JP7601039B2 - High-strength steel plates and automobile bodies - Google Patents

Description

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

Conventionally, zinc-plated steel sheets have been widely used as steel sheets for automobiles. This is because the sacrificial anticorrosive effect of zinc can significantly improve the corrosion resistance of steel sheets after painting. In recent years, efforts have been made to increase the strength of steel sheets for automobiles from the viewpoints of reducing _CO2 emissions from automobiles and ensuring safety.

However, it is known that increasing the strength of steel makes it more susceptible to a phenomenon known as delayed fracture. This delayed fracture is more likely to occur as the strength of the steel increases, and is particularly noticeable in high-strength steel with a tensile strength of 1180 MPa or more. Delayed fracture is a phenomenon in which high-strength steel is subjected to a static load stress (load stress less than the tensile strength) and, after a certain amount of time has passed, suddenly undergoes brittle fracture with little or no apparent plastic deformation.

In the case of steel sheets, it is known that this delayed fracture occurs due to residual stress when pressed into a specified shape and hydrogen embrittlement of the steel in areas of stress concentration. The hydrogen that causes this hydrogen embrittlement is thought to be hydrogen that in most cases penetrates into the steel from the external environment and diffuses there; typically, hydrogen that is generated during corrosion of the steel sheet penetrates into the steel and diffuses there. In general, galvanized steel sheets penetrate more hydrogen during corrosion than cold-rolled steel sheets, and are exposed to a more severe environment for the occurrence of delayed fracture.

In particular, among automotive structural components, the side sills (rockers), lower center pillars (B-pillars), bumpers and other automotive body lower structural components, as well as chassis structural components such as lower arms, are subject to severe corrosive environments and are particularly in need of measures to prevent delayed fracture.

In order to prevent such delayed fracture in high-strength steel sheets, for example, Patent Document 1 studies ways to reduce the delayed fracture susceptibility by adjusting the structure and components of the steel sheets. In addition, Patent Document 2 studies high-strength hot-dip galvanized steel sheets that prevent delayed fracture.

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

However, the technology disclosed in Patent Document 1 does not change the amount of hydrogen that penetrates into the steel sheet from the external environment, so although it is possible to delay the occurrence of delayed fracture, it is not possible to prevent delayed fracture itself. Furthermore, even if the steel composition and microstructural structure were adjusted based on the technology disclosed in Patent Document 2, sufficient delayed fracture resistance could not be obtained.

As a result, high-strength steel plates that are excellent in both corrosion resistance and delayed fracture resistance have not yet been put into practical use.

The object of the present invention is to solve the problems of the conventional technology as described above and to provide a high-strength steel plate with excellent delayed fracture resistance and corrosion resistance suitable primarily for strength components of automobiles.

In order to solve the above problems, the present inventors have conducted extensive research and studies into means for preventing delayed fracture by suppressing hydrogen penetration into the steel sheet. As a result, they have found that by coating the surface of the base steel sheet with a Zn- _SiO2 composite plating layer having a specific coating amount and specific _SiO2 concentration, it is possible to significantly suppress hydrogen penetration into the steel sheet, prevent delayed fracture of the steel sheet, and also obtain excellent corrosion resistance.

The present invention has been made based on these findings, and has the following gist.
[1] A steel sheet having a tensile strength of 1180 MPa or more and a Zn- _SiO2 composite plating layer coated on the surface of the steel sheet;
The Zn- _SiO2 composite plating layer has a coating amount of 0.10 g/m2 ^or more;
And the _SiO2 concentration in the Zn- _SiO2 composite plating layer is 0.1 mass% or more and less than 10 mass%.
[2] The high-strength steel sheet according to [1], characterized in that the coating weight of the Zn- _SiO2 composite plating layer is 20 g/m2 or ^more .
[3] The high-strength steel plate according to [1] or [2], characterized in that the _SiO2 concentration is 0.5 mass% or more and less than 5 mass%.
[4] The high-strength steel sheet according to any one of [1] to [3], characterized in that the coating amount of the Zn- _SiO2 composite plating layer is less than 90 g/ ^m2 .
[5] The high-strength steel plate according to any one of [1] to [4], characterized in that the base steel plate has a component composition containing, in mass%, C: 0.03% or more and 0.45% or less, Si: 0.01% or more and 3.00% or less, Mn: 0.5% or more and 3.5% or less, P: 0.050% or less, S: 0.0050% or less, Al: 0.001% or more and 1.500% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities.
[6] The high-strength steel plate according to [5], characterized in that the base steel plate further contains, in mass%, one or more selected from Cu: 1.000% or less, B: 0.0050% or less, Nb: 0.100% or less, Ti: 0.200% or less, V: 0.500% or less, Mo: 1.000% or less, Cr: 1.000% or less, Ni: 1.00% or less, W: 0.200% or less, Zr: 0.200% or less, Hf: 0.200% or less, Co: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0100% or less, Sn: 0.200% or less, Sb: 0.200% or less, and REM: 0.005% or less.
[7] An automobile body or automobile suspension part using the high strength steel plate according to any one of [1] to [6].
[8] An automobile body using the high-strength steel plate according to any one of [1] to [6] in one or more locations of a side sill, a lower part of a center pillar, and a bumper.
[9] An automobile chassis using the high strength steel plate according to any one of [1] to [6] in a lower arm portion.

The high-strength steel plate of the present invention has excellent delayed fracture resistance that effectively suppresses delayed fracture, and also has excellent corrosion resistance. The high-strength steel plate of the present invention is suitable for use in strength members for automobiles and building materials. In particular, it can be used for automobile body lower structural members such as side sills, lower center pillars, and bumpers, which require high strength and are subject to severe corrosive environments, and for automobile chassis structural members such as lower arms.

1 is a diagram showing a schematic diagram of a test piece for evaluating delayed fracture used in Examples. FIG. 2 is an explanatory diagram showing the steps of a combined cycle corrosion test carried out in the examples.

The high-strength steel sheet of the present invention has a substrate steel sheet having a tensile strength of 1180 MPa or more, and a Zn- _SiO2 composite plating layer coated on the surface of the substrate steel sheet, the Zn- _SiO2 composite plating layer having a coating weight of 0.10 g/ ^m2 or more and a _SiO2 concentration in the Zn- _SiO2 composite plating layer of 0.1 mass% or more and less than 10 mass%, and has excellent delayed fracture resistance and corrosion resistance.

The steel plate (base steel plate) that serves as the substrate for the high-strength steel plate of the present invention has a tensile strength of 1180 MPa or more, preferably 1320 MPa or more.

Delayed fracture occurs when hydrogen that has penetrated into the steel diffuses or concentrates in areas of stress concentration, causing embrittlement, and so depends on the strength of the steel. In other words, delayed fracture is essentially less likely to occur in steel plates with low tensile strength. Thus, the effects of the present invention can be obtained with steel plates with low tensile strength, but are most pronounced in steel plates with a tensile strength of 1180 MPa or more, and even more pronounced in steel plates with a tensile strength of 1320 MPa or more.

Automobile body lower structural components such as side sills, lower center pillars, and bumpers, as well as automobile chassis structural components such as lower arms, are required to have a tensile strength of 1180 MPa or more, as they require high collision safety and a lightweight structure. Furthermore, the corrosive environment of the lower structures is such that mud and salt water accumulate as the vehicle travels, making corrosion more likely to progress, and therefore they require superior corrosion resistance compared to the automobile body upper structure.

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

The composition and structure of the base steel sheet are not particularly limited. The rolling method is also 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, it is preferable to use a cold-rolled steel sheet, considering that steel sheets are used in the automotive and building material fields, and are particularly widely used in the automotive field. 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 1320 MPa or more.

The base steel sheet preferably used in the present invention may have any chemical composition and steel structure as long as it has the desired tensile strength. In order to improve various properties such as mechanical properties, for example, solid solution strengthening by adding interstitial solid solution elements such as C and N, or substitutional solid solution elements such as Si, Mn, P, and Cr, precipitation strengthening by carbonitrides such as Ti, Nb, V, and Al, chemical composition modification such as adding strengthening elements such as W, Zr, Hf, Co, B, Cu, and rare earth elements, and recovery at a temperature where recrystallization does not occur may be performed. These structural or organizational modifications can be performed alone or in combination with one another, such as strengthening by annealing, or partial recrystallization strengthening where unrecrystallized regions are left without complete recrystallization, strengthening by transformation structures such as single-phase bainite or martensite or a composite structure of ferrite and these transformed structures, grain refinement strengthening represented by the Hall-Petch equation: σ = σ0 + kd-1/2 (where σ is stress, σ0:, k are material constants) where d is the ferrite grain size, and processing strengthening by rolling, etc.

The chemical composition of such a base steel sheet may, for example, contain, by mass, C: 0.03% to 0.45%, Si: 0.01% to 3.00%, Mn: 0.5% to 3.5%, P: 0.050% or less, S: 0.0050% or less, Al: 0.001% to 1.500%, and N: 0.0100% or less, and further, by mass, Cu: 1.000% or less, B: 0.0050% or less, Nb: 0.100% or less, Ti: 0.200% or less, V: 0.500% or less, %, Mo: 1.000% or less, Cr: 1.000% or less, Ni: 1.00% or less, W: 0.200% or less, Zr: 0.200% or less, Hf: 0.200% or less, Co: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0100% or less, Sn: 0.200% or less, Sb: 0.200% or less, REM: 0.005% or less, and the balance is Fe and unavoidable impurities. Note that P, S, and N may exceed 0% due to production technology constraints.

The chemical composition of the steel sheet of the present invention will be described below. Note that percentages for components refer to mass % unless otherwise specified.

C: 0.03% or more and 0.45% or less C has the effect of increasing the strength of the steel sheet. To obtain this effect, 0.03% or more of C is preferable. On the other hand, if the C content exceeds 0.45%, the weldability, which is necessary when used as a material for automobiles and home appliances, deteriorates. Therefore, the C content may be 0.03% or more and 0.45% or less.

Si: 0.01% to 3.00% Si is an effective element for strengthening steel and improving ductility, and in order to obtain this effect, 0.01% or more of Si is preferable. On the other hand, if the Si content exceeds 3.00%, Si forms an oxide on the surface, which deteriorates the plating appearance. Therefore, the Si content may be set to 0.01% to 3.00%. More preferably, it is set to 0.01% to 2.00%.

Mn: 0.5% to 3.5% Mn is a useful element for improving hardenability and strength of steel sheet. This effect cannot be obtained if the Mn content is less than 0.5%. On the other hand, if the Mn content exceeds 3.5%, segregation of Mn occurs and workability is reduced. Therefore, the Mn content may be 0.5% to 3.5%. From the viewpoint of improving workability, Mn is preferably 3.0% or less.

P: 0.050% or less P is one of the elements that is inevitably contained, and in order to make it less than 0.005%, there is a concern of an increase in cost, so it is preferable that it is 0.005% or more. On the other hand, if the P content exceeds 0.050%, the delayed fracture resistance of the steel sheet after forming is deteriorated through deterioration of local ductility due to grain boundary embrittlement associated with P segregation to the austenite grain boundary during casting. Therefore, it is preferable to reduce the P content as much as possible, and the P content may be 0.050% or less. The P content is preferably 0.020% or less.

S: 0.0050% or less S is an element that is inevitably contained during the steelmaking process. However, if a large amount is contained, weldability deteriorates. Therefore, the S content may be 0.0050% or less. From the viewpoint of improving delayed fracture resistance, it is preferable that S is 0.0020% or less. Since there is a concern that costs will increase in order to make S 0.0001% or less, it is preferable that S is 0.0002% or more.

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%, this purpose is not achieved. On the other hand, if the Al content exceeds 1.500%, Al and N combine to form nitrides. The nitrides precipitate on the austenite grain boundaries during casting, causing grain boundary embrittlement and deteriorating delayed fracture resistance. Therefore, the Al content may be 0.001% to 1.500%.

N: 0.0100% or less N combines with Al to form nitrides. Nitrides precipitate on the austenite grain boundaries during casting, causing embrittlement and deteriorating delayed fracture resistance. Therefore, the N content is set to 0.0100% or less. From the above viewpoint, N is preferably set to 0.0050% or less. In order to set N to 0.0001% or less, there is a concern of increased costs, so it is preferable that N is 0.0005% or more.

Cu: 1.000% or less Cu has the effect of suppressing dissolution of the steel sheet in a corrosive environment and reducing the amount of hydrogen that penetrates into the steel. On the other hand, if the Cu content exceeds 1.000%, the cost increases. Therefore, the Cu content may be 1.000% or less. In order to obtain the above effect, the Cu content is preferably 0.005% or more.

B: 0.0050% or less The addition of B has the effect of promoting hardening. On the other hand, if the B content exceeds 0.0050%, the inclusions increase and the delayed fracture resistance deteriorates. Therefore, the B content may be set to 0.0050% or less.

Nb: 0.100% or less When Nb is 0.005% or more, the effect of improving strength can be obtained. On the other hand, when the Nb content exceeds 0.100%, the inclusions increase, deteriorating the delayed fracture resistance property and inviting an increase in cost. Therefore, when Nb is contained, the Nb content may be 100% or less, and is preferably 0.005% or more. From the viewpoint of reducing inclusions, it is more preferable that Nb is 0.050% or less.

Ti: 0.200% or less When Ti is 0.005% or more, the effect of improving strength can be obtained. On the other hand, when the Ti content exceeds 0.200%, inclusions increase and delayed fracture resistance deteriorates. Therefore, when Ti is contained, the Ti content may be 0.200 or less, and is preferably 0.005% or more. From the viewpoint of reducing inclusions, Ti is more preferably 0.080% or less.

V: 0.500% or less Fine carbides formed by combining V and C are effective in improving delayed fracture resistance by acting as precipitation strengthening and hydrogen trapping sites for the steel sheet, and may be contained as necessary. If the V content exceeds 0.500%, carbides may be excessively precipitated, which may deteriorate the strength-ductility balance. For this reason, the V content is preferably 0.500% or less. More preferably, it is 0.100% or less, and even more preferably, it is 0.050% or less. In the present invention, in order to obtain the above effects, it is preferable to contain 0.005% or more of V.

Mo: 1.000% or less When Mo is 0.005% or more, the effect of improving strength can be obtained. On the other hand, if the Mo content exceeds 1.000%, it will lead to an increase in costs. Therefore, when Mo is contained, the Mo content may be 1.000% or less, and is preferably 0.005% or more.

Cr: 1.000% or less When Cr is 0.005% or more, the hardenability effect is obtained. On the other hand, when the Cr content exceeds 1.000%, Cr is concentrated on the surface, which deteriorates the weldability. Therefore, when Cr is contained, the Cr content may be 1.000% or less, and preferably 0.005% or more.

Ni: 1.00% or less Ni has an effect of promoting the formation of residual γ phase when it is 0.005% or more. On the other hand, if the Ni content exceeds 1.00%, it will lead to an increase in costs. Therefore, if Ni is contained, the Ni content may be 1.00% or less, and is preferably 0.005% or more.

W: 0.200% or less When W is 0.005% or more, the effect of improving strength can be obtained. On the other hand, when the W content exceeds 0.200%, the inclusions increase, deteriorating the delayed fracture resistance property and inviting an increase in cost. Therefore, when W is contained, the W content may be 0.200% or less, and preferably 0.005% or more. From the viewpoint of reducing inclusions, W is more preferably 0.030% or less.

Zr: 0.200% or less When Zr is 0.005% or more, the effect of improving strength can be obtained. On the other hand, when the Zr content exceeds 0.200%, the inclusions increase, deteriorating the delayed fracture resistance property and inviting an increase in cost. Therefore, when Zr is contained, the Zr content may be 0.200% or less, and is preferably 0.005% or more. From the viewpoint of reducing inclusions, it is more preferable that Zr is 0.030% or less.

Hf: 0.200% or less When Hf is 0.005% or more, the effect of improving strength can be obtained. On the other hand, when the Hf content exceeds 0.200%, the inclusions increase, deteriorating the delayed fracture resistance property and inviting an increase in cost. Therefore, when Hf is contained, the Hf content may be 0.200% or less, and preferably 0.005% or more. From the viewpoint of reducing inclusions, it is more preferable that Hf is 0.030% or less.

Co: 0.500% or less Co has the effect of improving ductility when it is 0.005% or more. On the other hand, if the Co content exceeds 0.500%, the cost increases significantly. Therefore, if Co is contained, the Co content may be 0.500% or less, and is preferably 0.005% or more.

Ca: 0.0050% or less When Ca is 0.0002% or more, it has the effect of spheroidizing inclusions and improving delayed fracture resistance. On the other hand, when the Ca content exceeds 0.0050%, the inclusions increase, deteriorating the delayed fracture resistance and inviting an increase in costs. Therefore, when Ca is contained, the Ca content may be 0.0050% or less, and preferably 0.0002% or more.

Mg: 0.0100% or less When Mg is 0.0002% or more, it has the effect of spheroidizing inclusions and improving delayed fracture resistance. On the other hand, when the Mg content exceeds 0.0100%, the inclusions increase, deteriorating delayed fracture resistance and increasing costs. Therefore, when Mg is contained, the Mg content may be 0.0100% or less, and preferably 0.0002% or more.

Sn: 0.200% or less Sn has the effect of suppressing decarburization of the surface layer and improving fatigue resistance when it is 0.002% or more. On the other hand, if the Sn content exceeds 0.200%, it embrittles the grain boundary, deteriorating delayed fracture resistance and increasing costs. Therefore, if Sn is contained, the Sn content may be 0.200% or less, and preferably 0.002% or more.

Sb: 0.200% or less Sb has the effect of suppressing decarburization of the surface layer and improving fatigue resistance when it is 0.002% or more. On the other hand, if the Sb content exceeds 0.200%, it embrittles the grain boundary, deteriorating delayed fracture resistance and increasing costs. Therefore, if Sb is contained, the Sb content may be 0.200% or less, and preferably 0.002% or more.

REM: 0.005% or less When REM (Rare-Earth-Metal) is 0.0002% or more, it has the effect of spheroidizing inclusions and improving delayed fracture resistance. On the other hand, when the REM content exceeds 0.0050%, the inclusions increase, deteriorating delayed fracture resistance and increasing costs. Therefore, when REM is contained, the REM content may be 0.005% or less, and preferably 0.0002% or more.

The balance is Fe and unavoidable impurities.

If the content of B, Cu, Nb, Ti, Mo, Cr, Ni, W, Zr, Hf, Co, Ca, Mg, Sn, Sb, or REM described above as the preferred range of the components is less than the lower limit, the component is considered to be included as an unavoidable impurity.

The thickness of the base steel sheet that serves as the substrate in the present invention is not particularly limited, but is preferably 0.8 to 6.0 mm, and more preferably 1.2 to 4.0 mm.

The high-strength steel sheet of the present invention is a steel sheet (base steel sheet) as described above, the surface of which is coated with a specific Zn- _SiO2 composite plating layer, i.e., a Zn- _SiO2 composite plating layer having a coating amount of 0.10 g/m2 ^or more and a _SiO2 concentration of 0.1 mass% or more and less than 10 mass%.

According to the results of the research and examination by the present inventors, regarding the penetration of hydrogen into the inside of zinc-plated steel sheet during the corrosion process, the inventors considered that the generation of a large amount of hydrogen on the base steel due to the sacrificial corrosion protection action of the Zn plating during the corrosion process in wet conditions is largely responsible for the delayed fracture of the steel sheet. Furthermore, they discovered that in order to suppress the penetration of hydrogen into the inside of the steel sheet, it is particularly important to suppress the penetration of hydrogen into the steel sheet during the corrosion process. And, they considered that in the Zn-SiO ₂ composite plating layer, the SiO ₂ forms a corrosion product with a high corrosion suppression effect during the corrosion process, which can suppress the penetration of hydrogen into the steel sheet.

From the above estimated mechanism, it was found that in order to form a corrosion product effective for suppressing hydrogen penetration, the coverage of the Zn- _SiO2 composite plating layer on the steel sheet surface must be 0.10 g/m2 or ^more , and the _SiO2 concentration of the Zn- _SiO2 composite plating layer must be 0.1 mass% or more and less than 10 mass%. In addition, when applied to members that are particularly susceptible to corrosion, it is preferable to set the coverage of the Zn- _SiO2 composite plating layer to 20 g/m2 or ^more .

If the coverage of the Zn-SiO ₂ composite plating layer is less than 0.10 g/m ² , it is not possible to achieve both a sufficient hydrogen penetration suppression effect and an effect of improving corrosion resistance. For example, if the coverage of the Zn-SiO ₂ composite plating layer is very small compared to 0.10 g/m ² , the amount of hydrogen penetration can be reduced, but the desired corrosion resistance cannot be obtained. In addition, if the coverage of the Zn-SiO ₂ composite plating layer is less than 90 g/m ² , it is preferable because it is possible to prevent a significant increase in manufacturing costs. Therefore, the coverage of the Zn-SiO ₂ composite plating 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. More preferably, it is 40 g/m ² or more and 60 g/m ² or less. If it is 40 g/m ² or more and 60 g/m ² or less, good delayed fracture resistance and corrosion resistance can be stably obtained.

The high-strength steel sheet of the present invention may be one in which the above-mentioned Zn-SiO ₂ composite plating layer is coated on one side of the base steel sheet, or may be one in which the Zn-SiO 2 composite plating layer is coated on both sides of the base steel sheet.

The coating weight of the Zn- _SiO2 composite plating layer can be calculated by dissolving the Zn- _SiO2 composite plating layer on the base steel sheet and calculating the mass difference of the steel sheet before and after dissolution.
When the Zn- _SiO2 composite plating layer is coated on only one side of the substrate steel sheet, the coating amount of the Zn- _SiO2 composite plating layer can be measured by dividing the above mass difference (g) by the area of one side of the substrate steel sheet (the above mass difference (g) / area of one side of the substrate steel sheet ( ^m2 )).
In addition, when the Zn- _SiO2 composite plating layer is coated on both sides of the base steel sheet, the coating amount of the Zn- _SiO2 composite plating layer can be measured by dividing the above mass difference (g) by twice the area of one side of the base steel sheet (the above mass difference (g)/(area of one side of the base steel sheet ( ^m2 ) × 2)).

In addition, if the SiO ₂ concentration in the Zn-SiO ₂ composite plating layer is less than 0.1 mass%, dense corrosion products having sufficient protective properties during the corrosion process are not formed, and hydrogen penetration into the steel sheet cannot be suppressed. In addition, if it is 10 mass% or more, the plating adhesion deteriorates, the plating is easily peeled off during processing, and delayed fracture resistance cannot be exhibited. Therefore, the SiO ₂ concentration in the Zn-SiO ₂ composite plating layer is 0.1 mass% or more and less than 10 mass%, and preferably 0.5 mass% or more and less than 5 mass%.
The _SiO2 concentration can be measured by ICP emission spectrometry using a solution obtained by dissolving the Zn- _SiO2 composite plating layer.

The method for covering the surface of the base steel sheet with the Zn- _SiO2 composite plating layer is not particularly limited, and any known method can be applied. For example, an electroplating method (Zn- _SiO2 composite electroplating method) can be used.

In the case of electroplating (Zn- _SiO2 composite electroplating), the _SiO2 concentration of the Zn- _SiO2 composite plating layer can be changed by adjusting the concentrations of Zn and _SiO2 contained in the plating bath, and the coating amount of the Zn- _SiO2 composite plating layer can be changed by adjusting the electrolysis time. It is also known that it is possible to include a high concentration of _SiO2 in the plating layer by adding 100 to 3000 ppm of nitrate ions to the plating bath.

The Zn- _SiO2 composite plating layer of the present invention may contain any additive element within a range that does not impair the effects of the present invention. Examples of the optional additive elements include Al, Fe, Ni, Mo, V, and W.

As described above, the high-strength steel sheet of the present invention may be one in which the above-mentioned Zn- _SiO2 composite plating layer is coated on one side of the base steel sheet, or may be one in which the Zn-SiO2 composite plating layer is coated on both sides of the base steel sheet.

The manufacturing method of the base steel sheet used as the substrate is not particularly limited. In order to facilitate 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- _SiO2 composite plating layer will be briefly described by way of an example.

Steel having a predetermined composition is melted and cast into a slab by continuous casting according to a conventional method. The resulting slab is then 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 coiled at 500 to 650°C. This is followed by pickling and cold rolling at a rolling reduction of 30 to 70%. Thereafter, if necessary, cleaning treatments are performed according to conventional methods, such as washing with an alkali or a mixed solution of an alkali, a surfactant, and a chelating agent, electrolytic washing, hot water washing, and drying, followed by heating at 750 to 900°C and rapid cooling to adjust 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 at an elongation rate of about 0.01 to 0.5% according to a conventional method, and a Zn-SiO2 composite plating layer is coated on the surface of the cold-rolled steel sheet thus obtained by electroplating so that the coating amount is 0.10 g/ _m2 ^or more and the _SiO2 concentration in the Zn- _SiO2 composite plating layer is 0.1 mass% or more and less than 10 mass%. In this way, 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 cover the surface of a cold-rolled steel sheet with a Zn- _SiO2 composite plating layer, if there is a risk of hydrogen penetrating into the steel sheet and the Zn- _SiO2 composite plating layer during the plating process, a treatment may be carried out, as necessary, by holding the steel sheet and the Zn-SiO2 composite plating layer at a temperature of about 100 to 300°C for about 24 hours after the plating process to remove hydrogen that has penetrated into the steel sheet and the Zn- _SiO2 composite plating layer.

As explained above, the high-strength steel plate of the present invention has excellent delayed fracture resistance and corrosion resistance.

The high-strength steel plate of the present invention has excellent delayed fracture resistance and excellent corrosion resistance, making it suitable for use in automobile body or suspension parts.

The high-strength steel plate of the present invention is particularly suitable for use in areas such as side sills, lower center pillars, and bumpers, which require excellent delayed fracture resistance and corrosion resistance.

In addition, the high-strength steel plate of the present invention has excellent delayed fracture resistance and excellent corrosion resistance, making it useful for use in the lower arm portion of an automobile chassis.

In the present invention, the delayed fracture resistance was evaluated by a corrosion test specified in SAE J2334 shown in Figure 2 using the test specimen shown in Figure 1. Details of the test specimen and the corrosion test will be described in the Examples.

The test pieces used as the base steel sheets were cold-rolled steel sheets (tensile strength 1480 MPa) with a thickness of 1.2 mm and each of the component compositions A to G in Table 1. These cold-rolled steel sheets were immersed in toluene and ultrasonically cleaned for 5 minutes to remove the rust-preventive oil, and then Zn- _SiO2 composite electroplating was applied to form a Zn- _SiO2 composite plating layer on both surfaces of the steel sheets.

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

The electroplating solution used was one containing 65 g/L of divalent zinc ions added as sulfate, 30 g/L of sodium sulfate, 1.6 g/L of sodium nitrate, and 20 g/L of _SiO2 dispersed therein. The _SiO2 concentration of the Zn- _SiO2 composite plating layer was changed by adjusting the current density in the range of 10 to 80 A/ ^dm2 , and the coating amount of the Zn- _SiO2 composite plating layer was changed by adjusting the electrolysis time, to produce steel sheets No. 1 to 23 and 25 to 44.

A steel sheet (No. 24) that did not undergo the above plating process was used as one of the comparative examples.

The coverage of the Zn-SiO ₂ composite plating layer was determined by immersing the steel sheet in hydrochloric acid to dissolve the Zn-SiO ₂ composite plating layer, and measuring the difference in mass of the steel sheet before and after dissolution. The end point of dissolution of the Zn-SiO ₂ composite plating layer was determined by adding an inhibitor to hydrochloric acid to prevent iron from dissolving. By adding an inhibitor to hydrochloric acid, gas is generated while the Zn-SiO ₂ composite plating layer is dissolving, whereas _{gas generation ends when the dissolution of the Zn-SiO 2 composite} plating layer ends. The end of the gas generation was determined by visually observing the end of the gas generation.

The SiO ₂ concentration of the Zn-SiO ₂ composite plating layer was measured by dissolving the alloy layer with hydrochloric acid and measuring the Si concentration in the solution by ICP emission spectrometry.

The test pieces obtained as described above were subjected to the following evaluations. The results are shown in Table 2 together with the composition of the Zn- _SiO2 composite plating layer.

(1) Evaluation of delayed fracture resistance A test piece (30 mm x 99.5 mm) prepared by grinding was bent at 180° with a radius of curvature of 4 mmR, and the bent test piece 1 was restrained with a bolt 2 and a nut 3 so that the inner distance was 8 mm, as shown in Figure 1, to fix the shape of the test piece, thereby obtaining a test piece for delayed fracture evaluation. The test piece for delayed fracture evaluation prepared in this way was subjected to a maximum of 80 cycles of complex cyclic corrosion testing, in which one cycle (24 hours) is performed in the order of "Step 1: Salt water immersion (15 minutes in a solution containing 0.5 mass% NaCl, 0.1 mass% CaCl ₂ , and 0.075 mass% NaHCO ₃ at 25°C)", "Step 2: Drying (17 hours and 45 minutes in an environment of 60°C and 50% RH)", and "Step 3: Wetting (6 hours in an environment of 50°C and 100% RH)" as shown in Figure 2. The above one cycle was performed on weekdays (Monday to Friday). On non-weekdays (Saturday and Sunday), only drying (24 hours in an environment of 60°C and 50% RH) was performed, and was not included in the number of cycles performed. Before the salt water immersion process, which was the first process of each cycle, the presence or absence of cracks was visually checked, and the number of cycles in which cracks occurred was measured. In addition, this test was performed on three specimens of each steel plate, and the evaluation was performed using the average value. The evaluation was performed based on the number of cycles and on the following criteria. A test in which the number of cycles in which cracks occurred was 30 or more was considered to have passed (○ or ◎).
◎: 40 cycles or more ○: 30 cycles or more, less than 40 cycles △: 10 cycles or more, less than 30 cycles ×: Less than 10 cycles

(2) Evaluation of corrosion resistance The samples evaluated for delayed fracture resistance were visually inspected for the presence or absence of red rust before each cycle of salt water immersion, and the number of cycles for which red rust had occurred was measured. Evaluation was based on the number of cycles and was made according to the following criteria. A sample with no red rust occurring after five cycles was rated as pass (○).
○: No red rust occurs after 5 cycles ×: Red rust occurs within 5 cycles

According to Table 2, all of the steel sheets of the present invention examples have excellent delayed fracture resistance and corrosion resistance. Among them, it was found that steel sheets Nos. 10 to 12, 15, 17 to 19, 21, and 25 to 48, in which the coverage of the Zn-SiO ₂ composite plating layer is 20 g/m ² or more and less than 90 g/m ² and the SiO ₂ concentration in the Zn-SiO ₂ composite plating layer is 0.1 mass % or more and less than 10 mass %, have excellent corrosion resistance and very excellent delayed fracture resistance.
In contrast, the steel sheets of the comparative examples are inferior in either or both of delayed fracture resistance and corrosion resistance.

1 Test piece 2 Bolt 3 Nut

Claims (6)

A steel sheet having a tensile strength of 1180 MPa or more and a Zn- _SiO2 composite plating layer coated on the surface of the steel sheet, the Zn- _SiO2 composite plating layer having a coating weight of 20 g/m2 ^or more and 50 g/m2 ^or less per side ,
And the _SiO2 concentration in the Zn- _SiO2 composite plating layer is 0.5 mass% or more and less than 5 mass%. 2. The high-strength steel plate according to claim 1, characterized in that the base steel plate has a chemical composition containing, in mass%, C: 0.03% or more and 0.45% or less, Si: 0.01% or more and 3.00% or less, Mn: 0.5% or more and 3.5% or less, P: 0.050% or less, S: 0.0050% or less, Al: 0.001% or more and 1.500% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities. The high-strength steel plate according to claim 2, characterized in that the base steel plate further contains, in mass %, one or more selected from Cu: 1.000% or less, B: 0.0050% or less, Nb: 0.100% or less, Ti: 0.200% or less, V: 0.500% or less, Mo: 1.000% or less, Cr: 1.000% or less, Ni: 1.00% or less, W: 0.200% or less, Zr: 0.200% or less, Hf: 0.200% or less, Co: 0.500% or less, Ca: 0.0050% or less, Mg: 0.0100% or less, Sn: 0.200% or less, Sb: 0.200% or less, and REM: 0.005% or less . An automobile body or suspension part, using the high strength steel plate according to any one of claims 1 to 3 . 4. An automobile body using the high strength steel sheet according to claim 1 in at least one of a side sill, a lower part of a center pillar, and a bumper. 4. An automobile chassis using the high strength steel plate according to claim 1 in a lower arm portion.

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