JP2011001611A - Steel sheet excellent in delayed fracture resistance, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high tensile strength steel sheet having a tensile strength of ≥1,180 MPa, which has excellent delayed fracture resistance and is suitable as a strength member used for an automobile field and building materials by sticking copper to the surface of a steel sheet, and to provide the same steel sheet.SOLUTION: Regarding the production method, to a cold rolled steel sheet having a tensile strength of ≥1,180 MPa, a Cu or Cu based alloy film of 1 to 2,000 mg/mper side is stuck by the well-known method such as an electroless plating method, an electroplating method and a deposition method. The steel sheet is obtained by the method.

Description

The present invention relates to a steel sheet having excellent delayed fracture resistance. More specifically, the present invention relates to a high-strength steel sheet having a tensile strength of 1180 MPa (about 120 kgf / mm ² ) or more, which is applied to a strength member mainly used in the automobile field and the building material field, and requires delayed fracture resistance. .

Conventionally, cold-rolled steel sheets have been used as steel sheets for automobiles because of demands regarding the accuracy and flatness of the sheet thickness. In recent years, from the viewpoint of reducing CO ₂ emissions and ensuring safety in automobiles, The strength of the steel sheet is increased.

  However, it is known that the phenomenon of “delayed fracture” is likely to occur when the strength of the steel material is increased, and this phenomenon becomes remarkably severe as the strength increases, particularly in a high-strength steel having a tensile strength of 1180 MPa or more. It becomes. “Delayed fracture” means that high strength steel is subjected to static load stress (load stress less than tensile strength) and after a certain amount of time, there is almost no compositional deformation. This is a phenomenon where sudden brittle fracture occurs.

  In the case of a steel sheet, this “delayed fracture” is known to be caused by residual stress when formed into a predetermined shape by press working and hydrogen embrittlement of the steel in such a stress concentration portion. The hydrogen that causes hydrogen embrittlement is considered to invade into the steel from the external environment in most cases and diffuse, and is typically hydrogen that intrudes as the steel material corrodes.

  In order to prevent such delayed fracture in a high-strength steel plate, for example, as described in Patent Document 1, studies are made to weaken delayed fracture sensitivity by adjusting the structure and components of the steel plate. However, when such a method is used, there is no effect of suppressing the amount of hydrogen entering the steel sheet from the outside environment, and even if it is possible to delay the occurrence of delayed fracture, the delayed fracture itself is suppressed. I can't. That is, in order to substantially improve delayed fracture, a method for controlling the hydrogen penetration amount itself into the steel sheet is necessary.

  From such a viewpoint, Patent Document 2 discloses a technique for suppressing delayed fracture by suppressing the amount of hydrogen intrusion into the steel sheet by performing Ni or Ni-based alloy plating on the cold-rolled steel sheet. .

However, when the Ni or Ni-based alloy described in Patent Document 2 is electroplated, hydrogen generated during plating remains in the steel sheet, which may cause delayed fracture. Furthermore, the Ni or Ni-based alloy plating layer in Patent Document 2 has a relatively large adhesion amount of 5 mg / mm ² or more, and when it is subjected to press working while being plated on the steel plate surface, There is a high possibility that the adhesion is weak and the plated layer is damaged during processing, and the intended effect cannot be obtained.

JP2004-231992 JP-A-6-346277

  Accordingly, an object of the present invention is to provide a steel sheet having excellent delayed fracture resistance, which solves the problems in the prior art as described above. It is another object of the present invention to provide a high-tensile steel sheet having a tensile strength of 1180 MPa or more and excellent in delayed fracture resistance, which is suitable as a strength member mainly used in the automotive field and building materials.

In order to solve the above-mentioned problems, the present inventors have made extensive studies and researches on means for preventing delayed fracture by suppressing the penetration of hydrogen into the steel sheet. As a result, it was found that a relatively small amount of Cu or a Cu-based alloy is adhered to the surface of the steel sheet, so that the amount of hydrogen intrusion into the steel material can be significantly suppressed and delayed fracture of the steel material can be suppressed.
The present invention has been made based on the above findings, and the gist thereof is as follows.

That is, in the present invention for solving the above-mentioned problems, 1 mg / m ² or more and 2000 mg / m ² or less of Cu or Cu-based alloy (in the case of a Cu-based alloy, the above amount is a Cu-converted value) on one surface of the steel sheet. It is a steel sheet excellent in delayed fracture resistance characterized by being adhered.

  The present invention also shows a steel sheet excellent in delayed fracture resistance, wherein the steel sheet is a cold-rolled steel sheet having a tensile strength of 1180 MPa or more.

  The present invention also shows a steel sheet excellent in delayed fracture resistance, characterized in that Cu or a Cu-based alloy adhered to the steel sheet surface is a discontinuous film that does not completely cover the steel sheet surface. It is.

This invention which solves the said subject is a manufacturing method of the steel plate excellent in the delayed fracture resistance characterized by making 1 mg / m < ² > or more and 2000 mg / m < ² > or less of Cu or Cu base alloy adhere to the steel plate surface. is there.

  The present invention also shows a method for producing a steel sheet having excellent delayed fracture resistance, which uses a cold-rolled steel sheet as the steel sheet.

  According to the present invention, it is possible to provide a steel plate, particularly a high-tensile cold-rolled steel plate, which can effectively suppress delayed fracture, and it is possible to reduce the plate thickness by reducing the corrosion allowance. This makes it possible to reduce the weight of various structures and is extremely valuable industrially.

It is a figure which shows schematic shape of the test piece for evaluation used in the case of delayed fracture evaluation in an Example. It is a figure explaining the process of the combined cycle corrosion test performed in the Example.

  Hereinafter, the present invention will be described in detail based on embodiments.

  The steel plate that serves as a substrate for the steel plate having excellent delayed fracture resistance according to the present invention is not particularly limited in terms of its chemical composition and structure, or rolling method, and may be various types.

However, these are used in the automotive field and construction materials such as cold-rolled steel sheet is desirable especially often used in the automotive field and the like, among others, the tensile strength, 980 MPa (about 100 kgf / mm ²⁾ or more, particularly, 1180 MPa ( A cold-rolled steel sheet of about 120 kgf / mm ² ) or more is desirable.

  That is, even if Cu according to the present invention is attached to a steel sheet having a tensile strength of, for example, 980 MPa or less, the various properties of the steel sheet are not affected, and therefore, it may be applied to a steel sheet having a tensile strength of 980 MPa or less. However, since a steel sheet having a low tensile strength is essentially unlikely to cause delayed fracture, it is preferable to apply it to the high-strength steel as described above, because it causes an increase in cost by attaching Cu according to the present invention.

  As described above, in the high strength cold rolled steel sheet preferably used in the present invention, in order to improve various properties such as mechanical properties, for example, interstitial solid solution elements such as C and N, and Si, Mn, P , Solid solution strengthening by addition of substitutional solid solution elements such as Cr, precipitation strengthening by charcoal / nitrides such as Ti, Nb, and V, and addition of strengthening elements such as W, Zr, Hf, Co, B, rare earth elements Such as chemical composition modification, strengthening by recovery annealing at a temperature at which recrystallization does not occur, partial recrystallization strengthening that leaves unrecrystallized regions without complete recrystallization, bainite or martensite single phase or ferrite and these Strengthening by transformation structure such as complex organization of transformation structure, Hall-Petch formula when ferrite grain size is d: σ = σ0 + kd1 / 2 (where σ: stress, σ0, k: material constant) The structural or structural modification such as grain refinement strengthening or work strengthening by rolling or the like represented by the above is carried out singly or in combination, but as described above, the chemical composition and metal of the steel sheet used in the present invention The structure is not particularly limited and may have any composition and structure as long as it has a desired tensile strength.

  As an example of the composition of such a high-strength cold-rolled steel sheet, for example, C: 0.1 to 0.4 mass%, Si: 0 to 2.5 mass%, Mn: 1 to 3 mass% , P: 0 to 0.05 mass%, S: 0 to 0.005 mass%, and the balance being Fe and inevitable impurities, Cu, Ti, V, Al, Cr, etc. Of course, the present invention is not limited to these.

  Moreover, as what is commercially available as a high-strength cold-rolled steel plate, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (above, manufactured by JFE Steel) SAFC1180D (manufactured by Nippon Steel Corp.) and the like can be exemplified without limitation.

  Further, although not particularly limited, the thickness of the steel plate serving as a substrate in the present invention is, for example, about 0.8 to 2.5 mm, more preferably about 1.2 to 2.0 mm. It is.

  The steel sheet excellent in delayed fracture resistance according to the present invention is formed by adhering a relatively small amount of a predetermined amount of Cu or a Cu-based alloy to the surface of the steel sheet as described above.

  According to the research and examination results of the present inventors, it is considered that the hydrogen intrusion into the steel sheet during the corrosion process greatly contributes to the oxidation / reduction reaction of Fe rust in a wet environment. That is, in order to suppress hydrogen intrusion, it is important to form so-called “stable rust” that makes Fe rust difficult to change. In order to form stable rust, Cu addition to steel is effective, and in the field used as weather-resistant steel, Cu-added steel is widely used and a great effect is obtained. However, in the field used as weather resistant steel, weathering steel can be used because processing is not severe. However, for example, in a field where strong processing such as the automobile field is forced, it becomes easy to cause cracking of the steel material by processing, so it is not preferable to add Cu as a steel plate component. Moreover, when adding Cu as a steel plate component, in order to obtain an effect equivalent to that of the present invention, an excessive amount of addition is required, which leads to an increase in cost and is not industrially preferable.

  From such a viewpoint, in order to efficiently contain Cu in the corrosion product generated in the surface layer portion of the steel plate, it is efficient to attach Cu or a Cu-based alloy to the steel plate surface, for example, by plating.

The so-called copper-plated steel sheet has an appearance like a copper plate and is conventionally used as, for example, a roof of a house, an outer wall, a decoration material, etc., because it is stronger and cheaper than a copper plate. Yes. Furthermore, it uses copper corrosion resistance and is used in gasoline tanks and various pipes. However, it is known that in such a copper-plated steel sheet, when a defect exists in the copper plating film, Fe exposed in the defect portion becomes an anode and causes pitting corrosion. Therefore, conventionally, when copper plating is performed for the purpose of improving the corrosion resistance by copper plating, a sufficient film thickness that does not cause a defect in the copper plating film, for example, 3 μm or more, when converted to an adhesion amount, approximately 27000 mg / m ² or more was required.

  In the present invention, the purpose is not to improve the corrosion resistance by forming such a conventionally known barrier-type Cu film, but to form stable rust on the surface of the steel sheet by including Cu in the corrosion product of iron. Is essential to suppress hydrogen intrusion into the steel. In addition, when the steel material surface is covered with a thick Cu film, hydrogen that has entered the steel material is difficult to be released to the outside even if, for example, a baking treatment or the like is performed. Furthermore, when the steel plate surface is covered with such a thick Cu film, if there are defects such as pinholes in the Cu film, the area of Cu is larger than the area of Fe exposed to the outside world. Therefore, pitting corrosion is likely to occur in the steel sheet, which may promote corrosion.

  From such a viewpoint, in the present invention, it is important to form a thin Cu film to be adhered to the steel sheet surface, and it is particularly desirable to form a discontinuous film that does not completely cover the steel sheet surface. That is, it is preferable to have a shape that is spot-like or partially present, for example, is partially dotted in an island shape on the steel plate surface.

Specifically, it is necessary that the adhesion amount per one side of Cu to the steel sheet surface is 1 mg / m ² or more and 2000 mg / m ² or less. When the adhesion amount of Cu is less than 1 mg / m ² , stable rust is not formed, and the delayed fracture behavior is the same as when Cu is not added. On the other hand, when the adhesion amount is more than 2000 mg / m ^2, not only the effect of suppressing delayed fracture is obtained, but also the Cu film formed on the surface is deficient during pressing, for example, during continuous pressing in automobile production It is not preferable because it becomes a defect. Furthermore, as described above, since the area of Cu is larger than the area of Fe exposed to the outside world, it is not preferable because it easily causes pitting corrosion and promotes corrosion.

Although not particularly limited, a more preferable Cu deposition amount of the order 20 mg / ^{m 2} or more 300 mg / ^{m 2} or less. In particular, when it is 300 mg / m ² or less, the following extremely high effect of suppressing hydrogen intrusion due to a discontinuous film in which Cu or a Cu-based alloy does not completely cover the steel sheet surface can be obtained.

  In addition, as described above, when the Cu film is discontinuously spotted or partially present on the steel surface, Fe and Cu are exposed to the external corrosive environment. Since the reduction potential difference is large, until the above-described stabilized rust is formed, electrochemical corrosion occurs with Fe serving as the anode and Cu serving as the cathode. Accordingly, since hydrogen is generated on the Cu side which is the cathode region, hydrogen intrusion to the steel material side is suppressed.

That is, Cu is known to have a 3.4 mass% Cu ₂ O eutectic, and this eutectic structure is distributed in the Cu lump according to the oxygen content. As described above, the hydrogen generated on the Cu side by the corrosion reaction until the stabilization rust is formed diffuses and penetrates into Cu, reduces Cu ₂ O to generate water (water vapor), and forms a Cu film. It is thought to cause hydrogen embrittlement. However, since hydrogen embrittlement occurs on the Cu film side in this way, hydrogen is consumed and hydrogen diffusion to the steel sheet side is suppressed, so that hydrogen embrittlement on the steel material side is effectively prevented. .

  From this point of view, as the Cu attached to the steel surface, copper containing oxygen in the form of copper (I) oxide, for example, tough pitch copper containing 0.02 to 0.05 atomic% of oxygen is desirable. . In addition, even when copper that does not contain oxygen such as deoxidized copper and oxygen-free copper is formed, oxygen is diffused and penetrated into copper by being exposed to a thermal history in an external environment (under an oxidizing atmosphere). Such copper can be used sufficiently if an economic viewpoint is not taken into consideration.

In obtaining the steel sheet excellent in delayed fracture resistance according to the present invention, the Cu deposited on the steel sheet may be not only a simple substance of Cu but also a Cu-based alloy. Here, the Cu-based alloy refers to an alloy containing one or more of Fe, Co, Zn, Cr, Mn, Ni, and Mo in Cu at 20% by mass or less of the alloy. If it is within the range, the desired effect of the present invention can be sufficiently achieved. In addition, when using such Cu base alloy, the above-mentioned adhesion amount of 1 mg / m ² or more and 2000 mg / m ² or less is a conversion value in Cu contained in these alloys.

  The method for adhering Cu or a Cu-based alloy to the steel surface is not particularly limited and can be carried out by various known methods. For example, electroplating, electroless plating, An evaporation method or the like can be used.

The steel sheet excellent in delayed fracture resistance according to the present invention obtained through such a process typically contains 1 mg / m ² or more and 2000 mg / m ² or less of Cu or a Cu-based alloy on one surface per side. An amount (in the case of a Cu-based alloy, the above amount is a value converted to Cu) is adhered, and has a tensile strength of 980 MPa or more, more preferably 1180 MPa or more. In addition, the above-mentioned Cu or Cu-based alloy film may be deposited on only one surface of the steel plate, or may be deposited on both surfaces.

As described above, the method for producing a steel sheet excellent in delayed fracture resistance according to the present invention as described above is a Cu or Cu-based alloy having 1 mg / m ² or more and 2000 mg / m ² or less per side on the steel sheet surface. It consists of a very simple process of adhering.

  In addition, as above-mentioned as a steel plate used as a substrate, it is not specifically limited, The manufacturing history is also arbitrary.

  In order to facilitate understanding of the present invention, for example, a series of processes from steelmaking in the case of forming a Cu or Cu-based alloy film according to the present invention on a cold-rolled steel sheet will be briefly described with an example below. However, as a manufacturing process of the steel plate used as a substrate, of course, it is not limited to the following examples.

The steel of a predetermined component is melted and made into a slab by continuous casting according to a conventional method. Next, the obtained slab is heated at a temperature of 1100 to 1300 ° C. in a heating furnace, hot-rolled at a finishing temperature of 750 to 950 ° C., and wound up at 500 to 650 ° C. Subsequently, after pickling, cold rolling is performed at a rolling reduction of 30 to 70%. After that, if necessary, according to a conventional method, after performing cleaning treatment such as washing with an alkali or a mixed solution of an alkali and a surfactant and a chelating agent, electrolytic washing, warm water washing, and drying, at 750 to 900 ° C. Heat treatment, rapid cooling, and adjustment of steel sheet hardness. Furthermore, if necessary, a cold-rolled steel sheet having a desired tensile strength is obtained by performing temper rolling of about 0.01 to 0.5% according to a conventional method. On the surface of the cold-rolled steel plate thus obtained, Cu or a Cu-based alloy is added in an amount of 1 mg / m ² or more to 2000 mg / m ^{2 by} an appropriate method such as electroplating, electroless plating, or vapor deposition. By attaching a predetermined amount of ² or less, a cold-rolled steel sheet having excellent delayed fracture resistance according to the present invention is obtained.

  In addition, when depositing Cu or a Cu-based alloy on the surface of a cold-rolled steel sheet, when a plating method, particularly an electroplating method is used, hydrogen penetrates into the steel material and the copper or copper-based alloy film during the plating process. Since there is a fear, if necessary, after the plating treatment, a heat treatment at a temperature of about 100 to 300 ° C. may be performed to remove the hydrogen that has entered the steel material and the copper or copper-based alloy film. .

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the scope of the present invention is not limited to these examples.

  Table 1 shows the components of the steel sheet used as the test material. Using the steel plate with a thickness of 1.5 mm as it is cold-rolled, it is immersed in a copper sulfate solution (Cu sulfate: 0.1 g / L or 10 g / L) to provide a Cu plating film on both sides of the steel plate. did. Here, the amount of Cu plating was changed by changing the dipping time, and the amount of Cu plating deposited on the steel sheet was measured by fluorescent X-ray analysis using a calibration curve prepared with a standard plate with a known amount of adhesion. .

  Moreover, the steel plate which did not perform a plating process was also prepared as a comparison control. The strength of the obtained steel sheet was 1480 MPa regardless of the presence or absence of plating.

The Cu adhesion amounts obtained as described above were 0.5 mg / m ² (Comparative Example 1), 1 mg / m ² (Example 1), 10 mg / m ² (Example 2), 100 mg / m ² ( Example 3), 1000 mg / m ² (Example 4), 3000 mg / m ² (Comparative Example 2), 10000 mg / m ² (Comparative Example 3) Cu-plated steel sheet and no plating treatment as a comparative control The following evaluation was performed on each steel plate (Comparative Example 4). The obtained results are shown in Table 2.

(1) Evaluation of workability Each of the above-described Cu-plated steel plate and non-plated steel plate was sheared to a width of 35 mm x a length of 100 mm, and subjected to grinding until the width was 30 mm, thereby preparing a test piece. Next, the test piece was subjected to bending using a three-point bending tester. The bending curvature was 4 mmR and the workability was evaluated. The evaluation was performed by visually observing the outer surface of the bending top, and the presence or absence of a plating film was evaluated according to the following criteria.
○: No missing plating film.
Δ: Fine cracks are observed on the surface.
X: Peeling of the plating film is recognized.

(2) Delayed Fracture Evaluation After the grinding process in the same manner as described above, the test piece 1 produced by bending is fastened using bolts 2 and nuts 3 as shown in FIG. A test piece for delayed fracture evaluation was obtained. For the delayed fracture evaluation test piece produced in this way, a combined cycle corrosion test (see FIG. 2) consisting of drying, wetting, and salt water soaking processes defined in SAE J2334 defined by the American Society of Automotive Engineers, Up to 80 cycles were performed. The presence or absence of cracks in the steel sheet on the outer surface of the test piece was visually inspected before the salt water immersion process in each cycle, and the crack generation cycle was measured. In addition, this test was performed on three specimens of each steel plate, and the average value was evaluated. Evaluation was performed according to the following criteria from the number of cycles.
○: 70 cycles or more.
Δ: 40 cycles or more and less than 70 cycles.
X: Less than 40 cycles.

Examples 1 to 4 and Comparative Examples 1 to 3 are examples in which the amount of Cu plating applied to the steel sheet surface was changed, and Comparative Example 4 was an example in which the steel sheet surface was not subjected to Cu plating. When attention is paid, as is apparent from the results shown in Table 2, it can be seen that Comparative Example 1 and Examples 1 to 3 having a small amount of plating have no lack of plating film and have excellent workability. In Example 4 where the plating amount was 1000 mg / m ² , some fine cracks were observed, and in Comparative Examples 2 and 3 where the plating amount was 3000 mg / m ² or more, peeling of the film was observed and the plating amount increased. As a result, the tendency for the workability to deteriorate was revealed.

Next, focusing on delayed fracture evaluation, Comparative Example 1 showed a tendency to slightly improve delayed fracture characteristics as compared with Comparative Example 4 that was not subjected to plating treatment, but because the plating amount was small. It can be seen that sufficient effects are not obtained. Further, it is clear that Comparative Examples 2 and 3 in which part of the plating film was removed during processing showed a delayed fracture characteristic lower than that of Comparative Example 4 which was a comparative control which was not subjected to the plating treatment. became. On the other hand, in Examples 1 to 4 according to the present invention in which a predetermined amount of Cu plating was adhered, the delayed fracture characteristics were all good. In particular, in Examples 1 to 3 in which the plating adhesion amount is 300 mg / m ² or less, the effect of suppressing hydrogen intrusion due to the Cu film being a discontinuous film that does not completely cover the steel sheet surface is large, and delayed fracture characteristics. Was very good.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel plate which suppresses delayed fracture can be provided, and it becomes possible to apply in a wide field | area centering on the motor vehicle field | area or a building material field | area.

1 Test piece 2 Bolt 3 Nut

Claims (5)

Delayed fracture resistance, characterized in that 1 mg / m ² or more and 2000 mg / m ² or less of Cu or a Cu-based alloy (in the case of a Cu-based alloy, the above amount is a Cu-converted value) is adhered to the steel sheet surface. Steel sheet with excellent properties.   The steel plate excellent in delayed fracture resistance according to claim 1, wherein the steel plate is a cold-rolled steel plate having a tensile strength of 1180 MPa or more.   The steel sheet excellent in delayed fracture resistance according to claim 1 or 2, wherein the Cu or Cu-based alloy adhered to the steel sheet surface is a discontinuous film that does not completely cover the steel sheet surface. A method for producing a steel sheet having excellent delayed fracture resistance, wherein 1 mg / m ² or more and 2000 mg / m ² or less of Cu or a Cu-based alloy is adhered to one surface of the steel sheet. The method for producing a steel sheet having excellent delayed fracture resistance according to claim 4, wherein a cold-rolled steel sheet is used as the steel sheet.

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