JP4613618B2 - High-strength cold-rolled steel sheet excellent in deep drawability and its manufacturing method - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet used for automobiles, home appliances, etc., particularly a high-strength cold-rolled steel sheet having a tensile strength TS of 340 to 590 MPa and excellent in deep drawability and a method for producing the same. .

  Conventionally, for automotive parts that are difficult to form such as side outer panels and door inner panels, IF soft cold-rolled steel sheet (SPC270E, which has excellent deep drawing formability with TS of about 270 MPa and r value of 1.8 to 2.0. F) has been used. On the other hand, in recent years, due to further increasing needs for weight reduction of automobile bodies, application of high-strength cold-rolled steel sheets of IF having TS of 340 to 590 MPa is being advanced to these difficult-to-form parts. However, these parts are currently mass-produced TS of 340 to 390 MPa, high strength cold-rolled steel sheet with an r value of about 1.7, TS is around 440 MPa, high strength cold-rolled steel sheet with an r value of about 1.5, And TS is around 590 MPa, and when trying to form using high strength cold-rolled steel sheet with r value of about 1.0, cracking is likely to occur at the drawing part. It is the fact that it is applied only to. Therefore, a high-strength cold-rolled steel sheet having a TS of 340 to 590 MPa and a higher r value is required. Specifically, an r value of TS of 340 to 400 MPa and a value of 1.8 or more is desired, and an r value of TS of 400 to 590 MPa and a value of 1.55 or more, preferably 1.7 or more is desired.

  Up to now, as a method of increasing the r value, C steel and N steel are reduced as much as possible, and IF steel with a large amount of Ti and Nb added is used. A method is known in which N is reduced as much as possible and the precipitates are coarsened to promote the formation and growth of recrystallized grains having a texture favorable to the r value during annealing. As a similar method, in Patent Document 1 and Patent Document 2, C and N are reduced as much as possible, and Ti steel is used to generate Ti (C, S), which is advantageous for r value during annealing. A method of developing is disclosed.

  However, the method disclosed in Patent Document 1 is intended for soft cold-rolled steel sheets having a TS of 260 to 300 MPa, and the high strength of IF having a TS of 340 MPa or more to which a large amount of existing P or Mn is added. When applied to cold-rolled steel sheets, a large amount of P-compounds such as Fe-Ti-P and Fe-Nb-P are formed at the grain boundaries during winding after hot rolling, and because of the large amount of Mn itself, the r value Is significantly reduced.

  In addition, in the method disclosed in Patent Document 2, a high-strength cold-rolled steel sheet for deep drawing having a TS of 340 MPa or more to which P is added in a large amount has been proposed, but due to casting segregation of P during press forming. In some cases, cracks may occur due to the uneven structure in the thickness direction.

  On the other hand, a method for improving the r value by devising a manufacturing method has also been proposed. For example, Patent Document 3 discloses a method of performing finish rolling while lubricating in the α region during hot rolling.

  Patent Document 4 discloses a method of applying 1 to 50% rolling in a temperature range of 550 to 750 ° C. during annealing.

  In Patent Document 5, the amount of Si, Mn, and P of Nb and B-added steels is controlled, 0.3 to 5% rolling is added after pickling, cold rolling and annealing, and pickling is performed again and the galvanizing line is passed. A method is disclosed.

However, these methods all require a special manufacturing process, resulting in an increase in manufacturing cost and a decrease in productivity. That is, the method of Patent Document 3 requires recrystallization annealing of a hot-rolled steel sheet finish-rolled in the α region. The method of Patent Document 4 requires a rolling facility that can withstand high temperatures in an annealing furnace. In the method of Patent Document 5, pickling, annealing, and temper rolling need to be performed twice.
JP-A-6-108155 Japanese Patent No. 3291639 JP 7-188776 A JP-A-9-279249 Japanese Unexamined Patent Publication No. 2001-131643

  The object of the present invention is that TS is 340 to 400 MPa, r value is 1.8 or more, TS is 400 to 590 MPa, r value is 1.55 or more, preferably 1.7 or more, without requiring a special manufacturing process. It is providing the high-strength cold-rolled steel plate excellent in the deep drawing formability, and its manufacturing method.

  The present inventors investigated the influence of various alloy elements on the r value of IF high-strength cold-rolled steel sheet, and obtained the following knowledge.

  i) Conventionally, it is considered that the solid solution strengthening ability is small, and Al that has been hardly studied for IF steel has been actively included as a solid solution strengthening element, that is, the amount of sol. When added in a larger amount than in the case of a high-strength cold-rolled steel sheet, the r value is significantly improved. In particular, this effect is remarkable when 0.4% or more of Mn is added.

  ii) The addition of Si and P is effective for improving the r value.

  iii) A high r value can be obtained by optimizing the amount of P, sol. Al, Ti and, if necessary, the amount of Nb and the coiling temperature after hot rolling.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] By mass%, C: 0.015% or less, Si: 1.5% or less, Mn: 0.4 to 3%, P: 0.15% or less, S: 0.02% or less, sol.Al: 0.1 to 1%, N: 0.01 A high-strength cold-rolled steel sheet excellent in deep drawability, characterized by comprising% or less, Ti: 0.2% or less, the balance being Fe and inevitable impurities, and satisfying the following formula (1) .
1 ≦ ([Ti] / 48) / ([C] / 12 + [N] / 14)… (1)
Here, [M] represents the content (mass%) of the element M.

[2] In the above [1], high strength cold rolling excellent in deep drawability characterized by further containing Nb: 0.02% or less by mass% and satisfying the following formula (2): Steel sheet .
1 ≦ ([Nb] / 93 + [Ti] / 48) / ([C] / 12 + [N] / 14)… (2)
Here, [M] represents the content (mass%) of the element M.

[3] Oite to [1], further containing, by mass%, B: High-strength cold-rolled steel sheet excellent in deep drawability, characterized by containing 0.003% or less.
[4] By mass%, C: 0.015% or less, Si: 1.5% or less, Mn: 0.4 to 3%, P: 0.15% or less, S: 0.02% or less, sol.Al: 0.1 to 1%, N: 0.01 % Or less, Ti: 0.050 to 0.2%, Nb: 0.02% or less, B: 0.003% or less, comprising the balance Fe and inevitable impurities, and satisfying the following formula (2) High strength cold-rolled steel sheet with excellent drawability.
1 ≦ ([Nb] / 93 + [Ti] / 48) / ([C] / 12 + [N] / 14)… (2)
Here, [M] represents the content (mass%) of the element M.

[ 5 ] In any one of the above [1] to [ 4 ], the high strength cold-rolled steel sheet excellent in deep drawability, wherein the mass% is sol.Al: 0.2 to 0.7%.

[ 6 ] A high-strength cold-rolled steel sheet excellent in deep drawability , wherein Si and P satisfy the following formula (3) in any one of [1] to [ 5 ] .
0.3 ≦ [Si] + 10 × [P] ≦ 1.4 (3)
Here, [M] represents the content (mass%) of the element M.

[7] In any one of the above [1] to [6], further, by mass, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, and Mo: 0 high-strength cold-rolled steel sheet excellent in deep drawability, characterized in that it contains at least one kind selected from among lower .3% or.

[8] In any one of the above [1] to [7], the steel sheet further contains Sb and satisfies the following formula (4). .
0.002 ≦ [Sb ] ≦ 0.2 (4)
Here, [ Sb ] represents the content (% by mass) of the element Sb .

[ 9 ] Using the steel having the composition according to any one of [1] to [ 8 ] above, a step of heating to a slab heating temperature of 1080 to 1350 ° C., and heating the steel slab to a finishing temperature (Ar3 (Transformation point -20) to (Ar3 transformation point +150) The step of hot rolling at ℃ to make a hot-rolled steel sheet, and the hot-rolled steel sheet, the following formula (5) when Nb is not added , Nb Is added at a winding temperature CT satisfying the following formula (6), and the hot-rolled steel sheet after the winding is cold-rolled at a reduction rate of 50 to 90% to cold-rolled steel sheet And a step of continuously annealing the cold-rolled steel sheet at 750 to 870 ° C. or box annealing at 600 to 750 ° C. Manufacturing method .
480 ≦ CT ≦ 580 + 0.17 / {([Ti] + 0.08 × [sol.Al]) × [P]}… (5)
480 ≦ CT ≦ 580 + 0.17 / {(0.6 × [Nb] + [Ti] + 0.08 × [sol.Al]) × [P]}… (6)
Here, [M] represents the content (mass%) of the element M. In the present specification, “%” indicating the component of steel is “% by mass”.

  In the present invention, the high-tensile cold-rolled steel sheet is a cold-rolled steel sheet having a tensile strength (TS) of 340 to 590 MPa.

  According to the present invention, a high-strength cold-rolled steel sheet excellent in deep drawability can be obtained, which contributes to weight reduction of press parts, improvement in rigidity, reduction in the number of parts, etc. by application of high-strength steel pipes.

  Details of the present invention and reasons for limitation will be described below.

  First, the influence of Si, Mn, P, Al, Cu, and Ni on the r value of IF high-strength steel sheet was investigated. As a result, Si and P have the effect of slightly increasing the r value by increasing the addition amount, and Al has the effect of further increasing the r value significantly, especially its effect. Was found to be particularly prominent in steels with Mn above 0.4%. Cu and Ni tended to slightly increase the r value, and Mn was found to degrade the r value. These causes are not necessarily clear, but for Mn, the affinity with C is strong, so the effect of fixing C in the grains to lower the r value and the Ar3 transformation point during hot rolling Thus, it is presumed that the r value is lowered by the effect of refining the precipitate and delaying the precipitation (increasing the solid solution C). On the other hand, since Al is an element that increases the transformation point, contrary to Mn, precipitation by hot rolling is promoted and the r value is considered to be improved. However, since the effect of improving the r value by adding Al is large, the effect of changing the solid solution state of the solid solution C in the grains and the effect of changing the rolling structure also contribute to the improvement of the r value. It is inferred that

  Therefore, in order to investigate in more detail the range of material improvement by adding Al, the relationship between the amount of sol.Al and the r value was investigated.

1) sol.Al amount and r value
In order to investigate the relationship between the amount of sol.Al and the r value, the following test was conducted.

  C: 0.002%, Si: 0.25%, P: 0.08%, S: 0.007%, Nb: 0.015%, Ti: 0.03%, N: 0.002%, B: 0.001%, sol.Al content 0.01 ~ A steel slab with 1.2% and Mn content changed to 0.6-1.8% was heated to 1250 ° C and then hot rolled to form a hot-rolled steel plate with a thickness of 3 mm, which was then wound at 580 ° C for 1 hr. did. This hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.75 mm, subjected to continuous annealing at 820 ° C. for 60 sec, and temper rolled with an elongation of 0.7%. And the r value and TS were measured with the following method with respect to the obtained steel plate.

  JIS No. 5 test piece is taken from the rolling direction, 45 ° direction with respect to the rolling direction and 90 ° direction with respect to the rolling direction, and the r value and TS for each direction are measured, and within the steel plate surface expressed by the following formula The average value of was obtained.

Average = ([T ₀ ] + 2 [T ₄₅ ] + [T ₉₀ ]) / 4
Here, [T ₀ ] is the r value or TS in the rolling direction, [T ₄₅ ] is in the 45 ° direction with respect to the rolling direction, and [T ₉₀ ] is in the 90 ° direction with respect to the rolling direction.

  FIG. 1 shows the relationship between the amount of sol.Al, the r value, and TS. In FIG. 1, black circles are the results when the Mn content is 1.8%, and white circles are the results when the total amount of sol.Al and Mn is 1.8%.

  From FIG. 1, when the Mn amount is 1.8%, the r value is 1.55 or more when the sol.Al is 0.1% or more, 1.7 or more when 0.2 to 0.7%, and decreases when it exceeds 0.7%. TS exceeds 460 MPa at sol.Al of 0.1% or more, and increases monotonically with the amount of sol.Al. At this time, the increase in TS per 1% of sol.Al content is 35 MPa. Since this is almost equivalent to the solid solution strengthening ability of Mn, if the total amount of sol.Al and Mn is 1.8%, the relationship between TS and r value when the strength like white circle is kept constant can be obtained. From this, it can be seen that a higher r value can be obtained with the same strength by adding sol.Al and reducing Mn.

  If the sol.Al exceeds 1%, fine AlN precipitates at the austenite grain boundaries during continuous casting of the slab, embrittles the grain boundaries, and cracks on the slab surface during straightening of the slab and subsequent rough rolling. Is likely to occur. Such cracks on the surface of the slab are likely to cause scale-related surface defects, and the surface quality of the final product is significantly deteriorated.

  From the above results, even if TS exceeds 400 MPa, a high r value of 1.55 or more, preferably 1.7 or more can be obtained if the amount of sol.Al is 0.1 to 1%, preferably 0.2 to 0.7%. Become.

Here, the reason why a high r value can be obtained when the amount of sol.Al is 0.1 to 1% is considered as follows. That is, since Al is an element that raises the Ar ₃ transformation point, after transformation from austenite to ferrite during hot rolling, precipitation of carbides is promoted in the high-temperature α region, and solid solution C decreases and carbides become coarse. Therefore, a recrystallized texture preferable for the r value is formed during annealing, and the r value is improved. In addition, it is speculated that changes in the cold-rolled structure due to Al contribute to the improvement of the r value.

2) Si, P amount and r value Next, based on the above results, the effect of Si and P composite addition on Al-added steel was investigated. In order to investigate the relationship between Si, P amount and r value, C: 0.002%, Mn: 1%, S: 0.007%, sol.Al: 0.25%, Nb: 0.02%, Ti: 0.01%, N: A test similar to the case of 1) was performed using a steel slab with 0.002%, B: 0.001%, Si amount changed to 0.005 to 1.5%, and P amount changed to 0.003 to 0.15%.

  FIG. 2 shows the relationship between [Si] + 10 × [P] and the r value. In addition, the number in a figure represents Si amount.

  From FIG. 2, it is possible to obtain a higher r value by adding Si and P in steel added with Al, and the effect of improving the r value is as high as 1.7 or more when the following equation (2) is satisfied. It can be seen that the r value is obtained.

0.3 ≦ [Si] + 10 × [P] ≦ 1.4 (2)
Here, [M] represents the content (mass%) of the element M.

  However, when the amount of Si and 10 × P exceeds 1.5%, the r value deteriorates greatly, so the amounts of Si and P are 1.5% or less and 0.15% or less, respectively.

  In addition, when alloying hot-dip galvanizing is applied to the high-strength cold-rolled steel sheet of the present invention, these elements are liable to cause poor adhesion of plating, so the Si content is 0.5% or less and the P content is 0.08% or less. It is preferable. Further, since Si and P are effective elements for strengthening the solid solution of ferrite, the Si content is preferably 0.003% or more and the P content is preferably 0.01% or more.

3) Other ingredients
C: C combines with Ti and Nb to form carbides. If the amount exceeds 0.015%, the amount of this carbide increases and the r-value decreases remarkably, so the C amount is 0.015% or less, preferably 0.008% or less, more preferably less than 0.004%. However, since C has the effect of increasing the strength by precipitation strengthening as TiC and NbC, for example, it is effective to contain 0.004% or more in a steel having a TS of around 440 MPa. That is, if the C amount is 0.004 to 0.008% and Ti or Nb is added in an atomic ratio of 1.0 or more with respect to C, the strength can be increased while suppressing the decrease in the r value. If the C content is less than 0.0005%, the ferrite grains become coarse during annealing, and surface roughness is likely to occur during press molding. Therefore, the C content is preferably 0.0005% or more.

  Mn: Mn is an element that increases the strength by solid solution strengthening, and is an indispensable element for IF high-strength cold-rolled steel sheet. In order to obtain TS of 340 MPa or more, the Mn content needs to be 0.4% or more. On the other hand, when the amount exceeds 3%, the r value is remarkably lowered, so the Mn amount is 3% or less, preferably 2% or less, more preferably 1.5% or less.

The reason why the r value decreases as the amount of Mn increases is not necessarily clear, but it is thought that Mn interacts with solute C to decrease the r value. In addition, Mn lowers the Ar ₃ transformation point, so it refines the carbides that precipitate during hot rolling, and delays the precipitation of carbides to increase the solute C. It is presumed that the structure does not form and the r value decreases. From the above, the Mn content is 0.4% or more and 3% or less.

  S: S is present in steel as sulfide. If the amount exceeds 0.02%, ductility is deteriorated, so the S amount is 0.02% or less, preferably 0.01% or less. From the viewpoint of descaling properties, the S amount is preferably 0.004% or more.

  N: When the N content exceeds 0.01%, fine AlN, NbN, Nb (C, N) precipitates at the austenite grain boundaries during continuous casting of the slab, embrittles the grain boundaries, and during the slab casting or subsequent roughing. Cracks are likely to occur on the slab surface during rolling. Therefore, the N content is 0.01% or less. The smaller the N content, the better. However, the current steelmaking technique has a limit of about 0.001%.

  Ti: Ti has the effect of improving the r-value by reducing the crystal grains after hot rolling or by forming precipitates with C and N to reduce the solid solution C and N. In order to fully exhibit the effect of Ti, it is necessary to add Ti so as to satisfy the following formula (1).

1 ≦ ([Ti] / 48) / ([C] / 12 + [N] / 14)… (1)
Here, [M] represents the content (mass%) of the element M.

  However, even if the Ti content exceeds 0.2%, the increase in r value is small, so the Ti content is 0.2% or less. In addition, when alloying hot-dip galvanizing is performed on the high-strength cold-rolled steel sheet of the present invention, the Ti content is preferably 0.04% or less from the viewpoint of preventing uneven plating. In order to reliably obtain a high r value by adding Ti, the Ti content is preferably 0.005% or more.

  The balance is Fe and inevitable impurities.

  In addition to the above components, it is preferable to add 0.002% or more of Nb in order to obtain a higher r value. In this case, it is necessary to adjust the amounts of Nb, Ti, C, and N so as to satisfy the following formula (3).

1 ≦ ([Nb] / 93 + [Ti] / 48) / ([C] / 12 + [N] / 14)… (3)
Here, [M] represents the content (mass%) of the element M.

  However, if the Nb content exceeds 0.02%, fine NbN and Nb (C, N) precipitate at the austenite grain boundaries during slab continuous casting, embrittle the grain boundaries, and during slab casting and subsequent rough rolling. Cracks are likely to occur on the slab surface. Therefore, the Nb content is 0.02% or less.

  Furthermore, B can be added to improve secondary work embrittlement resistance. In this case, addition of 0.0001% or more is preferable. However, if the B content exceeds 0.003%, the effect of improving the secondary work brittleness resistance is small, and conversely, the r value decreases and the rolling load increases. Therefore, when added, the B content is 0.003% or less.

In addition, at least one element selected from Cu, Ni, Cr, and Mo can be added to further increase the strength, improve the secondary work brittleness resistance, and improve the r value. In this case, Cu: 0.03% or more, Ni: 0.03% or more, Cr: 0.03% or more, Mo: preferably added on 0.05% or more.
However, if the amounts of Cu and Cr exceed 0.5%, the surface quality is deteriorated. Therefore, when added, the amounts of Cu and Cr are both set to 0.5% or less. If the Ni content exceeds 0.5%, the cost will increase significantly. Therefore, when Ni is added, the Ni content is 0.5% or less. Mo has little adverse effect on secondary work brittleness resistance and is effective for high strength. However, if the Mo amount exceeds 0.3%, the yield point is increased and the surface accuracy of the pressed parts is deteriorated. In this case, the Mo amount is 0.3% or less . Na us, when adding Cu is desirably be contained Cu in an amount equivalent to Ni.

Furthermore, it is effective to contain Sb and satisfy the following formula (4) in order to improve the appearance of galvanization, galvanization adhesion, fatigue characteristics, toughness of the drawn portion during press molding, and the like. . In the case of containing, Sb: preferably 0.002% or less on.

0.002 ≦ [Sb ] ≦ 0.2 (4)
Here, [ Sb ] represents the content (% by mass) of the element Sb .

The addition of Sb prevents surface nitriding and oxidation during annealing by slab heating, winding, box annealing furnace (BAF), continuous annealing line (CAL), hot dip galvanizing line (CGL), etc. Deterioration of plating adhesion is improved. Moreover, adhesion of zinc oxide in the zinc bath is prevented, and the plating appearance is improved. Furthermore, Sb reduces surface oxidation and suppresses deterioration of fatigue characteristics and toughness after drawing.

However, when the amount of Sb Waso exceeds 0.2%, the degradation of zinc coating adhesion and toughness, when it contains, at most 0.2%.

4) high-strength cold-rolled steel sheet manufacturing method of the present invention, using a steel having the above components, and heating the slab heating temperature 1080~ 1350 ℃, finishing the steel slab after heating temperature (Ar ₃ transformation point - 20) to (Ar ₃ transformation point +150) Hot rolling at a temperature of ℃ to form a hot-rolled steel sheet.If Nb is not added, the following formula (5) is added, and Nb is added. When winding, a step of winding at the winding temperature CT satisfying the following formula (6), a step of cold rolling the hot-rolled steel sheet after winding at a reduction rate of 50 to 90% to make a cold-rolled steel sheet, It is manufactured by a manufacturing method including a step of continuously annealing a cold-rolled steel sheet at 750 to 870 ° C or box annealing at 600 to 750 ° C.

480 ≦ CT ≦ 580 + 0.17 / {([Ti] + 0.08 × [sol.Al]) × [P]}… (5)
480 ≦ CT ≦ 580 + 0.17 / {(0.6 × [Nb] + [Ti] + 0.08 × [sol.Al]) × [P]}… (6)
Here, [M] represents the content (mass%) of the element M.

  The steel slab needs to have a slab heating temperature SRT of 1080 ° C. or higher before hot rolling in order to sufficiently dissolve the Fe-Ti—P and Fe—Nb—P compounds formed in the slab. However, if the temperature exceeds 1350 ° C, the surface quality deteriorates, so it is necessary to keep it below 1350 ° C.

  In order to obtain an excellent appearance, it is desirable to sufficiently remove not only the primary scale but also the secondary scale generated during hot rolling. During hot rolling, heating can be performed with a bar heater.

The finishing temperature FDT for hot rolling needs to be (Ar ₃ transformation point −20) to (Ar ₃ transformation point +150) ° C. in order to refine the structure after hot rolling.

  The coiling temperature after hot rolling has a great influence on the r value of the cold-rolled steel sheet of the present invention in which Al, P, Ti and, if necessary, Nb are added in combination. This is because, in the IF steel with P added, Fe-Ti-P and Fe-Nb-P conversion compounds, which are not preferable for the r value, are easily generated. In general, the r value is markedly improved when the coiling temperature is raised to make the precipitate coarse and the solid solution C is reduced. However, when the coiling temperature is increased beyond the appropriate temperature, the above-mentioned P compound is generated and the r value is remarkably lowered.

  Therefore, as a result of investigating the optimum coiling temperature for various types of Al, P, Ti, and optionally Nb added steel, the coiling temperature CT was 580 + 0.17 / {([ Ti] + 0.08 × [sol.Al]) × [P]}, when Nb is added, 580 + 0.17 / {(0.6 × [Nb] + [Ti] + 0.08 × [sol.Al]) × It has been found that when [P]} is exceeded, a P compound is formed and the r value is remarkably lowered. On the other hand, when the coiling temperature CT is lower than 480 ° C., the carbides are not sufficiently precipitated during the coiling even if the P compound is not formed, and the r value deteriorates. Therefore, the coiling temperature CT needs to satisfy the above formula (5) or (6).

  In addition, it is preferable to wind in the temperature range of (upper limit value −40) to (upper limit value) ° C. of the above formulas (5) and (6).

  In cold rolling, from the viewpoint of improving the r value, the rolling reduction needs to be 50 to 90%, preferably 65 to 80%.

The annealing temperature AT needs to be 750 to 870 ° C. when continuously annealing with Cal or CGL. At a temperature lower than 750 ° C., recrystallization becomes insufficient and a high r value cannot be obtained stably. Moreover, characteristics such as elongation are significantly deteriorated. When the temperature exceeds 870 ° C., a steel plate with a large amount of Mn will be annealed beyond the Ar ₃ transformation point, the strength will increase extremely, and the n value will deteriorate significantly. In order to stably obtain a higher r value and high elongation, it is preferable to anneal at a temperature of 820 ° C. or higher. Further, when box annealing is performed with BAF, the annealing temperature AT needs to be 600 to 750 ° C. because the annealing time is long.

  The cold-rolled steel sheet after annealing can be plated with zinc by electroplating or hot dipping as necessary. Examples of the plating containing zinc include zinc plating, alloyed zinc plating, and zinc-nickel alloy plating. It is also possible to apply an organic film treatment after plating.

Steel Nos. A to X shown in Table 1 were melted and then continuously cast into a 230 mm thick slab. The slab was reheated at the heating temperature SRT shown in Table 2, then hot-rolled to a thickness of 3.2 mm at the finishing temperature FDT shown in Table 2, and wound at the winding temperature CT shown in Table 2. This hot-rolled sheet was cold-rolled to a thickness of 0.8 mm, then annealed with CAL, CGL, and BAF at the annealing temperature AT shown in Table 2, temper rolled with an elongation of 0.8%, and steel sheets No. 1-34 Was made. In CGL, the annealed steel sheet was immersed in a hot dip galvanizing bath at 460 ° C, and then alloyed at 500 ° C in an in-line alloying furnace. The plating basis weight was 45 g / m ² per side.

  The r value and TS were measured for the steel sheet obtained above by the same method as in FIG. In addition, surface quality was investigated by visually inspecting surface defects.

  The obtained results are also shown in Table 2.

  In both Tables 1 and 2, [Nb] in the uppermost formula is 0 when Nb is not added.

In the steel sheet Nos. 1 to 11 and 17 to 24, which are examples of the present invention, the TS is 340 to 400 MPa, the r value is 1.8 or more, the TS is 400 to 590 MPa, and the r value is 1.55 or more. The surface quality is also good. Further, it can be seen that the r value of the present invention example is remarkably higher than that of the comparative example having the same strength. In particular, when the amount of Mn exceeds 1%, the effect is noticeable.

  On the other hand, in the steel plates No. 25 to 34 of the comparative examples, TS is 340 to 400 MPa, r value of 1.8 or more, and TS is 400 to 590 MPa, r value of 1.55 or more cannot be obtained. Steel plates No. 27, 28 and 29 corresponding to conventional high-strength cold-rolled steel plates with a high Mn content have low r values. In Steel Plate Nos. 30, 31, 32, 33, and 34, the (Nb + Ti) / (C + N) ratio, C, Si, Mn, P, sol. Al, and Nb are each outside the range of the present invention, and the r value is low. Among them, in the case of steel plate No. 30 corresponding to the conventional low carbon high strength cold-rolled steel plate in which C amount and (Nb + Ti) / (C + N) ratio are not optimized and solute C and Mn coexist, sol. Even if Al is increased, a high r value cannot be obtained. Further, the steel plates No. 31 and 34 in which Nb, Nb and sol.Al are outside the scope of the present invention have poor surface quality.

  Compared with steel plate No. 31 corresponding to the conventional soft cold-rolled steel plate SPC270F and steel plate No. 32 to which a large amount of sol. It can be seen that the effect of improving the r value is small.

  It is also suitable for press molding in automobiles or home appliances that require excellent material properties.

It is a figure which shows the relationship between the amount of sol.Al, r value, and TS. It is a figure which shows the relationship between [Si] + 10x [P] and r value.

Claims (9)

In mass%, C: 0.015% or less, Si: 1.5% or less, Mn: 0.4 to 3%, P: 0.15% or less, S: 0.02% or less, sol. Al: 0.1 to 1%, N: 0.01% or less, Ti: 0.2% or less, the balance being Fe and inevitable impurities, and satisfying the following formula (1) A high-strength cold-rolled steel sheet with excellent deep drawability.
1 ≦ ([Ti] / 48) / ([C] / 12 + [N] / 14) (1)
Here, [M] represents the content (mass%) of the element M. The high strength cold-rolled steel sheet having excellent deep drawability according to claim 1, further comprising Nb: 0.02% or less in mass% and satisfying the following formula (2): .
1 ≦ ([Nb] / 93 + [Ti] / 48) / ([C] / 12 + [N] / 14) (2)
Here, [M] represents the content (mass%) of the element M. The high-strength cold-rolled steel sheet having excellent deep-drawing formability according to claim 1, further comprising, by mass%, B: 0.003% or less. In mass%, C: 0.015% or less, Si: 1.5% or less, Mn: 0.4 to 3%, P: 0.15% or less, S: 0.02% or less, sol. Al: 0.1 to 1%, N: 0.01% or less, Ti: 0.050 to 0.2%, Nb: 0.02% or less, B: 0.003% or less, the remainder Fe and inevitable A high-strength cold-rolled steel sheet excellent in deep drawability, characterized by comprising general impurities and satisfying the following formula (2).
1 ≦ ([Nb] / 93 + [Ti] / 48) / ([C] / 12 + [N] / 14) (2)
Here, [M] represents the content (mass%) of the element M. Furthermore, by mass%, sol. The high-strength cold-rolled steel sheet having excellent deep drawability according to any one of claims 1 to 4, wherein Al: 0.2 to 0.7%. Si and P satisfy | fill following formula (3), The high-strength cold-rolled steel plate excellent in the deep drawability in any one of Claims 1-5 characterized by the above-mentioned.
0.3 ≦ [Si] + 10 × [P] ≦ 1.4 (3)
Here, [M] represents the content (mass%) of the element M. Moreover, in mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, and Mo: at least one kind selected from among lower 0.3% or less A high-strength cold-rolled steel sheet excellent in deep-drawing formability according to any one of claims 1 to 6. Furthermore , Sb is contained and the following formula (4) is satisfied, The high-strength cold-rolled steel sheet excellent in deep drawability in any one of Claims 1-7 characterized by the above-mentioned.
0.002 ≦ [Sb ] ≦ 0.2 (4)
Here, [ Sb ] represents the content (% by mass) of the element Sb . Using the steel having the composition according to any one of claims 1 to 8, a step of heating to a slab heating temperature of 1080 to 1350 ° C, and a steel slab after the heating to a finishing temperature (Ar3 transformation point -20) to (Ar3 transformation point +150) Hot rolling at a temperature of 150 ° C. to form a hot-rolled steel sheet, the hot-rolled steel sheet, the following formula (5) when Nb is not added, and the following when Nb is added: A step of winding at a winding temperature CT satisfying the following formula (6), a step of cold rolling the hot-rolled steel sheet after the winding at a reduction rate of 50 to 90% to form a cold-rolled steel sheet, A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, characterized by comprising a step of continuously annealing a rolled steel sheet at 750 to 870 ° C or box annealing at 600 to 750 ° C.
480 ≦ CT ≦ 580 + 0.17 / {([Ti] + 0.08 × [sol.Al]) × [P]} (5)
480 ≦ CT ≦ 580 + 0.17 / {(0.6 × [Nb] + [Ti] + 0.08 × [sol.Al]) × [P]} (6)
Here, [M] represents the content (mass%) of the element M.

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