JP2002088447A - High strength galvanized steel sheet having excellent workability and plating property and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high strength galvanized steel sheet having good workability and high strength and capable of obtaining good plating in the case galvanizing is performed by using an equipment such as a continuous galvanizing line. SOLUTION: A slab having a composition containing, by weight, 0.01 to 0.20% C, <=1.0% Si, >1.5 to 3.0% Mn, <=0.10% P, <=0.05% S, <=0.10% Al and <=0.010% N and also containing one or more kinds selected from Ti, Nb and V by 0.010 to 1.0% in total, and the balance Fe with inevitable impurities is hot-rolled, is coiled at 750 to 450 deg.C and as it is or is further cold-rolled, and the obtained hot rolled sheet or cold rolled sheet is heated to >=750 deg.C, and in the process of cooling from the same temperature, galvanizing is performed to control the ferritic structure and bandlike structure of the plated steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet (including a high-strength galvannealed steel sheet) suitable for use as an inner plate and an outer plate of an automobile. .

[0002]

2. Description of the Related Art In recent years, there has been an increasing tendency to apply a high-strength hot-dip galvanized steel sheet as a steel sheet for automobiles from the viewpoints of safety, weight reduction and fuel efficiency of automobiles, and further improvement of the global environment. . Among them, in order to produce high-strength hot-dip galvanized steel sheets, in particular, the plating properties are good, and after passing through a hot-dip galvanizing bath or further alloying treatment, the desired strength and workability are obtained. Must be obtained.

Generally, to increase the strength of a steel sheet, M
Solid solution strengthening elements such as n, Si and P and precipitation strengthening elements such as Ti, Nb and V are added. It is known that when a steel sheet to which such an element is added is treated in a continuous hot-dip galvanizing line (CGL), the galvanizability is deteriorated. As described above, since the content of the alloying element has an opposing action in terms of strength and plating properties, it is extremely difficult to produce a high-strength galvanized steel sheet having good plating properties in a continuous galvanizing line. Was something. Further, since high strength hot-dip galvanized steel sheets generally have poor properties related to workability such as elongation, it has been even more difficult to produce hot-dip galvanized steel sheets with good workability.

[0004] As a conventional high-strength steel sheet with improved workability, a multi-structure steel sheet containing a low-temperature transformed phase (including retained austenite) containing martensite as a main phase in a ferrite base is known. This composite structure steel sheet is non-ageed at room temperature, has a low yield ratio, and has excellent workability and bake hardenability after processing. It is manufactured by heating at (α + γ) range temperature and then quenching by water cooling or gas cooling. .

[0005]

However, when the composite structure steel sheet is hot-dip galvanized or further alloyed at a temperature of about 500 ° C., the martensite dispersed in the ferrite matrix is tempered. The tensile strength and elongation decrease, the upper yield point appears, and the yield ratio increases,
Furthermore, yield elongation occurs. Tempering softening is more likely to occur as the amount of alloying elements such as Mn and Si decreases,
On the other hand, when there are many of these alloy elements, hot-dip galvanizing property will fall. After all, even in composite structure steel sheets,
Since martensite is tempered in the plating process, it has been difficult under the prior art to achieve both workability and high strength, which are the characteristics of the martensite, and to exhibit good plating properties.

Accordingly, the present invention provides both good workability and high strength even when hot-dip galvanizing using equipment such as a continuous hot-dip galvanizing line in order to solve the above-mentioned problems of the prior art. It is an object of the present invention to propose a method for producing a high-strength hot-dip galvanized steel sheet that satisfies and can provide good plating. A specific object of the present invention is to provide an index indicating workability and high strength as TS: 590 MPa or more, and TS × El.
Value: good plating can be obtained while satisfying 15000 MPa ·% or more.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems. As a result, the inventors have found that even if Mo and Cr are not added, a residual austenite phase and a tempered martensite phase are not included. Also found that high-strength hot-dip galvanized steel sheets with excellent workability and plating properties could be manufactured,
The present invention has been completed.

That is, the present invention has the following configuration as a gist. (1) In mass%, C: 0.01 to 0.20%, Si: 1.0% or less, Mn: more than 1.5 to 3.0%, P: 0.10% or less, S: 0.05% or less, Al: 0.10% or less, N: 0.010% It contains the following and contains one or more selected from Ti, Nb and V in a total amount of 0.010 to 1.0%, with the balance being composed of Fe and unavoidable impurities and the area ratio of the ferrite phase. Is 50% or more and the average crystal grain size of the ferrite phase is
A metal structure having a thickness of 10 μm or less and a band-like structure composed of the second phase satisfying a relationship of Tb / T ≦ 0.005 (where, Tb: average thickness of the band-like structure in the thickness direction, T: steel plate thickness). A high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, characterized by having:

(2) In the above (1), the steel composition further contains C
A high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, characterized by comprising a composition containing one or two of u and Ni in a total of 3.0% or less.

(3) The workability according to (1) or (2), wherein the steel composition further comprises one or two of Ca and REM in a total content of 0.001 to 0.01%. High strength hot-dip galvanized steel sheet with excellent plating properties.

(4) A slab comprising the steel composition described in any one of (1) to (3) above is hot-rolled to 750 to 45
Winding at 0 ° C, and then performing cold rolling as it is or further, heating the obtained hot rolled sheet or cold rolled sheet to 750 ° C or more, and performing hot dip galvanizing during cooling from this temperature. A method for producing a high-strength hot-dip galvanized steel sheet with excellent workability and plating properties.

(5) A slab having the steel composition described in any one of (1) to (3) above is hot-rolled to 750 to 45
Winding at 0 ° C., and then performing cold rolling as it is or further, heating the obtained hot-rolled sheet or cold-rolled sheet to 750 ° C. or more, performing hot-dip galvanizing during cooling from this temperature, A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, characterized by performing an alloying treatment.

(6) A slab comprising the steel composition described in any one of (1) to (3) above is hot-rolled to 750 to 45
It is wound at 0 ° C. and then cold-rolled as it is or further, and the obtained hot-rolled sheet or cold-rolled sheet is once heated to 750 ° C.
High-strength hot-dip galvanized steel sheet with excellent workability and plating properties, characterized in that it is heated to above, cooled, and further heated to 700 ° C or more, and hot-dip galvanized during cooling from this temperature. Manufacturing method.

(7) A slab having the steel composition described in any one of (1) to (3) above is hot-rolled to 750 to 45
It is wound at 0 ° C. and then cold-rolled as it is or further, and the obtained hot-rolled sheet or cold-rolled sheet is once heated to 750 ° C.
Heating and cooling to above, further heating to 700 ° C or higher, hot-dip galvanizing in the middle of cooling from this temperature, and then performing alloying treatment Manufacturing method of high-strength hot-dip galvanized steel sheet.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In these inventions, the following (a)
The effect of (b) is exhibited synergistically, without adding a large amount of Mn and Si, and without adding Mo or Cr as a strengthening element, and also systematically retaining the retained austenite phase and tempered martensite. Even if no phase is contained, the γ grains before cooling can be made finer, which promotes the concentration of C and Mn from the α phase to the γ phase, effectively transforms the γ phase into martensite, and This makes it possible to produce a high-strength hot-dip galvanized steel sheet having excellent properties and plating properties. (a) α (ferrite) crystal grains due to the pinning action of crystal grain boundary movement by carbides such as TiC, NbC, and VC generated by the addition of one or more selected from Ti, Nb, and V Can be reduced to 10 μm or less. For this reason, when this is further heated in a process before plating,
The effect of suppressing γ grains generated and grown in the two-phase region of α (ferrite) + γ (austenite) or coarsening of γ particles in the single-phase region of γ (austenite). (b) The second which exists before heating and contains a large amount of C and Mn
When the thickness of the band-like structure composed of phases is Tb / T ≦ 0.005
(However, Tb: average thickness in the thickness direction of the band-like structure,
(T: steel plate thickness). In the present invention, due to such a synergistic effect, it is not necessary to substantially contain Cr harmful to the plating property, so that the plating property is extremely good, and since Mo is not added, the band-like structure existing before heating is not present. In the case of the single CGL (hot-dip galvanizing) method, which has a relatively small thickness and is disadvantageous from the viewpoint of plating properties, it is possible to produce a high-strength hot-dip galvanized steel sheet having good workability without applying much high-temperature heating. .

Next, the experimental results based on the present invention will be described. (Experiment 1) Chemical composition is 0.08% C-0.01% Si-1.9% Mn-
0.011% P-0.002% S-0.04% Al-0.0022% N-0.02
A 30 mm thick sheet bar of% Ti-0.05% Nb was heated to 1200 [deg.] C. to form a hot rolled sheet of 2.8 mm thick in five passes. Then, 400 ° C, 650 ° C as processing equivalent to winding temperature (CT)
For 1 hour each. Then, after pickling, cold-rolled to a 1.4 mm cold-rolled sheet, heated and maintained at 700 ° C to 850 ° C for 1 minute, cooled to 500 ° C at a rate of 10 ° C / s, and hot-dip galvanized. After holding for 40s, 550 ℃ at 10 ℃ / s
Until the alloying treatment, and immediately cooled to room temperature at a rate of 10 ° C./s. Thereafter, temper rolling at a reduction of 1.0% was performed. The obtained hot-dip galvanized steel sheet was subjected to JIS
The tensile properties (TS, YS, El) were examined using a No. 5 tensile test piece, and the plating properties were examined. Plating properties were evaluated according to the following criteria. ：: No non-plating defect (good plating property) Δ: Partial occurrence of non-plating defect (slightly good plating property) ×: Many occurrence of non-plating defect (poor plating property) The results obtained are shown in FIG. From FIG. 1, the winding temperature is 65
At 0 ° C, when the heating temperature before plating is 750 ° C or more,
It can be seen that TS: 590 MPa or more and El: 25% or more can be achieved.

(Experiment 2) Using a cold-rolled sheet having a thickness of 1.6 mm with the same components as in Experiment 1 and a CT equivalent treatment changed from 400 ° C. to 700 ° C., and kept at 750 ° C. for 1 minute (first heating). , 10 ℃
/ S cooled to room temperature, pickled, held at 750 ° C for 1 minute (second heating), cooled to 10 ° C / s to 500 ° C, hot-dip galvanized, and held for 40s After, 10 ℃ / s
To 550 ° C for alloying treatment and immediately 10 ° C / s
At room temperature. Thereafter, temper rolling at a reduction of 1.0% was performed. With respect to the obtained hot-dip galvanized steel sheet, the tensile properties and the plating properties were examined in the same manner as in Experiment 1. As a result, heating twice (first heating and second heating)
2 (o in FIG. 2), as shown in FIG. 2, both the tensile properties and the plating properties are further higher than those in the experiment (● in FIG. 2) similar to Experiment 1 in which only one heating was performed. It can be seen that it can be improved.

From the above experiments, it is found that even when the strength is increased by the high Mn content, the plating properties and mechanical properties can be improved by high-temperature winding, high-temperature heating before plating, or two heat treatments. I understood. The following are conceivable reasons for obtaining such an effect. In other words, high-temperature winding and two heat treatments produce an internal oxide layer directly below the steel sheet, which suppresses the concentration of Mn, which is harmful to the plating property, on the steel sheet surface. Mn surface concentrated layer that is harmful to plating properties is removed by pickling before the second heating, high-temperature heating before plating is C,
It is conceivable that the band structure having a high Mn concentration is dissolved / dispersed, which advantageously acts on the formation of the second phase such as martensite.

Next, the reasons for limiting the component composition and the production conditions in the present invention to the above ranges will be described. (The component composition is represented by mass%) C: 0.01 to 0.20% C is one of the important basic components of steel. In particular, in the present invention, carbides of Ti, Nb, and V are precipitated to contribute to an increase in strength. In addition, it is an element that contributes to improvement in strength through a bainite phase and a martensite phase generated at a low temperature. If the C content is less than 0.01%, not only the precipitates described above but also the bainite phase and the martensite phase are hardly formed, while the C content is 0.20%.
If the content is too high, the spot weldability deteriorates, so the content range is set to 0.01 to 0.20%. The preferred C amount is 0.03 to
0.15%.

Si: 1.0% or less Si is an element that improves the workability such as elongation by reducing the amount of solid solution C in the α phase. Since the plating property is impaired, the upper limit is made 1.0%. The preferred amount of Si is 0.5% or less. Since lowering the Si content to less than 0.005% involves an increase in cost, the lower limit is preferably set to 0.005%.

Mn: more than 1.5 to 3.0% Mn is one of the important components in the present invention, and is an element that suppresses transformation in a composite structure and stabilizes the γ phase. However, when the content is less than 1.5%, the effect is not obtained. On the other hand, when the content exceeds 3.0%, the spot weldability and the plating property are significantly impaired. Therefore, Mn is more than 1.5 to 3.0%, preferably
It is contained in the range of 1.6 to 2.5%.

P: 0.10% or less P is an effective element for achieving high strength at low cost, but if it exceeds 0.1%, the spot weldability is significantly impaired, so the upper limit is made 0.10%. It is desirable that the P content be suppressed to 0.05% or less. Since lowering the P content to less than 0.001% involves an increase in cost, the lower limit is 0.00%.
It is desirable to keep it to 1%.

S: 0.05% or less S causes hot cracking at the time of hot rolling and also induces breakage in the nugget of the spot welded portion. Therefore, it is desirable to reduce S as much as possible. Therefore, in the present invention, the upper limit is 0.05%
The following is assumed. It is more preferable to suppress the content to 0.010% or less. Since lowering the S content to less than 0.0005% involves an increase in cost, the lower limit is desirably set to 0.0005%.

Al: 0.10% or less Al is an effective element as a deoxidizing agent at the steel making stage and fixing N which causes aging deterioration as AlN. However, if the content exceeds 0.10%, the production cost increases, so the Al content must be suppressed to 0.10% or less. The preferred content is 0.050% or less. In addition, the amount of Al is 0.005
%, The deoxidation tends to be insufficient, so the lower limit is 0.00
5% is desirable.

N: 0.010% or less N not only causes aging deterioration but also increases the yield point (yield ratio) and yield elongation, so it is necessary to suppress it to 0.010% or less. Note that a preferable N amount is 0.0050% or less. In addition, since lowering the N content to less than 0.0005% involves an increase in cost, the lower limit is preferably limited to 0.0005%.

Ti, Nb, and V: 0.01 to 1.0% in total Ti, Nb, and V are elements that form carbides and are effective in increasing the strength of steel.
1.01.0%. These elements are 0.01% in total
The above effect can be obtained with the above content. However, if the content exceeds 1.0%, disadvantages in cost are caused, and the amount of fine precipitates becomes too large, so that recovery and recrystallization after cold rolling is suppressed, and ductility (elongation) is suppressed. ). Therefore, these elements are 0.
It is contained in the range of 01 to 1.0%, preferably 0.010 to 0.20%.

Cu, Ni: not more than 3.0% in total Cu and Ni are elements that form a second phase such as martensite and are useful elements for increasing the strength of steel, and are added as necessary. However, if the total content exceeds 3.0%, not only is the cost increased, but also the yield point is lowered, which is disadvantageous when a high yield ratio is required. For this reason, the contents of Cu and Ni are set in a range of 3.0% or less in total. The preferred content range is 0.010 to 3.
It is in the range of 0%.

Ca, REM: 0.001 to 0.01% in total Ca and REM have the effect of controlling the morphology of inclusions and sulfides and improving the hole-expanding property. Therefore, it is preferable to contain 0.001% or more in total. . However, even if the content exceeds 0.01% in total, the effect is saturated and the cost is increased. For this reason,
The total content of Ca and REM should be 0.001-0.01%.
A more preferable content range is 0.002 to 0.00 in total.
5%.

Ferrite phase: 50% or more in area ratio The present invention is directed to a steel sheet for automobiles requiring high workability, and secures necessary ductility and stretch flangeability when the ferrite phase is less than 50% in area ratio. It will be difficult to do.
In the case where better ductility is required, it is desirable that the ferrite fraction be 75% or more in terms of area ratio. The ferrite includes not only so-called ferrite but also bainitic ferrite and acicular ferrite which do not include precipitation of carbide. The observation method and evaluation method of the ferrite phase are as follows: embedded in resin so that the cross section of the steel sheet becomes the observation surface; "aqueous solution obtained by adding 1 g of sodium pyrosulfite to 100 ml of pure water";
A solution obtained by mixing 4 g of picric acid with respect to 1 ml in a 1: 1 ratio was immersed in a solution at room temperature for 120 seconds and etched to obtain a ferrite phase (black portion) and a second phase (white portion). And the area ratio of ferrite was determined using an image analyzer with a magnification of 1000 times.

Average grain size of ferrite phase: 10 μm (0.0
1 mm) or less When heating to the α + γ two-phase region by annealing, if the ferrite grain size is more than 10 μm, the austenite grains generated from the ferrite grain boundary naturally increase. Of course,
These large austenite grains are transformed into a relatively large second phase such as martensite or bainite during cooling, and serve as crack initiation points to reduce hole expandability. Therefore,
In the present invention, the grain size of the ferrite is set to 10 μm or less in order to make the second phase finer and improve hole expandability. here,
The average crystal grain size is calculated from the value calculated by the quadrature method specified in ASTM from the photograph of the cross-sectional structure and the nominal grain size similarly obtained by the cutting method (for example, Umemoto et al., Described in Heat Treatment 24 (1984) 334). Adopt the larger one. In the present invention, the type of the second phase (martensite, bainite, pearlite, cementite, etc.) does not need to be particularly limited.

Band-like structure: Thickness of Tb / T ≦ 0.005 The band-like structure is a concentrated layer of C and Mn in a steel having a large amount of C and Mn, mainly agglomerated along crystal grain boundaries in a cooling step of the slab. Are formed by forming a second phase group which is stretched at the time of hot rolling or subsequent cooling and formed in a row or a layer in the rolling direction and the sheet width direction. The reason that the ratio Tb / T between the average thickness Tb and the plate thickness T of these band-like structures is 0.005 or less is that when the Mn content is large as in the present invention, C and Mn are mainly contained in the structure of the hot-rolled sheet. This is because it becomes difficult to produce a high-strength steel sheet in which hard martensite is uniformly dispersed in a ferrite base material. Therefore, in order to efficiently manufacture high-strength steel sheets,
It is necessary to disperse C and Mn concentrated in the band-like second phase, and the standard is the ratio of the average thickness Tb of the band-like structure to the plate thickness T, and Tb / T ≦ If it is 0.005, good results can be obtained. Average thickness of band-like structure: Tb is embedded in a resin such that the cross section of the steel plate becomes an observation surface, immersed in a 3% nital solution at room temperature for 15 seconds, etched, and image-analyzed at a magnification of 1000 times by The thickness of each of the row-shaped and layered second phase structures was measured at 20 points, and the thickness was determined from the average value of the 20 points.

Next, the manufacturing conditions in the present invention will be described. A steel slab having the above-described composition is hot-rolled in a conventional manner and wound at 750 to 450 ° C. If the winding temperature is lower than 450 ° C., carbides such as TiC and NbC are hardly generated, and the strength tends to be insufficient. Further, it is difficult to form an internal oxide layer immediately below the surface of the steel sheet, and it is not possible to suppress Mn concentration on the steel sheet surface. On the other hand, if it is wound above 750 ° C, the scale thickness becomes thicker and the pickling efficiency becomes worse, and the tip, center and rear ends in the longitudinal direction of the coil, and the edges and the center in the coil width direction are removed. This is because the material variation becomes large. The preferred winding temperature is 700 to 55
0 ° C.

The hot-rolled sheet is descaled by pickling if necessary, and hot-rolled or after further cold-rolled,
Heat and cool to 750 ° C or higher in a continuous hot-dip galvanizing line. Hot-dip galvanizing is performed during cooling. In the case where heating is performed twice, first of all, 750 in a continuous annealing facility or the like.
After heating to 1 ° C or higher (first heating) and cooling, then heating in a continuous hot-dip galvanizing line to 700 ° C or higher (second heating), cooling, and during hot-dip galvanizing, preferably at 600 to 420 ° C I do.

Prior to plating, Mn and the like concentrated in the band-like structure are dispersed by heating once to a temperature range of 750 ° C. or more (preferably 750 to 900 ° C.) and cooled, thereby improving the efficiency. By forming a composite structure of ferrite and martensite, it is possible to improve workability.
That is, when the Mn content is high as in the present invention, a band-shaped second phase structure is easily formed in the hot-rolled sheet,
The concentration of Mn or the like in the γ phase decreases, which is disadvantageous for forming a complex structure. Therefore, if the thickness of the band-like structure is reduced and finely dispersed, the temperature in the γ phase during the continuous hot-dip galvanizing line plating process or at around 500 ° C during the alloying process is reduced. Since the amount of enrichment of Mn or the like increases, the martensite phase can be suitably dispersed in the ferrite base.

When the second heating is performed, the second heating is performed at 700 ° C. or more. The second heating is necessarily performed in a continuous hot-dip galvanizing line. The second heating temperature is 70
If the temperature is lower than 0 ° C., the surface of the steel sheet is not reduced in the continuous hot-dip galvanizing line, and plating defects are likely to occur.
The second heating temperature is preferably in the range of 750 to 800 ° C. In the case where heating is performed twice, it is preferable to remove a surface-concentrated layer of Mn or the like generated in the first heating and then to perform pickling in order to enhance plating properties thereafter. After the above-mentioned heating step, hot-dip galvanizing is performed, and in some cases, alloying may be performed after hot-dip galvanizing.

[0036]Example 1  300mm thick continuous cast slab with the chemical composition shown in Table 1.
Is heated to 1200 ° C and after 3 passes of rough rolling, 7 stands
Winding as a hot-rolled sheet with a thickness of 2.5 mm with a finishing mill
Was. After pickling, leave the hot rolled sheet as it is or
After cold rolling to 1.2 mm, (1) First heating in continuous annealing line
-Pickling-Second heating in continuous hot-dip galvanizing line-
Lead plating or (2)
Hot-zinc plating
The sample collected was alloyed. these
The manufacturing conditions are shown in Tables 2 and 3.

[0037]

[Table 1]

CGL conditions after heating were as follows: the average cooling rate of the steel sheet from heating to plating was 10 ° C./s, and the steel sheet was immersed in a plating bath (bath composition: 0.15% Al—Zn, bath temperature: 470 ° C.). (Immersion time: 1 second), and then adjusted to a basis weight of 60 g / m ² by gas wiping. Thereafter, the mixture is heated to 490 ° C. and maintained for 20 seconds.
s to 200 ° C. Using the obtained steel sheet as a test material, mechanical properties, plating properties, spot weldability, and the like were investigated. The results are shown in Tables 2 and 3.

The mechanical properties, plating properties, alloying properties, and spot weldability were evaluated by the following methods.・ Mechanical properties (investigated by tensile test and hole expansion test) JIS Z 2204 sampled from steel plate in the direction perpendicular to the rolling direction
, The yield strength (YS), tensile strength (TS), elongation at break (El), and yield elongation (YEl) were measured using the No. 5 test piece specified in JIS Z 2241. The stretch flangeability was measured by measuring the hole expansion ratio (λ) by the method specified in JFS T1001. -Good plating: No non-plating defects Slightly good: Some non-plating defects occurred Bad: Many non-plating defects occurred-Good alloying treatment: Good with no alloying unevenness Good: Good with slight alloying unevenness Poor: Extremely uneven alloying ・ Spot weldability For spot welding, welding electrode: Dome tip diameter 6φ, electrode pressing force: 3.10 kN, welding current: 7 kA, pressurization time: 25 cy
c, setup time: 3 cyc, welding time: 13 cyc, holding time: 25 cyc. After welding, JIS
Tensile load (TSS) by tensile shear test of Z 3136
And a tensile load (CTS) by a cross-shaped tensile test according to JIS Z 3137, a tensile shear load of 8787 N or more, which is a standard tensile shear load for a sheet thickness of 1.2 mm, and a ductility ratio (CTS /
Those having a TSS) of 0.25 or more were evaluated as “excellent”, and those not satisfying these values were evaluated as “poor”.

From Tables 1 to 3, the invention examples show that TS: 590 to
At 690 MPa level, it has a tensile property of El: 25% or more.
When the value of S × El was 15,000 MPa ·% or more, the TS × El balance was good, and it was found that there was no problem with plating properties, alloying properties, and spot weldability.

[0041]

[Table 2]

[0042]

[Table 3]

[0043]Example 2  300mm thick continuous cast slab with the chemical composition shown in Table 4.
Is heated to 1200 ° C and after 3 passes of rough rolling, 7 stands
The results are shown in Table 5 as a 3.0 mm thick hot rolled sheet by a finishing mill.
Wound at temperature. After pickling, hot rolled sheet or hot rolled
After further rolling the sheet to a thickness of 1.2 mm, (1) Continuous annealing line
First heating in the oven-pickling-continuous galvanizing line
Second heating-galvanizing or (2) continuous hot-dip galvanizing
Plating in the process of heating-zinc plating
Further, a sample taken from a portion was alloyed. This
Table 5 shows these manufacturing conditions. In addition, the CGL strip after heating
The average cooling rate of the steel sheet from heating to plating
10 ° C / s, plating bath (bath composition: 0.15% Al-Zn, bath
Temperature: 470 ° C) (immersion time: 1 second)
60g / m by swiping²Was adjusted to the basis weight.
Then, heat to 490 ° C and hold for 20 seconds.
Cooling was performed at a uniform cooling rate of 20 ° C / s to 200 ° C. Got
Using steel sheet as a test material, mechanical properties, plating properties, spot
Weldability was examined in the same manner. The result is displayed
5 is also shown. As a result, the invention example shows that the TS × El
Good strength and high strength,
Problems with workability, alloying processability, spot weldability
It turned out there was no.

[0044]

[Table 4]

[0045]

[Table 5]

Example 3 The chemical composition shown in Table 6 was used, and the thickness was 300
The continuous cast slab having a thickness of 3 mm was heated to 1200 ° C., subjected to three passes of rough rolling, and wound up as a hot-rolled sheet having a thickness of 3.0 mm at a temperature shown in Table 7 by a finishing mill having seven stands. After pickling,
The sheet was cold-rolled to a thickness of 1.2 mm, plated in a process of first heating in a continuous annealing line, pickling, second heating in a continuous hot-dip galvanizing line, and galvanizing, followed by alloying. Table 7 shows the manufacturing conditions. In addition, C after heating
The GL conditions were as follows: the average cooling rate of the steel sheet from heating to plating was 10 ° C./s, and the plating bath (bath composition: 0.15% Al-Z
n, bath temperature: 470 ° C.) (immersion time: 1 second), and then adjusted to a basis weight of 60 g / m ² by gas wiping. Thereafter, the mixture was heated to 490 ° C. and maintained for 20 seconds, and then cooled to 200 ° C. at an average cooling rate of 20 ° C./s. Using the obtained steel sheet as a test material, mechanical properties, plating properties, spot weldability, and the like were similarly examined. The results are shown in Table 7. As a result, the invention example is TS × E
It was found that there was no problem in plating properties, alloying treatment properties, and spot weldability, despite good l balance and high strength.

[0047]

[Table 6]

[0048]

[Table 7]

[0049]

As described above, according to the present invention, a high-strength hot-dip galvanized steel sheet (high-strength galvannealed steel sheet) having no problem in plating properties, having a low yield ratio and a good TS × El balance. (Including steel plate). Therefore, the present invention makes it possible to reduce the weight and fuel consumption of an automobile, and greatly contributes to the improvement of the global environment. Incidentally, the present invention can be applied to hot-rolled steel sheets and cold-rolled steel sheets without plating, and also to electrogalvanized steel sheets,
Similar effects can be expected.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of heating temperature in a continuous galvanizing line on tensile strength (TS), yield strength (YS), elongation (El), and plating properties.

FIG. 2 is a graph showing the influence of the winding temperature and the presence or absence of two heating operations on tensile strength (TS), yield strength (YS), elongation (El), and plating properties.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Pref. (No address) Inside Mizushima Steel Works, Kawasaki Steel Corporation (72) Inventor Akino Shinohara 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture EA09 EA13 EA15 EA16 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EA36 EB05 EB07 EB08 EB09 EB11 FE01 FE02 FE03 FF02 FH01 FJ05 FK02 GA05 GA07 JA06

Claims (7)

[Claims] 1. Mass%, C: 0.01 to 0.20%, Si: 1.0% or less, Mn: more than 1.5 to 3.0%, P: 0.10% or less, S: 0.05% or less, Al: 0.10% or less, N: 0.010% or less, and one or two or more selected from Ti, Nb and V in a total content of 0.010 to 1.0%, with the balance being composed of Fe and unavoidable impurities. The area ratio is 50% or more, and the average crystal grain size of the ferrite phase is
A metal structure having a thickness of 10 μm or less and a band-like structure composed of the second phase satisfying a relationship of Tb / T ≦ 0.005 (where, Tb: average thickness of the band-like structure in the thickness direction, T: steel plate thickness). A high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, characterized by having: 2. The steel according to claim 1, wherein the steel composition further comprises one or more of Cu and Ni containing 3.0% or less in total. High-strength galvanized steel sheet. 3. The steel composition according to claim 1, wherein the steel composition further comprises one or two of Ca and REM in a total amount of 0.001.
A high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, characterized by having a composition containing 1 to 0.01%. 4. A slab comprising the steel composition according to any one of claims 1 to 3, which is hot-rolled and wound at 750 to 450 ° C., and then subjected to cold rolling as it is, High-strength hot-dip galvanized steel sheet with excellent workability and plating properties, characterized in that the obtained hot-rolled sheet or cold-rolled sheet is heated to 750 ° C or higher and hot-dip galvanized while cooling from this temperature. Manufacturing method. 5. A slab comprising the steel composition according to any one of claims 1 to 3, which is hot-rolled, wound at 750 to 450 ° C., and then cold-rolled as it is, The obtained hot rolled sheet or cold rolled sheet is heated to 750 ° C. or higher, and hot-dip galvanized while cooling from this temperature.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, which is followed by alloying. 6. A slab comprising the steel composition according to any one of claims 1 to 3, which is hot-rolled and wound at 750 to 450 ° C., and then cold-rolled as it is. The obtained hot rolled sheet or cold rolled sheet is once heated to 750 ° C or higher, cooled, and further heated to 700 ° C or higher,
A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, wherein hot-dip galvanizing is performed during cooling from this temperature. 7. A slab comprising the steel composition according to any one of claims 1 to 3, which is hot-rolled, wound at 750 to 450 ° C., and then subjected to cold rolling as it is, The obtained hot rolled sheet or cold rolled sheet is once heated to 750 ° C or higher, cooled, and further heated to 700 ° C or higher,
A method for producing a high-strength hot-dip galvanized steel sheet having excellent workability and plating properties, wherein hot-dip galvanizing is performed during cooling from this temperature, followed by alloying treatment.