JP2017115205A - MANUFACTURING METHOD OF HOT DIP Zn-Al-Mg ALLOY PLATED STEEL PLATE HAVING EXCELLENT PLATING ADHESION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot dip Zn-Al-Mg alloy plated steel plate having an excellent plating adhesion by using a steel plate as a plating substrate, the steel plate being imparted by a hot dip metal embrittlement crack resistance by adding B to a relatively high-Mn-based high-strength steel.SOLUTION: When a maximum arrival temperature of a steel plate surface is defined as "a reduction heat treatment temperature" inside a furnace of a reduction heat treatment performed in a case where a steel plate is introduced to a hot dip Zn-Al-Mg alloy plating bath, the reduction heat treatment is performed to secure a plating adhesion by suppressing coverage of the steel plate surface with BN in the reduction heat treatment, the reduction heat treatment being performed at a dew point within an area (including a boundary) enclosed by lines connecting points of A(-75,800)-B(-45,900)-C(-10,900)-D(-10,750)-E(-75,750)-A in an orthogonal coordinate system of a real number scale where an x-axis is a dew point within the furnace and a y-axis is the reduction heat treatment temperature (°C) and under a condition satisfying the reduction heat treatment temperature.SELECTED DRAWING: Figure 2

Description

  The present invention is a steel plate containing B as a plating base plate and further using a steel type containing one or more of Si, Mn, and Ti, and subjected to hot-dip Zn-Al-Mg alloy plating. The present invention relates to a method for producing a hot-dip Zn—Al—Mg alloy-plated steel sheet that has improved embrittlement cracking and plating adhesion.

  Although the hot-dip Zn-based plated steel sheet is widely used in various applications, when welding is performed on the Zn-based plated steel sheet, cracks may occur in the weld heat affected zone, which may be a problem. This phenomenon is generally called “molten metal embrittlement cracking”, and it is considered that the molten plating component acts on the grain boundaries of the steel sheet to cause brittle fracture (grain boundary fracture).

  Among Zn-based plated steel sheets, molten Zn-Al-Mg-based steel sheets are used in various corrosion resistance applications including building materials because they are excellent in corrosion resistance. Recently, a hot-dip Zn-Al-Mg-based steel sheet has been increasingly applied as a substitute for a conventional Zn-plated steel sheet. However, the molten Zn—Al—Mg-based plated steel sheet tends to cause molten metal embrittlement cracking more easily than the conventional hot-dip galvanized steel sheet.

  Therefore, it is known that it is effective to apply a plating original plate containing B as a technique for improving resistance to molten metal embrittlement cracking (Patent Document 1).

JP 2003-003238 A JP 2006-097063 A JP 2011-214041 A JP 2011-231346 A JP 2008-07842 A JP 2014-208902 A Japanese Patent No. 5799819

  The hot-dip Zn-Al-Mg-based plated steel sheet has been applied in various applications taking advantage of its high corrosion resistance, and the need for the alloy-plated steel sheet has also increased in the use of high-tensile steel sheets. Patent Document 2 discloses a technique for manufacturing a molten Zn—Al—Mg-based plated steel sheet using a steel type for a high-strength steel sheet containing a relatively large amount (around 2% by mass) of Mn as a plating base sheet. However, no special consideration is given to the resistance to molten metal embrittlement cracking, and when this is used for welding, molten metal embrittlement cracking may become a problem.

Patent Documents 3 and 4 also disclose a technique for manufacturing a molten Zn—Al—Mg-based plated steel sheet using a high-strength steel sheet containing a relatively large amount (1% by mass or more) of Mn as a plating base sheet. . The plating base plate targeted by this technology contains B in order to improve the molten metal embrittlement cracking resistance. However, a relatively high Mn-based high strength steel type containing B is used for the plating base plate. Then, it is disclosed that the new problem that the adhesiveness of a molten Zn-Al-Mg type plating layer tends to fall arises. If a steel sheet with poor plating adhesion is subjected to bending, plating peels off at the bent part, causing trouble. In this technology, by strictly controlling the conditions of the holding time of the reduction heat treatment and the reduction heat treatment temperature, the reduction heat treatment is completed before B diffuses to the surface in a large amount even for a plating original plate containing B. This solves the problem of poor plating adhesion.
However, as described in Patent Document 3, when a large amount of P is added, workability, spot weldability, and toughness are reduced, and when Cr and Mo are added, the cost of the plating original plate is increased. Further, in the weak oxidation heat treatment described in Patent Document 4, if the time of the reduction heat treatment is too long, it causes surface concentration of Si, Mn, B, and if it is too short, an Fe oxide film remains on the steel sheet surface. There is a problem that it is insufficient for securing plating adhesion. In production equipment that continuously performs reduction heat treatment and hot dipping, the sheet passing speed of the steel sheet may be reduced for operational reasons. Even in such a case, it is advantageous that the plating adhesion can be ensured even if the time for the reduction heat treatment is increased.

Patent document 5 is disclosing the technique which manufactures a hot-dip Zn-Al-Mg type plated steel plate by using as a plating original plate the high strength steel plate containing a lot of Mn (1.5 mass% or more). Although the plating original plate targeted by this technology does not contain B, it is disclosed in this document that there is a problem that non-plating or plating adhesion tends to be reduced when a molten Zn—Al—Mg based plating layer is applied. Has been. In this technique, the problem of non-plating and plating adhesion is solved by controlling the reducing atmosphere in the reduction heat treatment to bring SiO ₂ in the surface layer portion of the steel sheet into an internal oxidation state.
However, when the oxygen partial pressure PO ₂ is increased, Si and Mn are internally oxidized. On the other hand, Fe is oxidized, which is insufficient for suppressing non-plating and ensuring plating adhesion. Further, when Fe by adjusting the oxygen partial pressure PO ₂ is the condition not oxidized, Si to Si and Mn internal oxidation becomes insufficient surface of the steel sheet, occurs Mn concentrated, non plating inhibiting and plating adhesion It will be insufficient for securing the safety.

Patent Document 6 discloses a technique for manufacturing a hot-dip Zn—Al—Mg-based plated steel sheet using a relatively high Si, Mn-based high-strength steel sheet containing B as a plating base sheet. In this technique, the problem of plating adhesion is solved by defining the coiling temperature at the time of hot rolling the steel sheet, forming an internal oxide, and controlling the subsequent reduction heat treatment conditions.
However, it is cumbersome to control the coiling temperature in the hot rolling process, and when rolling in the cold rolling process, the internal oxide layer is simultaneously rolled to reduce the thickness of the internal oxide. If the time is too long, the surface concentration of Si, Mn, and B is caused, which is insufficient for securing the plating property.

Patent Document 7 discloses a technique for producing a hot-dip Zn-based plated steel sheet using a relatively high Si, Mn-based high-strength steel sheet as a plating original sheet. In this technique, the range of the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ and the steam partial pressure PH ₂ O in the preheating step, heating step, and soaking step in annealing treatment, heating time, heating By controlling the plate temperature at the time, the problem of plating wettability and pick-up resistance is solved.
However, in this method, since it is necessary to control the hydrogen partial pressure PH ₂ and the water vapor partial pressure PH ₂ O in each step, it can be performed only with equipment that can control them, and when such equipment is not provided. Has the problem of requiring new equipment.

  In the hot-dip Zn-Al-Mg alloy-plated steel sheet, which uses a high-strength steel sheet as the plating base plate, it is effective to use steel added with B as the plating base plate in order to improve the resistance to molten metal embrittlement cracking. It is. However, when a relatively high Mn high-strength steel type with B added is used for the plating base plate, there is a new problem that the adhesion of the molten Zn-Al-Mg alloy plating layer may be significantly reduced. did. If a steel sheet with poor plating adhesion is subjected to bending, the plating layer peels off at the bent portion, causing trouble.

  In consideration of the above-mentioned background, the present invention uses a steel plate provided with a resistance to molten metal embrittlement cracking by adding B to a relatively high Mn-based high-strength steel grade, and has excellent plating adhesion. Another object of the present invention is to produce a hot-dip Zn—Al—Mg alloy-plated steel sheet.

  The above-mentioned purpose is intended for a plating base plate containing B in a relatively high Mn-based high-strength steel type, and conditions for reduction heat treatment (dew point, reduction heat treatment temperature) performed when introduced into a molten Zn-Al-Mg alloy plating bath. This is achieved by suppressing the steel sheet surface from being covered with BN during the reduction heat treatment and ensuring plating adhesion.

  That is, in the present invention, the chemical composition of the steel sheet is, in mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.50 to 3.00%, Ti: Plating steel sheet containing at least one selected from the group of 0.010 to 0.150%, B: 0.0003 to 0.0100%, N: less than 0.010%, and the balance Fe and unavoidable impurities Subsequent to the reduction heat treatment, the original plate was subjected to molten Zn-Al-Mg alloy plating in which Al: 0.1 to 22.0% by mass, Mg: 0.05 to 10.0%, and the balance being Zn. In producing the hot-dip Zn—Al—Mg alloy-plated steel sheet, in the reduction heat treatment step, when the maximum reached temperature of the steel sheet surface in the reduction heat treatment furnace is defined as “reduction heat treatment temperature”, the dew point in the furnace ( ° C) as x-axis and reduction heat treatment temperature (° C) as y-axis In the real scale xy orthogonal coordinate system, A (−75,800) −B (−45,900) −C (−10,900) −D (−10,750) −E ( -75, 750) Molten Zn-Al-Mg excellent in plating adhesion by performing reduction heat treatment under conditions satisfying the dew point (including the boundary) and the reduction heat treatment temperature in the region surrounded by the straight line connecting -A Alloy-plated steel sheets can be manufactured.

  In the above, the plating base plate further contains at least one selected from the group of mass%, Cr: 1.00% or less, Nb: 0.10% or less, Mo: 0.50% or less. It does not matter. Further, the molten Zn—Al—Mg alloy plating further comprises at least one selected from the group consisting of Ti: 0.10% or less, B: 0.05% or less, and Si: 2.0% or less by mass%. It may be contained.

  The inventors of the present invention, when subjecting a plating base plate containing B to a relatively high Mn-based high-strength steel grade according to the above production method, by setting the dew point and the reduction heat treatment temperature in the reduction heat treatment furnace, The inventors have found that a hot-dip Zn—Al—Mg-based plated steel sheet having excellent plating adhesion can be obtained by not being covered with BN.

  According to the present invention, in a steel sheet containing B in a relatively high Mn-based high-strength steel grade that has been subjected to hot Zn-Al-Mg alloy plating having high corrosion resistance, "melting metal embrittlement cracking resistance" and " A material with improved “plating adhesion” is realized. Production of a hot-dip Zn-Al-Mg alloy-plated steel sheet having both of these properties simultaneously has been difficult, but the present invention can be used in the application of hot-dip Zn-plated steel sheets used for bending and welding. This contributes to the widespread use of Zn—Al—Mg plated steel sheets.

The schematic diagram which shows the boss-welding test performed in order to evaluate a melt-proof embrittlement countermeasure cracking property. The graph which shows the appropriate range of the "dew point" and the "reduction heat treatment temperature" of the reduction heat processing in invention steel. The graph which shows the appropriate range of "dew point" and "reduction heat treatment temperature" of reductive heat treatment with reference steel.

  In this specification, “%” in the chemical composition of the plating original plate and the hot dipping means “% by mass” unless otherwise specified.

[Plating plate]
In the present invention, an original plating plate obtained by adding B to a high-strength steel type containing a relatively large amount of Mn is used. Its chemical composition is as follows.
C: 0.01 to 0.20%
C is a basic element responsible for the strength of the steel sheet, and in the present invention, a steel type having a C content level of 0.01% or more is targeted. You may manage to use the thing of C content of 0.10% or more. However, since excessive C content reduces ductility and weldability, the C content is limited to 0.20% or less.

Si: 0.01% to 1.00%
Since Si in the steel sheet has an effect of increasing the strength of the steel material by solid solution strengthening, a content of 0.01% or more is required. However, the Si content is limited to 1.00% or less as a result of various studies because it also causes a Si oxide film harmful to plating properties to be generated on the surface of the steel sheet.

Mn: 0.50 to 2.50%
Mn in the steel sheet has the effect of increasing the strength of the steel material by solid solution strengthening, and also has the effect of stabilizing the austenite and promoting the generation of transformation phases such as martensite. In order to stabilize the characteristics, and if the Mn content is less than 0.5%, the plating adhesion due to BN does not deteriorate, so the Mn content needs to be about 0.50% or more. However, addition of a large amount of Mn causes a decrease in workability and plating properties, so the Mn content is limited to 2.50% or less.

Ti: 0.010 to 0.150%
Ti is a strong nitride-forming element and is an important element for fixing N in the plating original plate as TiN. By fixing N, the amount of free B is secured, and the effect of improving the resistance to molten metal embrittlement cracking by free B is exhibited. As a result of the study, it is necessary to secure a Ti content of 0.010% or more in order to sufficiently exhibit the above-described effects. More preferably, the content is 0.020% or more. However, even if Ti is added excessively, the above effect is saturated, and addition of a large amount of Ti becomes a factor that deteriorates the workability of the steel material. For this reason, Ti content is restrict | limited to 0.150% or less.

B: 0.0003-0.0100%
B is an element effective for suppressing molten metal embrittlement. The effect is considered to be brought about by the fact that B is segregated as free B to the grain boundary and the interatomic bonding force increases. For that purpose, it is necessary to secure a B content of at least 0.0003% or more. More preferably, the B content is 0.0005% or more. However, excessive B addition causes formation of borides and deterioration of workability, so the B content is limited to 0.0100% or less.

N: Less than 0.010% N reacts with B to form a boride, which causes a reduction in the amount of free B effective in improving the resistance to molten metal embrittlement cracking. As a result of various studies, the N content is limited to a range of less than 0.010%.

Cr: 1.00% or less Cr in the steel sheet has an effect of increasing the strength of the steel sheet by solid solution strengthening, and is also effective in suppressing melting metal embrittlement cracking resistance. However, if it is added in a large amount, it causes a decrease in workability, so it is limited to 1.00% or less. More preferably, it is 0.50% or less.

Nb: 0.10% or less, Mo: 0.50% or less Nb and Mo are elements that contribute to increasing the strength of a steel sheet by solid solution strengthening, and containing a large amount of Nb and Mo is longer at higher temperatures. It is advantageous to obtain good mechanical properties when time-reducing heat treatment conditions are used. However, since the addition of a large amount of these elements causes a decrease in workability, it is preferable to contain Nb in a range of 0.10% or less and Mo in a range of 0.50% or less. Preferably, Nb is 0.05% or less and Mo is 0.20% or less.

  In the present invention, a hot-rolled steel plate or a cold-rolled steel plate having the above chemical composition can be used as the plating base plate. In the case of a hot-rolled steel sheet, the slab to be subjected to hot rolling, the finishing temperature, and the winding temperature are not particularly limited, and may be as usual, but the surface oxide scale needs to be sufficiently removed. When cold rolling is performed, cold rolling is performed in accordance with a conventional method after hot rolling, and finished to a predetermined plate thickness. The plate thickness may be selected in the range of 0.6 to 4.5 mm, for example, depending on the application.

[Reduction heat treatment]
Before introducing the plating base plate into the molten Zn—Al—Mg alloy plating bath, a reduction heat treatment is usually performed to activate the steel plate surface. In a continuous hot dipping line at a mass production site, reduction heat treatment and hot dipping are continuously performed. In many cases, this reduction heat treatment process not only activates the surface of the plating original sheet but also serves as an annealing process for adjusting the metal structure of the steel sheet to a final structure. Therefore, various heat patterns are adopted according to the purpose. Further, depending on the operation status of the line, the speed (line speed) of the steel strip passing through the heat treatment furnace may be adjusted within a range that does not hinder activation and annealing.

As described above, when a steel sheet containing B, which is a relatively high Mn-based high-strength steel type, is subjected to hot-dip Zn—Al—Mg alloy plating, a problem may occur in plating adhesion. The inventors investigated in detail the state of the plating layer / steel substrate interface after hot dipping in order to investigate the cause. As a result, when the dew point of the reduction heat treatment is high (−40 ° C. or higher) with a steel plate containing B in a relatively high Mn-based high-strength steel grade, or a steel plate not containing B, a low Mn content (0.50%) In the steel sheet containing B, a continuous Fe—Al alloy layer was formed at the plating layer / steel base interface, and the adhesion of the plating layer was ensured through this alloy layer. In contrast, when a steel sheet containing B in a relatively high Mn-based high-strength steel type has a low dew point of the reduction heat treatment and a high reduction heat treatment temperature, an Fe—Al alloy layer is formed at the plating layer / steel substrate interface. Many parts were not seen. And it turned out that the plating layer and the steel base are not joined in the part.

  Therefore, in order to grasp the surface state of the plating original plate immediately before being immersed in the molten Zn—Al—Mg alloy plating bath, the steel plate sample was subjected to reduction heat treatment under various conditions, and then the surface was observed and analyzed. According to this, in the reduction heat treatment conditions and steel types that provide good plating adhesion, the Mn—B based complex oxide was only scattered on the steel sheet surface. On the other hand, when the dew point of the reduction heat treatment was low and the reduction heat treatment temperature was high, it was the same that the Mn-B-based composite oxide was scattered on the steel sheet surface, but BN further covered the entire surface. It became clear that it was covered. In such a surface portion covered with BN, the reaction between Fe in the steel substrate and Al in the Zn-Al-Mg alloy plating bath is hindered, and as a result, poor adhesion to the plating layer is likely to occur. It was guessed.

  From such knowledge, when performing hot-dip Zn-Al-Mg alloy plating using a steel plate containing B, which is a relatively high Mn-based high-strength steel type, as a plating base plate, reduction heat treatment of pre-plating treatment is performed on By performing in a condition range that BN does not cover, the plating adhesion can be improved. Specifically, good plating adhesion can be stably realized by strictly controlling the combination of “dew point” and “reduction heat treatment temperature” of the reduction heat treatment within an appropriate range.

  In order to sufficiently activate the plating original plate surface, it is effective to expose the steel plate surface in a reducing atmosphere of 700 ° C. or higher. As a result of detailed examination, when the maximum temperature reached on the surface of the steel sheet in the furnace in a reducing atmosphere is defined as `` reducing heat treatment temperature '', good plating adhesion is achieved by the `` dew point '' and `` reducing heat treatment temperature '' during the reduction heat treatment. It is possible to define the condition range of the reduction heat treatment that can be stably realized. In actual operation, it is possible to control the appropriate condition range of “dew point” and “reduction heat treatment temperature” based on the heat curve data of the steel sheet surface temperature measured in advance in the reduction heat treatment furnace of the production line used. Is possible.

  When a steel plate having a chemical composition in the above-described range is used as a plating base plate, A (−75,800) −B (−45,900) −C shown in FIG. 2 is set as an appropriate range of (dew point, reduction heat treatment temperature). It was found that the conditions within the region (including the boundary) surrounded by the straight line connecting (−10,900) −D (−10,750) −E (−75,750) −A can be adopted. Within this range, a condition that also serves as final annealing of the steel sheet may be applied.

  When the recrystallization annealing also serves as the recrystallization annealing, a condition in which the recrystallization temperature is reached or higher up to the inside of the steel sheet may be adopted in each of the above condition ranges. In the case of the target steel type, it is desirable that the reduction treatment temperature (maximum surface temperature) be 750 ° C. or higher in each of the above condition ranges.

As an atmosphere of the reduction heat treatment, an atmosphere conventionally used as a pretreatment for hot dipping can be applied. For example 5~50% H ₂ -N ₂ atmosphere can be exemplified.

[Hot zinc plating]
The plating original plate that has undergone the above reduction heat treatment is introduced into a molten Zn—Al—Mg alloy plating bath without being exposed to the atmosphere.

  Al in the plating bath is effective for improving the corrosion resistance of the plated steel sheet, and suppresses the generation of Mg oxide-based dross in the plating bath. The effect is recognized when the Al content of the hot dipping bath is 1.0% by mass or more. On the other hand, when the Al content exceeds 22.0%, a brittle Fe—Al alloy layer grows excessively at the interface between the plating layer and the steel substrate, which causes a decrease in plating adhesion. In order to ensure excellent plating adhesion, the Al content is preferably 15.0% or less, and may be controlled to 10.0% or less.

  Mg in the plating bath exhibits an effect of significantly increasing the corrosion resistance of the plated steel sheet by generating a uniform corrosion product on the surface of the plating layer. It is also effective in improving plating adhesion. These effects are manifested when the Mg content of the hot dipping bath is 0.05% or more, and in order to obtain a particularly remarkable effect, it is more preferable to ensure a Mg content of 1.0% or more. On the other hand, when the Mg content exceeds 10.0%, Mg oxide-based dross tends to occur. In order to obtain a higher quality plating layer, the Mg content is preferably 5.0% or less, and may be controlled to 4.0% or less.

When Ti and B are contained in the hot dipping bath, the formation and growth of the Zn ₁₁ Mg ₂ phase that gives speckled appearance defects in the hot-dip Zn—Al—Mg alloy plated steel sheet is suppressed. Further, the addition of these elements increases the degree of freedom in manufacturing conditions during hot dipping. For this reason, 1 type or 2 types of Ti and B can be added as needed. It is more effective to add 0.002% or more in the case of Ti and 0.001% or more in the case of B. However, if the Ti content is excessive, Ti—Al based precipitates are generated in the plating layer, and if the B content is excessive, Al—B based or Ti—B based precipitates are generated in the plating layer. And become coarse. These precipitates are factors that impair the appearance of the plating layer surface. Therefore, when Ti is added to the plating bath, it is necessary to be within the range of 0.10% or less, and more preferably 0.01% or less. Moreover, when adding B, it is necessary to set it as 0.05% or less of range, and it is more preferable to set it as 0.005% or less.

  When Si is contained in the hot dipping bath, excessive growth of the Fe—Al alloy layer generated at the interface between the steel substrate and the plating layer is suppressed, and the workability of the hot-dip Zn—Al—Mg plated steel sheet is improved. Is advantageous. Moreover, Si is effective in preventing the black change of the plating layer and maintaining the glossiness of the surface. Therefore, Si can be contained as necessary. When Si is contained, it is more effective to set the Si content of the hot dipping bath to 0.005% or more. However, since excessive Si content increases the amount of dross in the hot dipping bath, the Si content in the plating bath is limited to 2.0% or less.

  In general, it is unavoidable that Fe is mixed in the hot dipping bath because the steel sheet is immersed and passed. The Fe content in the Zn—Al—Mg alloy plating bath is generally allowed up to about 2.0%. In the plating bath, for example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr and Mn may be mixed, but the total content thereof is 1 It is desirable to manage to 0.0% or less.

It is desirable to adjust the plating adhesion amount within a range of ^{20 to} 300 g / m ² per one side of the steel sheet.

Steel having 16 kinds of chemical compositions shown in Table 1 is melted, the slab is heated to 1250 ° C., extracted, and then hot rolled at a finish rolling temperature of 800 ° C. and a winding temperature of 420 to 500 ° C. Thus, a hot-rolled steel strip having a thickness of 2.5 mm was obtained. Next, the hot-rolled steel strip was pickled and cold-rolled to prepare a cold-rolled steel plate having a thickness of 1.0 mm.
Next, each cold-rolled steel sheet is subjected to reduction heat treatment at various dew points and reduction heat treatment temperatures, and then immersed in a molten Zn-Al-Mg alloy plating bath without being exposed to the atmosphere. A hot dip galvanized steel sheet having a coating adhesion amount of about 90 g / m ² was obtained. The experimental conditions are as follows in Table 2.

[Reduction heat treatment]
Atmospheric gas; 30% H ₂ -N ₂ atmosphere, dew point and the heat treatment temperature: TABLE 2
-Holding time (time for holding above 750 ° C): 32 seconds

[Hot plating]
・ Bath composition; Table 2
・ Bath temperature: 400 ℃
・ Bath immersion time: 2 seconds

[Evaluation of plating adhesion]
A bending test piece having a width of 15 mm was cut out from the obtained plated steel sheet, and a 90 ° V bending test was performed using a punch having a tip radius R = 2 mm. The width direction of the specimen (= direction of the bending axis) was made to coincide with the rolling direction. Regarding the test piece after the bending test, a cellophane adhesive tape defined in JIS Z1522 was applied to the outer periphery of the bending portion, and then peeled off. ), Other than that was judged as x (plating adhesion; poor). The same kind of plating sample was subjected to a bending test with n = 3, and the result of the test piece with the worst evaluation was adopted as the result of the sample. The results are shown in Tables 2 and 3.

[Evaluation of molten metal embrittlement crack resistance]
A 100 mm × 75 mm sample was cut out from the plated steel sheet, and this was used as a test piece for evaluating the maximum weld crack length caused by molten metal embrittlement by arc welding.
In the welding test, “boss welding” for producing a boss weld member having an appearance as shown in FIG. 1 was performed, and the cross section of the welded portion was observed to examine the occurrence of cracks. That is, a boss (projection) 2 made of mild steel having a diameter of 20 mm and a length of 25 mm was set up vertically in the center of the plate surface of the test piece 1, and the boss 2 was joined to the test piece 1 by arc welding. The welding conditions were welding current: 220 A, welding voltage 25 V, welding speed 0.2 m / min, shielding gas: CO2, and shielding gas flow rate: 20 L / min. YGW12 was used for the welding wire.
After the welding start point made one round around the boss and after the welding start point was passed, welding was continued further to make a portion 4 where the weld beads 3 overlapped.

After the boss welding, the test piece and the boss were cut so as to include the bead overlap portion 4, the cross section was embedded, and the bead overlap portion was observed with an optical microscope. When a crack was observed in the portion of the test piece 1 in the cross section, the length of the crack was measured. When a plurality of cracks were observed, the longest crack length was defined as the “maximum crack length”. This crack occurs along the prior austenite grain boundary of the weld heat affected zone, and it is determined that this crack is a molten metal embrittlement crack. Evaluation of the resistance to molten metal embrittlement cracking was a pass (◯) when the maximum crack length was 0.1 mm or less, and a reject (x) when it exceeded 0.1 mm.
The evaluation results are shown in Table 4. Steels A to L passed, but four types of steels M to P failed.

  It can be seen that good plating adhesion can be obtained within the range of the reduction heat treatment defined in the present invention.

1 Test piece 2 Boss 3 Weld bead 4 Bead overlap

Claims (3)

The chemical composition of the steel sheet is% by mass: C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.50 to 3.00%, Ti: 0.010 to 0 150%, B: 0.0003 to 0.0100%, N: containing one or more selected from the group of less than 0.010%, the steel sheet consisting of the remainder Fe and unavoidable impurities as a plating original plate,
Following the reduction heat treatment,
Molten Zn-Al-Mg alloy-plated steel sheet by applying molten Zn-Al-Mg alloy plating in which mass: Al: 0.1-22.0%, Mg: 0.05-10.0%, the balance being Zn In manufacturing
In the reduction heat treatment step,
When the maximum temperature reached on the surface of a steel sheet in a reduction heat treatment furnace is defined as “reduction heat treatment temperature”, the dew point (° C.) in the furnace is the x axis and the reduction heat treatment temperature (° C.) is the y axis. In the xy orthogonal coordinate system, A (−75,800) −B (−45,900) −C (−10,900) −D (−10,750) −E (−75,750) shown in FIG. 2. ) It is characterized in that the reduction heat treatment is performed under conditions satisfying the dew point (including the boundary) and the reduction heat treatment temperature in the region surrounded by the straight line connecting -A.
A method for producing a hot-dip Zn—Al—Mg alloy-plated steel sheet having excellent plating adhesion. The chemical composition of the steel sheet further includes one or more selected from the group consisting of Cr: 1.00% or less, Nb: 0.10% or less, and Mo: 0.50% or less in terms of mass%. And
The manufacturing method of the hot dip Zn-Al-Mg alloy plating steel plate excellent in plating adhesiveness of Claim 1. The molten Zn-Al-Mg alloy plating further contains at least one selected from the group consisting of Ti: 0.10% or less, B: 0.05% or less, Si: 2.0% or less in terms of mass%. It is characterized by
The manufacturing method of the hot dip Zn-Al-Mg alloy plating steel plate excellent in plating adhesiveness of Claim 1 or 2.

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