JP4676923B2 - High strength and high ductility hot dip galvanized steel sheet excellent in corrosion resistance and welding strength and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in ductility, plating adhesion and corrosion resistance, and welding strength, and a method for producing the same, which are widely used in industries such as automobiles, electrical machinery, and building materials. In the following, as a typical application, description will be made focusing on the case where it is used for an automobile body.

  In recent years, the use of high-strength steel sheets having excellent ductility for vehicle bodies has been expanding from the viewpoint of achieving both improved collision safety and improved fuel efficiency. However, in general, the ductility tends to be deteriorated by increasing the strength of a steel material, and a steel sheet having improved ductility while maintaining high strength is required to be applied to automobile parts having a high degree of processing.

  Under such circumstances, a retained austenitic steel is known as a steel plate having both strength and elongation. This steel material is a steel material in which C is concentrated in austenite in the manufacturing process and is stabilized and left to room temperature, and its use as an automotive material is expanding. In conventional retained austenitic steel, Si was added. However, when the Si content exceeds 0.3%, the Sendzimer method using a plating bath containing Al significantly reduces plating wettability and causes non-plating. However, it has a problem that the appearance quality is deteriorated and the corrosion resistance is deteriorated. This phenomenon is considered to be caused by the fact that Si oxide is concentrated on the surface of the steel sheet during reduction annealing, and the wettability of molten zinc to Si oxide is poor. Also, in general continuous annealing equipment that produces residual austenitic steel material, austenite is stabilized by maintaining the temperature in the range of 350 to 550 ° C. after annealing and cooling. The equipment is generally not equipped with equipment that can be kept isothermally, which is performed in a continuous annealing equipment, and it has been difficult to manufacture residual austenitic steel subjected to hot dip plating both in terms of components and manufacturing process.

  As a means for solving such a problem, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5, etc., reduce the Si amount, and increase the Al addition amount as an alternative. A method for producing a retained austenitic steel capable of imparting is disclosed. Patent Document 6 also discloses a method for producing a steel sheet having good stretch flangeability by making the austenite shape of retained austenitic steel into a lath shape. However, there is not enough time to maintain the temperature in the range of 350 to 550 ° C. after hot rolling, cold rolling, and annealing cooling, and the austenite structure is not sufficiently defined to ensure a high balance between strength and ductility. It is considered difficult to meet the increasing need for moldability.

In order to stabilize the retained austenite phase, in general, the amount of C added to the steel sheet often exceeds 0.1% by mass, and Mn is also added to ensure strength. Then, the hardenability of the steel sheet is increased, and the weld metal and heat-affected zone (HAZ) after welding becomes a hardened structure such as martensite and bainite. When the hardness of the weld metal or HAZ is high, properties such as the strength, toughness, and fatigue strength of the welded portion deteriorate. Conventionally known steel sheets have been considered in terms of strength, ductility, and plating properties, but have not necessarily stepped into the weld properties. Since the steel sheet used in the automobile body is always welded after forming to form a vehicle body structure, it can be said that it is an essential condition to form a sound weld in order to keep the vehicle body performance good and for a long time. That is, no matter how excellent the ductility and corrosion resistance of the steel sheet are, if the welded part properties are poor, the steel sheet does not satisfy the requirements to be provided.
As described above, no steel sheet has been developed that takes into account the high strength and high ductility, the problem of Si addition, the corrosion resistance improvement, and the welded portion characteristics.

JP-A-6-145788 Japanese Patent Laid-Open No. 6-145892 JP-A-10-130776 JP-A-11-131145 JP 2000-256788 A JP 2002-309334 A

  An object of the present invention is to solve the above-mentioned problems, and to provide a high-strength hot-dip galvanized steel sheet that has both corrosion resistance and high ductility, and good weld strength, as well as a method for producing the same, by preventing unplating and surface defects. And

In order to solve the above-mentioned problems, the present inventor has intensively studied, and by controlling the shape of retained austenite grains in the steel, both the strength and elongation of the steel material can be demonstrated to a high degree, By reducing the hardness, it is possible to improve the properties of the welded part such as toughness and fatigue strength, and to further balance the amount of addition of Al, O, other C, Si, Mn in the steel and its amount Thus, the present invention has been completed by finding out that both strength and ductility and weld properties can be maintained at a high level.
The gist of the present invention is as follows.

(1) Steel component is mass%,
C: 0.1 to 0.3%
Si: 0.001 to less than 0.200%,
Mn: 1.0 to 3.0%
Al: 0.5 to 2.0%,
P: 0.001 to 0.300%,
S: 0.0001 to 0.1000%,
O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
A galvanized steel sheet comprising the balance Fe and unavoidable impurities, wherein the microstructure of the steel is 30 to 90% ferrite phase in volume fraction, 5% or more bainite, 10% or less martensite, And 5 to 30% of the retained austenite phase, the carbon in the retained austenite is 1.0% or more by mass%, and the average aspect ratio (maximum width / minimum width) of the retained austenite grains is 2 or less. A high strength and high ductility hot dip galvanized steel sheet having excellent corrosion resistance and welding strength.

( 2 ) The microstructure of the steel sheet is mainly composed of ferrite with a volume fraction of 30 to 90%, the average particle size is 20 μm or less, and the second phase is retained austenite with a volume fraction of 5 to 30%. The average particle size of the second phase is 10 μm or less, and further consists of bainite having a volume fraction of 5% or more. High strength and high ductility with excellent corrosion resistance and weld strength according to (1) Hot dip galvanized steel sheet.

( 3 ) Steel is further mass%,
Mo: 0.001 to 1.0%,
Cr: 0.001 to 25.000%,
Ni: 0.001 to 10.000%,
Cu: 0.001 to 5.000%,
Co: 0.001 to 5.000%,
W: 0.001 to 5.000%
A high strength and high ductility hot dip galvanized steel sheet excellent in corrosion resistance and weld strength according to (1) or (2) , characterized by containing one or more of the above.

( 4 ) The steel further contains 0.001 to 1.000% of one or more of Nb, Ti, V, Zr, Hf, Ta in total by mass% (1) A high-strength, high-ductility hot-dip galvanized steel sheet excellent in corrosion resistance and weld strength according to any one of (3) .

( 5 ) Steel is further excellent in corrosion resistance and weld strength according to any one of (1) to (4) , characterized by containing B: 0.0001 to 0.1000% by mass%. High strength and high ductility hot dip galvanized steel sheet.

( 6 ) Any one of (1) to ( 5 ), wherein the steel further contains 0.0001 to 1.0000% of one or more of Y, Rem, Ca, Mg, and Ce in mass%. A high-strength, high-ductility hot-dip galvanized steel sheet excellent in corrosion resistance and weld strength according to item 1.

( 7 ) In the production of the high-strength hot-dip galvanized steel sheet according to any one of (1) to (6) , a cast slab made of the steel sheet component according to any one of claims 1 to 6 is used. Hot-rolled steel sheet as it is cast or once cooled to 1200-1300 ° C., then subjected to rough hot rolling at a total rolling reduction of 60-99% at 800-1200 ° C., finish-rolled and wound After pickling, cold rolling, and then annealing for 10 seconds to 30 minutes in a temperature range of Ac ₁ (° C.) or more and Ac ₃ +50 (° C.) or less, and then reaching the maximum temperature during annealing: Tmax (° C.) When cooling to a temperature range of Tmax-200 ° C to Tmax-100 ° C at a cooling rate of 10 ° C / s or less, and subsequently cooling to a temperature of 350-500 ° C at a cooling rate of 5 ° C / second or more. After holding for 5 to 900 seconds within the range, Immersed in Kki bath, high strength and high manufacturing method of ductility galvanized steel sheet having excellent corrosion resistance and weld strength, characterized in that cooling to room temperature.

( 8 ) After immersion in the plating bath, the alloying treatment is performed in a temperature range of 400 to 550 ° C. and cooled to room temperature. (7) High strength and high ductility hot dip galvanizing excellent in corrosion resistance and welding strength A method of manufacturing a steel sheet.

( 9 ) The method for producing a high strength and high ductility hot dip galvanized steel sheet having excellent corrosion resistance and welding strength according to (7) , wherein the cold rolling rate is in the range of 40 to 80%.

  The high-strength hot-dip galvanized steel sheet of the present invention has excellent corrosion resistance, particularly corrosion resistance in a chlorine-containing environment, and excellent workability, and also has good welded part properties, so it is extremely useful for building materials, home appliances, automobile body applications, etc. It is valid.

  The inventors of the present invention have found by studying earnestly that it is extremely important to satisfy the following three conditions in order to retain excellent ductility and weldability while the steel sheet has high strength.

  First, the shape control of the retained austenite grains in the steel is extremely important in order to fully exhibit both the strength and elongation of the steel material. That is, it was clarified that the average aspect ratio (maximum width / minimum width) of austenite grains needs to be 2 or less. Clearly, in order to reduce the aspect ratio of retained austenite, the addition of Al and O plays an especially important role in steel sheet components, and it is necessary to maintain the isothermal holding after heating and cooling for a sufficient time in the manufacturing conditions. did.

  Next, in order to improve the characteristics of the welded part such as toughness and fatigue strength, it is necessary to reduce the hardness of the welded part. Addition of Al and O in the steel is essential in order to reduce the hardness of the weld zone, and the oxide of Al that can exist stably even at the melting point reduces the hardenability of the weld zone. It is possible to reduce the height.

Furthermore, in order to maintain both strength and ductility and weld properties at a high level, the inventors have found that the amount of addition of Al, O, and other C, Si, and Mn in the steel and the balance of the amounts are important. That is, it was found that it is important to satisfy the relationship of the following formula (A).
0 ≦ 2 (Al + Mn / 2 + 100 × O) × (2.2 × Al—Mn—O) —C—Si / 30 (A)

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting each steel plate component will be described.

  C: In order to ensure the strength (TS) of 590 MPa or more, the lower limit of the C amount was 0.1% by mass. In particular, the retained austenite is an indispensable additive element necessary to ensure a sufficient amount and stability. On the other hand, the upper limit capable of maintaining the welded portion characteristics was set to 0.3% by mass.

  Si: 0.001% or more to ensure manufacturability and strength on material, and the upper limit is made less than 0.200% because the plating property is deteriorated. Addition exceeding this often causes scale damage, leading to deterioration of the plating appearance and reduction of the yield of the steel sheet. Si, like C, is an element that enhances hardenability and degrades weld properties. On the other hand, since Si has the effect of improving the wettability of the molten metal and improving the toe shape of the weld metal, it is desirable to add 0.030% or more.

Mn: The range of 1.0 to 3.0% by mass can secure TS of 590 MPa or more by setting 1.0% by mass or more, and the upper limit of 3.0% by mass is This is because addition exceeding the above has an adverse effect on elongation. Mn alone enhances hardenability and degrades weld properties, but it lowers hardenability by forming MnO in steel and combining with Al ₂ O ₃ to form ferrite nuclei during welding cooling. It is considered possible.

Al: Al is an especially important element for improving the weldability and the balance between strength and ductility. The reason why Al is in the range of 0.5 to 2.0% by mass is 0.5% by mass or more for the purpose of deoxidation and suppressing the formation of carbides and stabilizing retained austenite because of low Si. did. As will be described later, a part of Al forms Al ₂ O ₃ in the steel sheet, which can reduce the hardenability of the weld and improve weld properties. On the other hand, excessive addition adversely affects plating wettability and manufacturability, so 2.0% was made the upper limit. From the viewpoint of plating adhesion, it is desirable that the content be 1.7% or less.

O: O is also an important element in the present invention. The amount of O was 0.0005 to 0.0100% by mass. By setting the amount of O to 0.0005% or more, the hardenability of the welded portion is lowered and the hardness of the welded portion can be lowered, and since extremely low is also economically disadvantageous, this is set as the lower limit. In particular, in order to improve the welded portion characteristics, it is desirable to set it to 0.0008% or more. On the other hand, the production of steel sheets exceeding 0.0100% has technical difficulties such as excessively generated oxides causing nozzle clogging during refining and casting, and the precipitation amount of Al ₂ O ₃ is large. Since the effect of Al addition is reduced, this is set as the upper limit. From the viewpoint of ease of manufacture in refining and casting, 0.0080% or less is desirable.

Furthermore, the effect of improving the weld properties by adding both Al and O will be described. The retained austenitic steel including the steel of the present invention has a high C content (0.1% or more) in order to secure a stable residual γ phase at room temperature, and a high Mn (1.0% or more) in order to ensure strength. C, Mn, and the like are elements that enhance the hardenability of the steel material, and the welded portions (here, the weld metal and the heat affected zone (HAZ)) are hardened as these elements increase. The structure and hardness of the welded part also depends on the cooling rate after welding heating. When the cooling rate is high, the welded part becomes hard martensite and bainite, and the hardness is high, while when the cooling rate is low, soft ferrite and Perlite is transformed and hardness decreases. As the welded portion is hardened, characteristics such as toughness and fatigue strength of the welded portion deteriorate. By adding Al and O in the above range, Al ₂ O ₃ which is an appropriate amount of Al ₂ O ₃ is formed in the steel has a high melting point, does not dissolve even when welding heat, stably exist. Al ₂ O ₃ hinders the growth of austenite grains in the weld metal during welding cooling, so that the austenite grains before transformation become fine grains. Fine graining causes a decrease in hardenability. Furthermore, since Al ₂ O ₃ becomes a nucleus for generating ferrite in the welded portion, the hardenability of the welded portion is lowered, that is, the welded portion hardness can be lowered. However, it is said that an oxide in which MnO is combined with Al ₂ O ₃ becomes a ferrite-forming nucleus of the weld during cooling of the weld, and Al ₂ O ₃ .MnO may also be formed in the present invention. On the other hand, it is known that the hardenability cannot be lowered with an oxide of Si or an oxide of Si and Mn. From this fact, it is particularly important to add Al instead of Si.

Furthermore, the present inventors have found that the aspect ratio of retained austenite can be reduced to 2 or less over a wide range of production conditions by adding Al or O within the scope of the present invention. The reason for this is presumed that not only Al but also oxides such as Al ₂ O ₃ promoted ferrite transformation and bainite transformation during production, and also had a favorable influence on the shape of retained austenite.

By the way, the amount of O dissolved in the molten steel decreases with the amount of Al up to about 0.5 mass% of Al. This is because the affinity between Al and O is strong during refining, that is, at a high temperature at which the steel material is melted, and Al ₂ O ₃ is formed in the molten steel. The coarsened Al ₂ O ₃ floats on the surface of the molten steel and forms slag with other oxides and is eliminated. Therefore, the amount of O in the steel plate that is the final product also remains at a low level. However, when the Al amount exceeds about 0.5%, it becomes possible to increase the amount of O in which molten steel can be dissolved as the Al amount increases. That is, it is possible to increase the amount of O of the final product. However, it is difficult to increase the amount of O to a level at which the weld characteristics can be improved even if the amount of Al is 0.5% or more in a normal manufacturing method. Conventionally, increasing the amount of O to form an oxide in the base material has been avoided because it deteriorates the ductility and toughness of the base material. However, the present invention is novel in that it has found out conditions that can improve the welded portion characteristics within a range in which ductility and toughness are not deteriorated. In order to secure the amount of O specified in the present invention with the base material, in addition to adding 0.5% or more of Al, it is necessary to devise a manufacturing method such as blowing O into molten steel during refining.

  The reasons for limiting other elements will be described.

The amount of N was 0.001 to 0.010% by mass. When the N content is 0.001% or more, the hardenability of the heat affected zone (HAZ) is lowered during welding due to the formation of AlN in the steel sheet, and the effect of improving the weld zone characteristics appears and the extreme reduction is economical. This is also a disadvantage, so this is the lower limit. On the other hand, if it exceeds 0.010%, the amount of precipitated AlN increases, and the effect of Al addition decreases, so this was made the upper limit. In addition, AlN melts in the molten steel and does not reprecipitate in a general welding cooling process. Therefore, unlike Al ₂ O ₃ , it does not contribute to a decrease in the hardenability of the weld metal. However, since HAZ has a maximum temperature within the region below the melting temperature and a cooling method with a high cooling rate, for example, laser welding or spot welding, HAZ AlN is not completely dissolved. The coarsening can be suppressed and the hardenability of the HAZ can be lowered.

  The amount of P in the range of 0.001 to 0.300% by mass is that the strengthening effect appears at 0.001% by mass or more and the extremely low is economically disadvantageous. This is considered as the lower limit because it contributes to the improvement of corrosion resistance. On the other hand, the upper limit of 0.300% by mass is because addition exceeding this amount adversely affects weldability, manufacturability during casting and hot rolling.

  The reason for setting the amount of S in the range of 0.0001 to 0.1000% by mass is that the extremely low reduction is economically disadvantageous, so 0.0001% by mass is set as the lower limit, and 0.1000% by mass is set. The upper limit is because addition of an amount exceeding this adversely affects weldability, manufacturability during casting and hot rolling.

  Furthermore, the steel targeted by the present invention can contain one or more of Mo, Cr, Ni, Cu, Co, and W for the purpose of further improving the strength.

  Mo: It may be added because corrosion resistance can be improved and strengthened. Since it is low Si, in addition to suppressing pearlite and carbide precipitation adversely affecting the strength ductility balance instead of Si, it is effective for improving the corrosion resistance of the base material, so 0.001% or more was made the lower limit. On the other hand, excessive addition causes a decrease in ductility, so the upper limit was made 1.0%. However, addition of 0.200% or less is desirable in order to secure a high strength ductility balance.

  The Cr content in the range of 0.001 to 25.000% by mass means that a strengthening effect appears at 0.001% by mass or more, and that the upper limit is 25.000% by mass. This is because the processability is adversely affected.

  The reason why the amount of Ni is in the range of 0.001 to 10.000% by mass is that the strengthening effect appears at 0.001% or more, and the upper limit of 10.000% by mass is the addition of an amount exceeding this. This is because it adversely affects workability.

  The amount of Cu in the range of 0.001 to 5.000 mass% is that the strengthening effect and the corrosion resistance improving effect appear at 0.001 mass% or more, and the upper limit of 5.000 mass% is this This is because addition of an excessive amount adversely affects processability and manufacturability.

  The amount of Co in the range of 0.001 to 5.000% by mass is that the strengthening effect appears at 0.001% by mass or more, and the upper limit is 5.000% by mass. This is because the processability is adversely affected.

  The amount of W is in the range of 0.001 to 5.000% by mass because the strengthening effect appears at 0.001% by mass or more, and the upper limit is 5.000% by mass. This is because the processability is adversely affected.

  Furthermore, the steel targeted by the present invention can contain one or more of Nb, Ti, V, Zr, Hf, and Ta, which are strong carbide forming elements, for the purpose of further improving the strength.

  These elements are extremely effective in forming fine carbides, nitrides or carbonitrides and strengthening the steel sheet, so that one or two or more of these elements may be added in a total amount of 0.001% by mass as necessary. The above addition was made. On the other hand, since it inhibits ductility deterioration and concentration of C in retained austenite, the upper limit of the total amount of one or more types is set to 1% by mass.

  B can also be added as needed. B is effective for strengthening grain boundaries and increasing the strength of steel by adding 0.0001% or more, but when the amount of addition exceeds 0.1% by mass, the effect is saturated and more than necessary. In order to increase the strength of the steel sheet and lower the workability, and further improve the hardenability of the welded portion, the upper limit was made 0.1 mass%.

  Y, Rem, Ca, Mg: Added for the purpose of suppressing the formation of Si-based internal grain boundary oxidation phases that degrade the wettability of plating. Grain boundary oxides are not formed like Si-based oxides, but relatively fine oxides can be dispersed and formed. Here, Rem refers to an element having an element number of 21, 39, or 57-71. One or two or more elements from the element group were added in an amount of 0.0001% or more. On the other hand, excessive addition lowers the manufacturability such as castability and hot workability and the ductility of the steel sheet product, so 1.0 mass% was made the upper limit.

  Next, a preferred microstructure for securing and improving the strength / ductility balance of the steel sheet will be described.

  In order to sufficiently ensure the workability, it is desirable that the main structure is a ferrite phase having a volume fraction of 30% or more, preferably 40% or more. An increase in the volume fraction of ferrite increases the ductility but leads to a decrease in strength, so the upper limit is made 90%.

  Considering high strength and stabilization of austenite, it is desirable to contain 5% or more of bainite, and it is desirable to contain 10% or more in order to ensure a high strength / ductility balance. The upper limit is not particularly defined, but if it is excessively contained, the ductility is lowered, so 50% or less is desirable.

  Further, in order to achieve both high strength and high ductility, a composite structure including a retained austenite phase is used. In order to achieve both high strength and high ductility, the retained austenite phase is desirably contained in a volume ratio of 5% or more, preferably 10% or more. The upper limit is set to 30% or less because the carbon concentration in the austenite decreases as the volume ratio increases, and the ductility deteriorates. The carbon concentration in the retained austenite is an index for stabilizing austenite, and if it exceeds 1.0% by mass, it is stable to temperature and processing, and exhibits an excellent balance between strength and ductility. . There is no particular upper limit, but it is determined by the balance between the volume fraction of the austenite phase and the carbon concentration of the other phase.

  Further, martensite may be included for higher strength, but the upper limit is made 10% or less in order to ensure high ductility.

  Next, the average particle size of the phase will be described.

  The shape of the retained austenite grains, which is a very important factor for the retained austenitic steel to exhibit high ductility while having high strength, is described. In order for the steel sheet to achieve high strength and high ductility, it is necessary to control the shape of retained austenite grains. That is, it has been found by intensive investigation that high strength and high ductility can be exhibited when the average aspect ratio (maximum width / minimum width) of austenite grains is 2 or less in the cross-sectional structure of the steel material. On the other hand, when the average aspect ratio exceeds 2, the effect of improving ductility is small, so it is defined as 2 or less. The reason for high ductility when the average aspect ratio is 2 or less is not necessarily clear, but it is considered important that the austenite phase close to a circle is dispersed around the ferrite grains with a certain interval or more. When deformation such as uniaxial tensile test or draw forming is applied to the steel of the present invention, ferrite in the structure bears strain mainly at the initial stage of deformation, and work hardening proceeds, and from the middle stage of deformation, from the retained austenite to martensite. It is presumed that the transformation is increased and hard martensite is produced, whereby the strain is dispersed in the surrounding softer phase, that is, excellent ductility is achieved. However, when the average aspect ratio of retained austenite exceeds 2, that is, when retained austenite exists so as to surround the periphery of ferrite grains, the transformation of austenite to martensite is caused by the transfer of stress between austenite and ferrite from the early stage of deformation. It is considered that the strain dispersion effect from the middle stage of deformation is small, that is, the ductility remains at a low level. Patent Document 6 also discloses a method of manufacturing a steel sheet that focuses on the shape of retained austenite and makes the shape lath and has good ductility and stretch flangeability, but it is difficult to exhibit high elongation. Seem.

  Moreover, in order to ensure the high ductility of a steel plate, the average particle diameter of a ferrite is 20 micrometers or less, and the average particle diameter of the austenite which is a 2nd phase is prescribed | regulated as 10 micrometers or less. As the ferrite grain size increases, the retained austenite average grain size also increases. If the residual austenite average particle size exceeds 10 μm, the processing is remarkably stabilized and the ductility deteriorates, so 10 μm was set as the upper limit. In order to reduce the average retained austenite particle size, the upper limit of the average particle size of ferrite is 20 μm. If it exceeds 20 μm, the residual austenite particle size is in an increasing function of the Feyrat particle size, so that it is difficult to produce a residual austenite average particle size of 10 μm or less.

  In addition to the above, the case of containing one or more of carbides, nitrides, sulfides, and oxides as the remaining structure of the microstructure is also within the category of the steel sheet of the present invention. The seeds or more are preferably 1.0% or less in volume fraction. In addition, based on the results of the above-mentioned microstructure, including ferrite, austenite, bainite, martensite, and remaining structure, observation of existing positions, average particle diameter (average equivalent circle diameter, 20 field observations or more by the following method, according to JIS The average aspect ratio and the occupancy ratio are measured using a Nital reagent, a repeller reagent, and a reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473. It is possible to quantify by observing an optical microscope of 500 to 1000 times by corroding a simple cross section. However, in the case of austenite only, the occupancy ratio is measured by the integrated intensity of ferrite (200) plane and (211) plane and the austenite (200) plane, (220) plane, and (311) plane by MoKα ray. Was measured and calculated. The carbon concentration in austenite is obtained by calculating the lattice constant from the reflection angles of the (111), (200), and (220) surfaces of austenite with Cu Kα rays, and the relational expression between the lattice constant and the carbon concentration in the austenite. (R. C. Ruhl et al., Trans. AIME, page 245, (1969) 241).

Next, the following formula (A) will be described. In order to ensure the strength and ductility of the steel material at a high level, reduce the hardness of the welded portion, and improve the toughness and fatigue strength of the welded portion, it is preferable that the composition satisfy the relationship of the following formula (A): I understood.
0 ≦ 2 (Al + Mn / 2 + 100 × O) × (2.2 × Al—Mn—O) —C—Si / 30 (A)

In order to improve the toughness and fatigue strength of the welded part, the joint performance is sufficient because cracks and cracks easily develop unless the shock absorption energy is 100 J / cm ² or more in the Charpy test of the welded part at 0 ° C. Become. As a result of intensive studies, the inventors tested using a steel type of Example 1 to be described later. As shown in FIG. 1, the inventors aimed at a fusion line centered on the plate thickness under the condition that the HAZ hardness was a test load of 500 gf. It can be seen that when the Vickers hardness is 350 or less, the impact absorption energy can be secured at 100 J / cm ² or more. In order to ensure the HAZ hardness at 350 or less, as shown in FIG. A) It has been found that the equation requires zero or more. This is presumed to be because oxide formation by Al, Mn, and O in the component balance equation (A) and C and Si affect the hardenability, and Al, Mn, and O form oxides and quench the weld. On the other hand, if Mn and O are excessive relative to Al, the effect of ductility improvement due to Al addition is reduced, so C and Si increase the hardenability. A negative term was used, and the coefficient was determined from the influence margin.

  For the test evaluation of the welded HAZ part, a welding current of 150 A and a welding speed of 0.9 m / min were selected so that the steel sheet was melted and solidified with a TIG arc to reproduce the welding, and through welding could be performed in the thickness direction. . Since the HAZ hardness at this time has a good correlation with the actual weld zone toughness, it is used as an index of weld zone characteristics.

A method for producing a high-strength hot-dip galvanized steel sheet having the above components and structure will be described below.
When producing the invention steel by cold rolling and annealing after hot rolling, the slab adjusted to the specified components is cast or re-heated to 1200-1300 ° C after cooling to reduce segregation of Mo, etc. To do. On the other hand, when heating above 1300 ° C., local oxidation may be remarkably accelerated, so this was set as the upper limit of the heating temperature.

After that, a hot reduction of 60% or more is applied by rough hot rolling in a temperature range of 800 to 1200 ° C., finish rolling and pickling, and after cold rolling, annealing is performed including a plating step to obtain a final product. . Although it is desirable that the total rolling reduction is high, it is set to 99% or less due to equipment constraints. Moreover, it is desirable that the finishing temperature of hot rolling is not less than the Ar ₃ transformation temperature determined by the chemical composition of the steel sheet. In this case, recrystallization after hot rolling is promoted and an excellent balance between strength and ductility is obtained.

  Furthermore, the coiling temperature after hot rolling cooling can be reduced by reducing the load during cold rolling by setting the bainite transformation start temperature determined by the chemical composition of the steel material, but when the total rolling reduction of cold rolling is small, the coiling temperature is reduced. There is no particular need to consider. On the other hand, the total rolling reduction ratio of the cold rolling is set based on the relationship between the final sheet thickness and the cold rolling load, but if it is 40% or more, the final steel sheet characteristics are not deteriorated. Although it is desirable that the total rolling reduction of the cold rolling is high, it is set to 80% or less due to equipment constraints.

When annealing after cold rolling, the annealing (maximum) temperature is the temperature Ac ₁ and Ac ₃ determined by the chemical composition of the steel, that is, from 10 seconds in the temperature range of Ac ₁ (° C.) to Ac ₃ +50 (° C.). Annealing for 30 minutes. If it is less than Ac ₁ (° C.), the amount of austenite generated during annealing is not at all or small, so that a sufficient austenite phase cannot be left in the steel sheet finally, so this was made the lower limit of the annealing temperature. Moreover, even if the annealing temperature exceeded Ac ₃ +50 (° C.), the upper limit of the annealing temperature was set to Ac ₃ +50 (° C.) in order to improve the manufacturing cost without improving the properties of the steel sheet. The annealing time at this temperature requires 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the effect is not only saturated but also the cost is increased. It is known that Ac ₁ (° C.) and Ac ₃ (° C.) can be calculated by, for example, the following equations.
Ac ₁ = 723-10.7 × Mn% + 29.1 × Si%,
Ac ₃ = 910-203 × (C%) ^0.5 % + 44.7 × Si% + 31.5 × Mo%
−30 × Mn% −11 × Cr% + 400 × Al%

  By the subsequent primary cooling, the transformation from the austenite phase to the ferrite phase is promoted, and C is concentrated in the untransformed austenite phase, thereby stabilizing the austenite phase. The cooling rate at this time shall be 10 degrees C / s or less. When the cooling rate exceeds 10 ° C./s, ferrite transformation does not occur sufficiently, and it becomes difficult to secure the retained austenite phase in the final steel sheet, or the hard phase such as martensite becomes large, This was the upper limit. Although there is no particular lower limit, setting it to less than 1 ° C./s leads to an increase in the required production line length and causes manufacturing disadvantages such as a significant reduction in production speed.

  When the ultimate temperature in the primary cooling is Tmax (° C.) when the maximum temperature during annealing is set to less than Tmax−300 ° C., pearlite is generated during cooling and the ductility deteriorates. . On the other hand, if the primary cooling is stopped at a temperature higher than Tmax-100 ° C., the ferrite transformation does not proceed sufficiently, so this was made the upper limit.

  The subsequent rapid cooling of the secondary cooling requires a cooling rate of 5 ° C./second or more in order to suppress pearlite transformation and precipitation of iron carbide during cooling. However, since it is difficult to make the cooling rate over 100 ° C./second in terms of equipment capacity, 5-100 ° C./second was set as the range of the secondary cooling rate. Then, it cools and hold | maintains to the temperature of the range of 350-500 degreeC with the cooling rate of 5-100 degreeC / second or more. This holding is essential to stabilize the austenite phase remaining in the steel sheet at room temperature, and it is essential to further increase the carbon concentration in the austenite by transforming a part of the austenite phase into bainite. Even if the cooling stop temperature of the secondary cooling is less than 350 ° C. or more than 500 ° C., the reaction of bainite transformation becomes extremely slow or does not occur at all, so the above temperature range was set. It has been found that maintaining the above temperature range for 5 to 900 seconds, desirably 150 seconds or more, allows the bainite transformation to proceed and exhibits an excellent balance between strength and ductility. If the holding time is less than 5 seconds, the bainite transformation does not progress, and if it exceeds 900 seconds, carbides are generated and the ductility deteriorates.

  Further, the steel of the present invention is immersed in a plating bath after being maintained in the above temperature range, and alloyed. The plating bath temperature is usually in the range of 420 to 480 ° C. If the temperature of the steel plate immediately before being immersed in the plating bath, that is, the holding temperature is lower by 30 ° C. or more than the temperature of the plating bath, the temperature of the plating bath is lowered when entering the plating bath of the steel plate, resulting in a large operational problem. However, the problem of lowering the plating bath temperature can be solved by installing a heating device immediately before the plating bath and heating it before immersion in the plating bath. In addition, the immersion time in the plating bath of a steel plate is usually within 30 seconds, and this immersion time may be included in the above holding time; 5 to 900 s.

  In the subsequent alloying treatment, if the treatment temperature exceeds 550 ° C., carbides are generated, and it becomes difficult to leave a sufficient residual austenite phase, and the strength / ductility balance deteriorates. Therefore, in order to perform sufficient alloying treatment after secondary cooling, the alloying treatment temperature is preferably in a temperature range of 400 ° C to 550 ° C.

The plating adhesion amount is not particularly limited, but is preferably 5 g / m ² or more in terms of one-side adhesion amount from the viewpoint of corrosion resistance. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.

  Hereinafter, the present invention will be described in more detail with reference to examples.

The steel sheets having the compositions shown in Table 1-1 and Table 1-2 were heated to 1200 to 1300 ° C., and thereafter, rolling was performed at 900 ° C. or higher and the total rolling reduction was 60% or more to complete the finish rolling and wound up. The hot rolled steel sheet was pickled and then cold rolled to a thickness of 2.0 mm. Ac ₁ (° C.) or higher Ac ₃ +50 (° C.) after heating and holding in a 10% H ₂ -N ₂ atmosphere, when reaching the maximum temperature during annealing: Tmax (° C.), 10 ° C./s It is cooled to a temperature range of Tmax-200 ° C. to Tmax-100 ° C. at the following cooling rate, and subsequently cooled to a temperature of 350 to 500 ° C. at a cooling rate of 5 ° C./second or more. After being held for 900 seconds, it was immersed in a plating bath, and the plated steel sheet was held at a temperature range of 400 to 550 ° C. for 15 seconds to 20 minutes as an Fe—Zn alloying treatment.

  Furthermore, some steel plates were used, and steel plates having thicknesses and strengths shown in Table 1-1 and Table 1-2 were melted and solidified with a TIG arc to reproduce welding. At this time, the welding current was 150 A, and the welding speed was 0.9 m / min. After producing the joint, the hardness aimed at the fusion line at the center of the plate thickness was defined as the HAZ hardness, and this was compared as an index for estimating the toughness of the weld. The case where the HAZ hardness was Vickers hardness of 350 or less was determined as “OK” (good), and the case where the HAZ hardness was higher than “NG” (defective). Test conditions and results are shown in Tables 2 and 3.

  From Tables 2 and 3, the steel of the present invention is excellent in the weld zone (HAZ) softening effect, corrosion resistance, and strength / ductility balance.

  On the other hand, the comparative examples that do not satisfy the scope of the present invention have high HAZ hardness, low appearance scores, and poor weld strength / elongation balance. Further, the steel sheet produced in the scope of the claims of the present invention has the microstructure described above, and is excellent in appearance, strength and elongation balance.

It is a figure which shows the relationship between impact absorption energy (J / cm < ² >) and HAZ hardness (Vickers hardness Hv). It is a figure which shows the relationship between HAZ hardness (Vickers hardness Hv) and the value of (A) Formula.

Claims (9)

Steel component is mass%,
C: 0.1 to 0.3%
Si: 0.001 to less than 0.200%,
Mn: 1.0 to 3.0%
Al: 0.5 to 2.0%,
P: 0.001 to 0.300%,
S: 0.0001 to 0.1000%,
O: 0.0005 to 0.0100%,
N: 0.001 to 0.010%
A galvanized steel sheet comprising the balance Fe and unavoidable impurities, wherein the microstructure of the steel is 30 to 90% ferrite phase in volume fraction, 5% or more bainite, 10% or less martensite, And 5 to 30% of the retained austenite phase, the carbon in the retained austenite is 1.0% or more by mass%, and the average aspect ratio (maximum width / minimum width) of the retained austenite grains is 2 or less. A high strength and high ductility hot dip galvanized steel sheet having excellent corrosion resistance and welding strength. The microstructure of the steel sheet is composed of ferrite with a volume fraction of 30 to 90% as a main phase, the average particle diameter is 20 μm or less, and the second phase consists of retained austenite with a volume fraction of 5 to 30%, 2. The high-strength and high-ductility hot-dip galvanized steel excellent in corrosion resistance and weld strength according to claim 1, wherein the second phase has an average particle size of 10 μm or less and further comprises bainite having a volume fraction of 5% or more. steel sheet. Steel is more mass%,
Mo: 0.001 to 1.0%,
Cr: 0.001 to 25.000%,
Ni: 0.001 to 10.000%,
Cu: 0.001 to 5.000%,
Co: 0.001 to 5.000%,
W: 0.001 to 5.000%
The high strength and high ductility hot dip galvanized steel sheet excellent in corrosion resistance and weld strength according to claim 1 or 2 , characterized by containing at least one of the following. Steel, further by mass%, Nb, Ti, V, Zr, Hf, according to claim 1 to 3, characterized in that it contains 0.001 to 1.000% in total of one or more of Ta A high-strength, high-ductility hot-dip galvanized steel sheet excellent in corrosion resistance and weld strength according to any one of the items. The steel further contains, in mass%, B: 0.0001 to 0.1000%, high strength and high ductility excellent in corrosion resistance and weld strength according to any one of claims 1 to 4. Hot dip galvanized steel sheet. Steel, with further mass%, Y, Rem, Ca, Mg, according to any one of claims 1 to 5, characterized in that it contains 0.0001 to 1.0000% of one or more of Ce High strength, high ductility hot dip galvanized steel sheet with excellent corrosion resistance and weld strength. In the production of high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 6, claim 1 any one component cast slab casting remain or once cooled consisting of steel sheet according to Section 6 After heating to 1200 to 1300 ° C. again, after adding a reduction of 60 to 99% of the total rolling reduction by rough hot rolling at 800 to 1200 ° C., pickling the hot rolled steel sheet wound by finish rolling, Cold-rolled, and then annealed in a temperature range of Ac ₁ (° C.) or higher and Ac ₃ +50 (° C.) or lower for 10 seconds to 30 minutes, and then the maximum attained temperature during annealing: Tmax (° C.) is 10 ° C. / It is cooled to a temperature range of Tmax−200 ° C. to Tmax−100 ° C. at a cooling rate of s or less, and subsequently cooled to a temperature of 350 to 500 ° C. at a cooling rate of 5 ° C./second or more. After holding for 900 sec, in plating bath Pickles, and high strength and high manufacturing method of ductility galvanized steel sheet having excellent corrosion resistance and weld strength, characterized in that cooling to room temperature. 8. The production of a high strength and high ductility hot dip galvanized steel sheet excellent in corrosion resistance and weld strength according to claim 7 , wherein the alloying treatment is performed in a temperature range of 400 to 550 ° C. after immersion in the plating bath and cooled to room temperature. Method. The method for producing a high strength and high ductility hot dip galvanized steel sheet excellent in corrosion resistance and weld strength according to claim 7 , wherein the cold rolling rate is in the range of 40 to 80%.

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