JP5625442B2 - High-strength steel sheet with a tensile strength of 1180 MPa or more with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a steel sheet having excellent delayed fracture resistance. More specifically, it is a steel sheet applied to strength members mainly used in the automotive field and building material field, and is a steel sheet excellent in delayed fracture resistance, particularly a high-strength steel sheet excellent in delayed fracture resistance and having a tensile strength of 1180 MPa or more. It is about.

Cold rolled steel sheets have been used for automobile steel sheets because of demands on plate thickness accuracy and flatness. However, in recent years, the high strength of automobile steel sheets has been increasing from the viewpoint of reducing CO ₂ emissions and ensuring safety. It is planned.

  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 “delayed fracture” becomes remarkably intense with an increase in the strength of the steel material, in particular, a high tensile strength of 1180 MPa or more. It becomes remarkable with the strength steel. “Delayed fracture” means that high strength steel is subjected to static load stress (load stress less than tensile strength) and, when a certain amount of time has passed, there is almost no plastic 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 work and hydrogen embrittlement of the steel at the stress concentration portion. The hydrogen that causes this hydrogen embrittlement is considered to be hydrogen that has penetrated and diffused into the steel from the outside environment in most cases. Invading and spreading.

  In order to prevent such delayed fracture in a high-strength steel plate, for example, in Patent Document 1, studies are made to weaken delayed fracture sensitivity by adjusting the structure and components of the steel plate. However, with this method, the amount of hydrogen that enters the steel sheet from the outside environment does not change, so that delayed fracture can be delayed, but delayed fracture itself cannot be prevented.

JP 2004-231992 A

  Accordingly, the present invention solves the above-described problems in the prior art and provides a high-tensile steel sheet having a tensile strength of 1180 MPa or more, excellent in delayed fracture resistance, and suitable as a strength member mainly used in the automotive field and building material field. The task is to do.

  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 hydrogen entering the steel sheet. As a result, it has been found that by coating the steel sheet surface with a small amount of Sn or an alloy mainly composed of Sn, hydrogen penetration into the steel sheet can be significantly suppressed and delayed fracture of the steel sheet can be suppressed. .

The present invention has been made on the basis of the above findings, and the gist of the present invention is that a steel sheet surface is coated with an alloy mainly composed of Sn or Sn having a metal Sn content of 10 mg / m ² or more and 2000 mg / m ² or less. The Sn-based plated steel sheet having excellent delayed fracture resistance and a tensile strength of 1180 MPa or more.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength cold-rolled steel plate which has the tensile strength of 1180 Mpa or more which can suppress the penetration | invasion of hydrogen to the inside of a steel plate, and can prevent delayed fracture effectively can be provided. Further, since the thickness of the steel sheet can be reduced by reducing the corrosion margin of the steel sheet, the weight of the strength member applied to the automobile field and the building material field can be reduced.

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

  Hereinafter, the present invention will be described in detail.

  The steel plate used as the substrate of the steel plate excellent in delayed fracture resistance of the present invention is a steel plate having a tensile strength of 1180 MPa or more. If the tensile strength is 1180 MPa or more, the chemical composition and steel structure are not particularly limited, and the rolling method and the like are not particularly limited, and either a hot-rolled steel sheet or a cold-rolled steel sheet may be used. However, among these, a high-strength cold-rolled steel sheet having a tensile strength of 1180 MPa or more, which is often used in the automobile field, building materials field, etc., particularly in the automobile field, is preferable, and a high-strength cold-rolled steel sheet having a tensile strength of 1340 MPa or more is further included. preferable. A steel sheet with low tensile strength is essentially less susceptible to delayed fracture. This is because the effect of the present invention is manifested even in a steel sheet having a low tensile strength, but is remarkably exhibited in a steel sheet having a tensile strength of 1180 MPa or more, and more prominently exhibited in a steel sheet having a tensile strength of 1340 MPa or more.

The high-strength cold-rolled steel sheet preferably used in the present invention may have any composition and structure as long as it has a desired tensile strength. In order to improve various properties such as mechanical properties, for example, C, Solid solution strengthening by addition of interstitial solid solution elements such as N and substitutional solid solution elements such as Si, Mn, P, Cr, precipitation strengthening by charcoal / nitrides such as Ti, Nb, V, Al, W, Zr, Chemical composition modification such as addition of strengthening elements such as Hf, Co, B, Cu, rare earth elements, strengthening by recovery annealing at a temperature at which recrystallization does not occur, or non-recrystallized regions without complete recrystallization Remaining partial recrystallization strengthening, bainite or martensite single phase or strengthening by transformation structure such as composite structure of ferrite and these transformation structures, Hal when ferrite grain size is d Formula ^{-Petch: σ = σ0 + kd -1/2} ( wherein sigma: stress, .sigma.0, k: material constant) grain refinement strengthening represented by the organizational or structural modification such as machining strengthening by rolling or the like alone Or in combination.

  As an example of the composition of such a high-strength cold-rolled steel sheet, in mass%, C: 0.1 to 0.4%, Si: 0 to 2.5%, Mn: 1 to 3%, P: Examples include 0 to 0.05%, S: 0 to 0.005%, and the balance being Fe and inevitable impurities, or further containing Cu, Ti, V, Al, Cr and the like.

  Examples of commercially available high-strength cold-rolled steel sheets having the above-described tensile strength include, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (and above, JFE Steel Corp.), SAFC1180D (Shin Nippon Steel Co., Ltd.) and the like.

  Although not particularly limited, the thickness of the steel plate serving as a substrate in the present invention is preferably about 0.8 to 2.5 mm, and more preferably about 1.2 to 2.0 mm.

  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.

  When Sn is added to steel, stable rust can be formed by the inclusion of Sn in the corrosion product produced in the steel surface layer. Therefore, in applications such as steel structures for building materials, Sn-added steel is used as weather-resistant steel. It has been. However, when Sn is added to the steel, the workability decreases. The steel structure of building materials can be used for Sn-added steel because the processing is not strict, but for example, in applications where processing is severe such as automotive strength members, the steel tends to break due to processing, It is not preferable to add Sn as a steel component.

In the present invention, the surface of a steel sheet having a tensile strength of 1180 MPa or more is coated with an alloy mainly composed of Sn or Sn having a metal Sn content of 10 mg / m ² or more and 2000 mg / m ² or less.

When the surface of the steel sheet is coated with 10 mg / m ² or more and 2000 mg / m ² or less of Sn, a film in which Sn is discontinuously present on the surface of the steel sheet, that is, a film in which Sn is discontinuously present in an island shape, or Sn is partially In other words, it is a film in which spots that do not exist are present, or a film in which these are mixed. When the Sn coating on the surface of the steel sheet is as described above, Fe and Sn are exposed to the external corrosive environment, and stable rust is caused by the inclusion of Sn in the corrosion product generated in the steel surface layer. Is formed. When stable rust is formed, hydrogen intrusion into the steel sheet during the corrosion process is suppressed. Further, since the redox potential difference between Fe and Sn is large, until the stable rust is formed, Fe is an anode, Sn is a cathode, and electrochemical corrosion occurs, and hydrogen is a cathode region. Since it occurs on the side, hydrogen intrusion to the steel material side is suppressed.

When the amount of Sn is less than 10 mg / m ² , stable rust is not formed, so that hydrogen penetration into the steel sheet during the corrosion process cannot be suppressed, and delayed fracture resistance is poor. When the Sn amount exceeds 2000 mg / m ² , the Sn film formed on the surface is peeled off during pressing, and the effect of suppressing delayed fracture cannot be obtained, and defects during continuous pressing in the manufacture of strength members for automobiles This is not preferable. Furthermore, since the area of Sn becomes larger than the area of Fe exposed to the external corrosive environment as described above, it is not preferable because pitting corrosion is likely to occur and corrosion is promoted.

As described above, even when the stable rust is formed by covering the steel sheet surface with Sn of 10 mg / m ² or more and 2000 mg / m ² or less as the amount of metal Sn, In addition, since hydrogen intrusion into the steel sheet during the corrosion process is suppressed, excellent delayed fracture resistance can be obtained. In addition, the formation of stable rust makes it possible to reduce the plate thickness by reducing the corrosion margin of the steel plate, and it is possible to reduce the weight of the strength member applied to the automobile field and the building material field. A more preferable amount of metal Sn is 50 mg / m ² or more and 300 mg / m ² or less.

  Sn-plated steel sheets obtained by applying Sn plating to steel sheets include so-called tinplate, which is mainly used in food canning applications. In this application, the strength of the steel sheet has been increased to reduce the weight of the can, but the proof stress is 690 MPa even with the highest strength double cold-rolled material DR10. There is no problem. The tin plate usually has a thickness of 0.6 mm or less.

The coating on the steel sheet may be not only Sn alone but also an alloy mainly composed of Sn. An alloy mainly composed of Sn refers to an alloy containing one or more of Fe, Co, Zn, Cr, Mn, Ni, and Mo in Sn in an amount of 20% by mass or less of the alloy. If it is a thing inside, there exists an effect of this invention. When coating an alloy mainly composed of Sn, the amount of Sn in the alloy mainly composed of Sn may be 10 mg / m ² or more and 2000 mg / m ² or less.

  The method for coating the steel material surface with Sn or an alloy mainly composed of Sn is not particularly limited, and can be performed by a known method. For example, electroplating, electroless plating, vapor deposition The law etc. can be used.

  The above-described Sn or an alloy mainly composed of Sn may be coated only on one surface of the steel plate or may be coated on both surfaces.

  The manufacturing method of the steel plate used as a substrate is not particularly limited.

  In order to facilitate understanding of the present invention, for example, a series of processes from steelmaking in the case where the surface of a cold-rolled steel sheet is coated with Sn or an alloy mainly composed of Sn 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 illustrations.

A steel having a predetermined component composition is melted and slab is formed 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%. Thereafter, if necessary, purifying treatment is performed by 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 according to a conventional method, and then at 750 to 900 ° C. Heat treatment, rapid cooling, and adjustment of the tensile strength of the steel sheet. Furthermore, if necessary, a cold-rolled steel sheet having a desired tensile strength is obtained by performing temper rolling with an elongation ratio of about 0.01 to 0.5% according to a conventional method. The surface is coated with Sn or an alloy mainly composed of Sn by a method such as electroplating, electroless plating, or vapor deposition so that the amount of metal Sn is 10 mg / m ² or more and 2000 mg / m ² or less. Thus, a high-strength cold-rolled steel sheet having excellent delayed fracture resistance according to the present invention can be obtained.

  In addition, when coating the surface of a cold rolled steel sheet with Sn or an alloy mainly composed of Sn, when using a plating method, particularly an electroplating method, the steel sheet and the alloy film mainly composed of Sn or Sn are applied during the plating process. When there is a risk of hydrogen intruding, if necessary, after the plating treatment, a baking treatment is performed at a temperature of about 100 to 300 ° C., and hydrogen penetrates into the steel film and the alloy film mainly composed of Sn or Sn. You may perform the process which removes.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Table 1 shows the components of the steel sheet used as the test material. Using a cold-rolled steel sheet having a thickness of 1.5 mm, immersed in toluene and subjected to ultrasonic cleaning for 5 minutes, divalent tin ion concentration: 30 g / L, phenolsulfonic acid concentration: 5 g / L (in terms of sulfuric acid), Using a solution having a sulfuric acid concentration of 60 g / L, Sn plating was performed at a current density of 30 A / dm ² . At that time, the Sn plating amount was changed by changing the electrolysis time. The amount of Sn plating deposited on the steel plate was measured using a calibration plate quantified using fluorescent X-rays. Furthermore, 4 g / L of copper sulfate as a copper concentration was added to the bath, and Sn—Cu alloy plating was performed. Moreover, the steel plate which did not perform a plating process was also prepared as a comparison control. The strength of the steel sheet was 1480 MPa regardless of the presence or absence of plating.

The Sn adhesion amount obtained as described above was 5 mg / m ² (Comparative Example 1), 10 mg / m ² (Invention Example 1), 100 mg / m ² (Invention Example 2), 1000 mg / m ² (Invention Example). 3) Each Sn-plated steel sheet of 3000 mg / m ² (Comparative Example 2), Sn-Cu plated steel sheet of Sn adhesion amount of 100 mg / m ² (Invention Example 4), and plating treatment as a comparative control were not performed. The following evaluation was performed on the non-plated steel sheet (Comparative Example 3). In addition, Cu content rate of Sn-Cu alloy plating was 20% with respect to the film weight. The obtained results are shown in Table 2.

(1) Evaluation of workability Each of the above-mentioned Sn-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 in order to remove residual stress during shearing. A piece was made. Next, the test piece was subjected to 180 ° bending using a three-point bending tester to evaluate workability. The bending radius of curvature is 4 mmR and 180 ° bending. Evaluation was made after bonding damplon tape ("Damplon" is a registered trademark) after bending, peeling off, bonding the tape to a copper plate, and measuring the Sn intensity using fluorescent X-rays. The plating film peeling amount was determined and evaluated according to the following criteria.
○: No plating film peeling.
Δ: Plating film peeling amount is less than 5% of plating film amount.
X: Plating film peeling amount is 5% or more of the plating film amount.

(2) Evaluation of delayed fracture resistance Bending test piece 1 was prepared by bending a test piece prepared by grinding in the same manner as in (1) above at 180 ° with a radius of curvature of 4 mmR, as shown in FIG. As described above, the bending test piece 1 was fastened with the bolt 2 and the nut 3 so that the inner distance was 8 mm, the shape of the test piece was fixed, and the test piece for delayed fracture resistance evaluation was obtained. Combined cycle corrosion test consisting of drying, wetting, and salt water soaking processes defined in SAE J2334 established by the American Society of Automotive Engineers for the test piece for delayed fracture resistance evaluation thus produced (see FIG. 2) Was performed up to 80 cycles. The presence or absence of cracks was visually inspected before the salt water immersion process of 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: 30 cycles or more and less than 70 cycles ×: less than 30 cycles

  Inventive Examples 1 to 4 to which Sn plating within the scope of the present invention was adhered did not peel off the plating film, and all showed results of good delayed fracture resistance. On the other hand, Comparative Example 1 in which the Sn plating amount is less than the range of the present invention showed a tendency that the delayed fracture resistance was slightly improved as compared with Comparative Example 3 in which the plating treatment was not performed. Compared to -4, the delayed fracture resistance was inferior. Further, in Comparative Example 2 in which the plating amount exceeded the range of the present invention, peeling of the film was observed by bending, and a result that the delayed fracture resistance was slightly lowered as compared with Comparative Example 3 in which the plating treatment was not performed was obtained. . Further, even when Sn—Cu plating was applied as the Sn alloy, good delayed fracture resistance was exhibited.

  ADVANTAGE OF THE INVENTION According to this invention, the high-tensile steel plate which suppresses delayed fracture can be provided, and it becomes applicable 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 (1)

Plating excellent in delayed fracture resistance having a tensile strength of 1180 MPa or more, characterized in that the surface of the steel sheet is coated with Sn or an alloy mainly composed of Sn or Sn of 10 mg / m ² or more and 2000 mg / m ² or less as the amount of metal Sn steel sheet.

Images