JP4975406B2 - High-strength galvannealed steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-tensile alloyed hot-dip galvanized steel sheet useful as a member used in automobiles, buildings, electrical equipment, and the like, and a method for producing the same.

  2. Description of the Related Art In recent years, automobiles have been desired to improve fuel efficiency from the viewpoint of impact on the environment, and for this reason, the weight of the vehicle body has been reduced. Further, in order to maintain safety even if the vehicle body is lightened, demands for steel plates (high-tensile steel plates) having higher strength than those conventionally used in various members constituting the vehicle body are increasing. However, when the strength is increased, the ductility and shape freezing properties are liable to decrease. Therefore, a steel plate having high strength and good shape freezing properties and ductility is required.

  In order to improve the balance between strength and ductility, it is known that the addition of Si to steel is very effective. Furthermore, high ductility high-tensile steel sheet (hereinafter referred to as “residual austenite type”) that contains a large amount of ferrite-forming element Si or Al and austenite-forming element Mn and that uses large elongation (TRIP effect) due to strain-induced transformation of retained austenite. Developments such as “high-tensile steel plate” or simply “residual austenitic steel plate” have been made.

  However, this retained austenitic high-strength steel sheet has a high yield ratio (YR) and has a large shape change due to elastic recovery after forming, so it cannot be said that it has sufficient performance from the viewpoint of shape freezing property.

  In order to produce this retained austenitic steel sheet, after the annealing treatment, the holding time in the temperature range of 350 to 600 ° C. (hereinafter, holding in this temperature range is referred to as “low temperature holding”, and the holding time at that time is It is important to lengthen (low temperature holding time) to promote bainite transformation, to concentrate and stabilize C in austenite, and to leave austenite to room temperature.

  At this time, if the content of Si or Al is small, cementite is precipitated during the bainite transformation, and C is not concentrated in the austenite and is not stable. Even when the low temperature holding time is short, the bainite transformation is not sufficient, and austenite is not stabilized. If austenite is not stabilized, a part of austenite undergoes martensitic transformation during cooling, making it difficult to obtain the TRIP effect and reducing ductility.

  Further, development of a steel plate of dual phase steel (hereinafter referred to as “DP steel”) having a composite structure of ferrite and martensite having a low YR even though the strength is the same is also in progress. This steel sheet is superior in shape freezing property to the retained austenitic steel sheet, but has a lower ductility than the retained austenitic steel sheet. In this steel sheet as well, generally, Si is often added to ensure ductility.

  On the other hand, from the viewpoint of improving corrosion resistance and appearance, plating materials have been applied as automobile members, and hot dip galvanized steel sheets are currently used for many members. However, many existing hot-dip galvanizing facilities have a short low-temperature holding line length. For this reason, when the base steel sheet is a retained austenite steel sheet with a high Si content, the low temperature holding time before the hot dip galvanization must be extended after the reduction annealing treatment. Speed is reduced and productivity is significantly reduced.

  Further, if the low temperature holding time is increased, even in a reducing atmosphere, the oxide of Si is concentrated on the surface of the steel sheet, so that the plating wettability and the plating adhesion are deteriorated. From the above, when using steel to which Si is added as the base steel plate, there is a concern about a decrease in productivity and a deterioration in plating adhesion.

  The retained austenitic steel sheet containing a large amount of Si and a method for producing the same are disclosed in, for example, Patent Documents 1 to 5. However, in order to obtain such a steel sheet, as described above, it is necessary to take a long low temperature holding time, not only the productivity is remarkably lowered, but also the shape freezing property, plating wettability and plating adhesion. There is also a problem.

  Patent Document 6 discloses a hot-dip galvanized steel sheet having a low Si and Al content and excellent plating adhesion, and a manufacturing method thereof. The steel sheet structure is bainite and ferrite, or bainite, ferrite, and martensite. . Further, Patent Document 7 discloses a steel plate containing a small amount of Si but containing 0.07 to 0.7% of Al and a method for producing the same, and the steel plate structure is martensite containing ferrite and retained austenite. It is. Such a steel plate is excellent in shape freezing property and plating adhesion, but is not sufficiently ductile.

  Patent Document 8 discloses a retained austenitic steel sheet having a low Si content and a high Al content. As described above, the retained austenitic steel sheet has not only poor shape freezing properties, but also components and production. This is not preferable because the characteristics vary greatly depending on conditions (particularly, low temperature holding time and final cooling conditions).

The problems of the retained austenitic steel and DP steel are summarized as follows.
[Residual austenitic steel]
(A) Since the yield ratio (YR) is high, the shape freezing property is poor.
(B) In manufacturing, it is necessary to make the low temperature holding time long in order to stabilize austenite and leave austenite to room temperature, so that the plating adhesion and productivity are poor. On the other hand, ductility is improved by the TRIP effect of retained austenite. On the other hand, when the low temperature holding time is set to a short time, a part of martensite is mixed, and the TRIP effect is hardly exhibited and the ductility is lowered.
[DP steel]
(A) The yield ratio is low and the shape freezing property is excellent, but the ductility is inferior to the retained austenitic steel.

  As mentioned above, high-tensile hot-dip galvanized steel sheets with excellent shape freezing properties and plating adhesion and excellent ductility are practical even if the amount of Si added is small and the low-temperature holding time is short under manufacturing conditions. The current situation is that they have not been realized, and in order to promote the application of high-tensile steel sheets, there is a need to solve these problems.

Japanese Patent Laid-Open No. 05-70886 Japanese Patent Laid-Open No. 06-145788 Japanese Patent Laid-Open No. 11-131145 Japanese Patent Laid-Open No. 2001-140022 JP 2001-303229 A JP 05-125485 A JP 2000-345288 A JP 05-247586 A

  The present invention has been made in view of such circumstances, and the object thereof is a high-tensile alloyed hot-dip galvanized steel sheet excellent in shape freezing property, plating adhesion and ductility and having a tensile strength of 450 MPa or more, and production thereof. It is to provide a method.

In order to solve the above-mentioned problems, the present inventors have investigated in detail the influence of the components and annealing conditions on the material of the steel sheet, and as a result, have obtained the following knowledge.
(A) The base steel sheet is less likely to exhibit the TRIP effect even when residual austenite is present when martensite is generated after a low temperature retention time after annealing.

  However, bainite transformation is performed during low-temperature short-time holding, and furthermore, the stability of austenite is adjusted by balancing the concentration of C in austenite with Si and Al and the Mn content, thereby suppressing austenite residue. By promoting the formation of martensite containing a large amount of C (hereinafter referred to as “high C martensite”), it is possible to ensure better ductility than ferrite + martensitic steel (DP steel).

Moreover, since the yield ratio (YR) can be kept low, the shape freezing property is also good. The ideal metal structure is ferrite + bainite + high C martensite. At this time, it is preferable that the retained austenite is small.
(B) For the production of high C martensite, the low temperature holding temperature and the low temperature holding time, and the cooling rate after the alloying treatment are important.
(C) Al is an element that is easily oxidized like Si, but Al raises the Ac ₃ transformation point more than Si, so that C enrichment to austenite tends to proceed even at the same annealing temperature and annealing time. Therefore, the low-temperature holding time after the annealing treatment can be shortened and the generation of oxide can be suppressed, so that a decrease in plating adhesion can be avoided. When Si is added, it must be kept at a low temperature for a long time, resulting in deterioration of plating adhesion due to oxide formation.
(D) In order to satisfy the base material property of being excellent in shape freezing property and ductility and the plating adhesion simultaneously, it is preferable to positively add Al.
(E) When Ni is adhered to the steel sheet surface as a pretreatment, the plating adhesion is improved.

The present invention has been completed based on the above findings, and the gist thereof is the following (1) high-tensile galvannealed steel sheet and (2) a method for producing the steel sheet.
(1) By mass%, C: 0.10 to 0.30%, Si: 0.2% or less, Mn: 1.0 to 3.0% and Al: 0.5 to 2.0% In addition, Si, Al and Mn satisfy the following formula (1), the balance is made of Fe and impurities, P in the impurities is 0.1% or less, S is 0.1% or less, and N is 0.020. % High-strength alloy comprising a zinc alloy plating layer containing Fe: 7 to 15% by mass on a steel plate (base material steel plate) containing 3 to 50% martensite by volume% Hot-dip galvanized steel sheet.

  Si (%), Al (%), and Mn (%) in the formula (1) represent the contents (mass%) of Si, Al, and Mn in the base steel sheet, respectively.

2 ≦ Si (%) + Al (%) + Mn (%) ≦ 4 (1)
The base steel sheet described in (1) is further, in mass%, Ni: less than 2.0%, Co: less than 2.0% and Cu: less than 1.0% (these are “group 1 components”). 1) or more, Ti: less than 0.1%, Nb: less than 0.1% and V: less than 0.2% (these are referred to as “second group components”) Any one or more of Mo: less than 1.0%, Cr: less than 1.0% and B: less than 0.01% (these are referred to as “group 3 components”) An element belonging to any one or more of the above three groups may be included.

In particular, the steel sheet according to (1) further includes Fe: 7 to 15% in mass% on the base steel sheet containing any one or more of the components of the third group. It can be set as the high tension alloying hot-dip galvanized steel plate provided with a zinc alloy plating layer.
(2) The method for producing a high-tensile galvannealed steel sheet according to (1), wherein the steel sheet having the chemical composition according to (1) is sequentially subjected to the following treatments (A) to (F).
(A) Treatment for annealing for 30 to 600 seconds in a two-phase coexistence temperature range of 700 to 900 ° C. (B) Treatment for cooling to a temperature range of 350 to 550 ° C. at a cooling rate of 3 to 200 ° C./second (C) The temperature Treatment for 10 to 90 seconds in region (D) Treatment for immersion in hot dip galvanizing bath (E) Treatment for 5 to 180 seconds in temperature range of 470 to 600 ° C. (F) Cooling rate of 4 ° C./second or more Treatment for cooling to 250 ° C. or lower If the treatment for adhering Ni to the steel sheet surface is performed before the treatment (A), the plating adhesion is improved.

The high-tensile alloyed hot-dip galvanized steel sheet of the present invention is a steel sheet having excellent shape freezing property, plating adhesion and ductility and high strength, and is suitable as a member used in automobiles, architecture, electrical equipment and the like. This alloyed hot-dip galvanized steel sheet can be easily produced by the method of the present invention.

  Hereinafter, the high-tensile galvannealed steel sheet of the present invention (the invention of (1) above) and the manufacturing method thereof (the invention of (2) above) will be described in detail. Note that “%” of the chemical component content of the base steel sheet, “%” of the Fe content in the plating film, and “%” of the Al concentration in the plating bath all mean “mass%”.

  In the high-tensile galvannealed steel sheet according to the present invention, the chemical composition of the base steel sheet is defined as described above for the following reason.

C: 0.10 to 0.30%
In the base steel sheet of the high-tensile galvannealed steel sheet of the present invention, ductility is enhanced by generating martensite containing a large amount of C, and the balance between strength and ductility is improved as compared with DP steel. Therefore, C is an essential element. The content of C may be appropriately determined according to the target steel plate strength. To achieve the tensile strength of 450 MPa or more targeted by the present invention and to improve the ductility as compared with DP steel, the content of C is 0. It is necessary to contain 10% or more. The upper limit of the C content is 0.30% in order to ensure good spot weldability.

Si: 0.2% or less Si does not dissolve in cementite and suppresses precipitation of cementite. As described above, it is an important element for adjusting the stability of austenite by accelerating bainite transformation in which cementite is difficult to form during holding at a low temperature and concentrating C in austenite. However, if the Si content is increased, the low temperature must be maintained for a long time, and the plating adhesion is lowered. Therefore, the content is set to 0.2% or less. The lower limit of the Si content is not defined by Si alone, but is defined by the total content including Al and Mn, as will be described later.

Mn: 1.0-3.0%
Mn not only increases the strength of the steel sheet, but is an austenite-generating element and is an important element that directly affects the stability of austenite. It also has the effect of suppressing the formation of pearlite during cooling from high temperatures. In order to obtain these effects, it is necessary to contain at least 1.0%, and within that range, the Mn content may be appropriately adjusted according to the tensile strength of the target base steel sheet. The upper limit of the Mn content is 3.0% from the viewpoint of cost and melting in a converter.

Al: 0.5 to 2.0%
Al is also used as a deoxidizing material, and at the same time, like Si, promotes bainite transformation in which cementite is difficult to form during low temperature holding, and concentrates C in austenite to adjust the stability of austenite. It is an important element. Since the low-temperature holding time can be shortened, the adhesion of the plating can be improved as compared with the case of containing Si, and therefore Al is actively used in the present invention.

  In order to finally obtain high-C martensite, it is necessary to contain 0.5% or more of Al. However, excessive addition deteriorates the adhesion and weldability of the plating, so the upper limit is made 2.0%.

2 ≦ Si (%) + Al (%) + Mn (%) ≦ 4 (formula (1) above)
By balancing the concentration of C in austenite with Si and Al and the Mn content, the stability of austenite can be adjusted, the austenite residue can be suppressed, and the formation of high C martensite can be promoted. .

  When the value of Si (%) + Al (%) + Mn (%) is less than 2, the stability of austenite becomes low, and pearlite is generated from austenite during cooling from a high temperature during annealing and during holding at a low temperature. Or, austenite decomposes into ferrite and cementite, and a desired structure cannot be obtained.

  On the other hand, when the value of Si (%) + Al (%) + Mn (%) exceeds 4, the stability of austenite becomes too high, austenite remains, the amount of martensite decreases, and the desired amount (volume) 3% to 50%) cannot be obtained, so that not only ductility is lowered, but also YR (yield ratio) is increased and shape freezing property is deteriorated. Therefore, it is necessary to adjust the contents of Si, Al, and Mn so as to satisfy the relationship of 2 ≦ Si (%) + Al (%) + Mn (%) ≦ 4. Desirably, adjustment is made so as to satisfy the relationship of 2.5 ≦ Si (%) + Al (%) + Mn (%) ≦ 3.5.

  The base steel sheet of the high-tensile galvannealed steel sheet of the present invention is composed of Fe and impurities in the balance other than the components described above. As impurities, it is necessary to suppress the upper limit of P, S and N.

P: 0.1% or less P is an element inevitably contained in steel as an impurity, and is preferably as low as possible. In particular, if the content exceeds 0.1%, ductility deterioration of the steel sheet becomes remarkable, so the P content is set to 0.1% or less.

S: 0.1% or less S is an element unavoidably contained in the steel as an impurity, and the lower one is desirable. In particular, if it exceeds 0.1%, precipitation of MnS becomes prominent, and not only the ductility of the steel sheet is inhibited, but also Mn added as an austenite stabilizing element is consumed as MnS. The S content is 0.1% or less.

N: 0.020% or less N is an element that is unavoidably contained in the rope as an impurity, and its content is preferably low. If the N content exceeds 0.020%, the amount of Al consumed as AlN is large, not only the effect of Al is diminished, but also the ductility deterioration due to AlN becomes significant, so the upper limit of the N content Is 0.020%.

  The base steel plate of the high-tensile alloyed hot-dip galvanized steel plate (1) further includes the components of the first group (Ni: less than 2.0%, Co: less than 2.0% and Cu: 1.0% Any one or more of the components of the second group (Ti: less than 0.1%, Nb: less than 0.1% and V: less than 0.2%). And any one of the three groups of the third group (Mo: less than 1.0%, Cr: less than 1.0% and B: less than 0.01%) It may contain an element belonging to one or more groups. Appropriate ranges for the effects and contents of these components are as follows.

Ni: Less than 2.0%, Co: Less than 2.0% and Cu: Less than 1.0% Ni, Co and Cu are austenite-generating elements and, at the same time, improve the strength of the steel sheet, like Mn. It is an element. Moreover, since all are less oxidized than Fe, it has the effect of concentrating on the steel plate surface and preventing the deterioration of the plating adhesion due to oxidation of Si or Al, so it may be added as necessary.

  When these are added, excessive addition leads to an increase in cost, so the contents of Ni and Co are each less than 2.0%. Cu is made less than 1.0% from the viewpoint of preventing hot cracking. The lower limit of the content is not particularly defined, but in order to obtain a remarkable effect due to the addition, it is desirable to contain at least 0.1% of Ni, Co and Cu.

Ti: less than 0.1%, Nb: less than 0.1% and V: less than 0.2% Ti, Nb, and V not only improve the strength of the steel sheet, but also when galvanizing alloying treatment is performed Is an effective element for improving the alloying speed, and may be added as necessary. When added, excessive addition not only causes deterioration of ductility, but also increases the yield ratio (YR) and degrades the shape freezeability, so the content of Ti and Nb is 0.1% each. Less than. About V, since the effect of addition is small compared with Ti and Nb, the content is made less than 0.2%. The lower limit of the content is not particularly defined, but in order to obtain a remarkable effect by addition, it is desirable to contain Ti and Nb in an amount of 0.005% or more and V in an amount of 0.01% or more, respectively.

Mo: less than 1.0%, Cr: less than 1.0% and B: less than 0.01% Mo, Cr and B suppress the formation of pearlite during cooling from a high temperature during the annealing treatment. It is an effective element that promotes formation, and may be added as necessary. In the case of adding Mo and Cr, 1.0% or more is added, and if B is added 0.01% or more, the effect is not only saturated but also the cost is increased. And Cr are each less than 1.0%, and B is less than 0.01%.

  Although the lower limit of these contents is not particularly defined, in order to obtain a remarkable effect by addition, Mo and Cr are each contained in an amount of 0.1% or more, and B is contained in an amount of 0.0005% or more. desirable.

  The base steel sheet of the high-tensile galvannealed steel sheet of the present invention further contains 3 to 50% martensite by volume%. In order to satisfy both shape freezing property and ductility at the same time, it is important that high C martensite is present in the base material, and for that purpose, it is necessary to contain 3% or more of martensite by volume%. It is. However, if the content exceeds 50%, the strength becomes too high, ductility deteriorates, and high C martensite decreases. A desirable content of martensite is 5 to 35%.

  The martensite content (abundance) mentioned here is measured by volume etching by observation with an optical microscope by performing color etching with a 4% picric acid ethyl alcohol solution and a 1% aqueous sodium pyrosulfite aqueous solution. It is the value calculated | required by this.

  The high-tensile galvannealed steel sheet of the present invention is a steel sheet provided with a zinc alloy plating layer containing 7 to 15% Fe on the above steel sheet. The content of Fe in the plating layer is regulated within the above range. When the Fe content exceeds 15%, it becomes difficult to ensure plating adhesion and workability, and when the content is less than 7%, it is good. This is because it is impossible to ensure a good spot weldability.

  Next, a method for producing the high-tensile galvannealed steel sheet of the present invention (the invention of (2) above) will be described.

  First, as a base material steel plate, a cold-rolled steel plate (cold rolled steel plate) having the above chemical composition is used. Hot rolling and cold rolling for obtaining this steel plate may be performed by a known method, but if the particle size of the base material becomes too large or too small, desired properties as the base material steel plate cannot be obtained. Therefore, it is desirable that the coiling temperature in the hot rolling process is 700 ° C. or lower and the cold rolling rate is in the range of 40 to 80%.

  In order to make the surface of the base steel plate suitable for hot dipping, a pretreatment is performed by a known method such as washing with an alkaline aqueous solution or surface grinding with a nylon brush or the like.

If a treatment for adhering Ni to the steel sheet surface is performed at the time of this pretreatment, the plating adhesion is remarkably improved. In this case, the adhesion amount of Ni to the steel sheet surface is desirably 1 g / m ² or less from the viewpoint of economy.

  Subsequently, the base steel sheet is heated to a temperature range in which two phases (ferrite phase + austenite phase) coexist at 700 to 900 ° C. and annealed for 30 to 600 seconds (the treatment (A) above). This annealing treatment is usually performed in a reducing atmosphere.

  As the reducing atmosphere, a gas atmosphere containing 5 to 30% by volume of hydrogen, the balance being nitrogen, and a dew point in the range of −60 to 0 ° C. is preferable. In addition, in order to improve a material characteristic, you may perform a pre-annealing by a continuous annealing line or batch type annealing after cold rolling.

  In the annealing treatment, if the annealing temperature is less than 700 ° C. or the annealing time is less than 30 seconds, recrystallization hardly occurs and cementite does not dissolve, so the properties of the steel sheet deteriorate.

  On the other hand, when the annealing temperature exceeds 900 ° C., not only the crystal grains become coarse, but also the volume fraction of austenite during annealing increases, and not only the C content in the martensite to be finally produced decreases. In addition, an increase in manufacturing cost due to an increase in furnace temperature is inevitable. In addition, if the annealing time exceeds 600 seconds, the crystal grains become coarse, the line speed decreases, and the productivity decreases, which is not preferable.

  The base steel sheet after the annealing treatment is cooled at a cooling rate of 3 to 200 ° C./second to a temperature range of 350 to 550 ° C. (that is, a low temperature holding temperature range) near the plating bath temperature (the treatment of (B) above). , Hold for 10 to 90 seconds (low temperature hold) in that temperature range (process (C) above).

  When the cooling rate after the annealing treatment is lower than 3 ° C./second, pearlite or cementite is generated from austenite during cooling, and a desired metal structure cannot be obtained. When the cooling rate is faster than 200 ° C./second, it becomes difficult to control the cooling rate, and a uniform structure cannot be obtained.

  When the low temperature holding temperature is less than 350 ° C., low C martensite is generated during cooling after annealing, and when the low temperature holding temperature is higher than 550 ° C., bainite transformation does not occur and austenite is transformed into pearlite. Desired material properties cannot be obtained.

  When the low temperature holding time is less than 10 seconds, the bainite transformation does not occur and the concentration of C into austenite does not proceed, so that it becomes low C martensite and ductility decreases. When the low temperature holding time exceeds 90 seconds, as described above, not only the productivity is lowered, but also the plating adhesion due to the generation of oxide is deteriorated, and the high structure which is an ideal structure of the present invention. The formation of C martensite is suppressed, leading to a reduction in ductility and poor shape freezeability.

  The base steel plate after the predetermined low-temperature holding is performed is immersed in a hot dip galvanizing bath to form a galvanized layer on the surface of the steel plate (processing (D) above).

  The plating bath temperature is set to 430 ° C. or more for easy adjustment of the plating adhesion amount, and 550 ° C. or less for avoiding evaporation of Zn and facilitating maintenance of the plating bath. The adjustment of the plating adhesion amount after lifting from the plating bath may be performed by a commonly used method such as a gas drawing method.

  Then, after performing the alloying process hold | maintained for 5 to 180 second in the temperature range of 470-600 degreeC (process of said (E)), it cools to the temperature of 250 degrees C or less with the cooling rate of 4 degrees C / second or more ( Process (F) above).

  When the alloying treatment temperature is less than 470 ° C., alloying does not occur. When the alloying treatment temperature exceeds 600 ° C., austenite is decomposed into cementite and ferrite and the characteristics are deteriorated.

  If the alloying treatment time is less than 5 seconds, alloying does not occur and plating adhesion deteriorates. On the other hand, if the alloying treatment time exceeds 180 seconds, not only austenite decomposes into cementite and ferrite and the characteristics deteriorate, but also the line speed decreases as in the case where the low temperature holding time exceeds the specified range. , Productivity decreases.

  When the cooling rate after alloying is less than 4 ° C./second, martensite is hardly generated during cooling, and bainite or ferrite and cementite are generated. Therefore, the cooling rate after the alloying treatment is set to 4 ° C./second or more. This cooling is performed using nitrogen and industrial gas (air). Furthermore, normal mist cooling may be performed.

  The high-tensile alloyed hot-dip galvanized steel sheet of the present invention has high strength, good shape freezing and ductility, and excellent plating adhesion, so that it is a material for home appliances, building materials, automobile bodies, etc. It is suitable as. In particular, when the steel sheet is coated and used in the construction field, the high-tensile galvannealed steel sheet of the present invention can exhibit excellent performance and economy.

  The steel types shown in Table 1 were melted in a vacuum melting furnace, and a forged ingot (thickness: 20 mm) was hot-rolled to a plate thickness of 4.0 mm at a finishing temperature of 850 ° C. Then, it hold | maintained at 650 degreeC for 30 minutes, and furnace-cooled to room temperature with the cooling rate of 20 degreeC / h. The hot-rolled steel sheet was pickled and cold-rolled to obtain a cold-rolled steel sheet for hot dip galvanizing with a thickness of 1.2 mm.

This cold-rolled steel sheet is used as a base steel sheet, and this is hot-dip galvanized using a vertical hot-dip plating simulator so that the coating amount is 60 mg / m ² , and then alloyed using a salt bath. And various alloyed hot-dip galvanized steel sheets were prepared.

  Table 2 shows the conditions in each step of hot dip galvanizing. In Table 2, the cooling rate (that is, the cooling rate after the reduction annealing treatment) in the “reduction annealing treatment” column is “cooling rate CR1”, and the cooling rate in the “alloying treatment” column is “cooling rate CR2”. I write.

  About these alloyed hot-dip galvanized steel sheets, the volume fraction of martensite is obtained, tensile tests are performed, tensile strength (TS) and elongation (El) are measured, and shape freezeability and plating adhesion are investigated. did. The volume ratio of martensite was determined by the method described above.

In the investigation of the shape freezing property, a hat mold test was performed under the following test conditions, and the spring back amount (angle) of the punch shoulder was measured.
〔Test conditions〕
Sample size: width 50mm x length 250mm
Punch shoulder radius: 10mm
Die shoulder radius: 10mm
Punch width: 50mm
Molding height: 60mm
Molding speed: 60 mm / min
The plating adhesion was examined by performing a tape peeling test on the outside of the die shoulder portion of the sample after the above-described shape freezing investigation, observing the peeled tape with a magnifying glass, and evaluating the following criteria.
◎ mark: Plating peeling is not observed, very good ○ mark: Slight plating peeling is observed, but there is no problem in plating adhesion, good X mark: a large amount of peeling is observed, and the defect investigation results are shown in Table 3 . In addition, changed conditions (reduction annealing temperature and cooling rate CR1, low temperature holding temperature and time, cooling rate CR2 after alloying treatment) among the hot dip galvanizing conditions shown in Table 2 are also shown in Table 3. It was.

  As shown in Table 3, the galvannealed steel sheet of the present invention has high strength, good ductility, and “TS (tensile strength) × El (elongation)” as an index of strength / ductility balance. It shows a high value. The alloyed hot-dip galvanized steel sheet of the present invention has good shape freezing properties and excellent plating adhesion. Moreover, No. of the example of this invention which made Ni adhere to the steel plate surface by plating pretreatment. In 4 and 14, an improvement in plating adhesion was observed.

  On the other hand, No. of the comparative example which made Si content of base-material steel plate 0.63% and 1.00% exceeding the prescription | regulation (0.2% or less) of this invention. In Nos. 22 and 23, the tensile properties were excellent, but the plating adhesion was poor because the oxide of Si was concentrated on the steel sheet surface.

  No. of the comparative example which made annealing temperature 650 degreeC lower than prescription | regulation (700-900 degreeC) of this invention. In No. 9, recrystallization and transformation did not occur, the tensile properties were deteriorated, and the shape freezing property was also poor.

  In Comparative Example No. 1, the cooling rate CR1 after the annealing treatment was set to 1 ° C./s, which was lower than the regulation (3-200 ° C./s) of the present invention. No. 6 and Comparative Example No. 1 in which the low-temperature holding temperature was set to 650 ° C. higher than the provision of the present invention (350-550 ° C.). In No. 2, pearlite transformation occurred during cooling or holding at low temperature, and tensile properties deteriorated.

  In Comparative Example No. 1 in which the low-temperature holding time was 1 second shorter than the regulation of the present invention (10 to 90 seconds). In No. 12, bainite transformation hardly occurred during holding at low temperature, and most of the austenite caused martensitic transformation, and the tensile properties deteriorated.

  In the comparative example, the cooling rate CR2 after the alloying treatment was set to 1 ° C./s lower than the regulation of the present invention (4 ° C./s or more). In No. 15, austenite decomposed into ferrite and cementite during cooling, martensite was not generated, and tensile properties deteriorated.

The high-tensile alloyed hot-dip galvanized steel sheet of the present invention is a steel sheet having excellent shape freezing property, plating adhesion and ductility, and a high strength with a tensile strength of 450 MPa or more, and is used for automobiles, architecture, electrical equipment, etc. It is suitable as a member to be used. This alloyed hot-dip galvanized steel sheet can be easily produced by the method of the present invention. Thereby, it can utilize widely as an optimal plated steel plate for automobiles etc., and its manufacturing method.

Claims (4)

Containing, by mass%, C: 0.10 to 0.30%, Si: 0.2% or less, Mn: 1.0 to 3.0% and Al: 0.5 to 2.0%, and Si, Al, and Mn satisfy the following formula (1), the balance is Fe and impurities, P in the impurities is 0.1% or less, S is 0.1% or less, and N is 0.020% or less. Further, a high-strength alloyed molten zinc comprising a zinc alloy plating layer containing Fe: 7 to 15% by mass% on a steel sheet containing 3 to 50% martensite by volume% Plated steel sheet.
2 ≦ Si (%) + Al (%) + Mn (%) ≦ 4 (1) In addition to the component according to claim 1, the composition further contains at least one of Mo: less than 1.0%, Cr: less than 1.0%, and B: less than 0.01% by mass%. In addition, Si, Al and Mn satisfy the following formula (1), the balance is composed of Fe and impurities, P in the impurities is 0.1% or less, S is 0.1% or less, and N is 0.00. A high tension characterized by comprising a zinc alloy plating layer containing Fe: 7-15% in mass% on a steel sheet containing -20% or less and further containing 5-50% martensite in volume%. Alloyed hot-dip galvanized steel sheet.
2 ≦ Si (%) + Al (%) + Mn (%) ≦ 4 (1) The steel sheet having the chemical composition according to claim 1 or 2 is subjected to the following treatments (A) to (F) in sequence: The high-tensile galvannealed steel sheet according to claim 1 or 2 Production method.
(A) Treatment for annealing in a two-phase coexistence temperature range of 700 to 900 ° C. for 30 to 600 seconds (B) Treatment for cooling to a temperature range of 350 to 550 ° C. at a cooling rate of 3 to 200 ° C./second (C) The temperature Treatment for 10 to 90 seconds in region (D) Treatment for immersion in hot dip galvanizing bath (E) Treatment for 5 to 180 seconds in temperature range of 470 to 600 ° C. (F) Cooling rate of 4 ° C./second or more Cooling to 250 ° C or lower   The method for producing a high-tensile galvannealed steel sheet according to claim 3, wherein Ni is adhered to the surface of the steel sheet before the treatment (A).