JP5860500B2 - High-strength Zn-Al-Mg-based plated steel sheet with excellent resistance to molten metal embrittlement and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength Zn—Al—Mg-based plated steel sheet excellent in corrosion resistance, molten metal embrittlement characteristics and local ductility and used as a structural member such as automobile parts and building materials, and a method for producing the same.

  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 plated steel sheet is used as compared with a general galvanized steel sheet, grain boundary 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.

  The applicant of the present application has made various studies on measures for suppressing molten metal embrittlement cracking of Zn—Al—Mg based plated steel sheets and invented Patent Documents 1 and 2. Patent Documents 1 and 2 are inventions made mainly for the purpose of improving corrosion resistance by hot dipping and suppressing molten metal embrittlement cracking during welding with Ti, B, Cr, Nb, and the like.

  On the other hand, in the case of steel sheets for automobiles, the weight reduction of the vehicle body using high-tensile steel sheets has been achieved for the purpose of improving the global environment and reducing CO2 emissions, but in particular, the high strength has a tensile strength of 590 MPa or more. There are not many findings regarding a method for obtaining a Zn—Al—Mg based steel sheet having slag.

  For example, Patent Document 3 discloses a method for producing a hot-dip Zn—Al—Mg plated steel sheet having a tensile strength of 590 MPa or more. However, although a composite structure having ferrite + martensite as a main structure can be obtained, the strength difference between ferrite and martensite is too large, resulting in a problem of poor local elongation performance. At the same time, no knowledge has been disclosed regarding a method for suppressing the resistance to molten metal embrittlement cracking.

JP 2003-3238 A JP 2008-184665A JP 2006-97063 A

Thus, in the above-mentioned document, although there are obtained individual measures for improving the resistance to embrittlement of molten metal and a method for producing a high-strength steel sheet mainly composed of ferrite + martensite, it has a high strength of 590 MPa or more. However, no knowledge has been obtained regarding the Zn—Al—Mg-based plated steel sheet excellent in both the molten metal embrittlement resistance and the local ductility and the manufacturing method thereof.
The object of the present invention is to provide a Zn-Al-Mg-based plated steel sheet having a high strength of 590 MPa or more and having excellent properties both in resistance to molten metal embrittlement and local ductility, and a method for producing the same. To do.

  As a result of detailed investigations by the present inventors, 0.08 to 0.15% Ti is added to steel obtained by appropriately adjusting C, 0.05 to 0.20%, Si, Mn, etc. as a base steel plate. However, when the composite structure is strengthened, the strength of ferrite increases due to precipitation of fine TiC and the like, and the difference in strength between ferrite and martensite decreases. We found that the ductility can be greatly improved. It was also found that the resistance to molten metal embrittlement can be improved even at a strength level of 590 MPa or more by adding a trace amount of one or more of B, Nb, Mo, and Cr on the premise of addition of Ti. Furthermore, it has been found that a martensite structure can be easily obtained by adding Cu and Ni, so that a strength of 590 MPa or more can be stably obtained.

  That is, according to the present invention, by mass%, Al: 3 to 22%, Mg: 1 to 10%, the balance being a steel plate that has been subjected to hot dipping consisting of inevitable impurities and Zn, , C: 0.05 to 0.20%, Si: 0.05 to 1.5%, Mn: 1.0 to 2.5%, P: 0.005 to 0.050%, S: 0.01 % Or less, acid-soluble Al: 0.005 to 0.10%, Ti: 0.05 to 0.15%, B: 0.0002 to 0.01%, the balance from Fe and inevitable impurities Depending on the case, Nb: 0.005 to 0.10%, Mo: 0.05 to 0.5%, Cr: 0.05% to 1.0% 1 and 2 or more of the above and / or Cu: 0.05-0.5%, Ni: 0.05-0.5%, a metal Zn-Al-Mg based plating excellent in molten metal embrittlement resistance and local ductility, characterized in that the weaving has a structure containing ferrite with a volume ratio of 50% or more and martensite with a volume ratio of 5 to 45%. A steel sheet and a method for manufacturing the same are provided.

  Inevitable impurities in the plating bath include Ti, B, Si, and Fe. These are Ti: 0.1% by mass or less, B: 0.05% by mass or less, Si: 2% or less, and Fe: You may contain in 2% or less of range.

  ADVANTAGE OF THE INVENTION According to this invention, the welded structural member which has the sound welding part which uses the Zn-Al-Mg type plated steel plate excellent in corrosion resistance is provided. Furthermore, it has a high strength of 590 MPa or more, and it can improve the local ductility that is advantageous for bending processing and hole expansion processing required for automotive parts, and it is also effective for reducing the weight of the vehicle body as an automotive steel plate. Applicable to

  The reasons for limiting the chemical components and metal structure of the base steel sheet according to the present invention and the reasons for limiting the manufacturing conditions for obtaining the metal structure will be described below. In the present invention, “%” means “% by mass” unless otherwise specified.

C: 0.05-0.20%
The strength of the martensite structure is greatly influenced by the C content. In order to obtain the strength level of 590 MPa or more targeted in the present invention, at least 0.05% or more must be added. However, if the content exceeds 0.20%, the strength level is increased, but the local ductility is inferior. Therefore, in the present invention, C is limited to 0.05 to 0.20%.

Si: 0.05 to 1.5%
Si is effective in increasing the strength of ferrite by solid solution strengthening. In order to obtain the effect, addition of 0.05% or more is necessary. However, if it exceeds 1.5%, the hot dipping property deteriorates. Therefore, in this invention, it is limited to Si: 0.05-1.5%.

Mn: 1.0 to 2.5%
Mn is effective for increasing the strength of ferrite by solid solution strengthening, and also effectively acts to stabilize austenite and promote the formation of low-temperature transformation phases such as martensite. In order to obtain these effects, addition of 1.0% or more is necessary. However, if it exceeds 2.5%, the hot dipping property deteriorates. Therefore, in the present invention, Mn is limited to 1.0 to 2.5%.

P: 0.005 to 0.05%
P is effective in increasing the strength of ferrite by solid solution strengthening, and also acts effectively in suppressing melting metal embrittlement during welding by segregating at grain boundaries. In order to obtain the effect, addition of 0.005% or more is necessary. However, if added over 0.05%, the low temperature toughness deteriorates. Therefore, in this invention, it limits to P: 0.005-0.05%.

S: 0.01% or less S generates sulfides that are harmful to processability, and therefore needs to be reduced as much as possible. Since up to 0.01% is acceptable, the upper limit is limited to 0.01% in the present invention.

Acid-soluble Al: 0.005 to 0.10%
Acid-soluble Al is added as a steel deoxidizer. In order to obtain the effect, addition of 0.005% or more is necessary. However, even if added over 0.1%, the effect is saturated and the manufacturing cost is increased. Therefore, in this invention, it is limited to acid-soluble Al: 0.005-0.10%.

Ti: 0.05 to 0.15%
Ti is an effective element for precipitating in the ferrite as TiC or the like to increase the strength of the ferrite structure and improve the local ductility. In order to obtain the effect, addition of 0.05% or more is necessary. However, even if added over 0.15%, the improvement effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, Ti is limited to 0.05 to 0.15%.

B: 0.0002 to 0.01%
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 0.05% or more of Ti is added, at least 0.0002% of addition is necessary in order to obtain the above-described effect by B. However, even if added over 0.01%, the effect is saturated and the manufacturing cost is increased. Therefore, in the present invention, B is limited to 0.0002 to 0.01%.

Nb: 0.005 to 0.10%
Mo: 0.05-0.5%
Cr: 0.05-1.0%
Similarly to B, Nb, Mo and Cr are segregated at the austenite grain boundaries during high-temperature heating and are effective elements for improving the resistance to molten metal embrittlement. In order to obtain this effect, Nb: 0.005% or more, Mo: 0.05% or more, and Cr: 0.05% or more are necessary when added alone. However, even if Nb: 0.10%, Mo: 0.5% and Cr: 1.0% 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, from the viewpoint of manufacturing cost, the total is 0.5% or less. Is desirable. Therefore, in the present invention, one or more of Nb: 0.005 to 0.10%, Mo: 0.05 to 0.5%, and Cr: 0.05 to 1.0% are added. Since Mo and Cr also have an action of suppressing transformation from austenite to ferrite, they are effective in stably obtaining a martensite structure. However, since this increases the manufacturing cost, the present invention is selectively added as necessary in consideration of the above-described resistance to molten metal embrittlement.

Cu: 0.05 to 0.5%
Ni: 0.05-0.5%
Since Cu and Ni also have an action of suppressing transformation from austenite to ferrite, it is effective in stably obtaining a martensite structure. In order to obtain this effect, it is necessary to add 0.05% or more in any case. However, even if added in excess of 0.5%, the effect is saturated and the manufacturing cost is increased, so the upper limit of the amount added is set to 0.5%. The same effect can be obtained by single addition or combined addition. However, in the case where two types are added together, it is desirable that the total content is 0.6% or less from the viewpoint of manufacturing cost. Therefore, in the present invention, one or two of Cu: 0.05 to 0.5% and Ni: 0.05 to 0.5% are added.

In the present invention, the slab having the above chemical components may be hot-rolled under normal hot rolling conditions, and there is no special restriction. However, the hot rolling finish temperature is not higher than the Ar3 point. Therefore, it is desirable to set it at Ar3 or higher.
The coiling temperature may be appropriately selected according to the required strength level, but is preferably 500 to 700 ° C. from the viewpoint of securing a ferrite volume ratio of 50% or more.
Moreover, there is no restriction | limiting in particular also in pickling conditions, What is necessary is just the general conditions which can remove the scale produced | generated by hot rolling completely.

  In the present invention, hot-rolling, pickling, and hot-dip galvanizing can be performed immediately in a continuous pickling line. Cold rolling can be performed before. In this case, the cold rolling rate need not be particularly limited, and may be set as necessary. However, if it is less than 30%, the productivity is inferior, and cold rolling exceeding 80% is a single cold rolling. Therefore, it is desirable that the cold rolling rate is 30 to 80%.

  When the heating temperature in the continuous hot dipping line is less than 730 ° C., the amount of austenite at the time of heating is not sufficient, and a sufficient amount of martensite cannot be secured after cooling. In addition, the higher the heating temperature, the more austenite during heating, and it becomes easier to obtain martensite by subsequent cooling, but the effect is saturated even if the plate is passed at a temperature exceeding 900 ° C., and the manufacturing cost is returned. Invite the rise.

  If the average cooling rate to 650 ° C. after heating in the plating line is less than 3 ° C./second, not only ferrite transformation but also pearlite transformation occurs, so that it becomes impossible to secure 5% or more martensite. On the other hand, if it exceeds 10 ° C./second, the ferrite transformation does not proceed sufficiently and the ferrite fraction may be less than 50%. Therefore, the average cooling rate up to 650 ° C. is limited to 3 to 10 ° C./second.

  If the average cooling rate to 440 ° C. is less than 10 ° C./second following the cooling to 650 ° C., pearlite transformation or bainite transformation occurs and 5% or more of martensite cannot be obtained. Therefore, the average cooling rate from 650 ° C. to 440 ° C. is limited to 10 ° C./second or more.

  In the present invention, after the cooling described above, immersion plating is performed by immersion in a Zn-Al-Mg-based hot dipping bath maintained at a temperature of 440 ° C or lower. When the bath temperature exceeds 440 ° C., pearlite transformation occurs and the amount of martensite decreases. Therefore, it was limited to 440 ° C. or lower.

The hot dip plating bath is made of, for example, Al: 4 to 10% by mass, Mg: 1 to 4%, and the balance is made of Zn and inevitable impurities. Inevitable impurities include 0.002 to 0.20 mass% Ti, about 0.001 to 0.1 mass% B, and the like. Moreover, it is desirable to adjust the plating adhesion amount in a range of ^{20 to} 300 g / m ² per one side of the steel plate.

  As for the metal structure of the obtained plated steel sheet, good local ductility cannot be obtained unless the volume fraction of ferrite is 50% or more. Further, if the volume fraction of martensite is less than 5%, sufficient strength cannot be obtained, and if it exceeds 45%, the local ductility deteriorates. Therefore, the ferrite volume fraction is limited to 50% or more and the martensite volume fraction is limited to a range of 5 to 45%.

  A steel slab having the composition shown in Table 1 was heated to 1200 ° C., hot-rolled and pickled to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. The hot rolling finishing temperature was 850 ° C., and the winding temperature was 580 ° C. A part of the obtained hot-rolled steel sheet was pickled and then rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

Each steel plate was heat-treated under the annealing conditions shown in Table 2, and then introduced into a plating bath composed of Al: 6% by mass, Mg: 3% by mass, balance: Zn and unavoidable impurities, and the amount of plating deposited per side: 45 g / M ² hot-dip Zn—Al—Mg plated steel sheet was produced. The surface appearance of the plated steel plate was observed, and those with non-plating were evaluated as plating property: x, and those having good surface properties were evaluated as plating property: ◯.

A JIS No. 5 test piece was cut out from the manufactured plated steel plate in parallel with the rolling direction and subjected to a tensile test at room temperature. Further, in order to evaluate local ductility, a 2 mmV notch was given to the end face at the intermediate position of the parallel part of the JIS No. 5 test piece, and the elongation at break with a distance of 10 mm between the scores centered on the notch part was measured as Elv. In the present invention, those having an Elv of 10% or more were judged good.
Moreover, the metal structure of the base steel plate of each plated steel plate was observed with a scanning electron microscope, and image analysis of 10 fields of view was performed at 1,000 times to calculate a ferrite area ratio and a martensite area ratio.

Moreover, a 100 mm x 75 mm sample was cut out from the obtained plated steel sheet, and it was set as the 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. YGW12 was used as the welding wire, and the circumference of the boss was made one round from the welding start point, and after passing the welding start point, the bead was further piled up and welding was completed when the welding proceeded a little. The welding conditions were 200 A, 22 V, welding speed 0.2 m / min, shielding gas: CO ₂ , and shielding gas flow rate: 20 L / min.

 At the time of welding, as shown in FIG. 2, the test piece 3 was placed at the center of the plate surface of the restraint plate 4 having a size of 120 mm × 95 mm × plate thickness 4 mm, and the entire circumference of the test piece 3 was previously welded and joined. This joined body was fixed on a horizontal test bench 5 with a clamp, and boss welding was performed in this state.

  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. The maximum crack depth was evaluated as ○ when the maximum crack depth was 0.2 mm or less, and x when the maximum crack depth exceeded 0.2 mm. Table 2 collectively shows the tensile test results, the image analysis results, the plating property evaluation results, and the maximum crack depth in boss welding thus obtained.

  As can be seen from Table 1 and Table 2, in the Zn-Al-Mg-based plated steel sheet according to the chemical composition and metal structure according to the scope of the present invention, all have high tensile strength (TS): 590 MP or higher, Elv shows a good value of 10% or more and is excellent in plating properties. Further, it can be seen that the maximum crack depth during boss welding shows a good value of 0.2 mm or less.

On the other hand, No. using K steel with low C content. 11, both the ferrite and martensite are smaller than the scope of the present invention as a result of an increase in the proportion of bainite in the metal structure, and as a result, a strength level of 590 MPa or more cannot be obtained. Moreover, in L steel (No. 12) with insufficient Ti content, Elv shows a value of less than 10%.
In M steel (No. 13) and O steel (No. 15) containing Si and Mn exceeding the scope of the present invention, although tensile properties and resistance to molten metal embrittlement cracking are good, non-plating is performed. Occurs and the plating property is poor. In the steel P with no Ti added (No. 16), Elv was reduced and a large crack penetrating the plate thickness was formed by boss welding. Further, even in the case of Q steel (No. 17) in which the B addition amount is less than the range of the present invention, molten metal embrittlement cracks penetrating the plate thickness occurred.

  Even when A steel and G steel whose steel components are within the scope of the present invention are used, No. 2 having a low annealing heating temperature during plating. 18, and no. In No. 21, recrystallization does not occur sufficiently and the amount of transformation to austenite becomes small, so that the martensite volume fraction is also small. As a result, Elv decreased significantly. No. with a small cooling rate to 650 ° C. No. 19, 22 and 650 ° C. to 440 ° C. with a low cooling rate. In 20 and 23, ferrite and pearlite transformation proceeds, and a sufficient amount of martensite cannot be obtained. As a result, it becomes impossible to secure a strength level of 590 MPa or more.

The figure which showed typically the shape of the boss | hub welding member. The figure which showed typically the restraint method of the test piece at the time of performing boss welding.

DESCRIPTION OF SYMBOLS 1 Boss 2 Clamp 3 Test piece 4 Restraint plate 5 Experimental stand 6 Weld bead 7 Weld bead 8 of a test piece perimeter welding part 8 Overlap part of a weld bead 9 Cut surface

Claims (4)

  C: 0.05-0.20%, Si: 0.05-1.5%, Mn: 1.0-2.5%, P: 0.005-0.050%, S: 0.01% or less, acid-soluble Al: 0.005 to 0.10%, Ti: 0.05 to 0.15%, B: 0.0002 to 0.01%, the balance being Fe and inevitable In addition, the metal structure is a structure containing ferrite with a volume ratio of 50% or more and martensite with a volume ratio of 5 to 45%, and tensile strength (TS): 590 MPa or more, Elv 10% or more. A high-strength Zn-Al-Mg-based plated steel sheet excellent in molten metal embrittlement characteristics and local ductility.   It is characterized by containing one or more of Nb: 0.005 to 0.10%, Mo: 0.05 to 0.5%, Cr: 0.05% to 1.0% by weight%. A high-strength Zn-Al-Mg-based plated steel sheet having excellent molten metal embrittlement characteristics and local ductility according to claim 1.   It contains 1 type or 2 types of Cu: 0.05-0.5% and Ni: 0.05-0.5% by weight%, The resistance-resistance described in Claim 1 or Claim 2 characterized by the above-mentioned. A high-strength Zn-Al-Mg-based plated steel sheet excellent in molten metal embrittlement characteristics and local ductility. The slab having the chemical composition according to any one of claims 1 to 3 is hot-rolled and pickled, or cold-rolled following pickling, and then passed through a continuous hot-dip plating line. After heating to a temperature range of 730 ° C. to 900 ° C. in a line reduction annealing furnace, it is cooled to 650 ° C. at an average cooling rate of 3 to 10 ° C./s, and further, an average cooling rate: After cooling to a temperature range, the bath temperature is introduced into a molten Zn-Al-Mg plating bath having a temperature of 440 ° C or lower, and the metal structure is characterized by a volume ratio: ferrite with a volume ratio of 50% or more and a volume ratio of 5-45%. High-strength Zn-Al-Mg-based plating excellent in molten metal embrittlement characteristics and local ductility, characterized by having a martensite structure and tensile strength (TS): 590 MPa or more and Elv 10% or more Steel plate manufacturing method

Images