High-strength cold-rolled steel sheet

Abstract

A steel sheet mainly suitable for strength members of automobiles or building materials, which has a tensile strength of 1,180 MPa or more, and which is excellent in delayed fracture resistance and primary rust prevention performance. The steel sheet includes a coating, placed on a surface of a cold-rolled steel sheet with a tensile strength of 1,180 MPa or more, containing one or more metalates selected from molybdates and tungstates and a P compound. The sum of the coating weights of the metalates in terms of Mo and W is 10 mg/m.sup.2 to 1,000 mg/m.sup.2 and is preferably 50 mg/m.sup.2 to 1,000 mg/m.sup.2. The coating weight of the P compound in terms of P is 10 mg/m.sup.2 to 1,000 mg/m.sup.2.

Claims

1. A high-strength cold-rolled steel sheet suitable for automobiles or building materials, the steel sheet comprising: a cold-rolled steel sheet having a tensile strength of 1,180 MPa or more; and a coating disposed directly on and in direct contact with a surface of the cold-rolled steel sheet, the coating consisting essentially of a P compound and at least one metalate selected from the group consisting of molybdates and tungstates, wherein a sum of coating weight of the at least one metalate in terms of at least one of Mo and W is in a range of 10 mg/m.sup.2 to 1,000 mg/m.sup.2 and a coating weight of the P compound in terms of P is in a range of 10 mg/m.sup.2 to 1,000 mg/m.sup.2.

2. The high-strength cold-rolled steel sheet according to claim 1, wherein the sum of the coating weight of the at least one metalate in terms of Mo and W is in a range of 50 mg/m.sup.2 to 1,000 mg/m.sup.2.

3. The high-strength cold-rolled steel sheet according to claim 1, wherein the P compound is at least one selected from the group consisting of phosphoric acid, pyrophosphoric acid, phosphonic acid, and hypophosphorous acid, and the molybdates are at least one selected from the group consisting of sodium molybdate, ammonium molybdate, and sodium phosphomolybdate.

4. The high-strength cold-rolled steel sheet according to claim 1, wherein the P compound is at least one selected from the group consisting of phosphoric acid, pyrophosphoric acid, phosphonic acid, and hypophosphorous acid, and the tungstates are at least one selected from the group consisting of sodium tungstate, potassium tungstate, and zirconium tungstate.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view of a specimen, used in an example according to an embodiment, for delayed fracture evaluation.

(2) FIG. 2 is an illustration showing steps of a combined cyclic corrosion test performed in an example according to an embodiment.

DETAILED DESCRIPTION

(3) In steel sheets excellent in delayed fracture resistance according to the disclosed embodiments, steel sheets (base steel sheets) serving as substrates have no particular limitation on the chemical composition, the metallographic microstructure, a rolling method, or the like and may be arbitrary ones. Among them, cold-rolled steel sheets which are used in the automotive field and the building material field and which are often used particularly in the automotive field are preferable. In particular, a high-tensile strength cold-rolled steel sheet, having a tensile strength of 1,180 MPa (about 120 kgf/mm.sup.2) or more, concerned about the occurrence of delayed fracture under an air corrosion environment is important. Even if the disclosed embodiments are applied to a steel sheet with a tensile strength of less than 1,180 MPa and a coating containing a specific metalate and a P compound is formed on a surface thereof, various properties of the steel sheet are not affected. However, steel sheets with low tensile strength are unlikely to have delayed fracture, forming a coating according to the disclosed embodiments leads to an increase in cost.

(4) In high-strength cold-rolled steel sheets, the following modifications are applied alone or in combination for the purpose of enhancing properties such as mechanical properties. The modification are, for example, microstructural or structural modifications such as solid solution hardening by the addition of an interstitial solute element such as C or N or a substitutional solute element such as Si, Mn, P, or Cr; precipitation hardening by a carbide or nitride of Ti, Nb, V, or the like; chemical compositional modifications by the addition of a strengthening element such as W, Zr, Hf, Co, B, a rare-earth element, or the like; hardening by recovery annealing at a temperature at which crystallization does not occur or partial recrystallization hardening allowing an unrecrystallized region to remain without recrystallization; hardening due to a transformation microstructure by forming a bainite or martensite single phase or a composite microstructure of ferrite and these transformation microstructures; grain refinement hardening given by the Hall-Petch equation σ=σ.sub.0+kd.sup.−1/2 (where σ: stress, σ.sub.0, k: material constant) when d is the ferrite grain size; and work hardening by rolling or the like. The chemical composition and metallographic microstructure of a steel sheet used in the disclosed embodiments are not particularly limited as described above and one having a predetermined tensile strength may have any chemical composition and metallographic microstructure.

(5) Examples of the composition of such a high-strength cold-rolled steel sheet include, but are not limited to, one containing C: 0.1 mass % to 0.4 mass %, Si: 0 mass % to 2.5 mass %, Mn: 1 mass % to 3 mass %, P: 0 mass % to 0.05 mass %, and S: 0 mass % to 0.005 mass %, the remainder being Fe and inevitable impurities; those obtained by adding one or more of Cu, Ti, V, Al, and Cr to this; and the like.

(6) Commercially available examples of the high-strength cold-rolled steel sheet include, but are not limited to, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (the above being manufactured by JFE Steel Corporation), SAFC1180D (manufactured by NIPPON STEEL & SUMITOMO METAL CORPORATION), and the like.

(7) The thickness of a cold-rolled steel sheet serving as a substrate is not particularly limited, is preferably, for example, about 0.8 mm to 2.5 mm, and is more preferably about 1.2 mm to 2.0 mm.

(8) A steel sheet excellent in delayed fracture resistance according to the disclosed embodiments includes a coating, placed on a surface of the above cold-rolled steel sheet, containing a P compound and one or more metalates selected from molybdates and tungstates.

(9) Examples of the molybdates include sodium molybdate, ammonium molybdate, sodium phosphomolybdate, and the like. Examples of the tungstates include sodium tungstate, potassium tungstate, zirconium tungstate, and the like. In the disclosed embodiments, as one or more selected from the molybdates and the tungstates, one or more of these may be contained.

(10) Examples of the P compound include phosphoric acid, pyrophosphoric acid, phosphoric acid, hypophosphorous acid, and the like. In the disclosed embodiments, as the P compound, one or more of these may be contained.

(11) The sum of the coating weights of the metalates in the coating in terms of metal (Mo, W) is set to 10 mg/m.sup.2 to 1,000 mg/m.sup.2. When the coating weight is less than 10 mg/m.sup.2, the effect of reducing the amount of generated hydrogen is low and no delayed fracture resistance can be exhibited. From this viewpoint, the lower limit of the coating weight is preferably 50 mg/m.sup.2. On the other hand, when the coating weight is more than 1,000 mg/m.sup.2, costs are high, though a function for delayed fracture resistance does not decrease. This is not preferable. From this viewpoint, the upper limit of the coating weight is preferably 500 mg/m.sup.2.

(12) The coating weight of the P compound in the coating in terms of P is set to 10 mg/m.sup.2 to 1,000 mg/m.sup.2. When the coating weight is less than 10 mg/m.sup.2, the formation of a reaction layer with the steel sheet is not sufficient and therefore there is no visible improvement in delayed fracture resistance over a long period of time. In consideration of the formation of the reaction layer, the lower limit of the coating weight is preferably 50 mg/m.sup.2. On the other hand, when the coating weight is more than 1,000 mg/m.sup.2, costs are high, though a function for delayed fracture resistance does not decrease. This is not preferable. From this viewpoint, the upper limit of the coating weight is preferably 500 mg/m.sup.2. Incidentally, the coating weight of each metal component in the coating is measured by a method described in an example.

(13) In the disclosed embodiments, reasons why the delayed fracture resistance is improved by forming the coating, which contains the P compound and one or more metalates selected from the molybdates and the tungstates and, are not necessarily clear but are probably due to a mechanism below.

(14) In the course of dry/wet corrosion, a hydrogen generation reaction among cathodic reactions is dominant in an acidic region and therefore the amount of generated hydrogen increases. As a result, the amount of hydrogen penetrating into the steel sheet increases to cause delayed fracture. On the other hand, it is known that the molybdates and the tungstates are present in the form of having a double bond with oxygen and therefore have an easily reducible nature. Therefore, it is conceivable that, since the coating, which contains the above-mentioned metalates, is present on a surface layer, a portion of the cathodic reactions is consumed in reducing components (metalates) and therefore the amount of generated hydrogen decreases. Hence, it is conceivable that the amount of hydrogen penetrating into the steel sheet decreases, resulting in an improvement in delayed fracture resistance.

(15) Furthermore, since the coating contains the P compound and therefore forms the reaction layer with a surface of the steel sheet, the coating can be made strong. Although the molybdates and the tungstates have the effect of reducing the amount of penetrating hydrogen in the course of corrosion as described above, the molybdates and the tungstates alone have low water resistance and therefore the coating is dissolved during moistening in a corrosion test; hence, there is no visible improvement in delayed fracture resistance over a long period of time. However, since the P compound is contained, excellent delayed fracture resistance is obtained over a long period of time. At the same time, excellent primary rust prevention performance can be obtained by forming the coating, which is strong, on a surface of the steel sheet.

(16) A method for forming the coating on a surface of the cold-rolled steel sheet is not particularly limited and is, for example, a method in which the cold-rolled steel sheet surface is coated with a surface treatment solution containing the above-mentioned components (the metalates and the P compound), followed by heating/drying. The surface treatment solution, which is coated on the cold-rolled steel sheet surface, can be prepared by dissolving or dispersing the above-mentioned components (the metalates and the P compound) in a solvent (water and/or an organic solvent).

(17) A method for coating the cold-rolled steel sheet surface with the surface treatment solution may be any one of an application method, an immersion method, and a spraying method. In the application method, any one of coating means such as a roll coater (a three-roll method, a two-roll method, or the like), a squeeze coater, and a die coater may be used. The adjustment of the application quantity, the homogenization of the appearance, or the equalization of the thickness can be performed by an air knife method or a roll drawing method after application treatment, immersion treatment, or spraying treatment using a squeeze coater or the like.

(18) After coating is performed using the surface treatment solution as described above, heating/drying is usually performed without water washing and may be performed after coating treatment. A method for heating/drying the coated surface treatment solution is arbitrary and, for example, a means such as a dryer, a hot blast stove, a high-frequency induction heater, or an infrared oven can be used. The heating/drying treatment is preferably performed at an attained temperature of 40° C. to 300° C., desirably within the range of 40° C. to 160° C. When the heating/drying temperature is lower than 40° C., the drying time is long and coating unevenness may possibly occur. However, when the heating/drying temperature is high, the strength is reduced by changing the material quality controlled in an annealing step or a function as an inherent high-strength steel may possibly be reduced. From this viewpoint, the heat treatment time is preferably short and the temperature range is preferably 300° C. or less.

EXAMPLES

(19) The following sheets were used as base steel sheets: cold-rolled steel sheets (as-cold-rolled steel sheets), containing components such as C: 0.191 mass %, Si: 0.4 mass %, Mn: 1.56 mass %, P: 0.011 mass %, and S: 0.001 mass %, the remainder being Fe and inevitable impurities, having a tensile strength of 1,520 MPa and a thickness of 1.5 mm.

(20) Oil sticking to surfaces of the cold-rolled steel sheet was ultrasonically removed using a mixture of toluene and ethanol. In a coating method, surface treatment solutions for forming coatings were prepared by dissolving blend components (metalates and P compounds) shown in Table 1 in water (pure water) and were applied to surfaces of the steel sheets, followed by heating/drying in a high-frequency induction heater, whereby steel sheets of Examples and Comparative Examples were obtained. The coating weight of each metal component in a corresponding one of the coatings was measured by X-ray fluorescence using steel sheets in which the coating weight of each metal component was known as reference sheets.

(21) The steel sheets obtained in the above manner were evaluated for delayed fracture resistance by a technique below. The results are shown in Table 1 together with the coating configuration. Incidentally, a steel sheet (No. 1 which was a comparative example) provided with no coating was similarly evaluated for properties.

(22) Evaluation of Delayed Fracture Resistance

(23) The steel sheets of the Examples and Comparative Examples were sheared to a width of 35 mm and a length of 100 mm and were ground to a width of 30 mm, whereby specimens were obtained. As shown in FIG. 1, each specimen 1 was bent to a U-shape and was constrained with a bolt 2 and a nut 3 such that the shape of the specimen was fixed, whereby a specimen for delayed fracture evaluation was obtained. The specimen, prepared in this manner, for delayed fracture evaluation was subjected to a combined cyclic corrosion test (refer to FIG. 2), specified in SAE J2334 defined by Society of Automotive Engineers, including drying, moistening, and saltwater immersion steps up to 20 cycles. Whether cracking had occurred was visually checked before the saltwater immersion step of each cycle, whereby the number of cycles until the occurrence of cracking was measured. This test was performed for three samples of each steel sheet and the average thereof was used for evaluation. Evaluation was made on the basis of standards below from the number of cycles until the occurrence of cracking and a symbol (Good, Fair, or Poor) was given. As shown in Table 1, the case of the comparative example provided with no coating was four cycles; hence, the symbols “Good” and “Fair” were set to a preferable range. In Table 1, the fact that the number of cycles until the occurrence of cracking is 20 or more means that cracking did not occur in the results of the examples.

(24) Good: 15 cycles or more

(25) Fair: 10 cycles to less than 15 cycles

(26) Poor: less than 10 cycles

(27) Evaluation of Primary Rust Prevention Performance

(28) The steel sheets of Examples and Comparative Examples were sheared to a size of 50 mm×50 mm. The specimens were subjected to the above combined cyclic corrosion test (refer to FIG. 2). Evaluation was made on the basis of standards below from the area fraction of red rust observed after the first cycle and a symbol (Good or Poor) was given. Incidentally, the symbol “Good” was set to a preferable range.

(29) Good: the area fraction of observed red rust being less than 50%

(30) Poor: the area fraction of observed red rust being 50% or more

(31) TABLE-US-00001 TABLE 1 Coating configuration Delayed fracture Metalate P compound resistance Coating Coating Number of weight weight cycles until Primary rust *1 *2 occurrence prevention No. Type (mg/m.sup.2) Type (mg/m.sup.2) of cracking Evaluation performance Category 1 — — — — 4 Poor Poor Comparative example 2 Sodium molybdate 5 Phosphoric 5 8 Poor Poor Comparative acid example 3 Sodium molybdate 10 Phosphoric 10 12 Fair Good Example acid 4 Sodium molybdate 10 Phosphoric 5 9 Poor Poor Comparative acid example 5 Sodium molybdate 50 Phosphoric 50 15 Good Good Example acid 6 Sodium molybdate 500 Phosphoric 500 19 Good Good Example acid 7 Sodium 200 Phosphoric 500 16 Good Good Example phosphomolybdate acid 8 Ammonium 1000 Pyrophosphoric 200 20 or more Good Good Example molybdate acid 9 Sodium tungstate 600 Phosphoric 1000 20 or more Good Good Example acid 10 Calcium tungstate 300 Phosphoric 500 18 Good Good Example acid 11 Zirconium 50 Phosphoric 500 15 Good Good Example tungstate acid *1 The coating weight in terms of metal (Mo, W). *2 The coating weight in terms of P.

(32) In Table 1, Example Nos. 3 and 5 to 8 have a coating containing a molybdate and a P compound and Example Nos. 9 to 11 have a coating containing a tungstate and a P compound within the scope of the disclosed embodiments. All the examples according to disclosed embodiments are provided with excellent delayed fracture resistance and primary rust prevention performance.