JP2017201049A - High-strength stainless steel sheet excellent in workability and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength stainless steel sheet which improves an r value by controlling a crystal orientation of a base phase while increasing a strength in a ferrite single phase steel, and is excellent in workability by improving hole expansibility, and to provide a method for manufacturing the same.SOLUTION: A stainless steel sheet contains one or two kinds of 0.001-0.03% C, 0.001-0.03% N, 0.05-3.0% Si, 0.1-15.0% Mn, 0.05% or less P, 0.01% or less S, 10% or more and less than 18 Cr, 0.30% or less Ti and 0.50% or less Nb, where the total of Ti and Nb is 8(C+N) to 0.75%, γ(gamma potential) is 65-85%, a ferrite grain size is 20 μm or less, and n×(1+r) is 0.40 or more. In the expression, n represents an n value (work hardening index), rrepresents the minimum r(Lankford) value, and γis evaluated by using a Castro.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength stainless steel plate excellent in workability and a method for producing the same, particularly used for automobile parts that require both workability and strength. Particularly, the present invention relates to a vehicle body such as an automobile, a motorcycle, a bus, and a railway vehicle. The present invention relates to structural cold-rolled steel sheets such as undercarriages, pillars, bumpers, and front side members, and cold-rolled steel sheets for fastening parts such as fixtures, flanges, clamps, and bands.

  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, and application of stainless steel, which is a high corrosion resistance steel, is being studied. When stainless steel containing Cr is applied, weight reduction by reducing rust allowance and omission of painting are the focus of application. Further, from the viewpoint of ensuring the safety of passengers, it is required to improve the collision safety, but it is necessary to achieve both the above-mentioned weight reduction of the vehicle body. As a measure for improving the collision safety, it is effective to increase the strength of the steel plate constituting the member, and there is a possibility that both safety and light weight can be achieved by applying the high strength stainless steel plate. A problem in applying a high-strength material to the structural member is securing workability. This is because if the workability is reduced due to the increase in strength, it becomes difficult to mold into a complex shaped part. In particular, the high-strength stainless steel sheet has many problems that cause cracks during hole expansion processing, and has left a problem when it is subjected to hole expansion processing in structural members of automobiles, buses, and railway vehicles.

  Conventionally, a martensitic stainless steel plate that is strengthened by quenching is known as the stainless steel plate for the structural member, but there is a problem in workability of the member because the ductility is extremely low. On the other hand, S304 and S301 are used as the austenitic stainless steel sheet. These are excellent in ductility, and high work hardening characteristics utilizing work-induced transformation can be obtained. However, it contains a large amount of Ni and is expensive, and depending on the environment, stress corrosion cracking becomes a problem, and the reliability as a structural material may be lowered.

  Among such problems, Patent Document 1 discloses a method for producing a high-ductility, high-strength, multiphase chromium-based stainless steel sheet having small in-plane anisotropy. Here, for steel strips containing 10% to 14% Cr, in-plane differences in strength and ductility can be achieved by adjusting the cooling rate by defining the heat treatment conditions and setting the structure during finish annealing heating as a ferrite + austenite structure. The feature is to reduce the directivity. However, when processing as a structural component as described above, particularly in hole expansion processing, deep drawability becomes a problem in addition to ductility, and there is a problem that the r value, which is an index of deep drawability, is low.

  Patent Document 2 discloses a structural stainless steel sheet containing Cr: 11 to 15%, having a main phase of a ferrite phase, an excellent hole expansion property of 2 to 20% of a martensite phase, and a tensile strength exceeding 600 MPa. Is disclosed. Here, it is shown that the elongation at break is 15% or more and the hole expansion rate is 70% or more due to the two-phase structure of the product structure. By expanding the martensite phase in the soft ferrite phase, the hole expansion is achieved. It is characterized by reducing the carbonitride that becomes the starting point of cracking during processing. However, there is a case where sufficient hole expansibility cannot be obtained only by making the product structure into two phases by a normal manufacturing method, and martensite itself may be a starting point of cracking. There is also a problem that the deep drawability becomes insufficient. This is because not only the volume fraction of martensite but also the plastic anisotropy due to the dispersion state and the crystal orientation of the ferrite phase are affected.

  Patent Document 3 includes Cr: 9 to 13%, has a metal structure including ferrite phase of 70% or more, carbonitride and martensite phase of less than 30%, and has an elongation flange with a tensile strength of 600 to 900 MPa. A Cr-containing high-strength cold-rolled steel sheet having excellent properties (similar to hole expandability) and a method for producing the same are disclosed. In this steel, the martensite phase remains in the product as in Patent Document 2 to improve stretch flangeability (hole expandability is 100% or more). Depending on the orientation, sufficient stretch flangeability could not be obtained by simply making it two-phased.

  Patent Document 4 discloses a chromium-containing steel for automobiles containing Cr: 7 to 15% and having excellent intergranular corrosion resistance and hole expansibility. Here, the content of various components is adjusted in order to make the hole expansion ratio 90% or more, but the influence of the structure and crystal orientation is not defined, and the hole expansion property is remarkably only by the definition of the components. There was a case where it varied. Further, the conditions at the time of manufacturing the cold-rolled steel sheet are only premised on the manufacturing method of the Cr-containing steel such as ordinary stainless steel, and the manufacturing conditions for obtaining high workability are insufficient.

Patent Document 5 includes Cr: 10 to 18%, and in the manufacturing process of a stainless steel plate having a composition exhibiting a two-phase region of ferrite and austenite, 20 to 80% of martensite phase is generated in the first heat treatment step. A method for producing a high-strength stainless steel sheet having excellent ductility by forming a ferrite single phase structure by performing heat treatment after cold rolling below the A _c1 transformation point is disclosed. In this manufacturing method, the crystal orientation of the ferrite phase of the product plate is not sufficiently developed, the plastic anisotropy is not sufficiently developed, and the hole expandability is sometimes inferior. Moreover, since an alloy element for fixing carbon and nitrogen is not added, hard carbonitride may be generated.

  Patent Document 6 includes Cr: 10 to 18%, and controls the texture of the ferrite phase of the parent phase after cold rolling and annealing by making the structure of the cold rolled material into a fine lath martensite structure. And the manufacturing method of the high intensity | strength stainless steel plate excellent in the workability which becomes a ferrite + martensite structure | tissue which grows a specific crystal orientation is disclosed. However, even in this production method, the average r value is about 0.9 to 1.2, and the workability is still insufficient.

JP 63-169334 A JP 2005-272938 JP 2006-1118018 A JP 2004-43964 A JP 2004-323960 A JP 2008-138270 A

  As described above, in the Cr-containing stainless steel sheet, various studies have been made on increasing the strength and improving the hole expansibility, but it has been impossible to stably achieve both strength and workability. In addition, when a ferrite + martensite duplex steel is used, defects are generated at the interface between hard martensite and soft martensite, and the hole expandability may be deteriorated. It is desirable. In view of the above, an object of the present invention is to solve the problems of the prior art and to provide a Cr-containing stainless steel plate having high strength and excellent workability and a method for producing the same.

  In the present invention, the r value is obtained by controlling the crystal orientation of the parent phase while increasing the strength of the ferrite single-phase steel based on a completely different idea from the conventional technique of improving hole expansion by ferrite + martensite biphasic. It is an object to improve the hole expanding property.

  In order to solve the above-mentioned problems, the present inventors have carefully studied the strength and workability of Cr-containing stainless steel that becomes a two-phase ferrite phase and austenite phase at a high temperature from the viewpoint of metallographic structure. And the technique which improves workability from the viewpoint different from a prior art (especially r value used as a parameter | index of deep drawability and hole expansibility) was discovered. That is, among the above-mentioned documents, Patent Documents 2 and 3 have a technique for improving the hole expansibility by making the metal structure into a two-phase, but greatly depends on the deformation characteristics of the ferrite phase as a parent phase, and only the two-phase is provided. Then, it was found that sufficient characteristics could not be obtained. It was also found that the hole expandability can be improved by increasing the strength while maintaining the ferrite single phase structure and further improving the r value by texture control. Specifically, recrystallization and phase transformation were used at the same time in controlling the microstructure of the final product. As a result, while maintaining the ferrite single phase, [1] high strength while ensuring ductility by refining the structure, and [2] ferrite phase ({111} effective in improving the workability of the ferrite phase) <011>, {211} <011>) and the suppression of a specific crystal orientation ({311} <136>) that lowers workability, and improves workability (r value and hole expandability) Can be achieved. At that time, by examining the crystal orientation relationship between the ferrite phase and the second phase in each manufacturing process and making the microstructure of the cold-rolled material a fine lath martensite structure, the crystal orientation of the ferrite phase after cold-rolling and annealing can be changed. It controls and improves workability (r value, hole expansibility).

The gist of the present invention for solving the above problems is as follows.
(1) In mass%, C: 0.001 to 0.03%, N: 0.001 to 0.03%, Si: 0.05 to 3.0%, Mn: 0.1 to 15.0 %, P: 0.05% or less, S: 0.01% or less, Cr: 10% or more and less than 18%, Ti: 0.30% or less, Nb: 0.50% or less 2 types are included, the total of Ti and Nb is 8 (C + N) to 0.75%, the balance is made of Fe and inevitable impurities, γ _p (gamma potential) is 65 to 85%, X In the crystal orientation strength of the ferrite phase at the center of the plate thickness by line diffraction, the {111} <011> crystal orientation strength is 3.0 or more, {211} <011> the crystal orientation strength is 3.0 or more, {311} <136 > The ratio of crystal orientation strength and {100} <011> crystal orientation strength ({311} <136> Orientation strength / {100} <011> crystal orientation strength) is 2.5 or less, ferrite grain size is 20 μm or less, and ferrite phase area ratio is 90% or more. Strength stainless steel sheet. Note that γ _p is evaluated using the Castro equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X. Zero if it contains only inevitable impurities.
(2) Further, by mass%, B: 0.0002 to 0.0030%, Al: 0.030 to 0.300%, Mo: 0.1 to 2.0%, Ni: 0.1 to 1. 2%, Cu: 0.1 to 2.0%, V: 0.05 to 1.00%, Sn: 0.005 to 0.500%, W: 0.005 to 3.00%, Co: 0 .01-0.30%, Sb: 0.005-0.500%, Ta: 0.01-0.10%, Ga: 0.0002% -0.1000%, REM: 0.001-0. The high-strength stainless steel plate having excellent workability as described in (1), which contains 200% of one kind or two or more kinds.
(3) The high-strength stainless steel plate excellent in workability as described in (1) or (2), wherein the average r value is 1.5 or more.
(4) The high strength excellent in workability according to any one of (1) to (3), wherein r _min is 1.0 or more and n × (1 + r _min ) is 0.40 or more. Stainless steel sheet.
n is an n value (work hardening index), and r _min is a minimum r (Rankford) value.
(5) The tensile strength is 440 MPa or more, the elongation at break is 20% or more, and the hole expansion ratio is 100% or more. The high excellent workability according to any one of (1) to (4) Strength stainless steel sheet.
(6) Using the slab having the component composition described in (1) or (2), in hot rolling, rough rolling is performed at a slab heating temperature of 1100 to 1200 ° C, and finishing rolling is started at a temperature of 900 ° C or more. Finish rolling so that the temperature is 800 ° C. or higher and the difference is within 200 ° C., winding at 600 ° C. or higher, cold rolling, and heat-treating at 700 to 1000 ° C. as an annealing treatment after cold rolling. The method for producing a high-strength stainless steel plate excellent in workability according to any one of (1) to (5), wherein a cooling rate after heat treatment is 1 to 10 ° C./sec.
(7) The said stainless steel plate is a high strength stainless steel plate excellent in workability as described in any one of (1)-(5) used for automotive structural components.
(8) The high-strength stainless steel plate excellent in workability according to any one of (1) to (5), wherein the stainless steel plate is used for automobile fastening parts.

  As is clear from the above description, according to the present invention, it is possible to provide a Cr-containing steel sheet having high strength and excellent workability without adding particularly expensive alloy elements, particularly automobiles, buses, railways, etc. By applying it to structural members and fastening parts related to transportation, it can greatly contribute to environmental measures and safety improvements.

It is a figure which shows crystal orientation distribution, r value, and hole expansibility. It is a figure which shows the relationship between n * (1 + _rmin ) of a product board, and hole expansibility. It is a figure which shows the relationship between the crystal grain diameter of a product board, and tensile strength.

The reason for limitation of the present invention will be described below.
The component range of steel will be described.

  C deteriorates moldability and corrosion resistance. Further, if C is high, the crystal orientation strength (texture) of the ferrite phase required in the present invention is difficult to obtain and martensite remains in the steel. Therefore, the lower the content, the better. Was 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering the manufacturing cost and intergranular corrosion properties of the weld, 0.002 to 0.02% is desirable.

  N deteriorates formability and corrosion resistance in the same way as C. If N is high, the crystal orientation strength (texture) of the ferrite phase required in the present invention is difficult to obtain and martensite remains in the steel. Therefore, the lower the content, the better, and the upper limit was made 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering the manufacturing cost, workability and corrosion resistance, 0.005 to 0.015% is desirable.

  Si is sometimes added as a deoxidizing element, and also improves oxidation resistance. However, Si is a solid solution strengthening element. Excessive addition rapidly reduces ductility, and {311 due to slip system limitations. }, The upper limit was made 3.0%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.05%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.1 to 1.0% is desirable.

  Mn, like Ni, is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. It also contributes to improved scale adhesion and suppression of abnormal oxidation. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, when excessively added, MnS is formed to lower the corrosion resistance and martensite remains in the steel, so the upper limit was made 15.0%. Furthermore, if considering the manufacturing cost and corrosion resistance, 1.0 to 5.0% is desirable.

  Since P is a solid solution strengthening element like Si, the lower the content, the better. Therefore, the upper limit was made 0.05%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.01 to 0.02% is desirable.

  Since S is an element that degrades corrosion resistance, the upper limit was made 0.01%. Furthermore, if considering the manufacturing cost and suppression of crevice corrosion when used as a part, 0.0005 to 0.0050% is desirable.

  Cr is an element that improves corrosion resistance and oxidation resistance. When considering the exhaust part environment, 10% or more is necessary from the viewpoint of suppressing abnormal oxidation. On the other hand, excessive addition of Cr causes hardening and deteriorates formability. Further, since Cr is a ferrite stabilizing element, if added excessively, austenite phase transformation does not occur. Furthermore, from the viewpoint of cost increase, the upper limit was made less than 18%. In addition, when considering the sheet breakage and workability at the time of manufacturing the steel sheet due to the manufacturing cost and toughness deterioration, 10.5% or more and less than 15% are desirable.

The present invention contains one or two of Ti: 0.30% or less and Nb: 0.50% or less, and the total of Ti and Nb is in the range of 8 (C + N) to 0.75%. Here, Ti, Nb, C, and N mean content (mass%) of each element.
Ti is an element added to combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. Moreover, since addition exceeding 0.30% hardens by solute Ti and toughness deteriorates, the upper limit was made 0.30%. Furthermore, if considering the manufacturing cost, 0.06 to 0.25% is desirable.
Nb, like Ti, is an element added to combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. In addition to improving workability and high-temperature strength, it is added as necessary to suppress crevice corrosion and promote repassivation. However, excessive addition causes hardening and deteriorates moldability, and also delays recrystallization, so the upper limit was made 0.50%. Furthermore, if considering the manufacturing cost, 0.05 to 0.3% is desirable.
Further, if the sum of Ti and Nb is less than 8 (C + N), excess C and N are dissolved in the steel and hardened, so the sum of Ti and Nb is 8 (C + N) or more. Furthermore, if the total of Ti and Nb exceeds 0.75%, solid solution Ti, solid solution Nb, and Nb and Ti carbonitrides and intermetallic compounds cause hardening and deteriorate toughness and formability. , Ti and Nb total is 0.75% or less.

  This invention can contain the following elements further as needed.

  B is an element that improves the secondary workability of the product by segregating at the grain boundaries. In addition to suppressing vertical cracks during secondary processing of exhaust system parts, 0.0002% or more is preferably added in order to prevent cracks in winter. However, excessive addition causes a decrease in workability and corrosion resistance, so the upper limit was made 0.0030%. Furthermore, if considering the refining cost and ductility reduction, 0.0003 to 0.0015% is desirable.

  In addition to being added as a deoxidizing element, Al has an effect of suppressing oxide scale peeling. Since this effect is manifested at 0.030% or more, the lower limit was made 0.030%. On the other hand, addition of 0.300% or more causes a decrease in elongation, weld penetration, and surface quality deterioration, so the upper limit was made 0.300%. Furthermore, if considering the refining cost and the pickling property at the time of manufacturing the steel sheet, 0.050 to 0.150% is desirable.

  Mo is an element that improves the corrosion resistance, and is an element that suppresses crevice corrosion, particularly when it has a crevice structure. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. Further, if it exceeds 2.0%, the moldability is remarkably deteriorated and the manufacturability is deteriorated, so the upper limit was made 2.0%. Considering the alloy cost and productivity, 0.1 to 0.5% is desirable.

  Ni is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. It also promotes crevice corrosion suppression and repassivation. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, excessive addition hardens and deteriorates moldability, and stress corrosion cracking tends to occur, so the upper limit was made 1.2%. In view of the raw material cost, 0.2% to 1.0% is desirable. More desirably, the upper limit is 0.8%, which may be 0.5% or less.

  Cu, like Ni and Mn, is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. Moreover, in order to promote suppression of crevice corrosion and repassivation, it adds as needed. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, excessive addition hardens and deteriorates toughness and moldability, so the upper limit was made 2.0%. Considering alloy cost and productivity, 0.15 to 1.0% is preferable.

  V is added as necessary to suppress crevice corrosion. Since this effect appears from 0.05% or more, the lower limit was made 0.05%. However, excessive addition hardens and deteriorates moldability, so the upper limit was made 1.00%. In consideration of the raw material cost, 0.10 to 0.50% is desirable.

  Sn contributes to the improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. However, since addition of more than 0.500% may cause slab cracking during steel sheet manufacture, the upper limit is made 0.500%. Furthermore, if considering the refining cost and manufacturability, 0.003 to 0.300% is desirable.

  W contributes to improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. However, the upper limit is set to 3.00% because it hardens by addition of more than 3.00% and leads to toughness deterioration and cost increase at the time of manufacturing the steel sheet. Furthermore, if refining costs and manufacturing methods are taken into consideration, 0.01 to 0.10% is desirable.

  Co contributes to improving the high-temperature strength, so 0.01% or more is added as necessary. Since addition of more than 0.30% leads to toughness deterioration and cost increase at the time of manufacturing the steel sheet, the upper limit is made 0.30%. Furthermore, if refining costs and manufacturability are taken into consideration, 0.01 to 0.10% is desirable.

  Sb is an element that segregates at the grain boundary to increase the high temperature strength. Since this is expressed from 0.005% or more, the lower limit was made 0.005%. However, if it exceeds 0.500%, Sb segregation occurs and cracks occur during welding, so the upper limit is made 0.500%. Considering high temperature characteristics, production cost and toughness, 0.03 to 0.30% is desirable. More desirably, it is 0.050 to 0.200%.

  Ta is combined with C and N to contribute to the improvement of toughness, so 0.01% or more is added as necessary. However, the addition of more than 0.10% increases the cost, so the upper limit is made 0.10%. Furthermore, if considering refining costs and manufacturability, 0.01 to 0.08% is desirable.

  Ga may be added at 0.1000% or less for improving corrosion resistance and suppressing hydrogen embrittlement. The lower limit is made 0.0002% from the viewpoint of sulfide and hydride formation. Preferably it is 0.0010% or more.

  REM (rare earth element) is effective in improving oxidation resistance, and is added in an amount of 0.001% or more as necessary. Moreover, even if added over 0.200%, the effect is saturated and the corrosion resistance is lowered by sulfide of REM, so 0.001 to 0.200% is added. Considering the workability and manufacturing cost of the product, it is desirable that the lower limit is 0.002% and the upper limit is 0.10%. REM follows the general definition. It is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu). It may be added alone or as a mixture.

  The other components are not particularly defined in the present invention, but Hf may be added in an amount of 0.001% to 1.0% in order to improve the high temperature strength. Moreover, you may contain Bi 0.001 to 0.02% as needed. Note that it is desirable to reduce general harmful impurity elements such as As and Pb as much as possible.

In the steel sheet of the present invention, γ _p is adjusted to 65% or more within the above-described component range. If γ _p is too low, it is necessary to heat the ferrite-austenite transformation at a high temperature, which makes it difficult to obtain the crystal orientation strength (texture) of the ferrite phase required in the present invention and to refine the crystal grains. Since it becomes difficult, it was made 65% or more. Further, if γ _p is too high, martensite formation is unavoidable in the steel, and the steel sheet is not made mainly of a ferrite phase. Furthermore, considering the balance between the material and the crystal grain size, γ _p is preferably 72% or more and 80% or less. γ _p is evaluated using the Castro equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X. Zero if it contains only inevitable impurities.

  Next, the relationship between texture and workability (r value, n value, hole expandability) will be described.

In the present invention, the crystal orientation of the ferrite phase of the parent phase is extremely important for workability, and the average crystal orientation distribution in the thickness direction of the steel sheet contributes to the improvement of the r value, and the basic molding of the structural material It has been found that it has a beneficial effect on improving the hole expandability, which is one of the characteristics. FIG. 1 shows steel A (0.005% C-0.43% Si-1.85% Mn-0.022% P-0.0010% S-0.23% Ni-11.0% Cr-0. 79% Cu-0.05% Al-0.011% N, γ _p = 77) and steel B (0.005% C-0.42% Si-0.66% Mn-0.021% P-0 .0004% S-0.16% Ni-10.9% Cr-0.07% Al-0.009% N, γ _p = 59). Steel A is cold-rolled (plate thickness: 1 mm) and annealed (A: 800 ° C. × 60 sec) after hot rolling. On the other hand, steel B is cold-rolled (plate thickness 1 mm) and annealed (900 ° C. × 60 sec) after hot rolling.

  Here, the texture is measured using an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.), and using the Mo-Kα ray, the central region of the plate thickness (combination of mechanical polishing and electrolytic polishing) (200), (110), (211) positive pole figure of the present) was obtained, and ODF (orientation distribution function) was obtained from these data using spherical harmonics.

  FIG. 1 shows the strength distribution of the crystal orientations of the steel A and the steel B with contour lines. {111} <011> improves the r value, and {211} <011> has {111} < Although the r value is lower than 011>, it is an orientation that increases the r value in the 45 ° direction and is as strong as 3.0 or more. {311} <136> reduces the r value, and {100} <011> has a small effect on the r value, but is caused by the suppression of the production of {311} <136> by the phase transformation. The crystal orientation. This steel A has a strong {111} <011> strength of 3.0 or more and a {211} <011> strength of 3.0 or more, a {311} <136> crystal orientation strength and a {100} <011> crystal. The ratio of orientation strength ({311} <136> crystal orientation strength / {100} <011> crystal orientation strength) is 2.5 or less, and the average r value is as high as 1.9. On the other hand, since the crystal orientation strength of steel B is different from that of steel A, the average r value is as low as 1.4. If the average r value is 1.5 or more, it has satisfactory processability as a structural member. Therefore, the {111} <011> strength of the ferrite phase is 3. 0 or more, and the ratio of {311} <136> crystal orientation strength to {100} <011> crystal orientation strength is 2.5 or less. By setting the texture satisfying this condition, it is possible to provide a high workability stainless steel sheet having an average r value of 1.5 or more.

  That is, according to the present invention, in the crystal orientation strength of the ferrite phase at the center of the plate thickness by X-ray diffraction, the {111} <011> crystal orientation strength is 3.0 or more, and the {211} <011> crystal orientation strength is 3.0 or more. , {311} <136> crystal orientation strength and {100} <011> crystal orientation strength ratio ({311} <136> crystal orientation strength / {100} <011> crystal orientation strength) is defined as 2.5 or less. To do.

The average r value was evaluated by taking a JIS13B tensile test piece from the product plate and applying a 14.4% strain in the rolling direction, the rolling direction and 45 ° direction, and the rolling direction and 90 ° direction (2). It calculated using Formula (3) and Formula (3).
r = ln (W ₀ / W) / ln (t ₀ / t) (2)
Here, W ₀ is the plate width before tension, W is the plate width after tension, t ₀ is the plate thickness before tension, and t is the plate thickness after tension.
r _m = (r ₀ + r ₄₅ + r ₉₀ ) / 4 (3)
Here, r _m is the average r value, r ₀ is the r value in the rolling direction, r ₄₅ is the r value in the rolling direction and the 45 ° direction, and r ₉₀ is the r value in the rolling direction and the 90 ° direction.
The minimum r value (r _min ) is the r value having the smallest value among r ₀ , r ₄₅ , and r ₉₀ .

ここで、ｅ₁＝０．０５、ｅ₂＝０．１５はそれぞれ二点の塑性ひずみ（−）、Ｆ₁、Ｆ₂はそれぞれｅ₁、ｅ₂に対応した試験力（Ｎ）である。 The evaluation of the n value, that is, the work hardening index, was obtained by taking a JIS No. 13 B tensile test piece from the product plate and obtaining a stress strain curve in accordance with JIS Z 2253, and then having a test force (% plastic strain of 5% and 15%). N) was calculated and calculated using equation (4).

Here, e ₁ = 0.05 and e ₂ = 0.15 are two points of plastic strain (−), and F ₁ and F ₂ are test forces (N) corresponding to e ₁ and e ₂ , respectively.

The hole expandability is evaluated in accordance with JIS Z 2256. A 60 ° conical punch is pushed into a punched hole of φ10mm to gradually expand the hole, and when the hole cracks, the punch is stopped and the hole diameter changes. From this, the hole expansion rate λ was obtained using the equation (5).
λ = 100 × (D−D ₀ ) / D ₀ (5)
Here, D is the hole diameter after the hole expansion test, and D ₀ is the hole diameter before the hole expansion.

  The steel plate used for the automotive structural component targeted by the present invention is required to have high strength of 440 MPa or more and high hole expansibility in addition to a high average r value. In the present invention, as described above, as a structure mainly composed of ferrite, [1] high strength while ensuring ductility by refining the structure, and [2] ferrite phase effective for improving workability of the ferrite phase. A texture is realized to improve workability (r value and hole expandability).

If the hole expansion ratio is 100% or more, it has satisfactory workability as a steel material for automobile structural parts. As shown in FIG. 2, the hole expansibility correlates with the n value and the minimum r value (r _min ), and the vehicle structure is such that n × (1 + r _min ) satisfies the condition of 0.40 or more. It is possible to provide a high-workability stainless steel plate having a hole expansion rate of 100% or more suitable for steel for parts. In FIG. 2, the hole expansion rate is 100% or more, and the others are black squares. More desirably, n × (1 + r _min ) is 0.45 or more. If the minimum r value is 1.0 or more, n × (1 + r _min ) can be set to 0.40 or more.

  Next, the crystal structure and the high temperature strength will be described.

  The structure form at this time needs to be a structure mainly composed of ferrite as described above. The ferrite main structure in the present invention is a structure having a ferrite phase ratio (area ratio) of 90% or more. Desirably, it is 95% or more. A single-phase structure with a ferrite phase ratio of 100% is more desirable. The ferrite phase ratio was measured by EBSD (Electron Back Scattering Diffraction) and image analysis. The amount of martensite produced was determined quantitatively by measuring the fraction of ferrite phase and martensite using TSL OIM (Orientation Imaging Microscopy) analysis software. If the ferrite phase ratio is less than 90%, the r value becomes poor or unmeasurable and the hole expansion rate becomes poor.

  At this time, the crystal grain size of the ferrite is 20 μm or less. As shown in FIG. 3, a high-strength stainless steel plate having a tensile strength of 440 MPa or more, which is a generally required strength level as a high-strength member, even if it is a ferrite structure by setting the crystal grain size to 20 μm or less. Can be provided. More desirably, the crystal grain size is 10 μm or less. In FIG. 3, the strength is 440 MPa or more, and the others are ●. The crystal grain size is an average crystal grain size. The crystal grain size of ferrite was measured by the EBSD method. The measurement condition of the particle diameter is a condition of 0.3 to 0.6 μm step at a measurement magnification of 1000 times, and the obtained data is set to one circle boundary with an orientation difference of 15 ° or more by OIM analysis software. The equivalent diameter was calculated. The value obtained by arithmetic average of the obtained equivalent circle diameter was defined as the crystal grain size.

  In addition, the tensile elongation at break is also important for hole expansibility. However, since sufficient hole expansibility is obtained at a break elongation of 20% or more, and processing as a high-strength structural member is possible, it is preferable to break Elongation is 20% or more. More desirably, it is 25% or more.

The present invention contains the above-described components of the present invention and, as described above, has a texture of the present invention and a crystal structure having a ferrite grain size of 20 μm or less and a ferrite phase area ratio of 90% or more. r value is 1.5 or more, n × (1 + r _min ) is 0.40 or more, tensile strength is 440 MPa or more, elongation at break is 20% or more, hole expansion rate is 100% or more, and has high workability and high temperature strength. Stainless steel sheet can be realized.

  Next, a manufacturing method will be described.

  A slab having the above-described component composition of the present invention is cast. The cast slab is heated at 1100 to 1200 ° C. From the viewpoint of the development of the texture of the cold-rolled / annealed sheet, the hot-rolled sheet should have a larger amount of lath martensite, and the generated amount is preferably over 80%. Further, if the heating temperature is too low, scale generation is reduced and the surface quality is deteriorated due to seizure of the rolling roll and the steel material, so the lower limit temperature is set to 1100 ° C. From the viewpoint of furnace performance and economy, the slab heating temperature was set to 1100 to 1200 ° C. Furthermore, considering productivity and surface wrinkles, 1120 ° C. to 1160 ° C. is desirable.

  After the slab heating, in the hot rolling process, multiple passes of rough rolling are performed, and finish rolling consisting of a plurality of stands is performed in one direction. After rough rolling, finish rolling is performed at a high speed and the coil is wound up. In the present invention, in order to obtain a fine structure during winding, the rough rolling temperature (the slab heating temperature) and the winding temperature are defined. In order to prevent cracks and breaks during production, it is important to make the structure of the hot-rolled sheet a fine structure. Further, by cold-rolling a hot-rolled sheet having a refined structure, the development of the {311} <136> orientation is suppressed while the {111} <011> crystal orientation and the {211} <011> crystal orientation are developed. In addition, the ratio of the {311} <136> and {100} <011> crystal orientations can be made 2.5 or less. However, if the winding temperature is too low, recrystallization and phase transformation do not occur during winding, and thus finishing rolling must be performed at a high temperature and at a high speed. Therefore, the finish rolling temperature is set such that the start temperature is 900 ° C. or higher, the end temperature is 800 ° C. or higher, and the difference is within 200 ° C. In addition, the coiling temperature is 600 ° C. or higher. Thereby, the crystal grain size and tensile strength of the steel sheet can be within the scope of the present invention in combination with the cold-rolled sheet annealing conditions described later. In the present invention, the hot-rolled plate thickness may be selected as appropriate, but about 2 to 15 mm is desirable in consideration of the winding shape, plate thickness accuracy, and surface properties.

  After the hot rolling is completed, the following cold rolling and cold rolling sheet annealing are performed without performing hot rolling sheet annealing. By omitting the hot-rolled sheet annealing, the crystal grain size is within the range of the present invention in combination with the cold-rolled sheet annealing conditions described later, and the texture of the steel sheet can be a preferable structure of the present invention.

  In order to obtain the structure after cold-rolled sheet annealing, it is necessary to anneal in the ferrite + austenite two-phase region. At this time, if the annealing is performed at an excessively high temperature, martensite is generated in the steel and the ductility is lowered. Further, the {111} crystal orientation of the cold-rolled plate also undergoes phase transformation, and the {111} crystal orientation of the product plate becomes weak. Therefore, the upper limit temperature was set to 1000 ° C. On the other hand, if the heating temperature is too low, recrystallization does not occur sufficiently, the cold-rolled texture remains in the steel, and the workability (r value and hole expandability) decreases. Therefore, the heat treatment temperature was set to 700 to 1000 ° C. More desirably, the temperature is 750 to 900 ° C. Even if the annealing temperature is within the range of the present invention, if the cooling rate is too high, martensite is generated, the ferrite area ratio is reduced, and the r value and the hole expandability are lowered. If the cooling rate is too slow, ferrite crystal grains grow and become coarser to 20 μm or more. Therefore, the upper limit of the cooling rate after annealing is 10 ° C./sec or less and the lower limit is 1 ° C./sec or more. Basically, it is desirable to cool by air, but if it becomes a ferrite single phase structure even if cooled by water, it may be cooled by water.

  As in the present invention, in order to develop a texture in the product plate and obtain a high r value and high hole expansion characteristics, recrystallization and phase transformation (ferrite phase → austenite phase → ferrite phase) are performed in the middle process. I found it important to do it at the same time. In this invention, it pickles without performing hot-rolled sheet annealing, and uses for a cold-rolling process as a cold-rolling raw material. This is different from a normal manufacturing method (usually hot-rolled sheet annealing is performed). In a normal manufacturing method, a method of obtaining a sized recrystallized structure by performing hot rolling plate annealing in a ferrite single phase region is general. On the other hand, in this invention, since hot-rolled sheet annealing is not performed, r value improves by carrying out cold rolling with the martensite structure | tissue produced | generated at the hot rolling process remaining, and annealing after cold rolling. In addition, when sufficiently fine martensite is generated, hot-rolled sheet annealing may be performed. When the steel of the present invention is heated in the austenite region during hot rolling or high-temperature hot-rolled sheet annealing, it causes martensitic transformation during cooling, but in the component system of the present invention, it is a lath-like extremely fine structure, This is probably because the {111} orientation easily develops from the vicinity of the grain boundary of the fine structure during the subsequent cold rolling and annealing. The {111} -oriented grains are crystal orientations that improve the r-value of ferritic steel having a BCC crystal structure. Generally, however, when a two-phase structure of hard martensite and soft ferrite phase is cold rolled, The martensite phase introduces non-uniform deformation in the ferrite phase, and {111} oriented grains are difficult to generate. However, in the component system of the present invention, since the morphology of the lath martensite structure is extremely fine, the effect of developing ferrite having {111} crystal orientation from fine lath martensite during transformation and recrystallization during cold-rolled sheet annealing is manifested. I think that. The expression of this texture may depend on the hardness difference between the martensite phase and the ferrite phase, and it is considered that the hardness balance of both phases is appropriate in the component system of the present invention. Further, during annealing after cold rolling, by annealing in the ferrite + austenite two-phase region, {111} oriented grains that are difficult to undergo phase transformation are recrystallized while {100} <011>, which tends to undergo phase transformation, are recrystallized. Let The processed {100} <011> produces {311} <136> recrystallized grains that greatly reduce the r value when recrystallized, but suppresses the formation of these recrystallized grains by transforming to an austenite phase. It becomes possible to do. Further, the phase transformed {100} <011> transforms into a ferrite phase or a martensite phase having an orientation in the vicinity of the original {100} <011> due to the orientation memory effect during cooling. In order to bring about this azimuth memory effect, it is considered that the component system of the present invention is appropriate because the increase in strength due to the addition of Mn is important. Further, the {111} oriented grains remaining without undergoing phase transformation are recrystallized into {111} oriented grains during annealing. From the above, it is possible to obtain a high workability steel in which the {111} orientation is developed and the formation of {311} <136> is suppressed.

  As described above, using a slab having a component composition defined in the present invention, in hot rolling, rough rolling is performed at a slab heating temperature of 1100 to 1200 ° C., and finish rolling is started at a temperature of 900 ° C. or higher and an end temperature of 800 ° C. After finishing rolling so that the difference is within 200 ° C., winding at 600 ° C. or more, cold rolling, heat treatment at 700-1000 ° C. as an annealing treatment after cold rolling, and after heat treatment The cooling rate of 1 to 10 ° C./sec realizes the crystal orientation strength (aggregate structure) of the ferrite phase defined in the present invention, the ferrite grain size is 20 μm or less, and the ferrite phase area ratio is 90% or more. The quality of the tensile strength is 440 MPa or more, the breaking elongation is 20% or more, and the hole expansion rate is 100% or more. Further, the rmin can be 1.0 or more, the ferrite phase area ratio can be 90% or more, and the average r value can be 1.5 or more.

  In addition, about the manufacturing method of a steel plate, what is necessary is just to select suitably about conditions other than prescribe | regulated by this invention. For example, no special equipment is required for hot rolling conditions, hot rolled sheet thickness, product sheet thickness, cold rolled sheet annealing atmosphere, pass schedule, cold rolled rate, roll diameter in cold rolling, and existing equipment is used efficiently Just do it. Further, temper rolling or tension leveler may be applied after cold rolling and annealing.

  The stainless steel plate of the present invention is preferably used for automobile structural parts or automobile fastening parts. This is because it has high strength and good workability required for stainless steel plates for automobile structural parts and automobile fastening parts. Here, examples of the automobile structural component include a suspension, a chassis, an arm, and a member. Further, examples of the automobile fastening part include a flange and a bracket.

  Steels having the composition shown in Table 1 were melted and cast into slabs. The cast slab was hot-rolled under the conditions shown in Table 2 to obtain a hot rolled coil having a thickness of 5 mm. Thereafter, pickling was performed without annealing the hot-rolled coil. Only Comparative Example B2 in Table 2 is subjected to hot-rolled sheet annealing. Then, it cold-rolled to 1 mm thickness, annealed and pickled on the conditions shown in Table 2, and was set as the product board. The product plate thus obtained was subjected to a tensile test (tensile strength, elongation at break), r-value / n-value measurement, hole expansion test, and texture evaluation. The results are shown in Table 3. The test conditions were as described above. In the column “311/100” regarding the texture in Table 3, the ratio of {311} <136> crystal orientation strength and {100} <011> crystal orientation strength ({311} <136> crystal orientation strength / {100} <011> crystal orientation strength).

Steel No. 1 in Table 1 A1 to A23 in Tables 1 to 23, Table 2 and Table 3 are examples of the present invention. As is apparent from the results, when the steel having the component composition defined in the present invention is produced by the method of the present invention, the crystal orientation strength (texture) of the ferrite phase defined in the present invention is realized, and the ferrite grains The diameter is 20 μm or less, the ferrite phase area ratio is 90% or more, the average r value is 1.5 or more, the minimum r value is 1.0 or more, and n × (1 + r _min ) is 0.40 or more. Is 440 MPa or more, the elongation at break is 20% or more, and the hole expansion rate is 100% or more.

Steel No. 1 in Table 1 24 to 36, B1 to B20 in Tables 2 and 3 are comparative examples.
Since B1 has an excessively cold-rolled sheet annealing temperature, the ferrite area ratio is insufficient, and {211} <011> crystal orientation strength is low, r value cannot be measured, and workability is poor. In B7 and B8, γ _p was too low, the texture was out of the range of the present invention, and the average r value was not achieved. In B11, C was too high, and in B12, N was too high.

B2 was subjected to hot-rolled sheet annealing, B4 had a low coiling temperature, B5 had a slow cooling rate after cold-rolled sheet annealing, and the crystal grain size exceeded 20 μm, and the tensile strength was not achieved. Further, γ _p was too low for B7 and B8, Cr was too high for B10, both had a crystal grain size exceeding 20 μm, and the tensile strength was not achieved.

B3 had a low cold-rolled sheet annealing temperature, B14 had a high Nb content and was off, had a large hot-rolling finish rolling temperature difference, had unrecrystallized steel sheets, and all had poor workability. B6 has a too slow cooling rate after cold-rolled sheet annealing, B9 has too high γ _p , B11 has too high C, B12 has too high N, B13 has too low Ti and Nb, both of which are ferrite area ratios In addition, the average r value, minimum r value, and n × (1 + r _min ) did not reach or could not be measured, and the workability was poor. In B15 to B17, any one of Cu, Si, and Ni was too high, and the workability was poor.

Claims (8)

In mass%, C: 0.001 to 0.03%, N: 0.001 to 0.03%, Si: 0.05 to 3.0%, Mn: 0.1 to 15.0%, P : 0.05% or less, S: 0.01% or less, Cr: 10% or more and less than 18%, Ti: 0.30% or less, Nb: 0.50% or less And the total of Ti and Nb is 8 (C + N) to 0.75%, the balance is made of Fe and inevitable impurities, and γ _p (gamma potential) is 65 to 85%,
In the crystal orientation strength of the ferrite phase at the center of the plate thickness by X-ray diffraction, {111} <011> crystal orientation strength is 3.0 or more, {211} <011> crystal orientation strength is 3.0 or more, {311} <136> crystal orientation strength and ratio of {100} <011> crystal orientation strength ({311} <136> crystal orientation strength / {100} <011> crystal orientation strength) is 2.5 or less,
A high-strength stainless steel plate excellent in workability characterized by having a ferrite particle size of 20 μm or less and a ferrite phase area ratio of 90% or more.
Note that γ _p is evaluated using the Castro equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X. Zero if it contains only inevitable impurities.   Furthermore, in mass%, B: 0.0002 to 0.0030%, Al: 0.030 to 0.300%, Mo: 0.1 to 2.0%, Ni: 0.1 to 1.2%, Cu: 0.1-2.0%, V: 0.05-1.00%, Sn: 0.005-0.500%, W: 0.005-3.00%, Co: 0.01- 0.30%, Sb: 0.005 to 0.500%, Ta: 0.01 to 0.10%, Ga: 0.0002% to 0.1000%, REM: 0.001 to 0.200% The high-strength stainless steel sheet having excellent workability according to claim 1, comprising one or more kinds.   The high-strength stainless steel sheet having excellent workability according to claim 1 or 2, wherein an average r value is 1.5 or more. The high-strength stainless steel plate with excellent workability according to any one of claims 1 to 3, wherein r _min is 1.0 or more and nx (1 + r _min ) is 0.40 or more. .
Here, n is an n value (work hardening index), and r _min is a minimum r (Rankford) value.   The high-strength stainless steel excellent in workability according to any one of claims 1 to 4, wherein the tensile strength is 440 MPa or more, the elongation at break is 20% or more, and the hole expansion ratio is 100% or more. steel sheet.   In the hot rolling using the slab having the component composition according to claim 1 or 2, rough rolling is performed at a slab heating temperature of 1100 to 1200 ° C, and finish rolling is started at a temperature of 900 ° C or higher and an end temperature of 800 ° C. After finishing rolling so that the difference is within 200 ° C., winding at 600 ° C. or more, cold rolling, heat treatment at 700-1000 ° C. as an annealing treatment after cold rolling, and after heat treatment The manufacturing method of the high strength stainless steel plate excellent in workability of any one of Claims 1-5 characterized by making the cooling rate of 1-10 degrees C / sec.   The high-strength stainless steel plate excellent in workability according to any one of claims 1 to 5, wherein the stainless steel plate is used for automobile structural parts.   The high-strength stainless steel plate excellent in workability according to any one of claims 1 to 5, wherein the stainless steel plate is used for automobile fastening parts.

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