JP5213386B2 - Ferritic / austenitic stainless steel sheet with excellent formability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a ferrite-austenitic stainless steel sheet excellent in formability and a method for producing the same. According to the present invention, a ferrite and austenitic stainless steel sheet having excellent formability can be manufactured without containing a large amount of expensive and rare element Ni, which can contribute to resource protection and environmental protection. It is considered a thing.

  Stainless steel can be broadly classified into austenitic stainless steel, ferritic stainless steel, and two-phase (ferrite / austenite) stainless steel. Austenitic stainless steel contains 7% or more of expensive Ni, and many steel types have excellent formability. Ferritic stainless steel contains little Ni and generally has a much lower formability than austenitic stainless steel. On the other hand, two-phase (ferrite / austenite) stainless steel has a relatively low Ni content, and many types of steel have an intermediate position between austenitic stainless steel and ferritic stainless steel in terms of formability and corrosion resistance. Yes. However, in recent years, a technology having formability close to that of austenitic stainless steel has also been developed in ferritic / austenitic stainless steels by utilizing the work-induced martensitic transformation of the austenitic phase during plastic working. Patent Document 1 describes a technique in which the tensile elongation at break is increased by the TRIP phenomenon using a stainless steel containing a retained austenite phase whose main constituent phase is a ferrite phase. Patent Document 2 describes a method of increasing the tensile elongation by defining the stability of the austenite phase. Patent Document 3 discloses a technique for increasing the total elongation in a tensile test by defining the austenite phase fraction and the amounts of C and N in the austenite phase.

  However, in Patent Document 1, as shown in the Examples, the tensile elongation at break is 34 to 42%, and the elongation at break is not necessarily high. In an actual forming process, it is often determined that forming is not possible when the thickness of the steel sheet is reduced (necking) even if the steel sheet is broken and no “crack” occurs. That is, the “uniform elongation”, which is the uniform deformation limit, determines the formability from the “breaking elongation” in the tensile test, but it is unclear to what level the uniform elongation is. In Patent Document 2, the tensile elongation at break is up to 46%, and in Patent Document 3, the elongation at break up to 71% is described in the examples. There is no mention of elongation at all.

  As described above, knowledge for improving the ductility of ferritic / austenitic stainless steel exists, but all are methods for improving tensile elongation at break. Since elongation at break consists of uniform elongation and local elongation, it may be considered that the elongation at break increases by increasing the local elongation. However, if the uniform elongation does not increase, the actual formability is not improved. In the above technique, there is no description about a technique for improving uniform elongation which is important in actual molding.

JP-A-10-219407 JP-A-11-71643 JP 2006-169622 A

  In view of the technical background as described above, the present invention does not contain a large amount of expensive and rare element Ni, and a ferrite-austenitic stainless steel having a high “uniform elongation” which is a factor governing actual formability. It is intended to provide a thin plate and a method for manufacturing the same.

  In order to investigate the metal structure factor governing uniform elongation, the present inventors have melted various ferritic / austenitic stainless steels in the laboratory, and after annealing, A thin steel plate was manufactured by performing hot rolling. As a result of investigating the relationship between the metal structure of the obtained thin steel sheet and the uniform elongation after the tensile test, the following knowledge was obtained.

The characteristics of austenite grains in steel types with high uniform elongation are:
“(1) The crystal grain size is small and the shape is almost spherical (not expanded in the rolling direction).
(2) The distance between the nearest austenite grains is narrow.
(3) There is an appropriate value for the austenite stability calculated from the chemical composition in the austenite phase. "
Met. Details are described below.

  First, 0.006 to 0.030% C-0.10 to 0.85% Si-1.0 to 3.0% Mn-0.022 to 0.039% P-0.0002 to 0.0035% To manufacture a thin steel sheet by annealing and cold rolling after hot-rolling 10 steel types having a composition of S-20.1 to 21.0% Cr-0.08 to 0.12% N The manufacturing conditions such as the hot rolling conditions and the annealing temperature were changed. A JIS No. 13 B tensile test piece was taken in parallel with the rolling direction from the obtained 1 mm-thick thin steel plate, and the uniform elongation was measured by a method based on JIS Z 2201. Moreover, the metal structure of the cross section perpendicular | vertical to the rolling width direction (L cross section) of the center position of the rolling width direction of a thin steel plate was investigated by EBSP, and the phase was identified. Data obtained from EBSP was classified into ferrite grains (BCC phase) and austenite grains (FCC phase) for each crystal grain, and first the austenite phase ratio was measured. Further, portions where the crystal orientation difference between adjacent measurement points is 15 ° or more are regarded as crystal grain boundaries and indicated by black lines. A measurement example is shown in FIG. FIG. 1A shows the BCC phase, and FIG. 1B shows the FCC phase in white.

  Furthermore, the crystal grain size and aspect ratio of each grain of austenite grains (FCC phase) were measured, and the distance between nearest grains was measured for austenite grains. The closest intergranular distance is the distance between the center positions of each austenite grain, and the smallest value is the nearest intergranular distance of the grain. As for the center position of each crystal grain, when the length in the rolling direction of the grain is L and the length in the plate thickness direction is H, the position of 1 / 2L and 1 / 2H is the center position of the crystal grain. The distance between the closest grains was measured for 100 austenite grains, and the average value thereof was obtained.

Moreover, the chemical composition in the austenite grain was investigated using EPMA. The Md value was calculated from the obtained chemical composition as an index of the stability of the austenite phase. Here, Md is an index representing austenite stability calculated by the following equation (2). The coefficients of this calculation formula are based on Nohara et al.'S formula (see Iron and Steel 63 (1977) p. 772). [] In a formula shows the composition measured by EPMA of each element. However, for C, quantification in the austenite phase is difficult with EPMA, and thus an average composition {} is indicated. The “average composition” mentioned here represents an average composition contained in the steel regardless of the phase, and is determined by a combustion-infrared absorption method described in JIS G 1211.
Md = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [ Mo] ... (2)
The Md value is determined by the chemical composition in the austenite grains. Therefore, the Md value can be adjusted by changing the chemical composition in the austenite grains by, for example, the annealing temperature or the annealing time.

  N, Cu, Ni and Mn are concentrated in the austenite phase, that is, elements having a higher concentration in the austenite phase than in the ferrite phase, so that these can decrease the Md value by increasing the amount of addition. Can do. In general, the composition of the austenite phase is not an equilibrium composition determined by the annealing temperature. This is because it takes time to diffuse each element to distribute to the austenite phase and ferrite phase at a certain annealing temperature. Therefore, by increasing the holding time in the final annealing step, the equilibrium composition is approached (the concentration of N, Cu, Ni, and Mn in the austenite phase increases), so increasing the holding time also reduces the Md value. This is an effective method. However, if the holding time is 30 minutes, the equilibrium composition is almost reached.

  C is an element that lowers the Md value, and the Md value can be lowered by increasing the amount of addition. C is also an element concentrated in the austenite phase, but it is difficult to measure the concentration in the austenite phase. In the present invention, C uses the average composition in the formula for calculating the Md value. Therefore, the holding time during annealing does not affect the Md value of the present invention.

  The influence of Si and Cr on the Md value cannot be clearly stated. That is, since these elements are effective to the Md value with a negative coefficient, when these elements are viewed alone, the Md value decreases as the amount added increases. However, when the amounts of Si and Cr are high, the concentration in the austenite phase such as Mn, Ni, and Cu decreases, so the Md value may increase conversely. The degree of influence of Cr and Si varies depending on the concentrations of Mn, Ni, Cu and other elements and annealing conditions.

  As described above, the Md value is determined by the chemical composition in the austenite grains. The chemical composition in the austenite grains also changes depending on the austenite phase rate. That is, when the austenite phase ratio is low, the concentration of austenite-forming elements in the austenite phase increases, and therefore the Md value tends to decrease. On the other hand, when the austenite phase ratio is low, the concentration of the austenite-forming element in the austenite phase is low, so the Md value increases. In addition, the austenite phase ratio varies with temperature. The components specified in the present invention have the highest austenite phase rate at 1000 ° C. to 1150 ° C., and the austenite phase rate decreases when the temperature is higher or lower than that.

  In addition, the higher the absolute value of uniform elongation, the higher the formability, but if the uniform elongation is 30% or more, it is at a higher level than ferritic stainless steel, and if it is 40% or more, it is a well-formable austenitic stainless steel. Almost the same shape can be processed.

  First, the relationship between the volume fraction of austenite phase (austenite phase ratio) and uniform elongation was investigated for all data. The relationship between the austenite phase and the uniform elongation during the tensile test is shown in FIG. The uniform elongation has a suitable range for the austenite phase ratio, and the uniform elongation decreases if it is too high or too low. In order to ensure a uniform elongation of 30% or more, the austenite phase ratio needs to be 10% or more and less than 50%. Preferably it is 15 to 40%.

  Next, FIG. 3 shows the relationship between the Md value and the uniform elongation for data with an austenite phase ratio of 10% or more and less than 50%. In order to obtain a good uniform elongation, the Md value also has an appropriate range similar to the austenite phase ratio. The uniform elongation shows a high value of 34 to 44% when the Md value is in the range of -10 to +110, but such a high uniform elongation cannot be obtained in a range outside this range. However, the variation in uniform elongation is large only with the Md value, and it is considered that other tissue factors also affect the uniform elongation.

Since the crystal grain size and shape of the austenite grains are considered to affect the uniform elongation, the data of Md values from −10 to +110 in FIG. 3 are described as “Austenite grains having a grain size of 15 μm or less and a shape aspect ratio of less than 3. The ratio of the total austenite grain ”X ₁ (%) was measured, and the relationship with the uniform elongation u-EL (%) was investigated. The result is shown in FIG. As shown in FIG. 4, the higher the ratio, the higher the uniform elongation. When the ratio is 90% or more, a better uniform elongation can be obtained.

Further, data with a uniform elongation of 37% or more in FIG. 4 was extracted, and the average distance X ₂ (μm) between the austenite grains and the nearest elongation measured as described above and the uniform elongation u−EL (%) The relationship is shown in FIG. The uniform elongation increases as the average value of the distance from the nearest grain decreases, and the uniform elongation becomes extremely high when the distance is 12 μm or less.

  This invention is based on the said knowledge, Comprising: The summary of the invention is as follows.

( 1 ) In mass%,
C: 0.002 to 0.100%,
Si: 0.05 to 2.00%,
Mn: 0.05 to 5.00%,
P: less than 0.050%,
S: less than 0.010%,
Cr: 17 to 25%,
N: 0.010 to 0.150%,
Containing, Ri is Do iron and unavoidable impurities balance,
The volume fraction of the austenite phase is 10% or more and less than 50%, the Md value calculated from the chemical composition in the austenite phase satisfies the following formula (1), and the crystal grain size is in the cross section perpendicular to the rolling width direction. The ratio of austenite grains having a shape aspect ratio of less than 3 accounts for 90% or more of the total number of austenite grains, and the average distance between the nearest austenite grains in the same cross section is 12 μm or less. Ferritic / austenitic stainless steel sheet with excellent formability.
−10 ≦ Md ≦ 110 (1)
(Where Md = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18 .5 [Mo], [] is the composition (mass%) in the austenite phase, {} is the average composition (mass%))

( 2 ) Furthermore, in mass%,
Ni: 5.00% or less,
The ferritic / austenitic stainless steel sheet having excellent formability as described in ( 1 ) above, containing one or more of Cu: 5.00% or less, Mo: 5.00% or less.

( 3 ) Furthermore, in mass%,
Nb: 0.50% or less,
Ti: 0.50% or less,
The ferritic / austenitic stainless steel sheet having excellent formability according to the above (1) or (2) , characterized by containing one or two of the following.

( 4 ) Furthermore, in mass%,
Ca: 0.0030% or less,
Mg: 0.0030% or less,
The ferrite-austenitic stainless steel sheet excellent in formability according to any one of the above (1) to ( 3 ), characterized by containing one or two of the above.

( 5 ) Continuously casting the steel of the component according to any one of (1) to ( 4 ) above, and heating the obtained steel slab at 1150 ° C. or more and less than 1250 ° C. before hot rolling T ₁ (° C.) After being heated in, the hot rolled sheet obtained by carrying out rolling at a temperature of 1000 ° C. or higher and holding at a rolling ratio of 30% or more for 1 pass or more and maintaining a total rolling rate of 96% or more in hot rolling. the was annealed at T ₁ -100 ° C. or more T ₁ ° C. below the temperature, the cold-rolled was performed thereafter, has excellent formability which comprises carrying out final annealing at 1000 ° C. C. to 1100 ° C. ferrite -Manufacturing method of austenitic stainless steel sheet.
(6) wherein the cold-rolled according to (5), and characterized in that to carry out intermediate annealing at a heating temperature _T 1 -100 ° C. or more _{T 1} ° C. below temperatures below 1150 ° C. or higher 1250 ° C. A method for producing ferritic / austenitic stainless steel sheets with excellent formability.

  According to the present invention, it is possible to obtain a ferritic / austenitic stainless steel sheet excellent in formability, particularly uniform elongation, without containing a large amount of Ni. Conventionally, an austenitic stainless steel sheet containing a large amount of Ni is provided. Since it can be applied to parts that have been used, it contributes greatly to the global environment in terms of saving Ni resources.

  The present invention is described in detail below.

  First, the metal structure that is an important element of the present invention will be described.

  The volume fraction of the austenite phase is 10% or more and less than 50%: Since the proportion of the austenite phase is required to be 10% or more in order to obtain good uniform elongation as described above, this is set as the lower limit. Further, the higher the austenite phase ratio, the higher the uniform elongation does not become, and when it exceeds 50%, the uniform elongation is conversely lowered. The austenite phase ratio is preferably measured by classifying phases using EBSP, extracting only austenite grains, and measuring the area ratio. At this time, the measurement range is 200 μm × 200 μm or more. In the present invention, the austenite phase ratio is important as an index of formability (uniform elongation). The reason for this is considered as follows. The austenite phase causes a work-induced martensitic transformation during the forming process and contributes to an increase in uniform elongation. At this time, if the amount of transformation is small, the uniform elongation decreases. The reason why the uniform elongation is low when the austenite phase ratio exceeds 50% is not clear at this stage, but it is presumed that deformation concentrates on the soft ferrite phase compared to the austenite phase.

Md value calculated from the chemical composition in the austenite phase is −10 to 110: In the present invention, the property of the austenite phase is also defined. That is, the Md value calculated from the chemical composition in the austenite phase satisfies the following formula (1).
−10 ≦ Md ≦ 110 (1)
(Where Md = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18 .5 [Mo], [] is the composition (mass%) in the austenite phase, {} is the average composition (mass%))
The chemical composition in the austenite phase from which Md is calculated is measured by EPMA. [] In the above Md calculation formula indicates the composition (mass%) in the austenite phase measured by EPMA of each element. However, since it is difficult for C to be measured by EPMA, the average composition (% by weight) is shown instead of the composition in the austenite phase. When the Md value is less than −10 and more than +110, good uniform elongation cannot be obtained. The reason why the uniform elongation affects the Md value is considered as follows. The Md value is an index representing the stability of the austenite phase, that is, it can be said to represent the amount of strain necessary for causing the processing-induced martensitic transformation. If the amount of strain is too small, the processing-induced martensitic transformation is completed at the initial stage of molding, and sufficient ductility cannot be maintained at the latter stage of molding, which is important for the possibility of molding. If the strain amount is too large, uniform deformation is completed before the strain amount is reached, and the processing-induced martensitic transformation cannot be effectively utilized. Therefore, there exists an appropriate Md value range in which processing-induced martensite transformation occurs during molding.

  The proportion of austenite grains having a crystal grain size of 15 μm or less and a shape aspect ratio of less than 3 is 90% or more of the total number of austenite grains. The metallographic characteristics of austenite grains when good uniform elongation is obtained include fine grains In addition, the austenite grains having a crystal grain size of 15 μm or less and a shape aspect ratio of less than 3 are 90% or more of the total number of austenite grains. When there are many crystal grains having a crystal grain size of more than 15 μm, the uniform elongation becomes low. The lower limit is not particularly required, but if the thickness is 1 μm or less, the cost in the manufacturing process is greatly increased. Therefore, the lower limit is preferably 1 μm.

  The shape of the crystal grains is also an important factor. In the present invention, the aspect ratio of each grain is measured by observing from the L cross section (the plane observed in the plate width direction). The proportion of crystal grains having a value of less than 3 is important. At this time, since the uniform elongation tends to decrease when the aspect ratio is 3 or more, the aspect ratio condition defined as the tissue factor was set to less than 3. The method for measuring the aspect ratio is a value obtained by dividing the length of the longest side of each grain by the length orthogonal thereto. Therefore, the lower limit of the aspect ratio is 1. The number of crystal grains for measuring the crystal grain size and aspect ratio is 100 or more. In the present invention, it became clear for the first time that the austenite grain size and grain shape affect the uniform elongation, but the reason for this is unclear at this stage. However, it is considered that the deformation mode (dislocation density, deformation band, presence / absence of twins, etc.) in the austenite grains was affected, and this changed the work-induced martensitic transformation behavior.

  Average distance between nearest austenite grains is 12 μm or less: Since the distance between nearest austenite grains also affects uniform elongation, the average distance is made 12 μm or less. If it exceeds 12 μm, the uniform elongation decreases, so this was made the upper limit. The lower limit is not particularly specified. The nearest intergranular distance is the intersection of the rolling direction length center line and the plate thickness direction length center line of each austenite grain, and the smallest value of the distances between the center positions of each grain is the center of each grain. The distance between the closest grains. This is defined as the “average distance between nearest austenite grains” with the average value of the results of measurement for 100 crystal grains or more. The reason why the average distance between the nearest austenite grains affects the uniform elongation is considered as follows. Considering the process in which strain is introduced into austenite grains during the deformation process and deformation-induced martensitic transformation occurs, and the deformation spreads to the surroundings when a certain level of strain is reached. The site transformation propagates to several crystal grains and is continuously generated to obtain a high uniform elongation.

  Uniform elongation is an important index representing formability in the present invention. The uniform elongation is measured by taking a JIS No. 13 B tensile test piece in parallel with the rolling direction and using a method according to JIS Z 2241.

  In the present invention, the state of the ferrite grains is not particularly specified. However, when the crystal grain size of the ferrite phase is coarse, the above-mentioned austenite inter-grain distance is increased, so the crystal grain size is 25 μm on average. The following is desirable, and also when the shape aspect ratio is large, the austenite intergranular distance becomes large, so it is desirable that it is less than 3.

  The reasons for limiting the components are described below. In addition, "%" shown below represents mass%.

C: 0.002 to 0.100%,
C is an element that greatly affects the stability of the austenite phase. If added over 0.100%, the uniform elongation may decrease. Further, in order to promote precipitation of Cr carbide, it causes the occurrence of intergranular corrosion, so 0.100% was made the upper limit. In view of corrosion resistance, it is preferable to lower C. However, considering the existing facility capacity, a large increase in cost is caused to reduce the C content to less than 0.002%. Preferably, it is 0.002 to 0.8%.

Si: 0.05 to 2.00%,
Si may be used as a deoxidizing element or may be added to improve oxidation resistance. However, addition of over 2.00% leads to hardening of the material and lowers the uniform elongation, so this was made the upper limit. Preferably it is 1.6% or less. Moreover, in order to reduce Si extremely, the cost at the time of refining is increased, so the lower limit was made 0.05. Preferably it is 0.08%.

Mn: 0.05 to 5.00%,
Mn concentrates in the austenite phase and plays an important role in changing the stability of the austenite phase. However, addition of a large amount not only lowers the uniform elongation, but also lowers the corrosion resistance and hot workability, so the upper limit was made 5.00%. If it is less than 0.05%, the cost in the refining process is increased, so this is set as the lower limit. From the viewpoint of corrosion resistance, the lower one is preferable, and the upper limit is more preferably 3.00%. Furthermore, it is desirable that the upper limit be 2.80%.

P: less than 0.050%,
P is an element inevitably mixed in, and is difficult to reduce because it is contained in raw materials such as Cr. However, if it is contained in a large amount, the formability is lowered, so the upper limit is set to 0.00. Although the content is less than 050%, the lower the value, the more preferably 0.035% or less.

S: less than 0.010%,
S is an element inevitably mixed in, and may combine with Mn to form inclusions, which may serve as the starting point of the cracking, so the upper limit was made less than 0.010%. Since it is preferable from the viewpoint of corrosion resistance, the lower the content, the better it is 0.0020% or less.

Cr: 17 to 25%,
Cr is an element necessary for ensuring corrosion resistance, and it is necessary to add 17% or more. However, the upper limit is set to 25% because a large amount of addition leads to hot working cracks and increases the cost of the refining process. Preferably it is 17 to 22%.

N: 0.010 to 0.150%,
N, like C, is an element that greatly affects the stability of the austenite phase. Moreover, since it has the effect of improving corrosion resistance when it exists in solid solution, it shall be added 0.010 or more. However, when adding over 0.150%, the uniform elongation may be reduced, and Cr nitride is likely to precipitate, conversely causing a decrease in corrosion resistance, so this was made the upper limit. Preferably it is 0.03 to 0.13%.

  Further, the following elements can be selectively added.

Ni: 5.00% or less,
Ni is an austenite stabilizing element and is an important element for adjusting the stability of the austenite phase. Moreover, since it has the effect of suppressing hot working cracks, it may be added in an amount of 0.10% or more when exhibiting these effects. Addition exceeding 5.00% leads to an increase in raw material cost, and it may be difficult to obtain a two-phase structure of austenite and ferrite, so this was made the upper limit. Preferably it is 3.00% or less.

Cu: 5.00% or less Cu, like Ni, is an austenite stabilizing element and an important element for adjusting the stability of the austenite phase. Moreover, since it has the effect of improving corrosion resistance, you may add 0.10% or more. However, addition over 5.00% promotes cracking during hot working and lowers corrosion resistance, so this was made the upper limit.

Mo: 5.00% or less Since Mo is an element that improves corrosion resistance, it may be selectively added. Addition of 0.10% or more shows the effect of improving corrosion resistance, so it is preferable to add more. However, if it exceeds 5.00%, the uniform elongation decreases and the raw material cost increases greatly, so this was made the upper limit.

Nb: 0.50% or less,
Nb has the effect of preventing the weld heat-affected zone from becoming coarse, so 0.03% or more of addition is necessary to exert the effect that may be added. good. However, addition over 0.50% lowers the uniform elongation, so this was made the upper limit.

Ti: 0.50% or less,
Ti, like Nb, may be added in an amount of 0.03% or more in order to prevent the weld heat-affected zone from becoming coarse and to further solidify the solidified structure into fine equiaxed crystals. However, addition over 0.50% lowers the uniform elongation, so this was made the upper limit.

Ca: 0.0030% or less,
Ca may be slightly added for desulfurization and deoxidation. Since the effect is exhibited by addition of 0.0002% or more, this may be added as the lower limit. However, addition of more than 0.0030% tends to cause hot working cracks and lowers corrosion resistance, so this was made the upper limit.

Mg: 0.0030% or less,
Mg has an effect of refining the solidified structure as well as deoxidation, and may be added. In order to exert these effects, 0.0002% or more must be added, and this may be added as the lower limit. Moreover, since addition over 0.0030% brings about the cost increase in a steelmaking process, this was made the upper limit.

  Next, the reason for limiting the manufacturing method will be described. As described above, in order to obtain good uniform elongation, it is necessary to control the metal structure, but such a metal structure is not obtained only by the chemical composition. In the present invention, such a metal structure was obtained by setting the production conditions as described below.

As a hot rolled material, a steel piece obtained by continuous casting is used. Heating temperature _{T 1} of the prior hot rolling is set to less than 1250 ° C. 1150 ° C. or higher. Ear cracks occur during hot rolling when the temperature is lower than 1150 ° C., so this was set as the lower limit. If the heating temperature is higher than 1250 ° C, the austenite grain size after the final annealing tends to be large, and the steel slab is easily deformed in the heating furnace, and wrinkles are likely to occur during hot rolling. did.

In the middle of hot rolling, rolling is carried out for at least one pass so as to hold the rolling at a temperature of 1000 ° C. or higher and a rolling reduction of 30% or more and subsequently hold it for 30 seconds or longer. In order to obtain a metal structure for obtaining good uniform elongation in the present invention, a fine graining process utilizing recrystallization is required during hot rolling. In order to cause hot recrystallization in ferritic / austenitic stainless steel, this reduction step is required. When the rolling temperature is less than 1000 ° C., the austenite grain size becomes coarse in the metal structure after cold rolling annealing even if holding for 30 s or more after a reduction of 30% or more in one pass, and the uniform elongation during the tensile test is not good. It will be enough. In addition, both the rolling reduction and the time between passes have a great influence on the recrystallization behavior, but in order to obtain fine austenite grains with a small aspect ratio after cold rolling annealing, the rolling reduction during hot rolling is 30% or more. The subsequent holding time needs to be 30 s or longer.
Further, the total rolling rate of hot rolling is set to 96% or more. If it is less than 96%, the crystal grains after cold rolling and annealing become coarse and the distance between austenite grains becomes large, so that the uniform elongation becomes insufficient. Annealing temperature of the hot rolled sheet, and be carried out between the T ₁ ° C. from a heating temperature T ₁ -100 ° C. before hot rolling. When the temperature is lower than T ₁ -100 ° C., the aspect ratio of the crystal grain after cold rolling and annealing becomes large. When the temperature is T ₁ ° C. or higher, the crystal grain size after cold rolling and annealing becomes coarse, and the target metal The structure cannot be obtained, and the uniform elongation during the tensile test is lowered.

Moreover, you may implement what is called cold rolling twice which repeatedly performs cold rolling and annealing. An intermediate annealing temperature in should be the same as T ₁ -100 ° C. or more T ₁ ° C. or less and hot rolled sheet annealing.

  The final annealing temperature is 1000 ° C. or higher and 1100 ° C. or lower. When the temperature is lower than 1000 ° C., the shape aspect ratio of the austenite and ferrite grains becomes large, or the Md value falls outside the proper range and the uniform elongation decreases. On the other hand, when the temperature exceeds 1100 ° C., the γ phase ratio decreases, the Md value falls outside the proper range, or the crystal grain size becomes too large.

  Examples are shown below.

After melting the steel types shown in Table 1, a 1.0 mm-thick thin steel sheet was produced through the steps of hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing. In producing the steel sheet, the thickness of the material, the heating temperature of the hot rolling, the rolling pass schedule, the time between rolling passes, the hot rolled sheet annealing temperature, the final annealing temperature and the time can be changed to change the metal structure, This time, the final annealing temperature was changed, and the annealing time was 60 seconds. From the obtained product plate, a tensile test was performed to measure uniform elongation. From the thin steel sheet / L cross-sectional metallographic structure, phase identification by EBSP, investigation of grain size and shape aspect ratio, and measurement of the distance between nearest grains between austenite grains were carried out. Each condition is as described above. The obtained metal structure was measured for γ phase ratio, Md value, X ₁ and X ₂ , and the relationship with uniform elongation is shown in Table 2 together with the production conditions.

The symbols in Table 2 are as shown below.
T ₁ : heating temperature before hot rolling (° C.),
N: The number of times of performing rolling for 30 s or more following the reduction having a reduction rate of 30% or more at 1000 ° C. or more in the hot rolling process,
R: Total hot rolling reduction (%),
T ₂ : hot rolled sheet annealing temperature (° C.),
T ₃ : final annealing temperature (° C.)
X ₁ : ratio of austenite grains having a crystal grain size of 15 μm or less and a shape aspect ratio of less than 3 to the total austenite grains,
X ₂ : The average value of the distance between each austenite grain and the nearest grain,
Md: From the composition in the austenite phase (average composition of C only), a value calculated by the following formula:
Md = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18.5 [ Mo]
Here, [] is the composition (mass%) in the austenite phase, and {} is the average composition (mass%).
Condition 1a is an example of the present invention, and good uniform elongation is obtained. In condition 1b, since T ₂ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention. Further, in condition 1c, since T ₁ does not satisfy the scope of the present invention, X ₁ deviates from the present invention.

In condition 2a, R does not satisfy the scope of the present invention, so X ₂ is out of the present invention. Condition 2b is an example of the present invention, and good uniform elongation is obtained. Conditions 2c because T ₃ does not satisfy the present invention range, gamma phase ratio and X ₂ is out of the present invention.

Condition 3a does not satisfy X ₁ from the present invention because T ₃ does not satisfy the scope of the present invention. Condition 3b is an example of the present invention, and good uniform elongation is obtained. Conditions 3c because N does not satisfy the present invention range, X ₁ is out of the present invention.

In Condition 4a, T ₃₁ and R do not satisfy the scope of the present invention, so X ₁ falls outside the scope of the present invention. Condition 4b is an example of the present invention, and good uniform elongation is obtained. Since conditions 4c is the T ₂ does not satisfy the present invention range, X ₂ is out of the present invention.

Condition 5a is an example of the present invention, and good uniform elongation is obtained. In condition 5b, T ₂ and T ₃ do not satisfy the scope of the present invention, so that the γ phase ratio and X ₁ are outside the scope of the present invention. Condition 5c does not satisfy X ₁ because T ₁ does not satisfy the scope of the present invention.

In condition 6a, R does not satisfy the scope of the present invention, so X ₂ is out of the present invention. Condition 6b is an example of the present invention, and good uniform elongation is obtained. Condition 6c does not satisfy Md and X ₂ because T ₂ and T ₃ do not satisfy the scope of the present invention.

Condition 7a does not satisfy X ₁ from the present invention because T ₃ does not satisfy the scope of the present invention. Condition 7b is an example of the present invention, and good uniform elongation is obtained. Conditions 7c Since N does not satisfy the present invention range, X ₁ is out of the present invention.

In condition 8a, T ₃ , N, R and T ₃ do not satisfy the scope of the present invention, so that the γ phase ratio, Md and X ₂ are out of the present invention. Condition 8b is an example of the present invention, and good uniform elongation is obtained. In condition 8c, since T ₂ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention.

Condition 9a is an example of the present invention, and good uniform elongation is obtained. Conditions 9b is X ₁ Since T ₂ does not satisfy the present invention range is out of the present invention. In condition 9c, since T ₁ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention.

Conditions 10a because R does not satisfy the present invention range, X ₁ is out of the present invention. Condition 10b is an example of the present invention, and good uniform elongation is obtained. Conditions 10c because the _{T 3} do not satisfy the present invention range, Md and _{X 2} is out of the present invention.

In condition 11a, T ₃ does not satisfy the scope of the present invention, so that the γ phase ratio and X ₁ are outside the scope of the present invention. Condition 11b is an example of the present invention, and good uniform elongation is obtained. Conditions 11c because N does not satisfy the present invention range, X ₁ is out of the present invention.

In the condition 12a, since T ₁ and N do not satisfy the scope of the present invention, X ₁ is excluded from the present invention. Condition 12b is an example of the present invention, and good uniform elongation is obtained. In condition 12c, since T ₂ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention.

Condition 13a is an example of the present invention, and good uniform elongation is obtained. In condition 13b, since T ₂ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention. In condition 13c, since T ₁ and N do not satisfy the scope of the present invention, X ₁ falls outside the scope of the present invention.

Condition 14a is an example of the present invention, and good uniform elongation is obtained. In condition 14b, since T ₂ does not satisfy the scope of the present invention, X ₁ and X ₂ depart from the present invention. In condition 14c, T ₁ does not satisfy the scope of the present invention, and thus X ₁ departs from the present invention.

  In any of the conditions 15a, 15b, and 15c, the component system does not satisfy the scope of the present invention, so that good uniform elongation cannot be obtained.

In the example of the present invention, good uniform elongation can be obtained. In the comparative example, any of the γ phase ratio, Md value, X ₁ , and X ₂ does not satisfy the condition, and the uniform elongation is low.

It is the figure which classified the EBSP measurement result into the BCC phase and the FCC phase, (a) shows the BCC phase, and (b) shows the FCC phase in white display. It is a figure which shows the relationship between (gamma) phase rate and uniform elongation (u-EL). It is a figure which shows the relationship between Md value and uniform elongation (u-EL). Crystal grain size is 15μm or less and a shape aspect ratio of less than 3 the austenite grains is a graph showing the relationship percentage of total austenite grains (X ₁₎ and uniform elongation (u-EL). It is a diagram showing the relationship between the average value of the distance between nearest grains of each austenite grain (X ₂₎ and uniform elongation (u-EL).

Claims (6)

% By mass
C: 0.002 to 0.100%,
Si: 0.05 to 2.00%,
Mn: 0.05 to 5.00%,
P: less than 0.050%,
S: less than 0.010%,
Cr: 17 to 25%,
N: 0.010 to 0.150%,
Containing, Ri is Do iron and unavoidable impurities balance,
The volume fraction of the austenite phase is 10% or more and less than 50%, the Md value calculated from the chemical composition in the austenite phase satisfies the following formula (1), and the crystal grain size is in the cross section perpendicular to the rolling width direction. The ratio of austenite grains having a shape aspect ratio of less than 3 accounts for 90% or more of the total number of austenite grains, and the average distance between the nearest austenite grains in the same cross section is 12 μm or less. Ferritic / austenitic stainless steel sheet with excellent formability.
−10 ≦ Md ≦ 110 (1)
(Where Md = 551-462 ({C} + [N])-9.2 [Si] -8.1 [Mn] -13.7 [Cr] -29 [Ni] -29 [Cu] -18 .5 [Mo], [] is the composition (mass%) in the austenite phase, {} is the average composition (mass%)) Furthermore, in mass%,
Ni: 5.00% or less,
Cu: 5.00% or less Mo: 5.00% or less of one or containing two or more, characterized in claim 1 moldability excellent ferrite-austenite stainless steel sheet according. Furthermore, in mass%,
Nb: 0.50% or less,
Ti: 0.50% or less,
The ferritic / austenitic stainless steel sheet having excellent formability according to claim 1 or 2 , characterized in that it contains one or two of the following. Furthermore, in mass%,
Ca: 0.0030% or less,
Mg: 0.0030% or less,
The ferritic / austenitic stainless steel sheet excellent in formability according to any one of claims 1 to 3 , characterized by containing one or two of the following. The steel of the component according to any one of claims 1 to 4 is continuously cast, and the obtained steel slab is heated at a heating temperature T ₁ (° C) of 1150 ° C or higher and lower than 1250 ° C before hot rolling, and then 1000 ° C. held over at least 30% of the reduction ratio of 30s or subsequent to reduction with performing rolling one or more passes, the hot-rolled sheet obtained as the total rolling reduction of 96% or more hot rolling T ₁ -100 ° C. and annealing at least T ₁ ° C. below the temperature, the cold-rolled was performed thereafter, excellent ferrite-austenite stainless steel sheet in formability, characterized by carrying out final annealing at 1000 ° C. C. to 1100 ° C. Manufacturing method. The cold rolling according to claim 5 is performed at a heating temperature T of 1150 ° C. or higher and lower than 1250 ° C. ₁ -100 ℃ or more T ₁ A method for producing a ferritic / austenitic stainless steel sheet excellent in formability, characterized by carrying out intermediate annealing at a temperature of ℃ or less.

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