JP2007327116A - High-strength hot-dip aluminum-plated steel sheet for fuel tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip aluminum-plated steel sheet having both sufficient strength and deep drawing workability which are required for a fuel tank to possess. <P>SOLUTION: The base steel sheet has a basic composition comprising 0.001 to 0.010% C, 1.5% or less Si, 0.5 to 2.5% Mn, 0.15% or less P, 0.01% or less N, and 0.01 to 0.30% Ti. The hot-dip aluminum-plated steel sheet has a plated layer of an Al-Si alloy containing 3 to 13% Si on the base steel sheet. The base steel sheet can further include one or more elements selected from among 0.01 to 0.10% Nb, 0.0002 to 0.0020% B, 0.5% or less Cu and 0.5% or less Cr. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength hot-dip aluminized steel sheet for fuel tanks that is excellent in deep drawing workability, secondary work brittleness resistance, and low temperature toughness of welds.

Steel materials used for automobile fuel tanks are required to have excellent deep drawing workability because they are processed into a complicated tank shape. Considering that the fuel tank is an important safety part, it is also important that the secondary work brittleness resistance after molding and the low temperature toughness of the welded part are excellent.
A Pb—Sn plated steel sheet called a turn sheet has been conventionally used as a material for a fuel tank, but a hot-dip aluminized steel sheet is being used instead of a turn sheet having a large environmental load.

In addition, studies are underway to reduce the weight of fuel tanks by using thin steel plates to improve fuel economy. However, in the conventional hot-dip Pb—Sn-plated steel sheet (Patent Document 1) and hot-dip aluminum-plated steel sheet (Patent Document 2), an ultra-low carbon IF steel having a tensile strength of 270 MPa is used to ensure deep drawing workability. Therefore, it is impossible to achieve both thinness and weight reduction and securing of desired strength.
Japanese Patent Publication No.57-61833 Japanese Patent Laid-Open No. 9-156027

The inventors of the present invention have studied various alloy designs in order to obtain a hot-dip aluminized steel sheet having a tensile strength of 390 MPa or more and excellent deep drawing workability so that the necessary strength can be obtained even if the wall thickness is reduced. As a result, it has been found that when Si, Mn, and P are added to the ultra-low carbon IF steel, the strength is increased and the deep drawability is improved. Further, it has been found that the addition of Mn and Ti improves the low temperature toughness of the welded portion, and the addition of B and Nb further improves the secondary work brittleness resistance and the low temperature toughness of the welded portion.
The present invention provides a high-strength hot-dip galvanized steel sheet for fuel tanks based on such knowledge and excellent in deep drawing workability, secondary work brittleness resistance, and low-temperature toughness of welds at a tensile strength of 390 MPa or more. With the goal.

The high-strength hot-dip aluminized steel sheet for fuel tanks of the present invention has C: 0.001 to 0.010 mass%, Si: 1.5 mass% or less, Mn: 0.5 to 2.5 mass%, P: 0 An Al—Si alloy plating layer of Si: 3 to 13% by mass is provided on a steel plate having a basic composition including 0.15% by mass or less, N: 0.01% by mass or less, and Ti: 0.01 to 0.30% by mass. ing.
A steel plate can contain Nb: 0.01-0.10 mass%. Moreover, you may add 1 type, or 2 or more types of B: 0.0002-0.0020 mass%, Cu: 0.5 mass% or less, and Cr: 0.5 mass% or less.

Effects and embodiments of the invention

Tensile strength: At 390 MPa or more, the optimum component design of the steel used for the fuel tank material was studied in order to satisfy deep drawing workability, secondary work brittleness resistance after press working, and low temperature toughness of welds. As a result, when Si, Mn, and P are added to the ultra-low carbon IF steel, both high strength and deep drawing workability can be achieved. Furthermore, the addition of Nb and B results in secondary work brittleness resistance after press working, We have found that the low temperature toughness is improved and have come to the optimal component design that satisfies the required characteristics of the fuel tank.
Hereinafter, the alloy components, contents and the like defined in the present invention will be described.

C: 0.001 to 0.010 mass%
The lower the C content, the more advantageous the deep drawing workability and ductility. In particular, it is important to suppress to 0.010% by mass or less in order to obtain good deep drawing workability. However, if it is less than 0.001% by mass, the fine carbides contributing to the improvement of the tensile strength are reduced and the production cost is increased. Preferably, the C content is determined in the range of 0.002 to 0.007% by mass.

Si: 1.5% by mass or less The effect of increasing the strength of the steel sheet without excessively degrading the deep drawing workability is exhibited, and the effect of addition of Si is seen at 0.01% by mass or more, but 1.5% by mass. Excessive Si exceeding the range causes deep drawing workability and surface properties of the hot-dip aluminized steel sheet. The deterioration of surface properties caused by excessive Si results in the formation of a Si-based oxide film on the surface of the steel sheet in the aluminum plating process, which inhibits the Al-Fe reaction in the plating bath and causes plating defects called non-plating. There is a cause. In a system using Si as a solid solution strengthening element, a relatively high Si content of 0.5% by mass or more is set.

Mn: 0.5 to 2.5% by mass
It is a component that contributes to increasing the strength of the steel sheet without excessively degrading the deep drawing workability, and enables a reduction in the amount of P added that causes embrittlement of grain boundaries. It also has the effect of refining crystal grains during the transformation that occurs during the cooling process during welding and improving the low temperature toughness of the weld. Such an effect is seen when Mn is added in an amount of 0.5% by mass or more, but excessive Mn exceeding 2.5% by mass is harmful to the deep drawing workability by lowering the elongation and rankford value (r value) of the steel sheet. is there. Preferably, Mn content is defined in the range of 0.6-2.0 mass%.

P: 0.15% by mass or less Similar to Si and Mn, it is an effective component for increasing the strength. If 0.01% by mass or more, the effect of adding P becomes remarkable, but excessive addition exceeding 0.15% by mass The grain boundary embrittlement causes the secondary work embrittlement resistance after press working and the low temperature toughness of the weld. In the component system in which P is a solid solution strengthening element, a relatively high P content of 0.1% by mass or more is set.
N: 0.01% by mass or less Fixed to TiN by reaction with Ti to improve the Rankford value. However, excess N causes an increase in the amount of Ti necessary for fixing TiN, so the upper limit of the N content is set to 0.01% by mass (preferably 0.007% by mass) from the viewpoint of effective use of Ti. .

Ti: 0.01 to 0.30% by mass
It produces carbonitride and exhibits the effect of improving deep drawing workability and low temperature toughness of welds, and the effect of addition is seen at 0.01% by mass or more. However, excessive addition exceeding 0.30% by mass saturates the effect of improving the low temperature toughness of the welded part, and causes the steel material cost to increase.
Nb: 0.01 to 0.10% by mass
It is an alloy component that is added as necessary, and is a component that becomes carbonitride as well as Ti and improves deep drawing workability and low temperature toughness of welds. When combined with Ti, composite carbides of Ti and Nb are formed. The deep drawability and the low temperature toughness of the weld are further improved. Such an effect becomes significant when Nb: 0.01% by mass or more. However, if it exceeds 0.10% by mass, an effect commensurate with the increase cannot be obtained and the steel material cost increases.

B: 0.0002 to 0.0019 mass%
It is an alloy component added as necessary, contributes to the improvement of hardenability, and also exhibits the effect of further improving the low temperature toughness of the weld zone. Moreover, it preferentially segregates over grain boundaries in steel rather than P, which also helps to improve secondary work brittleness resistance after press working. Such an effect is seen when B is added in an amount of 0.0002% by mass or more, but excess B exceeding 0.0019% by mass inhibits the growth of crystal grains and causes the Lankford value and elongation to decrease.

Cu: 0.5% by mass or less, Cr: 0.5% by mass or less An alloy component that is added as necessary. All improve the pickling property after hot rolling and improve the surface properties of the hot-dip aluminized steel sheet. Has the effect of making However, excessive addition causes a significant increase in strength and a decrease in ductility, so both were made 0.5 mass% (preferably 0.4 mass%) as the upper limit.

Al-Si alloy plating layer The steel material prepared to a predetermined composition is sent to a continuous hot dipping equipment through hot rolling, annealing / pickling, and cold rolling. In the continuous hot dipping equipment, the steel sheet surface is activated by annealing in a reducing atmosphere, and then introduced into a hot dipped aluminum plating bath. The molten aluminum plating bath contains 3 to 13% by mass of Si, and the bath composition is reflected in the Al—Si alloy plating layer. Si contained in the plating layer has an action of suppressing generation and growth of a hard and brittle Al—Si alloy layer. The formation / growth suppression effect of the alloy layer is observed at Si: 3% by mass or more, but is saturated at 13% by mass, and even if it is added more than that, it adversely affects the corrosion resistance and adhesion of the plating layer.

A slab having the composition shown in Table 1 (invention example) and Table 2 (comparative example) was hot-rolled to a hot rolling finish temperature of 900 ° C., a winding temperature of 500 ° C. to a thickness of 3.2 mm, and then annealed. The plate was pickled and cold-rolled to a thickness of 1.2 mm. The obtained cold-rolled steel strip was fed into a continuous hot dipping plating equipment, and an Al—Si alloy plating layer of Si: 9% by mass with a plating adhesion amount of 80 g / m ² was provided on the steel plate surface.

A test piece was cut out from a hot-dip aluminized steel sheet and subjected to a tensile test, a secondary processing test, and a weld strength test.
In the tensile test, a tensile test was performed using a No. 5 test piece in accordance with JIS Z2241, and the tensile strength, total elongation, and Rankford value (r value) were determined. R ≧ 1.5 passed, r <1.5 failed. It was determined to pass.

  In the secondary processing test, the test piece was drawn with a drawing ratio of 2. by a hydraulic molding tester using a die having a punch diameter of 33 mm, a punch shoulder R of 4.5 mm, a die diameter of 35.4 mm, and a die shoulder R of 3 mm. After processing into a cup shape at 0 and cooling the cup to various temperatures, a cone punch with an apex angle of 60 degrees was placed on the cup and a weight of 6.3 kg was dropped from a height of 2 m onto the cone punch. . Measure the temperature at which the test piece brittlely fractured due to the drop impact of the weight, and evaluate the secondary work brittleness with the brittle fracture temperature: -40 ° C or less ◎, -40 ~ -20 ° C ○, -20 ° C or more × did.

In the impact test of the welded portion, a seam weld impact test piece having the size and shape shown in FIG. 1 was prepared by imitating the flange of the fuel tank, and tested at −40 ° C. using a Charpy impact tester. Impact values obtained in shock test 100 J / cm ² or more ◎, evaluate the low temperature toughness of the weld the 80~100J / cm ² ○, the 60~80J / cm ² △, as × less than 60 J / cm ² did.

As can be seen from the investigation results in Table 3, Invention Examples Nos. 1 to 16 have a tensile strength of 390 MPa or more, a Rankford value of 1.5 or more, and both tensile strength and deep drawability. It was excellent in secondary work brittleness resistance and low temperature toughness of welds.
In contrast, Comparative Example No. 17 with a large amount of Si, Comparative Example No. 18 with a large amount of Cu, Comparative Example No. 19 with a large amount of P, Comparative Example No. 20 with a large amount of Mn, Comparative Example with a large amount of C In No. 21 and Comparative Example No. 22 with a large amount of Cr, physical properties satisfying all of the Rankford value, secondary work brittleness resistance, and the low temperature toughness of the welded part could not be obtained. In Comparative Examples No. 23 and 24 where the amount of Ti was too small, the low temperature toughness of the welded portion was inferior.

A seam weld impact specimen prepared to investigate the low temperature toughness of welds.

Claims (2)

  C: 0.001 to 0.010 mass%, Si: 1.5 mass% or less, Mn: 0.5 to 2.5 mass%, P: 0.15 mass% or less, N: 0.01 mass% or less , Ti: 0.01 to 0.30% by mass, the balance being Fe and an unavoidable impurity steel sheet is provided with a Si: 3 to 13% by mass Al—Si alloy plating layer. High strength hot-dip aluminized steel sheet for fuel tanks.   Steel sheet is further Nb: 0.01-0.10 mass%, B: 0.0002-0.0020 mass%, Cu: 0.5 mass% or less, Cr: 0.5 mass% or less The high-strength hot-dip aluminized steel sheet for fuel tanks according to claim 1 containing

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