WO2004104235A1

WO2004104235A1 - Warm rolling method

Info

Publication number: WO2004104235A1
Application number: PCT/JP2004/007277
Authority: WO
Inventors: Shiro Torizuka; Eijiro Muramatsu; Tadanobu Inoue; Kotobu Nagai
Original assignee: National Institute For Materials Science
Priority date: 2003-05-20
Filing date: 2004-05-20
Publication date: 2004-12-02
Also published as: CN100366761C; US20060191613A1; TWI266659B; JP4221497B2; KR20060035603A; EP1642988B1; CN1791688A; EP1642988A4; EP1642988A1; JP2004346420A; KR100749381B1; TW200510084A

Abstract

A multi-directional warm-rolling method for manufacturing a superfine grain steel material with a superfine crystal grain structure of 3 μm or smaller in average grain size. When rolling of two passes or more is performed for a steel material in the rolling temperature range of 350 to 800˚C, a rolling by an oval shape hole die and a rolling by the other shape hole die are performed at least one time so that a large amount of strain can be introduced into the material by a simple means with less section reduction rate and less number of passes. Steel materials having the superfine crystal grain structure and excellent strength and ductility can be manufactured by this method.

Description

Description Warm rolling method Technical field

The invention of this application relates to a new warm rolling method for producing an ultrafine grained steel material having an ultrafine grain structure having a grain size of 3 x m or less and excellent in strength and ductility.

Background art

It is thought that ultrafine grained steel can significantly increase the strength without adding alloying elements, and at the same time, can significantly reduce the ductile-brittle transition temperature. In order to realize this ultrafine-grained steel industrially, we have invented a method of warm multi-pass rolling (Reference 1) and a method of multi-directional working (Reference 2).

However, if warm multi-directional rolling could be facilitated, ultra-fine grained steel would be used more widely, but this was not always easy in the process of the inventors' investigation.

This is because it is necessary to introduce a certain amount of strain into the material as a piece of technical IT. For example, the critical strain is

1.5-2.3, preferably about 3. However, in the case of a strain of 3, the reduction in area is equivalent to 95%, and large deformation must be performed. When trying to obtain a round bar with a diameter of 10 mm as a final product, it is necessary to warm work from a diameter of 45 mm.In order to introduce this large strain in a temperature range where deformation resistance is high, There was a problem that the material had to be enlarged and the number of rolling passes was inevitably increased.

Therefore, if a large strain can be introduced into a material with a smaller area reduction ratio and a smaller number of passes, an ultrafine structure can be obtained more easily, and industrially Thus, there are many advantages such as higher rolling efficiency. '

The inventors of this application have also proposed a method of compressing from multiple directions with an anvil (Reference 2) and a two-direction rolling reduction technique for multidirectional rolling. However, although multi-directional machining is a method that can efficiently introduce large strains, machining from at least two directions involved certain technical difficulties.

Reference 1: JP 2000-3098 50

Reference 2: JP 200 1—2409 1 2

Therefore, the invention of this application has been made in view of the above background, and further expands the knowledge obtained from the previous studies by the inventors, and realizes a large distortion by simpler means. Provides a new warm multi-directional rolling method that can introduce the steel into the material with a smaller area reduction rate and a smaller number of passes, and has an ultra-fine grain structure resulting in excellent strength and ductility. It is an object of the present invention to provide a manufacturing method.

Disclosure of the invention

The invention of this application solves the above-mentioned problems. First, there is provided a warm rolling method for producing an ultrafine grained steel material having an ultrafine grain structure with an average grain size of 3 μπι or less. When rolling two or more passes in steel at a rolling temperature range of 350 to -800 in a steel material, at least one or more rolls with one-hole shape die and another shape die shape The second feature is to provide a warm rolling method characterized by performing rolling by an opal-shaped die, followed by rolling by another type of die. A rolling method is provided.

Thirdly, the present invention provides a warm rolling method characterized in that the other shape of the hole is a square shape or a round shape.

Fourth, any of the above-mentioned warm rolling methods, in which, when パス> 2, of the total number of passes, rolling is performed twice or more and at most ΝΖ2 or less with an oval hole die. The fifth method is to perform continuous two-pass rolling. Sixth, the warm rolling method according to any one of the above, characterized in that, in the two-pass rolling in which the shape of the hole is one pallet and the angle, the area reduction after the square hole rolling from the material is 20% or more. Seventh, rolling is a combination of two-pass rolling in which the shape of the hole is an opal and a corner. The present invention provides a warm rolling method characterized in that a reduction in area is 60% or more in a single rolling operation.

Eighth, the invention of the present application is characterized in that it includes a rolling step in which the maximum minor axis length of the material after rolling in the opal hole type is 75% or less of the material-to-side length before oval rolling. Ninth, in any one of the above-mentioned warm rolling methods, a plastic strain of 1.5 or more is introduced into at least a 50% by volume region inside the material. The tenth rolling method is a warm rolling method characterized by introducing a plastic strain of 2 or more in a region of 90% by volume or more inside the material. The first method is as follows. The rolling condition parameter Z expressed by the formula is 11 or more (when the structure just before rolling is ferrite, bainite, martensite, pearlite, etc., and the Fe crystal structure is bcc) or 20 or more (just before rolling). The structure is austenite and the crystal structure of Fe is fcc). The warm rolling method of a,

ε: strain

t: Time from start to end of rolling (s)

T: Rolling temperature C, for multi-pass rolling, the average of rolling temperatures in each pass)

Q: Use 254,000 when the structure just before rolling is ferrite, payinite, martensite, or pearlite as the matrix, and 300,000 when austenite is the matrix. The first and second aspects provide a warm rolling method characterized in that the reduction ratio of the initial material and the area after final rolling is 90% or less, and the thirteenth is an average crystal in the C section or the L section. Warm rolling, which is characterized by the production of ultrafine-grained steel with a grain size of 3 microns or less; ^ Specializes in producing certain ultra-fine grained steel A warm rolling method is provided.

The invention of this application having the features described above has been completed based on new findings obtained by the study of the inventor. In other words, it has been known that caliber rolling, in which rolling is performed using a roll having a grooved groove, is generally used as a method for manufacturing a steel bar, and the shape of the grooved shape is square (square, square). Diamond type), opal type and round type. By performing caliber (groove roll) rolling in the warm temperature range, an ultrafine-grain ferrite-based structure can be obtained by multi-pass rolling (Reference 1). It has been found that the use of the opal hole type is effective for making the L section of the steel bar (cross section parallel to the longitudinal direction of the bar) ferrite grain equiaxed.

As a result of earnest research by the inventor, recently, by performing force ripper rolling combining an opal hole type with other types of holes such as a square shape and a round shape in an appropriate temperature range, even with a relatively small area reduction rate, We discovered that large strains could be introduced into the material and were able to establish it as a technology.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing a hole type according to the first embodiment.

Figure 2 is a photograph showing the C section of the bar after rolling.

Figure 3 is a mesh diagram of the material. .

FIG. 4 is a view showing the plastic strain after one pass.

FIG. 5 is a diagram showing the plastic strain after the two-pass square hole type.

FIG. 6 is a diagram showing the plastic strain after three passes.

FIG. 7 is a diagram showing the plastic strain after the 4-pass square hole type.

FIG. 8 is a diagram showing the plastic strain after 5-pass opal.

FIG. 9 is a diagram showing the plastic strain after a 6-pass round hole.

FIG. 10 is an SEM image of the tissue after the two-pass square hole type.

1 1 is a SEM image of the tissue after the 4-pass square hole type.

FIG. 12 is an SEM image of the tissues of Examples 2 to 4. FIG. 13 is a diagram showing a hole type.

Figure 14 is a photograph showing the C section of the bar after rolling.

Figure 15 is a SEM image of the tissue.

FIG. 16 is a SEM image of the tissue of Comparative Example 1.

FIG. 17 is a diagram showing the relationship between the parameter Z and the average particle size. BEST MODE FOR CARRYING OUT THE INVENTION

The invention of this application has the features as described above, and the embodiment will be described below.

As described above, the warm rolling method of the invention of this application is to provide a steel material having an ultrafine grain structure with an average grain size of 3 im or less by combining rolling of an oval hole type and another type of hole type. It can be manufactured. The groove roll used for rolling in this case is an opal hole type and a different type of hole type.

Here, as for the oval hole type groove roll, the hole formed by the upper die and the lower die is not a circular (round), but a so-called circular (round) flattened shape. I have. Other types of pores to be combined with the opal pores may be corners, diamonds, rounds, or various similar ones.

In the invention of this application, as a warm rolling method for producing an ultrafine grained steel material having an ultrafine grain structure having an average grain size of 3 z / m or less, a rolling temperature range of 350 When rolling in two or more passes in a temperature range of _800, at least one or more rolling operations using an opal-shaped die and rolling using another type of die are performed.

In practice, rolling with an oval-shaped die is performed after rolling with an oval-shaped die, or rolling with an opal-type die is performed when N> 2 out of the total number of passes: N. It is preferable that the number of times is equal to or more than N times and the number of times is equal to or less than N 2 and that continuous two-pass rolling is performed. For example, in the case of combining an oval hole type and a square type, the shape of the hole includes rolling two or more times in a combination of opal corners in all rolling passes. Rolling by corners is included in the middle of the combination of overruling angles, 4 passes of overruling angle per opal-angle, and 6 passes of overruling angle-one val-angle-one overlug angle are considered. Of course, even in this case, the corners may be round or rhombic.

In the rolling method of the invention of the present application, the micro local orientation difference generated by introducing a large strain by warm working becomes the origin of the ultrafine crystal grains, and during the recovery process occurring during or after the working, At the same time as the dislocation density in the inside decreases, a grain boundary is formed and an ultrafine grain structure is formed. However, if the temperature is low, the recovery is not enough, and a processed structure with a high dislocation density remains. On the other hand, if the temperature is too high, the crystal grains become coarse due to discontinuous recrystallization or normal grain growth, and an ultrafine grain structure of 3 mm or less cannot be obtained. Therefore, the rolling temperature is limited to 350 to ~ 800. According to the invention of this application, ultrafine crystal grains are generated from the processed grains flattened by warm working, and increase with the increase in strain. However, almost all of the microstructures are formed of ultrafine crystal grains. To get it, you need at least 1.5 strain.

More specifically, by introducing a plastic strain of 1.5 or more, or even 2 or more, into a region of at least 50% by volume inside the material, ultrafine grains can be formed in that region. Desirably, by introducing a plastic strain of 2 or more in a region of 90% or more inside the material, an ultrafine grain region can be formed in that region.

The greater the strain introduced, the greater the misorientation angle between the fine grains. That is, large-angle grain boundaries increase. When strain 3 can be introduced, the ratio of large-angle grain boundaries to fine-grain boundaries becomes sufficient. Therefore, if the region with a strain of 3 or more is 50% or more, preferably 80% or more of the entire cross section, the mechanical properties are excellent. The steel bar is completed.

In addition to processing in the main rolling direction and combining it with rolling in another direction at an angle of about 90 °, processing strain is imparted in at least two directions to disperse the orientation of ultrafine crystal grains. The ratio of large-angle grain boundaries can be increased.

And the inventors' research so far revealed that the average grain size of the ultrafine grains formed by the high-strength working depends on the working temperature and the strain rate. The crystal grain size becomes finer as the rolling condition parameter Z in the above equation (1), which is a function of the processing temperature and the strain rate, increases. In order to obtain a structure with an average grain size of 1 m or less, it is necessary to set the rolling condition parameter Z over a certain critical value. As a result of an experiment using a single-pass large-strain compression process using a small sample, the critical value was about 11 for bcc-structured iron (ferrite, payinite, martensite, pearlite, etc.), and for the fcc-structure (austenite). It turned out to be about 20 (Fig. 17).

Note that the strain (ε) in the equation (1) may be a true strain that is industrially simple. For example, assuming that the initial area of a steel bar is S o and the C cross-sectional area after rolling is S, the area reduction R is

R = (S o-S) / S o (2)

Is represented by Then, the true strain ε

ε = — L η (1— R)

Is represented by Also, instead of the true strain, one calculated by the finite element method (for example, Keisaburo Harumi, et al. “Introduction to the Finite Element Method” (Kyoritsu Publishing Co., Ltd .: March 15, 1990) may be used. More specifically, the calculation of the plastic strain can be performed by the float in Table 1 below. Flow of plastic strain calculation

1 Acquire stress-strain curve corresponding to material processing temperature

2 Preparation for finite element calculation

(1) Create a mesh on the workpiece

(2) Determine contact conditions Friction coefficient = 0.3 Coulomb condition

(3) Determine stress-strain curve and material properties

3 (1) — Calculate with the general finite element code, for example, ABAQUS, based on the conditions of (3). The plastic strain ε is expressed by the following formula, and each strain increment is calculated by the general finite element method code.

άε _χ άε ife _z : y, z strain increment

d _Yxy d _yz d: Shear strain increment

From the above, in the warm rolling method of the invention of this application, the rolling conditions are desirably set so that the parameter Z is 11 or more (bcc structure) or 20 or more (fcc structure). .

Further, in the invention of this application, as a preferred embodiment, in a two-pass rolling of oval hole rolling and a square hole rolling of a material, a two-pass rolling in which a hole shape is a single pallet and a corner is performed, and a reduction in area is achieved. 20% or more, or a combination of two-pass rolling with a groove shape of a flat and a corner, 40% or more for two rolling operations, and 60% or more for a combination of three rolling operations. For example, it is possible to include a rolling step in which the maximum minor axis length of the material after rolling in the oval hole type is 70% or less of the material side length before the opal rolling.

Regarding the composition of the steel material to which the warm rolling method of the invention of the present application can be applied, the mechanism for increasing the strength by phase transformation is not used at all, and the addition of alloying elements for increasing the strength is not required. The composition of the steel may be limited Instead, for example, a steel material having a wide range of components, such as a steel type having no phase transformation, such as a ferritic single-phase steel or an austenitic single-phase steel, can be used. More specifically, for example, if the composition is

C: 0.001% or more, 1.2% or less,

S i: 0.1% or more and 2% or less,

Mn: 0.1% or more and 3% or less,

P: 0.2% or less,

S: 0.2% or less,

A 1: 1.0% or less,

N: 0.02% or less,

Cr, Mo., Cu, and Ni are 30% or less in total,

Nb, T i, V are less than 0.5% in total,

B: 0.01 or less,

A composition having no alloying element added, such as the balance of Fe and unavoidable impurities, can be shown as an example. Of course, the above-mentioned alloying elements such as Cr, Mo, Cu, Ni, Nb, Ti, V, and B can be added beyond the above range as necessary. May not be included at all.

Therefore, an embodiment will be described below and will be described in more detail. Of course, the invention is not limited by the following examples. Example

Table 2 below shows the chemical compositions of the test steels used in the examples (the balance being Fe). Table 2 Chemical composition of test steel (mass%)

A ferrite with an average ferrite grain size of 5 microns and a 24 mm square bar with a pearlite structure having the composition shown in Table 2a was rolled at a rolling temperature of 5200-45 Ot using the hole shape shown in Fig. 1. Pascalipa rolling was performed. The outline of the hole size (mm) in Fig. 1 is shown in Table 3 below. Table 3 Long axis Short axis Polarity radius

1 pass' opal 54 1 2 64

3 Pass' Opal 41 9 49

5 passes, opal 1 9 1 0 1 2

6 pass ■ Round Diameter: 1 2 Figure 2 shows the change in cross-sectional shape and the reduction in area for each pass of rolling. The area reduction rate when a square bar of material 24 x 24 mm is rolled with the opal hole type in the first pass is 37%, and the area reduction rate when rolled with the square hole type in the second pass is 37%. 2 1%, 15% reduction in area when rolled in oval hole form in 3rd pass, 24% in area reduction when rolled in square hole form in 4th pass, 5th pass The area reduction rate when rolling with the opal hole type is 13%, and the area reduction rate when rolling with the round shape in the sixth pass is 12%. Also, the reduction rate of the material from the material to the 17 mm square bar in the second pass is 44%, The area reduction rate from the material to the 13 mm square bar in the fourth pass is 71%, and the area reduction rate from the material to the 12.5 mm round bar in the sixth pass is 80%.

Figures 3 to 9 show the plastic strain distribution inside the material calculated by the finite element method. From Fig. 5, it can be seen that there is already a region in the material that exceeds plastic strain 1.5 in the two-pass rolling of opal corner. Its area ratio is 75%. As shown in Fig. 6, the area with plastic strain of 2 or more occupies 92% of the total after the three-pass rolling with opal-one oval, and after the four-pass rolling with one opal-one oval angle shown in Fig. 7. It can be seen that the region with a plastic strain of 3 or more accounts for 95% of the total. Further, when rolling is performed with the single circle shown in FIG. 9, the plastic strain becomes 3 or more in the 100% region.

The area reduction rate after 2 passes is about 44% (If the area reduction R is simply converted to true strain e, e = -ln (R / 100), e = 0.67), 71% after 4 passes (Reduction of the area reduction to simply true strain is 1.23), and 80% after 6 passes (1.61 when the area reduction is simply converted to true strain). It can be seen that very large plastic strain has occurred. The reason for this is that the rolling of the combination of the oval hole type and the square hole type generates a strain much larger than the strain calculated from simple reduction in cross section.

Figures 10 and 11 show SEM photographs of the tissue. Fine ferrite grains of 1 micron or less are generated in the areas (1) and (2) in Fig. 10 corresponding to Fig. 5, and no fine grains are generated in the area (3). According to the micrograph of FIG. 11 corresponding to FIG. 7, almost the entire area is composed of ultrafine ferrite grains of 1 micron or less.

Table 4 shows the mechanical properties of the 13 mm square material after 4 passes. The properties of the 24-square bar before rolling are also shown in the it comparison. It had twice the yield strength, did not break brittlely even at liquid nitrogen temperature, and had an energy absorption of J. _ _ 4 Ductile embrittlement Absorbed energy, part piperite Yield strength Tensile strength

Tag ½1 degree (J) Curse hardness Particle size (μηι) (MPa) (MPa)

(.C) -120 ° C (-) Example 1 0.5 840 850 -196> 118 290 Example 4 0.6 800 810 -196> 80 270-310 Comparative Example 25 460 580 -40 0

Examples 2-4>

Figure 2 shows a ferrite with an average ferrite grain size of 5 microns and a 24 mm square steel bar with a pearlite structure with the composition shown in Table 2a at rolling temperatures of 400, 600 and 700 shown in Fig. 1 (1), (2) ) Two-pass caliber rolling was performed using the hole shape. Figures 1.2 (a), (b), and (c) show the SEM microstructure of the central part of the steel bar (the part corresponding to ① in Fig. 10). The average grain size is 0.5, 1, and 1.5 microns. Ferrite grain size is obtained.

A ferrite with an average ferrite grain size of 2.0 microns and a bar of 15 mm in diameter with a pearlite structure having the composition shown in Table 2b was rolled at a rolling temperature of 450-550 at a rolling temperature of 450-550 using a hole die shown in Fig. 13. And 6-pass rolling was performed to a diameter of 8 mm. Table 5 shows the dimensions of the caliper. Figure 14 shows the change in cross-sectional shape and the area reduction rate for each rolling pass. Fig. 15 shows a SEM photograph of the structure after 6 passes, and it was found to have a fine ferrite grain structure despite the area reduction of about 74%. Regarding the mechanical properties, the Vickers hardness is also shown below the photograph in Fig. 15, but excellent properties of 270-310 and a tensile strength of 80 OMPa or more are obtained. ¾ 5 Long axis Short axis Polarity radius

1 passOpal 31 6.8 38

3 passesOpal 27 5.3 35.9

5 Pass' Opal 15 6.5 10.7

6 pass, round Diameter: 8 Comparative Example 1>

A ferrite with an average ferrite grain size of 5 microns and a 24 mm square bar with a pearlite structure with the composition shown in Table 2a was rolled at a rolling temperature of 500 at a rolling temperature of 500 mm until it became 13 mm square using the die shown in Fig. 1. Then, 7-pass caliper rolling was performed with a reduction in area of 70% (strain 1.2). Rolling by opal hole type is not included. As shown in the SEM photograph of Fig. 16, no fine grains were formed in the center of the bar.

Comparative Example 2>

After heating a steel bar with a composition of Table 2b with a diameter of 115 mm to 90 Ot: at a rolling temperature of 870-850, a square hole die was used to reduce the surface area to 94 mm (94%). Caliper rolling with a strain of 3.1) was performed. Rolling by oval type is not included. The average particle size was 5 m and no fine particles were formed. The mechanical properties are shown in Table 2. The yield strength and tensile strength were 480 and 560 MPa. Industrial applicability

As explained in detail above, the invention of this application provides a new warm multi-directional rolling method that enables a large amount of strain to be introduced into a material with a smaller area reduction ratio and a smaller number of passes by a simpler means. And a method for producing a steel material having an ultra-fine grain structure and excellent strength and ductility. Can be offered.

Claims

The scope of the claims

1. A warm rolling method for producing ultrafine grained steel having an ultrafine grain structure having an average grain size of 3 / zm or less, wherein the rolling temperature range is 350 to 800 A hot rolling method characterized by performing at least one or more rolling operations using an opal-shaped groove and rolling using another shape when performing rolling in two or more passes in the temperature range described above.

2. It is recommended that rolling using a hole with a different shape is followed by rolling with a hole with another shape.

2. The warm rolling method according to claim 1, wherein:

3. The warm rolling method according to claim 1, wherein the other shape is a square or round shape.

4. The number of total passes: N, where N> 2, the warm rolling of any one of claims 1 to 3, characterized in that the rolling by the oval hole die is performed twice or more and at most NZ 2 or less. Method.

5. The warm rolling method according to any one of claims 1 to 3, wherein continuous two-pass rolling is performed.

6. The temperature reduction according to claim 5, wherein, in the two-pass rolling of the two-pass rolling in which the shape of the groove is one-palm and the angle, the area reduction rate after the square hole rolling from the material is set to 20% or more. Rolling method.

7. Rolling in a combination of two-pass rolling in which the hole shape is equal to the corner and corner, a rolling reduction of 40% or more with two rolling combinations and a rolling reduction of 60% or more with three rolling combinations. The warm rolling method according to any one of claims 1 to 3, wherein the hot rolling is performed.

8. The method according to any one of claims 1 to 7, characterized in that the method includes a rolling step in which the maximum minor axis length of the material after rolling in the oval hole form is 75% or less of the material-to-side length before the opal rolling. Warm rolling method.

9. Apply a plastic strain of 1.5 or more to at least 50% by volume of the material. The warm rolling method according to any one of claims 1 to 8, wherein the hot rolling is performed.

10. The warm rolling method according to claim 9, wherein a plastic strain of 2 or more is generated in a region of 90% by volume or more inside the material.

1 1. Rolling condition parameter Z expressed by the following equation (1) is 11 or more (when the microstructure immediately before rolling is ferrite, bainite, martensite, pearlite, etc., and the Fe crystal structure is bcc) or 2 The warm rolling method according to any one of claims 1 to 10, wherein the structure is 0 or more (when the structure immediately before rolling is austenite and the crystal structure of Fe is fcc).

ε: strain

t: Time from start to end of rolling (s)

T: Rolling temperature (In the case of multi-pass rolling, the rolling temperature of each pass is averaged.)

Q: Use 254 000 when the microstructure just before rolling is ferrite, payinite, martensite, or pearlite as the mother phase, and 300,000 when the austenite is used as the mother phase.

12. The warm rolling method according to any one of claims 1 to 11, wherein an initial material and a reduction in area after final rolling are set to 90% or less.

13. The warm rolling method according to any one of claims 1 to 12, wherein an ultrafine-grained steel having an average crystal grain size of a C section or an L section of 3 microns or less is manufactured.

14. The warm rolling method according to any one of claims 1 to 12, wherein an ultrafine-grained steel having an average crystal grain size of a C section or an L section of 1 micron or less is produced.