JP2008184685A - Zn-Al-Mg BASE PLATED STEEL SHEET HAVING EXCELLENT MOLTEN METAL EMBRITTLEMENT CRACK RESISTANCE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Zn-Al-Mg base plated steel sheet provided with the properties that molten metal embrittlement cracks can be stably suppressed even under severe welding conditions. <P>SOLUTION: In the Zn-Al-Mg base plated steel sheet having excellent molten metal embrittlement crack resistance, a steel sheet substrate (plating original sheet) is composed of a steel having a composition comprising, by mass, 0.01 to 0.3% C, ≤1.5% Si, 0.05 to 2.0% Mn, ≤0.1% P, ≤0.005% N, ≤0.1% Ti, 0.0003 to 0.01% B and 0.5 to 3.0% Cr, and, if required, comprising one or more selected from ≤0.3% Nb, ≤1.0% V, ≤1.0% Mo and ≤1.0% Zr, or further comprising one or more selected from ≤1.0% Cu and ≤1.0% N, and the balance Fe with inevitable impurities, and satisfying inequality(1): Ti≥3.43×N (1). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot-dip Zn—Al—Mg-based plated steel sheet in which the occurrence of molten metal embrittlement cracks in a weld heat affected zone is suppressed when subjected to welding.

  The Zn—Al—Mg based steel sheet is excellent in corrosion resistance as compared with a general galvanized steel sheet. Conventionally, it has been difficult to produce high-quality molten Zn-Al-Mg plated steel sheets, but in recent years, manufacturing technology for molten Zn-Al-Mg plated steel sheets on an industrial scale has been established. Hot-dip Zn—Al—Mg plated steel sheets have been used for various structural members as well as materials.

  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. According to the investigations so far, when using a Zn-Al-Mg plated steel sheet, fine intergranular cracks are likely to occur in the weld heat affected zone (HAZ) as compared to a general galvanized steel sheet. I know empirically. This crack is caused by a tensile stress caused by the expansion and contraction of the material during welding, and is a kind of phenomenon called a molten metal embrittlement crack. It is considered that the Zn—Al—Mg-based plating component melted at the time of welding increases the sensitivity of molten metal embrittlement cracking compared to the case of zinc plating.

  In order to improve the molten metal embrittlement cracking resistance of the Zn—Al—Mg plated steel sheet, various studies have been made so far. Patent Documents 1 and 2 disclose that the metal structure of the base steel is a mixed structure of ferrite and bainite, pearlite, or martensite, and suppresses intrusion of the molten metal into the grain boundary by complicating the crystal grain boundary. Is disclosed. Patent Document 3 uses steel plate materials to which Ti, Nb, V, Mo, Zr, etc. are added, and precipitates of these elements having a pinning action are dispersed to suppress the growth of crystal grains in the austenite region during welding. In addition, there is disclosed a technique for preventing cracks by utilizing the action of these elements segregating at grain boundaries after welding. Patent Document 4 uses a base steel on which a strong and thin film in which Cr, Cu, Ni and the like are concentrated together with Si is used to suppress the penetration of hot-dipped metal into crystal grain boundaries on the steel surface. A technique is disclosed.

JP 2004-315847 A JP 2004-315848 A JP 2006-97129 A JP 2006-89787 A

Any of the methods described in the above patent documents is effective in improving the resistance to molten metal embrittlement cracking of the Zn—Al—Mg based steel sheet. However, according to further investigations by the present inventors, in actual welding work, there are cases where molten metal embrittlement cracks cannot be stopped depending on the welding conditions even if the methods of these documents are adopted. all right.
An object of this invention is to provide the Zn-Al-Mg type plated steel plate with the property which can suppress a molten metal embrittlement crack stably also on severe welding conditions.

As a result of detailed studies by the inventors, it has been found that the above object can be achieved by adopting steel in which Ti and B are added in combination and the Cr content is increased to a certain level or more as a base steel sheet. That is, in the present invention, in a plated steel sheet that has been hot-plated consisting of Al: 3 to 22%, Mg: 1 to 10%, and the balance being substantially Zn in mass%, the base steel sheet (plating original sheet) is C in mass%. : 0.01-0.3%, Si: 1.5% or less, Mn: 0.05-2.0%, P: 0.1% or less, N: 0.005% or less, Ti: 0.1 %: B: 0.0003-0.01%, Cr: 0.5-3.0%, and in some cases, Nb: 0.3% or less, V: 1.0% or less, Mo: 1 1.0% or less and Zr: 1.0% or less of one or more, or Cu: 1.0% or less and Ni: 1.0% or less of one or more, the balance Fe and inevitable Zn-Al-Mg-based plated steel sheet having excellent resistance to molten metal embrittlement cracking, which is made of steel having a composition satisfying the following formula (1): It is provided.
Ti ≧ 3.43 × N (1)

  Here, the term “hot-plated substantially consisting of Zn” means that the corrosion resistance of the Zn—Al—Mg-based plating is not hindered and the hot-dip plated steel sheet can be manufactured itself. This is to allow mixing of elements other than. For example, Ti, B, Si, and Fe are included as elements generally contained in the molten Zn—Al—Mg plating bath, and these are Ti: 0.1 mass% or less, B: 0.05 mass. %, Si: 2% or less, Fe: 2% or less. In addition, “the balance is substantially composed of Zn” includes a case where “the balance is composed of Zn and inevitable impurities”, and includes “at least one of Ti, B, Si, and Fe in the above content range, Is comprised of Zn and inevitable impurities. The value of the content of the element represented by mass% is substituted for the element symbol in the formula (1).

  According to the present invention, in various uses where steel having a C content of 0.01 to 0.3% is used, a welded structure having a sound welded portion using a Zn-Al-Mg plated steel sheet having excellent corrosion resistance. A member is provided. Accordingly, the present invention brings about an improvement in the corrosion resistance level while maintaining reliability in various welded structural members that have conventionally used galvanized steel sheets.

  In the present invention, by using a steel containing an appropriate amount of Ti, B, and Cr as a base steel plate that is a plating base plate, molten metal embrittlement cracking during welding of a molten Zn-Al-Mg plated steel plate is prevented. To do. The component elements of the base steel sheet will be described below.

  C is an element necessary for ensuring the strength of the steel material, and the C content is adjusted according to the required strength level. For example, as a building material application, steel having a high strength level in which the C content is adjusted to 0.07 to 0.3% by mass is suitable for a column reinforcing material or a member where a frame is joined. Further, steels having a C content of 0.01 to less than 0.07% by mass are often used in automobile members and other applications that require relatively good workability. When the C content increases, the workability decreases with decreasing ductility, so it is necessary to set the upper limit of the C content according to the application. In the present invention, C: 0.01 to 0.3% by mass of steel is an object.

  Si is an element that dissolves in the ferrite phase and is effective in increasing the strength of the steel material. In order to obtain the effect sufficiently, it is desirable to secure a Si content of 0.01% by mass or more. However, when the Si content is increased, ductility is reduced, or Si is concentrated on the surface of the steel material, thereby reducing the plating property. For this reason, in this invention, Si content is restrict | limited to the range of 1.5 mass% or less, and it is more preferable to set it as 0.5 mass% or less.

  Mn is an element that prevents embrittlement due to S and is effective in improving strength. In order to obtain these effects, a Mn content of 0.05% by mass or more is ensured. However, when the Mn content is increased, workability and weldability are deteriorated, or Mn is concentrated on the steel material surface and the plating property is decreased. For this reason, in this invention, Mn content is restrict | limited to the range of 2.0 mass% or less, and it is more preferable to set it as 0.5 mass% or less.

  Since P adversely affects ductility, it is better for P to be used in applications where high workability is required. However, since P has an effect of increasing the strength, it can be added in a range that does not adversely affect the workability and the plating property in applications where high strength is required. For example, P content of 0.01% by mass or more is advantageous for increasing the strength, but the upper limit is limited to 0.1% by mass for the above reasons, and more preferably 0.05% by mass or less. .

  Since S causes embrittlement of the steel, the S content is preferably suppressed to 0.03 mass% or less.

  N reacts with B in the steel to form BN. When B is consumed due to the formation of BN, it becomes difficult to sufficiently secure the content (effective B amount) of B, which plays an important function in order to improve the resistance to molten metal embrittlement cracking. For this reason, it is important to keep the N content as low as possible. In the present invention, formation of BN is suppressed by fixing N with Ti described later, but the N content still needs to be suppressed to 0.005 mass% or less.

Ti is a strong nitride-forming element and has a function of suppressing the formation of BN by fixing N as TiN as described above. As a result of various studies, in order to sufficiently exhibit such a Ti function, it is necessary to ensure the Ti content so as to satisfy the following expression (1).
Ti ≧ 3.43 × N (1)
This formula (1) indicates the Ti content necessary for fixing almost the entire amount of N present in the steel. When the Ti content is low beyond the formula (1), it is difficult to sufficiently secure the effective B amount.
It was also found that Ti has the effect of promoting the grain boundary segregation of Cr. For this reason, it is effective also in enhancing the improvement effect of the resistance to molten metal embrittlement cracking by Cr described later. However, excessive Ti content causes a decrease in workability and an increase in manufacturing cost of the steel material, so the Ti content is limited to a range of 0.1% by mass or less.

  B is an element effective for suppressing molten metal embrittlement cracking. The effect is considered to be due to the fact that B segregates at the grain boundaries and increases the interatomic bonding force. In the analysis by SIMS, it is confirmed that B is significantly segregated at the austenite grain boundary by heating the steel of the present invention to a high temperature austenite region. In order to exert such a grain boundary strengthening function by B, as described above, by reducing N or fixing N by Ti, it is possible to suppress the consumption of B accompanying BN formation and secure an effective B amount. is important. In addition, the B content (total amount) must be 0.0003 mass% or more, and more preferably 0.001 mass% or more. However, since excessive addition of B causes deterioration of workability, the B content (total amount) is limited to 0.01% by mass or less.

  It has been found that Cr significantly contributes to the suppression of molten metal embrittlement cracking by segregating at austenite grain boundaries at high temperatures. As shown in Patent Document 4, conventionally, there has been a steel containing Cr and B as a base steel plate of a Zn—Al—Mg based steel plate. However, when such conventional steel was used, there were cases where molten metal embrittlement cracks occurred in actual welding work. As a result of detailed studies, the inventors have found that the above problem can be solved by increasing the Cr content level compared to the conventional steel and adding Ti. Specifically, the addition amount of Ti and B is as described above, and the addition amount of Cr is extremely effective to ensure a Cr content of 0.5% by mass or more, preferably more than 0.5% by mass. I found out that Thus, the mechanism by which the resistance to molten metal embrittlement is remarkably improved by adding Ti and B and increasing the amount of Cr is not clarified at the present time, but a certain amount or more of B and Cr are simultaneously high in temperature. By segregating at the austenite grain boundaries, the synergistic effect of B and Cr lowers the grain boundary energy than in the case of conventional steel and increases the interatomic bonding force at the grain boundaries. It is inferred that the resistance increased significantly. However, if Cr is excessively contained, the workability of the steel material is lowered and the toughness is also adversely affected, so the Cr content is limited to a range of 3.0% by mass or less.

  Each element of Nb, V, Mo, Zr, Cu, and Ni also has the effect of suppressing molten metal embrittlement cracks by segregating at high temperature austenite grain boundaries, so one or more of these elements may be added as necessary can do. The effect becomes more remarkable by the combined addition with B and Cr. In order to sufficiently bring out the above-described action of each element, Nb: 0.01% by mass or more, V: 0.05% by mass or more, Mo: 0.05% by mass or more, Zr: 0.05% by mass or more, It is particularly effective to set Cu: 0.05% by mass or more and Ni: 0.05% or more. However, even if these elements are added excessively, the effect of suppressing molten metal embrittlement cracking is saturated, resulting in a decrease in the toughness and workability of the steel material and an increase in manufacturing costs. Nb: 0.3% or less, V: 1.0% or less, Mo: 1.0% or less, Zr: 1.0% or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less Do in range.

  The base steel plate having the above composition can be produced by a conventional method in a general steel plate production line. Thereafter, hot-dip Zn—Al—Mg plating is performed by a known method to obtain a plated steel sheet. The Zn—Al—Mg based plating applied to the present invention is as follows.

  Al in a plating layer has the effect | action which improves the corrosion resistance of a plated steel plate. Moreover, it has the effect | action which suppresses generation | occurrence | production of Mg oxide type dross by containing Al in a plating bath. In order to obtain these effects sufficiently, the Al content of the hot dipping needs to be 3% by mass or more, and more preferably 4% by mass or more. On the other hand, if the Al content exceeds 22% by mass, the growth of the Fe—Al alloy layer becomes remarkable at the interface between the plating layer and the base steel sheet, and the plating adhesion deteriorates. In order to ensure excellent plating adhesion, the Al content is preferably 15% by mass or less, and more preferably 10% by mass or less.

  Mg in the plating layer exhibits a function of generating a uniform corrosion product on the surface of the plating layer and remarkably increasing the corrosion resistance of the plated steel sheet. In order to fully exert its action, the Mg content of the hot-dip plating needs to be 1% by mass or more, and it is desirable to ensure 2% by mass or more. On the other hand, when the Mg content exceeds 10% by mass, an adverse effect of easily generating Mg oxide-based dross increases. In order to obtain a higher quality plating layer, the Mg content is preferably 5% by mass or less, and more preferably 4% by mass or less.

When Ti and B are contained in the hot dipping bath, the generation and growth of the Zn ₁₁ Mg ₂ phase which gives speckled appearance defects in the hot-dip Zn—Al—Mg based steel sheet are suppressed. Even if Ti and B are each contained alone, the effect of suppressing the Zn ₁₁ Mg ₂ phase is produced, but it is desirable to contain Ti and B in combination in order to greatly relax the degree of freedom of the production conditions. In order to sufficiently obtain these effects, it is effective that the Ti content of the hot dipping is 0.0005 mass% or more and the B content is 0.0001 mass% or more. However, if the Ti content is excessively large, Ti—Al-based precipitates are generated in the plating layer, and irregularities called “bumps” are generated in the plating layer, thereby impairing the appearance. For this reason, when adding Ti to a plating bath, it is necessary to make it the content range of 0.1 mass% or less, and it is desirable to set it as 0.01 mass% or less. On the other hand, when the B content is excessively large, Al—B or Ti—B based precipitates are generated and coarsened in the plating layer, and irregularities called “bumps” are also generated to impair the appearance. For this reason, when adding B to a plating bath, it is necessary to make it the content range of 0.05 mass% or less, and it is desirable to set it as 0.005 mass% or less.

  When Si is contained in the hot dipping bath, the growth of the Fe—Al alloy layer is suppressed, and the workability of the hot dipped Zn—Al—Mg plated steel sheet is improved. Moreover, Si in the plating layer is effective in preventing the black change of the plating layer and maintaining the gloss of the surface. In order to sufficiently bring out such an action of Si, it is effective to set the Si content of the hot dipping to 0.005 mass% or more. However, since the amount of dross in the hot dipping bath increases when Si is added excessively, the content range is 2.0% by mass or less when Si is contained in the plating bath.

  In general, it is unavoidable to mix Fe in the hot dipping bath because the base steel plate is immersed and passed. In the Zn—Al—Mg based plating, Fe is allowed to be contained up to about 2% by mass. The plating bath may contain, as other elements, for example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr, and Mn, but their total content The amount is desirably 1% by mass 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 plate. The amount of plating adhesion can be controlled using a gas wiping nozzle in accordance with the production of a general galvanized steel sheet. The wiping gas or the atmosphere gas during solidification of the plating layer can be air (atmosphere). That is, an air cooling method can be adopted. If the plating bath temperature exceeds 550 ° C., the evaporation of zinc from the bath becomes remarkable, so that plating defects are likely to occur and the amount of oxidized dross on the bath surface increases, which is not preferable.

Steels having the compositions shown in Tables 1 to 4 are melted in a vacuum melting furnace to produce an ingot, and a steel sheet having a thickness of 3.2 mm is cooled by a process of hot forging → hot rolling → pickling → cold rolling. A rolled sheet was produced. After annealing the cold-rolled plate in a hydrogen-nitrogen mixed gas at 700 ° C., keep it in a state where it is not exposed to the air, and immediately immerse it in one of the following plating baths to perform hot-dip Zn—Al—Mg plating. did. The “%” below is mass%.
[Plating bath a] Al: 6%, Mg: 3%, Zn: balance [Plating bath b] Al: 6%, Mg: 3%, Ti: 0.002%, B: 0.0005%, Si: 0 0.01%, Fe: 0.1%, Zn: balance [Plating bath c] Al: 6%, Mg: 3%, Si: 0.1%, Fe: 0.1%, Zn: balance [Plating bath d ] Al: 12%, Mg: 8%, Si: 0.1%, Fe: 0.1%, Zn: balance

The plating bath temperature was about 400 ° C., and a hot-dip Zn—Al—Mg-based plated steel sheet was obtained by pulling up from the plating bath and adjusting the plating adhesion amount to about 90 g / m ² per one side of the steel sheet. The cooling when the plating layer is solidified is air cooling.

A 100 mm × 75 mm sample was cut out from the obtained plated steel sheet, and this was used as a test piece for evaluating the maximum weld crack length caused by molten metal embrittlement.
The welding test was performed by “boss welding” for producing a boss welded member having an appearance as shown in FIG. 1 and examining the cross section of the welded portion 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. As the welding wire, YGW12 was used. After the welding start point, the circumference of the boss was made one round, and after passing the welding start point, the bead was further piled up and the welding was completed when the welding proceeded a little. That is, the weld bead 6 overlaps between the welding start point and the welding end point. The welding conditions were a welding current: 217 A, a welding voltage of 25 V, a welding speed of 0.2 m / min, a shielding gas: CO ₂ , and a shielding gas flow rate: 20 L / min. A member after welding composed of the boss 1, the test piece 3, and the weld bead 6 is called a “boss welded member”.

  However, the welding was performed in a state where the test piece 3 was restrained as shown in FIG. That is, the test piece 3 was placed at the center of the plate surface of the restraint plate 4 (SS400 steel material defined in JIS) having a size of 120 mm × 95 mm × 4 mm, and the entire circumference of the test piece 3 was welded to the restraint plate 4 in advance. The integrated test piece 3 / restraint plate 4 assembly was fixed on a horizontal test bench 5 by two clamps 2, and the boss welding was performed in this state. In this specification, the boss welding performed in such a restrained state is referred to as “restraint boss welding”. According to this method, since the test piece 3 is integrated with the restraint plate 4 by all-around welding, the expansion / contraction caused by heat input during boss welding is restrained. Stress makes it easier for weld cracks to occur during boss welding, enabling clear evaluation of weld cracks.

After welding the restraint boss, the joined surface of the boss 1 / test piece 3 / restraint plate 4 is cut at the cut surface 9 passing through the central axis of the boss 1 and passing through the overlap portion 8 of the weld bead. About the metal structure of the test piece 3 (namely, the base steel plate which is a plating steel plate base material) part of the welding bead vicinity was observed with the microscope. About the crack observed in the part of the test piece 3 in the cross section by the microscopic observation, the length of the crack from the surface on the boss weld side of the test piece 3 to the tip of the crack is measured, and the measured value for the longest crack is obtained. “Maximum crack length”. Considering the strength and fatigue characteristics of the welded part, those having a maximum crack length of 0.2 mm or less were accepted. Such a crack in the base steel sheet occurs along the prior austenite grain boundary of the weld heat affected zone, and this is judged to be a “molten metal embrittlement crack”.
The results are shown in Tables 1-4. FIG. 3 illustrates the relationship between the Cr content of the base steel sheet and the maximum crack length for the steels shown in Tables 1 and 4. In addition, all S content of each base steel plate is 0.03 mass% or less.

  In the base steel sheet of the present invention example containing Ti, B, and Cr in an appropriate range, any of the molten metal embrittlement cracks in the vicinity of the weld is remarkably suppressed, and cracks that exceed the maximum crack length of 0.2 mm Did not occur. In this restraint boss welding test, the Zn-Al-Mg-based plated steel sheet, in which molten metal embrittlement cracking is stably suppressed in this way, has a strength and fatigue characteristics of the welded portion as compared with the conventional similar-plated steel sheet. The reliability is greatly improved, and it is suitable for various materials such as building materials and automobiles.

  In contrast, B1 steel, B5 steel (which is a steel corresponding to the invention of Patent Document 3), and B6 steel (which is a steel corresponding to the invention of Patent Document 1) are Cr-containing. Due to the insufficient amount or the addition of Cr, molten metal embrittlement cracking exceeding the maximum crack length of 0.2 mm occurred in this constrained boss welding test. Due to the lack of B content in the B2 steel, molten metal embrittlement cracks exceeding the maximum crack length of 0.2 mm were generated. B3 steel, B4 steel, B8 steel, B9 steel, and B10 steel are considered to have been unable to ensure sufficient effective B amount due to the generation of BN due to the Ti content deviating from formula (1) and low. Although the amount and the Cr content were appropriate, it was not possible to suppress molten metal embrittlement cracking accompanying welding of the Zn—Al—Mg based steel sheet. B7 steel (which is a steel corresponding to the invention of Patent Document 4) has a low Cr content, and the Ti content is too low from the formula (1), so that the maximum crack length is 0.2 mm. Excessive molten metal embrittlement cracking occurred.

The figure which showed typically the shape of the boss | hub welding member. Sectional drawing which showed typically the restraint method of the test piece at the time of performing restraint boss welding. The graph which illustrated the relationship between Cr content and the maximum crack depth of a base steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Boss 2 Clamp 3 Test piece 4 Restraint plate 5 Test bench 6 Weld bead 7 Weld bead of all-around weld part of test piece 8 Overlap part of weld bead 9 Cut surface

Claims (4)

In a plated steel sheet that has been hot-plated with Al: 3 to 22% in mass%, Mg: 1 to 10%, and the balance substantially consisting of Zn, the base steel sheet is C: 0.01 to 0.3% in mass%. , Si: 1.5% or less, Mn: 0.05-2.0%, P: 0.1% or less, N: 0.005% or less, Ti: 0.1% or less, B: 0.0003- 0.01%, Cr: 0.5 to 3.0%, balance Fe and unavoidable impurities, and made of steel having a composition satisfying the following formula (1) Excellent Zn-Al-Mg plated steel sheet.
Ti ≧ 3.43 × N (1)   The base steel plate is further composed of steel containing at least one of Nb: 0.3% or less, V: 1.0% or less, Mo: 1.0% or less, and Zr: 1.0% or less. The Zn-Al-Mg-based plated steel sheet having excellent resistance to molten metal embrittlement cracking according to claim 1.   3. The molten metal embrittlement crack resistance according to claim 1, wherein the base steel plate is made of steel containing at least one of Cu: 1.0% or less and Ni: 1.0% or less. Zn-Al-Mg plated steel sheet with excellent properties.   The hot-dip plating contains Al: 3 to 22% and Mg: 1 to 10% by mass, and further Ti: 0.1% by mass or less, B: 0.05% by mass or less, Si: 2% or less, The Zn-Al- having excellent resistance to molten metal embrittlement cracking according to any one of claims 1 to 3, wherein Fe: one or more of 2% or less is contained, and the balance is composed of Zn and inevitable impurities. Mg-based plated steel sheet.

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