JP2005171324A - Method for producing stainless steel restraining development of two-phase structure formation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a stainless steel strip excellent in the workability in soft quality by setting annealing temperature as higher as possible while preventing two-phase structure formation. <P>SOLUTION: When the stainless steel forming ferrite+austenite two-phase structure at the high temperature range exceeding Ac<SB>1</SB>transformation point, is continuously annealed, to the annealed stainless steel, an X-ray diffraction is applied and the X-ray diffraction profile is obtained and thus, the transformation from the austenite to the martensite is judged from the spreading of width in the X-ray diffraction profile. When the transformation from the austenite to the martensite is decided, the temperature-rising condition is relaxed with the lowering of the temperature in the furnace, the transportation speed of the steel strip, etc. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a stainless steel with good material properties by preventing the formation of a two-phase structure due to over-annealing when annealing a stainless steel exhibiting a two-phase structure in a high temperature range.

  Stainless steel is used in a wide range of fields, including kitchen equipment, manufacturing equipment, vehicle structural materials and equipment, and building materials, utilizing its excellent corrosion resistance. Stainless steel strips or stainless steel plates (hereinafter collectively referred to as stainless steel strips) are formed into target shapes according to applications by deep drawing, overhanging, stretch flange forming, bending, cold forging, etc. With the diversification of products, there is an increasing demand for high workability.

  In order to cope with the sophistication of processing, materials with excellent processability are being developed. Ductility is one of the important factors that affect workability, but ductility is significantly affected by manufacturing conditions. For this reason, even if a steel type exhibiting excellent ductility in the material itself is developed, the original performance is not sufficiently exhibited unless the manufacturing conditions are appropriate. Among the manufacturing conditions, recrystallization annealing has a great influence on the material properties, and the aim of recrystallization annealing is to finish the metal structure with a high ductility. The proper metal structure in this case is a metal structure in which no crystal distortion remains inside, and an annealing condition for heating at a sufficient temperature or time is adopted so that an appropriate metal structure is obtained.

Considering that the stainless steel strip is manufactured in a continuous line from the viewpoint of productivity, if the annealing time is set to be long in order to obtain an appropriate metal structure, the line speed becomes slow and the productivity is hindered. Therefore, a method is selected in which the temperature of the annealing furnace is set high, and a sufficient temperature is applied to the stainless steel strip that is being passed through at high speed.
However, when ferritic stainless steel such as SUS430 and martensitic stainless steel such as SUS410 and SUS420J2 are heated at a high temperature beyond the Ac ₁ transformation point, which is the temperature at which reverse transformation starts when the temperature rises, ferrite + austenite Presents a two-phase structure. Since the austenite phase is transformed into a hard martensite phase in the process of cooling the stainless steel strip to room temperature, the dual phase structure becomes a ferrite + martensite dual phase structure. As a result, the obtained stainless steel strip is a hard material with poor ductility.

Since the hot-rolled material having a two-phase structure of ferrite and austenite is hard, it is softened prior to processing into a product shape or cold rolling. In softening, a hot rolled material is charged into a box-type annealing furnace, and the martensite phase is decomposed into a ferrite phase and a carbide by heating for a long time below the Ac ₁ transformation point. Moreover, since the cold-rolled stainless steel strip is work-hardened, it is softened by intermediate annealing or finish annealing. However, when the stainless steel strip is heated to a temperature exceeding the Ac ₁ transformation point due to fluctuations in furnace conditions during the softening treatment, a ferrite + martensite two-phase structure with low ductility is obtained again.
Stainless steel strips with reduced ductility cannot be discerned in appearance, and abnormalities may be detected by mechanical tests of product elongation and hardness at the final stage. Since the stainless steel strip in which an abnormality is detected at the final stage needs to be scrapped or subjected to a second heat treatment, the manufacturing cost increases significantly.

In order to prevent material abnormalities due to the formation of ferrite + martensite two-phase structure, the structure of the heat-treated stainless steel strip is measured in-line and annealed to prevent the formation of ferrite + austenite two-phase structure at high temperatures. It is conceivable to control the conditions. A recrystallized sensor using magnetostriction has been developed as a means for detecting the state in which the Ac ₁ transformation point is exceeded and a ferrite + austenite two-phase structure is formed after annealing. However, since both the soft ferrite phase and the hard martensite phase are ferromagnetic materials, the detection accuracy of the recrystallization sensor is low, and a sufficiently reliable measurement result cannot be obtained. No other practical means for measuring the ferrite + austenite two-phase structure in-line has been reported.
Therefore, in actual operation, over-annealing is avoided by passing a stainless steel strip at a low furnace temperature, and recrystallization annealing is a bottleneck for improving productivity.

The present invention has been devised to solve such a problem. By adopting X-ray diffraction that can determine the presence or absence of a two-phase structure by observing a stainless steel strip after annealing, For this reason, even if annealing is performed under conditions where the steel strip temperature is as close to the Ac ₁ transformation point as possible, there is no generation of a ferrite + austenite two-phase structure, and a stainless steel strip having an appropriate metal structure with excellent ductility is produced at high production. It aims at manufacturing with the property.

In order to achieve the object, the present invention performs X-ray diffraction by subjecting the annealed stainless steel to X-ray diffraction when continuously annealing a stainless steel having a ferrite + austenite two-phase structure in a high temperature range exceeding the Ac ₁ transformation point. A profile is obtained, and the austenite → martensite transformation is determined from the width of the X-ray diffraction profile, and when the austenite → martensite transformation is determined, the temperature rise condition of the stainless steel is relaxed.
Actions such as a decrease in the furnace temperature and an increase in the sheet feeding speed are adopted to ease the temperature rise conditions.

In the present invention, the presence or absence of the ferrite + austenite two-phase structure is determined by continuously X-ray diffracting the surface of the annealed stainless steel strip. When the ferrite + austenite two-phase structure begins to be detected, the steel strip temperature is lowered below the Ac ₁ transformation point by actions such as lowering the furnace temperature and increasing the sheeting speed, thereby suppressing the formation and growth of the ferrite + austenite two-phase structure. . Application of X-ray diffraction to tissue observation is a known technique (Non-Patent Document 1), but the present invention uses this for the determination of the occurrence of a two-phase structure, and an appropriate metal structure rich in ductility. This makes it possible to produce stainless steel strips with
The Japan Society of Materials Science X-ray Material Strength Committee “The 23rd X-ray Material Strength Discussion Papers” pp. 20-26

When the stainless steel strip is heated to a temperature range exceeding the Ac ₁ transformation point, an austenite phase is generated by reverse transformation, and the austenite phase is transformed into a hard martensite phase after cooling. Transformation strain generated in the stainless steel strip due to martensitic phase transformation is detected by a change in the width of the X-ray diffraction profile. That is, the two-phase organization of the stainless steel strip is determined from the change in the width of the X-ray diffraction profile.
In the unstrained state of the crystals on the surface of the stainless steel strip, an X-ray diffraction profile is obtained under conditions that satisfy the Bragg equation nλ = 2dsinθ (FIG. 1a). That is, an X-ray diffraction profile appears at a position of 2θ that satisfies the diffraction condition with the crystal lattice spacing d and the wavelength λ of the characteristic X-ray used for diffraction. The X-ray diffraction profile is measured with a distribution having a width around a theoretical 2θ position due to the spread of the wavelength λ of characteristic X-rays and the scattering factor of the measurement optical system.

When the tensile stress in the elastic region acts and a uniform strain occurs in the crystal, the unstrained profile moves to the position θ + Δθ so as to satisfy the Bragg equation according to the strain change d−Δd (FIG. 1b). . In this case, the width of the X-ray diffraction profile is not changed, and only the diffraction angle θ is moved.
When random stress acts and non-uniform strain occurs in the crystal, diffracted X-rays are detected with a distribution dispersed in the range of θ ± Δθ so that the Bragg equation is satisfied according to various strain changes d ± Δd. (FIG. 1c). Therefore, the X-ray diffraction profile becomes wider.

  When a stainless steel strip in which the austenite phase is transformed into the martensite phase is X-ray diffracted, an X-ray diffraction profile broadened by shear strain generated in the crystal is obtained. That is, the surface of the annealed stainless steel strip is X-ray diffracted and the change in the width of the X-ray diffraction profile is measured to determine the presence or absence of a two-phase structure due to over-annealing. When the annealing conditions are adjusted so as not to have a ferrite + austenite two-phase structure in a high temperature range according to the determination result, there is no austenite → martensite transformation, and a soft and highly ductile stainless steel strip is obtained.

When the surface of the stainless steel strip is X-ray diffracted by an optical system using a concentrated beam with high resolution, an accurate X-ray diffraction profile can be obtained. However, the concentrated beam method is affected by the geometrical conditions of the optical system, specifically the distance between the steel strip surface and the X-ray source and the convergent part of the diffracted X-rays. Easy to be incorporated into diffraction profiles. In this respect, the parallel X-ray method using parallel X-rays that are not easily affected by the geometric conditions of the optical system is suitable at the production line site.
Regarding strain measurement using parallel X-rays, a method of measuring strain of grain-oriented electrical steel sheets (Patent Document 1) diffracts an X-ray source with a single crystal such as Si to reduce the divergence angle to 100 seconds or less. Linearization is necessary, and X-ray intensity is reduced by parallel X-ray conversion. Therefore, it cannot be used for determination of two-phase organization that requires industrially high-speed continuous measurement.
JP-A-8-297104

In addition, although a proportional counter such as a scintillation counter is frequently used to detect the diffracted reflected X-ray, this method requires scanning the 2θ range where the diffraction line appears, and takes a long time for measurement. Therefore, it is preferable to use a position sensitive proportional counter that can obtain an X-ray diffraction profile in a short time without the need for scanning.
Specifically, the X-ray emitted from the X-ray tube 1 is passed through the collimator 2, the measurement sample S is irradiated as the incident X-ray Lin, and the diffracted X-ray Lref from the measurement sample S is converted into a position sensitive proportional counter. 3 to detect. The position sensitive proportional counter 3 is provided with a linear core wire along the linear X-ray inlet, and detects the time difference occurring at both ends of the core wire, thereby reducing the position resolution in the core direction (one-dimensional direction). It is a leaning X-ray detector. The diffraction X-ray Lref from the measurement sample S can also be detected by a CCD-X-ray detector in which a charge coupled device (CCD) is linearly arranged on the X-ray light receiving surface.

Various stainless steel strips having the compositions shown in Table 1 were used as measurement samples S for X-ray diffraction. Sample Nos. 1 to 3 are SUS430 having different Ac ₁ transformation points, and sample Nos. 4 to 6 are martensitic stainless steels having different C contents, such as SUS410, SUS410S, and SUS420J2.

Each sample had a plate thickness of 1 mm and was sheared to a 50 mm square to form a test piece, and a thermocouple was attached to the center of the test piece by welding. The test piece was placed in the Elema Electric Furnace, heated to the target material temperature, and then subjected to heat treatment for air cooling.
The heat-treated test piece was pickled to measure the surface hardness, and an X-ray diffraction profile was obtained by X-ray diffracting the surface of the test piece near the hardness measurement position. In addition, since the temperature of the test piece is increased by radiant heat irradiation during the heat treatment, and the information obtained by X-ray diffraction is the diffraction information of the extreme surface layer portion of the test piece of 10 μm or less, the structure change information of the extreme surface layer portion is obtained The surface hardness was measured at a load of 1 kg.

  In the X-ray diffraction, a sealed cathode of Cr counter-cathode was used for the X-ray tube 1 and 40 kV-30 mA was loaded. At this time, the focal spot size had a diameter of about 0.5 mm. X-rays from the X-ray tube 1 were sent to a 90 mm long single or double pinhole collimator 2 to make the incident X-ray Lin close to a parallel beam. The pinhole diameter of the collimator 2 was changed in the range of 0.1 to 4 mm in order to examine the influence of the divergence angle. The X-ray diffraction plane was α (211) and 2θ was about 155 degrees.

  About the thing using the single collimator 2 with a pinhole diameter of 4 mm, the divergence angle was set to 0.137 degree (0.0024 rad), and the influence which the divergence angle had on the measurement result was investigated. If the X-ray intensity is the same even if the divergence angle is changed by changing the pinhole diameter, the accuracy will hardly change even if the divergence angle is reduced and the resolution is improved. It means that the prolongation becomes obvious. Further, when the pinhole diameter is reduced and the divergence angle is reduced, the X-ray irradiation area is reduced, the number of crystal grains contributing to X-ray diffraction is reduced, and the continuity of the X-ray diffraction profile is lowered.

Even when the single collimator 2 having a pinhole diameter of 4 mm is used, the continuity of the X-ray diffraction profile is lowered when the crystal grain size is large. Therefore, the number of crystal grains contributing to X-ray diffraction was increased by reciprocating the measurement sample S with a stroke of 20 mm. The reciprocating motion of the measurement sample S corresponds to an operation for continuously measuring the stainless steel strip moving in the production line. The measurement time was set to 30 seconds.
When the X-ray diffraction profile was measured for Sample Nos. 1 to 3 and the relationship between the width of the X-ray diffraction profile and the heat treatment temperature was investigated, the relationship of FIG. 3 was established between the two. In the unstrained state, the diffraction lines of Kα1 and Kα2 are sufficiently separated, and the diffraction lines of Kα1 and Kα2 overlap and change in shape as the strain increases, so that the width of the X-ray diffraction profile is the highest of 1 The width of the X-ray diffraction profile in the range up to / 3 was read. As apparent from FIG. 3, there are temperatures (1) to (3) where the width of the X-ray diffraction profile begins to increase as the annealing temperature increases, and the temperatures (1) to (3) are the sample Nos. 1 to 3, respectively. It corresponded to an Ac ₁ transformation point of 3.

Similarly, when the relationship between the surface hardness and the width of the X-ray diffraction profile was obtained for sample Nos. 1 to 3, the relationship of FIG. 4 was established between the two. In the test piece annealed at a temperature not reaching the Ac ₁ transformation point, the surface hardness was in the vicinity of 165 HV, and the width of the X-ray diffraction profile was concentrated in the vicinity of 1.2 degrees. On the other hand, in the specimen annealed at a temperature exceeding the Ac ₁ transformation point, the surface hardness is increased and the width of the X-ray diffraction profile is increased, and the increase of the surface hardness and the width of the X-ray diffraction profile is increased. Was in a linear relationship.
The result of FIG. 4 shows that when the martensite phase is generated by over-annealing between two different materials to form a two-phase structure, the surface hardness and the width of the X-ray diffraction profile increase as the hard martensite phase increases. Means. In other words, the presence or absence of a two-phase structure that causes an increase in hardness or a decrease in ductility can be determined by whether or not the 1/3 width of the X-ray diffraction profile is within 1.3 degrees under the present measurement conditions.

Samples Nos. 4 to 6 were annealed at various heat treatment temperatures, and the annealed stainless steel strip was X-ray diffracted to investigate the relationship between the 1/3 width of the X-ray diffraction profile and the heat treatment temperature. As can be seen from the investigation results in FIG. 5, the 1/3 width of the X-ray diffraction profile spreads around the temperature (4) to (6), respectively, and the temperature (4) to (6) is the sample No. 4 It corresponded to each Ac ₁ transformation point.
The surface hardness of the annealed samples Nos. 4 to 6 was measured and the relationship with the annealing temperature was determined. As shown in FIG. 6, the specimens annealed at a temperature that did not reach the Ac ₁ transformation point were almost constant. Although the surface hardness was shown, the surface hardness of the test piece annealed beyond the Ac ₁ transformation point increased rapidly. However, in sample No. 6, the hardness level below the Ac ₁ transformation point is higher than the hardness level of the other test pieces. This is a SUS420J2 steel sample No. 6 containing 0.294% by mass and a relatively large amount of C, and a large amount of hard carbide generated in the heat treatment state before box annealing (box type annealing) is after finish annealing. This is also due to the remaining.

The two-phase structure that brings about an increase in hardness occurs when the stainless steel strip is heated to a temperature exceeding the Ac ₁ transformation point during intermediate annealing or finish annealing. Therefore, when the difference ΔHV between the hardness increased by forming a two-phase structure by heating at a temperature exceeding the Ac ₁ transformation point and the hardness before the Ac ₁ transformation is arranged by 1/3 width of the X-ray diffraction profile, The relationship shown in FIG. 7 was obtained.
Since the increase in the hardness difference ΔHV is caused by the formation of the martensite phase, a plastic strain corresponding to the formation of the martensite phase is generated, and the width of the X-ray diffraction profile is widened. The hardness difference ΔHV and the 1/3 width of the X-ray diffraction profile have a similar linear relationship between materials having different C contents. Therefore, the presence or absence of a two-phase structure that causes an increase in hardness or a decrease in ductility can be determined by whether or not the 1/3 width of the X-ray diffraction profile is within 1.3 degrees under the present measurement conditions.

The method for determining the martensitic transformation from the X-ray diffraction profile is not limited to stainless steel exhibiting a ferrite + austenite two-phase structure at high temperatures, but has an Ac ₁ transformation point, and the austenitic phase generated at high temperatures undergoes martensitic transformation. Thus, the present invention can be similarly applied to a steel material in which lattice distortion occurs. In addition, the measurement surface for irradiating X-rays has almost no change in the width of the X-ray diffraction profile even when the oxide scale is attached, pickled or brightly annealed. Line diffraction is possible.

A total of 20 coils of SUS430 and SUS420J2 annealed pickling materials with a plate thickness of 0.25 to 2.5 mm and a plate width of 1200 mm are manufactured on the actual line, and the surface hardness and X-ray diffraction profile was measured. Of the 20 coils, 3 coils were intentionally raised in furnace temperature, passed through a condition where the steel strip temperature slightly exceeded the Ac ₁ transformation point, and the two-phase structure was advanced.
FIG. 8 shows the measurement results of the surface hardness and X-ray diffraction profile of the actual line reclamation material. The stainless steel strip passed through under normal conditions had a surface hardness in the range of 150 to 170 HV, and a 1/3 width of the X-ray diffraction profile was also in the range of 1.2 to 1.37 degrees. When specimens cut from these stainless steel strips were subjected to a mechanical test, they showed normal proof stress and tensile strength, but the elongation was a high value of 30% or more.

On the other hand, among the coils that passed through under conditions where the steel strip temperature exceeded the Ac ₁ transformation point, those in which the A coil was two-phase textured by over-annealing had a surface hardness of 180 HV or more and 1/3 of the X-ray diffraction profile. The width also spread over 1.4 degrees. When a specimen was cut out from the vicinity of the hardness measurement point of the A coil and subjected to a mechanical test, the elongation was reduced to 22%, and the proof stress and tensile strength were also increased.
The coil B had a normal surface hardness of 165 HV at the center of the coil, but 180 HV or higher at both ends of the coil. From this, it can be seen that both end portions of the coil are two-phase textured by over-annealing. Two-phase organization is also confirmed by the fact that the 1/3 width of the X-ray diffraction profile extends to 1.4 degrees or more.

  In the C coil, the surface hardness on the work stand WS side is 172 HV, which is higher than that of other parts, and the 1/3 width of the X-ray diffraction profile is widened to 1.42 degrees. It seems that two-phase structure was formed by annealing. When the test piece cut out from the vicinity of the hardness measurement point of the C coil was mechanically tested, the center part and the drive side DS side showed normal proof stress and tensile strength, and the elongation was as high as 32%. On the other hand, on the work side WS side, the elongation was as low as 29%, the yield strength was slightly reduced, and the tensile strength was a normal value. The tensile test result on the work side WS side is a characteristic tendency when a martensitic transformation occurs in a very small part and a two-phase structure is formed. The result of the martensitic transformation appears as the disappearance of the yield point and the decrease in elongation. ing.

  In Example 1 and Example 2, the value of the 1/3 width of the X-ray diffraction profile at the two-phase organization boundary is different. This difference is that in Example 1, since the sample was prepared in the laboratory, almost no distortion was introduced into the sample. In Example 2, a slight distortion was introduced into the sample by contact with the driving rubber roll when the actual line was passed. It is derived from that.

  A SUS430 cold-rolled material having a thickness of 1 mm was passed through the actual line where the furnace temperature of the continuous annealing pickling equipment was maintained at 950 ° C. and 1050 ° C., and the sheet passing speed was gradually increased from 20 m / min. The X-ray diffraction profile measurement part of the same X-ray diffractometer as in Example 1 was attached to the exit side of the pickling equipment, and the 1/3 width of the X-ray diffraction profile was measured. Moreover, the test piece was cut out from the stainless steel strip after passing, and the surface hardness was measured.

  From the measurement results shown in FIG. 9, it was confirmed that there was high consistency between the surface hardness and the 1/3 width of the X-ray diffraction profile. It can be seen that at a furnace temperature of 950 ° C., the material temperature rose too much and the two-phase structure was formed when the plate passing speed did not reach 35 m / min. Moreover, since the stainless steel strip was not sufficiently heated when the plate passing speed was increased, insufficient softening was observed. When the plate passing speed was 65 m / min, the surface hardness was 170 HV and the 1/3 width of the X-ray diffraction profile was about 1.4 degrees. The elongation of this portion was about 27%, which was a lower value compared to the elongation of 32% when passed through at 35 to 50 m / min. The low elongation value is caused by annealing corrosion due to a delay in temperature rise at the center of the plate thickness when the plate passing speed is increased. Actually, the hardness at the center of the plate thickness was as high as 185 HV.

At a furnace temperature of 1050 ° C., the material temperature increased excessively and a two-phase structure was formed unless the plate passing speed reached 65 m / min. In addition, when the plate passing speed was increased, the stainless steel strip was not sufficiently heated and softening was insufficient. At a plate passing speed of 95 m / min, the surface hardness was about 170 HV and the 1/3 width of the X-ray diffraction profile was about 1.4 degrees. The elongation of this portion was also about 26%, which was a lower value than the elongation of 32% when the sheet was passed at 65 to 75 m / min. Further, the hardness at the center of the plate thickness was as high as 190 HV, and it was in a state of insufficient annealing.
Thus, by measuring the width of the X-ray diffraction profile in the actual line material as well, it is possible to detect the two-phase structure caused by over-annealing and to accurately determine the ductility reduction after annealing.

  As described above, in the present invention, the disorder of the crystal lattice that occurs when the austenite phase of the ferrite + austenite two-phase structure in the high temperature region transforms into the martensite phase during the cooling process is determined by the X-ray diffraction profile. ing. By adjusting to an annealing condition that does not cause an abnormal structure in accordance with the determination result, an increase in hardness and a decrease in ductility due to the formation of martensite phase are avoided, and a soft stainless steel having excellent workability is manufactured. In addition, high-speed operation is possible under conditions where the annealing temperature is as high as possible, so that productivity is also improved.

Schematic diagram showing the relationship between crystal state and X-ray diffraction profile Schematic diagram of an optical device for measuring X-ray diffraction profiles Graph showing relationship between annealing temperature and 1/3 width of X-ray diffraction profile Graph showing the relationship between surface hardness and 1/3 width of X-ray diffraction profile Graph showing relationship between annealing temperature and 1/3 width of X-ray diffraction profile Graph showing the relationship between annealing temperature and surface hardness Graph showing the relationship between the change in surface hardness and the 1/3 width of the X-ray diffraction profile Graph showing the relationship between measurement position, surface hardness, and 1/3 width of X-ray diffraction profile of actual line production materials A graph showing the relationship between the plate feed speed, hardness and 1/3 width of the X-ray diffraction profile of a stainless steel strip passed through a real line

Claims (2)

When continuously annealing a stainless steel with a ferrite + austenite two-phase structure in a high temperature range exceeding the Ac ₁ transformation point, the annealed stainless steel is X-ray diffracted to obtain an X-ray diffraction profile, and the X-ray diffraction profile is widened. A method for producing a stainless steel with suppressed two-phase structure, characterized in that the austenite → martensite transformation is determined and the temperature rise condition of the stainless steel is relaxed when the austenite → martensite transformation is determined.   The manufacturing method of Claim 1 which relieves the temperature rising condition of stainless steel by the fall of a furnace temperature, and / or the raise of a plate passing speed.

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