JP4254663B2 - High strength thin steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength thin steel sheet having a tensile strength of 780 MPa or more, which is excellent in bending workability and mold galling resistance, and a method for producing the same.

  In recent years, in order to improve the fuel efficiency of automobiles or the safety of passengers in the event of a collision, it has been actively studied to apply high-strength steel sheets with a tensile strength of 780 MPa or more to automobile parts, mainly reinforcing members. Yes. For this reason, demands for workability of high-strength thin steel sheets are becoming increasingly severe. However, a normal high-strength thin steel sheet is inferior in workability at the time of forming a part, so that improvement is required.

As a high-strength thin steel sheet excellent in workability, a composite structure steel sheet having ferrite as a main phase and a low-temperature transformation phase such as martensite or bainite as a second phase has been proposed. For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having a composite structure containing ferrite as a main phase, having a tensile strength of 80 kgf / mm ² or more and a yield ratio of 60% or less. This steel sheet has excellent workability with a tensile strength-elongation balance (TS × El) of 17000 to 25000 MPa ·%. However, in this way, the high-strength thin steel sheet using a hard low-temperature transformation phase tends to form cracks at the interface between the hard phase and the soft phase, so that there is a problem that the bendability is not sufficient.

As a high-strength thin steel sheet excellent in bendability, a steel sheet having a high structure uniformity mainly composed of low-temperature transformation phases such as bainite and martensite has been proposed. For example, Patent Document 2 presents an ultra-high strength cold-rolled steel sheet having a bainite structure fraction of 90% or more and a tensile strength of 1200 N / mm ² or more. This steel sheet can be bent at a bending angle of 90 ° and a bending inner radius of 0.5 mm, and exhibits excellent bendability. However, since the structure mainly composed of bainite and martensite is not hardened and deforms non-uniformly, there is a problem that severe processing with a bending angle of 90 ° and a bending radius of about 0 mm cannot be performed in the processing of sheet parts. .

  As a high-strength thin steel sheet excellent in bendability, a steel sheet having a structure mainly composed of ferrite only in the surface layer portion has been proposed. For example, Patent Document 3 discloses an ultra-high-strength cold-rolled steel sheet that includes a soft layer of C: 0.1 wt. This steel sheet has an excellent bending property because the surface layer portion is softened by decarburization annealing, thereby allowing close bending. Patent Document 4 discloses an ultra-high-strength cold-rolled steel sheet having a layer mainly composed of ferrite as a surface layer of a steel sheet and a layer mainly composed of martensite bainite as an inner layer. This steel sheet exhibits excellent workability because the surface layer is mainly composed of ferrite by decarburization annealing. However, since the galling resistance deteriorates when the surface is softened like these steel sheets, the compatibility between workability and galling resistance has not been achieved in the prior art.

On the other hand, since workability is affected by the surface roughness, it has been studied to improve the workability by adjusting the surface roughness. For example, Patent Document 5 discloses a technique for improving workability by appropriately adjusting the surface roughness of a steel plate. This technique secures good slidability and improves workability by adjusting the surface roughness imparted by control of skin pass rolling to a thin steel plate produced by continuous annealing. In addition, since the mold galling resistance is also affected by the surface roughness, it has been studied to improve the mold galling resistance by adjusting the surface roughness. For example, Patent Document 6 discloses a technique for improving the mold galling resistance by controlling the geometric shape of the steel sheet surface. This technique improves the resistance to galling regardless of the surface pressure generated in the plate during press molding. However, if the tensile strength required as a reinforcing member is 780 MPa or more, it is difficult to control the surface roughness and geometric shape of the steel sheet, and even if controlled, it will be hard-worked, so the workability will deteriorate. There was a problem.
JP-A-4-236671 Japanese Patent Laid-Open No. 5-105959 JP-A-5-195149 Japanese Patent Laid-Open No. 10-1307825 JP-A-6-99202 JP-A-9-29304

  An object of the present invention is to provide a high-strength thin steel sheet excellent in workability at a tensile strength of 780 MPa or more and excellent in galling resistance and a method for producing the same. In the steel sheet according to the present invention, the target value of workability is a minimum bending radius of 0.2 t or less.

  In order to provide a steel sheet having the above characteristics, the present inventors have repeatedly studied from the viewpoints of steel composition, steel surface properties, steel structure, and production conditions. As a result, by adjusting the steel composition and manufacturing conditions to an appropriate range, the Vickers hardness of the surface layer portion having a depth from the front and back surfaces of the steel sheet of 0.05 mm is in the range of 100 to 250 Hv, and (depth from the front and back surfaces). Vickers hardness at a position of 0.2 mm) x 0.8 or less, Vickers hardness variation of inner layer part from depth of 0.2 mm from the front and back surfaces is 100 Hv or less, a structure mainly composed of bainite or martensite, surface roughness Has found that a high strength thin steel sheet excellent in workability such as bendability and mold galling resistance can be obtained without lowering the strength level by making the steel sheet in the range of 0.4 to 1.2 μm in Ra. Completed the invention.

  According to the present invention, it is possible to simultaneously achieve high strength, workability, and improvement of mold galling resistance, which could not be achieved with the conventional technology, tensile strength is 780 MPa or more, and minimum bending radius is 0.2 t or less. It is possible to provide a high-strength thin steel sheet with excellent workability and anti-scoring property characterized by having the following mechanical properties, which contributes to reducing the weight of automobile body parts and improving collision safety Is remarkable. Also, since the wear of the skin pass roll is reduced, the productivity of high strength steel sheets of 780 MPa or more can be improved.

Next, various conditions defined in the present invention will be described.
First, the reason for limiting the chemical composition of the high-strength thin steel sheet of the present invention will be described. In the following description, “%” indicating a chemical composition represents “% by mass” unless otherwise specified.

(C: 0.06-0.25%)
C is an austenite stabilizing element and effectively acts to strengthen the transformation structure that forms the hard phase and strengthens the steel. In order to ensure a tensile strength of 780 MPa or more, at least 0.06% or more is contained. However, if it exceeds 0.25%, the workability and weldability deteriorate significantly. For this reason, C content was limited to 0.06 to 0.25% of range. A preferred lower limit is 0.07%, and a preferred upper limit is 0.15%.

(Si: 0.005-1.0%)
Si is an element that contributes to strength improvement. In the present invention, Si is contained in an amount of 0.005% or more. However, if the content exceeds 1.0%, the nugget portion at the time of spot welding is cured and the toughness deteriorates. For this reason, Si content was made into 0.005-1.0%. Si is an element that promotes the decarburization reaction, and its effect is observed at 0.2% or more, so it is preferably 0.2 to 1.0%.

(Mn: 1.0-2.7%)
Mn is an austenite stabilizing element and effectively acts to strengthen the transformation structure that forms the hard phase and strengthens the steel. In order to ensure a tensile strength of 780 MPa or more, at least 1.0% or more is contained. However, if the content exceeds 2.7%, the structure becomes uneven and the bendability deteriorates. For this reason, Mn content was made into 1.0 to 2.7%. The preferred lower limit is 1.2% and the preferred upper limit is 2.6%.

(P: 0.02% or less)
P is an unavoidable impurity, and if contained excessively, a non-uniform structure is formed, so that workability deteriorates, so it is desirable to reduce it as much as possible. Therefore, the P content is set to 0.02% or less. Preferably it is 0.015% or less. In addition, it is preferable to make a minimum into 0.005% from a viewpoint of the steel manufacturing cost and effect which are required for reduction of P content.

(S: 0.01% or less)
S is an unavoidable impurity and exists as a sulfide in the steel and becomes a stress concentration source, so that workability deteriorates. For this reason, it is desirable to reduce the S content as much as possible. However, if it is 0.01% or less, even a high-strength material as intended in the present invention does not adversely affect bendability. In addition, Preferably it is 0.005% or less.

(Sol.Al: 0.01-0.08%)
Al is an element added to deoxidize steel and effectively acts to improve the cleanliness of the steel. In order to remove silicate inclusions and improve processability, the content of sol.Al is 0.01% or more. However, if the content exceeds 0.08%, the oxide inclusions increase, so the surface properties and workability deteriorate. Therefore, the sol.Al content is set to 0.08% or less. The preferred lower limit is 0.02, and the preferred upper limit is 0.06%.

(N: 0.0003-0.01%)
N is an unavoidable impurity, and if it is excessively contained, coarse nitrides are precipitated, so that workability is deteriorated. For this reason, it is desirable to reduce the N content as much as possible. However, if it is 0.01% or less, even a high-strength material as intended in the present invention does not adversely affect workability. For this reason, N content was made into 0.01% or less. In addition, Preferably it is 0.005% or less, More preferably, it is 0.003% or less.

(Cr: 0.1-0.5%, Mo: 0.1-0.5%, B: 0.0005-0.003%)
In this invention, the said element can be contained 1 type or 2 types or more as needed.
Cr is an element that effectively acts to strengthen the transformation structure that enhances hardenability, produces a hard phase and strengthens the steel, and its effect is recognized with a content of 0.1% or more. However, if the content exceeds 0.5%, the effect becomes saturated and the cost increases. For this reason, Cr content was made into 0.1 to 0.5%.

  Mo is an element that effectively acts to strengthen the transformation structure that enhances hardenability, produces a hard phase and strengthens the steel, and the effect is recognized with a content of 0.1% or more. However, if the content exceeds 0.5%, the effect becomes saturated and the cost increases. For this reason, Mo content was made into 0.1 to 0.5%.

  B is an element that effectively acts on strengthening the transformation structure that enhances hardenability, produces a hard phase and strengthens the steel, and the effect is recognized with a content of 0.0005% or more. However, if the content exceeds 0.003%, not only the effect is saturated, but also the deformation resistance of hot rolling becomes large, which makes it difficult to produce. For this reason, B content was made into 0.0005 to 0.003%. A preferred lower limit is 0.0007%, and a preferred upper limit is 0.002%.

(Nb: 0.01 to 0.1%, V: 0.01 to 0.1%, Ti: 0.01 to 0.1%, one or more)
In the present invention, one or more of the above elements can be contained as required. These elements are elements that effectively work to improve the bendability by refining the crystal grains, and the effect is recognized when each element contains 0.01% or more. However, if the content exceeds 0.1%, the precipitates in the steel become coarse and the workability deteriorates. For this reason, Nb content was 0.01 to 0.1%, V content was V: 0.01 to 0.1%, and Ti content was Ti: 0.01 to 0.1%.

  The balance other than the above components is Fe and impurities. Examples of impurities include O, Cu, and Ni, and the contents of O: 0.006% or less, Cu: 0.05% or less, and Ni: 0.05% or less are acceptable.

Next, the reason for limiting the surface layer structure of the high strength thin steel sheet of the present invention will be described.
The Vickers hardness at a position where the depth from the front and back surfaces of the high-strength thin steel sheet of the present invention having the above composition is 0.05 mm is 100 to 250 Hv, and (Vickers at a position where the depth from the front and back surfaces is 0.2 mm) Hardness) x 0.8 or less. When the Vickers hardness at the position is less than 100 Hv, the mold galling resistance deteriorates. When the Vickers hardness at the position is more than 250 Hv or (Vickers hardness at a depth of 0.2 mm from the front and back surfaces) × 0.8, the surface ductility is lowered and the bendability is deteriorated.

  Here, the surface layer portion is C (gds) ((C component value measured by GDS): 0.5 × C (gds-bulk) (the depth from the front and back surfaces of the steel sheet measured by GDS is 0.1 mm to 0.15 mm) It is preferable to include 0.002 to 0.05 mm of a C-deficient layer equal to or less than the average value of C component values.Mn (gds) (Mn component value measured by GDS): 0.8 × Mn ( It is more preferable to include an Mn-deficient layer having a depth from the front and back surfaces of the steel sheet measured by GDS of 0.1 mm to 0.15 mm (average value of Mn component values) or less. Thus, not only the Vickers hardness at the position is easily controlled within the range of 100 to 250 Hv, but also a remarkable improvement in workability is recognized.

Next, the reason for limiting the structure of the inner layer portion of the high strength thin steel sheet of the present invention will be described.
The high-strength thin steel sheet of the present invention having the components and surface properties described above has a Vickers hardness variation of 100 Hv or less in the inner layer portion on the sheet thickness center side from the position where the depth from the front and back surfaces is 0.2 mm, and bainite and martensite. It is a structure containing 80% or more of the total area ratio.

  When the variation in Vickers hardness exceeds 100 Hv, deformation tends to concentrate in the soft region and not only the bendability deteriorates, but also the workability subjected to bending and bending back processing deteriorates remarkably. For this reason, the variation in the Vickers hardness of the inner layer portion was set to 100 Hv or less.

  In addition to bainite and martensite, for example, when ferrite and residual austenite are contained in a total area ratio of 20% or more, microcracks are likely to occur at the phase interface, and the bendability is significantly reduced. For this reason, bainite and martensite are contained in a total area ratio of 80% or more. Here, bainite and martensite may have an area ratio of 0%.

  Here, according to the present invention, the reason that “workability” and “galling resistance” are compatible is that not only the bendability is improved by making the surface a soft structure, but also the roughness of the steel sheet surface. This is because adjustment can be facilitated and anti-galling characteristics can be improved. In addition, the high strength thin steel sheet having a tensile strength of 780 Mpa or more is difficult to control the surface roughness of the steel sheet because it is hard, and therefore, the control of the surface roughness of the steel sheet has not been studied in the past. .

Next, the reason for limiting the surface roughness of the high-strength thin steel sheet of the present invention will be described.
The surface roughness of the high-strength thin steel sheet according to the present invention is in the range of 0.4 to 1.2 μm in Ra. By making Ra 0.4 μm or more, not only the slidability is improved and the workability is improved, but also the mold galling resistance is improved. However, when Ra exceeds 1.2 μm, stress tends to concentrate on the recesses on the surface of the steel sheet and workability deteriorates. For this reason, the surface roughness was set to a range of 0.4 μm or more and 1.2 μm or less in terms of Ra. The surface roughness of the steel sheet can be adjusted by a cold rolling process before annealing and a skin pass rolling process after annealing.

Next, the reasons for limiting the production conditions of the high strength thin steel sheet of the present invention will be described.
The high-strength thin steel sheet of the present invention can be produced by subjecting a cold-rolled steel sheet obtained by hot rolling from a steel slab or steel ingot having the above-described steel composition to continuous annealing described later. As for the production conditions, conventional methods may be used. For example, when hot rolling, descaling, cold rolling and continuous annealing are performed on a steel ingot or steel slab having the above-described steel composition, it is preferable to do the following.

The molten steel having the above steel composition is melted by a generally known melting method such as a converter or an electric furnace, and is made into a steel material such as a slab by continuous casting or casting-slab rolling. Continuous casting is desirable from the viewpoint of productivity.
Next, the steel material is subjected to hot rolling. When the steel material is produced by a continuous casting method, it may be directly subjected to hot rolling after continuous casting, or continuous casting or casting-splitting. When manufacturing by rolling, after cooling to a suitable temperature once, you may use for a hot rolling by heating with a heating furnace. When heating in a heating furnace, the heating temperature is preferably 1000 to 1300 ° C. When heated above 1000 ° C, not only hot rolling is easy, but also a Mn-deficient layer is formed on the surface, and the Vickers hardness at a position where the depth from the front and back surfaces of the steel sheet after continuous annealing is 0.05 mm is 100 It becomes easy to control to ~ 250Hv. On the other hand, when the heating temperature exceeds 1300 ° C., the scale loss increases.

  The finishing temperature in hot rolling is preferably in the range of 800 to 950 ° C. When the finishing temperature is less than 800 ° C., the deformation resistance during rolling is large, the structure becomes a non-uniform band structure, and the workability after cooling annealing may deteriorate. On the other hand, when the temperature exceeds 950 ° C., grain growth occurs in subsequent cooling, and a uniform fine structure may not be obtained. The coiling temperature after hot rolling is preferably in the range of 500 to 700 ° C. When the coiling temperature is less than 500 ° C., hard bainite and martensite are generated, and subsequent cold rolling may be difficult. When the coiling temperature exceeds 700 ° C., the structure becomes a non-uniform band structure, and the workability after continuous annealing may deteriorate.

  The hot-rolled steel sheet is pickled by a normal method and then cold-rolled to obtain a cold-rolled steel sheet. In order to refine the structure of the cold-rolled annealed plate and improve workability, it is preferable that the rolling reduction of cold rolling is 40% or more. In addition, you may adjust the roughness of the steel plate surface by the cold rolling before continuous annealing. In that case, it is preferable to roll the final pass at a roll surface roughness Ra of 2.0 μm or more and a reduction amount of 8 μm or more.

(Soaking conditions for cold-rolled steel sheet: 700 ° C to (Ac ₃ transformation point – 20 ° C) over 20 seconds, heated (Ac ₃ transformation point – 20 ° C) to (Ac ₃ transformation point + 20 ° C) ) Within 10 seconds)
The annealing of the cold-rolled steel sheet is continuous annealing. First, the temperature range from 700 ° C. to (Ac ₃ transformation point −20 ° C.) is heated for 20 seconds or more, and (Ac ₃ transformation point −20 ° C.) to (Ac ₃ transformation). Hold for 10 seconds or longer in the range of (+ 20 ℃).

By heating the temperature range from 700 ° C to (Ac ₃ transformation point-20 ° C) over 20 seconds, the steel plate surface is softened by decarburization, and it is easy to provide the desired surface roughness on the steel plate surface. In addition, the workability of the steel sheet is improved. Once the cold-rolled steel sheet is heated to the range of (Ac ₃ transformation point −20 ° C.) to (Ac ₃ transformation point + 20 ° C.) to form austenite single phase or a structure close to austenite single phase, bainite and martensite are formed. A cold-rolled annealed steel sheet having a uniform structure containing 80% or more in total area ratio and excellent workability is obtained. If the holding temperature is lower than (Ac ₃ transformation point−20 ° C.), the influence of the cold-rolled structure remains and a band structure is formed, and the workability is significantly deteriorated. On the other hand, if the holding temperature exceeds (Ac ₃ transformation point + 20 ° C.), the structure becomes coarse and not only a cold-rolled annealed sheet having a uniform structure can be obtained, but also re-coalizing, so the C content in the surface layer portion increases. It becomes difficult to make the surface layer portion have a desired hardness. For this reason, the holding temperature of the cold rolled steel sheet was set to (Ac ₃ transformation point−20 ° C.) or more and (Ac ₃ transformation point + 20 ° C.) or less. Further, when the holding temperature exceeds the Ac ₃ transformation point, the structure becomes more uniform and the workability is improved. Therefore, the holding temperature is preferably set to Ac ₃ transformation point to (Ac ₃ transformation point + 20 ° C.).

The Ac ₃ transformation point temperature is confirmed by analyzing the thermal expansion curve.
In addition, segregation of substitutional elements such as Mn is reduced by holding for 10 seconds or more in the range of (Ac ₃ transformation point −20 ° C.) to (Ac ₃ transformation point + 20 ° C.), and the structure of the cold-rolled annealed plate Becomes uniform and processability is improved. For this reason, the annealing conditions of the cold-rolled steel sheet are held at (Ac ₃ transformation point −20 ° C.) to (Ac ₃ transformation point + 20 ° C.) for 10 seconds or more. However, since holding for a long time causes coarsening of the particle size, it is preferable to hold for 10 seconds to 300 seconds in the range of (Ac ₃ transformation point−20 ° C.) to (Ac ₃ transformation point + 20 ° C.).

The annealing atmosphere is not particularly defined, but for example, annealing in an oxygen-containing atmosphere or a high dew point atmosphere is preferable because it can be decarburized and annealed, and the C content in the surface layer portion is reduced.
(Cooling conditions for cold-rolled steel sheet)
The cold-rolled steel sheet is then cooled at an average cooling rate from 650 ° C. to 450 ° C. at 20 to 200 ° C./second to a cooling stop temperature range of 200 to 450 ° C. When the cooling rate is less than 20 ° C / sec, it becomes difficult to secure a tensile strength of 780 MPa or more, and when the cooling rate is slow, not only ferrite but also pearlite is likely to be generated, resulting in a non-uniform structure. As a result, workability deteriorates. However, when the cooling rate exceeds 200 ° C./second, the surface layer portion becomes hard, so the average cooling rate from 650 ° C. to 450 ° C. of the cold-rolled steel sheet is set to 20 to 200 ° C./second.

  In the present invention, the cold-rolled steel sheet is cooled to a cooling stop temperature range of 200 to 450 ° C. When the cooling stop temperature exceeds 450 ° C., it becomes difficult to secure a tensile strength of 780 MPa or more. On the other hand, when the temperature is lower than 200 ° C., not only the flatness of the steel sheet deteriorates due to heat shrinkage and transformation expansion, but also the workability deteriorates due to the hardening of the surface, so the cooling stop temperature is set to 200 ° C. to 450 ° C.

  After continuous cooling to the cooling stop temperature, hold in the temperature range of 200 to 450 ° C. for 30 seconds to 10 minutes, and then cool to room temperature. In order to decompose the austenite phase and make the structure uniform, a holding time of 30 seconds or more is required. However, holding for more than 10 minutes leads to waste of energy and reduced productivity.

Furthermore, in order to adjust the roughness of the steel sheet surface, skin pass rolling is performed after continuous annealing. In that case, the surface roughness Ra of the roll is set to 1.0 μm to 4.0 μm, and the elongation of the steel sheet is set to 0.1 to 1%.
If the surface roughness Ra of the roll is less than 1.0 μm, it becomes difficult to obtain a desired surface roughness of the steel sheet, so that the mold galling resistance deteriorates. On the other hand, if the surface roughness Ra of the roll exceeds 4.0 μm, the roll and the steel plate may be seized, making it difficult to obtain a desired surface roughness of the steel plate, and further deteriorate the workability. Therefore, the surface roughness Ra of the roll is set to 1.0 μm to 4.0 μm.

  When the elongation rate of skin pass rolling is less than 0.1%, the surface roughness of the roll is not sufficiently transferred to the steel sheet, the desired surface roughness of the steel sheet cannot be obtained, and the mold galling resistance deteriorates. On the other hand, when the elongation rate of skin pass rolling exceeds 1.0%, workability deteriorates. Therefore, the elongation rate of skin pass rolling is set to a range of 0.1 to 1.0%.

  In this way, by adjusting the steel material components, hot rolling, annealing after cold rolling, and optimization of skin pass rolling conditions, the Vickers hardness at a position where the depth from the front and back surfaces of the steel sheet is 0.05 mm is 100 to 250 Hv And (Vickers hardness at a depth of 0.2 mm from the front and back surfaces) × 0.8 or less, variation in Vickers hardness at the inner layer part on the thickness side from the position of 0.2 mm depth from the front and back surfaces is 100 Hv A high-strength thin steel sheet in which the inner layer contains bainite and martensite in a total area ratio of 80% or more, the surface roughness of the steel sheet is 0.4 to 1.2 μm in Ra, and the tensile strength of the steel sheet is 780 MPa or more. It becomes.

  Examples of the present invention are shown below.

  A steel slab having the chemical composition shown in Table 1 was heated to 1250 ° C., subjected to hot rolling under the conditions shown in Table 2, wound up, and then cooled to obtain a hot-rolled steel sheet (3.5 mm thick). . Next, the hot-rolled steel sheet was pickled and cold-rolled to a thickness of 1 mm to obtain a cold-rolled steel sheet.

Thereafter, the cold-rolled steel sheet was heated to the temperature shown in Table 2, and after annealing, after cooling under the conditions shown in Table 2, skin pass rolling was performed. While measuring the Ac ₃ transformation point temperature of the cold-rolled steel sheet having the components shown in Table 1, the obtained cold-rolled annealed plate was measured for Ac ₃ transformation point, surface roughness measurement, hardness measurement, structure observation, tensile test. A bending test was conducted. The test method is shown below.

(experimental method)
(1) Measurement of Ac ₃ transformation point temperature By collecting specimens from various cold-rolled steel sheets and analyzing the change in expansion coefficient when heated from room temperature to 1000 ° C at 10 ° C / s, the Ac ₃ transformation point temperature was determined. It was measured.

(2) Surface roughness measurement
Ra was measured based on the method specified in JIS-B0601.
(3) Hardness measurement Vickers hardness of various cold-rolled annealed plates in the rolling direction and in the direction perpendicular to the rolling direction was measured. The average hardness at a position where the depth from the steel sheet surface was 0.05 mm was defined as the hardness of the surface layer portion. The indentation load was 9.8 × 10 ⁻² N, 10-point hardness was measured at intervals of 200 μm, and the average hardness was calculated. When the indentation load is 9.8 × 10 ⁻² N or more, the indentation becomes large and the hardness of the surface layer portion cannot be measured.

The average hardness at a position where the depth from the steel sheet surface is 0.2 mm was defined as the internal hardness. In order to compare with the hardness of the surface layer portion, the indentation load was 9.8 × 10 ⁻² N, 10 point hardness was measured at intervals of 200 μm, and the average hardness was calculated.

  Further, the hardness variation at the position where the depth is 0.2 mm was regarded as the hardness variation of the inner layer portion. The indentation load was 0.49 N, 10-point hardness was measured at intervals of 200 μm, and the difference in hardness between the maximum value and the minimum value was regarded as hardness variation.

(4) Microstructure observation Specimens were sampled from the rolling direction of various cold-rolled annealed sheets and the direction perpendicular to the rolling direction, and the cross-sectional view in the rolling direction and the cross-sectional structure perpendicular to the rolling direction were photographed with an optical microscope or electron microscope for image analysis. Was used to measure the fraction of each phase.

(5) Tensile test JIS No. 5 tensile test specimens were sampled from the direction perpendicular to the rolling direction of various cold-rolled annealed plates and examined for tensile properties (tensile strength TS, elongation El).

(6) Bending test JIS No. 3 bending test specimens with the direction perpendicular to the rolling direction as the longitudinal direction were taken from various cold-rolled annealed plates, and the bendability was investigated by the V-block method in accordance with the provisions of JIS Z 2248. At that time, a pusher with an apex angle of 90 ° was pushed so that the burr was inside. The correctness after the test was visually checked, and the minimum radius of the metal fitting that was not cracked after the test was divided by the plate thickness and normalized to calculate the minimum bend radius. In addition, a press fitting having a radius of 2 mm, 1 mm, 0.5 mm, 0.2 mm, and 0 mm was used.

(7) Scoring resistance evaluation test For anti-scratching characteristics, a cylindrical punch with a diameter of 50 mm is used, and antirust oil (NOX-RUST (registered trademark) 550HN, manufactured by Parka Kosan Co., Ltd.) is applied 1 g / m ² The steel sheets coated on both sides were subjected to cylindrical drawing with a drawing ratio of 1.8, and the molded products at the time of forming 20 sheets were visually evaluated to distinguish them from those having no problem in appearance and those having defects. However, when the steel sheet strength TS is 1180MPa or more, it is difficult to draw the cylinder, so the hat forming shown in Fig. 1 is performed and the molded product when 20 sheets are formed is visually evaluated. And was evaluated separately. Those with no problem are indicated with “◯”, and those with no problem are indicated with “X”.

(8) Spot weldability evaluation test The steel plates of the same steel type were stacked and spot welded to produce spot weld specimens. Spot welding was performed under the conditions of electrode: double R type, applied pressure: 3900 N, energization: 16 cycles (current 7.5 kA), and holding: 3 cycles. The spot weldability is indicated by ◯ when there is no breakage in the nugget (the portion melted at the time of spot welding and then solidified) when the spot weld specimen is peeled with the chisel, and when there is, it is indicated by ×.

  These results are shown in Table 3. The steel sheet of the present invention has a Vickers hardness of 100 to 250 Hv at a position where the depth from the front and back surfaces is 0.05 mm and (Vickers hardness at a position where the depth from the front and back surfaces is 0.2 mm) x 0.8 or less, The Vickers hardness variation in the inner layer part from the position where the depth from the front and back surfaces is 0.2mm to the center side of the plate thickness is 100Hv or less, and the inner layer part contains bainite and martensite in a total area ratio of 80% or more. In the range of 0.4 to 1.2 μm in surface roughness Ra, the steel sheet has a tensile strength of 780 MPa or more and a minimum bending radius of 0.2 t or less. .

  On the other hand, steel plates No. 1 and No. 2 of the comparative examples have chemical components that are out of the scope of the present invention, and cannot ensure a predetermined strength. Steel plate No. 16 has a chemical component that is out of the scope of the present invention, and has a non-uniform structure, and therefore has poor bendability. Steel plate No. 27 has a chemical component outside the scope of the present invention, and the toughness deteriorates because the nugget portion hardens when welded. The steel plate 33 has a chemical component that is out of the scope of the present invention and is not only poor in bendability but also deteriorates toughness because the nugget portion is hardened when welded.

  Steel plate No. 3 has manufacturing conditions that are out of the scope of the present invention, and not only cannot ensure a predetermined strength, but also has poor bendability due to its non-uniform structure. In No. 25, the manufacturing conditions are out of the scope of the present invention, and the predetermined strength cannot be secured. Steel plates No. 5 and No. 7 have manufacturing conditions that are out of the scope of the present invention, and the surface is hard and the bendability is not only poor, but also the surface roughness of the steel plate is small and the anti-galling property is also poor. Steel plate No. 8 has a manufacturing condition deviating from the scope of the present invention, and has a non-uniform structure, and therefore has poor bendability. Steel plate No. 9 has manufacturing conditions that are out of the scope of the present invention, and the rolling rate of skin pass rolling is large, so the surface ductility deteriorates and the bendability is poor. Steel plate No. 10 has manufacturing conditions that are out of the scope of the present invention and has poor bendability. Steel plates No. 17 and No. 21 have poor bendability because the manufacturing conditions are out of the scope of the present invention and the structure is uneven. Steel plate No. 29 has manufacturing conditions that are out of the scope of the present invention, and since the surface is hard, not only the bendability is bad, but also the surface roughness of the steel plate is small and the die-squeeze characteristics are poor. Steel plate No. 30 has poor bendability because the manufacturing conditions deviate from the scope of the present invention, and the stress and strain of the surface layer portion are not relaxed when heat-treated. Steel plate Nos. 13 and 14 have manufacturing conditions that are out of the scope of the present invention, the surface roughness of the steel plate is small, and the galling property is poor.

It is explanatory drawing which shows a mold galling test condition.

Claims (4)

  In mass%, C: 0.06-0.25%, Si: 0.005-1.0%, Mn: 1.0-2.7%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.01-0.08% and N: 0.0003- A steel sheet containing 0.01% and having a chemical composition consisting of the remaining Fe and impurities, the Vickers hardness at a position where the depth from the front and back surfaces of the steel sheet is 0.05 mm is 100 to 250 Hv, and (from the front and back surfaces) Vickers hardness at a position where the depth is 0.2 mm) × 0.8 or less, variation in Vickers hardness in the inner layer portion on the thickness side from the position where the depth from the front and back surfaces is 0.2 mm is 100 Hv or less, The inner layer contains bainite and martensite in a total area ratio of 80% or more, the surface roughness of the steel sheet is 0.4 to 1.2 μm in Ra, and the tensile strength of the steel sheet is 780 MPa or more. Strength thin steel sheet.   The chemical composition further contains one or more of Cr: 0.1 to 0.5%, Mo: 0.1 to 0.5%, and B: 0.0005 to 0.003% by mass%. The high-strength thin steel sheet described in 1.   The chemical composition further contains one or more of Nb: 0.01 to 0.1%, V: 0.01 to 0.1%, and Ti: 0.01 to 0.1% by mass%. Or the high-strength thin steel sheet according to 2; Steel having the chemical composition according to any one of claims 1 to 3 is subjected to hot rolling, descaling, and cold rolling, and then the obtained steel sheet is subjected to 700 ° C to (Ac ₍₃ transformation point-20 ° C) is heated for 20 seconds or more and held in the range of (Ac ₃ transformation point-20 ° C) to (Ac ₃ transformation point + 20 ° C) for 10 seconds or more. Set the average cooling rate to 450 ° C to 20 to 200 ° C / sec, cool to 200 to 450 ° C, hold in the temperature range of 200 ° C to 450 ° C for 30 seconds to 10 minutes, then cool to room temperature, continuous annealing A method for producing a high-strength thin steel sheet, characterized by performing skin pass rolling with a work roll having a surface roughness Ra of 1.0 to 4.0 μm and an elongation of 0.1 to 1%.

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