%), and after soaking this cold rolled sheet for 1 minute at each temperature between 800 and 1100 ℃, from that temperature to 100 ℃ Average cooling rate 20 ℃ / sec
The martensite content (% by volume) and hardness (HV) of the finished heat-treated material obtained when the finishing heat treatment is performed in the form of temperature are shown in relation to the heating temperature during the finishing heat treatment (in the figure, A, B, C represent 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, the steel C in which the amounts of Mn, Ni, and Cu are below the specifications of the present invention has a narrow temperature range in which the temperature range where the amount of martensite and hardness are saturated is on the high temperature side.
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 variation, which is even more advantageous. By controlling the strength level 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 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 production of the multi-phase structure material is the same as that explained 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,
A multi-phase in which fine ferrite and martensite formed by transformation from austenite by quenching are uniformly mixed by the finishing heat treatment of heating and quenching in the two-phase region of ferrite and austenite after hot-rolled sheet annealing and cold rolling. This was achieved by forming an organization. In other words, the strength (hardness) of hard martensite and the ductility of soft ferrite were obtained. By mixing both phases finely and uniformly, the in-plane anisotropy of strength and ductility was reduced. It was possible. 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 multiphase steel strip obtained by the method of the present invention will be concretely described by giving examples of the method of the present invention in 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,
Cold rolling without intermediate annealing to form a cold-rolled steel strip with a plate thickness of 0.7 mm (cold rolling rate 80.6%). Continuous finishing in a continuous heat treatment furnace under the finishing heat treatment conditions shown in Table 4. Heat treatment (however, Comparative Example No. 4 was batch annealing treatment in a box furnace) was performed. The properties of the obtained steel strip material 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 a small 0.2% proof stress, tensile strength and anisotropy of elongation, and no ridging is 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 Mn, Ni, and Cu contents of 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 also within the scope of the present invention, but the C content and Ni content of the steel are specified by the present invention.
0.405% and 5, higher than 0.10% and 4.0%.
Since it is steel No. 9 containing 07% 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.

For the steel strips of Comparative Examples Nos. 1, 3, 4 and 5, ridging was observed in the tensile test pieces after fracture, whereas the multiphase structure steel strips of the present invention were ridged. It can be seen that there is no occurrence of cracks and that processing such as press molding 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 (8)

[Claims] 1. In 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: 10.0% or more Steel containing 20.0% or less, N: 0.12% or less, O: 0.02% or less, Cu: 4.0% or less, and the balance being Fe and inevitable impurities, and 0.01% ≦ C + N ≦ 0.20% 0.5 % S Ni + (Mn + Cu) / 3 ≤ 5.0% A steel slab that satisfies the relationship of 0% is manufactured, and this is hot rolled to manufacture a hot-rolled steel strip. One-time cold rolling without intermediate annealing Cold rolling process to produce cold rolled steel strip by cold rolling to thickness, and then pass the obtained cold rolled steel strip through a continuous heat treatment furnace to obtain Ac ₁ point or more.
For two-phase temperature of 1100 ℃ or less ferrite + ausnite
After holding for 10 minutes or less, a continuous finishing heat treatment process, in which the finishing heating heat treatment is performed to cool from the maximum heating temperature to 100 ° C at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less. A method for producing a high-strength dual-phase chromium stainless steel strip having excellent ductility. 2. The heating temperature in the continuous finishing heat treatment step is Ac
₁ point + 100 ° C or more and 1100 ° C or less
The manufacturing method described in the item. 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. The cold rolling ratio in the cold rolling process is 30% or more 95
% Or less Claims 1, 2, or 3
The manufacturing method described in the item. 5. 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: 10.0% 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, M of 1.0% or less
O, 0.10% or less REM, 0.20% or less Y one or more kinds, and the balance of Fe and inevitable impurities, 0.01% ≦ C + N ≦ 0.20% 0.5% ≦ Ni + ( Mn + Cu) /3≦5.0% of the steel slab is manufactured and hot-rolled to manufacture hot-rolled steel strip. It is cooled to the product sheet thickness by single cold rolling without intermediate annealing. Cold rolling process to produce cold-rolled steel strip by hot rolling, and then pass the obtained cold-rolled steel strip through a continuous heat treatment furnace to obtain Ac ₁ point or more.
For two-phase temperature of 1100 ℃ or less ferrite + ausnite
After holding for 10 minutes or less, a continuous finishing heat treatment process, in which the finishing heating heat treatment is performed to cool from the maximum heating temperature to 100 ° C at an average cooling rate of 1 ° C / sec or more and 500 ° C / sec or less. A method for producing a high-strength dual-phase chromium stainless steel strip having excellent ductility. 6. The heating temperature in the continuous finishing heat treatment step is Ac
₁ point + 100 ° C or more and 1100 ° C or less
The manufacturing method described in the item. 7. The heating temperature in the continuous finishing heat treatment step is 85.
The manufacturing method according to claim 5, which is 0 ° C or higher and 1100 ° C or lower. 8. The cold rolling ratio in the cold rolling process is 30% or more 95
% Or less, Claims 5, 6, or 7
The manufacturing method described in the item.