JP3584623B2 - Thin steel sheet with excellent corrosion resistance - Google Patents

Description

【００２７】
本発明に係る鋼板及び比較例鋼板の特性は、上記の焼鈍後の鋼板から採取した試料で引張り試験、成形性を評価するランクフォード値の測定、耐食性のための暴露試験を行い、評価した。
その際、引張り試験は、日本工業規格（ＪＩＳ Ｚ ２２０１、記載５号）に規定された定形試験片を製作して行われた。また、該鋼板のｒ（ランクフォード）値は、１５％の引張り予歪みを与えた後、３点法にて測定し、Ｌ方向（圧延方向）、Ｄ方向（圧延方向に４５度方向）、Ｃ方向（圧延方向に９０度方向）の平均値を、下記式によって求めた。
【００２８】
ｒ＝（ｒ_Ｌ ＋２ｘｒ_Ｄ ＋ｒ_Ｃ ）／４
さらに、耐食性は、前記した方法と同様に、鋼板の大気暴露を６ケ月間行い、暴露後の腐食試験前後の重量減及び腐食試験後の最大孔食深さで評価した。
これら試験の結果を表２に、また板表面から板厚の３％以内までの成分組成を表３に示す。表２及び表３によれば、本発明に係る鋼板は、全て優れた耐食性を有すると同時に、深絞り性も良いことがわかる。また、表２から明らかなように、本発明に係る鋼板は、高いｒ値を示している。一方、比較例として示した鋼板は、少なくとも加工性か、耐食性かのいずれか一方が劣っている。
【００２９】
【表２】

【００３０】
【表３】

【００３１】
【発明の効果】
以上説明したように、本発明により、鋼板中央部での成分組成と、表面から板厚の３％以内の厚さまで（表層）の成分組成のうちのＳｉ、Ｍｎ、Ａｌ含有量とを適切に調整し、耐食性及び深絞り性に優れた薄鋼板を提供することが可能となった。その結果、従来のように、「めっき」を施したり、Ｃｕ等の特殊元素を添加することなく、耐食性及び加工性に優れた薄鋼板が安価に提供できるようになった。ただし、厳しい耐食性の要求される鋼板に対しては、本発明に係る鋼板に、さらに「めっき」を施し、その要求に応じることも可能である。
【図面の簡単な説明】
【図１】鋼板表層のＳｉ含有量と該鋼板の孔食深さとの関係を示す図である。
【図２】鋼板表層のＭｎ含有量と該鋼板の孔食深さとの関係を示す図である。
【図３】鋼板表層のＡｌ含有量と該鋼板の孔食深さとの関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thin steel sheet made of ultra-low carbon steel, and more particularly, to a steel sheet which has excellent corrosion resistance even when subjected to bending, press forming, and drawing forming for an automobile body.
[0002]
[Prior art]
In recent years, demands for reducing fuel consumption of automobiles have been increasing for reasons such as energy saving and preservation of the global environment. In order to respond to these demands, there has been a strong demand for weight reduction of vehicle bodies. In order to reduce the weight of a vehicle body, a high-strength steel plate is required so that the required strength can be obtained even when the plate thickness is reduced. However, when the thickness of such a high-strength steel sheet is reduced, the corrosion resistance of the steel sheet tends to be reduced, and another problem of preventing a reduction in the corrosion resistance has become important.
[0003]
On the other hand, steel sheets having excellent corrosion resistance have been used for structural materials for automobiles, especially for underbody members. It is more excellent in terms of corrosion resistance because automobiles and the like are exposed to corrosive environments such as seawater splashes in coastal areas, road surface freezing in cold regions and salt sprayed for snow melting and perforated corrosion. This is because a steel sheet having characteristics has been required. As a steel sheet focused on corrosion resistance, various surface-treated steel sheets such as a hot-dip galvanized steel sheet have been developed and used.
[0004]
However, in the case of undercarriage parts or the like that are easily peeled off from painting or “plating”, the corrosion resistance effect of the adopted steel sheet is not yet sufficient, and it is necessary to improve the corrosion resistance of the steel sheet itself. Thus, for example, Japanese Patent Application Laid-Open No. Sho 62-243739 discloses a steel sheet in which Cu or Ni is added to a low carbon steel having a C content of 0.001 to 0.02% by weight to improve corrosion resistance. However, the addition of Cu or Ni rather deteriorates the surface properties of the steel sheet, requires maintenance such as grinding at the stage of the product sheet, and has a problem that productivity is reduced.
[0005]
In addition, there is a finding that "extremely purified steel has good corrosion resistance", but it is still difficult to reduce all the contained elements by current steelmaking technology. In other words, high-purity steel requires a lot of time for processing such as refining, and is disadvantageous in terms of cost and productivity. Therefore, a high-purity steel sheet aimed at improving corrosion resistance has not yet been put to practical use. In addition, there is an example of a steel sheet in which the S content is reduced to enhance the corrosion resistance (see CAMP-ISIJ, vol. 8 (1995) -1425), but it is very necessary to match the effect of adding Cu and P. Therefore, the idea of high purity is not fully utilized. In addition, such a highly purified steel sheet has a problem in that it has low strength and cannot be used for automobiles.
[0006]
[Problems to be solved by the invention]
In view of such circumstances, the present invention has sufficient corrosion resistance even when used for undercarriage members, reinforcing members, etc. of automobiles without performing “plating”, adding elements such as Cu and Ni, or performing high-purification. The purpose of the present invention is to provide the thin steel sheet shown in (1) at low cost and with high productivity.
[0007]
[Means for Solving the Problems]
The inventor has conducted intensive studies to achieve the above object, and found that if the composition of the same steel sheet is varied in the thickness direction, the corrosion resistance and deep drawability of the entire steel sheet become extremely good, and the present invention Was completed. That is, in the present invention, the component composition at the central portion of the plate thickness,
C: 0.003% by weight or less,
Si: more than 0.001% by weight to 0.03% by weight or less,
Mn: more than 0.001% by weight to 0.05% by weight or less,
S: 0.003% by weight or less,
Al: more than 0.002% by weight to 0.05% by weight or less,
N: 0.002% by weight or less,
O: 0.002% by weight or less,
And the sum of components other than iron is 0.10% by weight or less, and the component composition (surface layer) from the surface to a thickness of 3% or less of the plate thickness is:
Si: 0.001% by weight or less,
Mn: 0.001% by weight or less,
Al: 0.002% by weight or less,
This is a thin steel sheet having excellent corrosion resistance.
[0008]
In addition, the present invention, the component composition in the central part of the plate thickness,
Ti: 0.008% by weight or less,
Nb: 0.005% by weight or less,
One or two of these are additionally contained, or the component composition at the central portion of the plate thickness is further
B: 0.005% by weight or less,
Is also a thin steel sheet excellent in corrosion resistance characterized by additionally containing.
[0009]
In the present invention, since the composition of the thin steel sheet is a so-called “layer structure” as described above, the corrosion resistance of the entire steel sheet is significantly improved as compared with the conventional steel sheet. As a result, a thin steel plate that shows sufficient corrosion resistance even when used for undercarriage members, reinforcing members, etc. of automobiles without plating, addition of elements such as Cu and Ni, or special purification. Can be obtained at low cost and with high productivity.
[0010]
Hereinafter, embodiments of the present invention will be described, including the background of the research leading to the present invention.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the inventor assumed that C: 0.002% by weight, Si: 0.01% by weight, Mn: 0.03% by weight, S: 0.0015% by weight, Al: 0.024% by weight, and N: 0. A steel sheet having a composition of 0016% by weight, O: 0.001% by weight, the balance of iron and unavoidable impurities is heated and soaked at 1050 ° C., and then heated to a sheet thickness of 1.0 mm at a finishing temperature of 880 ° C. Rolled. Then, the steel sheet that had been hot-rolled without being pickled was subjected to reduction annealing for 20 seconds at an atmosphere temperature of 850 ° C. in a gas atmosphere of 10 vol% H ₂ -90 vol% N ₂ . This is intended to remove oxygen from the scale generated by hot rolling and adhering to the surface, changing the layer formed by the scale to a very Fe-rich composition. Further, in order to compare the properties with the above-described reduction-annealed steel sheet, a hot-rolled steel sheet annealed in an oxidizing atmosphere or an incomplete reducing atmosphere was also prepared.
[0012]
The steel sheet subjected to the heat treatment was exposed to the air for six months, and the corrosion resistance was evaluated by measuring the maximum pit depth before and after that. As a result, it was found that the corrosion resistance of the steel sheet annealed in the reducing atmosphere was much better than the corrosion resistance of the steel sheet annealed in the oxidizing or incomplete reducing atmosphere. When the cause of this improvement was examined in detail, it was discovered that there was a difference in the component composition between the central part and the surface part of the steel sheet depending on the annealing conditions. The difference was particularly remarkable in the contents of Si, Mn, and Al. In addition, the chemical analysis of the surface layer was performed on samples collected from within a thickness of 0 to 3% of the plate thickness.
[0013]
Examples of the measurement of the pit depth are shown in FIGS. 1 to 3 in relation to the amounts of Si, Mn, and Al in the surface layer. In FIGS. 1 to 3, the measured value when the reduction is sufficiently performed is indicated by a symbol 、, and the case where the reduction is incomplete is indicated by a symbol ○.
As is apparent from these figures, when the components of the surface layer satisfy simultaneously Si ≦ 0.001% by weight, Mn ≦ 0.001% by weight, and Al ≦ 0.002% by weight in the reduction annealing, the pit depth is reduced. Small, good corrosion resistance. In the surface layer, when any one of the three components deviates from the above range, the pitting depth becomes large, as indicated by the component values deviating from FIGS.
[0014]
Although the cause of this phenomenon is not clear at present, the oxide film formed by hot rolling was reduced by annealing, and the surface layer became very Fe-rich, and the strength and deep drawability were taken into consideration. It is only certain that this is related to the fact that the contents of Si, Mn, and Al contained in the surface layer were significantly reduced.
Therefore, the inventor made various studies based on the above experimental results, and determined the requirements as “a thin steel sheet excellent in corrosion resistance” according to the present invention as follows.
Component composition (1) Central part C of steel sheet: 0.003% by weight or less C is preferably as small as possible because the deep drawability is improved. In addition, with recent advances in steelmaking technology, there is no problem in setting the content to a so-called extreme bottom carbon region of 0.003% by weight or less, so the content was limited to 0.003% by weight or less.
[0015]
Si: more than 0.001% by weight to not more than 0.03% by weight Since Si has an effect of strengthening steel, it contains a necessary amount according to the strength desired by the user. If it exceeds, the deep drawability and the corrosion resistance deteriorate, so the upper limit was made 0.03% by weight. On the other hand, the lower limit is more than 0.001% by weight in order to obtain the strength of the molded product.
[0016]
Mn: more than 0.001% by weight to 0.05% by weight or less Since Mn has an effect of strengthening steel, it contains a necessary amount according to the strength desired by the user. Exceeding the above results in deterioration of deep drawability and corrosion resistance, so the upper limit was made 0.05% by weight. On the other hand, the lower limit is more than 0.001% by weight in order to obtain the strength of the molded product.
[0017]
S: 0.003% by weight or less S is preferably as small as possible because the deep drawability is improved. In addition, with recent advances in steelmaking technology, there is no problem in setting the content to a low region of 0.003% by weight or less, so the content was limited to 0.003% by weight or less.
Al: more than 0.002% by weight to 0.05% by weight or less Al is deoxidized in molten steel and is contained for improving the yield of carbon / nitride forming elements. If it exceeds, the deep drawability and the corrosion resistance deteriorate, so the upper limit was made 0.01% by weight. On the other hand, the lower limit is set to exceed 0.002% by weight in order to perform deoxidation with Al and adjust the oxygen amount in the steel to a predetermined amount.
[0018]
N: 0.002% by weight or less N is preferably as small as possible because the deep drawability is improved. Therefore, the range in which the current steelmaking technology can be realized is limited to 0.002% by weight or less.
O: 0.002% by weight or less O is also preferably as small as possible because the deep drawability is improved. Therefore, like N, the content is limited to 0.002% by weight or less.
[0019]
Ti: 0.008% by weight or less Ti is a carbon / nitride forming element, and precipitates and fixes solute C and N in steel as TiC and TiN to form a {111} orientation advantageous for deep drawability. To be added. However, even if it is added excessively, the effect as a carbon / nitride forming element saturates, and may rather increase the recrystallization temperature or deteriorate the surface properties. 0.0008% by weight or less. Preferably, 0.0005% by weight or more is added.
[0020]
Nb: 0.005% by weight or less Nb is a carbide forming element, and is added to precipitate and fix solid solution C in steel as NbC and form a {111} orientation advantageous for deep drawability. However, even if it is added in excess, the effect as a carbide-forming element is saturated, which may lead to a higher recrystallization temperature or a deterioration in surface properties. %. In addition, like Ti, addition of 0.0005% by weight or more is desirable.
[0021]
B: 0.0005% by weight or less B is added for improving the brittleness of secondary working when C is fixed by adding the above-mentioned Ti and Nb. The effect on secondary work brittleness saturates, and rather adversely affects deep drawability. Therefore, if added, the content is limited to 0.0005% by weight or less. The preferred lower limit is 0.00001% by weight.
[0022]
Since high-purity steels having a sum of components other than iron of 0.10% by weight or less have good corrosion resistance, the sum of the weight percentages of iron and unavoidable impurities has a great effect on corrosion resistance. Therefore, it is preferable to improve the purity of the steel sheet. However, in order to improve the purity, a long refining time is required and productivity is impaired. In addition, the strength of the steel is reduced, and the surface is easily roughened. Therefore, in the present invention, in order to prevent such adverse effects, the total content of additional elements other than Fe in the central part of the steel sheet is allowed up to a total of 0.10% by weight. To determine.
(2) From the plate surface to the thickness of 3% or less of the plate thickness (surface layer), the component composition in the surface layer is the most important requirement in the present invention. That is, in the component composition from the plate surface to a thickness of 3% or less of the plate thickness, the content of Si is set to 0.001% by weight or less, Mn to 0.001% by weight or less, and Al to 0.002% by weight or less. This is because they have been found to exhibit excellent corrosion resistance (see FIGS. 1 to 3).
[0023]
In addition, Si, Mn, and Al are included in necessary amounts as elements for strengthening the steel sheet, thereby securing deep drawability and strength. Therefore, the object is achieved by setting the above values at least from the surface layer to a thickness of 3% or less of the plate thickness.
Next, the reason why the surface layer of the steel sheet according to the present invention is set within the range of 3% or less of the sheet thickness from the sheet surface is that according to the experiment of the inventor, as described above, an excellent corrosion resistance effect is obtained in this range. Because. In addition, from the viewpoint of ensuring the strength of the steel sheet, it is not preferable to reduce Si, Mn, and Al over an unnecessary thickness. Therefore, the central part of the steel sheet in the present invention corresponds to the inner side other than the surface layer.
[0024]
Specifically, as described above, the method for reducing Si, Mn, and Al in the surface layer is to perform annealing of the steel sheet in a reducing atmosphere while the scale generated by hot rolling is adhered, thereby completely reducing the scale. is there. As for the reducing atmosphere, temperature, or annealing time, as shown in Examples described below, it is sufficient to use a known device and adopt known conditions.
Furthermore, in the present invention, it is not necessary to perform pickling after manufacturing a thin steel sheet of 1.2 mm or less, so that the pickling cost and the steps can be omitted. Further, depending on the use of the steel sheet, so-called CAL (continuous annealing) can be used. Line) or annealing in the “plating” line, there is no need for cold rolling, so that cold rolling cost can be reduced and the process can be omitted. Therefore, the present invention not only provides a steel plate having excellent corrosion resistance, but also enables cost reduction and productivity improvement through labor saving of the process, and is a truly revolutionary technology.
[0025]
【Example】
A steel slab having the chemical composition shown in Table 1 was heated and soaked at 1050 ° C., and then hot-rolled and finish-rolled to produce a hot-rolled steel sheet having a thickness of 1.0 mm. Then, the hot-rolled steel sheet was immediately charged into an annealing furnace having a furnace atmosphere of 90% N ₂ -10% H ₂ without pickling, and annealed at 900 ° C. for 20 seconds. Further, the annealed steel sheet was subjected to cold rolling, and again annealed under the condition of continuous annealing which is usually performed. Although the steel slabs in Table 1 include those out of the component range according to the present invention (indicated by underlines), here, these steel slabs are treated as comparative examples. Further, since the sheet thickness is small, the surface layer does not change significantly by cold rolling.
[0026]
[Table 1]

[0027]
The properties of the steel sheet according to the present invention and the comparative example steel sheet were evaluated by performing a tensile test, a measurement of a Rankford value for evaluating formability, and an exposure test for corrosion resistance on a sample collected from the above-described annealed steel sheet.
At that time, the tensile test was performed by producing a standard test piece specified in Japanese Industrial Standards (JIS Z 2201, description No. 5). The r (Rankford) value of the steel sheet was measured by a three-point method after giving a tensile prestrain of 15%, and was measured in the L direction (rolling direction), the D direction (45 degrees in the rolling direction), The average value in the C direction (90-degree direction in the rolling direction) was determined by the following equation.
[0028]
r = (r _L + 2 × r _D + r _C ) / 4
Further, the corrosion resistance was evaluated by exposing the steel sheet to the atmosphere for 6 months in the same manner as described above, and evaluating the weight loss before and after the corrosion test after the exposure and the maximum pit depth after the corrosion test.
Table 2 shows the results of these tests, and Table 3 shows the component compositions from the plate surface to within 3% of the plate thickness. Tables 2 and 3 show that the steel sheets according to the present invention all have excellent corrosion resistance and also have good deep drawability. Further, as is clear from Table 2, the steel sheet according to the present invention has a high r value. On the other hand, the steel sheet shown as a comparative example is inferior in at least one of workability and corrosion resistance.
[0029]
[Table 2]

[0030]
[Table 3]

[0031]
【The invention's effect】
As described above, according to the present invention, the component composition at the central portion of the steel sheet and the Si, Mn, and Al contents of the component composition (surface layer) from the surface to a thickness of 3% or less of the sheet thickness are appropriately adjusted. It is possible to provide a thin steel sheet which is excellent in corrosion resistance and deep drawability. As a result, a thin steel sheet excellent in corrosion resistance and workability can be provided at low cost without performing “plating” or adding a special element such as Cu as in the related art. However, it is also possible to apply a “plating” to the steel sheet according to the present invention for the steel sheet requiring strict corrosion resistance to meet the demand.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the Si content of a steel sheet surface layer and the pitting depth of the steel sheet.
FIG. 2 is a diagram showing the relationship between the Mn content of the steel sheet surface layer and the pitting corrosion depth of the steel sheet.
FIG. 3 is a diagram showing the relationship between the Al content in the surface layer of a steel sheet and the pitting corrosion depth of the steel sheet.

Claims (3)

The component composition at the center of the plate thickness is
C: 0.003% by weight or less,
Si: more than 0.001% by weight to 0.03% by weight or less,
Mn: more than 0.001% by weight to 0.05% by weight or less,
S: 0.003% by weight or less,
Al: more than 0.002% by weight to 0.05% by weight or less,
N: 0.002% by weight or less,
O: 0.002% by weight or less,
And the sum of components other than iron is 0.10% by weight or less;
The component composition from the surface to the thickness within 3% of the plate thickness is
Si: 0.001% by weight or less,
Mn: 0.001% by weight or less,
Al: 0.002% by weight or less,
Thin steel sheet with excellent corrosion resistance. In the component composition at the center of the plate thickness,
Ti: 0.008% by weight or less,
Nb: 0.005% by weight or less,
The steel sheet excellent in corrosion resistance according to claim 1, characterized in that one or two of them are additionally contained. In addition to the component composition at the center of the plate thickness ,
B: The thin steel sheet having excellent corrosion resistance according to claim 1 or 2, further comprising 0.005% by weight or less.

Images