JP3312103B2 - High strength hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hot-rolled steel sheet,
Particularly, the present invention relates to a hot-rolled steel sheet which is used for an automobile body and the like, and is subjected to hot-dip plating, electroplating, alloying treatment, chemical conversion treatment, chromate treatment and the like as required, and has high strength.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of emission control, it has been desired to reduce the weight of a vehicle body and reduce fuel consumption. One of the effective means for reducing the weight is to reduce the thickness of the plate. However, in order to ensure safety, it is necessary to increase the strength of the plate itself by reducing the thickness. And actually, Mn, Si, Mn, Cr, C, P, etc. are contained in steel, and the high strength of a steel plate is achieved.

[0003] Such a high-strength hot-rolled steel sheet is usually subjected to pickling treatment of an oxide film (commonly referred to as black scale) generated by rolling or heat treatment, and then to plating or chemical conversion treatment for imparting corrosion resistance. For example, when performing hot-dip plating, in order to remove an extremely thin, so-called invisible oxide film formed on the surface after the pickling treatment, a steel sheet is charged in an annealing furnace in advance, and N ₂ -H
_A reduction process is performed in a _two- gas atmosphere. However, when the temperature in the furnace is low and the reduction is insufficient, an oxide film remains and non-plating occurs. If the temperature at which the remaining oxide film is re-reduced is too high, the S in the steel after the reduction is too high.
i, Mn, Cr and the like are concentrated as oxides on the steel surface, and also in this case, non-plating occurs. That is, Si, Mn, C
If the content of r or the like is large, there is no appropriate reduction temperature range for suppressing non-plating.

[0004] Also, for example, when a large amount of Si, Mn, Cr or the like is contained in steel during electroplating or chemical conversion treatment, the "non-plating" phenomenon and the chemical conversion treatment liquid do not wet the steel sheet surface. A so-called “hajiki” often occurs. That is, a high-strength hot-rolled steel sheet having excellent plating properties, chemical conversion properties, and the like has been difficult to obtain easily. Among the above-mentioned problems, the improvement of hot-dip plating is described in JP-B-61-93.
No. 86 states that "N-coated on the surface of steel sheet prior to hot-dip plating.
i. Undercoat plating ”. However, in this method, C: 0.001 to 0.30 wt
%, Si: 0.001 to 3.0 wt%, Mn: 0.1 to
In the case of a steel sheet containing 3.0 wt% and Cr: 0.001 to 2.0 wt%, it is necessary to perform Ni plating so as to have an adhesion amount of 10 g / m ² or more. Invite a rise. When such a large amount of Ni plating is applied, even if the wettability of hot-dip galvanizing is improved, Si, Ni
Another problem arises in that defects caused by the frequent occurrence occur.

In addition to undercoating Ni, for example,
Japanese Patent Laying-Open No. 57-70268 proposes a technique of applying a base plating of Fe to the surface of a steel sheet prior to hot-dip plating. However, by adopting this method, it was possible to prevent the non-plating of the Si-added steel. However, in this case, it is necessary to perform Fe plating so that the adhesion amount becomes 5 g / m ² or more. It was economic.

Other techniques for improving the plating properties other than those described above include JP-A-55-122865 and JP-A-4-25.
No. 4531 discloses a method of “oxidizing a steel sheet in advance to form an iron oxide film, and then performing reduction annealing to suppress the formation of a film of an alloying element oxide before plating.”
In this method, the thickness of the iron oxide film was reduced to a certain value or less before plating by reduction annealing. However, the iron oxide film was excessively reduced by reduction annealing, and the alloy elements were concentrated on the surface and the plating was performed. However, there is a problem that the properties are poor, that is, the balance between the oxide film and the reduction amount is lost. Moreover, in order to prevent this excessive reduction, an enormous amount of iron oxide film is required, but the iron oxide film is peeled off by a rolling roll or the like. Therefore, during the subsequent reduction annealing, there has been a problem that the selective oxidation of the alloy element occurs to impair the plating property, and the peeled-off iron oxide film is scattered in the annealing furnace, which adversely affects the plating operation.

On the other hand, a steel sheet for automobiles is particularly required to have good pretreatment for coating, that is, good chemical conversion treatment. If the chemical conversion property is poor, the subsequent baking coating will not be successful. However, since a large amount of elements such as C, Si, Mn, and Cr are contained for the purpose of increasing the strength of the steel sheet, there has been a problem that the above-mentioned “repelling” occurs and the chemical conversion treatment property is significantly deteriorated. The reason is the oxide film (black scale)
This is because the Fe-based invisible oxide film formed on the surface of the steel sheet after pickling and removing contains SiO ₂ , MnO, Cr ₂ O _3, etc., and reduces the wettability of the steel sheet.

When such a hot-rolled steel sheet is used as a steel sheet for household electrical appliances other than automobiles, a so-called chromate treatment may be applied in order to improve corrosion resistance. However, usually, as in the case of the chemical conversion treatment, due to C, Si, Mn, Cr, etc. contained for increasing the strength of the steel sheet,
There is also a problem that chromate treatment property is remarkably deteriorated.

[0009]

The present invention invention is to solve the above, in view of such circumstances, and ingenuity to the base material to the steel plate, during the electroplating of "non-plating" and, at the time of formation process or during chromate treatment "cratering" It is an object of the present invention to provide a high-strength hot-rolled steel sheet which does not cause unevenness or “unevenness”.

[0010]

As described above, C,
Various surface treatments on high-strength hot-rolled steel sheets containing elements such as Si, Mn, Cr, etc.
In the case of performing chromate treatment, these elements are present as oxides on the surface during annealing or pickling of the steel sheet and hinder wettability with molten zinc or a chemical conversion treatment solution, so that non-plating occurs. Also, there is a correlation between the amount of surface enrichment of these elements and the plating properties and the degree of alloying. It was also confirmed that the smaller the amount of surface enrichment, the better the plating properties and the faster the alloying speed. Have been.

Therefore, the inventor has conducted detailed studies on the surface layer structure of a steel sheet that suppresses the surface concentration of the above elements. As a result, a high-strength hot-rolled steel sheet containing a specified component is contained in the crystal grain boundaries and / or crystal grains in the surface layer portion.
If various types of oxides of the specific component are generated in advance ,
Electric plating resistance, Kasei processability, chromate treatment properties were found to be dramatically improved. The present invention, which embodies this finding, a hot rolled steel sheet is subjected to an electroplating or chemical treatment or chromate treatment, C: 0.001~0.30wt% Si: 0.001~3 . 0wt% Mn: 0.1~3.0wt% Cr: containing 0.001~2.0wt%, the steel sheet and the balance of iron and unavoidable impurities steel
A high- grain material comprising an oxide containing at least one of Si, Mn, Cr, and P in a grain boundary and / or in a crystal grain from the plate surface layer to a depth of 0.1 to 100 μm. It is a high-strength hot-rolled steel sheet.

Further, in the present invention, the steel sheet may further comprise:
0.1 wt% or less, or the oxide is SiO ₂ , MnO, FeSiO ₃ , Fe ₂ SiO
₄ , MnSiO ₃ , Mn ₂ SiO ₄ , P ₂ O ₅ , Cr ₂
O ₃ , FeCr ₂ O ₄ , (Fe, Mn) O, (Fe, C
r) ₂ O ₃ , (Fe, Mn) SiO ₃ and (Fe, M
is a high-strength hot-rolled steel sheet, characterized in that n) selected from ₂ SiO ₄ is at least one.

Furthermore, the present invention, the oxygen content of the entire steel plate is also a high-strength hot-rolled steel sheet being higher than 1ppm than before to produce the oxide layer. According to the present invention, since the oxide of the specific component present in the surface layer portion, "non plating" during main Tsu key or Kasei process, causing a "cissing" and "unevenness" during chromate treatment without,
Electric plating resistance, Kasei processability, it becomes possible to produce a superior high-strength hot-rolled steel sheet to a chromate treatment resistance.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The oxides present in the crystal grain boundaries and / or in the crystal grains in the surface layer of the high-strength hot-rolled steel sheet according to the present invention are generated when the steel sheet is hot-rolled. In particular, it grows when the temperature at which the steel strip is coiled is high and the subsequent cooling rate is low. The observation result of the grain boundary oxide formed during the hot rolling is shown in FIG. 1 in a state where it exists immediately below the so-called black scale. In addition, it is considered that these oxides are generated when the steel strip itself is still at a high temperature in a state where the steel strip is wound in a coil shape and is shielded from the surface atmosphere.
That is, oxygen is dissociated from FeO or the like, which is the main component of the black scale, and the surface layer is internally oxidized by the dissociated oxygen. FIG. 2 shows the result of analyzing this oxide by EPMA. Peaks of Si, Mn, Cr, P, and O are found at the crystal grain boundaries, and the formation of these oxides is confirmed. Incidentally, the rolling conditions oxides are produced, the take-out Installing conditions, as described in the Examples below.

In the high-strength hot-rolled steel sheet according to the present invention, the surface layer is internally oxidized in advance as described above, and these elements are distributed as oxides in the steel. Therefore, Si in the invisible oxide film after pickling, Mn, since oxides of Cr is not contained, wettability of the steel sheet is not deteriorated, electroplating, chemical conversion, "cissing" upon chromate treatment and "uneven Does not occur.

[0016]

[0017] In this case, electroplating, as well as electro-galvanized, electro-zinc - nickel plating, electrolytic zinc -
Iron plating or the like may be used. As described above, since the oxide of Si, Mn, and Cr is not contained in the invisible oxide film after pickling, the wettability is not deteriorated,
This is because "repelling" and "unevenness" do not occur during the chemical conversion treatment and the chromate treatment.

[0018]

The same applies to the influence of the annealing temperature. When an oxide layer is formed, SiO ₂ , MnO, and C are contained in the Fe-based invisible oxide film formed on the steel sheet surface after pickling.
r ₂ O _{3 and the} like are not included. Therefore, to be reduced even to completely Fe system invisible oxide film in a low temperature annealing of about 550 ° C., In addition, since the surface segregation in 800 over ° C. high temperature annealing does not occur, can plating without any problem even if the former annealing instead .

Next, the reasons for limiting the components employed in the present invention and other conditions will be described. Si, to that a lower limit on the amount of Mn or Cr is used when less than this, Si is contained in the invisible oxide film after pickling, less oxide content of Mn or Cr, a normal cell base This is because electroplating is possible in a typical sulfuric acid bath or hydrochloric acid bath. The same applies to the chemical conversion property and the chromate treatment property, particularly because the chemical conversion treatment and the chromate treatment can be similarly performed without changing the conventional equipment and the conventional method . Its Reyue, the present invention is, S
The i content was 0.001 wt% or more, the Mn content was 0.1 wt% or more, and the Cr content was 0.001 wt% or more. Also, 0.0010 wt% or more of the C content is an unavoidable content in a normal steel making method.

On the other hand, the upper limit of the amount of Si is 3.0 wt%,
The upper limit of the amount is 3.0 wt% and the upper limit of the Cr amount is 2.0 wt%.
That is given by, beyond these numbers, even when pickling even when grilled blunt, oxide film produced on the surface of the steel sheet, the plating bath and the chemical conversion treatment liquid, for significantly reducing the adhesion between the chromate treatment liquid It is. Further, the upper limit of the C content is set to 0.30 wt%. If the upper limit is higher than that, the hardness of the steel sheet increases, the ductility decreases, and some of mechanical properties such as deep drawability decrease, which is not preferable. Because.

P is essential for high-strength hot-rolled steel sheets because P has little effect on deep drawability and can harden steel sheets, and B has a great effect on secondary work brittleness of steel. Element. However, these elements are zinc reduction after annealing and the electrical plating after pickling, chemical conversion treatment, does not significantly inhibit the wettability such as chromate. Furthermore, even if the degreasing pickling after the reduction annealing is not sufficient and the acid remains on the surface layer, "non-plating", "repelling", "chemical conversion treatment unevenness"
It is not likely to cause any problems. However, if P is contained in an excessively large amount, it may cause alloying delay, so that 0.10 wt% or less is desirable. For B, there is no particular limitation on the content.

In addition, the thickness of the oxide layer is 0.1 to 100
The reason why the thickness is limited to μm is that when the thickness is less than 0.1 μm, the amount of oxide itself is small, so that deterioration of wettability cannot be suppressed. When the thickness exceeds 100 μm, the oxide is brittle. This is because the mechanical properties of the steel sheet itself may be reduced. In addition, if an oxide layer is formed on the surface layer even if it is slight, the above-described effect of preventing “unplating” or the like can be obtained. When the oxygen content in steel is measured by some analysis method, for example, "impulse furnace melting-infrared absorption method",
In a steel sheet in which even a small amount of oxide layer is formed, the oxygen content of the entire steel sheet is 1 ppm lower than that before forming the oxide layer.
The above was increasing. If the amount of increase in oxygen was less than 1 ppm, it was not effective in improving the plating property and the like. Therefore, in the present invention, due to the presence of the internal oxide, the oxygen content of the entire steel sheet is reduced by 1 pp compared to before the oxide layer was formed.
m or more is also a condition of the oxide layer.

The presence or absence of oxide formation on the steel sheet according to the present invention is determined by the following method. The amount of oxygen in the entire hot-rolled steel sheet after pickling the black scale and the same hot-rolled steel sheet surface as 0.5 m
The surface was polished and removed, and the surface was polished. The amount of oxygen was compared with that of the polished surface. In addition, the presence or absence of oxide formation can be confirmed by polishing the surface of the steel sheet at the thickness portion and observing the surface layer of the steel sheet with an optical microscope as a black streak pattern in the oxide layer at the crystal grain boundary. Further, the polished surface may be lightly etched with a 1% nital solution for about 10 to 20 seconds.

[0025]

After EXAMPLES slab was hot-rolled to a steel sheet, pickling for mill scale removal, the seed surface treatment is sequentially performed, various surface-treated steel sheets such as those listed in Table 1 are produced Was. The conditions at that time are as follows: the heating temperature of the slab is 1200 to 1250 ° C .;
Temperature of the extending of the finish rolling is eight hundred sixty to nine hundred and ten ° C., coiling temperature of the steel strip is six hundred to seven hundred sixty ° C..

[0026] The electrical plating, chemical conversion treatment, is the time of chromate treatment, the black skin scale of hot-rolled steel sheet was used as the hot-rolled steel sheet after pickling removal process, each electroplating, chemical conversion treatment, performs a chromate treatment, The surface treatment properties are evaluated.

[0027]

[Table 1]

[0028] Also, for the surface treatment, a variety of electrical plating, chemical conversion treatment, chromate treatment was used. In addition,
Various surface treatments of plating or chemical conversion treatment or the like has been performed steel sheet, by visual observation, and the like whether there is "adhesion amount unevenness" is determined.

Among the various surface-treated steel sheets produced as described above, those corresponding to the hot-rolled steel sheets according to the present invention are shown in Table 2.
Table 3 collectively shows comparative examples of conventional steel plates and the like .
As apparent from these Tables, in steel sheets as comparative examples
“ Adhesion amount unevenness” occurs, and the oxide layer has a thickness of 150 μm.
If thick, the mechanical properties of the steel sheet itself deteriorates,
Various problems occurred. However, the steel sheet according to the present invention, "adhesion amount unevenness", let alone not occur deterioration any trouble in the mechanical properties of the steel sheet itself.

[0030]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[0034]

As described above, according to the present invention, a high-strength hot-rolled steel sheet which can be easily subjected to various surface treatments can be manufactured.

[Brief description of the drawings]

FIG. 1 is a view showing a cross section in a thickness direction of a hot-rolled steel sheet according to the present invention.

FIG. 2 is a view showing an analysis result by EPMA of the cross section.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-276060 (JP, A) JP-A-8-291367 (JP, A) JP-A-9-310163 (JP, A) JP-A-10- 17936 (JP, A) JP-A-9-310148 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C23C 2/00-2/40 C21D 9/46-9/48

Claims (4)

(57) [Claims] 1. A hot rolled steel sheet is subjected to an electroplating or chemical treatment or chromate treatment, C: 0.001~0.30wt% Si: 0.001~3.0wt % Mn: 0.1~ 3.0 wt% Cr: containing 0.001~2.0wt%, the steel sheet and the balance of iron and unavoidable impurities steel
A high- grain material comprising an oxide containing at least one of Si, Mn, Cr, and P in a grain boundary and / or in a crystal grain from the plate surface layer to a depth of 0.1 to 100 μm. High strength hot rolled steel sheet. 2. The steel sheet further contains P in an amount of 0.1 wt%.
The high-strength hot-rolled steel sheet according to claim 1, wherein the steel sheet contains: 3. The oxide is SiO ₂ , MnO, Fe.
SiO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ ,
P ₂ O ₅ , Cr ₂ O ₃ , FeCr ₂ O ₄ , (Fe, Mn) O,
(Fe, Cr) ₂ O ₃ , (Fe, Mn) SiO ₃ and (F
3. The high-strength hot-rolled steel sheet according to claim 1, wherein the high-strength steel sheet is at least one member selected from the group consisting of e, Mn) ₂ SiO ₄ . 4. Due to the presence of the oxide, the entire steel sheet has an acid.
Element content is 1 ppm compared to before forming the oxide layer.
The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the number increases .