JP7020446B2 - High-strength steel plate - Google Patents

Description

The present invention relates to a high-strength steel sheet having excellent delayed fracture resistance, and more specifically, a steel sheet suitable mainly for strength members for automobiles and building materials, and has excellent delayed fracture resistance and corrosion resistance, preferably. The present invention relates to a high-strength steel sheet having a tensile strength of 1180 MPa or more.

Conventionally, galvanized steel sheets are often used as steel sheets for automobiles because of the requirement for corrosion resistance. Specifically, galvanized steel sheets are often used as steel sheets for automobiles because the sacrificial anticorrosion action of zinc can significantly improve the corrosion resistance of steel sheets after painting. On the other hand, in recent years, from the viewpoint of reducing CO ₂ emissions of automobiles and ensuring safety, the strength of steel sheets for automobiles has been increased.

However, it is known that when the strength of steel is increased, a phenomenon called delayed fracture is likely to occur, and this delayed fracture becomes more severe as the strength of steel increases, especially in high-strength steel with a tensile strength of 1180 MPa or more. Become. In addition, delayed fracture is a state in which a high-strength steel material is subjected to static load stress (load stress less than the tensile strength), and when a certain period of time elapses, it appears suddenly with almost no plastic deformation. This is a phenomenon in which brittle fracture occurs.

It is known that this delayed fracture is caused by the residual stress when the steel sheet is formed into a predetermined shape by press working and the hydrogen embrittlement of the steel in the stress concentration portion. In most cases, the hydrogen that causes this hydrogen embrittlement is thought to be hydrogen that has penetrated into the steel from the external environment and diffused, and typically, the hydrogen generated during the corrosion of the steel sheet is the steel. It has invaded and spread inside. In general, galvanized steel sheets have a larger amount of hydrogen penetration during corrosion than cold-rolled steel sheets, and are prone to delayed fracture.

In order to prevent such delayed fracture in a high-strength steel sheet, for example, in Patent Document 1, studies have been made to reduce the delayed fracture sensitivity by adjusting the structure and composition of the steel sheet. Further, in Patent Document 2, a study has been made on a high-strength hot-dip galvanized steel sheet that prevents delayed fracture.

Japanese Unexamined Patent Publication No. 2004-231992 Japanese Unexamined Patent Publication No. 6-145893

However, in the method of Patent Document 1, since the amount of hydrogen invading the inside of the steel sheet from the external environment does not change, it is possible to delay the occurrence of delayed fracture, but it is not possible to prevent delayed fracture itself. Further, even if the steel composition and the microstructure are adjusted by the method of Patent Document 2, sufficient delayed fracture resistance has not yet been obtained.
As described above, it cannot be said that the technology for high-strength steel sheets having excellent corrosion resistance and delayed fracture resistance has been sufficiently established.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to obtain a high-strength steel plate which is mainly suitable for strength members for automobiles and building materials and has excellent delayed fracture resistance and corrosion resistance. To provide.

In order to solve the above problems, the present inventors have diligently studied and studied means for preventing delayed fracture by suppressing hydrogen invading the inside of the steel sheet. As a result, by coating the surface of the base steel sheet with a Zn—Mn alloy layer having a specific coating amount and Mn concentration, it is possible to significantly suppress hydrogen intrusion into the inside of the steel sheet and prevent delayed fracture of the steel sheet. Moreover, it was found that excellent corrosion resistance can also be obtained.

The present invention has been made based on such findings, and the gist thereof is as follows.
[1] It has a base steel sheet having a tensile strength of 1180 MPa or more and a Zn—Mn alloy layer coated on the surface of the base steel sheet.
The Zn—Mn alloy layer has a coating amount of 0.10 g / m ² or more and less than 90 g / m ² .
Moreover, a high-strength steel plate having a Mn concentration of 5% by mass or more and less than 50% by mass in the Zn—Mn alloy layer.
[2] The high-strength steel plate according to the above [1], wherein the coating amount of the Zn—Mn alloy layer is 20 g / m ² or more and less than 90 g / m ² .
[3] The high-strength steel plate according to the above [1] or [2], wherein the Mn concentration is 10% by mass or more and 30% by mass or less.
[4] The base steel plate is C: 0.03 to 0.4%, Si: 0.01 to 3.00%, Mn: 0.5 to 3.0%, P: 0.05 in mass%. % Or less, S: 0.005% or less, Al: 0.001 to 1.500%, N: 0.005% or less, Cu: 0.05 to 1.00%, and B: 0.001 to 0. It contains 005%, and further, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, V: 0 to 0.5%, Mo: 0.05 to 1.00%, It contains one or more selected from Cr: 0.001 to 1.000% and Ni: 0.05 to 1.00%, and has a component composition in which the balance is Fe and unavoidable impurities. The high-strength steel plate according to any one of the above [1] to [3].

The high-strength steel plate of the present invention has excellent delayed fracture resistance in which delayed fracture is effectively suppressed, and also has excellent corrosion resistance. The high-strength steel plate of the present invention is suitably used as a strength member for automobiles and building materials.

It is a figure which shows typically the test piece for delayed fracture evaluation used in an Example. It is explanatory drawing which shows the process of the composite cycle corrosion test performed in an Example.

The high-strength steel sheet of the present invention has a base steel sheet having a tensile strength of 1180 MPa or more and a Zn—Mn alloy layer coated on the surface of the base steel sheet, and the Zn—Mn alloy layer has a coating amount of 0.10 g. It is characterized in that it is at least / m ² and less than 90 g / m ² , and the Mn concentration in the Zn—Mn alloy layer is 5% by mass or more and less than 50% by mass, and is excellent in delayed fracture resistance and corrosion resistance.

The steel sheet (base steel sheet) used as a substrate for the high-strength steel sheet of the present invention is a steel sheet having a tensile strength of 1180 MPa or more, preferably 1340 MPa or more.
Delayed fracture occurs when hydrogen that has entered the steel diffuses or thickens in the stress concentration area, causing embrittlement, and thus depends on the strength of the steel. That is, a steel sheet having a low tensile strength is essentially less likely to undergo delayed fracture. Therefore, the effect of the present invention can be obtained even in a steel sheet having a low tensile strength, but it is remarkably exhibited in a steel sheet having a tensile strength of 1180 MPa or more, and more remarkably in a steel sheet having a tensile strength of 1340 MPa or more.

The above tensile strength can be measured in accordance with JIS Z 2241 (2011).

The composition and steel structure of the base steel sheet are not particularly limited. Further, 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 as the base steel sheet. Of these, cold-rolled steel sheets are preferably used in view of the fact that steel sheets are used in the fields of automobiles and building materials, and in particular, they are often used in the fields of automobiles. That is, the base steel sheet of the present invention is preferably a high-strength cold-rolled steel sheet having a tensile strength of 1180 MPa or more, and more preferably a high-strength cold-rolled steel sheet having a tensile strength of 1340 MPa or more.

The base steel plate preferably used in the present invention may have any composition and steel structure as long as it has a desired tensile strength, and in order to improve various properties such as mechanical properties, for example, C, N. Strengthening of solid solution by addition of intrusive solid solution elements such as Si, Mn, P, Cr, etc., precipitation strengthening by carbonic acid / nitride such as Ti, Nb, V, Al, W, Zr, Hf , Co, B, Cu, chemical composition modification such as addition of strengthening elements such as rare earth elements, strengthening by recovery baking at a temperature where recrystallization does not occur, or parts that leave unrecrystallized regions without complete recrystallization. Recrystallization reinforcement, reinforcement by transformation structure such as bainite or martensite monophasic formation or composite assembly of ferrite and these transformation structures, Hall-Petch formula when ferrite grain size is d: σ = σ0 + kd-1 / 2 ( In the formula, σ: stress, σ0, k: material constant) can be used alone or in combination for structural or structural modification such as granulation strengthening and processing strengthening by rolling or the like.

The composition of such a base steel plate is, for example, in terms of mass%, C: 0.03 to 0.4%, Si: 0.01 to 3.00%, Mn: 0.5 to 3.0%, and the like. P: 0.05% or less, S: 0.005% or less, Al: 0.001 to 1.500%, N: 0.005% or less, Cu: 0.05 to 1.00%, and B: 0 .001 to 0.005%, Nb: 0.005 to 0.050%, Ti: 0.005 to 0.080%, V: 0 to 0.5%, Mo: 0.05 to Contains one or more selected from 1.00%, Cr: 0.001 to 1.000%, and Ni: 0.05 to 1.00%, with the balance being Fe and unavoidable impurities. It can have a certain component composition.

Hereinafter, the composition of the steel sheet of the present invention will be described. In addition, the% display regarding a component shall mean mass% unless otherwise specified.

C: 0.03 to 0.4%
C has the effect of increasing the strength of the steel sheet. In order to obtain the effect, C of 0.03% or more is required. On the other hand, if the C content exceeds 0.4%, the weldability required for use as a material for automobiles and home appliances deteriorates. Therefore, the C content may be 0.03 to 0.4%.

Si: 0.01-3.00%
Si is an element effective for strengthening steel and improving ductility, and 0.01% or more of Si is required to obtain the effect. On the other hand, when the Si content exceeds 3.00%, Si forms an oxide on the surface and the plating appearance is deteriorated. Therefore, the Si content may be 0.01 to 3.00%. It is preferably 0.01 to 2.00%.

Mn: 0.5-3.0%
Mn is an element useful for enhancing hardenability and increasing the strength of steel sheets. The effect cannot be obtained when the Mn content is less than 0.5%. On the other hand, if the Mn content exceeds 3.0%, Mn segregation occurs and the workability is lowered. Therefore, the Mn content may be 0.5 to 3.0%.

P: 0.05% or less P is one of the elements inevitably contained, and in order to make it less than 0.005%, there is a concern that the cost will increase, so it should be 0.005% or more. desirable. On the other hand, when the P content exceeds 0.05%, the delayed fracture resistance of the molded steel sheet is deteriorated through the deterioration of local ductility due to the grain boundary embrittlement due to the P segregation to the austenite grain boundaries during casting. Therefore, the P content is preferably reduced as much as possible, and the P content may be 0.05% or less. It is preferably 0.02% or less.

S: 0.005% or less S is an element inevitably contained in the steelmaking process. However, if it is contained in a large amount, the weldability deteriorates. Therefore, the S content may be 0.005% or less.

Al: 0.001 to 1.500%
Al is added for the purpose of deoxidizing molten steel, but if the content is less than 0.001%, the purpose is not achieved. On the other hand, when the Al content exceeds 1.500%, Al and N are combined to form a nitride. The nitride precipitates on the austenite grain boundaries during casting and embrittles the grain boundaries, thereby deteriorating the delayed fracture property. Therefore, the Al content may be 0.001 to 1.500%.

N: 0.005% or less N combines with Al to form a nitride. When the N content exceeds 0.005%, the nitride precipitates on the austenite grain boundaries during casting, causing embrittlement and deteriorating the delayed fracture resistance. Therefore, the N content is set to 0.005% or less.

Cu: 0.05-1.00%
Cu has the effect of suppressing the melting of the steel sheet in a corrosive environment and reducing the amount of hydrogen invading the steel. If the Cu content is less than 0.05%, the effect cannot be obtained. On the other hand, if the Cu content exceeds 1.00%, the cost will increase. Therefore, the Cu content may be 0.05 to 1.00%.

B: 0.001 to 0.005%
When B is 0.001% or more, the quenching promoting effect can be obtained. On the other hand, if the B content exceeds 0.005%, the chemical conversion treatment property deteriorates. Therefore, the B content may be 0.001 to 0.005%.

Nb: 0.005 to 0.050%
When Nb is 0.005% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if the Nb content exceeds 0.050%, the cost will increase. Therefore, when it is contained, the Nb content may be 0.005 to 0.050%.

Ti: 0.005 to 0.080%
When Ti is 0.005% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if the Ti content exceeds 0.080%, the chemical conversion treatment property is deteriorated. Therefore, when it is contained, the Ti content may be 0.005 to 0.080%.

V: 0 to 0.5%
The fine carbide formed by combining V and C acts as precipitation strengthening of the steel sheet and as a trap site for hydrogen, and is effective in improving the delayed fracture resistance. Therefore, it may be contained as necessary. If the V content exceeds 0.5% by mass, carbides may be excessively precipitated and the strength-ductility balance may be deteriorated. Therefore, the V content is preferably 0 to 0.5%. It is more preferably 0.1% or less, still more preferably 0.05% or less. In the present invention, the content of V of 0.004% or more is preferable in order to obtain the above effect.

Mo: 0.05-1.00%
When Mo is 0.05% or more, the effect of strength adjustment (strength improvement) can be obtained. On the other hand, if the Mo content exceeds 1.00%, the cost will increase. Therefore, when it is contained, the Mo content may be 0.05 to 1.00%.

Cr: 0.001 to 1.000%
A quenching effect can be obtained when Cr is 0.001% or more. On the other hand, when the Cr content exceeds 1.000%, the surface of Cr becomes thickened, so that the weldability deteriorates. Therefore, when it is contained, the Cr content may be 0.001 to 1.000%.

Ni: 0.05-1.00%
When Ni is 0.05% or more, the effect of promoting the formation of the residual γ phase can be obtained. On the other hand, if the Ni content exceeds 1.00%, the cost will increase. Therefore, when it is contained, the Ni content may be 0.05 to 1.00%.

The balance is Fe and unavoidable impurities.
When the content of Nb, Ti, Mo, Cr, and Ni described as the above component is less than the lower limit, the component is assumed to be contained as an unavoidable impurity.

The thickness of the base steel plate as the substrate in the present invention is not particularly limited, but is preferably 0.8 to 2.5 mm, more preferably 1.2 to 2.0 mm.

In the high-strength steel sheet of the present invention, the surface of the steel sheet (base steel sheet) as described above is a specific Zn—Mn alloy layer, that is, the coating amount is 0.10 g / m ² or more and less than 90 g / m ² , and Mn. It is coated with a Zn—Mn alloy layer having a concentration of 5% by mass or more and less than 50% by mass.

According to the research and study results of the present inventors, regarding the invasion of hydrogen into the inside of the zinc-plated steel sheet in the corrosion process, in the corrosion process under wet conditions, the sacrificial anticorrosion action of Zn plating causes the base iron. It was considered that the generation of a large amount of hydrogen greatly contributed to the delayed fracture of the steel sheet. Further, in order to suppress the invasion of hydrogen into the steel sheet, it was found that it is particularly important to suppress the invasion of hydrogen into the steel sheet during the corrosion process. Then, in Zn-Mn plating, it was considered that hydrogen invasion into the steel sheet could be suppressed by forming a dense and highly protective corrosion product in the corrosion process by Mn.

From the above estimation mechanism, in order to form an effective corrosion product for suppressing hydrogen intrusion, the coating amount of the Zn—Mn alloy layer on the steel plate surface is 0.10 g / m ² or more and less than 90 g / m ² . It was found that the Mn concentration of the Zn—Mn alloy layer needs to be 5% by mass or more and less than 50% by mass. Further, when applied to a member particularly severe in corrosion, the coating amount of the Zn—Mn alloy layer is preferably 20 g / m ² or more and less than 90 g / m ² .

If the coating amount of the Zn—Mn alloy layer is less than 0.10 g / m ² , it is not possible to achieve both a sufficient effect of suppressing hydrogen intrusion and an effect of improving corrosion resistance. For example, when the coating amount of the Zn—Mn alloy layer is very small with respect to 0.10 g / m ² , the hydrogen intrusion amount can be reduced, but the desired corrosion resistance cannot be obtained. On the other hand, if it is 90 g / m ² or more, the manufacturing cost increases. Therefore, the coating amount of the Zn—Mn alloy layer is 0.10 g / m ² or more and less than 90 g / m ² , preferably 20 g / m ² or more and less than 90 g / m ² .

The high-strength steel sheet of the present invention may be one in which the above-mentioned Zn—Mn alloy layer is coated on one side of the base steel sheet, or may be one in which both sides of the base steel sheet are coated.
The coating amount of the Zn—Mn alloy layer can be calculated from the mass difference of the steel sheet before and after melting the Zn—Mn alloy layer on the base steel sheet.
When the Zn—Mn alloy layer is coated only on one side of the base steel sheet, the mass difference (g) is divided by the area of one side of the base steel sheet (mass difference (g) / area of one side of the base steel sheet (m ² )). Therefore, the coating amount of the Zn—Mn alloy layer can be measured.
When both sides of the base steel sheet are coated with the Zn—Mn alloy layer, the mass difference (g) is divided by a value twice the area of one side of the base steel sheet (the mass difference (g) / (one side of the base steel sheet). The coverage of the Zn—Mn alloy layer can be measured by the area (m ² ) × 2)).

Further, if the Mn concentration in the Zn—Mn alloy layer is less than 5% by mass, a corrosion product having sufficient protective properties is not formed in the corrosion process, and hydrogen intrusion into the steel sheet cannot be suppressed. On the other hand, when the amount is 50% by mass or more, the potential is larger than that of Zn and becomes low, which promotes hydrogen generation on the ground iron. Therefore, the Mn concentration in the Zn—Mn alloy layer is 5% by mass or more and less than 50% by mass, preferably 10% by mass or more and 30% by mass or less.
The Mn concentration can be measured by ICP emission spectroscopy using a solution obtained by dissolving the Zn—Mn alloy layer.

The method of coating the surface of the base steel plate with the Zn—Mn alloy layer is not particularly limited, and a known method can be applied. For example, an electroplating method (Zn—Mn alloy electroplating method), A non-electroplating method, a vapor deposition method, or the like can be used.

In the case of the electroplating method (Zn—Mn alloy electroplating method), the Mn concentration of the Zn—Mn alloy layer can be changed by adjusting the concentrations of Zn and Mn contained in the plating bath, and the electrolytic time. The coating amount of the Zn—Mn alloy layer can be changed by adjusting. Further, in the case of the electroless plating method, the Mn concentration of the Zn—Mn alloy layer can be changed by adjusting the concentrations of Zn and Mn contained in the plating bath, and the Zn concentration can be changed by adjusting the electrolytic time. -The coating amount of the Mn alloy layer can be changed. Further, in the case of the thin-film deposition method, the Mn concentration can be adjusted by using Zn and Mn as targets, and the coating amount of the Zn—Mn alloy layer can be changed by adjusting the vapor deposition time.

As described above, the high-strength steel sheet of the present invention may be one in which the above-mentioned Zn—Mn alloy layer is coated on one side of the base steel sheet, or may be one in which both sides of the base steel sheet are coated.

The Zn—Mn alloy layer is basically composed of Zn and Mn, but is selected from V, Mo, and W as long as the condition that the Zn concentration is higher than the Mn concentration is satisfied. The above may be contained in a small amount (10% by mass or less in total).

According to X-ray diffraction, Mn in the Zn—Mn alloy layer is not oxidized and is in an alloy state. Since it is not in an oxidized state, it is presumed that it reacts at the time of corrosion to form the above-mentioned dense corrosion product.

The method for producing the base steel sheet used as the substrate is not particularly limited. In order to facilitate the understanding of the present invention, a series of processes from steelmaking in the case where the surface of a cold-rolled steel sheet is coated with a Zn—Mn alloy layer will be briefly described with an example.

Steel with a predetermined composition is melted and continuously cast into slabs according to a conventional method. Next, the obtained slab is heated in a heating furnace at a temperature of 1100 to 1300 ° C., hot rolled at a finishing temperature of 750 to 950 ° C., and wound at 500 to 650 ° C. Following this, after pickling, cold rolling with a rolling reduction of 30 to 70% is performed. Then, if necessary, according to a conventional method, a cleaning treatment of alkali or a mixed solution of an alkali with a surfactant and a chelating agent, electrolytic cleaning, hot water cleaning, and drying is performed, and then the temperature is set to 750 to 900 ° C. Heat treatment is performed, rapid cooling is performed, and the tensile strength of the steel plate is adjusted. Further, if necessary, a cold-rolled steel sheet having a desired tensile strength is obtained by performing temper rolling with an elongation rate of about 0.01 to 0.5% according to a conventional method, and the cold-rolled steel sheet thus obtained is obtained. The surface of the Zn—Mn alloy layer is coated with a Zn—Mn alloy layer by electroplating, electroless plating, vapor deposition, or the like, with a coating amount of 0.10 g / m ² or more and less than 90 g / m ² in the Zn—Mn alloy layer. The Mn concentration is coated so as to be 5% by mass or more and less than 50% by mass. Thereby, a high-strength cold-rolled steel sheet as an example of the high-strength steel sheet of the present invention can be obtained.

When a plating method, particularly an electroplating method, is used to coat the surface of the cold-rolled steel plate with the Zn—Mn alloy layer, and there is a risk that hydrogen may enter the steel plate and the Zn—Mn alloy layer during the plating process. If necessary, it may be held at a temperature of about 100 to 300 ° C. for about 24 hours after the plating treatment, and may be subjected to a treatment for removing hydrogen that has entered into the steel plate and the Zn—Mn alloy layer.

As described above, the high-strength steel plate of the present invention is excellent in delayed fracture resistance and corrosion resistance.
In the present invention, for excellent delayed fracture resistance, a ground test piece (30 mm × 99.5 mm) is bent 180 ° with a radius of curvature of 4 mmR, and bolts and nuts are used so that the inner spacing is 8 mm. For the delayed fracture test piece obtained by restraining and fixing, "salt water immersion (0.5 mass% NaCl, 0.1 mass% CaCl ₂ and 0.075 mass) specified in SAE J2334. "Drying ₍ 60 ° C., 17 hours 45 minutes under RH 50% environment)", "Wet (50 ° C., 6 hours under 100% RH environment)" A combined cycle corrosion test was carried out in order to make one cycle (24 hours), and the presence or absence of cracks was investigated before the salt water immersion process of each cycle, and the number of crack occurrence cycles was 30 or more. Point to.
Further, the excellent corrosion resistance means that the presence or absence of red rust is investigated before the salt water immersion step of each of the above cycles, and no red rust is generated after 5 cycles.

As the base steel sheet, a cold-rolled steel sheet having a thickness of 1.5 mm (tensile strength of 1480 MPa) having each component composition of A to F in Table 1 was used as a test piece. These cold-rolled steel sheets were immersed in toluene and ultrasonically cleaned for 5 minutes to remove rust-preventive oil, and then Zn—Mn alloy electroplating was performed. Formed.

The above tensile strength was measured according to JIS Z 2241 (2011).

As the electroplating solution, divalent manganese ion concentration: 10 g / L, divalent zinc ion concentration: 15 g / L was added as a sulfate, 180 g / L of trisodium citrate was added, and the pH was adjusted to 5.5 by sulfuric acid. The adjusted one was used. By adjusting the current density in the range of 10 to 80 A / dm ² , the Mn concentration of the Zn—Mn alloy layer is changed, and by adjusting the electrolysis time, the coating amount of the Zn—Mn alloy layer is changed. .1 to 20 and 22 to 41 steel plates were produced.

A steel sheet (No. 21) not subjected to the above plating treatment was used as one of the comparative examples.

The coating amount of the Zn—Mn alloy layer was determined from the difference in mass of the steel sheet before and after melting the Zn—Mn alloy layer by immersing the steel sheet in hydrochloric acid. The end point of dissolution of the Zn—Mn alloy layer was determined by adding an inhibitor that prevents the dissolution of iron to hydrochloric acid. By adding an inhibitor to hydrochloric acid, gas is generated while the Zn—Mn alloy layer is being melted, whereas gas generation is completed when the dissolution of the Zn—Mn alloy layer is completed. By visually observing the end of this gas generation, it was determined that the dissolution of the Zn—Mn alloy layer was completed.

The Mn concentration of the Zn—Mn alloy layer was measured by ICP emission spectroscopy from the solution obtained by dissolving the alloy layer with hydrochloric acid.

The test pieces obtained as described above were evaluated as follows. The obtained results are shown in Table 2 together with the composition of the Zn—Mn alloy layer.
(1) Evaluation of Delayed Fracture Resistance A test piece (30 mm × 99.5 mm) produced by grinding is bent 180 ° with a radius of curvature of 4 mmR, and as shown in FIG. 1, the bending test piece 1 is inside. The shape of the test piece was fixed by restraining it with bolts 2 and nuts so that the interval was 8 mm, and a test piece for delayed fracture evaluation was obtained. The test piece for delayed fracture evaluation thus prepared was immersed in salt water (0.5% by mass NaCl, 0.1% by mass CaCl ₂ , and 0.1% by mass CaCl 2 specified in SAE J2334 defined by the American Society of Automotive Engineers of Japan. Dry (60 ° C., 17 hours 45 minutes in a RH 50% environment), wet (50 ° C., 6 hours in a RH 100% environment) in a 25 ° C. solution with 0.075% by weight NaCl ₃ . A combined cycle corrosion test (see FIG. 2), which was carried out in order to make one cycle (24 hours), was carried out up to a maximum of 80 cycles. Before the salt water immersion process, which is the first step of each cycle, the presence or absence of cracks was visually investigated, and the number of crack occurrence cycles was measured. In addition, this test was carried out for each of the three steel plates, and the average value was used for evaluation. The evaluation was based on the number of cycles according to the following criteria. The case where the number of crack occurrence cycles is 30 or more is regarded as acceptable (○ or ◎). In this evaluation, the test was started on weekday Monday, and the number of cycles was evaluated only by the number of cycles on weekdays (Monday to Friday), and the number of cycles on holidays (Saturday and Sunday) was not evaluated.
⊚: 40 cycles or more ○: 30 cycles or more, less than 40 cycles Δ: 10 cycles or more, less than 30 cycles ×: less than 10 cycles

(2) Evaluation of corrosion resistance With respect to the sample subjected to the above-mentioned delayed fracture resistance evaluation, the presence or absence of red rust generation was visually investigated before the salt water immersion step of each cycle, and the number of red rust generation cycles was measured. The evaluation was based on the number of cycles according to the following criteria. The case where no red rust was generated after 5 cycles was regarded as acceptable (○). In this evaluation as well, the test was started on weekday Mondays, and the number of cycles was evaluated only on weekdays (Monday to Friday), not on holidays (Saturday, Sunday).
◯: No red rust occurred after 5 cycles ×: Red rust occurred in less than 5 cycles

According to Table 2, the steel sheets of the examples of the present invention all have excellent delayed fracture resistance and corrosion resistance. Among them, the steel sheet No. 1 in which the coating amount of the Zn—Mn alloy layer is 20 g / m ² or more and less than 90 g / m ² and the Mn concentration in the Zn—Mn alloy layer is 10% by mass or more and 30% by mass or less. It was found that 7, 9, 14, 15, 22 to 41 obtained excellent corrosion resistance and very excellent delayed fracture resistance.
On the other hand, the steel sheet of the comparative example is inferior in either or both of delayed fracture resistance and corrosion resistance.

1 Test piece 2 Bolt 3 Nut

Claims (3)

It has a base steel plate having a tensile strength of 1180 MPa or more and a Zn—Mn alloy layer coated on the surface of the base steel plate, and the base steel plate has a mass% of C: 0.03 to 0.4%. Si: 0.01 to 3.00%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.001 to 1.500%, N : 0.005% or less, Cu: 0.05 to 1.00%, and B: 0.001 to 0.005%, and further, Nb: 0.005 to 0.050%, V: 0 to One or more selected from 0.5%, Mo: 0.05 to 1.00%, Cr: 0.001 to 1.000%, and Ni: 0.05 to 1.00%. The Zn—Mn alloy layer contains a component composition in which the balance is Fe and unavoidable impurities, and the coating amount of the Zn—Mn alloy layer is 0.10 g / m ² or more and less than 90 g / m ² , and the Zn—Mn alloy layer is present. A high-strength cold- rolled steel sheet characterized in that the Mn concentration of the alloy layer is 5% by mass or more and less than 50% by mass. The high-strength cold- rolled steel sheet according to claim 1, wherein the coating amount of the Zn—Mn alloy layer is 20 g / m ² or more and less than 90 g / m ² . The high-strength cold- rolled steel sheet according to claim 1 or 2, wherein the Mn concentration is 10% by mass or more and 30% by mass or less.

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