JP2000328143A - Method for producing double-phase structure steel having fine structure - Google Patents

Abstract

(57) [Summary] C: A steel material containing 0.05 to 0.80 mass% and having a ferromagnetic phase ratio of 20 vol% or more at room temperature, a lower limit temperature: Ac ₁ point, and an upper limit temperature: Ac ₃ points. Or (Ac ₂ points + 100
C), and the steel material is subjected to a magnetic field application treatment of 0.1 to 20 T and a processing treatment in the temperature range of any lower temperature. [Effect] The steel structure is effectively refined to obtain a double phase structure steel material excellent in strength and ductility.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a dual phase steel having a microstructure, and more particularly to an advantageous combination of a working treatment and a magnetic field applying treatment, thereby advantageously improving the steel structure and consequently improving the mechanical properties. An attempt is made to make advantageous improvements.

[0002]

2. Description of the Related Art As desirable characteristics of steel materials, high strength and high ductility are required. However, generally, when the strength of steel materials is increased, ductility is reduced, and it is known that these two characteristics are hardly compatible. Have been. On the other hand, as means for achieving higher strength and higher toughness of steel materials, refinement of crystal grains has been proposed.

In the above case, a controlled rolling method is generally used as a method for refining the structure.
No. 3124, JP-A-63-128117, and the like. Also, CAMP-ISIJ vol.11 (1998) 566 discloses a method of compressing a martensitic steel material and further compressing the material in a direction perpendicular to the compression direction to obtain fine grains.

[0004] As described above, there has been provided a material aimed at improving strength and toughness by miniaturizing the steel structure.
However, the method of controlling the temperature and improving the characteristics by devising the rolling method has already reached its limits,
In particular, in the case of a sheet material or the like, there arise problems such as an increase in the number of grains having a flat shape in the rolling direction, or formation of a texture in which crystal grains are oriented in a certain direction. In the case of pipes and wires, the presence of elongated grains becomes remarkable. As a result, even if the steel structure is refined, not only the strength and toughness are not improved as expected, but also the ductility often deteriorates and the ductility anisotropy increases in many cases. In addition, the conventionally proposed method has a drawback that the rolling reduction and the like become very large because the rolling reduction is relatively high.

[0005] For this reason, a new method capable of miniaturizing crystal grains and controlling a multiphase structure is required. One of the methods is to use a magnetic field. For example, Palmai
Zoltan (Gepgyartastechnologia. 22 (1982) P.463)
Is a study showing the effect of magnetic field on steel, Fe-0.
0.57T is a heat treatment that reversely transforms a 60C-0.30Si-0.72Mn composition steel from a martensitic structure to an austenitic structure.
It is reported that when a magnetic field of (T is a unit indicating the strength of a magnetic field: Tesla) is applied, the ferrite phase is stabilized and the amount of residual ferrite increases. However, to date, no research has been made on controlling the structure of a steel material using a magnetic field.

Accordingly, the present inventors have recently worked on controlling the structure of steel using a magnetic field, in part because a superconducting magnet capable of applying a strong magnetic field has been developed. As a result, C:
0.1 to 0.8 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.2 to
2.5% by mass and Cr: 0.1 to 1.5% by mass, with the balance being Fe and unavoidable impurities, when a heat treatment is performed on a steel material in a magnetic field of 0.1 to 20T, the lower limit temperature is A.
c ₁ point, by heating at a temperature range with either lower temperature of the maximum temperature Ac ₃ point or (A ₂ points +100 ° C.), when in a magnetic field heating, reverse transformed austenite phase shape extending in the direction of the applied magnetic field And a method for controlling the structure of a dual phase steel in combination with a residual ferromagnetic phase such as a ferrite phase was developed and disclosed in Japanese Patent Application No. 10-134241. By the above method, the structure of the steel material can be controlled, and as a result, the mechanical properties can be advantageously improved.

[0007]

SUMMARY OF THE INVENTION The present invention is a further improvement of the above-mentioned technology. By combining heat treatment in a magnetic field and processing, the steel structure is effectively refined to further improve strength and ductility. An object of the present invention is to propose a method of manufacturing a steel material having a dual phase structure having a microstructure.

[0008]

As described above, when a dual-phase structure steel composed of a paramagnetic phase and a ferromagnetic phase is manufactured by a reverse transformation heat treatment process, the steel is formed by reverse transformation by applying a magnetic field. The ferromagnetic phase remaining without reverse transformation with the austenite phase can also have a cellular structure elongated in the direction of the magnetic field, and the structure of the steel material can be controlled by such heat treatment in a magnetic field. became.
However, miniaturization of the structure was not yet sufficient.

Therefore, the present inventors have conducted intensive studies on this point, and as a result, the lower limit temperature: Ac ₁ point and the upper limit temperature: Ac ₃
Point or (Ac ₂ points + 100 ° C), whichever is the lower temperature range, and after applying the above-mentioned magnetic field application treatment in such a temperature range, immediately subject the steel material to a processing treatment,
Further, it has been found that the intended purpose can be advantageously achieved by performing annealing thereafter. In addition, it has been found that the above-described processing need not necessarily be performed after the magnetic field application processing, and the same effect can be obtained even before the magnetic field application processing or during the magnetic field application processing. That is,
It has been found that it is extremely effective to effectively combine the heating to a predetermined temperature range, the magnetic field application processing, and the processing in order to reduce the size of the structure.
The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. C: A steel material containing 0.05% by mass or more and 0.80% by mass or less and having a ferromagnetic phase at room temperature of 20% or more by volume, a lower limit temperature: Ac ₁ point, an upper limit temperature: Ac ₃ points or (Ac ₂ points) +100
C), wherein the steel material is subjected to a magnetic field application treatment of 0.1 to 20 T and a processing treatment in the temperature range of any lower temperature, and a double phase structure having a fine braid. Method of manufacturing steel.

2. In the above item 1, the method for producing a dual-phase structure steel material having a fine structure is characterized in that the magnetic field application process is performed a plurality of times and the direction of application of the magnetic field at that time is sequentially changed.

3. In the above 1 or 2, the method for producing a dual-phase structure steel material having a fine structure, wherein a magnetic field is applied in an arbitrary direction perpendicular to the processing direction of the steel material when performing the magnetic field application treatment. Here, the processing direction refers to a direction in which the material is stretched by the processing, for example, a rolling direction in the case of rolling, and a tensile direction in the case of tensile processing.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, experimental results which led to the present invention will be described. C: 0.6 mass%, Si: 0.
Contains 2 mass% and Mn: 0.4 mass%, with the balance substantially
A hot-rolled steel sheet having a composition of Fe (the ratio of the ferromagnetic phase at room temperature: 95 vol%) and a thickness of 1.5 mm is not more than _three points Ac.
After rapidly heating to 45 ° C. and annealing for 5 minutes while applying a magnetic field of 5 T, the steel was quenched and cooled to room temperature. The application direction of the magnetic field is three in the rolling direction, the width direction and the thickness direction.
Conditions. Polish the cross section of the obtained sample to 3 vol.
After being corroded with an aqueous solution of alcohol, the photograph was taken under a microscope. 1 and 2 show micrographs of the structure when a magnetic field is applied in the rolling direction and the width direction. In the figure, the black parts are the austenite phase reversed by heat treatment (observed as martensite after quenching),
On the other hand, what looks white is the residual ferrite phase, and the characteristic feature is that the structure is oriented in the direction of applying a magnetic field regardless of the direction of hot rolling.

Similarly, FIG. 3 shows a photograph of the structure when a magnetic field is applied in the thickness direction. In this case as well, the tissue is oriented in the thickness direction, which is the direction in which the magnetic field is applied. From the above results, it was confirmed that the structure can be controlled by using the heat treatment and the magnetic field application treatment regardless of the history of hot rolling and the like.

Further, the steel material to which a magnetic field was applied in the thickness direction was immediately rolled in the direction in which the magnetic field was applied after the above-mentioned magnetic field application treatment, and was taken out when the rolling reduction reached 70%. The structure after the crystal annealing was observed by SEM. As a result, the presence of flat grains was extremely low, and 1
It was confirmed that the particles were fine particles of μm or less.

As described above, by heating a material having a ferromagnetic phase to a two-phase region and applying reverse magnetic transformation while applying a magnetic field, the structure can be controlled irrespective of the history of rolling and the like. In other words, it is possible to eliminate the texture formed by rolling, the flat grains extending in the rolling direction, and the like, and it is possible to further refine the structure by combining such a magnetic field application treatment with the processing. It was determined.

The mechanism of the phenomenon in which the structure of the steel material takes a shape elongated in the direction of the magnetic field by applying the magnetic field according to the present invention is considered as follows. In other words, when reverse transformation is performed under a magnetic field, a paramagnetic austenite phase nucleates inside the ferromagnetic ferrite phase, in which case the shape takes such a form that the increase in the overall magnetostatic energy is minimized. . Such a shape is considered to be a spheroid extending in the direction of the magnetic field. Then, a large number of such austenite nuclei are generated, grow and coalesce, the reverse transformation proceeds, and finally, the shape becomes elongated in the direction of applying the magnetic field. FIGS. 4A and 4B schematically show this mechanism.

Although the reason why the crystal grains can be further refined by combining such a magnetic field application process and a processing process has not yet been clearly elucidated, the magnetic field application process is linked with the processing process. It is considered that by performing the above, it is possible to effectively break the elongated grains and texture by rolling or the like, and as a result, it became possible to introduce more recrystallization nuclei.

Therefore, the present invention is applicable to any practical steel as long as the material is a dual phase steel composed of a paramagnetic phase and a ferromagnetic phase. However, those having the following composition and structure are particularly suitable. C: 0.05 to 0.8 mass% If the C content is less than 0.05 mass%, the austenite phase in the two-phase region is reduced, and improvement in mechanical properties cannot be expected. On the other hand, if the content exceeds 0.8 mass%, reverse transformation in the two-phase region as intended in the present invention does not occur, that is, the two-phase region of cementite, which is a paramagnetic phase, and austenite, which is a paramagnetic phase, becomes the above-mentioned cellular structure. Cannot be obtained, so the C content is limited to the range of 0.05 to 0.8 mass%.
The other components are not particularly limited, and the ratio of the ferromagnetic phase at room temperature is 20 vol.
%, As long as it is a component system that forms a phase exceeding 0.1%.

As described above, in the present invention, the volume ratio of the ferromagnetic phase at room temperature needs to be 20% or more in the steel structure. This is because if the ratio of the ferromagnetic phase is less than 20% at room temperature, even if the phase is very finely dispersed, the effect of controlling the structure by the magnetic field is small and does not lead to the refinement of the structure.

Next, the reason why the heating conditions of the steel material and the magnetic field application treatment conditions in the present invention are limited to the above ranges will be described. First, the lower limit of the strength of the magnetic field applied during the heat treatment was set to 0.1 T because the magnetic effect was small below 0.1 T, while the upper limit was set to 20 T
This is based on the strength of a magnetic field that can be industrially generated in a large space. The type of the magnetic field may be a static magnetic field or a low-frequency fluctuating magnetic field, but usually a DC static magnetic field is more advantageous.

Next, the magnetic field application temperature will be described.
Be Ac ₁ point or more temperature is needed to cause the reverse transformation is clear from the equilibrium diagram. Similarly, the reason why one of the upper limit temperatures is set to the Ac ₃ point is that if the temperature is higher than the Ac ₃ point, it becomes a single phase region of austenite and does not become a two-phase region of a paramagnetic phase and a ferromagnetic phase. Since the Ac ₃ point may increase at a rate of 1 to 2 ° C./T due to the application of the magnetic field, the Ac ₃ point is set in the state where the magnetic field is applied. In addition, another upper limit temperature from Ac ₂ point (Curie point)
100 ° C higher temperature means that when a strong magnetic field is applied, Ac ₂
Beyond this point, up to about + 100 ° C, the austenite phase, which is a paramagnetic phase, and the ferrite phase, which is a ferromagnetic phase, have the effect of forming a long cell structure extending in the direction of the magnetic field. Although it is not performed, about 1 to 20 minutes is preferable.

In the present invention, the timing of processing is the same as that of the miniaturization effect during, immediately before, or immediately after the application of a magnetic field. Further, the application of the magnetic field may be performed a plurality of times so that such processing is performed at two or more timings during, immediately before, and immediately after the application of the magnetic field. In the present invention, “immediately before the application of a magnetic field” means that the time from the end of the processing to the start of the application of the magnetic field is within 1 minute, and “immediately after the application of the magnetic field” means that the time from the end of the application of the magnetic field to the start of the processing is within 1 minute. Say.
In the case where the processing is performed immediately after the application of the magnetic field, it is advantageous to perform an annealing process thereafter.

The degree of processing is not particularly limited, but if the processing amount is too small, it is difficult to obtain a desired effect of miniaturization.
% Is preferable.

Furthermore, in the present invention, in order to further refine the crystal grains, it is preferable to apply a magnetic field to the steel material a plurality of times. In that case, the application direction of the magnetic field in any one of the magnetic field application processes may be different from the application direction of the magnetic field in the other application processes. As described above, if the application direction of the magnetic field is sequentially changed, a more uniform structure can be obtained with a plurality of directions in which the paramagnetic phase crystal grains generated by the reverse transformation extend.

Further, when the steel material is processed, the crystal grains are elongated and flattened in the processing direction, so that a magnetic field is applied in an arbitrary direction perpendicular to the processing direction, thereby extending in the direction perpendicular to the processing direction. By having a cellular structure, the generation of elongated grains and flat grains due to processing can be suppressed. For this reason, the direction of application of the magnetic field is preferably an arbitrary direction perpendicular to the processing direction of the steel material, and at least one direction is required even when the magnetic field application processing is performed a plurality of times and the magnetic field application direction is sequentially changed. It is preferable to apply a magnetic field in the direction perpendicular to the processing direction. In the present invention, the shape of the steel material is not particularly limited, and can be applied to any ordinary steel material such as a plate, a wire, a bar and a mold.

[0027]

EXAMPLES Example 1 Steel materials having the chemical compositions shown in Table 1 were prepared by vacuum melting. These steel materials were hot-rolled to a thickness of 4 mm, and then pickled to obtain test materials. Next, these steel materials were subjected to a rolling process. A superconducting magnet was installed immediately before the rolling mill, and a heating furnace was installed at a position where the maximum magnetic field of the magnet was obtained. A heat treatment in a magnetic field and a rolling treatment were performed using the above-described equipment. The application direction of the magnetic field is the plate thickness direction, and the heating temperature in the magnetic field is summarized in Table 2. The final thickness is 1.5 mm by rolling.
(Reduction rate 62%). Further, for some of the samples, the rolling reduction was reduced to 20%, and the same material was prepared.

A JIS No. 13 B tensile test piece was sampled from the obtained plate material so that the rolling direction and the direction perpendicular to the rolling direction (width direction) were the tensile direction, and the tensile strength (tensile speed: 10 mm /
s) and total elongation were measured. In addition, after polishing the rolled surface of the sheet, the structure was observed by SEM to see whether the grains were flat grains or equiaxed grains. In this case, the definition of the flat grains was defined as a case where the area of the grains was four times or more as compared with other fine grains. Further, the crystal grain size was examined in a range of 250 μm × 250 μm. If the proportion of flat grains exceeds 10%, the results are indicated as flat grains. The method for measuring the crystal grain size is as follows. First, EBSP (Electron B
(Ack Scattering Pattern) was used to measure in the range of 250 μm × 250 μm, and the orientation difference between adjacent crystal grains was examined. The average grain size of a crystal grain having an orientation difference of less than 15 ° was measured as one crystal grain. The experimental results thus obtained are also shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from the results shown in Table 2, No. 1 was tested using various steel materials obtained according to the present invention.
For ~ 14, the crystal grain size was as fine as about 1.4 μm or less, and the ratio of flat grains was low.
Both the tensile strength and the elongation showed a well-balanced mechanical property in both vertical directions. On the other hand, in all the experiments (Nos. 17 to 30) using the same steel material without applying a magnetic field, the ratio of flat grains exceeded 13 vol% and the crystal grain size was 6 μm or more. As a result, the mechanical properties were low, and the anisotropy of elongation in the direction perpendicular to the rolling direction also increased. From Table 2, it can be seen that the elongation value in the direction perpendicular to the rolling direction is 70% or less and the anisotropy is large in each of the comparative examples with respect to the elongation value in the direction parallel to the rolling direction. Steel H has only 18vol% ferromagnetic phase,
In both cases with magnetism (Nos. 15 and 16) and without magnetism (Nos. 31 and 32), crystal grains are large and characteristic values are low. The reason for this is that the effect of the magnetic field did not appear because the ferromagnetic phase was small. Furthermore, when the rolling reduction is reduced to 20% (N
o.33, 34), the grain size and mechanical properties are
When processing at a draft of 62% without applying a magnetic field (No.2
1, 22), and it was found that even if the amount of processing was significantly reduced, the same characteristics as those without a magnetic field were obtained.

Example 2 In this example, the effects of the heating temperature and the magnetic field strength were examined. C: 0.61 mass%, Si: 0.45 mass% and Mn: 0.
A steel material (Ac ₁ = 730 ° C., Ac ₂ = 755 ° C., Ac ₃ = 808 ° C .; ferromagnetic phase ratio: 96 vol%) containing 60 mass% and the balance being substantially Fe After rolling to a plate thickness of 4 mm, the plate was pickled and used as a test material. The subsequent experimental method was the same as in Example 1. Table 3 summarizes specific processing conditions and obtained results.

[0033]

[Table 3]

As is clear from Table 3, when the treatment was carried out according to the present invention (Nos. 35 to 39), fine crystal grains were obtained, and as a result, the mechanical properties were also good. . On the other hand, in No. 40, the heating temperature was too low, the reverse transformation did not progress, and the effect of the magnetic field was not recognized. No.41
On the contrary, the heating temperature was too high, crystal grains grew, and proper structure control could not be performed. In addition, No. 42
The magnetic field strength was too low and no effect of the magnetic field was observed.

Example 3 In this example, the effect of the order of rolling and magnetic field treatment was investigated. Steel type C shown in Table 1 was prepared and hot-rolled to a sheet thickness of 4 mm, and then pickled to obtain a test material. The experiment method performed the following five types of experiments by changing the installation positions of the rolling mill and the magnet. The rolling reduction was 62%, which was the same as in Example 1.・ No.43: Magnetic field heat treatment (magnetic field applied in sheet thickness direction)-rolling ・ No.44: Heating-rolling while applying magnetic field in sheet thickness direction ・ No.45: Magnetic field heat treatment (magnetic field applied in sheet thickness direction)-rolling −
No. 46: Magnetic field heat treatment (magnetic field applied in a direction inclined 20 ° from the initial magnetic field application direction) ・ No. 46: Magnetic field heat treatment (magnetic field applied in the thickness direction)-rolling-
No. 47: Magnetic field heat treatment (magnetic field applied in the direction inclined 60 ° from the initial magnetic field application direction) ・ No. 47: Magnetic field heat treatment (magnetic field applied in the thickness direction)-rolling-
Magnetic field heat treatment (magnetic field application in a direction inclined by 90 ° from the initial magnetic field application direction) Evaluation after the experiment was performed in the same manner as in Example 1. Table 4 shows the experimental conditions, and Table 5 shows the experimental results.

[0036]

[Table 4]

[0037]

[Table 5]

As is clear from Table 5, in each case, a fine equiaxed grain structure having a crystal grain size of 1.5 μm or less was obtained. In particular, when the magnetic field application direction was successively changed twice (Nos. 45 to 47), further miniaturization was achieved and the characteristics were good.

Example 4 In this example, a material having a rod shape was examined. That is, as a steel material, C: 0.1 mass%, Si:
Contains 0.2 mass%, Mn: 1.3 mass%, with the balance substantially
Steel bar with Fe composition (Ac ₁ = 715 ° C, Ac ₂ = 750 ° C, A
c ₃ = 858 ° C .; ferromagnetic phase ratio: 91 vol%). A superconducting magnet was installed immediately after the final finishing mill for rolling the steel bar, and a heating furnace was installed at a position where the maximum magnetic field of the magnet was reached. Rolling-heat treatment in a magnetic field-rapid cooling treatment was performed using such an apparatus. At this time, the direction of application of the magnetic field was set to be perpendicular to the rolling direction of the steel bar.

A test piece was cut out from the longitudinal direction of the rolled steel bar. The observation and evaluation of the crystal grain size were performed on both the cross section in the longitudinal direction and the cross section in the vertical direction and the vertical direction. First, the structure was observed by SEM to observe whether the grains were elongated grains or equiaxed grains. In this case, the definition of the elongated grain was defined as having an aspect ratio of 2 or more. When the proportion of the elongated grains exceeded 10% due to the influence of the processing, the result was entered as the elongated grains. The evaluation of the crystal grain size was performed in the same manner as in Example 1.
The tensile test and elongation were performed on JIS No. 13B tensile test pieces cut out with the longitudinal direction of the steel bar taken as the tensile direction.
For comparison, a similar structure observation and tensile test were also performed on a sample under the condition that no magnetic field was applied by a superconducting magnet and no heat treatment was performed by a heating furnace. Table 6 summarizes specific experimental conditions and experimental results.

[0041]

[Table 6]

As shown in Table 6, in the case of a method not using a normal magnetic field, the number of elongated grains increased in the longitudinal direction, and the tensile strength was at a level of 500 MPa. On the other hand, in the steel obtained according to the present invention, the crystal grains were fine in both the longitudinal and radial cross sections of the steel bar, and the mechanical properties were significantly improved.

[0043]

As described above, according to the present invention, it is possible to stably obtain a fine-structured dual-phase steel having excellent strength and ductility, which is extremely useful in industry.

[Brief description of the drawings]

FIG. 1 shows a hot rolled steel sheet containing 0.6 mass% of C:
745 ° C in the magnetic field (the direction of the applied magnetic field is parallel to the rolling direction)
It is a microscope structure photograph of the sample reverse-transformed for 5 minutes.

Fig. 2 C: Hot rolled steel sheet containing 0.6 mass%
745 ° C in the magnetic field (the direction of the applied magnetic field is perpendicular to the rolling direction)
It is a microscope structure photograph of the sample reverse-transformed for 5 minutes.

FIG. 3 shows a hot rolled steel sheet containing 0.6 mass% of C:
5 is a microstructure photograph of a sample reverse-transformed at 745 ° C. for 5 minutes in a magnetic field (magnetic field application direction is a plate thickness direction).

FIG. 4 is a schematic diagram showing a tissue orientation mechanism by applying a magnetic field.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Michio Shimomeme 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside Kawasaki Steel Research Institute (72) Inventor Yasunori Yonehana 1-Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Yoshio Abe (72) Inventor Yoshio Abe 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division of Nippon Steel Corporation (reference) 4K037 EA05 EA06 EA13 EA13 EA15 EA17 EA19 EA27 EA28 EA31 EB11 FG10 JA06

Claims (3)

[Claims] 1. A steel material containing C: 0.05 mass% or more and 0.80 mass% or less and having a ferromagnetic phase at room temperature of 20% or more in volume, a lower limit temperature: Ac ₁ point, and an upper limit temperature: Ac ₃ points. Or (A
c ₂ points + 100 ° C.), and heat-treating the steel material in the temperature range of 0.1 to 20 T in the temperature range of 0.1 to 20 T and processing the fine braid in the temperature range. A method for producing a steel having a dual phase structure. 2. The method according to claim 1, wherein the application of the magnetic field is performed a plurality of times, and the application direction of the magnetic field at that time is sequentially changed. 3. The dual-phase structure steel material having a fine structure according to claim 1, wherein a magnetic field is applied in an arbitrary direction perpendicular to a processing direction of the steel material when the magnetic field application process is performed. Manufacturing method.