JPH0225982B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to an aluminum diffusion steel sheet with excellent oxidation resistance and high-temperature strength, and a method for manufacturing the same. This article relates to an aluminum diffusion steel sheet used as a material that requires the following: and its manufacturing method. Conventional technology Aluminum-plated steel sheets have superior oxidation and corrosion resistance compared to ordinary steel sheets and galvanized steel sheets, so they are used as one of the parts that require oxidation and corrosion resistance, such as automobile parts and household appliances. It is used in the department. However, the oxidation resistance temperature of ordinary aluminum-plated steel sheets is 600°C or lower, and they cannot be used in areas that require higher temperatures. Therefore,
Today, for example, special public interest steel plates manufactured in 1973 are used as aluminum-plated steel sheets that can be used in environments exceeding 600℃.
For original plates of aluminum plated steel sheets disclosed in Publication No. 15454 and Japanese Patent Publication No. 52-33579,
Heat-resistant aluminum-plated steel sheets containing Ti or Cr have been developed and are commercially available. However, even with this developed heat-resistant aluminum-plated steel sheet, the maximum oxidation resistance temperature that can withstand use is 700℃, and at temperatures exceeding this, red rust spots often appear on the plated surface, and the inside of the steel sheet is also exposed to the surface. It is corroded by oxidation and cannot be used. Therefore, in recent years, for example, stainless steel materials (SUS410 series) have been used as automobile exhaust pipe materials, and in addition to oxidation and corrosion resistance at high temperatures of 700 to 800°C, high-temperature strength equivalent to that of stainless steel has been introduced. (Tensile strength at 600℃: 18Kg/mm2 ^or more,
Aluminum-plated steel sheets with improved oxidation resistance and high-temperature strength (elongation: 35% or more) are superior to conventional heat-resistant aluminum-plated steel sheets (tensile strength of 10 to 600℃).
11Kg/mm ² ), and early development is desired. As a technology to improve this oxidation resistance, PCT,
There is a disclosure in WO85/00386. This technology is
Original plate components are C0.001~0.25%, Mn0.2~0.8%,
Molten aluminum plated steel sheet with P0.015% or less and Ti0.01~1.0% in non-oxidizing gas at a temperature of 816~927
℃, heat treatment for 5 to 50 hours to remove Al of the plating layer.
Diffused into steel plate to improve oxidation resistance (704-982℃)
There is no consideration or description of techniques such as improving strength at high temperatures. Therefore, even with the above original plate composition, even though it is high C,
C alone cannot provide the high-temperature strength necessary for safe design in high-temperature usage environments, and even if it has excellent oxidation resistance, it cannot be used for members exposed to high temperatures such as automobile exhaust pipes. Problems to be Solved by the Invention In view of the above-mentioned circumstances, the present invention aims to provide an aluminum diffusion steel sheet that has excellent oxidation resistance and high-temperature strength at temperatures as high as 700 to 800°C. be. Means for Solving the Problems That is, the present inventors have developed a product that has oxidation resistance and high-temperature strength, and also has excellent workability and weldability, which are necessary characteristics for manufacturing actual crosspieces such as automobile exhaust pipes. In order to develop a material with aluminum-plated steel sheets as a base, we conducted research using aluminum-plated steel sheets as a base, and found that in order to satisfy oxidation resistance at 800℃, aluminum-plated steel sheets with a specific composition range must be heat-treated (hereinafter referred to as heating) in nitrogen gas. 0.5 to 10% Al enriched with Al on the surface of the steel sheet through mutual diffusion of Al in the plating layer and Fe in the base metal.
Al diffusion layer of α-Fe containing 15μ or more and Al on top of it,
It was found that it is sufficient to form a Fe, Si alloy layer of 20μ or more. In addition, high temperature strength (tensile strength at 600℃: 18Kg/
mm ² or more, elongation: 35% or more), and to satisfy the processability and weldability characteristics required for actual crosspiece production.
Mn is an element that suppresses coarsening of crystal grains at high temperatures.
It has been found that in order to add one or both of P, Ti, Si, and B to steel and to improve oxidation resistance, workability, and weldability, it is necessary to limit the range of the added components. Ta. Furthermore, the conditions for manufacturing an excellent material that satisfies the above characteristics are as follows: As mentioned above, aluminum-plated steel sheets with a specific composition range are manufactured by heating diffusion A (5 hours, 800°C) as shown in Figure 1. , B (100 hours,
800℃), C (100 hours, 700℃) and D (10 hours,
It was also found that the process can be carried out within the time and temperature range within the figure (700°C) in a nitrogen atmosphere that prevents oxidation of the steel sheet surface and increases the thickness of the diffusion layer. The present invention was made based on the above findings, and the gist thereof is that Mn is 0.6 to 1.5% by weight,
P0.03~0.1%, Al0.01~0.08%, Ti0.08~0.25%
Including Si0.3% or less, B0.003% or less1
Contains one or two species, C0.01% or less, N0.004%
An α-Fe Al diffusion layer containing 0.5 to 10% Al on the surface of a steel sheet, with the remainder being substantially Fe.
An aluminum diffusion steel sheet with excellent oxidation resistance and high temperature strength, characterized by having a thickness of 15 to 50μ and an Al, Fe, and Si alloy diffusion layer of 20 to 50μ on top of the diffusion layer.
and Mn0.6~1.5%, P0.03~0.1%, by weight%
Contains Al0.01~0.08%, Ti0.08~0.25%, and
Contains one or two of Si 0.3% or less, B 0.003% or less, limited to C 0.01% or less, N 0.004% or less,
A steel plate, the remainder of which is essentially Fe, plated with aluminum containing 3 to 15% Si is heat treated in a nitrogen atmosphere at a temperature and time within the range surrounded by points A, B, C, and D in Figure 1. The present invention provides a method for manufacturing an aluminum diffusion steel sheet that has excellent oxidation resistance and high-temperature strength. However, A (5 hours, 800°C), B (100 hours, 800°C), C (100 hours, 700°C), and D (10 hours, 700°C). The present invention will be explained in detail below. Function First, we will discuss the reason for limiting the steel composition of the target aluminum diffusion steel sheet. First of all, Mn is an element that increases strength at room temperature and high temperature.
To ensure a value of mm ² or more, it is necessary to add 0.6% or more to the steel. However, if Mn increases too much, the moldability of the material deteriorates, so the upper limit was set at 1.5%. Like Mn, P is an element that improves the strength at room temperature and high temperature, and the greater the amount added, the higher the strength. However, if the amount added exceeds 0.1%, for example, cracks may occur in spot weld nuggets, and in arc welding, weldability will deteriorate, such as a significant decrease in the ductility of the weld, so the upper limit of the amount of P added may be It was set at 0.1%. Additionally, although the reason for this composite addition of P, Mn, and Ti is not clear, steel sheets are given reactivity in aluminum plating baths, and defects such as pinholes in the plating layer are reduced. . Therefore,
An aluminum diffusion steel sheet made from this plated steel sheet can have a uniform and thick Al diffusion layer. Therefore, P that satisfies these properties
The lower limit of 0.03% is required. Al is used as a pre-dissolution deoxidizer to improve Ti addition yield. Al remaining on the plated original plate
If the amount is less than 0.01%, the addition yield of Ti will be poor and the Ti concentration in the steel will also become unstable. On the other hand,
When the Al amount is 0.01% or more, the Ti addition yield is good and stable, but when it exceeds 0.08%, aluminum oxide is generated on the steel plate surface during the plating process, which deteriorates the wettability of the aluminum plating bath.
This may cause unsatisfactory areas to appear in the plating layer of the diffusion material. Therefore, the amount of Al remaining in the steel plate is set to 0.01
~0.08% range. Ti fixes C and N contained in steel and improves the workability of steel sheets. Furthermore, when the amount of Ti added is 0.08% or more, coarsening of crystal grains in the original plate is suppressed by heat diffusion treatment at high temperature. Therefore, in order to satisfy high-temperature strength at high temperatures as well as excellent workability, high strength, and weldability of materials cooled from high-temperature environments to room temperature, composite addition of Ti and the aforementioned Mn and P is effective. It is. However, when this Ti addition exceeds 0.25%, the plating wettability of the aluminum-plated steel sheet, which is the diffusion material, deteriorates rapidly.
The appropriate range of addition amount was set at 0.08 to 0.25%. Next, one or both of Si and B are included to increase the strength of the aluminum diffusion steel sheet at room temperature and high temperature. However, although the material strength increases as the amount of Si added increases, if the amount added exceeds 0.3%, Si will be generated on the surface of the steel sheet during the manufacturing process of aluminum-plated steel sheet, which is a diffusion material.
The wettability of the plating deteriorates due to the oxide, and plating defects occur, making it impossible to obtain a good diffusion material. Therefore, it is necessary to reduce the amount of Si added to 0.3% or less. B precipitates as a B compound at grain boundaries, and has the effect of suppressing coarsening of crystal grains at high temperatures and improving strength accordingly. However, when the amount added is large,
For example, quenching occurs due to the welding heat associated with the construction of automobile exhaust pipes, etc., which causes excessive hardening and impairs the ductility of the welded part. Therefore, the amount added
It was set to 0.003% or less. On the other hand, C in steel is an effective component for increasing high-temperature strength, but it deteriorates the wettability of aluminum plating on steel sheets, causing unplated areas and deteriorating oxidation resistance. Furthermore, when the amount of Ti added is high, which is one of the elements for preventing grain coarsening in steel sheets at high temperatures, the amount increases and the workability also deteriorates. Also,
In the heat diffusion treatment, the thickness of the diffusion layer generated is larger when the carbon is low, so extremely low carbon is preferable for improving oxidation resistance. Therefore, in the present invention,
C is a harmful component and is regulated to 0.01% or less. In addition, N may cause Ti to be added more than necessary,
Decreases the plating wettability of steel sheets. Furthermore, since it is an interstitial element like C, it is a harmful element that inhibits the formation of an Al diffusion layer. Therefore, the amount added
It is limited to 0.004% or less, but the lower the content, the better. Next, as mentioned above, the steel sheet of the present invention produces an Al diffusion layer and an Al, Fe, Si alloy layer in the diffusion treatment.
diffuses into the base steel and forms a solid solution of up to 10% of Al, forming a soft α-Fe Al diffusion layer with a Vickers hardness of 120 to 290, and an outermost layer containing more than 10% Al. , hard Al with a Bitkers hardness of 600 to 1200,
Fe, Si alloy layer (Fe ₃ Al, FeAl identified by X-ray diffraction)
It generates. These layers are made of α-Fe Al
Although the diffusion layer can withstand severe processing, Al, Fe, and Si alloy layers containing more than 10% Al easily crack under light processing. This cracking adversely affects oxidation resistance. Therefore, the amount of Al in the Al diffusion layer needs to be soft α-Fe containing 10% or less. In addition, regarding the oxidation resistance of this Al diffusion layer, we tested the relationship between the thickness of the layer and the amount of Al by heating it in the atmosphere at 800°C for 50 hours and then cooling it for 1 hour five times. is 0.5% or more and the layer thickness is 15μ or more, the above oxidation resistance can be met.
This is because Al in the Al diffusion layer reacts with O ₂ in the atmosphere to form an anti-oxidation film of Al ₂ O ₃ on the surface of the steel sheet. Therefore, based on the above results, the Al concentration range of the Al diffusion layer was set to 0.5 to 10%, and the lower limit of the layer thickness was set to 15μ. Regarding the upper limit layer thickness, although the thicker the layer, the better the oxidation resistance tends to be. However, the formation of an Al diffusion layer exceeding 50 μm reduces operational efficiency, so the upper limit layer thickness was set at 50 μm. Next, on the outermost surface of this steel plate, an alloy diffusion layer containing Al, Fe, and Si of 20 to 50 μm is formed.
Diffuses Al into steel and increases the thickness of the Al diffusion layer, contributing to improved oxidation resistance. The thickness of this alloy layer was set to 20 to 50μ.
If it is less than that, the uniformity of the surface gloss of the aluminum diffusion steel sheet is lost, the appearance is poor, and the feel is also poor. In addition, although those with a thickness exceeding 50μ have excellent surface gloss, this alloy layer is hard and cracks easily occur on the steel plate surface with light processing, and the thick layer also causes the alloy layer to peel off. It is. Next, a method for manufacturing an aluminum diffusion steel sheet according to the present invention will be described. First of all, the reason why the amount of Si in Al, which is a plating component of the aluminum diffusion steel sheet material, was limited to 3 to 15% is as follows. That is, if an unsatisfied portion exists in the aluminum-plated steel sheet that is the raw material, Al is not diffused in that portion during the diffusion heat treatment and it exists as an oxidation-resistant defect. Therefore, it is necessary to prevent the occurrence of this unsightly part. Therefore, in order to prevent the occurrence of defects, Al
Adding Si to the plating bath lowers the melting point and improves the wettability with the steel plate.As a result of investigating the range of the appropriate amount of Si to add, it was found that if the Si content is less than 3%, the wettability between the Al plating bath and the steel plate is poor. The Si content decreases rapidly and many imperfections occur on the surface of the steel sheet, but on the other hand, when the amount of Si added exceeds 15%, the amount of Si increases near the interface between the Al-plated layer and the base steel of the aluminum-plated steel sheet. It was found that many cracks occurred and the cracked areas behaved in the same way as unsealed areas, adversely affecting oxidation resistance. For these reasons, the amount of Si in the Al plating bath should be
It was set at 15%. Next, the reasons for determining the atmosphere, temperature, and time range of the main heating diffusion treatment conditions in the manufacturing method of the present invention will be described. First, air, H ₂ , 5
~75% _H2 - _N2 , Ar, and nitrogen were selected and diffused at 700-800℃ for 50 hours in these atmospheres, and the appearance of the surface of the diffusing material and line analysis of the cross-sectional abrasive material by EPMA were used to evaluate the diffusion process. The layer thickness was measured. As a result, many point-like red rust oxides are generated on the surface of the diffusion steel sheet treated in air at temperatures of 700°C or higher, resulting in poor oxidation resistance. Therefore, in order to prevent this oxidation, it is considered that the diffusion atmosphere should be a non-oxidizing gas. However, _H2, which is non-oxidizing
For unknown reasons, in a 5-75% H ₂ -N ₂ (mixed) atmosphere, Al on the outermost surface of the heat-diffused material
The Fe and Si alloy layers are peeled off, and the EPMA results show that the formation of an Al diffusion layer is extremely small, and a longer treatment time is required to achieve the thickness of 15μ or more required to improve oxidation resistance. This atmosphere is therefore unsuitable. On the other hand, in the heat diffusion treatment in nitrogen and Ar atmospheres, even at a high temperature of 800°C, there is no red rust or peeling of the alloy layer on the surface of the steel sheet, and the surface appearance is excellent. However, regarding this diffusion layer, the reason for the formation of microcracks due to thermal strain in the outermost alloy layer and the thickness of the Al diffusion layer is currently unknown, but microcracks are much more likely to occur in a nitrogen atmosphere than in an Ar atmosphere. It has been found that the Al diffusion layer is easy to generate. Therefore, the optimal atmosphere for the heating and diffusion treatment is nitrogen gas. Therefore, in the present invention, nitrogen was selected as the heat treatment atmosphere gas. Next, various temperatures and times were investigated as conditions for heat diffusion treatment in a nitrogen atmosphere using an aluminum-plated steel sheet of the steel composition of the present invention.
As a result, the appropriate ranges for these temperatures and times are A (5 hours, 800°C) and B (100 hours,
800℃), C (100 hours, 700℃) and D (10 hours,
700℃), there is no coarsening of the steel plate's crystal grains and no microcracks in the steel plate's surface alloy layer.
Easily generates Al diffusion layer, has excellent oxidation resistance,
It has been found that this range is indicative of high-temperature strength, workability, and weldability. In this figure, the CD line indicates the line at the heating temperature of 700°C, and at temperatures below this it is not possible to generate an Al diffusion layer of 15μ or more, which is necessary to satisfy the oxidation resistance of 800°C. area. The BC line is a line indicating heating for 100 hours at a heating temperature of 700°C or higher and 800°C or lower. If the heating time is longer than this, an Al diffusion layer will form and become thicker, showing a good tendency for oxidation resistance.
A heating time exceeding 100 hours lowers operational efficiency, so the upper limit heating time was set at 100 hours. The AB line indicates the line at a heating temperature of 800°C; above this line, Al diffusion layers are easily formed, but grains become coarser, which has a significant negative impact on high-temperature strength and workability. Therefore, the upper limit temperature was set at 800℃. The AD line is a straight line connecting coordinate A (5 hours, 800°C) and coordinate D (10 hours, 700°C), and in the range to the left of this straight line, the 15μ This is a region where the above Al diffusion layer cannot be generated. The aluminum-plated steel sheet, which is the material of the aluminum diffusion steel sheet of the present invention, is processed through general ingot-forming or continuous casting, followed by hot rolling, pickling, cold rolling, crystallization annealing, and Sendzimir method molten aluminum plating line. It can be easily manufactured by EXAMPLES The effects of the present invention will be illustrated in more detail with reference to Examples below. Example: Steel with the components shown in Table 1 was melted in a vacuum melting furnace (300Kg).
A 300Kg steel ingot was melted and forged to a thickness of 40Kg.
This was then hot-rolled into a hot-rolled sheet of 8 mmt, and further cold-rolled to produce a coil of a thin steel sheet of 1.6 mmt. This coil is attached to a Sendzimir method molten aluminum plating line (plating bath: 10%Si-90%Al).
A molten aluminum surface-treated steel sheet was prepared through the process, which was wound into a tight coil and placed in a batch furnace, and various heat diffusion treatments shown in Table 1 were performed in a nitrogen atmosphere. Evaluation tests were conducted on the material. In addition, the thickness of the Al diffusion layer and alloy layer in the table is
Using the results of line analysis by EPMA, the grain size number was determined in accordance with JIS G0552. In addition, regarding the evaluation test method of formability and weldability, which are important factors in making actual bars, and the usage environment under high temperature, press formability is 80mmΦ diameter and 40mm depth.
The evaluation and weldability of I-developed butt TIG welding (current
95A, voltage 11V, speed 300mm/min, arc length 1.5mm)
This welded part was evaluated using the Erichsen test, which is a three-dimensional ductility evaluation, and the high temperature strength was determined at the tensile test temperature.
The tensile strength and elongation of a 1.6 mmt flat plate specimen (JIS G0567) were evaluated at 600℃, and the oxidation resistance was determined using an aluminum diffusion material (dimensions: 1.6t x 50w x 150Lmm) in consideration of application to automobile exhaust pipes. A 180° bending test piece with a diameter of 42.7 mm was prepared and heated at 800°C x 50 in the atmosphere.
Heating for 1 hour and cooling for 1 hour were repeated 5 times for evaluation. In the above evaluation tests, 800℃ oxidation resistance and
A material that has a tensile strength of 18 Kg/mm ² or higher at 600°C and a high-temperature strength with an elongation of 35% or higher, and also exhibits excellent formability and weldability when making actual crosspieces such as automobile exhaust pipes. This was evaluated as meeting the intended purpose of the present invention. These results are also listed in Table 1. As is clear from the table, when No. 1 and No. 2 general aluminum-killed and capped steel aluminum-plated steel sheets were heat-diffused at 700°C for 50 hours, the crystal grains of the original sheets became coarse for both steels. turned into
Moreover, due to the high carbon content, the formation of an Al diffusion layer that prevents oxidation is small, and the press formability, high temperature strength, and oxidation resistance are poor, and it cannot be used as a material for aluminum diffusion steel sheets. Therefore, regarding the limitation and range of the base plate components, when looking at the appropriate heating and diffusion conditions of 750°C x 50 hours, the No. 4 high C material, which is outside the scope of the present invention, has improved high temperature strength but Al Less formation of diffusion layer,
Poor oxidation resistance. Material No. 5 with a small amount of Mn and No. 7 with a small amount of P both have low high temperature strength, material No. 6 with a large amount of Mn has poor press formability, but material No. 6 with a large amount of P has low strength.
Even with No. 8 material, press formability and weldability are poor. In addition, material No. 9 with a small amount of Ti had poor evaluation characteristics in all cases, and material No. 10 with a large amount of added Si or No. 11 with a large amount of Si had a defective part on the steel plate during the aluminum plating process. occurs, this portion exists as a defect in the heat diffusion treatment, red rust occurs in the oxidation resistance test, and the oxidation resistance deteriorates. As for B, the ductility of the welded part of material No. 12, which has a large additive amount, is poor at 5.1 mm in Erichsen value, and cannot withstand processing such as pipe expansion and bending during construction. Therefore,
For processing including welded parts, it is necessary to ensure an Erichsen value of 9 mm or more. In addition, No. 15 material with a large amount of N has Al
Oxidation resistance is poor because the diffusion layer is not thicker than 15μ. On the other hand, materials No. 3, No. 13 and No. 14 are aluminum diffusion materials within the composition range of the present invention and have all the properties of the evaluation test, namely press formability, weldability,
Excellent high temperature strength and oxidation resistance. Next, looking at the range of appropriate heating and diffusion treatment conditions for aluminum-plated steel sheets having the composition range of the present invention,
When the diffusion temperature of materials No. 16 to 18 is as high as 850°C, the original plate crystal grains become coarse, resulting in poor press formability, weldability, and high-temperature strength. Next, increase the diffusion temperature to 800-700℃
For materials No. 19 to No. 30 that were lowered for 3 to 100 hours, No. 19 at 800℃ for 3 hours, No. 23 at 750℃ for 6 hours, and No. 27 at 700℃ for 8 hours. , the treatment time is short relative to the treatment temperature, and the Al diffusion layer that prevents oxidation is thin, less than 15μ, so oxidation resistance is poor. In comparison, heat-diffusion treated materials No. 20-22, No. 24-26 and No. 28-30 produced an Al diffusion layer of 15μ or more and were excellent in all property evaluations including oxidation resistance. . Furthermore, since the diffusion temperature of materials No. 31 and No. 32 is as low as 650° C., the formation of an Al diffusion layer is less than 15 μm even after treatment for 50 to 100 hours, and the oxidation resistance is poor. Note that material No. 33 is an aluminum-plated steel sheet containing the ingredients of the present invention that is not subjected to heat diffusion treatment. Therefore, since this steel sheet does not have an Al diffusion layer,
Oxidation resistance is poor.

【table】

[Table] ◎: Example of the present invention
*Some unmet parts exist

【table】

[Table] Effects of the Invention As is clear from the above examples, according to the present invention, an aluminum diffusion material having excellent oxidation resistance and high temperature strength, which has all of oxidation resistance, high temperature strength, workability, and weldability. It made it possible to provide steel plates, and it has made an extremely large contribution to industry.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between temperature and time of the heating diffusion treatment in the manufacturing method of the present invention.

Claims (1)

[Claims] 1% by weight, Mn0.6-1.5%, P0.03-0.1%,
Contains Al0.01~0.08%, Ti0.08~0.25% and more
Contains one or two of Si 0.3% or less, B 0.003% or less, limited to C 0.01% or less, N 0.004% or less,
Al0.5~ on the surface of the steel plate, the remainder of which is essentially Fe.
It has an Al diffusion layer of 15 to 50 μ of α-Fe containing 10%, and a 20 to 50 μ of Al, Fe, and Si alloy diffusion layer on top of it.
An aluminum diffusion steel sheet with excellent oxidation resistance and high-temperature strength, characterized by its 50μ. 2 In weight%, Mn0.6-1.5%, P0.03-0.1%,
Contains Al0.01~0.08%, Ti0.08~0.25%, and
Contains one or two of Si 0.3% or less, B 0.003% or less, limited to C 0.01% or less, N 0.004% or less,
3~15% Si on the surface of the steel plate, the remainder of which is essentially Fe
oxidation resistance and high-temperature strength by heat-treating a steel plate plated with aluminum containing Excellent method for manufacturing aluminum diffusion steel sheets. However, A (5 hours, 800℃) B (100 hours, 800℃) C (100 hours, 700℃) D (10 hours, 700℃)