JP4211520B2 - High strength and high ductility galvanized steel sheet with excellent aging resistance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength, high-ductility galvanized steel sheet excellent in aging resistance and applicable to a car interior / exterior panel and the manufacturing method thereof.

  In recent years, with the background of environmental problems and the like, weight reduction of automobiles by applying high-strength steel sheets has been promoted. This movement has spread not only to the undercarriage parts of automobiles but also to the inner and outer plate panels. For example, some types of vehicles using 390MPa and 440MPa class high-strength steel plates for the side panels are beginning to appear.

  The inner and outer panel materials for automobiles are required to have excellent press formability, dent resistance, surface strain resistance, secondary work brittleness resistance, surface properties, aging resistance, and the like. As described above, it is necessary to satisfy a very high level of required performance, and it is the most difficult steel plate to be developed on the material supply side.

  Automakers are now in the midst of global expansion while merging and collaborating. Therefore, it is considered that the need to supply panel application materials to various parts of the world will increase. In that case, it is expected that aging will deteriorate due to exposure to high temperatures during transportation by sea. Therefore, it is extremely important for the panel application material to have excellent aging properties.

  Conventionally, IF (Interstitial-Free) steel has been mainly used as an automotive inner and outer panel material. This is because IF steel has a high r value, exhibits excellent deep drawability, and does not have a solid solution element (solid solution C, N), and therefore has non-aging properties. However, since it does not have bake-hardening properties, dent resistance, which is one of the most important characteristics for panel materials, is insufficient.

  Regarding the strengthening of IF steel, for example, in JP-A-7-216452, Japanese Patent Application Laid-Open No. 7-216452, an ultra-low carbon steel compounded with Ti, Nb, and B is added to a solid solution strengthening element such as Si, Mn, or P A high-strength cold-rolled steel sheet for processing reinforced by the above has been proposed. In this case, a high r value is obtained while securing a tensile strength of 45 to 50 kgf / mm 2 by optimizing the Mn addition amount with respect to the Si and P addition amounts.

  On the other hand, bake-hardening type high-strength steel sheets have been developed to provide excellent dent resistance while maintaining the deep drawability of IF steel. This is based on the idea that the dent resistance is improved by the solid solution strengthening element and the bake hardenability is provided by leaving the solid solution C and N in the steel.

With regard to increasing the strength of such a bake hardened steel sheet, for example, JP-A-5-78784 proposes a Ti-containing ultra-low carbon high strength cold-rolled steel sheet having high paint bake hardenability (BH property). Has been. This is because a good r value and elongation can be obtained at a tensile strength of 35 to 50 kgf / mm ² by positively adding Mn and Cr and controlling the amounts of Si and P.

In addition, as an example of a composite structure type galvanized steel sheet, Japanese Patent Application Laid-Open No. 2002-30347 proposes a method for producing a high-strength hot-dip galvanized (plated) steel sheet capable of imparting excellent ductility. Has been. This steel sheet is a low-carbon steel that contains Si, Mn, and P, and starts rapid cooling at an average cooling rate of 100 ° C / sec or higher within 2.5 seconds after hot rolling until the temperature reaches 700 ° C or lower and over 500 ° C. It is characterized by winding after cooling and then pickling or further cold rolling to perform continuous hot dip galvanizing and annealing at 720 ° C or higher.
Japanese Unexamined Patent Publication No. 7-216452 JP-A-5-78784 JP 2002-30347 A

  However, the above prior art has the following problems. For example, the technique described in Patent Document 1 only increases the strength with a solid solution strengthening element based on IF steel, and the quality of plating cannot be expected. In this case, the amount of Mn is suppressed to improve the r value, so a large amount of Si and P must be added to ensure strength, which has a significant adverse effect on surface properties, plating quality, and resistance to secondary work brittleness. Effect.

  Furthermore, in this case, in addition to the surface properties, there is a problem of insufficient dent resistance (no BH property) due to IF steel, and it is impossible to put it into practical use as a galvanized steel sheet for automotive inner and outer panel panels. It is. In addition, simply aiming to increase the r value is rather disadvantageous from the viewpoint of yield strength in a biaxial region as an automotive inner / outer panel material mainly for stretch forming.

  Although the technique described in Patent Document 2 has bake hardenability, after all, only the strength is increased by a solid solution strengthening element based on a Ti-added ultra-low carbon steel. Dent resistance is easily imparted because of the bake hardened steel sheet, but aging resistance must be sacrificed in order to obtain high dent resistance by the principle of bake hardenability. As long as such a material design is used, it is impossible to achieve both dent resistance and aging resistance.

  Furthermore, also here, the surface properties and plating quality are inevitably deteriorated due to the solid solution strengthening element, and in recent years, it can no longer be applied as an automotive inner / outer panel material with particularly strict surface requirements. Further, as described above, simply increasing the r value is not suitable as an automotive inner / outer panel material mainly for stretch forming that requires dent resistance.

  As disclosed in the examples, the technique described in Patent Document 3 is basically a low-carbon steel with a high C (Table 1: 0.05 to 0.18% C) and a high-strength steel. This is completely different from the low-strength DP steel for automobile inner and outer panel intended by the present invention. Further, the hot dip galvanizing is a heat treatment condition equivalent to a normal continuous hot dip galvanizing line, and is not described except for the soaking temperature, and is not touched particularly from the viewpoint of structure control.

  An object of the present invention is to solve the above-mentioned problems and to provide a high-strength, high-ductility galvanized steel sheet excellent in aging resistance and applicable to a vehicle inner / outer panel and the like and a method for producing the same.

In order to solve the above problems, the present invention has a chemical component of mass%, C: 0.005% or more and less than 0.04%, Si: 0.5% or less, Mn: 0.5-3.0% , P: 0.08% or less, S: 0.03% or less, Al: 0.01 to 0.10%, N: 0.01% or less, and the balance is composed of iron and inevitable impurities, and the structure is an average grain It is composed of a ferrite having a diameter of 4 to 15 μm and a second phase having a volume ratio of less than 10%. 80% or more of the second phase is precipitated at the grain boundary, and further 80% or more of the second phase precipitated at the grain boundary. Is a martensite particle having an average particle size of less than 3 μm, and the martensite particles distributed in the grain boundary satisfy that the average interval L between two adjacent particles satisfies the following inequality with respect to the average particle size d of the ferrite. Provided is a high strength and high ductility galvanized steel sheet having excellent aging resistance.
0.79d <L <3.1d (1)

In the galvanized steel sheet of the present invention, Cr: 1% or less, Mo: 1% or less, V: 0.5% or less, B: 0.0002 to 0.003%, Ti: 0.1% as chemical components Hereinafter, one or more of Nb: 0.1% or less may be contained. Moreover, it can also have an organic film further on the plating surface.

  In order to manufacture a galvanized steel sheet having both high strength and high ductility and excellent aging resistance, which is impossible with the prior art, the present invention is applied to an automotive interior / exterior panel from a completely new viewpoint different from the conventional one. It was made as a result of working on the development of materials and earnestly examining it. In particular, by conducting elaborate experiments focusing on the microstructure at the electron microscope level and conducting repeated studies, it was determined that the composite structure was mainly composed of ferrite and martensite and the distribution status was important. It was.

  That is, as described above, the volume ratio and particle size of ferrite and martensite, the martensite formation site, the distribution form, and the distribution interval are extremely important. By controlling these within a predetermined range, It was clarified that high strength and high ductility galvanized steel sheets with excellent aging properties were obtained.

  Furthermore, in order to produce galvanized steel sheets with excellent characteristics suitable for such automotive interior and exterior panels, it is essential to combine chemical composition balance with cooling conditions after hot rolling and to precisely control annealing conditions. I found. Based on these findings, we have clarified for the first time optimum values of chemical components and production conditions to achieve the above-mentioned microstructure. Hereinafter, individual configurations of the invention will be described.

C: 0.005% or more and less than 0.04%
C is an element necessary for ensuring the tensile strength of steel, and it is necessary to contain a certain amount. However, if the C content is 0.04% or more, the formability is lowered, which is not suitable as a steel sheet for automobiles to which the present invention is mainly applied, and is not preferable from the viewpoint of weldability. On the other hand, in order to form a martensite phase having a constant volume ratio, it is necessary to be at least 0.005% or more, depending on the content of other elements. Therefore, the C content is within the range of 0.005% or more and less than 0.04%.

  From the viewpoint of formability, it is more desirable that the C content be less than 0.035%. In order to form the martensite phase more stably and to further improve the aging resistance, which is the object of the present invention, the C content is desirably 0.01% or more.

Si: 0.5% or less
Si is an effective element for securing strength and obtaining a martensite phase stably. However, if Si is added in excess of 0.5%, the plating adhesion and surface properties are significantly deteriorated. Therefore, the Si content is 0.5% or less. The Si amount is desirably 0.25% or less for further improving the plating adhesion and surface properties, and 0.1% or less for obtaining higher plating quality.

Mn: 0.5-3.0%
Mn is an element that is effective in precipitating S in steel as MnS and preventing hot cracking of the slab. In the present invention, in order to form a martensite phase, 0.5% or more of Mn for improving hardenability is added. However, if the amount of Mn exceeds 3.0%, not only does the slab cost increase significantly, but the workability also deteriorates. Therefore, the Mn content is set in the range of 0.5% to 3.0%. In order to stably obtain the martensite phase, it is desirable to add 1% or more of Mn. On the other hand, from the viewpoint of cost and workability, the Mn content is preferably less than 2.5%.

P: 0.08% or less
P is an element that is very effective for increasing the strength of steel. In the present invention, as compared with conventional steel sheets for panels based on ultra-low carbon steel, high secondary work embrittlement resistance can be maintained due to residual solute C, so that relatively high P addition can be allowed. However, if P is added in an amount exceeding 0.08%, the secondary work brittleness resistance after press forming is deteriorated, and the alloying processability after galvanization is lowered. Therefore, the P content is 0.08% or less. From the viewpoint of secondary work embrittlement resistance and alloying reaction, the P content is preferably 0.05% or less.

S: 0.03% or less
S is preferable because it reduces hot workability and increases the hot cracking sensitivity of the slab. If the amount of S exceeds 0.03%, the workability deteriorates due to the precipitation of fine MnS. Therefore, the S amount is 0.03% or less. From the viewpoint of workability, it is desirable to further reduce the S content to 0.015% or less.

Al: 0.01-0.1%
Al contributes to deoxidation of steel and also has a role of fixing unnecessary solid solution N in the steel as a nitride. This effect is not sufficient if the Al content is less than 0.01%, while an effect commensurate with the added amount cannot be obtained even if the Al content exceeds 0.1%. Therefore, the Al content is set in the range of 0.01 to 0.1%.

N: 0.01% or less
Since N cannot be left in a solid solution state from the viewpoint of aging, it is preferable that N is small. If the N content exceeds 0.01%, the ductility and toughness deteriorate due to the formation of excess nitride. Therefore, the N content is 0.01% or less.

Cr, Mo: 1% or less each when added
Cr and Mo are effective elements for improving hardenability and stably obtaining a martensite phase. It is also effective in suppressing softening of the heat affected zone (HAZ) in welding. However, if the added amount of Cr and Mo exceeds 1%, the HAZ hardness increases too much. Therefore, when Cr and Mo are added, each content should be 1% or less.

V: When added, 0.5% or less
V is effective in suppressing softening of the weld heat affected zone (HAZ). However, if the amount of V exceeds 0.5%, the HAZ hardness increases too much. Therefore, when adding V, it is 0.5% or less.

B: When added, 0.0002 to 0.003%
B is an element effective for improving hardenability, and is added in an amount of 0.0002% or more in order to stably generate a low temperature transformation phase. However, even if added over 0.003%, an effect commensurate with the cost cannot be obtained. Therefore, when adding B, it is set within the range of 0.0002 to 0.003%.

Ti, Nb: 0.1% or less each when added
Ti and Nb form nitrides and fix N. Improving formability can be expected by fixing N with Ti and Nb instead of Al. However, even if each exceeds 0.1%, an effect corresponding to the cost cannot be obtained. Therefore, when adding Ti and Nb, respectively, it is made 0.1% or less. However, if Ti and Nb are added in excess of the amount necessary to fix N, Ti and Nb will form carbides and solid solution C will decrease, so the low-temperature transformation phase (martensite) will be generated stably. It becomes difficult. Therefore, it is preferable that the amount of Ti and Nb added is not more than the amount necessary for immobilization of N (for example, the chemical equivalent of Ti + Nb to N).

Microstructure: Ferrite + Martensite As mentioned above, high strength and high ductility galvanized steel sheets with excellent aging resistance are difficult to obtain with conventional methods of increasing strength based on IF steel or bake hardened steel . For that purpose, first, it is necessary to use a composite steel of ferrite and martensite. As a result of further detailed examination, it is important to appropriately control the dispersion state of martensite with respect to ferrite. Details thereof will be described below.

Ferrite particle size: 4-15μm
Control of the ferrite grain size is particularly important. When the ferrite particle size is less than 4 μm, even if the martensite particles described below are within the scope of the invention, the surface strain resistance is deteriorated and the ductility required for molding the inner and outer panel of an automobile cannot be obtained. Accordingly, the ferrite grain size is set to 4 μm or more, preferably 5 μm or more. On the other hand, if the ferrite particle size exceeds 15 μm, it causes a rough skin during press molding and deteriorates the surface properties. Accordingly, the ferrite particle diameter d is set in the range of 4 to 15 μm, preferably in the range of 5 to 15 μm.

Second phase volume ratio: less than 10% The second phase referred to in the present invention refers to martensite, bainite, and retained austenite. When the volume fraction of the second phase is 10% or more, the steel sheet for an automobile inner / outer panel targeted by the present invention does not have sufficient press formability. Accordingly, the volume fraction of the second phase is less than 10%. From the viewpoint of moldability, the second phase volume fraction is preferably less than 8%.

Precipitation site of second phase: ferrite grain boundary precipitation is 80% or more The precipitation site of second phase is very important in the present invention. The second phase precipitated in the ferrite grains brings about a decrease in ductility, and this tendency becomes prominent when the ratio of intragranular precipitation exceeds 20%. Thus, in order to obtain the excellent balance between strength and ductility, which is the object of the present invention, it is necessary that 80% or more of the second phase occupy the ferrite grain boundary in terms of volume fraction. From the viewpoint of strength-ductility balance, it is desirable that the ferrite grain boundary precipitation ratio is 90% or more.

Martensite ratio: 80% or more of the second phase precipitated at the grain boundary Martensite precipitated at the ferrite grain boundary is compared with the second phase other than martensite in the case of intragranular precipitation or even at grain boundary precipitation. It was found to be very effective in eliminating the yield elongation YPEl. In particular, this effect becomes remarkable when the ratio of the martensite in the second phase precipitated at the grain boundaries is 80% or more. In the present invention, excellent aging resistance is defined as the absence of YPEl after an accelerated aging test (100 ° C. × 10 hr) corresponding to 30 ° C. × 6 months. From the viewpoint of aging resistance, the martensite ratio is preferably 90% or more.

Martensite particle diameter: less than 3 μm in average particle diameter Martensite particles precipitated at ferrite grain boundaries cause a decrease in ductility when the average particle diameter is 3 μm or more. Therefore, the average particle size of martensite is set to less than 3 μm. From the viewpoint of ductility, it is more desirable to be less than 2.5 μm.

Martensite particle average interval L: 0.79d <L <3.1d (d: ferrite particle size)
The distribution of martensite particles on the ferrite grain boundary has a great influence on the aging resistance. Martensite particles form a large number of dislocations in the vicinity of the interface in the adjacent ferrite when they are formed. When these movable dislocations move at a relatively low stress during plastic deformation, the deformation proceeds without yield point elongation YPEl. At this time, if the number of martensite particles on the ferrite grain boundary is too small or the distribution is not uniform, YPEl appears. On the other hand, when there are too many martensite particles, moldability is impaired.

  In this way, it is most important to obtain martensite particles uniformly distributed at predetermined intervals on the ferrite grain boundaries in order to obtain excellent aging resistance and a high level of formability suitable for panel forming. It is. When the average interval L of the martensite particles is used as an index representing the distribution state of the martensite particles on the ferrite grain boundary, the condition for achieving the object of the present invention is the following inequality (1) from the examples described later. Within range.

0.79d <L <3.1d (1)
Here, d represents the ferrite grain size.

  If the average interval L is 3.1d or more, the number of martensite particles on the ferrite grain boundary with respect to the ferrite grains is insufficient, and the aging resistance deteriorates. On the other hand, when the average interval L is the lower limit of 0.79 d or less, the number of martensite particles becomes excessive and the moldability deteriorates. Note that L <2.4d is preferable from the viewpoint of aging resistance, and 1.0d <L is preferable from the viewpoint of moldability.

  This formula (1) is represented by the average value L of the spacing between adjacent martensite particles, but it is desirable for both aging resistance and moldability that the martensite particles are distributed as uniformly as possible. Therefore, it is desirable that the individual martensite particle spacing satisfies the formula (1) as much as possible, and 70% or more of the individual particle spacings satisfy the formula (1) (here, L is the individual martensite particle spacing). ). Further, in order to improve aging resistance and moldability, it is desirable that 80% or more of the martensite particle spacing satisfies the formula (1).

The invention of the manufacturing method capable of obtaining the above-described high-strength and high-ductility galvanized steel sheet excellent in aging resistance is as follows. In the invention, the steel having the chemical composition of the above-mentioned invention is hot-rolled at an end temperature of ₃ or more points of Ar, cooling is started within 2 seconds after rolling, and a cooling rate of 70 ° C./s or more is started. ° C. and cooled to below, then coiling at 500 ° C. or higher, and facilities the cold rolling and galvanizing treatment, galvanizing treatment, the soaking temperature Ac 1 _~ a heating before plating _(Ac 1 + ₈₀ ℃) cutlet In the temperature range within 100 ° C up to the soaking temperature, the steel sheet is heated at a heating rate of 5 ° C / s or less, and then primary cooling is performed at a cooling rate of 3-15 ° C / s until immersion in the plating bath. Or after alloying treatment, secondary cooling is performed at a cooling rate of 3 ° C./s or more, and the structure is controlled to the above-described structure by these treatments. High strength and high ductility galvanized steel sheet It is a production method.

  This production method was made as a result of detailed investigations on the production conditions capable of obtaining the above-described high-strength and high-ductility galvanized steel sheet, and is essential for obtaining the steel sheet of the present invention with a high degree of structure control. It is a condition. Details will be described below.

End temperature of hot rolling: Ar ₃ points or more In hot rolling, the lower the end temperature, the more the formation of ferrite phase after rolling is promoted, and the structure becomes finer. However, when the end temperature of hot rolling falls below the Ar ₃ point, a band structure is formed, and ferrite and martensite whose structure is controlled after annealing cannot be obtained. Accordingly, the hot rolling end temperature is set to Ar ₃ or higher.

  Cooling after hot rolling is an important point in the structure control of the present invention. As described above, the present invention is a composite structure with a highly controlled structure as described above, as an applied material for inner and outer plate panels that is required to have the highest performance as a material for automobiles. For this purpose, it is first necessary to prevent the formation of a band structure as much as possible.

  The band structure formed in the hot rolling stage is not eliminated by the heat treatment (annealing) in the subsequent hot dip galvanizing process, and the martensite distribution cannot be finely and uniformly dispersed. As a result of intensive studies, it has been found that the formation of a band structure can be suppressed by controlling the cooling conditions after hot rolling, in particular, the rapid cooling start time, the cooling rate, and the rapid cooling end temperature. Next, these cooling conditions will be described.

Rapid cooling start time: within 2 seconds after rolling If the time until the rapid cooling start after hot rolling exceeds 2 seconds, ferrite formation starts at a high temperature and the structure becomes coarse. Therefore, rapid cooling starts within 2 seconds after rolling. Furthermore, in order to make the hot rolled sheet structure uniform, it is desirable to start quenching within 1.5 seconds after rolling.

Cooling rate: 70 ° C / s or more When the quenching cooling rate is less than 70 ° C / s, ferrite precipitates corresponding to segregation of alloy elements. The segregation of the alloy elements is layered by rolling, and proeutectoid ferrite and pearlite are generated in a layered manner to form a band structure. Therefore, the cooling rate after hot rolling is set to 70 ° C./s or more. From the viewpoint of band structure suppression, the cooling rate is preferably 100 ° C./s or more, and more preferably over 120 ° C./s.

Rapid cooling end temperature: 650 ° C. or less Rapid cooling (rapid cooling) after hot rolling may be performed up to the coiling temperature, but the rapid cooling can be terminated at a slightly higher temperature for controlling the coiling temperature. When the rapid cooling end temperature exceeds 650 ° C., a band structure is formed as described above due to the high temperature during subsequent cooling. Accordingly, the rapid cooling end temperature is set to 650 ° C. or lower. In addition, subsequent cooling is cooling for control of coiling temperature, and there is no restriction | limiting in particular about a cooling rate and a cooling method (air cooling, water cooling).

Winding temperature: 500 ° C or more Management of the winding temperature is particularly important. When the coiling temperature is less than 500 ° C., the structure of the hot-rolled sheet tends to be mainly bainite, and the recrystallized texture after cold pressure and annealing does not develop and hinders the improvement of the r value. Therefore, the coiling temperature is 500 ° C. or higher, preferably 550 ° C. or higher.

  The obtained hot-rolled sheet is pickled and then cold-rolled and then plated. Regarding the plating method, heat treatment conditions are important in obtaining the structure of the present invention. As heat treatment conditions, a heating rate before heating (annealing), a soaking temperature, a cooling rate for primary cooling (after annealing and before plating), and a cooling rate for secondary cooling (after plating) are controlled. Next, these heat treatment conditions will be described.

Temperature rising rate: 5 ° C./s or less in the temperature range within 100 ° C. until the soaking temperature The temperature rising rate for heating (annealing) before plating is a very important heat treatment condition for obtaining the structure of the present invention. If the heating rate exceeds 5 ° C / s in a temperature range of 100 ° C or less (soaking temperature -100 ° C to soaking temperature) up to the soaking temperature, the Ac ₁ transformation point must be exceeded before recrystallization proceeds sufficiently. become. For this reason, recrystallization and second phase precipitation (austenite formation) compete in the soaking stage, and finally it becomes impossible to precipitate martensite at the ferrite grain boundaries that are the object of the present invention. Further, a desirable ferrite particle size cannot be obtained.

  From the above, in the temperature range within 100 ° C up to the soaking temperature, the rate of temperature rise is 5 ° C / s or less. Further, it is more effective when the rate of temperature rise is 3 ° C./s or less. In addition, it is not necessary to carry out the slow heating of 5 degrees C / s or less in the low temperature side from this temperature range, and rapid heating can be performed.

Soaking temperature: Ac ₁ ~Ac ₁ + 80 ℃
The soaking temperature of annealing needs to be heated (soaking) to an appropriate temperature in order to obtain a structure composed of ferrite and the second phase. When the soaking temperature is less than the Ac ₁ transformation point, an austenite phase is not generated and martensite cannot be obtained. On the other hand, when the annealing temperature exceeds Ac ₁ + 80 ° C., the austenite phase increases and coarsens, element concentration to the second phase is not promoted, and fine martensite cannot be obtained. Accordingly, the soaking temperature in the range of Ac ₁ ~Ac ₁ + 80 ℃.

Furthermore, from the viewpoint of the high ductility and aging resistance targeted by the present invention, it is desirable that the soaking temperature be within the range of Ac ₁ +20 to Ac ₁ + 60 ° C. The soaking time is desirably secured for 20 seconds or more from the viewpoint of obtaining a preferable ferrite grain size and promoting element concentration in the austenite phase.

Cooling conditions for primary cooling: Cooling rate 3-15 ° C / s
In primary cooling until soaking after plating, the cooling rate is set to 3 ° C./s or more to prevent austenite from being decomposed into pearlite during the cooling process. Although it depends on the chemical component, it is more preferably 5 ° C./s or more. On the other hand, when the cooling rate is 15 ° C./second or more, element concentration in the second phase is suppressed, and martensite is hardly generated stably. Therefore, the cooling rate in the primary cooling is in the range of 3 to 15 ° C./s.

Cooling rate of secondary cooling: 3 ° C / s or more The cooling rate of secondary cooling after plating immersion or alloying is 3 ° C / s or more up to a temperature below the Ms point in order to obtain martensite stably. It is necessary to cool at a cooling rate. The cooling rate of the secondary cooling is preferably 5 ° C./s or more, although it depends on the chemical components, and is preferably 7 ° C./s or more in order to obtain martensite more stably. In addition, it is desirable to ensure this cooling rate to a temperature at least below the Ms point.

  The plated steel sheet thus manufactured is subjected to temper rolling with an elongation of about 0.2 to 1.5% to eliminate the yield point elongation. Moreover, you may perform an organic film process further to the plating surface.

  The present invention is excellent in ensuring high strength by limiting chemical components within a specific range, precisely controlling the hot rolling conditions and annealing conditions, and accurately building the final microstructure. It enables the production of galvanized steel sheets that have all the required performance as materials for applying to automotive inner and outer panels, including aging resistance and formability.

  In carrying out the invention, a steel slab having the above chemical components is directly rolled while being heated or heated continuously in a heating furnace or in a continuous casting state by an ordinary method for producing a thin steel plate. In cold rolling, the cold pressure ratio may be 50 to 85% within the normal operating range, but in order to obtain the ferrite grain size of the present invention more stably, the cold pressure ratio is 80% or less. It is desirable.

  FIG. 1 is a diagram showing the relationship between the obtained structure, the aging resistance, and the formability of the steel sheet having the chemical composition of the present invention, with various changes in the final structure depending on the manufacturing conditions. For the structure, the average distance L between two adjacent particles of the average particle diameter d of ferrite and the martensite particles distributed at the grain boundaries was determined using a scanning electron microscope. Specifically, 10 fields of view were taken at a magnification of 2000, and d and L were measured by image processing. In addition, L measured the length along a grain boundary.

  As for the aging resistance, as described above, a sample in which YPEl does not appear after the accelerated aging test (100 ° C. × 10 hr) corresponding to 30 ° C. × 6 months was set as OK. An accelerated aging test (100 ° C. × 15 hours) corresponding to 30 ° C. × 9 months was also conducted. Formability was evaluated by performing a bulge test using a 160 mmφ ball head punch and measuring the limit bulge height. The limit overhang height is influenced not only by the ductility of the material but also by the n value and further the r value (including Δr), and can be said to be a characteristic value for comprehensively evaluating the formability as a panel material. In addition, the presence or absence of rough skin was also evaluated during this test.

  With respect to the above results, in FIG. 1, the samples that are OK for all of the aging resistance, moldability, and rough skin resistance are indicated by ○ and ◎, and even one of the samples is NG (not OK) Is indicated by a cross. Among these, ◎ indicates a sample exhibiting particularly excellent characteristics. Note that YPEL does not appear in the accelerated aging test corresponding to the above-mentioned 30 ° C. × 9 months as a particularly excellent characteristic in aging resistance (◎).

  After melting steels having chemical components shown in Table 1, slabs were produced by continuous casting. Steel Nos. 1 to 10 are steels of the present invention, and steel Nos. 11 to 14 are comparative steels in which the C amount, Si amount, Mn amount, and P amount exceed the upper limit values, respectively.

After these steel slabs are heated to 1200 ° C, they are hot-rolled at the rolling end temperature above the Ar ₃ transformation point, and the subsequent cooling conditions (quenching start time, cooling rate, quenching end temperature) and coiling temperature are changed. To produce a hot-rolled steel sheet. The hot-rolled steel sheet was pickled and cold-rolled at a cold pressure rate of 70% to produce a cold-rolled steel sheet having a thickness of 0.8 mm.

  The cold-rolled steel sheet was passed through a hot dip galvanizing line and plated and alloyed. In this case, the treatment was performed by changing the heating rate, the soaking temperature, and the cooling rate of primary cooling (after annealing and before plating) and secondary cooling (after plating). The plated steel sheet was subjected to temper rolling (rolling ratio 0.2 to 1.5%) to eliminate the yield point elongation. The above production conditions are shown in Table 2.

  About the obtained galvanized steel sheet, the microstructure was photographed with a scanning electron microscope and analyzed in detail by image processing. The analysis items are the average grain size of ferrite, the volume fraction of the second phase, the proportion and ratio of grain boundary precipitation in the second phase, and the average interval between martensite grains precipitated at the grain boundary.

  Moreover, about these test materials, the tension test was extract | collected the JIS5 test piece in the C direction. Further, overhang property and aging resistance were also performed under the same conditions as in Example 1. The above survey results are also shown in Table 2. Here, the samples that are OK in terms of aging resistance and moldability are indicated by ◯ and ◎, and the samples that are NG (not OK) are indicated by ×. Among them, ◎ indicates a sample that showed particularly excellent characteristics as in Example 1.

  As shown in Table 2, test materials (invention examples No. 1 to 3, 7 to 8, 13 to 20) within the chemical composition range of the present invention (invention steel) and having the microstructure of the present invention. ) Satisfies both excellent extrudability and aging resistance at the same time.

  In Comparative Examples No. 4 to 6, since the continuous annealing condition during the plating treatment is outside the scope of the present invention, the average interval between the ferrite particle diameter or the martensite particles does not fall within the scope of the present invention. Sex has not reached the goal.

  In Comparative Examples No. 9 to 12, since the cooling condition or coiling condition after hot rolling is outside the scope of the present invention, either the average interval between martensite grains, the ratio of grain boundary precipitation or both are within the scope of the present invention. And either or both of the overhang property and the aging resistance have not reached the target.

  Further, in Comparative Examples No. 21 to 24, the chemical composition is outside the scope of the present invention (comparative steel) (No. 22 is the soaking temperature is also outside the scope of the present invention), and the microstructure defined by the present invention is not obtained , Either overhang, aging resistance or both have not reached the target.

  The galvanized steel sheet of the present invention can be used not only for automobiles but also in a wide range of fields such as household appliances.

The figure which shows the structure | tissue in the steel plate which has a chemical component of this invention, the relationship between aging resistance, and a formability.

Claims (5)

Chemical component is mass%, C: 0.005% or more and less than 0.04%, Si: 0.5% or less, Mn: 0.5-3.0%, P: 0.08% or less, S: 0 0.03% or less, Al: 0.01 to 0.10%, N: 0.01% or less, consisting of the balance iron and inevitable impurities, and having a structure with an average particle size of 4 to 15 μm and a volume ratio of less than 10% 80% or more of the second phase is precipitated at the grain boundary, and more than 80% of the second phase precipitated at the grain boundary is martensite particles having an average particle size of less than 3 μm. The martensite particles distributed at these grain boundaries have high strength and high aging resistance, characterized in that the average interval L between two adjacent particles satisfies the following inequality with respect to the average particle diameter d of the ferrite: Ductile galvanized steel sheet.
0.79d <L <3.1d   The high strength and high ductility galvanized steel sheet excellent in aging resistance according to claim 1, further comprising, as chemical components, Cr: 1% or less, Mo: 1% or less, V: 0.5% or less, B: 0.0002. A high strength and high ductility galvanized steel sheet excellent in aging resistance, characterized by containing at least one of -0.003%, Ti: 0.1% or less, and Nb: 0.1% or less.   The high-strength and highly ductile galvanized steel sheet with excellent aging resistance according to claim 1 or 2, further comprising an organic coating on the plating surface. The steel having the chemical composition according to claim 1 or 2 is hot-rolled at an end temperature of ₃ or more points of Ar, cooling is started within 2 seconds after rolling, and a cooling rate of 70 ° C / s or more is started. It cooled to 650 ° C. or less, then coiling at 500 ° C. or higher, and facilities the cold rolling and galvanizing treatment,
In the hot dip galvanizing treatment , the steel sheet is heated at a rate of temperature increase of 5 ° C./s or less in a temperature range of 100 ° C. to a soaking temperature Ac ₁ to (Ac ₁ + 80 ° C.) before heating. After that, the primary cooling at a cooling rate of 3 to 15 ° C./s is performed until immersion in the plating bath, and after the hot dip galvanization or after the alloying treatment, the secondary cooling is performed at a cooling rate of 3 ° C./s or more. A method for producing a high-strength and high-ductility galvanized steel sheet excellent in aging resistance, characterized in that the structure is controlled to the structure according to claim 1 by these treatments . The method for producing a high-strength and high-ductility galvanized steel sheet having excellent aging resistance according to claim 4 , wherein the plated surface is further subjected to an organic coating treatment.

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