JP4949124B2 - High strength duplex stainless steel sheet with excellent shape freezing property and method for producing the same - Google Patents

Description

  The present invention relates to structural members that require strength and shock absorbing performance, particularly shock absorbing members such as automobiles, bus front side members, pillars, and bumpers, as well as suspension members, railway vehicle bodies, and bicycle rims. The invention relates to an invention of a high-strength duplex stainless steel plate excellent in shape freezing used in the invention and a method for producing the same.

  In recent years, from the viewpoint of environmental problems, improvement in fuel consumption of transportation equipment such as automobiles, motorcycles, buses, and railway vehicles has become an essential issue. As one of the solutions, weight reduction of the vehicle body is actively promoted. The weight reduction of the vehicle body is largely due to the weight reduction of the material forming the member, specifically, the reduction of the thickness of the material plate. However, simply reducing the thickness of the material plate reduces the collision safety performance. For this reason, the strengthening of the material which comprises a member is progressing as a collision safety improvement measure, and the high strength steel plate is applied to the impact-absorbing member of a motor vehicle. Examples of the high-strength steel sheet include DP (Dual Phase) steel and TRIP (Transformation Induced Plasticity) steel having a multiphase structure as a metal structure. As described in Document 1, all of these steel types have been confirmed to have superior shock absorption characteristics as compared with solid solution strengthened steel and precipitation strengthened steel.

On the other hand, when forming a high-strength steel sheet as described above into a member, bend forming or deep drawing is performed, but the higher the strength, the greater the return after forming (hereinafter referred to as springback). It is difficult to obtain a predetermined shape, which is a big problem in using a high-strength steel plate. Patent Document 1 and Patent Document 2 are methods for reducing springback in a high-strength steel sheet. Patent Document 1 describes a high-strength steel sheet having a reduced springback by a steel composition and a manufacturing method, and a forming method for reducing springback by optimizing a clearance, a resin sheet, and the like when forming the steel plate. Patent Document 2 and Patent Document 3 describe a technique for improving the shape freezing property by controlling the texture of the product by defining the steel composition and the manufacturing method. However, the methods described in Patent Documents 1 to 3 have insufficient shape freezeability. In particular, as in Patent Documents 2 and 3, only by specifying the r value in the rolling direction or the r value in the direction perpendicular to the rolling direction, the cutting direction of the sample is regulated in some cases. The degree of freedom during molding decreased, and when the r value was 0.7 or less, the shape freezing property was insufficient.
Japan Plasticity Processing Society 228th Plastic Processing Symposium (2004), p15 JP 7-268484 A JP 2002-97545 A JP 2004-131754 A

  Although there was a method of imparting good shape freezing property to the steel plate itself, the improvement effect was not sufficient, which was an obstacle when applying a high strength steel plate. In view of such a background, an object of the present invention is to provide a high-strength duplex stainless steel sheet having a high degree of freedom during forming and excellent shape freezing property, and a method for producing the same.

  In order to solve the above problems, the present inventors have studied a steel composition and a production method, and as a result, have come to produce a high-strength stainless steel plate excellent in shape freezing property. Usually, the shape freezing property tends to deteriorate as the tensile strength increases. A hat bending test was performed on a stainless steel plate and a carbon steel plate having a thickness of 2.0 mm having various tensile strengths under the following conditions to produce a hat sample shown in FIG.

Here, for convenience, the test piece before the hat bending test is called a hat bending test piece, and the test piece after the hat bending test is called a hat sample.
Hat bending test piece size: 50mm x 260mm
Punch: Square shape, width 80mm, shoulder R5mm,
Dice: Square, width 85mm, shoulder R5mm
Wrinkle holding force: 6 tons Molding height: 70mm
Lubricating oil: Lubricating type rust preventive oil equivalent to JISK2246, which is coated on both sides with a selected mineral oil having a viscosity equivalent to SAE-30. The shape of the obtained hat sample is measured using a three-dimensional shape measuring machine, and point a , B, and c, the curvature of the vertical wall portion was measured as “wall warp amount: 1 / ρ”. Point b is a position corresponding to ½ of the total height when the line connecting the hat collar portions of the sample is the base, and points a and c are located 20 mm above or below point b, respectively. The wall warpage amount is the reciprocal of the radius (mm) of the circle passing through the points a, b, and c, and the smaller the value, the better the shape freezing property.

Take three tensile test pieces and three hat bend test pieces from the same sheet material so that the longitudinal direction of the tensile test piece and hat bend test piece is parallel and perpendicular to the rolling direction. A tensile test (r value measurement) and a hat bending test were performed, and the average value of each of the three test pieces was used as data. In addition, the r value and the wall warp amount 1 / ρ of the test pieces in which the longitudinal directions of the tensile test piece and the hat bending molded test piece are parallel to each other were evaluated in association with each other. The wall warp amount 1 / ρ obtained under the above conditions is a commercially available ordinary steel having a normal r value of about 1.0 (r value = 1.0 ± 0.2) without special structure control. Or, in a ferritic or austenitic stainless steel sheet, it has a good correlation with tensile strength, and can be expressed approximately by 1 / ρ = 7.75 × 10 ⁻⁶ [TS] indicated by a dotted line in FIG. It was. Here, [TS] is the tensile strength (N / mm ² ). That is, in a material having an r value of about 1.0, the shape freezing property is determined by the tensile strength. On the other hand, in the stainless steel in which the r value in both the rolling direction and the rolling and vertical direction is reduced to 0.5 or less by contriving the components and the manufacturing method in the stainless steel plate as in the present invention, the black triangle in FIG. As shown in the data shown, 1 / ρ can be significantly reduced. If 1 / ρ, an index of shape freezing property, can be reduced by 30% or more compared to a normal material having an r value of about 1.0, the generation of rejected products due to shape freezing failure during hat bending and the resulting productivity It was found that the decrease can be greatly suppressed, and the effect appears straightforward. For example, the incidence of rejected products is 1/10 or less. For this reason, the target of 1 / ρ was set to be lower than the solid line in the figure, which was reduced by 30% or more from the normal material.

  In yet another test, by controlling the annealing temperature and annealing time after hot rolling and cold rolling, the texture of the stainless steel is controlled to change the r value, and the wall warp amount 1 / ρ is changed. The relationship was investigated. That is, by setting the annealing temperature to 1050 ° C. or higher, the r value increases due to the development of a texture in which the (111) plane in the ferrite phase is substantially parallel to the plate surface, and conversely, the annealing temperature is less than 1050 ° C. By doing so, the r value can be lowered by suppressing the development of the texture in which the (111) plane in the ferrite phase is substantially parallel to the plate surface. Above 1050 ° C., the longer the annealing time is, the more the texture develops, and the r value increases. Above an annealing time, the increase in r value is saturated. As a result, it became clear that the amount of wall warp has a good correlation with the r value. In a stainless steel plate having a tensile strength of about 900 MPa (850 to 950 MPa), a product plate is produced by changing the components and hot-rolled sheet annealing conditions and cold-rolled sheet annealing conditions. After the hat bending test was conducted, the wall warpage amount was measured. FIG. 3 shows the correlation between the r value and the wall warp amount. When the r value is higher than 0.5, the wall warping amount substantially follows the equation (1) as shown by the dotted line in the figure, but when the r value becomes 0.5 or less, 1 / ρ suddenly increases. It turns out that it falls. Therefore, by making the r value 0.5 or less, a remarkably good shape freezing property can be obtained. Further, by setting the r value to 0.5 or less in the rolling direction at the time of manufacturing the plate and in the direction perpendicular to the rolling direction, the plate taking direction is not restricted, and the degree of freedom in forming becomes high.

The gist of the present invention is the following contents as described in the claims.
(1) By mass%, C: 0.001 to 0.200%, Si: 0.01 to 3.00%, Mn: 0.01 to 10.00%, P: less than 0.050%, S: 0.0001 to 0.0100%, Cr: 11.0 to 30.0%, Ni: 0.03 to 10.00%, N: 0.001 to 0.300%, the balance being Fe and inevitable The austenite phase ratio of the metal structure at room temperature is 5% or more and less than 80%, and the remaining metal structure is composed of a ferrite phase, a work-induced martensite phase or an inevitable precipitation phase, and has a tensile strength of A high-strength duplex stainless steel sheet excellent in shape freezing property, having 500 MPa or more, r value r _{0 in the} direction parallel to the rolling direction, and r value r _{90 in the} direction perpendicular to the rolling direction are both 0.5 or less. .
(2) By mass%, Cu: 0.10 to 5.00%, Mo: 0.10 to 5.00%, W: 0.10 to 5.00%, V: 0.10 to 5.00% 1 type or 2 types or more of these, The high intensity | strength duplex stainless steel plate excellent in the shape freezing property as described in said (1) characterized by the above-mentioned.
(3) By mass%, Ti: 0.005 to 0.500%, Nb: 0.005 to 0.500%, B: 0.0003 to 0.0050%, one kind or two or more kinds are contained. The high-strength duplex stainless steel sheet excellent in shape freezing property as described in (1) or (2) above.
(4) By mass%, Al: 0.003-0.500%, Mg: 0.00001-0.0050%, Ca: 0.00001-0.0050%, containing one or more. The high-strength duplex stainless steel sheet excellent in shape freezing property according to any one of (1) to (3).
(5) When producing the stainless steel plate according to any one of (1) to (4), after performing a heat treatment at a maximum temperature of 900 to 1050 ° C. and a holding time of 1 to 600 s after hot rolling. In the cooling process, the average cooling rate in the range of 900 ° C. to 600 ° C. is cooled to 5 ° C./s or higher, and then cold rolling is performed at a rolling rate of 30% or more and less than 90%. In the cooling process after the heat treatment with a holding time of 1 to 600 s, cooling is performed with an average cooling rate in the range of 900 ° C. to 600 ° C. being 5 ° C./s or more, which is excellent in shape freezing property A method for producing a high strength duplex stainless steel sheet.

  According to the present invention, it is possible to provide a high-strength duplex stainless steel sheet having excellent shape freezing property and a method for producing the same, which contributes significantly to reducing the weight of automobiles and the environment, reducing industrial impact, and the like, an industrially significant effect. Play.

First, the reasons for limiting the present invention will be described.
Metal structure: A high-strength duplex stainless steel sheet characterized in that the austenite phase ratio of the metal structure at room temperature is 5% or more and less than 80%.

  Duplex stainless steel sheets made of austenite, ferrite, or work-induced martensite can be easily controlled by appropriately adjusting the phase ratio according to the characteristics required by the customer. This is because they are excellent in toughness, impact absorption performance, corrosion resistance, and recyclability, and therefore meet the purpose of the present invention.

  Each phase ratio is controlled when the N amount is up to about 0.06% by mass, the Schaeffler Diagram is used, and the N amount is more than 0.06% by mass and not more than 0.30% by mass. Further, a Dulong diagram can be used in addition to the reference, and the amount of the added component can be changed while observing the balance between the Cr equivalent and the Ni equivalent. Specifically, the Cr, Mo, Si, and Nb amounts may be used in the calculation of the Cr equivalent, and the Ti, V, and W amounts may be added to the Cr equivalent calculation with a coefficient of 0.5 as necessary. On the other hand, Ni, C, N, and Mn may be used for the Ni equivalent calculation, and Cu may be added to the Ni equivalent calculation with a coefficient of 0.5 as necessary. However, the Schaeffler organization chart and the Dulong organization chart are organization charts of the weld metal to the last, and cannot be directly applied to the structure of the stainless steel sheet product of the present invention. However, the ferrite phase amount of the stainless steel plate after the final heat treatment, or the work-induced martensite phase amount after cold working it, qualitatively corresponds to the structure shown in the Schaeffler or Dulong structure chart, Both organization charts are important guidelines for chemical composition control.

  The reason for the limitation regarding the austenite phase ratio in the present invention is that when the austenite phase ratio is less than 5%, the strength of the steel sheet may be insufficient and the collision safety performance may be lowered, while the austenite phase ratio is 80% or more. In this case, the shape freezing property is poor. It is desirable that a part of the austenite phase is transformed into a work-induced martensite phase by cold working. This is because the work to increase the strength while maintaining the toughness at a high level and the shock absorbing ability can be expected. Most of the remaining metal structure other than the austenite phase is the ferrite phase or the work-induced martensite phase. As will be described later, the texture of the ferrite phase greatly affects the shape freezeability. Furthermore, it is set as the stainless steel which consists of an unavoidable precipitation phase. This is because, depending on the combination of additive elements, precipitates such as carbides, sulfides and nitrides may be deposited in the stainless steel sheet, or oxides generated during deoxidation may inevitably remain. It is.

The ferrite phase and the work-induced martensite phase have ferromagnetism, while the austenite phase is paramagnetic. Therefore, the phase ratio is measured using an electromagnetic measurement method. The phase can be determined in volume%. Since the amount of inevitable precipitate phase is negligible, the amount of austenite phase is 100% or a value obtained by subtracting the volume% of the ferrite phase or the work-induced martensite phase. In the measurement device for ferrite phase and work-induced martensite phase, the ferrite phase and work-induced martensite phase contained in a hemisphere with a radius of about 1 mm at the tip of the measurement terminal are measured, so a stainless steel plate thinner than 2 mm is used. In the case of a measurement object, two or more sheets are closely adhered and embedded in a resin, and the cross section is mirror-polished to a flat surface, followed by electrolytic polishing, and a layer having a surface polishing strain remaining is 10 μm. Dissolve the extent. This is because a work-induced martensite phase is newly generated on the surface due to mirror polishing, and the amount of ferrite phase and work-induced martensite phase inherent in the material cannot be measured accurately.
<Tensile strength>
The tensile strength is 500 MPa or more. This is because if it is less than this, the strength required for the high-strength member is not satisfied. The upper limit of the tensile strength is not particularly specified, but it is 1800 MPa as a level that can be achieved at present due to the combination of components and production method. The measuring method of tensile strength is measured by a tensile test based on JIS Z 2241 using a JIS 13B tensile test piece in a direction parallel to the rolling direction. The N number is assumed to be 3 or more and an average value is taken.
<R value> The r value r _{0 in the} direction parallel to the rolling direction and the r value r _{90 in the} direction perpendicular to the rolling direction are both 0.5 or less.
When the r value exceeds 0.5, good shape freezing property cannot be obtained. Since normal bending is performed in a direction parallel or perpendicular to the rolling direction, in the present invention, both the rolling direction, the parallel direction r ₀ and the vertical direction r ₉₀ are set to 0.5 or less. Further, the lower the r value, the more the shape freezing property is improved. Therefore, the lower limit is not particularly required. In order to ensure stable shape freezing property, it is preferably 0.1 to 0.5. The measuring method of r value is measured by a test based on JIS Z 2254 using a JIS 13B tensile test piece. The N number is assumed to be 3 or more and an average value is taken.

Hereinafter, the reasons for limitation regarding the components will be described. “%” Indicates mass%.
<C>
Since C may lower the corrosion resistance, the upper limit was made 0.200%. The lower limit was set to 0.001% in consideration of the load on decarburization during smelting. The range that can be stably produced is preferably 0.005 to 0.080%.
Si: If Si is added in a large amount, it will cause cracks during production and increase the rolling load, so the upper limit was made 3.00%. The lower limit is 0.01% considering the addition at the steelmaking stage.
Mn: If Mn is added in a large amount like Si, it will cause cracking at the time of manufacture, and Mn alarming property will be precipitated to deteriorate the corrosion resistance. Therefore, the upper limit of Mn is set to 10.00%. The lower limit was set to 0.01% as a level that can be reduced without applying a large load at the refining stage.
<P>
If P is present in a large amount, cracking is caused during cold press molding, and the cold workability is lowered. Therefore, the P content is less than 0.050%. Preferably, it is less than 0.040%.
S: When S is present in a large amount, sulfide is generated and becomes a starting point of corrosion. Therefore, the lower one is preferable, and the upper limit is set to 0.0100%. Although the lower one is preferable, the lower limit is set to 0.0001% in consideration of the load at the refining stage for desulfurization.
<Cr>
Cr is an element that improves the corrosion resistance. In the present invention, it plays an important role in controlling the metal structure (ratio of austenite phase and ferrite phase). From such a viewpoint, the lower limit was made 11.0%. Further, when added in a large amount, an intermetallic compound is formed and cracks are induced during production, so the upper limit was made 30.0%.
<Ni>
Ni, like Cr, has an important role in controlling the metal structure. In addition, it is an element that improves toughness, so the lower limit was made 0.03%. The upper limit is set to 10.00% because the austenite phase ratio increases excessively or the raw material cost increases due to the addition of a large amount.
<N>
N is an element that concentrates in the austenite phase at high temperatures and plays an important role in adjusting the austenite phase ratio, and also improves the corrosion resistance, so the lower limit was made 0.001%. However, a large amount of addition hardens the material and causes cracks during production. In addition, an N ₂ -containing gas pressurizing facility for adding a large amount of N during steelmaking is required, which causes a significant increase in production cost, so the upper limit was made 0.300%.

Further, the following elements may be selectively added.
Cu, Mo, W, and V: Cu, Mo, W, and V are elements that improve corrosion resistance, and in order to improve these, one or a combination of two or more may be added. Since the effect is exhibited at 0.10% or more, this is the lower limit. However, since the addition of a large amount tends to increase the rolling load at the time of production and produce production soot, the upper limit was made 5.00%.
Ti, Nb, and B: Ti, Nb, and B are elements that improve formability, and may be added singly or in combination of two or more as required. Since the effect of improving the formability is Ti: 0.005%, Nb: 0.005%, and B: 0.0003% or more, this is set as the lower limit. Addition of a large amount leads to an increase in production soot and a decrease in hot workability, so Ti: 0.500%, Nb: 0.500%, B: 0.0050% was made the upper limit.
Al, Mg and Ca: Al, Mg and Ca may be added for the purpose of deoxidation and desulfurization during refining. The effect is exhibited by Al: 0.003%, Mg: 0.0001%, and Ca: 0.0001%, which were set as lower limits. Moreover, since addition of a large amount leads to an increase in production soot and an increase in raw material cost, Al: 0.500%, Mg: 0.0050%, Ca: 0.0050% was made the upper limit.
The following explains the reasons for limitation regarding the manufacturing method.
Hot-rolled sheet annealing and cold-rolled sheet annealing: In both hot-rolled sheet annealing and cold-rolled sheet annealing, if the maximum temperature reached is less than 900 ° C, the austenite phase amount cannot be secured sufficiently during heat treatment, resulting in a decrease in product strength. May occur and shape freezeability may deteriorate. When the temperature is 1050 ° C. or higher, the recrystallization of the ferrite phase is promoted, and the crystal orientation that deteriorates the shape freezing property becomes dominant particularly in the texture during cold-rolled sheet annealing. That is, in the ferrite phase, the (111) plane develops almost parallel to the plate surface and the r value rises, and the shape freezeability of the product is significantly deteriorated. For this reason, both the temperature range of annealing of a hot-rolled sheet and annealing of a cold-rolled sheet was made into 900 to 1050 degreeC. In addition, in order to suppress the said texture reliably, it is desirable that both the temperature range of annealing of a hot-rolled sheet and annealing of a cold-rolled sheet shall be 900 degreeC or more and less than 1025 degreeC. In addition, if the holding time at the highest temperature reached during annealing is less than 1 s, a large amount of unrecrystallized region remains and the ductility of the product tends to decrease. On the other hand, if the holding time is 600 s or more, recrystallization proceeds too much, and the (111) plane of the ferrite phase develops parallel to the plate surface, the r value increases, and the shape freezing property deteriorates. Furthermore, the average cooling rate in the range of 900 ° C. to 600 ° C. in the cooling process needs to be 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, the phase ratio of the austenite phase decreases and the phase ratio of the ferrite phase increases, and carbonitride precipitates, causing precipitation strengthening, and good shape freezing property is It is because it cannot be obtained. In order to set the cooling rate to 5 ° C./s or higher, it is desirable to perform air-water cooling or forced air cooling. Since the point is to control the phase ratio of the austenite phase in this cooling process, the measurement range of the cooling rate is set to a range of 600 ° C. to 900 ° C. where the phase rate of the austenite phase is easily changed and the diffusion rate is fast. .
Cold rolling: If the rolling rate is less than 30%, the recrystallization of the ferrite phase does not proceed sufficiently after annealing after cold rolling, and a large amount of unrecrystallized region remains, and the ductility is impaired and good. Since shape freezing property was not obtained, this was made the lower limit. Moreover, since the load to a rolling mill becomes very large when rolling over 90%, this was made the upper limit. When considering the load on the rolling mill and the suppression of derivation of the rolling mill, the optimum cold rolling rate is 40 to 75%.

  In addition, it is good also as what is called 2 times cold rolling which implements cold rolling and annealing after implementing cold-rolled sheet annealing. The cold rolling and annealing conditions in that case shall follow the above. Furthermore, temper rolling may be performed after final annealing for the purpose of shape correction, strength adjustment, and the like.

Hereinafter, specific examples will be described. Steels having chemical compositions shown in Table 1 were melted and hot-rolled. The hot-rolled sheet was annealed, cold-rolled, and annealed under various conditions, and then a 2.0 mm-thick product sheet was obtained. Using the obtained product plate, a tensile test, an r value measurement test, and a shape freezing property evaluation test were performed. For the evaluation of the shape freezing property, a hat bending test similar to that described above was performed, and the wall warp amount 1 / ρ was determined. For the reason described in “Means for Solving the Problems”, the wall warp amount 1 / ρ is calculated from the tensile strength TS (N / mm ² ) of the material. 1 / ρ = 7.75 × 10 ^{− 6} x [TS]
It was judged that the shape freezing property was good when it was lower by 30% or more.

  Table 2 shows the manufacturing conditions, the austenite phase ratio of the product structure, and various characteristics. As is apparent from the examples, the examples of the present invention have a low amount of wall warp and exhibit extremely good shape freezing properties.

It is a cross-sectional schematic diagram of a hat-shaped sample. It is a figure which shows the relationship between wall curvature amount "1 / (rho)" after hat shaping | molding, and tensile strength TS. It is a figure which shows the relationship between wall curvature amount "1 / (rho)" and r value in the material whose tensile strength is about 900 MPa (850-950 MPa).

Claims (5)

% By mass
C: 0.001 to 0.200%,
Si: 0.01 to 3.00%,
Mn: 0.01 to 10.00%,
P: less than 0.050%,
S: 0.0001 to 0.0100%,
Cr: 11.0-30.0%,
Ni: 0.03 to 10.00%,
N: 0.001 to 0.300% is contained, the balance is Fe and inevitable impurities, the austenite phase ratio of the metal structure at room temperature is 5% or more and less than 80%, and the balance metal structure is the ferrite phase or consists strain-induced martensite phase or unavoidable precipitated phases, a tensile strength of more than 500 MPa, the rolling direction and parallel to the direction of the r value r ₀ and the rolling direction and the vertical direction of r value r ₉₀ is at both 0.5 or less A high-strength duplex stainless steel sheet with excellent shape freezing properties. % By mass
Cu: 0.10 to 5.00%,
Mo: 0.10 to 5.00%,
W: 0.10 to 5.00%,
The high-strength duplex stainless steel sheet excellent in shape freezing property according to claim 1, characterized by containing one or more of V: 0.10 to 5.00%. In mass% Ti: 0.005 to 0.500%,
Nb: 0.005 to 0.500%,
B: The high-strength duplex stainless steel sheet excellent in shape freezing property according to claim 1 or 2, characterized by containing one or more of 0.0003 to 0.0050%. % By mass
Al: 0.003 to 0.500%,
Mg: 0.00001 to 0.0050%,
The high-strength compound excellent in shape freezing property according to any one of claims 1 to 3, characterized by containing one or more of Ca: 0.00001 to 0.0050%. Phase stainless steel sheet.   In producing the stainless steel plate according to any one of claims 1 to 4, after the hot rolling, a heat treatment having a maximum ultimate temperature of 900 ° C or more and less than 1050 ° C and a holding time of 1s or more and less than 600s is performed. In the cooling process, after cooling with an average cooling rate in the range of 900 ° C. to 600 ° C. being 5 ° C./s or more, cold rolling is performed at a rolling rate of 30% or more and less than 90%, and then the maximum temperature reached 900 ° C. or more and 1050 ° C. It is cooled to an average cooling rate in the range of 900 ° C. to 600 ° C. at 5 ° C./s or higher in the cooling process after performing a heat treatment with a holding time of 1 s or more and less than 600 s at a temperature below 5 ° C. A method for producing excellent high-strength duplex stainless steel sheets.

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