JPH059493B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a hot-dip aluminum plated steel sheet with excellent strength and oxidation resistance at high temperatures, and a method for manufacturing the same. In particular, the present invention relates to a molten aluminum-plated low alloy steel sheet that can replace AISI 409 and 410 as a material for automobile exhaust gas systems, and a method for producing the same. Conventionally, molten aluminum-plated steel sheets are broadly classified into heat-resistant and corrosion-resistant steel sheets, and the former is usually called a molded aluminum-plated steel sheet, and the latter is called a molded aluminum-plated steel sheet. In heat-resistant type aluminum steel sheets, the presence of a small amount of Si in the Al coating suppresses the development of the Fe-Al alloy layer during high-temperature heating, thereby improving the heat resistance of the plated steel sheet. Ru. However, even with this type of aluminum-plated steel sheet, the service temperature of conventional aluminum-plated steel sheets is usually approximately 600° C. or less.
On the other hand, the aluminum-plated steel plate of the mold uses pure Al as a coating material, and has superior corrosion resistance compared to the mold, but inferior heat resistance. Such molten aluminum-plated steel sheets are usually made by applying molten aluminum plating to a cold-rolled steel sheet such as aluminum-killed steel or rimmed steel. Slabs are usually subjected to a hot rolling process, a descaling process, a cold rolling process, an annealing process and a molten aluminum plating process. It is generally carried out by passing a base cold-rolled steel sheet through a Sendzimer-type molten aluminum plating line. Japanese Patent Publication No. 53-15454 and the corresponding US Pat. No. 3,881,880 disclose that the carbon content is approximately
The base material is an aluminium-killed carbon steel of 0.03 to about 0.25% by weight to which sufficient titanium is added to fix all the carbon in the steel and leave about 0.1 to about 0.3% by weight of unbonded titanium. We suggest applying molten aluminum plating. According to this, carbon is precipitated as titanium carbide, and there is virtually no carbon solidly dissolved in the iron.Therefore, an iron base that is close to pure iron is obtained, and this aluminum-plated steel sheet is heated at high temperatures. It is taught that when the steel is heated to a temperature of 100%, Al in the plating layer easily diffuses into the steel base, and as a result, the high-temperature oxidation resistance of the steel base surface is enhanced. However, in recent years, the automobile manufacturing industry has developed materials with sufficient oxidation resistance at high temperatures as well as excellent high-temperature strength (for example, 600
Tensile strength at °C is 13Kgf/ ^mm2 or more, preferably 15Kg
f/mm ² or higher) and which can replace AISI 409 and 410 stainless steel, there has been a demand for hot-dip aluminum plated steel sheets. The aforementioned U.S. patent no.
No. 3,881,880 does not teach how a hot-dip aluminum-plated steel sheet that satisfies this high-temperature strength in addition to the oxidation resistance as taught can be industrially advantageously produced. One object of the present invention is to establish an industrially advantageous method for producing hot-dip aluminum-plated steel sheets with excellent strength and oxidation resistance at high temperatures. Another object of the present invention is to provide a hot-dip aluminum plated steel sheet that has excellent strength and oxidation resistance at high temperatures. According to the present invention, the advantageous effect of titanium addition on oxidation resistance taught in the above-mentioned Japanese Patent Publication No. 53-15454 and the corresponding US Pat. No. 3,881,880 is achieved by minimizing the carbon content It was found that even if appropriate amounts of Si and Mn were added as alloying elements to the base steel, the targeted high-temperature strength could be developed without being inhibited. This means that it is possible to obtain an iron base close to pure iron by minimizing the alloy components other than Ti, from the plating layer to the base material surface layer.
This is surprising considering the disclosure in the specification that Al is important in promoting Al diffusion. Si and
Both Mn is known as an element that increases the strength of steel, and it is also known that Ti-added steel has an increased secondary recrystallization temperature. Therefore, Ti addition
It is predictable that Si--Mn steel will be able to develop sufficient strength at high temperatures up to the secondary recrystallization temperature. However, such cold-rolled steel sheets of Ti-added Si-Mn steel are manufactured under normal conditions on a normal industrial production line, and then
Attempts to commercially produce hot-dip aluminum-plated steel sheets with superior strength and oxidation resistance at high temperatures by passing them through a Sendzimir-type hot-dip aluminum plating line equipped with in-line annealing equipment have failed. . The obtained product is
It had dotted areas and did not exhibit oxidation resistance at high temperatures. According to the present invention, Ti
If Si-Mn-added steel is used as the base material and the hot-rolling temperature in the manufacturing process is controlled to a low temperature that prevents oxidation of Si and Mn in the steel, it is possible to obtain sufficient oxidation resistance and strength at high temperatures. It has been found that industrial products of enhanced hot-dip aluminum coated steel sheets can be manufactured effectively. Thus, the present invention provides a method for industrially manufacturing hot-dip aluminum-plated steel sheets based on such Ti-added ultra-low carbon Si-Mn steel, in which carbon and nitrogen in the steel are sufficiently fixed and A slab of steel with sufficient titanium added so that unbonded titanium remains in the steel is manufactured and subjected to hot rolling, descaling, cold rolling, annealing and molten aluminum plating baths. In a normal method for manufacturing a hot-dip aluminum-plated steel sheet in which a hot-dip aluminum-plated steel sheet is obtained by sequentially subjecting to a dipping process, the slab contains, in weight percent, C: 0.020% or less, Si: 0.1 to 2.2%, Mn: 2.5. % or less, 1.9×(%Si)+0.9×(%Mn)≧1 and (%
Mn)≧0.5×(%Si), Ti; 0.1-0.5% and (%
Ti)/(%C+%N)≧10, Al; 0.01-0.1%,
Using a Ti-added Si-Mn steel slab consisting of N: 0.010% or less, the balance being Fe and unavoidable impurities, and controlling the temperature of the hot-rolled material wound in the hot rolling process to a sufficiently low temperature. Provided is a method for producing a hot-dip aluminum plated steel sheet, characterized in that a steel surface substantially free of internal oxidation layers is obtained after the descaling step. Ti-containing ultra-low carbon Si-Mn steel containing Si and Mn added as alloying elements in amounts within the range specified by the present invention is subjected to finish hot rolling, followed by heat treatment normally used for this type of steel. When winding at extended winding temperature,
During cooling after coiling (normally this cooling is done by standing), the scale that inevitably exists on the steel surface oxidizes the Si and Mn dissolved in the steel, resulting in the formation of Si and Mn. It was found that there is a problem in that Mn oxides precipitate at grain boundaries in the surface layer of steel, or at grain boundaries and inside grains. Such oxidation of Si and Mn within the steel is referred to herein as "internal oxidation." Internal oxidation is limited to the surface layer of steel, but depending on the conditions (steel composition, coiling temperature,
The depth ranges from several microns to several tens of microns depending on the cooling rate after winding, etc.). As mentioned above, Si and Mn oxides generated by internal oxidation precipitate at the grain boundaries of the steel surface layer or between the grain boundaries and within the grains, and although they do not necessarily form a continuous layer, the generated internal oxidation In this specification, the entire object is referred to as an "internal oxidation layer".
I will call it. This is completely different from the scale on the steel surface, both in composition and form. The details of the present invention will be explained below. FIG. 1a is a schematic enlarged cross-sectional view of the surface of a Ti-containing ultra-low carbon Si-Mn steel added with Si and Mn in amounts within the range defined by the present invention, just before hot rolling and winding. Scale 2 is generated on the surface of the steel base material 1. This scale 2 is usually called a secondary scale. The scale that forms on the surface of a slab heated to high temperatures in a heating furnace is called primary scale.
Most of it is removed from the surface during the rolling process.
Most of the secondary scale 2 is generated between the finish rolling mill and the coil winding machine. Figure 1b shows that the hot-rolled material with secondary scale 2 in Figure 1a is coiled at a temperature exceeding 600°C (for example, approximately 700°C).
FIG. 3 is a similar cross-sectional view when the film is wound up at 10°C and left to cool. During cooling, internal oxidation occurs at the grain boundaries and within the grains in the surface layer of the steel. This oxide is an oxide of Si and Mn produced by oxygen being supplied into the steel from the scale 2 made of iron oxide and reacting with Si and Mn in the steel. FIG. 1c is a similar cross-sectional view of the hot rolled material of FIG. 1b after descaling by pickling.
Although scale 2 is removed by pickling, the oxides precipitated within the grains remain. A portion of the oxide precipitated at the grain boundaries is removed, and gaps 3 are formed at the grain boundaries. FIG. 1d is a similar cross-sectional view of the descaled hot rolled material of FIG. 1c after cold rolling. The surface of the cold-rolled material is not smooth, and the gap 3 is enlarged and deformed by cold rolling. The internal oxidation layer remains even after cold rolling.
Rolling oil and other foreign substances used in the cold rolling process tend to enter the gap 3 on the surface of the expanded and deformed cold-rolled material, and these foreign substances may not be completely removed even by annealing on the plating line. . FIGS. 1e and 1f are similar sectional views after the cold rolled material in FIG. 1d has been passed through an in-line annealing type molten aluminum plating line to be plated with molten aluminum. If foreign matter that has entered the grain boundary "gap" on the surface of the cold-rolled material is not completely removed by annealing on the plating line, it may occur as shown in Figure 1 e.
As seen in Figure 2, the aluminum coating may not adhere to that area. In Figures 1e and f, 4 is an aluminum plating coating layer (Al-Si
layer), 5 is an Al--Fe--Si alloy layer formed at the interface between the aluminum plating coating layer 4 and the steel base material 1, and 6 in FIG. 1e indicates a defect. FIGS. 2a and 2b are micrographs (400x magnification) before and after plating of cases where such defects occurred. Even if such dissatisfaction does not occur, as shown in Figure 1 f,
The thickness of the Al-Fe-Si alloy layer 5 formed at the interface between the Al-Si coating layer 4 and the steel base material layer 1 tends to be larger than in the normal case. This is thought to be because the surface area of the cold-rolled material becomes larger than it appears due to the presence of the grain boundary "gap 3". If the thickness of the Al--Fe--Si alloy layer 5 is large, peeling of the plating tends to occur when a molten aluminum-plated steel sheet is processed. Moreover, the deep part (tip) of "gap 3" tends to become "void 7" even after plating. The presence of voids 7 also causes plating and peeling. FIG. 3a is a micrograph (magnification: 400 times) of the plating material corresponding to FIG. 1f. The internal oxide layer is not reduced by the reducing annealing atmosphere at the plating line and still remains after plating, as seen in the photograph of FIG. 3a and shown in the schematic diagram of FIG. 1f. Such internal oxidation layers (film-like oxides at grain boundaries and oxide particles within grains) prevent Al from diffusing from the plating layer into the base steel when the aluminum-plated steel sheet is heated to high temperatures. As a result, the high-temperature oxidation resistance that the addition of Ti had tried to improve was obstructed. Figure 3b shows the plating material shown in Figure 3a being placed in the atmosphere at 800°C.
This is a micrograph at the same magnification after heating at ℃ for 20 hours. According to the figure, it can be seen that the presence of an internal oxidation layer significantly inhibits the high-temperature oxidation resistance of the aluminum-plated steel sheet. Further, due to such high temperature heating, the internal oxidation layer itself also progresses deeper into the steel base material. In summary, if an internal oxidation layer consisting of Si and Mn oxides exists in the surface layer of the base material of a hot-dip aluminum coated steel sheet, (1) the smoothness of the cold-rolled material surface will be significantly impaired; (b) Increases the thickness of the Al-Fe-Si alloy intermediate layer due to increased surface area; (c) weakens the plating bond strength; When a galvanized steel sheet product is heated to high temperatures, this internal oxidation layer prevents the formation of an Al diffusion layer, deteriorating the product's high temperature oxidation resistance and causing the glare layer to peel off. Therefore, for the purpose of the present invention, it is important to obtain a steel surface substantially free of a high-temperature oxidation layer after the descaling process in producing a base steel for hot-dip aluminum-plated steel sheets. According to the present invention, this can be achieved by controlling the temperature of the steel sheet to be sufficiently low just before the steel sheet is wound onto the coiler during the hot rolling process. In order to find out where the upper limit of the permissible winding temperature lies, the following laboratory tests were conducted. Table 1 shows the test steel plates (thickness 1.0mm)
Indicates the chemical composition value of Each sample material was prepared from each molten steel by forging, hot rolling (thickness: 7.0 mm), grinding (thickness: 5.0 mm), and cold rolling (thickness: 1.0 mm).

[Table] A test steel plate was heated to a predetermined high temperature in the air for 20 hours, and the depth of the internal oxidation layer formed was measured by microscopic observation. Tests were conducted at 550, 600, 650 and 700°C. The results are shown in Figure 4.
Figure 4a shows the results at 550℃, but the internal oxidation layer is
Does not occur regardless of the Si and Mn contents. Figure 4b shows the results for the case of 600°C, and although some internal oxidation layers were formed in sample steel Nos. 5, 6 and 7, they were not formed in the other sample steels. in this case,
There is no difference due to increase or decrease in the content of Si and Mn, but this is probably due to the difference in the form of scale formed on the surface. At 650°C as shown in Fig. 4c, a deep internal oxidation layer is formed in all steels except for test steel Nos. 1 and 3, which have extremely low Si and Mn contents. At 700℃ in Figure 4 d, the test steel
In all cases other than No. 8, a deeper internal oxidation layer is formed. According to the test results, in order to obtain a steel surface with substantially no internal oxidation after the descaling process, the coiling temperature in the hot rolling process should be about 600°C or less, preferably about 570°C or less, and most preferably about 570°C or less. It seems necessary to control the temperature to below about 550°C. The lower limit of the winding temperature is not critical and depends on the coiler's capabilities. Usually about
Winding at temperatures lower than 400°C is not practical. In actual operation, the hot-rolled coil produced in the hot-rolling process is most commonly cooled by air cooling treatment while being wound, except in special cases.
This cooling time is usually 2 to 3 days. The formation of internal oxidation depends on the content of Si and Mn in the steel and the coiling temperature, but also affects the cooling rate of the hot-rolled coil. FIG. 5 conceptually shows the relationship between the occurrence of internal oxidation and the cooling curve of a hot-rolled coil. For steels with certain Si and Mn contents, the occurrence of internal oxidation is shown by curve A. That is, internal oxidation occurs under the conditions indicated by the points in the region above curve A. Curve B shows the cooling curve of the hot rolled coil. In the present invention, it is important to control the winding temperature sufficiently low so that curve B does not intersect with curve A. Hot rolling coiling temperature has traditionally had an important meaning as a prescription for controlling the physical properties of the obtained steel sheet. In the case of Ti-added low carbon steel, in which Ti is precipitated as Ti carbonitride to improve the ductility and workability of the steel, by increasing the coiling temperature, for example, temperatures exceeding 600℃, More preferably
The aim was to control the size of Ti carbonitrides within an appropriate range by increasing the temperature to 700°C or higher. In the present invention, carbon and nitrogen are fixed by Ti, but the strength of the steel is not increased to the target value by precipitation strengthening by carbonitrides (in the present invention, the C content is 0.02
%), Si and Mn
The purpose is to improve strength by actively adding an appropriate amount of . An aluminum-plated steel sheet was manufactured on an industrial scale by actually applying the coiling temperature conventionally recommended for Ti-added steel to the Ti-containing ultra-low carbon Si--Mn steel according to the present invention. However, as shown in Failure Example A of Example 1 below, this product was defective as an aluminum-plated steel sheet product. The inventors of the present invention have conducted repeated research to overcome this reality, and as mentioned above, they have discovered the cause of this problem in the internal oxidation layer, and have traditionally recommended this for Ti-added steel as a prescription to suppress the formation of this internal oxidation layer. They discovered that it is essential to use low-temperature winding as opposed to high-temperature winding. Note that the hot rolling process consists of rough rolling and finish rolling of the slab and winding it into a coil, and includes performing a primary scale removal process during this time. The descaling process after hot rolling is a normal chemical or mechanical descaling process, and is basically a process to remove secondary scale that is inevitably generated during the hot rolling process. Most commonly, pickling is carried out.
As mentioned above, the internal oxide layer is not removed in this descaling step. The cold rolling process is a process of cold rolling a descaled hot rolled steel plate to a desired thickness with or without intermediate annealing. In order to achieve the stated purpose of the method for producing a hot-dip aluminum-plated steel sheet according to the method of the present invention, the chemical composition values of the base steel applied to this method have a particular significance. The effect of each component of this base steel and the reason for limiting the content will be individually explained below. C is a component harmful to the high temperature oxidation resistance of aluminum plated steel sheets. The first harmful effect of C is that it significantly reduces the diffusion ability of Al from the plating coating layer into the base steel, which causes the plating to occur when the aluminum-plated steel sheet is heated to high temperatures. This is because it causes a large amount of pores and voids to be generated at the interface with the layer. According to the present invention, these pores and voids occur more often when the diffusion rate of Fe from the base steel into the plating layer is higher than the diffusion rate of Al from the plating layer into the base steel. They are thinking. The second harmful effect of C is that it increases the peelability of the plated layer at high temperatures. The reason for this is that O (oxygen) that has reached the surface of the base steel through defects and voids in the plating layer combines with C in the base steel to produce CO+CO ₂ , and this CO+
The present inventors believe that CO ₂ increases the internal pressure of the pores and voids generated at the interface between the base steel and the plating layer, thereby significantly reducing the interfacial strength between the base steel and the plating layer. is thinking. Such harmful effects of C can be eliminated by adding Ti to substantially eliminate solid solution C, that is, by fixing substantially all of the C as Ti carbide and precipitating it. If you don't have the top, you can avoid it. However, when attempting to decarburize using the normal converter steel manufacturing method, for example, there are cases where steel with an end point C value of 0.03 or 0.02% or more is obtained and this is fixed with Ti, and there is also a case where the converter molten steel is made in a vacuum. There is a big difference in the following points from the case where the C value is further lowered to an extremely low range by subjecting it to degassing treatment etc. and then fixing C with Ti. When the C content exceeds 0.02%, it becomes difficult to obtain a steel sheet product with stable strength properties and excellent surface cleanliness if sufficient Ti is added to fix the C content. For example, when enough Ti is added to fix 0.03 to 0.25% C as in the aforementioned US Pat. No. 3,881,880, the amount of Ti carbide (and Ti nitride) precipitated becomes very large. The form of this precipitation changes even with slight variations in various conditions during the hot rolling process or even the annealing process. and,
The strength and malleability properties of steel vary greatly depending on the form of this precipitation. Therefore, it becomes practically difficult to manufacture products with stable strength characteristics.
Further, when Ti is added to molten steel containing such a relatively high C content, scum is generated, which appears on the slab surface and remains even during subsequent rolling, which may cause surface flaws. In addition, increasing the amount of Ti added is economically disadvantageous. Although such a high C content can improve the strength characteristics of steel by using Ti carbonitrides, it actually causes various disadvantages. Therefore, in the present invention, we do not expect any improvement in strength due to Ti carbonitride, but we lower the C content to a low range and accordingly reduce the amount of Ti required to lower the secondary recrystallization temperature due to Ti addition. The objective of the present invention, which is to improve high-temperature strength, is achieved by maintaining the strength-improving effect of Si and Mn addition up to this secondary recrystallization temperature. . For this reason, in the present invention, it is necessary to make the C content as low as possible, and the upper limit is 0.02%, preferably 0.017%, and more preferably 0.015%. Such a very low C content can be achieved by adding a vacuum degassing process to the normal converter steel manufacturing process.
The lower limit of C content is not critical and can be achieved economically by combining a conventional converter with vacuum degassing equipment.
It can be 0.001% or more. Si is an element that contributes to improving high-temperature strength, which is the main objective of the present invention, and at the same time also contributes to high-temperature oxidation resistance. The high-temperature strength improvement effect of Si is due to solid solution strengthening, and the higher the Si content, the greater the effect. However, when the Si content exceeds 2.2%, although the high-temperature strength further increases, not only cold workability and weldability deteriorate, but also aluminum plating performance deteriorates significantly, making it impossible to obtain a sound aluminum plated coating. This becomes difficult. Therefore, in the present invention, the upper limit of the Si content is set to 2.2%. Furthermore, if the Si content is less than 0.1%, the effect on high temperature strength and high temperature oxidation resistance is extremely small, so the lower limit is set at 0.1%, but preferably,
The lower limit is 0.2%, more preferably 0.5%. Mn is an element that contributes to improving high temperature strength, which is the main objective of the present invention. The high-temperature strength improving effect of Mn is due to the solid solution strengthening effect, and the effect becomes larger as the Mn content increases. But Mn
If the content exceeds 2.5%, the high-temperature strength will further increase, but not only will the cold workability and weldability deteriorate significantly, but it will also deteriorate during high-temperature use of hot-dip aluminum coated steel sheets in the temperature range below 800℃. ,α
Since there is a risk that γ transformation may occur and cause a significant change in mechanical properties, the upper limit is set at 2.5%. Si content and Mn content are not independent of each other;
interrelated. It was found that in order to achieve satisfactory high temperature strength, the following relationship must be satisfied: 1.9×(%Si)+0.9×(%Mn)≧1. In order to achieve further improved high temperature strength, it is preferable that 1.2×(%Si)+0.6×(%Mn)≧1. It is important to make the material of the hot rolled material as homogeneous as possible in order to smoothly perform the subsequent cold rolling process and annealing process. To achieve this, it is necessary to perform hot rolling in the stable γ range, but as the Si content increases, the αγ transformation temperature increases, making it difficult to finish hot rolling in the stable γ range. .
On the other hand, as mentioned above, Mn has the effect of lowering the αγ transformation temperature. It was found that in order to finish hot rolling in the stable γ region, the relationship (%Mn) ≧ 0.5 × (%Si) must be satisfied. Figure 6 shows the Si content defined by the present invention and
The relationship with Mn content is shown. In the present invention, the shaded area in FIG. 6, that is, point A (0.1, 2.5),
Add Si and Mn in amounts represented by points within a pentagon defined by F (0.1, 0.9), G (0.43, 0.21), Q (2.2, 1.1) and D (2.2, 2.5). The straight line FG shown in Figure 6 represents 1.9×(%Si)+0.9×(%Mn)=1, and the straight line GQ represents (%Mn)=0.5×(%Si). Preferred Si content and Mn content are points A (0.1, 2.5), K (0.1, 1.47), L (0.67,
0.33), Q (2.2, 1.1) and D (2.2, 2.5). Straight line in Figure 6
KL represents 1.2×(%Si)+0.6×(%Mn)=1. As mentioned above, Ti is one of the basic elements that effectively diffuses Al in the plating layer into the base steel. In other words, C and N in the base steel are replaced by Ti(C,
N) By fixing Al as a precipitate, diffusion of Al from the plating layer into the base steel becomes extremely easy.
The amount of pores and voids generated at the interface between the base steel and the plating layer is drastically reduced. Due to this effect, after high-temperature heating, the surface of the aluminum-plated steel sheet according to the present invention has a thermally and chemically stable outermost layer (outermost surface layer of the plated steel sheet) mainly composed of Al ₂ O ₃ . An α-Fe layer containing a high concentration of Al covered with a dense oxide layer is formed, exhibiting excellent high-temperature oxidation resistance. When Ti is added in an amount 10 times or more the amount of (C+N), a sufficient amount of Ti can be present in the form of solid solution Ti in the base steel, further improving the high temperature oxidation resistance of the plated steel sheet. This effect is
α− containing the aforementioned high concentration of Al during high temperature heating
As Ti is selectively oxidized at the interface between the Fe layer (Al diffusion layer) and the oxide layer containing Al ₂ O ₃ as the main component, Ti is concentrated at the interface and Al ₂ O ₃ is This is thought to be because it makes the oxide layer, which is the main component, more stable and dense. Furthermore, since Ti increases the secondary recrystallization temperature, the ferrite grains of the base steel are stabilized even at high temperatures, and the solid solution strengthening effect of Si and Mn according to the present invention is maintained even at high temperatures. Become. The overall effect of Ti as described above does not increase even if the Ti content exceeds 0.5% and is added in large quantities, and it only causes deterioration of the surface quality of the base steel, so the upper limit value should be set. limited to 0.5%. Furthermore, if the Ti content is less than 0.1, even if it is sufficient to fix C and N in the base steel, the amount of solid solute Ti in the base steel will decrease, resulting in the above-mentioned problem.
Since this is insufficient to make the oxide layer containing Al ₂ O ₃ as the main component more stable and dense, the lower limit is set at 0.1%. Al is added for the purpose of deoxidizing molten steel, but
In the steel of the present invention, Ti is also important as a preheating deoxidizing element that can be added with good yield, and from this point of view, the lower limit value has been set.
It was set as 0.01%. Furthermore, if Al is added in excess of 0.1%, not only will the deoxidizing effect not be particularly improved, but there is also a greater risk of unnecessarily damaging the surface properties of the steel sheet, so the upper limit is set at 0.1%. In Ti-added steel such as the steel of the present invention, N is
Almost the entire amount is TiN during melting and solidification.
It is precipitated as a substance, and it hardly decomposes or aggregates in any subsequent steps. Therefore, in order to effectively utilize Ti, it is preferable to keep the N content as low as possible, but it is difficult to completely remove it with current steelmaking methods.
The N content is 0.010% or less. It should be noted that if P and S are contained in large amounts, they will impair cold or hot workability, so it is preferable that they be contained as little as possible. However, there is no problem as long as P and S are 0.04% or less and 0.04% or less, respectively, which are usually unavoidably contained. Next, the present invention will be further explained by giving specific examples. Example 1 This example illustrates the importance of the coiling temperature specified by the present invention in the commercial scale production of hot-dip aluminized steel sheets; A is a failure and B is a success. be. A (Control) Low carbon steel was produced in an 80-ton LD converter. Molten steel produced in a converter was subjected to ladle refining using the VAD method and decarburized by vacuum heating. With the addition of auxiliary materials of ferromanganese, ferrosilicone, aluminum and ferrotitanium, C; 0.013% in weight%;
Si; 1.00%, Mn; 1.13%, P; 0.022%, S;
0.006%, Ti; 0.26%, Sol.Al; 0.053%, N;
0.0030%, %Ti/(%C+%N)=16.3, balance Fe
and impurity steel. Seven slabs with a cross section of 199 mm x 940 mm and a length of 7900 mm are made from steel with the above composition using a vertical continuous casting machine.
I got the book. The slabs were left to cool while stacked. After removing scratches from each slab with scarf earth,
It was soaked for 4 hours in a heating furnace maintained at 1280°C and immediately hot rolled. Hot rolling finishing temperature is 900~920
The coiling temperature was 680-720°C, and the thickness of the hot-rolled sheet was 3.2 mm. After each hot-rolled coil was allowed to cool, it was descaled using a continuous pickling device using a hydrochloric acid bath. Each descaled hot-rolled sheet was used in a tandem four-high cold rolling mill to obtain a cold-rolled sheet with a thickness of 1.55 mm. After each cold-rolled sheet was subjected to a surface cleaning treatment, it was passed through a Sendzimir-type molten aluminum plating apparatus equipped with in-line annealing equipment to perform Al-Si (9% Si) plating. More specifically, the temperature of the strip in the line was up to 700°C in the NOF, followed by a temperature of 810-830°C in the HZ (heat zone). The atmosphere in the HZ was AX gas (decomposed ammonia gas). The plate, which passed through the HZ with a residence time of about 50 seconds, was subsequently cooled to the temperature of the Al--Si bath in an AX gas atmosphere and was continuously immersed in the Al--Si bath. The thus plated steel plate was adjusted to a total plating weight of 80 g/m ² on both sides using a pair of jet wipers, cooled appropriately, and then wound into a coil. The plated coil was temper rolled with a dull roll at an elongation rate of approximately 1.0%. The observation results were as follows. An internal oxidation layer was formed on both surface layers over the entire length of each hot-rolled coil, and the internal oxidation layer remained even after scale was removed by pickling. Spot-like defects were observed on the plated steel plate. A cross-sectional micrograph (400x magnification) of a portion of the plated steel plate with no imperfections is shown in Figure 3a. This photo shows that the internal oxidation layer remains even after plating. A steel plate shown in Figure 3a is placed in the atmosphere for 800 min
Figure 3b shows a cross-sectional micrograph (400x magnification) of the same material after heating at ℃ for 20 hours. According to this photograph, it can be seen that the Al diffusion layer was not formed on the surface layer of the steel base material, but that Fe scale was formed directly under the plating layer and that it was formed deeper from the internal oxidation layer. The plated steel sheet of this example cannot therefore be said to be an industrial product that satisfies the requirements for high temperature oxidation resistance. B (method of the present invention) In weight%, C; 0.009%, Si; 0.57%, Mn;
0.99%, P; 0.014%, S; 0.006%, Ti; 0.30%,
Sol.Al; 0.046%, N; 0.0033%, %Ti/(%C
+%N) = 23, the remaining Fe and impurity steel were obtained after vacuum degassing, the hot rolled plate thickness was 4.5 mm,
The same amount of molten aluminum plated steel sheets were produced by repeating the above operation described in A except that the hot rolling coiling temperature was 530 to 560°C. The observation results were as follows. No internal oxidation was observed in any of the hot rolled coils. There were no defects in the plated steel plate. Micrograph of cross section of galvanized steel plate (magnification x 400)
times) is shown in Figure 3c. According to this photo, it can be seen that internal oxidation is completely absent even in the galvanized steel sheet. In addition, the plated steel plate shown in Fig. 3c was exposed to 800°C in the atmosphere.
Cross-sectional micrograph after heating at °C for 20 hours (magnification ×
400x) is shown in Figure 3d. According to this photo,
An Al diffusion layer that contributes to high-temperature oxidation resistance was formed, and Fe was formed at the interface between the plating layer and the steel base material.
It can be seen that no scale was generated and no internal oxidation progressed at all. Example 2 This example is a laboratory experiment to illustrate that the composition of the base steel specified by the present invention is important for the high temperature strength and oxidation resistance of the product. Each 10 kg of steel with the composition shown in Table 2 is melted in a vacuum melting furnace, then cast, hot rolled, and cold rolled to obtain a steel plate with a thickness of 1.0 mm. After annealing, the material surface The oxide scale was removed, and after degreasing, aluminum plating (Al deposition amount 80 g/m ² ) was applied by immersing it in a molten Al bath (Al-9% Si) according to the usual molten aluminum plating conditions. . For each sample obtained in this way, the tensile property values at room temperature and 600
The strength (tensile strength) at ℃ was measured. The high-temperature oxidation resistance was also evaluated by the oxidation weight gain after repeating 10 times of holding the sample at 800°C in the atmosphere for 20 hours and then cooling it to room temperature. These test results are summarized in Table 2.

[Table] The following can be seen from Table 2. Samples A, B and C contain Si and
This is a comparative example in which the Mn content is outside the range of the present invention, and the Ti content and Ti/(C+N) ratio are varied. These three samples, whose Si and Mn contents are outside the range of the present invention, have uniformly low strengths at 600°C, regardless of the Ti content. Further, when comparing the oxidation weight gain of these three samples, the oxidation weight gain of sample C, which has the highest Ti content and the highest Ti/(C+N) ratio, is the lowest, and the effect of Ti on oxidation resistance is clear. However, this sample C has low high temperature strength and cannot achieve the object of the present invention. Sample D and sample E have Si content and
This is a comparative example in which the Mn content exceeds the upper limit of the range of the present invention. Sample D has high high temperature strength but low ductility at room temperature. This sample D had some imperfections. For this reason, oxidation weight gain is also high. Sample E has high high temperature strength and low oxidation weight gain, but the mechanical properties at room temperature tend to change greatly depending on the annealing conditions. Sample F is a comparative example in which Ti is not added, although the Si content and Mn content are within the range of the present invention. Although this sample has excellent high-temperature strength, it is inferior in high-temperature oxidation resistance. Samples G to K are within the scope of the present invention. Comparing these five samples with Sample C shows that Si and Mn contribute to improving the room temperature strength and high temperature strength of such Ti-added steel without impairing high temperature oxidation resistance.

[Brief explanation of the drawing]

FIG. 1 is a schematic enlarged cross-sectional view of the steel surface showing the behavior of the internal oxidation layer of a hot-dip aluminum coated steel sheet, and a to e represent the stages of manufacturing the steel sheet. Figure 2 is a micrograph showing the metallographic structure of a cross section of the base steel with an internal oxidation layer before and after aluminum plating (magnification: ×400 in both cases).
(a) shows before plating, and b shows after plating. Figure 3 is a micrograph showing a cross-sectional metal structure showing behavior in which the presence or absence of an internal oxidation layer influences high-temperature oxidation resistance (all magnifications are ×
400 times). Figure 4 shows the Si content and
FIG. 3 is a graphical representation of the results of an experiment investigating how Mn content and temperature affect the formation of an internal oxide layer. FIG. 5 is a diagram showing the relationship between the occurrence of internal oxidation and the cooling curve of a hot rolled coil. Figure 6 shows the Si content and Mn in the base steel specified in the present invention.
It is a figure showing mutual relationship with content.

Claims (1)

[Claims] 1. A steel slab to which a sufficient amount of titanium is added so that carbon and nitrogen in the steel are sufficiently fixed and unbonded titanium remains in the steel is produced, and this is hot-rolled. In a normal method for manufacturing a molten aluminum plated steel sheet, the slab is subjected to a step, a descaling process, a cold rolling process, an annealing process, and a molten aluminum plating bath immersion process to obtain a molten aluminum plated steel sheet in order, %, C: 0.020% or less, Si: 0.1 to 2.2%, Mn: 2.5% or less, 1.9×(%Si)+0.9×(%Mn)≧1 and (%
Mn)≧0.5×(%Si), Ti; 0.1 to 0.5% and (%Ti)/(%C+%N)
≧10, Al: 0.01 to 0.1%, N: 0.010% or less, the remainder consisting of Fe and unavoidable impurities, Ti-added Si-Mn
By using a steel slab and controlling the temperature of the hot-rolled material wound up in the hot rolling process to a sufficiently low temperature, a steel surface substantially free of internal oxidation layer can be obtained after the descaling process. A method for producing a hot-dip aluminum plated steel sheet, comprising: 2. The manufacturing method according to claim 1, wherein the temperature of the hot-rolled material to be wound is controlled to 600°C or less. 3. Claim 1 in which the Si content and Mn content in the slab satisfy the following relationship: 1.2×(Si%)+0.6×(%Mn)≧1
The manufacturing method according to item 1 or 2.