JP5641741B2 - High strength Zn-Al-Mg plated steel sheet with excellent bendability and molten metal embrittlement resistance - Google Patents

Description

  The present invention relates to a high-strength Zn-Al-Mg-based plated steel sheet having excellent corrosion resistance, bendability and molten metal embrittlement characteristics, which is useful as a member for automobiles, buildings, electrical equipment, and the like.

Since Zn-Al-Mg-based plated steel sheets have excellent corrosion resistance, the amount of use is increasing in fields such as automobiles, buildings, and electrical equipment. In such applications, it is important to have excellent workability in addition to corrosion resistance because it is molded into the required shape.
In particular, in the field of automobiles, high strength is required from the viewpoint of improving collision safety and improving fuel efficiency by reducing weight, and the need for a high strength steel plate of 780 MPa or more is also increasing.
In steel plates, generally, workability tends to decrease as the strength increases. For this reason, when expanding the application range of a high strength steel plate, it is necessary to manufacture a high strength steel plate with good workability.

  Furthermore, structural members such as automobile members and building materials are often assembled by welding plated steel sheets. In this case, the plating layer melts together with a part of the steel base during welding. When a Zn—Al—Mg based plated steel sheet is used as compared with a general galvanized steel sheet, intergranular cracking may occur in the weld heat affected zone. This intergranular crack is caused by the tensile stress generated by the expansion and contraction of steel accompanying heating and cooling during welding, and is a phenomenon called molten metal embrittlement cracking, and the component of the Zn-Al-Mg plating is It is considered that the sensitivity of molten metal embrittlement cracking is increased compared to the case of general Zn plating.

For example, in the manufacture of a Zn—Al—Mg-based plated steel sheet of Patent Document 1, the plating bath temperature is lower and the alloying treatment is unnecessary compared to the manufacture of an alloyed hot-dip galvanized steel sheet. Although it is said that a high-strength steel sheet mainly composed of ferrite and martensite can be obtained even with a Mn-based steel component, there is a problem that the bendability is inferior because the difference in strength between ferrite and martensite is too large. At the same time, no knowledge is disclosed regarding a method for suppressing resistance to embrittlement of molten metal.
In Patent Documents 2 and 3, there are disclosed Zn-Al-Mg hot-dip steel sheets for the purpose of improving corrosion resistance mainly by hot dipping and suppressing molten metal embrittlement cracking during welding with Ti, B, Cr, Nb, and the like. Proposed.

JP 2006-097063 A JP 2003-003238 A JP 2008-184665A

In each of the above-mentioned patent documents, although it is possible to individually obtain a measure for improving the resistance to melting metal embrittlement and a method for producing a high-strength steel sheet having a main structure of ferrite + martensite, it has a high strength of 780 MPa or more, The knowledge regarding the Zn-Al-Mg system plated steel plate excellent in both bendability and a molten metal embrittlement characteristic is not acquired.
The present invention has been devised to solve such problems, and finely adjusts the steel components and the structure even in the case of a Zn-Al-Mg based steel sheet exhibiting a tensile strength of 780 MPa or more. An object of the present invention is to provide a Zn—Al—Mg-based plated steel sheet excellent in both bendability and molten metal embrittlement resistance.

The high-strength Zn—Al—Mg-based plated steel sheet excellent in bendability and resistance to molten metal embrittlement according to the present invention achieves the object, C: 0.05 to 0.18 mass%, Si: 0.00. 1 to 0.8 mass%, Mn: 1.5 to 2.3 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, B: 0.0005 to 0.005 mass%, Ti: Containing 0.01 to 0.10% by mass, the balance being composed of Fe and unavoidable impurities, ferrite as the main phase and martensite or martensite as the second phase and 3% or less of bainite In addition, the ferrite has an average particle size of 8.0 μm or less, the martensite has an average particle size of 5.0 μm or less, an average aspect ratio of 0.7 or more, and an area ratio of martensite or martensite and bainite is 15; % With less than 45% metal structure When the difference between the maximum value and minimum value of the hardness on the sheet thickness ¼ line of the thickness direction cross section in the rolling direction is defined as the variation in hardness, the variation in the Vickers hardness is 40 HV or less, and the tensile strength is 780 MPa or more. The present steel is a base steel plate.

As a component composition, Nb: 0.01-0.10 mass%, Cr: 0.01-1.0 mass%, Mo: 0.01-0.5 mass% 1 type (s) or 2 or more types It may be included.
It is preferable that ferrite as the main phase is 60% or more and less than 75%, and martensite or martensite and 3% or less of bainite account for 25% or more and less than 40% as the second phase.

According to the present invention, by finely setting the component composition and the metal structure, even a high-strength Zn-Al-Mg-based plated steel sheet exhibiting a tensile strength of 780 MPa or more can reduce fluctuations in Vickers hardness. Thus, the steel sheet has excellent bendability and has excellent resistance to molten metal embrittlement.
Therefore, according to the present invention, automotive parts having excellent characteristics such as pillars, lockers, members, and the like can be provided at low cost by performing simple bending.

Perspective view explaining the shape of boss weld test material Sectional drawing explaining the procedure of producing a boss weld test material Diagram showing fluctuations in Vickers hardness in the thickness direction

In order to obtain a Zn-Al-Mg-based plated steel sheet exhibiting high strength exceeding 780 MPa, a so-called DP steel sheet having a dual phase structure in which subphase martensite is arranged in the main phase ferrite is used as the base steel sheet. It is valid. However, since a DP steel sheet generally has poor bendability, application to a member used after being subjected to bending has not progressed. Further, when a Zn—Al—Mg-based plated steel sheet is used after being welded, it may cause molten metal embrittlement cracking.
Therefore, the present inventors have proposed measures for improving bending workability and resistance to embrittlement of molten metal in order to expand the use of Zn-Al-Mg-based plated steel sheets with DP steel sheets as base steel sheets in the automotive field and building materials field. As a result of extensive studies, the present invention has been achieved.
The details will be described below.

The DP steel sheet is a steel sheet having a composite structure in which martensite is dispersed as a subphase in the main phase ferrite, and is excellent in ductility because it has soft ferrite. And martensite increases the strength of the material. However, there is a difference in deformability due to the large hardness difference between ferrite and martensite, and cracking occurs at the boundary between the two phases when deforming in one direction, resulting in poor bending workability. become. In addition, molten metal embrittlement is liable to occur in steel sheets subjected to Zn—Al—Mg plating.
Therefore, in the present invention, as a measure for improving the bending workability and the resistance to molten metal embrittlement in the Zn-Al-Mg-based plated steel sheet, the component composition of the base steel, particularly the content of Ti, B, etc., is finely adjusted to improve the resistance. The ferrite and martensite grain sizes are made fine and the shape of the martensite is equiaxed in order to improve the molten metal embrittlement characteristics and to suppress cracking at the boundary between the two phases by making the structure of the base steel uniform and refined. I decided to make it. Specifically, martensite as an auxiliary phase constituting the DP steel sheet has an average particle diameter of 5.0 μm or less and an average aspect ratio of 0.7 or more, preferably 0.8 to 1.0.

As a whole steel plate, C: 0.05-0.18 mass%, Si: 0.1-0.8 mass%, Mn: 1.5-2.3 mass%, P: 0.05 mass% or less, S: 0.01% by mass or less, B: 0.0005 to 0.005% by mass, Ti: 0.01 to 0.10% by mass, and Nb: 0.01 to 0.10% by mass as necessary. A DP steel sheet having a component composition containing one or more of Cr: 0.01 to 1.0% by mass, Mo: 0.01 to 1.0% by mass, and the balance of Fe and inevitable impurities. And ferrite as the main phase and martensite or martensite and bainite as the second phase, and the ferrite has an average particle size of 8.0 μm or less, and the martensite has an average particle size of 5.0 μm or less. Average aspect ratio of 0.7 or higher, martensite or martensite and bainite An area ratio of 15% or more and less than 45% was provided.
By using such a component composition and metal structure, it is possible to obtain a uniform structure with a Vickers hardness variation of 40 Hv or less, and high strength excellent in both bending workability and resistance to molten metal embrittlement. A Zn—Al—Mg plated steel sheet is obtained.

The reason for limitation of each requirement which comprises this invention steel plate is demonstrated. First, the component composition will be described. Hereinafter, mass% in the composition is simply referred to as%.
C: 0.05-0.18%
C is an essential element for increasing the strength of steel sheets. If the content is less than 0.05%, it is difficult to obtain a tensile strength of 780 MPa or more. However, addition exceeding 0.18% makes the structure non-uniformity remarkable and the bendability deteriorates. Therefore, C is set in the range of 0.05 to 0.18%.

Si: 0.1-0.8%
Si is an element that contributes to strength improvement without significantly degrading bendability. In the present invention, Si addition of 0.1% or more is required. However, if excessively added, an oxide is formed during heating in the plating line, and the plating property is deteriorated. Therefore, the Si amount is set to 0.1 to 0.8%.

Mn: 1.5 to 2.3%
Mn stabilizes austenite and contributes to the formation of martensite by suppressing the formation of pearlite during cooling after heating. If the content is less than 1.5%, the amount of martensite necessary to obtain a high strength of 780 MPa or more cannot be ensured. However, if it exceeds 2.3%, the band structure becomes prominent and the structure becomes non-uniform, so that the bendability deteriorates. Therefore, Mn is set to a range of 1.5 to 2.3%.

P: 0.05% or less P is an inevitable impurity element. However, if P is excessively contained, weldability and the like deteriorate, so 0.05% or less.

S: 0.01% or less S exists as a sulfide such as MnS, and if it is present in a large amount, the bendability deteriorates. For this reason, the S content is preferably as low as possible, but if it is 0.01% or less, the effect on bendability is small.

B: 0.0005-0.005%
B is an element that segregates at austenite grain boundaries during high-temperature heating and is effective in improving the resistance to molten metal embrittlement. It also works effectively to delay the transformation from austenite to ferrite and to obtain a hard martensite structure. In the present invention based on the premise that 0.01% or more of Ti is added, at least 0.0005% of addition is necessary in order to obtain the above-described effect by B. However, even if added over 0.005%, the effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, B is limited to 0.0005 to 0.005%.

Ti: 0.01 to 0.10%
Ti is an element that improves the uniformity of the structure by refining the structure and contributes to the improvement of strength without deteriorating the bendability by precipitation strengthening of carbides. In order to improve bendability by refining the structure, addition of 0.01% or more is necessary. However, if added over 0.10%, the recrystallization temperature rises remarkably, so 0.01 to 0.10%.

Nb: 0.01-0.10%, Cr: 0.01-1.0%, Mo: 0.01-0.5%,
Similarly to B, Nb, Cr and Mo are elements that segregate at austenite grain boundaries during high-temperature heating and are effective in improving the resistance to embrittlement of molten metal. In order to obtain this effect, Nb: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more are required when added alone. However, even if Nb: 0.10%, Cr: 1.0%, and Mo: 0.5% are added, the improvement effect is saturated and the manufacturing cost is increased. Even if two or more types are added in combination, the same effect can be obtained without impeding the effect. However, when two or more types are added in combination, the total is 0.5% or less from the viewpoint of manufacturing cost. Is desirable. Therefore, in the present invention, one or more of Nb: 0.01 to 0.10%, Cr: 0.01 to 1.0%, and Mo: 0.01 to 0.5% are added. In addition, since Cr and Mo also have an effect | action which suppresses the transformation from austenite to a ferrite, it is effective also in obtaining a martensitic structure stably. However, since the manufacturing cost is increased, the present invention is selectively added as necessary in consideration of the above-described resistance to molten metal embrittlement.

Next, the metal structure will be described.
The high-strength molten Zn—Al—Mg-based plated steel sheet of the present invention is based on a DP steel sheet having a composite structure in which martensite or martensite and bainite are dispersed as the second phase in the main phase ferrite.
Martensite or martensite and bainite as the second phase dispersed in the main phase ferrite of the base steel sheet is made 15% to less than 45% in total. If it is less than 15%, a tensile strength of 780 MPa or more cannot be obtained. On the other hand, if it is 45% or more, it becomes too hard and difficult to process. Preferably it is 25 to 40% of range.
As the second phase, only martensite is preferable, but bainite may be partially dispersed.

Average particle diameter of ferrite: 8 μm or less, average particle diameter of martensite: 5 μm or less In the present invention, the bendability is improved by making the structure fine. If the average particle sizes of ferrite and martensite become larger than 8 μm and 5 μm, respectively, a non-uniform structure tends to be formed and the bendability deteriorates.

Average aspect ratio of martensite: 0.7 or more In the present invention, in addition to making the structure fine, the martensite is rounded. The aspect ratio (short axis / major axis) of martensite is related to bendability. When martensite with a small aspect ratio increases, the non-uniformity of the structure increases and the bendability deteriorates. Therefore, it is set to 0.7 or more.

By having the component composition as described above and having the metal structure composed of ferrite as the main phase without residual austenite and martensite as the second phase, or martensite and bainite, Variations in tensile strength and Vickers hardness can be suppressed to 40 HV or less.
In addition, by suppressing the fluctuation | variation of Vickers hardness to 40 HV or less, the concentration of a deformation | transformation to a soft part is suppressed and bendability improves.

Finally, the manufacturing method of the high-strength molten Zn—Al—Mg based steel sheet according to the present invention will be briefly described.
After the steel comprising the above-described components of the present invention is melted by a usual method such as a converter or an electric furnace and the components are adjusted, a normal casting process is performed, and a bulk rolling process is performed as necessary. It is sufficient to perform rolling and pickling, followed by cold rolling and annealing, and then performing a molten Zn—Al—Mg based plating treatment.

Cold-rolled sheet annealing is performed in a hot dipping line, heated to a predetermined temperature for a predetermined time, cooled at a predetermined cooling rate, and then continuously immersed in a molten Zn-Al-Mg plating bath to perform hot dipping.
There are no special precautions until hot rolling and the subsequent pickling process. As before, the heating prior to hot rolling should be at a temperature such that the carbides are sufficiently dissolved in the matrix.

Heating temperature in the plating line: 730-780 ° C
If the heating temperature is less than 730 ° C., recrystallization is not completed and an unrecrystallized structure remains, and thus good bendability cannot be obtained. If it exceeds 780 ° C., the structure becomes coarse and the bendability deteriorates, so 730 to 780 ° C. is preferable.

Cooling rate after heating: 5 ° C./s or more If the cooling rate after heating is less than 5 ° C./s, some pearlite is generated, and it becomes difficult to obtain high strength of 780 MPa or more. In addition, the cooling rate is preferably 5 ° C./s or more from the viewpoint of reducing the ferrite grain size. In the present invention, by containing predetermined Ti and, if necessary, Nb, the average particle size of ferrite becomes 8 μm or less by selecting the cooling rate after heating in this way.
After that, as before, it is immersed in a molten Zn—Al—Mg based plating bath to perform molten Zn—Al—Mg based plating.

A slab having the chemical composition shown in Table 1 was hot-rolled at a heating temperature of 1250 ° C. and a finish rolling temperature of 880 ° C., and then wound at 550 ° C. to obtain a hot-rolled steel plate having a thickness of 3.5 mm. It was.
After pickling the hot-rolled steel sheet, it is cold-rolled to a sheet thickness of 1.4 mm (reduction rate: 60%) to obtain a cold-rolled steel sheet, and then the line speed in a continuous molten Zn—Al—Mg-based plating line: 110 m / min, heating temperature: 730-780 ° C., holding time: annealing in 35 seconds, cooling rate: after cooling at 10 ° C./s, immersed in a Zn—Al—Mg-based alloy plating bath with a bath temperature of 410 ° C., A Zn—Al—Mg plated steel sheet was obtained. The plating adhesion amount was 90 g / m ² per side.
The composition of the plating layer is as follows. Al: 6%, Mg: 3%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: balance

The obtained high-strength molten Zn—Al—Mg-based plated steel sheet was examined for structure observation, hardness test, tensile test, bending test, and molten metal embrittlement characteristics.
The metal structure was observed by a scanning electron microscope in the thickness direction in the rolling direction.
The average particle diameters of ferrite and martensite were determined according to G 0552.
In addition, the metallographic structure of the base steel sheet of each plated steel sheet is observed with a scanning electron microscope, image analysis is performed for 10 fields of view at 1000 times, and the martensite and martensite + bainite amount, the average aspect ratio of martensite (short axis / Long axis).
The measurement results are shown in Table 2. Although not shown in Table 2, bainite was observed in the No. 2 plated steel sheet, and the area ratio was 3%. The total amount of Ti and Nb present as precipitates in the matrix was 0. It was 0.04 mass% or less. The amount of Ti and Nb in the precipitate was determined by analyzing Ti and Nb in the residue obtained by dissolving the matrix.

The hardness test was a micro Vickers hardness tester with a load of 100 gf, and 30 points were measured at intervals of 150 μm on the sheet thickness ¼ line in the sheet thickness section in the rolling direction to measure the hardness distribution.
The difference between the maximum value and the minimum value of the obtained hardness was defined as hardness variation. A hardness variation of 40 HV or less was accepted.

In the tensile test, a JIS No. 5 test piece was cut out from the manufactured plated steel plate in parallel with the rolling direction, and JIS
In accordance with Z2241, it was subjected to a tensile test at room temperature.
In the bending test, a bending test piece having a longitudinal direction perpendicular to the rolling direction was taken, and a 90 ° V-block bending test was performed. After the test, the bent portion was visually observed from the outside of the bend, and the minimum tip R where no crack was observed was calculated as the limit bend R. A critical bending R of 1.0 mm or less was accepted.

The molten metal embrittlement characteristics were evaluated by conducting a welding test according to the following procedure.
A sample of 100 mm × 75 mm was cut out from the obtained plated steel sheet, and this was used as a test piece for evaluating the maximum crack depth resulting from molten metal embrittlement. In the welding test, “boss welding” was performed to create a boss weld material having the appearance shown in FIG. 1, and the cross section of the weld was observed to examine the occurrence of cracks.
That is, a boss (projection) 1 made of a steel bar having a diameter of 20 mm and a length of 25 mm was set up vertically at the center of the plate surface of the test piece 3, and the boss 1 was joined to the test piece 3 by arc welding. The welding wire is YGW12, and the circumference of the boss is made one round from the welding start point. After passing the welding start point, welding is further advanced to make the weld bead 6 and the weld bead overlap portion 8 past the welding start point. At that point, welding was finished. The welding conditions were 110 A, 21 V, welding speed 0.4 m / min, shield gas: CO ₂ , and shield gas flow rate: 20 L / min.
This is the test procedure for creating the boss weld shown in FIG.

In the welding, a test piece 3 previously joined to the restraint plate 4 was used. First, a constrained plate 4 (SS400 material stipulated in JIS) 120 mm × 95 mm × 4 mm thick is prepared, and the test piece 3 is placed at the center of the plate surface. It is welded to the restraint plate 4.
As shown in FIG. 2, the boss weld material is prepared by fixing the joined body (the test piece 3 and the restraint plate 4) on the horizontal test bench 5 with the clamp 2, and performing boss welding in this state. It is what I did.
After the boss welding, the boss 1 / test piece 3 / restraint plate 4 joined body is cut at a cut surface 9 passing through the central axis of the boss 1 and passing through the overlapping portion 8 of the beads. Observation was performed, and the maximum depth of cracks observed in the test piece 3 was measured. A maximum crack depth of 0.2 mm or less was evaluated as acceptable, and a sample having a maximum crack depth exceeding 0.2 mm was evaluated as unacceptable.
Table 3 summarizes the results of the hardness test, the bending test, the tensile test and the molten metal embrittlement characteristics obtained in this way. Further, FIG. 3 shows the Vickers hardness fluctuation state at 30 measurement positions of No. 1 of the present invention and No. 19 of the comparative example.

As can be seen from FIG. 3 showing the Vickers hardness fluctuation of No. 1 of the present invention example and No. 19 of the comparative example, No. 1 of the invention example has a hardness fluctuation as small as about 30 HV, No. 19 of the comparative example has a large hardness variation of 60 HV or more and has a non-uniform structure.
As can be seen from Tables 2 and 3, in the Zn-Al-Mg-based plated steel sheets No. 1 to No. 16 according to the chemical composition and metal structure according to the scope of the present invention, the average grain size of ferrite is 8 μm or less, The average particle size is 5 μm or less, the average aspect ratio of martensite is 0.7 or more, and the hardness variation of Vickers is 40 HV or less, resulting in a uniform fine structure. Therefore, the limit bending R in the 90 ° V block bending is 1.0 mm or less and has excellent bendability. Further, the maximum crack depth during boss welding has a good value of 0.2 mm or less.
On the other hand, since No.17-22 which is a comparative example has the steel component remove | deviated from the scope of the present invention, No.17 and No.18 have poor bendability, and No.19 has bendability and molten metal resistance. The embrittlement characteristics are poor, and No. 20 and No. 21 cannot obtain a strength of 780 MPa or more, and No. 22 has poor melt metal embrittlement resistance.

Claims (2)

C: 0.05-0.18 mass%, Si: 0.1-0.8 mass%, Mn: 1.5-2.3 mass%, P: 0.05 mass% or less, S: 0.01 Less than mass%, B: 0.0005-0.005 mass%, Ti: 0.01-0.10 mass%, with the balance being composed of Fe and inevitable impurities, ferrite as the main phase and It consists of martensite or martensite and 3% or less of bainite as two phases, and the ferrite has an average particle size of 8.0 μm or less, the martensite has an average particle size of 5.0 μm or less and 0.7 or more. It has a metal structure with an average aspect ratio, martensite or martensite and bainite area ratio of 15% or more and less than 45% . when the hardness variation difference, 40HV following Vickers Variations in hardness, and high-strength hot-dip Zn-Al-Mg plated steel sheet having excellent bending resistance and liquid metal embrittlement characteristics, characterized in that the steel exhibits a tensile strength of not less than 780MPa as a substrate steel sheet.   Furthermore, Nb: 0.01-0.10 mass%, Cr: 0.01-1.0 mass%, Mo: 0.01-0.5 mass% 1 type or 2 types or more are included. A high-strength molten Zn—Al—Mg-based plated steel sheet having excellent bendability and molten metal embrittlement characteristics.

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