JP2010053371A - Method for producing high-strength cold-rolled steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high-strength cold-rolled steel sheet which has excellent chemical-processing property and corrosion-resistance after electrostatic-deposition coating even in the case of a comparatively high content of Si. <P>SOLUTION: The cold-rolled steel sheet containing Si of 0.8-2.0 mass%, and Mn of 1.0-3.0 mass%, is first subjected to annealing in atmosphere of N<SB>2</SB>-H<SB>2</SB>in a furnace and at a ratio of PH<SB>2</SB>O/PH<SB>2</SB>of ≤1.0×10<SP>-3</SP>. Successively, a quenching and a tempering, are applied, and thereafter, a pickling is performed with an alternating current electrolysis having ≥1A/dm<SP>2</SP>current density. After passing through such sequential process, the high-strength cold-rolled steel sheet having strength of ≥590 MPa TS and excellent in the chemical-processing property and the corrosion-resistance after electrostatic-deposition coating, is obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength cold-rolled steel sheet used for automobile steel sheets and the like, and particularly has excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the Si content is relatively large. The present invention relates to a method for producing a high-strength cold-rolled steel sheet.

In recent years, there has been a demand for improving fuel efficiency of automobiles from the viewpoint of global environmental conservation. In addition, in order to protect passengers in the event of a collision, it is also required to improve the safety of automobile bodies. Therefore, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted.
In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is said that it is effective to increase the strength of component materials. Used actively.

  On the other hand, since the steel sheet for automobiles is subjected to severe processing, it is important to have excellent ductility as well as strength. As a high-strength steel plate having such mechanical properties, a dual-phase steel plate (DP steel plate) having a composite structure of ferrite and martensite, or transformation induced plasticity (TRIP) caused by retained austenite is used. The high ductility steel plate which was made is mentioned, and it has reached the stage of practical use. In addition, Si, Mn, Cr, P, and the like are used as additive elements in steel effective for increasing the strength. Among them, Si is frequently used because it is effective in achieving both high strength and high ductility.

By the way, a steel sheet used as an automobile part is generally subjected to a phosphate treatment as a base treatment prior to coating after being formed. By applying such a phosphate treatment, the adhesion of the coating film in the coating of the next step can be improved.
However, when the phosphate treatment is performed on the steel sheet to which Si or the like is added, there are the following problems. The steel sheet to which Si and the like are added is an atmosphere of reduction annealing carried out in a normal manufacturing process, Si, which is an easily oxidizable element, is preferentially oxidized and concentrated on the steel sheet surface, and a Si-containing oxide layer on the surface Form. And, when a phosphate treatment is performed on a steel sheet having such a Si-containing oxide layer formed on the surface, zinc phosphate crystals cannot be formed uniformly and finely, and the phosphate is partially It becomes a surface state in which crystals are deficient. Furthermore, when a coating such as electrodeposition coating is applied to the surface of such a steel sheet with poor phosphate treatment, a coating film with good adhesion cannot be obtained, or the corrosion resistance after coating is deteriorated.

In order to solve the above problems, various techniques have been proposed so far.
For example, Patent Document 1 discloses a method of forming an iron coating layer of ^{20 to} 1500 mg / m ² on a steel plate using an electroplating method. However, in this method, there is a problem that the cost is increased due to the additional steps required for the electroplating equipment.
In Patent Document 2, the Mn / Si ratio is defined, and in Patent Document 3, Ni is added to improve the phosphate treatment property. However, the effect depends on the Si content in the steel sheet, and it is considered that further improvement is necessary for the steel sheet having a high Si content.
Furthermore, in Patent Document 4, by setting the dew point during annealing to -25 to 0 ° C., an internal oxide layer made of an Si-containing oxide is formed within a depth of 1 μm from the surface of the steel sheet substrate, and the steel sheet surface length is 10 μm. A method is disclosed in which the proportion of the Si-containing oxide is 80% or less. However, in the case of the method described in Patent Document 4, although the chemical conversion treatment property is improved, there is a problem that the corrosion resistance after electrodeposition coating is poor because the Si-containing oxide exists immediately below the chemical conversion treatment film.
JP-A-5-320952 JP-A-6-104878 Japanese Patent No. 2951480 Japanese Patent No. 3803992

  In view of such circumstances, an object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the Si content is relatively large.

The gist of the present invention is as follows.
[1] Cold-rolled steel sheet containing Si: 0.8 to 2.0 mass%, Mn: 1.0 to 3.0 mass% is annealed, then quenched and tempered, and then subjected to electrolytic pickling so that TS ≧ 590 MPa in producing a high strength cold rolled steel sheet having a strength, the annealing, the ratio of the furnace atmosphere and N ₂ -H _2, H ₂ O partial pressure (PH ₂ O) and H ₂ partial pressure (PH ₂₎ The PH ₂ O / PH ₂ is maintained at 1.0 × 10 ⁻³ or less, and the electrolytic pickling is an alternating electrolysis having a current density of 1 A / dm ² or more. Method.

  In addition, in this specification,% which shows the component of steel is all mass%.

According to the present invention, a high-strength cold-rolled steel sheet having excellent chemical conversion properties and corrosion resistance after electrodeposition coating can be obtained even when the Si content is relatively large.
Moreover, since the said steel plate is obtained without providing new equipment etc., it is an industrially useful invention.

The steel plate targeted in the present invention is a high-strength cold-rolled steel plate having a TS (tensile strength) of 590 Mpa or more. Therefore, in this invention, Si: 0.8-2.0mass% and Mn: 1.0-3.0mass% are contained. Si and Mn are elements that increase strength without degrading workability, and are necessary to improve the balance between strength and workability.
In order to obtain TS of 590 MPa or more, addition of Si: 0.8 mass% or more and Mn: 1.0 mass% or more is necessary. On the other hand, if added excessively, embrittlement becomes significant, so the upper limit of Si is 2.0 mass% and the upper limit of Mn is 3.0 mass%.

Although the steel of the present invention can achieve the desired characteristics with the above additive elements, it is preferable to add the following elements in addition to the above additive elements. The preferred range when these elements are added is as follows.
The amount of C is preferably 0.01 to 0.20 mass%, more preferably 0.04 to 0.15 mass%. If the amount of C is less than the lower limit, the effect of strengthening due to solid solution, precipitation, fine graining, transformation, etc. is hardly seen, and if the amount of C exceeds the upper limit, solid solution, precipitation, fine graining, This is because the effect of strengthening by transformation or the like is saturated and costly.
The amount of P is preferably 0.03 mass% or less, and more preferably 0.02 mass% or less. This is because if the upper limit is exceeded, the corrosion resistance tends to deteriorate.
The amount of S is preferably 0.005 mass% or less, and more preferably 0.002 mass% or less. This is because if the upper limit is exceeded, the corrosion resistance tends to deteriorate.
In addition to the above, one or more of Ti, Nb, V, Mo, Cu, and Ni can be further contained as necessary.

And in this invention, the steel plate which consists of the said composition is annealed, Then, quenching and tempering are performed, and an electrolytic pickling process is carried out after that. By subsequently quenching and tempering the annealed cold-rolled steel sheet, the structure of the steel can be controlled to obtain a material with a balance between strength and elongation.
In this case, annealing, the furnace atmosphere and N ₂ -H _2, H ₂ O partial pressure (PH ₂ O) and H ₂ partial pressure _{(PH 2) PH 2 O /} PH 2 of 1.0 × the ratio of Hold below ^10-3 . The annealing temperature is preferably 750 to 900 ° C., and the tempering temperature is preferably 150 to 400 ° C. The annealing temperature and the tempering temperature can be appropriately selected according to the target material characteristics and are not particularly limited.
Electrolytic pickling is an alternating electrolysis with a current density of 1 A / dm ² or more.
These are the most important requirements in the present invention.

Hereinafter, the atmosphere in the annealing furnace and the electrolytic pickling conditions will be described in detail.
In general, it is known that, in such annealing involving quenching, if oxides formed on the surface remain in the annealing process and the quenching process, the chemical conversion treatment property is hindered. For this reason, it is necessary to remove the surface oxide by pickling after tempering. However, among surface oxides, Fe-based oxides such as hematite, magnetite, and wustite can be easily removed by ordinary pickling treatment immersed in an aqueous solution of sulfuric acid or hydrochloric acid. Removal is difficult.
Therefore, first, the present inventors examined removal of oxides containing Si and Mn. As a result, oxides containing only Si are difficult to remove even by alternating electrolytic pickling with alternating polarity, but complex oxides containing both Si and Mn do not have alternating electrolytic pickling with alternating polarity. I found that it was possible to remove by doing. It has also been found that oxides containing Si and Mn are formed in an annealing process at a high processing temperature and not formed in a tempering process at a relatively low temperature.
From the above, the present inventors considered that the fine structure of the surface oxide that affects the chemical conversion property can be controlled by controlling the atmosphere in the furnace at the time of annealing.
As a result, extremely important for the microstructure of the control surface oxides in the ratio of the partial pressure (PH ₂ O) and H ₂ partial pressure of H ₂ O which inevitably present in the furnace during annealing (PH ₂₎ I found out.
Figures 1 and 2 show that cold rolled steel sheet containing 1.4 mass% Si and 2.0 mass% Mn is H ₂ -5% H ₂ , dew point -35 ° C (PH ₂ O / PH ₂ = 3.8 × 10-3), And H ₂ -7% H ₂ , dew point -45 ° C (PH ₂ O / PH ₂ = 0.9 × 10-3) in a cross-sectional transmission electron microscope of a sample that was annealed at 830 ° C for 30 seconds It is a photograph (bright field image). The partial pressure of H ₂ O was converted from the value of the dew point measured with a dew point meter.
In FIG. 1, Mn ₂ SiO ₄ which is a complex oxide of Si and Mn is formed on the extreme surface layer of a steel sheet, and SiO ₂ which is an oxide composed only of Si is formed on the inner layer. On the other hand, in FIG. 2, Mn ₂ SiO _4, which is a complex oxide of Si and Mn, is only formed on the extreme surface layer of the steel sheet, and no SiO ₂ is formed inside. The reason why such a difference occurs even at the same annealing temperature is considered as follows.
FIG. 3 schematically shows the interface reaction during annealing. According to FIG. 3, first, O is generated on the surface of the steel sheet by the dissociation of H ₂ O molecules shown in Formula (1). Next, Si and Mn in the extreme surface layer are oxidized by the O, and the activities of Si and Mn in the steel near the surface decrease. The gradient of activity becomes a driving force, and Si and Mn diffuse from the inside toward the surface.
H ₂ O = H ₂ + O ----- (1)
At this time, since the Mn diffusion rate is slower than that of Si, an Mn-deficient layer is formed inside the extreme surface layer.
In addition, according to Fig. 4, when the ratio of PH ₂ O / PH ₂ is high, the potential of O generated on the steel sheet surface due to dissociation of H ₂ O molecules increases, so that diffusion of Si and Mn from the steel toward the surface The diffusion rate of O from the steel sheet surface to the inside is faster than the velocity, and SiO ₂ containing only Si is generated in the region of the Mn-deficient layer inside.
On the other hand, if the ratio of PH ₂ O / PH ₂ is low, the potential of O generated on the surface of the steel sheet due to the dissociation of H ₂ O molecules decreases, so the amount of oxidation itself decreases, and the combined oxidation of Si and Mn occurs on the extreme surface layer. Form only things.
Thus, it becomes possible to control the fine structure of the oxide generated on the surface by controlling PH ₂ O / PH ₂ during annealing.
Then, if based on the above results, since the SiO ₂ be subjected to electrolytic pickling is difficult removal, it can be removed by electrolytic pickling if only a composite oxide of Si and Mn, during annealing By optimizing the combination of PH ₂ O / PH ₂ control and electrolytic pickling, it is possible to create a surface with good chemical conversion treatment. That is, by controlling the surface oxide to be a complex oxide of Si and Mn and carrying out alternating electrolytic pickling, the surface oxide is removed and the chemical conversion treatment performance is improved.
As a result of further investigation based on these findings, in order to control the surface oxide to be a complex oxide of Si and Mn, in addition to a predetermined amount of Mn, PH ₂ O / PH ₂ It was found that 1.0 × 10 ⁻³ or less is necessary. If PH ₂ O / PH ₂ is at more than _{^{1.0 × 10 -3, Mn 2 SiO}} 4 in the surface layer electrode, and SiO ₂ is formed therein, SiO ₂ formed in the interior, the electrolytic pickling This is because removal is difficult.
The atmosphere in the furnace is N ₂ —H ₂ . The H ₂ concentration only needs to contain an amount capable of reducing the steel sheet surface, and is preferably 1% or more, more preferably 5% or more. Furthermore, the annealing temperature is preferably 750 ° C. or higher and 900 ° C. or lower. This is because if it is less than 750 ° C., sufficient strength cannot be obtained after quenching. On the other hand, if it exceeds 900 ° C., there is a risk of breakage in the furnace.
Electrolytic pickling is an alternating electrolysis with a current density of 1 A / dm ² or more.
When the current density is 1 A / dm ² or more, the surface oxide can be removed. Preferably it is 5 A / dm ² or more.
Then, alternating electrolysis of cathode → anode or anode → cathode is necessary. The pickling effect is small when the steel plate is held on the cathode. Conversely, if the steel plate is held on the anode, Fe eluted during electrolysis accumulates in the pickling solution, and the Fe concentration in the pickling solution increases. When such pickling solution adheres to the surface of the steel plate, problems such as dry dirt occur. For the above reasons, in the present invention, alternating electrolysis is used.
The pickling solution used for electrolytic pickling is not particularly limited, but nitric acid and hydrofluoric acid are not preferable because they are highly corrosive to equipment and require careful handling. Hydrochloric acid is not preferred because it may generate chlorine gas from the cathode. For this reason, use of sulfuric acid is preferable in consideration of corrosivity and environment. The sulfuric acid concentration is preferably 5 mass% or more and 20 mass% or less, and more preferably 10 mass% or more and 15 mass% or less. When the sulfuric acid concentration is less than 5 mass%, the electrical conductivity is lowered, so that the bath voltage during electrolysis is increased and the load on the power source is increased. On the other hand, if the sulfuric acid concentration exceeds 20 mass%, the loss due to dragout is large, which causes a problem in cost.
The temperature of the pickling solution is preferably 40 ° C. or higher and 70 ° C. or lower, more preferably 50 ° C. or higher and 60 ° C. or lower. Since the bath temperature rises due to heat generated by continuous electrolysis, it is difficult to maintain the temperature below 40 ° C. Moreover, it is a problem that temperature exceeds 70 degreeC from the viewpoint of durability of the lining of an electrolytic cell.
Other quenching and tempering conditions are not particularly limited. Tempering is preferably performed at a temperature of 150 to 400 ° C. This is because the elongation deteriorates below 150 ° C., whereas the hardness decreases below 400 ° C.
By producing as described above, a high-strength cold-rolled steel sheet having excellent chemical conversion properties and corrosion resistance after electrodeposition coating can be obtained. The reason why the high-strength cold-rolled steel sheet produced in this way has excellent chemical conversion treatment properties and excellent electrodeposition coating properties is that the Si-containing oxide in the plate thickness cross-sectional area from the steel plate surface to the plate thickness direction 1 μm This is probably because the ratio is as low as about 5% or less.
The existing ratio of the Si-containing oxide in the plate thickness cross-sectional region from the steel plate surface to the plate thickness direction of 1 μm can be confirmed by the following method.
The cross section in the direction perpendicular to the steel sheet surface is confirmed by a mapping process with an electron microscope at a magnification of 50,000 times or more and the area occupied by the Si-containing oxide in the area of the steel sheet surface length 10 μm and the surface and the surface thickness direction 1 μm. To do. Since the Si-containing oxide is formed in a layer on the surface layer or exists in the form of precipitates, the component of the part different from such a normal steel plate component is measured, and the area of the Si-containing oxide part is measured. do it.

Next, the present invention will be described in more detail with reference to examples. In addition, this invention is not restrict | limited by the following Example.
A slab containing C: 0.12 mass%, Si: 1.4 mass%, Mn: 1.9 mass% was hot-rolled to a sheet thickness of 2.5 mm and wound at a winding temperature of 600 ° C. Subsequently, the scale was removed and cold rolling was performed to a plate thickness of 1.2 mm.
The cold-rolled steel sheet obtained above was subjected to continuous annealing under the conditions shown in Table 1, and tempering at 300 ° C. for 140 seconds after water quenching. Subsequently, electrolytic pickling was performed at 40 ° C. in a 5% sulfuric acid aqueous solution under the conditions shown in Table 1 to obtain a test material. The TS of the obtained test material was 700 to 1000 MPa. TS was measured according to a JIS Z 2241 metal material tensile test method.

For the specimens obtained as described above, the Si-containing oxide in the plate thickness cross-sectional area from the steel plate surface to the plate thickness direction of 1 μm was measured by the method shown below, and after chemical conversion treatment and electrodeposition coating Corrosion resistance was evaluated.
<Si-containing oxide>
The cross section in the direction perpendicular to the steel sheet surface is confirmed by a mapping process with an electron microscope at a magnification of 50,000 times or more and the area occupied by the Si-containing oxide in the area of the steel sheet surface length 10 μm and the surface and the surface thickness direction 1 μm. To do. Since the Si-containing oxide is formed in a layer on the surface layer or exists in the form of precipitates, the component of the part different from such a normal steel plate component is measured, and the area of the Si-containing oxide part is measured. do it. The amount of Si-containing oxide was classified as follows.
◎: Less than detection limit (1%) ○: 1% or more and 5% or less △: More than 5%, less than 10% ×: More than 10% <Chemical conversion treatment evaluation>
A 150 × 70 mm test piece was cut out from the test material and subjected to chemical conversion treatment with PB-L3080 manufactured by Nihon Parkerizing. Next, a 10 × 10 mm test piece was cut out, and an SE image was observed at an acceleration voltage of 15 kV and a magnification of 1000 times using a scanning electron microscope. The chemical conversion processability was judged by the presence or absence of. The case where there was a skein even in one of the 5 fields of view was marked as x, and the case where there was no skein in the field of view was marked as ◯.
<Corrosion resistance after electrodeposition coating>
(Specimen creation procedure)
From a cold-rolled steel plate sample obtained by the above method, a test piece of 150 x 70 mm was cut out and subjected to chemical conversion treatment with PB-L3080 made by Nihon Parkerizing, followed by cationic electrodeposition coating with PN-150G made by Nihon Paint ( Baking conditions: 170 ° C. × 20 minutes, film thickness 25 μm). Thereafter, the end surface and the side not evaluated were sealed with Al tape, and a cross cut (cross angle 60 °) reaching the ground iron with a cutter knife was used as a test material.
Take out after immersion for 240 hours in 5% NaCl aqueous solution (55 ° C), wash with water, dry and then peel off the crosscut part, measure the peel width, ○ if less than 2.5mm on one side, if more than 2.5mm X was determined.

  The results obtained above are shown in Table 1, and FIG. 5 shows an observation example of good materials and NG materials.

  From Table 1, in the example of this invention, both chemical conversion property and the corrosion resistance after electrodeposition coating are excellent. On the other hand, in the comparative example, both chemical conversion property and corrosion resistance after electrodeposition coating are inferior.

  Since it is excellent in chemical conversion treatment and corrosion resistance after electrodeposition coating, it can be applied in a wide range of fields, especially automobiles. In particular, the present invention can be suitably used for a high-strength steel sheet having a relatively high Si content.

It is a cross-sectional transmission electron micrograph (bright field image) of a sample. It is a cross-sectional transmission electron micrograph (bright field image) of a sample. It is a figure which shows typically the interface reaction at the time of annealing. PH ₂ O / PH ₂ in annealing is a diagram schematically showing the effect on the interfacial reaction. It is a figure which shows the determination result of chemical conversion property. (Example 1)

Claims (1)

A cold rolled steel sheet containing Si: 0.8 to 2.0 mass% and Mn: 1.0 to 3.0 mass% is annealed, then quenched and tempered, and then subjected to electrolytic pickling to have a strength of TS ≧ 590 MPa. in producing a high-strength cold-rolled steel sheet, the annealing is the ratio of the furnace atmosphere and N ₂ -H _2, partial pressure (PH ₂ O) and H ₂ partial pressure of H ₂ O (PH ₂₎ PH _A method for producing a high-strength cold-rolled steel sheet, characterized in that ₂ O / PH ₂ is maintained at 1.0 × 10 ⁻³ or less, and the electrolytic pickling is alternating electrolysis with a current density of 1 A / dm ² or more.