JP5098583B2 - High formability and high strength cold-rolled steel sheet excellent in chemical conversion treatment and production method thereof - Google Patents

Description

  The present invention relates to a high workability, high strength cold-rolled steel sheet excellent in chemical conversion treatment and suitable for automobile frame members, reinforcing members, and the like, and a method for producing the same.

In recent years, various technologies have been studied from the viewpoint of preserving the global environment (that is, prevention of air pollution, prevention of global warming, etc.). For example, automobiles are required to improve fuel efficiency, and as part of this, weight reduction of the vehicle body is an important issue. On the other hand, it is necessary to improve the strength of the vehicle body from the viewpoint of protecting an occupant when a collision occurs.
Therefore, in order to obtain a vehicle body that achieves both weight reduction for improving fuel efficiency and high strength for improving safety, cold-rolled steel sheets with a tensile strength of 780 MPa or more (so-called high-tensile cold-rolled steel sheets) are used. It is used for a skeleton member and a reinforcing member of a vehicle body. If a high-tensile cold-rolled steel sheet is used, since it has high strength, it is possible to improve safety by suppressing deformation and breakage of the vehicle body, and to reduce the thickness of the members constituting the vehicle body and achieve weight reduction.

On the other hand, since the skeleton member and the reinforcing member of the body of an automobile are formed by press working, the cold-rolled steel plate of the material is required to have excellent workability. In general, the addition of alloying elements, refinement of crystal grains, generation of martensite phase, and the like are effective for increasing the strength of cold-rolled steel sheets. However, an increase in strength leads to deterioration of workability.
However, among alloying elements, Si is an element having an effect of increasing the strength while suppressing the deterioration of workability. Therefore, if Si is added to the cold-rolled steel sheet, both an increase in strength and an improvement in workability can be achieved. However, since Si is an easily oxidizable element, when the Si content increases, Si is preferentially oxidized by annealing in a reducing atmosphere performed in the normal cold-rolled steel plate manufacturing process. Concentrate in the vicinity of the surface layer. As a result, the oxide mainly composed of Si covers the cold-rolled steel sheet, thereby inhibiting the formation of chemical crystals in the chemical conversion treatment performed after annealing.

Chemical conversion property is an important characteristic required for using cold-rolled steel sheets with improved workability and strength (hereinafter referred to as high-workability high-strength cold-rolled steel sheets) as skeleton members and reinforcing members for automobile bodies. is there. Therefore, various techniques for improving the chemical conversion processability of high workability and high strength cold-rolled steel sheets with increased Si content have been studied.
For example, Patent Document 1 discloses that the ratio of Si oxides that adversely affect chemical conversion treatment by controlling the Si content / Mn content ratio of oxides formed on the surface of high workability, high strength cold-rolled steel sheets to 1 or less. A technique for improving the chemical conversion treatment performance by lowering the value is disclosed. In Patent Documents 2 to 4, in addition to the Si content / Mn content ratio of the oxide formed on the surface of the high workability and high strength cold-rolled steel sheet, the properties of the oxide (ie, dimensions, density, coverage) are described. A technique for improving chemical conversion processability by controlling is disclosed. All of these techniques are intended to improve the chemical conversion property by controlling the composition and properties of oxides formed on the surface of a high workability high strength cold-rolled steel sheet by defining the annealing conditions.

In addition, a technique for removing or destroying an oxide generated on the surface of a high workability, high-strength cold-rolled steel sheet has been studied. For example, in Patent Document 5, chemical conversion treatment is carried out by grinding 2.0 g / m ² or more of a high workability high strength cold-rolled steel sheet after annealing, then pickling with hydrochloric acid, and further grinding 0.1 to 0.3 g / m ^2. A technique for improving the performance is disclosed. Patent Document 6 discloses a technique for improving chemical conversion treatment properties by spraying solid particles having an average particle size of 30 to 300 μm.

For hot-rolled steel sheets, a technique for controlling the surface roughness has been studied. For example, in Patent Document 7, the average surface roughness Ra of a hot-rolled steel sheet that has been pickled after hot rolling is 0.3 μm or less and the number of pitting by pickling is an average of 5 within a square of 10 μm per side. A technique for improving the chemical conversion processability by setting the number to less than the number is disclosed. Patent Document 8 discloses a technique for improving chemical conversion treatment by setting the L value of diffuse reflection of a hot-rolled steel sheet that has been pickled after hot rolling to 65 or less.
JP-A-4-76060 JP 2005-187863 A JP 2005-290440 JP JP 2006-283130 A JP 2003-226920 A JP 2005-240148 A JP 2005-139486 JP 2007-75877

  However, in the techniques of Patent Documents 1 to 4, in order to control the ratio of the Si content and the Mn content of the oxide generated on the surface to a suitable range, the Si content of the high workability and high strength cold-rolled steel sheet is inevitably required. Quantity and Mn content are constrained. Therefore, the workability and strength of the high workability and high strength cold-rolled steel sheet are not sufficiently improved. Furthermore, since the composition and properties of the oxide are easily affected by annealing conditions (for example, dew point, hydrogen concentration, etc.), chemical conversion properties are not sufficiently improved.

In the technique of Patent Document 5, since grinding is performed twice, it is inevitable that productivity and yield are reduced and manufacturing cost is increased. The technique of Patent Document 6 also increases the manufacturing cost due to a decrease in yield.
The techniques of Patent Documents 7 and 8 suppress deterioration of chemical conversion property due to the roughening of the pickled hot-rolled steel sheet, and are different from techniques for improving the chemical conversion property of cold-rolled steel sheets. .

As described above, the technology for improving the chemical conversion property of the high workability and high strength cold-rolled steel sheet has not yet been put into practical use.
An object of this invention is to provide the high workability high-strength cold-rolled steel plate which has the outstanding chemical conversion property, and its manufacturing method.

  The present inventors have melted molten steel with various contents of C, Si, and Mn, and after continuous casting, hot rolling, cold rolling, and annealing are performed. The strength, workability and chemical conversion were investigated. As a result, in order to obtain a cold-rolled steel sheet that achieves both an increase in strength and an improvement in workability (that is, a high-workability high-strength cold-rolled steel sheet), it is necessary to contain 0.8% by mass or more of Si, while 3.0% It has been found that when the content exceeds mass%, a large amount of Si oxide is generated, so that the chemical conversion treatment performance is remarkably lowered.

Therefore, the present inventors have studied a technique for suppressing deterioration of chemical conversion property due to such Si oxide. As a result, chemical conversion treatment is performed on the annealed high workability high-strength cold-rolled steel sheet by subjecting it to temper rolling using a work roll having a surface with a predetermined geometric shape, without reducing productivity or yield. It was found that the sex could be improved.
Furthermore, the geometric shape provided on the surface of the work roll used in temper rolling was examined in detail. As a result, as shown in FIG. 1, the maximum depth from the waviness curve and the flat portion where the deviation from the waviness curve of the roughness profile curve of the high workability and high strength cold-rolled steel sheet is ± 2 μm or less. Excellent chemical conversion when temper rolling is carried out so that the concave portion of 5 μm or more is on the surface, the average area of the concave portion is 0.001 to 0.1 mm ² , and the area ratio of the concave portion is 5 to 50%. It was found that processability was obtained.

In addition, the area ratio of a recessed part is a ratio of the total area of a recessed part with respect to the surface area of a high workability high-strength cold-rolled steel plate.
The present invention has been made based on these findings.
That is, in the present invention, C: 0.05 to 0.30% by mass, Si: 0.8 to 3.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: A flat portion having a composition containing 0.008% or less, the balance being Fe and inevitable impurities, and having a deviation from the waviness curve of the roughness profile curve of ± 2 μm or less, and the maximum from the waviness curve It has a recess with a depth of 5 μm or more on the surface, the average area of the recess is 0.001 to 0.1 mm ² , and the area ratio of the recess is 5 to 50%. It is a rolled steel sheet.

  In the high workability high strength cold-rolled steel sheet of the present invention, in addition to the above-mentioned composition, 1% is selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 0.2%, and B: 0.0001 to 0.005% in mass%. It is preferable to contain seeds or two or more kinds. Furthermore, it is preferable to contain one or more selected from Ti: 0.005-0.05%, Nb: 0.005-0.05%, and V: 0.005-0.05%.

  Further, the present invention provides a method for producing a cold-rolled steel sheet in which hot rolling, cold rolling, and annealing are sequentially performed on a slab, and the slab has the above-described composition, and after completion of annealing, the 10-point average roughness Rz is 10 μm or more. And the manufacturing method of the high workability high-strength cold-rolled steel sheet which was excellent in the chemical conversion treatment property which performs temper rolling with the elongation rate of 0.10% or more using the work roll which has the surface whose peak count PPI is in the range of 10-400 It is.

  According to the present invention, a high workability high strength cold-rolled steel sheet having excellent chemical conversion properties can be obtained. This high workability and high strength cold-rolled steel sheet is suitable for use as a skeleton member or a reinforcing member of an automobile body.

First, the components of the high workability high strength cold-rolled steel sheet of the present invention will be described. The unit of the content of each element is mass%, and is simply expressed as%.
C: 0.05-0.30%
C is an important element having an effect of increasing the strength of the high workability high strength cold-rolled steel sheet. In order to obtain a desired tensile strength (ie, 780 MPa or more), the C content needs to be 0.05% or more. On the other hand, if it exceeds 0.30% by mass, the weldability of the high workability and high strength cold-rolled steel sheet is remarkably deteriorated. Therefore, C is in the range of 0.05 to 0.30%. When good weldability is required, it is preferably in the range of 0.05 to 0.20%. More preferably, it is 0.05 to 0.15%.

Si: 0.8-3.0%
Si is an important element that contributes to improving the strength of a high workability and high strength cold-rolled steel sheet by solid solution strengthening, and has an effect of increasing strength while suppressing deterioration of workability. In order to obtain desired tensile strength and workability, the Si content needs to be 0.8% or more. On the other hand, if it exceeds 3.0%, the effect of improving the strength is saturated and the workability deteriorates. Therefore, Si is within the range of 0.8 to 3.0%. Preferably it is 1.0 to 2.0%.

Mn: 1.0-3.0%
Mn is an element that contributes to improving the strength of the high workability and high strength cold-rolled steel sheet by solid solution strengthening, and enhances the hardenability of the austenite phase, and has the effect of stably maintaining the strength. In order to stably obtain the desired strength, the Mn content needs to be 1.0% or more. On the other hand, if it exceeds 3.0%, workability deteriorates. Therefore, Mn is within the range of 1.0 to 3.0%. Preferably it is 1.5 to 2.5%.

P: 0.10% or less P has an effect of improving the strength of a high workability, high strength cold-rolled steel sheet, and is added according to the required strength. However, if the P content exceeds 0.10%, the weldability of the high workability and high strength cold-rolled steel sheet deteriorates. Therefore, P is set to 0.10% or less. When good weldability is required, 0.05% or less is preferable.

S: 0.01% or less S is present as an inclusion in a high workability, high-strength cold-rolled steel sheet, and deteriorates workability (particularly stretch flangeability). Therefore, S needs to be reduced as much as possible. However, if the S content exceeds 0.01%, the adverse effect on stretch flangeability becomes significant. Therefore, the S content is 0.01% or less. When good stretch flangeability is required, 0.005% or less is preferable.

Al: 0.01 to 0.1%
Al is an element that is added as a deoxidizing element at the melting stage to increase the cleanliness of molten steel. Moreover, it has the effect | action which refines | miniaturizes the structure | tissue of a high workability high-strength cold-rolled steel plate. If the Al content is less than 0.01%, these effects cannot be obtained. On the other hand, if it exceeds 0.1%, the surface properties of the high workability and high strength cold-rolled steel sheet deteriorate. Therefore, Al is within the range of 0.01 to 0.1%.

N: 0.008% or less N is an impurity mixed in the high workability and high strength cold-rolled steel sheet. If the N content exceeds 0.008%, variations in strength occur. Therefore, N is set to 0.008% or less.
The above is an essential element of the high workability high strength cold-rolled steel sheet of the present invention, and desired properties can be obtained if the respective contents are maintained within a predetermined range. However, in order to further improve and stabilize the excellent properties of the high workability and high strength cold-rolled steel sheet, the following elements may be added.

One or more selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 0.2% and B: 0.0001 to 0.005%
Cr, Mo, and B are elements having an effect of improving the hardenability of the austenite phase, and are added as necessary. In order to obtain such an effect, Cr: 0.01% or more, Mo: 0.01% or more, B: 0.0001% or more are added. On the other hand, if it exceeds Cr: 1.0%, Mo: 0.2%, B: 0.005%, the workability of the high workability and high strength cold-rolled steel sheet deteriorates. In addition, the use of a large amount of expensive Cr, Mo, B increases the manufacturing cost of the high workability, high strength cold-rolled steel sheet. Therefore, when adding Cr, Mo, B, it is preferable that Cr: 0.01 to 1.0%, Mo: 0.01 to 0.2%, B: 0.0001 to 0.005%.

One or more selected from Ti: 0.005-0.05%, Nb: 0.005-0.05% and V: 0.005-0.05%
Ti, Nb, and V have a function of forming carbides in a high workability and high strength cold-rolled steel sheet and increasing the strength by precipitation strengthening. In order to obtain such an effect, Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.005% or more are added. On the other hand, when Ti exceeds 0.05%, Nb: 0.05%, and V: 0.05%, the workability of the high workability and high strength cold-rolled steel sheet deteriorates. In addition, by using a large amount of expensive Ti, Nb, and V, the manufacturing cost of the high workability and high strength cold-rolled steel sheet increases. Therefore, when adding Ti, Nb, and V, it is preferable that Ti: 0.005 to 0.05%, Nb: 0.005 to 0.05%, and V: 0.005 to 0.05%.

The effects of the present invention are not lost even if other alloy elements (for example, rare earth elements, Ni, Cu, Mg, Ca, Zr, etc.) are contained within a range that is added to a normal cold-rolled steel sheet.
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include Sb, Sn, Zn, Co and the like. The content of these elements is preferably Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, and Co: 0.1% or less.

Next, the geometric shape of the surface of the high workability high strength cold-rolled steel sheet of the present invention will be described.
High formability, high-strength cold-rolled steel sheet has a flat portion with a deviation of ± 2 μm or less from the waviness curve of the roughness profile curve and a concave portion with a maximum depth of 5 μm or more from the waviness curve. The average area of the recesses is 0.001 to 0.1 mm ² and the area ratio of the recesses is 5 to 50%. Such a geometric shape can be obtained by annealing a high workability high-strength cold-rolled steel sheet and then performing temper rolling using a work roll having a surface with a predetermined geometric shape.

  That is, the high workability high strength cold-rolled steel sheet 3 after annealing produces Si oxide 4 on the surface as shown in FIG. 2 (a), but temper rolling is performed using the work roll described above. As a result, the recesses 2 are formed on the surface of the high workability high strength cold-rolled steel sheet 3, and the Si oxide 4 is destroyed as shown in FIG. 2 (b). Next, when the high formability and high strength cold-rolled steel sheet 3 is subjected to chemical conversion treatment, as shown in FIG. 4 peels off. Thereafter, as shown in FIG. 2 (d), a chemical crystal 5 is formed on the surface of the high workability and high strength cold-rolled steel sheet 3, and further grows. In this way, excellent chemical conversion properties can be obtained.

  If the maximum depth of the recess 2 formed on the surface of the high workability high strength cold-rolled steel sheet 3 is less than 5 μm, the effect of destroying the Si oxide 4 cannot be obtained. Therefore, the maximum depth of the recess 2 is 5 μm or more. Preferably, it is 10 μm or more. On the other hand, if the maximum depth of the concave portion 2 exceeds 50 μm, there is a possibility that it becomes a starting point of a crack when performing press working. Therefore, the maximum depth of the recess 2 is preferably 50 μm or less. Preferably, it is 30 μm or less.

If the average area of the recesses 2 formed on the surface of the high workability high strength cold-rolled steel sheet 3 is less than 0.001 mm ² , the grain size (diameter 1 to 10 μm) of the chemical conversion crystal 5 may be smaller. There is a possibility that the crystal 5 is not generated and a gap is formed between the chemical conversion crystals 5. Accordingly, the average area of the recesses 2 is set to 0.001 mm ² or more. Preferably it is 0.002 mm ² or more. On the other hand, if the average area of the recesses 2 exceeds 0.1 mm ² , the distribution density of the recesses 2 is lowered and the effect of destroying the Si oxide 4 cannot be obtained. Therefore, the average area of the recess 2 is 0.1 mm ² or less. Preferably it is 0.05 mm ² or less. In addition, the area of a recessed part is an area including the filter wave | undulation curve cut off by the recessed part, and an average area is the average value.

  If the area ratio of the recesses 2 formed on the surface of the high workability and high strength cold-rolled steel sheet 3 is less than 5%, the distribution density of the recesses 2 is lowered and the effect of destroying the Si oxide 4 cannot be obtained. Therefore, the area ratio of the recess 2 is set to 5% or more. Preferably it is 10% or more. On the other hand, if the area ratio of the concave portion 2 exceeds 50%, the load of temper rolling for forming the concave portion 2 increases, so that a large-scale rolling mill is required, and the high workability and high strength cold-rolled steel sheet 3 is formed. Increases manufacturing costs. Therefore, the area ratio of the recess 2 is set to 50% or less. Preferably it is 40% or less. In addition, the area ratio of a recessed part is a ratio of the total area of a recessed part with respect to the surface area of a high workability high-strength cold-rolled steel plate.

Next, the manufacturing method of the high workability high strength cold-rolled steel sheet of the present invention will be described.
Using molten steel having a predetermined component, a slab is manufactured by a continuous casting method, an ingot-making / slabbing method, or a thin slab casting method. Next, hot rolling, cold rolling and annealing are sequentially performed. In manufacturing the slab, it is preferable to employ a continuous casting method in order to prevent macro segregation.

  The obtained slab is reheated and hot rolled. At that time, the slab is once cooled to room temperature and charged into the heating furnace, or the slab is reheated and hot rolled by a method such as charging the heating furnace as it is without cooling. If the reheating temperature is less than 1000 ° C, the deformation resistance of the slab is large, which hinders hot rolling. Therefore, the reheating temperature of the slab is preferably 1000 ° C. or higher. On the other hand, when the temperature exceeds 1300 ° C., the scale loss increases. Therefore, the reheating temperature of the slab is more preferably in the range of 1000 to 1300 ° C.

Further, direct feed rolling in which hot rolling is performed as it is after casting without applying slab reheating is also applicable. Even when direct rolling is performed, the temperature of the slab is preferably 1000 ° C. or higher. More preferably, it is 1000-1300 degreeC.
In hot rolling, rough rolling is performed as necessary, and then finish rolling is performed. When the temperature of the slab in finish rolling is less than 800 ° C., the structure of the hot-rolled steel sheet becomes non-uniform, and the workability in the subsequent cold-rolling and pressing processes deteriorates. Therefore, the temperature of the slab in finish rolling is preferably 800 ° C. or higher. On the other hand, when the temperature exceeds 1000 ° C., surface flaws due to entrainment of the scale are likely to occur. Therefore, the temperature of the slab in finish rolling is more preferably in the range of 800 to 1000 ° C.

Next, the hot rolled steel sheet subjected to hot rolling is wound around a coil. When the temperature of the hot-rolled steel sheet when wound on the coil exceeds 650 ° C., scale is generated after winding and the pickling load increases. Therefore, the temperature of the hot-rolled steel sheet when wound on a coil is preferably 650 ° C. or less.
The hot-rolled steel sheet thus obtained is pickled to remove the scale, and then cold-rolled. When the scale of the hot-rolled steel sheet is extremely small, pickling may be omitted and cold rolling may be performed. In cold rolling, the rolling reduction is preferably 20% or more from the viewpoint of ensuring the flatness of the surface of the obtained cold-rolled steel sheet and the uniformity of the structure.

Next, the cold rolled steel sheet is annealed. For annealing, it is preferable to use continuous annealing equipment from the viewpoint of improving productivity.
When the annealing holding temperature is less than 750 ° C., recrystallization does not occur sufficiently, and the workability may decrease. On the other hand, when it exceeds 900 ° C., the structure becomes coarse and the balance between strength and workability is lost. Therefore, the annealing holding temperature is preferably in the range of 750 to 900 ° C.

When the annealing holding time is less than 60 seconds, recrystallization becomes non-uniform. Accordingly, the holding time for annealing is preferably 60 seconds or more. More preferably, it is 120 seconds or more.
When the dew point of annealing exceeds -35 ° C, Si oxide is generated inside the cold-rolled steel sheet, and high workability and high strength are achieved by chemical conversion treatment and electrodeposition coating even after temper rolling after annealing. The corrosion resistance of the cold rolled steel sheet deteriorates. Therefore, the dew point of annealing is preferably −35 ° C. or less. More preferably, it is −40 ° C. or lower.

After annealing, the cold-rolled steel sheet is preferably cooled to 300 ° C. or lower at a cooling rate of 30 ° C./second or higher. If the cooling rate is less than 30 ° C./second, it is necessary to add a large amount of alloying elements in order to obtain sufficient strength, which leads to an increase in the manufacturing cost of the high workability and high strength cold-rolled steel sheet.
Furthermore, in order to improve workability, after quenching, it is preferable to hold at 100 to 450 ° C. for 1 to 30 minutes.

Annealed cold-rolled steel sheet is subjected to temper rolling using a work roll having a geometric shape surface with a 10-point average roughness Rz of 10 μm or more and a peak count PPI of 10 to 400, and high workability A high-strength cold-rolled steel sheet is obtained.
PPI is the number of uneven peaks per inch as defined by the SAE 911 standard, and is a value at a count level of ± 0.635 μm. When PPI is less than 10 or exceeds 400, the average area of the recesses of the high workability and high strength cold-rolled steel sheet cannot be controlled within the range of 0.001 to 0.1 mm ² and the area ratio of the recesses within the range of 5 to 50%. Therefore, the PPI is 10 to 400. Preferably it is 20-300.

  If Rz is less than 10 μm, the load required to provide a recess having a maximum depth of 5 μm or more on the surface of a high workability and high strength cold-rolled steel sheet becomes large. This increases the manufacturing cost of the rolled steel sheet. Therefore, Rz is 10 μm or more. Preferably it is 20 micrometers or more. The upper limit of Rz is not particularly defined, but if it exceeds 200 μm, the maximum depth of the recess becomes 50 μm or more, and the recess may be a starting point of cracking during press working. Therefore, Rz is preferably 200 μm or less. More preferably, it is 100 μm or less.

  If the elongation rate of temper rolling is less than 0.10%, even if a work roll in which Rz and PPI are adjusted within a predetermined range is used, the surface of the high workability high strength cold-rolled steel sheet has a predetermined average area and area ratio. It becomes difficult to provide the recess. Therefore, the elongation of temper rolling is preferably 0.10% or more. More preferably, it is 0.15% or more. The upper limit of the elongation rate is not particularly specified, but if it exceeds 0.5%, the ductility of the high workability and high strength cold-rolled steel sheet is significantly lowered. Therefore, the elongation rate is preferably 0.5% or less. More preferably, it is 0.4% or less.

In this way, recesses having an average area of 0.001 to 0.1 mm ² can be provided on the surface of the high workability and high strength cold-rolled steel sheet at an area ratio of 5 to 50%, and an improvement in chemical conversion treatment can be achieved.
Prior to temper rolling, it is preferable to remove surface oxides by pickling using hydrochloric acid, sulfuric acid, nitric hydrochloric acid or the like. By removing the oxide and performing temper rolling, wear of the work roll can be suppressed.

Further, after temper rolling, pickling with hydrochloric acid, sulfuric acid, nitric hydrochloric acid or the like is preferred to remove the surface oxide. By performing the chemical conversion treatment after removing the oxide, a further excellent chemical conversion treatment property can be obtained. On the other hand, from the viewpoint of die squeezing in press forming, it is preferable to leave a certain amount of Si oxide on the surface of the high workability and high strength cold-rolled steel sheet.
Furthermore, after temper rolling and pickling, or after temper rolling, Ni or the like may be attached within a range of 5 to 50 mg / m ² . Formation and growth of chemical crystals can be promoted by attaching Ni or the like, which is thought to promote the formation and growth of uniform and fine chemical crystals.

  Slabs having the components shown in Table 1 were produced by continuous casting, reheated to 1250 ° C., finish-rolled at 850 ° C. to form a hot-rolled steel sheet having a thickness of 3.0 mm, and wound on a coil at 600 ° C. Steel types A, B, and D to N in Table 1 are examples having components that satisfy the definition of the present invention, and steel type C is an example in which the Si content deviates from the definition of the present invention.

  The hot-rolled steel sheet was pickled and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Further, the cold-rolled steel sheet was annealed at 780 to 860 ° C. (dew point: −35 to −50 ° C.), gradually cooled to 550 to 720 ° C., then cooled to room temperature, and tempered at 150 to 400 ° C. Next, temper rolling was performed under the conditions shown in Table 2 to obtain a high workability, high strength cold-rolled steel sheet. No. 3 in Table 2 is an example using a steel grade C slab, No. 6 is an example in which the Rz of the work roll and the maximum depth of the recess of the high workability high strength cold rolled steel sheet are outside the scope of the present invention, No. .9 is an example in which the elongation ratio of temper rolling and the average area of the recesses of the high workability high strength cold rolled steel sheet are outside the scope of the present invention. No. 12 is the PPI of the work roll and the high workability high strength cold rolled steel sheet. This is an example in which the area ratio of the recesses is outside the scope of the present invention.

The mechanical properties, chemical conversion properties, and surface geometry of the high workability and high strength cold-rolled steel sheets obtained were investigated. The results are shown in Table 2. The survey method is as follows.
mechanical nature:
A JIS Standard No. 5 test piece was taken with the direction perpendicular to the rolling direction of the high workability, high strength cold-rolled steel sheet as the major axis, and a tensile test was conducted in accordance with the provisions of JIS Standard Z2241. The tensile strength and elongation are shown in Table 2 as TS and EL, respectively. Table 2 also shows the calculated value of TS × EL as an index indicating workability. If the TS × EL value is 18000 MPa ·% or more, the workability is good.

Surface geometry:
Using a scanning electron microscope with a three-dimensional shape measurement function, the maximum depth, average area, and area ratio from the flat portion of the recess were measured. Further, Ra was measured according to JIS standard B0601.
Chemical conversion:
After chemical conversion treatment of a high workability high strength cold-rolled steel sheet under standard conditions using a commercially available chemical conversion treatment agent (Palbond PB-L3020 manufactured by Nihon Parkerizing Co., Ltd.), the surface of the high workability high-strength cold-rolled steel sheet When five fields of view are observed with a scanning electron microscope at 500 times, a uniform chemical crystal with an area ratio of 95% or more is generated in all five fields of view (Good), and a gap with an area ratio of 5% or more is 1 What was recognized also in the visual field was evaluated as bad (x).

As is clear from Table 2, all of the inventive examples had a tensile strength (TS) of 780 MPa or more and a TS × EL value of 18000 MPa ·% or more. In other words, the invention examples are excellent in strength and workability. Furthermore, chemical conversion processability is also good.
On the other hand, in the comparative example, No. 3 uses a steel type whose Si content is outside the range of the present invention, so the TS × EL value is less than 18000 MPa ·%, and the workability is inferior. In No. 6, the Rz of the work roll and the maximum depth of the concave portion of the high workability high strength cold-rolled steel sheet are out of the scope of the present invention, so the removal of the Si oxide on the surface is not sufficient, and the chemical conversion processability is inferior. Yes. In No. 9, the elongation ratio of the temper rolling and the average area of the recesses of the high workability high strength cold-rolled steel sheet deviate from the scope of the present invention, so the removal of the Si oxide on the surface is not sufficient, and the chemical conversion processability is poor. ing. In No. 12, the PPI of the work roll and the area ratio of the recesses of the high workability and high strength cold-rolled steel sheet are out of the scope of the present invention, so the removal of the Si oxide on the surface is not sufficient, and the chemical conversion processability is inferior. .

It is a graph which shows typically the example of the filter waviness curve of the high workability high intensity cold-rolled steel plate of the present invention. It is sectional drawing which shows sequentially the process in which a conversion crystal produces | generates on the surface of a high workability high strength cold-rolled steel sheet.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Roughness cross-section curve 2 Concave part 3 High workability high-strength cold-rolled steel sheet 4 Si oxide 5 Chemical conversion crystal

Claims (4)

Contains C: 0.05 to 0.30%, Si: 0.8 to 3.0%, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.008% or less in mass% And the balance has a composition composed of Fe and inevitable impurities, and the deviation of the roughness profile curve from the filtered waviness curve is ± 2 μm or less, and the maximum depth from the filtered waviness curve is 5 μm. has a surface and a more recesses, the average area of the recesses is 0.001-0.1 mm ^2, high workability area ratio of the concave portion has excellent chemical conversion treatability, which is a 5-50% High strength cold rolled steel sheet.   In addition to the composition, the composition contains one or more selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 0.2%, and B: 0.0001 to 0.005% in mass%. The high workability high-strength cold-rolled steel sheet excellent in chemical conversion processability according to 1.   In addition to the composition, it contains one or more selected from Ti: 0.005-0.05%, Nb: 0.005-0.05%, and V: 0.005-0.05% by mass. A high workability high-strength cold-rolled steel sheet having excellent chemical conversion properties as described in 1 or 2. In the manufacturing method of the cold-rolled steel sheet which sequentially performs hot rolling, cold rolling, and annealing on the slab, the slab has the composition according to any one of claims 1 to 3, and after completion of the annealing, 10 For chemical conversion treatment characterized by temper rolling with an elongation of 0.10% or more using a work roll having a surface with a point average roughness Rz of 10 μm or more and a peak count PPI in the range of 10 to 400 An excellent method for producing high-strength cold-rolled steel sheets.

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