JP5000281B2 - High-strength stainless steel sheet with excellent workability and method for producing the same - Google Patents

Description

  The present invention relates to a stainless steel sheet used as a structural member that requires both workability and strength, and particularly relates to structural cold-rolled steel sheets such as automobiles, buses, railway vehicles, etc. It is.

  In recent years, from the viewpoint of environmental problems, improvement in fuel efficiency of transportation equipment such as automobiles, 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 below discloses a method for producing a high-ductility, high-strength, chromium-based stainless steel sheet having small in-plane anisotropy. Here, for the steel strip containing 10% to 14% of Cr, the heat treatment conditions are defined, and the structure at the time of finish annealing is adjusted to the cooling rate with the ferrite + austenite structure, so that the strength and ductility are in-plane. It is characterized by reducing the anisotropy. However, when processing as a structural part 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 listed below contains Cr: 11 to 15%, has a main phase of ferrite phase, is excellent in hole expansion property of 2 to 20% martensite phase, and has a tensile strength exceeding 600 MPa. A structural stainless steel plate excellent in workability is disclosed. Here, it is shown that the elongation at break is 15% or more and the hole expansion ratio 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 are cases where sufficient hole expansibility cannot be obtained only by making the product structure into two phases by a normal manufacturing method, and deep drawability other than the hole expansibility may be 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 shown below has a metal structure containing Cr: 9 to 13%, a ferrite phase of 70% or more, a carbonitride and a martensite phase of less than 30%, and a tensile strength of 600 to 900 MPa. A Cr-containing high-strength cold-rolled steel sheet excellent in stretch flangeability (similar to hole expandability) and a method for producing the same are disclosed. In this steel, 0.5% or more of Si was added as a solid solution strengthening element, but there was a problem that ductility was extremely low. Further, as in Patent Document 2, the martensite phase is left in the product to improve the stretch flangeability (hole expansibility is 100% or more), but depending on the crystal orientation of the ferrite phase of the parent phase as described above. However, it was not possible to obtain sufficient stretch flangeability by simply making two phases.

  Patent Document 4 listed below discloses a chromium-containing steel for automobiles that contains Cr: 7 to 15% and has excellent intergranular corrosion resistance and hole expansibility. Here, the content of various components is adjusted so that the hole expandability is 90% or more, but the effect of the structure and crystal orientation is not specified, and the hole expandability varies significantly only with the components. was there.

Further, in Patent Document 5 below, in the manufacturing process of a stainless steel plate having a composition exhibiting a ferrite-austenite two-phase region, 20 to 80% of a martensite phase is generated in the first heat treatment step, and the heat treatment after cold rolling is performed. A method for producing a high-strength stainless steel sheet having excellent ductility by heat treatment below the Ac1 transformation point to a ferrite single phase structure 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.
JP 63-169334 A JP 2005-272938 JP2006-1118018 JP2004-43964 JP 2004-323960 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. For these reasons, it is an object of the present invention to provide a Cr-containing stainless steel plate that solves the problems of the prior art and has high strength and excellent workability.

  In order to solve the above-mentioned problems, the present inventors carefully studied the strength and workability of Cr-containing stainless steel that becomes two phases of a ferrite phase and an austenite phase (cooled and martensite phase) at a high temperature from a metallographic viewpoint. Studied. And the technique which improves workability from the viewpoint different from a prior art (especially hole expansibility) was discovered. That is, among the above documents, Patent Document 2 and Patent Document 3 particularly have a technique for improving the hole expansibility by making the metal structure into two phases, but depends greatly on the deformation characteristics of the ferrite phase as the parent phase. It has been found that sufficient characteristics cannot be obtained only by the conversion. It was also found that the hole expandability can be improved by controlling the texture of the ferrite phase and suppressing deformation in the thickness direction. Specifically, in the final product, the r value and the hole expandability are improved by developing the specific crystal orientation of the ferrite phase of the parent phase. At that time, by refining the crystal orientation relationship between the ferrite phase and the second phase in each manufacturing process and forming the structure of the cold-rolled material into a fine lath martensite structure, the crystal orientation of the ferrite phase after cold-rolling and annealing can be changed. It is controlled to improve workability (r value that is an index of deep drawability).

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 0.5%, Mn: 0.05 to 5%, P: 0.05% or less, Ni: 0.3-5%, Cu: 0.01-3%, Cr: 10-18%, Al: 0.005-0.50%, the balance being Fe And {111} <011> with a crystal orientation strength of 3 or more, {211} <011> with a crystal orientation strength of 3 or more, and {100} <011> crystal orientation strength is less than 2 state, and are hole expansion ratio is 100% or more, ferrite + martensite der high strength stainless steel sheet excellent in workability characterized by Rukoto.
(2) Further, in mass%, Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, B: 0.0020% or less, or one or more of them A high-strength stainless steel plate excellent in workability according to (1), characterized in that it contains.
(3) The high-strength stainless steel plate excellent in workability as described in (1) or (2), further containing Mo: 2.0% or less by mass%.
(4) Excellent in workability as described in any one of (1) to (3), wherein the tensile strength is 500 MPa or more, the elongation at break is 15% or more, and the average r value is 0.9 or more. High strength stainless steel sheet.
(5) (1) to the hot-rolled steel sheet according to any one of (3) was heat-treated at 900 to 1100 ° C. subjecting the cold rolled after martensite, the ferrite was heat-treated at 750 to 900 ° C. + A method for producing a high-strength stainless steel plate excellent in workability as described in the above, characterized by having a martensite structure .

  The reason for limiting the present invention will be described below.

  C deteriorates formability and corrosion resistance, and excessive addition forms a hard martensite phase and causes problems such as plate breakage in the manufacturing process. Therefore, the lower the content, the better, and the upper limit is 0.03%. did. On the other hand, excessive reduction causes the transformation point to disappear, the martensite phase that is the point of the present invention is not generated, and the refining cost is increased, so the lower limit was made 0.001%. More preferably, 0.005 to 0.025% is good.

  N, like C, deteriorates formability and corrosion resistance, and excessive addition forms a hard martensite phase and causes problems such as plate breakage in the production process. Therefore, the lower the content, the better. 03%. On the other hand, excessive reduction causes the transformation point to disappear, the martensite phase that is the point of the present invention is not generated, and the refining cost is increased, so the lower limit was made 0.001%. More preferably, 0.005 to 0.020% is good.

  Si is a solid solution strengthening element and is an element effective for increasing the strength. However, the addition of more than 0.5% rapidly lowers the ductility, so the upper limit was made 0.5%. On the other hand, since it is added as a deoxidizing element and the effect is manifested from 0.05%, the lower limit was made 0.05%. More preferably, it is 0.1 to 0.3%.

  Mn is added as a deoxidizer. Moreover, since it is an austenite generating element, it makes it easy to generate martensite by heat treatment. Since these effects are manifested from 0.05%, the lower limit was made 0.05%. On the other hand, since it is a solid solution strengthening element, the workability is lowered, and MnS is formed to lower the corrosion resistance. Therefore, the upper limit was made 5%. More desirably, it is 0.5 to 4.0%.

  Since P is a solid solution strengthening element like Mn and Si, the lower the content, the better. Therefore, the upper limit was made 0.05%. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.01%. More preferably, 0.02 to 0.04% is good.

  Ni is an austenite-forming element and is effective for improving martensite and improving toughness and high-temperature salt corrosion resistance. Since these effects are manifested from 0.3%, the lower limit was made 0.3%. On the other hand, excessive addition increases the cost and may cause the occurrence of stress corrosion cracking, so the upper limit was made 5%. More preferably, 0.4 to 3% is good.

  For Cr, the lower limit is 10% from the viewpoint of corrosion resistance. On the other hand, addition of over 18% decreases workability and causes toughness deterioration, so it was made 10 to 18%. More preferably, 11 to 16% is good.

  Cu is an austenite-forming element and is effective for improving toughness and sulfuric acid resistance in addition to facilitating the formation of martensite. Since these effects are manifested from 0.01%, the lower limit was made 0.01%. On the other hand, excessive addition increases hardness and costs and may cause the occurrence of stress corrosion cracking, so the upper limit was made 3%. More preferably, 0.03 to 2% is good.

  In addition to being added as a deoxidizing element, Al forms an nitride and is therefore an effective element for improving workability. Since this effect is manifested from 0.005%, the lower limit was made 0.005%. On the other hand, excessive addition causes generation of surface flaws and deterioration of weldability, so the upper limit was made 0.5%. More preferably, 0.01 to 0.10% is good.

  Ti, Nb, and V are elements that combine with C and N to improve the corrosion resistance, or to bring about solid solution strengthening and precipitation strengthening. Therefore, Ti, Nb, and V may be appropriately added in accordance with the strength level and the corrosion resistance level. Since these effects are manifested from 0.05%, the lower limit is preferably 0.05%. On the other hand, excessive addition causes cost increase, manufacturability deterioration, surface quality deterioration and the like, so the upper limit was made 0.5%. More preferably, it is 0.1 to 0.3%.

  Since B is an element effective for increasing the strength, it may be added according to the strength level. Since this effect is manifested from 0.0002%, the lower limit is preferably made 0.0002%. On the other hand, excessive addition leads to deterioration of corrosion resistance and cost increase, so the upper limit was made 0.0020%. More desirably, the content is 0.0003 to 0.0010%.

  Mo is an element that improves the corrosion resistance, and may be appropriately added according to the corrosion resistance level depending on the use environment. Since the effect is manifested from 0.1%, the lower limit is preferably 0.1%. On the other hand, excessive addition causes deterioration of workability and cost increase, so the upper limit was made 2.0%. More preferably, 0.5 to 1.8% is good.

  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 0.02% C-0.13% Si-0.86% Mn-0.03% P-0.0005% S-0.36% Ni-11% Cr-0.03% Cu-0. The crystal orientation distribution, r value and hole expansion rate of .015% Al-0.017% N steel are shown. Steel A and steel B have the same components and a plate thickness of 1 mm. Steel A is heat-treated after hot rolling at a temperature of 1000 ° C. × 60 sec to form a martensite structure and then cold-rolled (plate thickness 1 mm) and annealed (700 ° C. × 60 sec). On the other hand, steel B is a steel single-phase structure formed by applying heat treatment at 700 ° C. × 60 sec in the heat treatment after hot rolling, and then cold-rolled (plate thickness 1 mm) and annealed (700 ° C. × 60 sec).

  Here, for the texture, an X-ray diffractometer (manufactured by Rigaku Denki Kogyo Co., Ltd.) was used, and information on the total plate thickness was obtained using Mo-Kα rays. As the samples, 22 1 mm-thick samples were stacked and subjected to mechanical polishing and electrolytic polishing on the plate thickness surface, and (200), (110), and (211) positive electrode dot diagrams of the ferrite phase were obtained by X-ray diffraction. These data were transferred to CAMP-ISIJ Vol. 18 (2005), 438, a three-dimensional crystal orientation density function in the plate surface direction was obtained. The crystal orientation distribution of the ferrite phase often exhibits a distribution in the thickness direction, and the crystal orientation distribution of only a specific plane in the thickness direction as in the past could not be sufficiently grasped. An orientation distribution is obtained. In addition, the r value was evaluated after collecting a JIS No. 13 B tensile specimen from a cold-rolled annealed plate and applying a strain of 7% in the rolling direction, the rolling direction and 45 ° direction, and the rolling direction and 90 ° direction (1 ).

r = ln (W ₀ / W) / ln (t ₀ / t) (1)
Here, W ₀ is the sheet width before tension, W is the sheet width after tension, t ₀ is the sheet thickness before tension, t is the sheet thickness after tension, r ₀ is the r value in the rolling direction, and r _45. Is the r value in the rolling direction and the 45 ° direction, and r ₉₀ is the r value in the direction perpendicular to the rolling direction. The evaluation of the hole expandability is performed by pushing a 60 ° conical punch into a φ12mm punch hole and expanding the hole little by little. When the hole cracks, the punch is stopped and the hole expansion rate λ is followed by the change in hole diameter. Obtained from the formula.

λ = 100 × (D−D ₀ ) / D ₀ (2)
Here, D is the hole diameter after the hole expansion test, and D ₀ is the hole diameter before the hole expansion. FIG. 1 shows the strength distribution of the crystal orientation of steel A and steel B with contour lines. Steel A improves the r value {111} <011> The strength is as strong as 3 or more, and the r value is reduced {100 } <011> is weak with less than 2. In addition, {211} <011> is a crystal orientation whose r value is lower than that of {111} <011>, but is an orientation that increases the r value in the 45 ° direction and is strong at 4 or more. This steel A has a high average r value of 1.0, and the hole expansion rate is as high as 110%. On the other hand, since the crystal orientation strength of steel B is different from that of steel A, the r value is as low as 0.7 and the hole expansion rate is as low as 80%. If the r value is 0.9 or more and the hole expansion ratio is 100% or more, it has satisfactory workability as a structural member. Therefore, in the crystal orientation strength of the entire plate thickness by X-ray diffraction, {111} of the ferrite phase The <011> strength was 3 or more, {211} <011> the strength was 3 or more, and the {100} <011> strength was less than 2. In this invention, the r value is improved by controlling the crystal orientation of the parent phase, and is different from the conventional technique for improving the hole expansion by the two phases of ferrite and martensite phase. Is to improve.

  The structure form at this time may be a ferrite single phase or a ferrite + martensite structure, and a martensite phase may be generated according to the strength level. That is, when a high-strength material is required, martensite may be generated, and a cold-rolled sheet annealing temperature described later may be adjusted. However, the strength level generally required for a high-strength member is 500 MPa or more, and the lower limit is 500 MPa. The tensile elongation at break is also important for hole expansibility. However, since the hole expansibility is sufficiently obtained at a break elongation of 15% or more, and processing as a high-strength structural member is possible, the elongation at break is reduced. 15% or more.

  Next, a manufacturing method will be described. As in the present invention, in order to develop a texture in a product plate and obtain a high r value and a high hole expansion characteristic, it has been found that it is important to generate a martensite phase in an intermediate process. In a normal manufacturing method, a process in which heat treatment is performed in a ferrite single phase region after hot rolling and annealing after cold rolling is adopted, but the r value is improved by generating a martensite phase in the heat treatment after hot rolling. FIG. 2 shows the relationship between the hot-rolled sheet heat treatment conditions and the r-value of the cold-rolled annealed sheet. The cold-rolled sheet annealing conditions were 700 ° C. at which the structure became a ferrite single phase and 800 ° C. at which two phases of a ferrite phase and a martensite phase were formed. FIG. 3 shows the relationship between the hot-rolled sheet heat treatment conditions and the hole expansion rate of the cold-rolled annealed sheet. From these, by heat-treating the hot-rolled sheet at 900 ° C. or higher, the r value is 0.9 or higher and the hole expansion ratio is 100% or higher regardless of the structure after cold rolling annealing. This is because when the steel of the present invention is heated and cooled in the austenite region, it causes martensitic transformation. However, since it is a lath-like extremely fine structure, it is from the vicinity of the grain boundary of the fine structure during cold rolling and annealing. } It is considered that the orientation is easy to develop. {111} orientation grains are crystal orientations that improve the r value of ferritic steel having a BCC crystal structure, but generally, when cold rolling a two-phase structure of hard martensite and soft ferrite phase, 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 rolling annealing appears. Conceivable. The expression of this texture may also 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. As shown in FIG. 2, by improving the r value according to the present invention, the material inflow property at the time of hole expansion is improved, and the hole expansion ratio is improved as shown in FIG. In the above-mentioned Patent Document 5, a technique for generating a 20 to 80% martensite phase in the first heat treatment step is disclosed, but only from the viewpoint of homogenization, which is different from the technical idea of the present invention. From the viewpoint of the texture development of the cold-rolled / annealed plate, the larger the amount of lath martensite, the better, the production amount is preferably more than 80%, and the heat treatment temperature is 900 to 1100 ° C.

  The structure after the cold-rolled sheet annealing may be either a ferrite single phase or a ferrite + martensite phase as described above. However, if the martensite phase is excessively generated, the ductility is remarkably reduced, so the heat treatment temperature is 700 to 900. C.

  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 related to transportation, it can contribute greatly to environmental measures and safety improvements.

  Steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to form a hot rolled coil having a thickness of 3 mm. Thereafter, the hot rolled coil was annealed and pickled, cold-rolled to a thickness of 1 mm, and annealed and pickled to obtain a product plate. The product plate thus obtained was subjected to a tensile test, r value measurement, and hole expansion test.

  As is apparent from Table 1, when steel having the component composition defined in the present invention is produced by this method, it is excellent in workability as compared with the comparative example.

  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.

It is a figure which shows crystal orientation distribution, r value, and hole expansibility. It is a figure which shows the relationship between the hot-rolled sheet heat processing temperature and structure | tissue (after hot-rolled sheet heat processing), and the average r value of a product board. It is a figure which shows the relationship between the hot-rolled sheet heat processing temperature and structure | tissue (after hot-rolled sheet heat processing), and the hole expansion rate of a product board.

Claims (5)

In mass%
C: 0.001 to 0.03%,
N: 0.001 to 0.03%,
Si: 0.05 to 0.5%,
Mn: 0.05-5%
P: 0.05% or less,
Ni: 0.3 to 5%,
Cu: 0.01 to 3%,
Cr: 10 to 18%,
Al: 0.005 to 0.50% is contained, the balance consists of Fe and inevitable impurities,
In the crystal orientation strength of the total thickness by X-ray diffraction, {111} <011> crystal orientation strength is 3 or more, {211} <011> crystal orientation strength is 3 or more, and {100} <011> crystal orientation strength is 2 less than in state, and are hole expansion ratio is 100% or more, ferrite + martensite der high strength stainless steel sheet excellent in workability characterized by Rukoto. Furthermore, in mass%,
Ti: 0.5% or less,
Nb: 0.5% or less,
The high-strength stainless steel plate with excellent workability according to claim 1, wherein one or more of V: 0.5% or less and B: 0.0020% or less are contained. Furthermore, in mass%,
The high-strength stainless steel plate with excellent workability according to claim 1 or 2, characterized by containing Mo: 2.0% or less.   The high strength excellent in workability according to any one of claims 1 to 3, wherein the tensile strength is 500 MPa or more, the elongation at break is 15% or more, and the average r value is 0.9 or more. Stainless steel sheet. The hot-rolled steel sheet according to any one of claims 1 to 3 is heat-treated at 900 to 1100 ° C to obtain a martensite structure, then cold-rolled, and heat-treated at 750 to 900 ° C to obtain ferrite + martensite. A method for producing a high-strength stainless steel sheet having excellent workability, which is characterized by having a structure .

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