JPH0572449B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a new industrial manufacturing method for high-strength multi-phase structure chromium stainless steel sheets or steel strips that are excellent in ductility and have low in-plane anisotropy in strength and ductility. The present invention provides a high-strength, ductile stainless steel plate or steel strip that can be used as a forming material and subjected to processing such as press forming. In the following description, "steel plate or steel strip" may be abbreviated as "steel plate", but "steel strip" is also included in the content of the present invention. [Prior Art] Chromium stainless steel containing chromium as a main alloy component includes martesitic stainless steel and ferritic stainless steel. Martensitic stainless steel has been well known as a high-strength chromium stainless steel sheet. For example, JIS G 4305 specifies seven types of martensitic stainless steel. These martensitic stainless steels have C: 0.8% or less (SUS410S) to 0.60 to 0.75%.
(SUS440A) contains higher C than ferritic stainless steel, and can be given high strength by quenching or quenching and tempering. Also, in this JIS G 4305, 0.26 to 0.40
Contains % C and 12.00-14.00% Cr
For SUS420J2, after quenching by rapid cooling from 980 to 1040℃, tempering by air cooling to 150 to 400℃
It is possible to obtain a hardness of HRC40 or higher, and
For SUS440A containing 0.60-0.75% C and 16.00-18.00% Cr, a hardness of HRC 40 or higher can be obtained by quenching by rapid cooling from 1010-1070℃ and then air-cooling tempering at 150-400℃. It has been shown that As described above, martensitic stainless steel can achieve high strength through heat treatment, but when it is shipped from material manufacturers as stainless steel plates or steel strips, it is shipped in an annealed state, and at that point JIS G 4305. As shown in Table 15, the strength and hardness are low. Therefore, heat treatments such as quenching, quenching-tempering, etc. are usually performed by the processing manufacturer. Ferritic stainless steel sheets, which are another type of chromium stainless steel, cannot be expected to harden very well through heat treatment, so the best way to increase their strength is to perform cold temper rolling after annealing to increase the strength due to work hardening. It may be measured. However, the reality is that ferritic stainless steel is not often used for applications that originally require high strength. [Problems to be solved by the invention] In martensitic stainless steel sheets, the structure after quenching or quenching-tempering treatment is basically martensitic structure, as its name suggests, and has very high strength and hardness. On the other hand,
Elongation is very low. Therefore, if a steel plate or steel strip is subjected to heat treatment before processing, subsequent processing becomes difficult. Therefore, heat treatment is often performed after processing the product into a shape that approximates the final product. In particular, processing such as press molding is not possible after heat treatment. In any case, in order to obtain high strength with martesitic stainless steel, a heat treatment process by the processing manufacturer is essential, which increases the burden on the processing manufacturer, and this inevitably increases the cost of the final product. There was a problem. 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 worsens, resulting in poor workability. The degree of increase in strength due to temper rolling is significantly higher in yield strength than in tensile strength. For this reason, when the rolling reduction becomes high, the difference between proof stress and tensile strength becomes smaller, and the yield ratio (= proof stress /
When the tensile strength (tensile strength) approaches 1, the plastic working range of the material becomes very narrow, and when the yield strength is high, the spring back becomes large and the shapeability after press working etc. becomes poor. Furthermore, tension-rolled materials have very large in-plane anisotropy in strength and elongation, and even with slight press working, the shape after working becomes poor. Furthermore, since the processing strain caused by rolling is larger closer to the surface of the plate, it is inevitable that strain distribution in the thickness direction of the tension-rolled material will become non-uniform. This results in uneven distribution of residual stress in the plate thickness direction,
In particular, ultra-thin steel plates may undergo shape changes such as warpage after punching or photo-etching, which is a major problem in applications that require high precision, such as electronic parts. In addition to the above-mentioned problems caused by skin-pass rolling, ferritic stainless steels also have the problem of ridging, which can be said to be an essential drawback. This becomes a big problem in applications where strength is important. [Means for solving the problem] The above-mentioned problem is solved by using chrome that has moderately high strength, good ductility and workability that can be processed into a desired shape, and has small anisotropy and does not cause ridging. This problem could be solved if stainless steel materials could be provided in the form of steel plates or steel strips by material manufacturers. In order to solve this problem, the present inventors have continued extensive research on chromium stainless steel from both the chemical composition and manufacturing conditions. As a result, unexpectedly, instead of the conventional finish annealing at a temperature in the ferrite single-phase region, that is, the annealing treatment applied to the steel plate or steel strip product, a finishing process consisting of heating to the ferrite + austenite two-phase region followed by a rapid cooling treatment was realized. Substantially all of the above problems can be solved if heat treatment is applied to chromium stainless steel steel plates or steel strip products (cold rolled steel plates or steel strips obtained by conventional hot rolling or cold rolling). We were able to obtain a wonderful result. Thus, the present invention provides that as an essential component
A cold-rolled steel plate or steel obtained by manufacturing a cold-rolled steel plate or steel strip of chromium stainless steel containing 0.10% by weight or less of carbon and 10.0-18.0% by weight of chromium through normal hot rolling and cold rolling. In-plane anisotropy characterized by heating the band to a temperature in which it becomes a two-phase region of ferrite + austenite, and then subjecting it to a finishing heat treatment in which it is cooled at a cooling rate of 5°C/sec or more and 1000°C/sec or less. The present invention provides a method for producing a chromium stainless steel sheet or steel strip having a high ductility, high strength, and a multi-phase structure (structure consisting of substantial ferrite and martensite) with a small amount of heat. According to the method of the present invention, not only virtually all of the above-mentioned problems are solved, but also the strength can be freely and easily adjusted by controlling the steel composition or the heating temperature and cooling rate during finishing heat treatment. This product provides an advantageous and new manufacturing technology for the industrial production of chromium stainless steel sheets or steel strip materials, and can be applied to martesitic stainless steel sheets or steel strips and ferritic stainless steel sheets that have been shipped to the market in the past. Another object of the present invention is to provide the market with a new chromium stainless steel material that has both ductility and strength properties that steel strips do not have, and has less in-plane anisotropy in ductility and strength. Traditionally, for example, even in SUS430, which is a representative steel type of ferritic stainless steel, austenite is generated when heated to a temperature in the two-layer region, and this austenite is transformed into martensite by rapid cooling, forming a two-phase structure of ferrite + martensite. It was known that it would become. However, in the production of chromium stainless steel sheets or steel strips that produce austenite at high temperatures, the heat treatment after cold rolling is limited to annealing at a temperature in the ferrite single-phase region; It is common sense to avoid this as it causes material deterioration such as a decrease in ductility, and it has not been considered at all in the actual production of steel plates or steel strips. Therefore, there have been detailed studies on the relationship between heating temperature, strength, and ductility, as well as the anisotropy of ductility and strength when finishing heat treatment as in the present invention is applied after cold rolling of chromium stainless steel. do not have. The content of the present invention will be specifically explained below by citing an example of the results of a finishing heat treatment test of two-phase region heating and rapid cooling on a cold-rolled chromium stainless steel material conducted by the present inventors. Steels A and B having the chemical composition shown in Table 1 are melted and hot-rolled under normal conditions to form a hot-rolled plate with a thickness of 3.6 mm. After annealing at 780 x 4 hours, cold-rolling is performed. A cold rolled plate with a thickness of 0.7 mm was obtained. Steel A
is a steel component that is classified as martensitic stainless steel in the conventional concept, and steel B is a steel component that is classified as ferritic stainless steel in the conventional concept.

[Table] After soaking the roller cold-rolled plate for 2 minutes at each temperature in the range of 750 to 1150℃, the average cooling rate was 20℃/sec.
Finishing heat treatment was performed by cooling to room temperature. A tensile test, a hardness test, and a microscopic observation of the metallographic structure were performed on each of the finished heat-treated materials obtained. Column a on the left side of Figure 1 shows the relationship between the heating temperature during the finishing heat treatment and the amount of martensite (volume %), hardness (Hv), tensile strength (Kgf/mm ² ), and elongation (%). Indicated. For comparison, b in the right column of FIG. 1 compares the properties of the same cold-rolled sheet with enhanced strength by temper rolling at rolling ratios of 70% and 80%. Regarding tensile strength and elongation, the value L in the rolling direction, the value D at 45° to the rolling direction, and the value D at 90° to the rolling direction.
Find the value T of each direction and calculate (L+2D+T)/4
The average value calculated by As is clear from Fig. 1a, both steel A and steel B have a heating temperature of
A rapid increase in hardness and tensile strength was observed from the range of 800 to 900℃ or higher (the temperature in the two-phase temperature range of ferrite + austenite), and the heating temperature was 950℃.
If the value exceeds 100%, the increase in hardness and tensile strength tends to be saturated. On the other hand, as hardness and tensile strength increase, elongation decreases; for example, steel A has an elongation of 15% when the heating temperature is 900°C (hardness
Hv: 265, tensile strength: 83 Kgf/mm ² ), which is 2.5% when the same hardness and tensile strength are obtained by temper rolling (Fig. 1b), which has extremely superior ductility. I can say that I am doing it. In order to clarify this point, Figure 2 shows the results of Figure 1 in terms of the relationship between tensile strength and elongation. As is clear from Figure 2, the elongation when heating in the ferrite + austenite two-phase temperature range and rapid cooling from this temperature as finishing heat treatment is significantly higher than that of the temper-rolled material at the same strength level. - It can be seen that the ductility balance is excellent. Furthermore, Table 2 shows the tensile strength and elongation in the three directions L, D, and T for the finish heat-treated material of Steel A heated at 950° C. and the 80% temper-rolled material.

[Table] As is clear from Table 2, the difference in tensile strength and elongation of the 950°C finish heat-treated material depending on the direction, that is, the anisotropy, is significantly smaller than that of the temper-rolled material.
For example, the tensile strength of heat-treated material finished at 950℃ is highest in the L direction at 96.1Kgf/mm ² and lowest in the D direction at 93.4Kg.
f/mm ² , and the difference is only 2.7 Kgf/mm ² , whereas in temper rolling, the difference between the highest T direction and the lowest L direction is as large as 19.8 Kgf/mm ² . The same is true for elongation; finish heat treatment shows high elongation and small anisotropy, while temper rolling shows low elongation and large anisotropy. Figure 3 is a photograph of the metallographic structure of Steel A, which was subjected to finishing heat treatment at 950°C. The white area in the photo is ferrite, and the black or gray area is martensite. As can be seen from this photo, this material has a dual-phase structure of ferrite and martensite. In other words, as seen in the above test results, a high ductility and high strength material with small anisotropy of strength and ductility was obtained in the two-phase region of ferrite + austenite after hot rolling and cold rolling. This can be achieved by finishing heat treatment in which the material is heated to a temperature of 100% and then rapidly cooled to basically form a multi-phase structure of ferrite and martensite, which is transformed from austenite by rapid cooling. The strength (hardness) of such a multiphase steel sheet depends on the amount (volume fraction) of the martensitic phase and the strength (hardness) of the martensitic phase. Therefore,
When implementing the method of the present invention, from the chemical composition perspective
Due to the balance between austenite-forming elements such as Mn, Ni, C, and N and ferrite-forming elements such as Cr, Si, and Al, the amount of austenite at high temperatures, that is, the volume fraction of the martensite phase after quenching, can be freely controlled. The strength (hardness) of the martensitic phase itself can be freely controlled by controlling the amounts of C and N, which have a large strengthening ability. Also, from the viewpoint of manufacturing conditions, the volume fraction and strength (hardness) of the martensitic phase can be controlled by controlling the heating temperature and cooling rate during the final heat treatment. If the amount of C in the chromium stainless steel to which the method of the present invention is applied is too high, a large amount of martensite phase will be formed after finishing heat treatment, and in some cases, 100% martensite will be formed, and the hardness of the martensite phase itself will decrease. Since it becomes very high, although high strength can be obtained, ductility decreases. Therefore, the upper limit of the amount of C in chromium stainless steel to which the method of the present invention is applied is preferably 0.10%. Regarding the amount of Cr, at least 10.0% should be contained as the minimum necessary amount to maintain the corrosion resistance of stainless steel, but if the amount of Cr is too high,
The upper limit is preferably 18.0% because the amount of austenite-forming elements necessary to generate martensitic phase and obtain high strength increases and the product becomes expensive. In order to achieve the above object in the present invention, it is an absolute condition that the heating temperature during the finishing heat treatment is in the ferrite+austenite two-phase region temperature. In the case of chromium stainless steel in which the present invention can be advantageously practiced, the lower limit temperature at which a ferrite+austenite two-phase structure is formed is approximately in the range of 800 to 900°C. In other words, it can be said that the effects of the present invention are best exhibited when the method of the present invention is applied to chromium stainless steel whose chemical composition has a lower limit temperature in the range of 800 to 900°C at which a ferrite + austenite dual-phase structure is formed. I can say that. Regarding the upper limit of the heating temperature during this finishing heat treatment, it is preferable to set the upper limit to 1100° C., since if the temperature is too high, the increase in strength will be saturated and it will also be disadvantageous in terms of manufacturing cost. Ferrite + during finishing heat treatment in the method of the present invention
The metallurgical significance of austenite two-phase input heating is as follows:
Three points can be mentioned: solid solution of Cr carbides and nitrides, generation of austenite phase, and concentration of C and N in the generated austenite. In the case of chromium stainless steel, all of these phenomena reach an almost equilibrium state within a short time, so the heating time during the finishing heat treatment in the present invention may be short, approximately 10 minutes or less. The fact that heating is required for a short time is very advantageous in terms of production efficiency and manufacturing cost in terms of actual operation of the method of the present invention. Regarding the cooling rate during finishing heat treatment, it is necessary to set the cooling rate to 5℃/sec or more in order to obtain a multi-phase structure of martensitic phase and soft ferrite phase, but it is necessary to set the cooling rate to over 1000℃/sec. is practically difficult. Therefore, in the present invention, cooling from two-phase temperature range heating is performed at a rate of 5 to 1000°C/sec.
Carry out cooling rates in the range of . Although this cooling rate may be set to the final cooling temperature to room temperature, it is not necessarily necessary to employ this cooling rate in the cooling process after transformation to a low-temperature transformation phase, that is, a martensitic phase. As a cooling method, a forced cooling method in which a gas and/or liquid cooling medium is sprayed onto the steel plate or steel strip, a roll cooling method using water-cooled rolls, etc. can be applied. The finishing heat treatment according to the present invention can be carried out by a continuous heat treatment method in which the cold rolled chromium stainless steel strip is passed through a continuous heat treatment furnace having a heating soaking zone and a quenching zone between the coil unwinding machine and the winding machine. can. Example Steel having the chemical composition shown in Table 3 was melted, hot-rolled to a thickness of 3.6 mm, hot-rolled at 780°C for 4 hours, and pickled to a thickness of 0.3 mm. cold rolled. The fourth test targeted at these cold-rolled plates.
Finish heat treatment was performed under the finish heat treatment conditions shown in the table. The material properties of the finished heat-treated material obtained were
Also listed in the table. In addition, comparative example No. 3 in Table 4
and 5 are examples of steel Nos. 2 and 3 in which the strength was increased by temper rolling. In this case, the thickness of the hot-rolled sheet was adjusted in advance by surface grinding so that the thickness after adding the indicated rolling reduction in the notes was 0.3 mm.

【table】

【table】

[Table] As is clear from Table 4, all the samples according to the present invention have high tensile strength, hardness, and good elongation. Furthermore, it is clear that the method of the present invention has small anisotropy in 0.2% proof stress, tensile strength, and elongation. On the other hand, in Comparative Example No. 1, the finishing heat treatment conditions are within the range specified by the present invention, but the C content of the steel is greater than the C amount specified by the present invention (C≦0.10%).
Since it is steel No. 5 with a content of 0.162%, the amount of martensite after finishing heat treatment is 100% as shown,
It has high strength but very low elongation. In comparative example No. 2, the heating temperature in the finishing heat treatment was 800℃.
℃, and at this heating temperature, Steel No. 2 does not enter the ferrite + austenite two-phase region, so the metal structure after finishing heat treatment is a ferrite single-phase structure without martensite, and the elongation is high. The strength and hardness of objects are low. In Comparative Example No. 3 and Comparative Example No. 5, the cold rolling rate after hot-rolled sheet annealing was controlled as described above, and the strength was increased by so-called temper rolling. In comparison, elongation relative to tensile strength (hardness) is significantly lower. In addition, the ratio of 0.2% proof stress to tensile strength, that is, the yield ratio, is high, and the 0.2% proof stress is
High anisotropy in tensile strength and elongation. Therefore, it is clear that the workability and shapeability after processing are inferior to the materials obtained by the examples of the present invention. Comparative example No. 4 has a cooling rate of 0.03 in the finishing heat treatment.
Since the temperature is very low at ℃/sec, no martensite is generated after heat treatment, and although elongation is high, strength and hardness are low. Next, present invention examples No. 1 to 4 and comparative example No. 1,
A press working test was conducted on the materials obtained in 3 and 5, and the success or failure of press work, the shape accuracy of the processed product, and the presence or absence of ridging were investigated. The results are shown in Table 5. In the test, a disk specimen with a diameter of 35 mm was used as the fourth
It was pressed into the shape shown in the figure. Two types of punch shoulder radius R were used: 0.2 mm and 0.6 mm. Also,
The shape accuracy of the processed product is evaluated by the flatness of the flange after processing.The processed product is placed on a horizontal table, and the unevenness of wrinkles or undulations on the flange is measured over the entire circumference. It is shown as the difference between the height and the lowest height.

【table】

[Table] As is clear from Table 5, all of the materials according to the examples of the present invention can be press-formed with either a punch shoulder radius of 0.2 mm or 0.6 mm, and
The shape accuracy after processing was good, the flatness was 20 μm or less, and no ridging was observed. On the other hand, the material according to Comparative Example No. 1 broke at the punch shoulder portion with punch shoulder radii of 0.2 mm and 0.6 mm, and could not be pressed. In addition, the materials according to Comparative Examples No. 3 and 5 have a punch shoulder radius of 0.2 mm.
It is impossible to press the punch with the shoulder radius of the punch.
Although press working was possible in the case of 0.6 mm, the shape accuracy was poor and ridging was observed. In general, it goes without saying that the smaller the flatness value of the flange is, the better the shape accuracy of the processed product is, but in the case of a normal processed product, it is required that the flatness of the flange is at least 30 μm or less. As seen in the above examples, steel No. 3 in Table 3.
Chromium stainless steel sheets with steel compositions such as Nos. 1 to 4, which have been subjected to the finishing heat treatment according to the present invention, have an elongation of 9% or more and a tensile strength of 80 Kgf/mm ² or more and ductility.
It is a material that has both high ductility and high strength with a good strength balance, and also has small in-plane anisotropy of ductility and strength, low proof stress, low yield ratio, and excellent press formability. In the field of conventional chrome stainless steel sheets, there has never been an example of a high-strength material with such good workability being shipped to the market in the form of steel plates or steel strips. Therefore, the present invention provides a new material steel plate or steel strip for the conventional chrome stainless steel plate field. The material according to the present invention is particularly useful as a high-strength material that requires processability into electronic parts, precision mechanical parts, etc., and can achieve great results in this field.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between the heating temperature of the finishing heat treatment according to the present invention, the amount of martensite, hardness, tensile strength, and elongation in comparison with that of the temper-rolled material.
The figure is a diagram showing the relationship between tensile strength and elongation for finish heat-treated materials and temper-rolled materials according to the present invention, and FIG. 3 is a micrograph showing the metal structure of chromium stainless steel subjected to finish heat treatment according to the present invention. FIG. 4 is an explanatory view of a disk-shaped workpiece subjected to the press working test described in the specification, viewed in cross section along its center line.

Claims (1)

[Claims] 1. 0.10% by weight or less of carbon and 10.0% by weight as essential components
A cold-rolled steel plate or steel strip of chromium stainless steel containing ~18.0% by weight of chromium is produced through normal hot rolling and cold rolling, and the obtained cold-rolled steel plate or steel strip is processed into two-phase ferrite + austenite. 5℃/sec or more from this temperature,
A method for producing a high ductility, high strength, dual-phase structure chromium stainless steel sheet or steel strip with small in-plane anisotropy, characterized by subjecting it to finishing heat treatment by cooling at a cooling rate of 1000°C/sec or less. 2. The manufacturing method according to claim 1, wherein the chromium stainless steel is a steel whose composition is adjusted so that the temperature in the two-phase region of ferrite + austenite exceeds 800°C. 3. The manufacturing method according to claim 1 or 2, wherein the heating temperature in the finishing heat treatment is 1100°C or less. 4. The manufacturing method according to claim 1, 2 or 3, wherein the heating time of the finishing heat treatment is 10 minutes or less. 5. The manufacturing method according to claim 1, 2, 3, or 4, wherein cooling in the finishing heat treatment is performed at a cooling rate and temperature sufficient to transform austenite into martensite.