JPH10310824A - Production of galvannealed steel sheet having strength increasing heat treatability after forming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel sheet capable of increasing tensile-strength or hard ness by executing short time heat treatment at a relatively low temp. after forming such as press-forming by specifying the compsn. and microstructure of a steel compositely added with suitable amounts of C, W, Cr and Mo and subjecting it to hot dip galvanizing and heating alloying treatment. SOLUTION: A steel contg., by weight, 0.01 to 0.08% C, 0.005 to 1.0% Si, 0.01 to 3.0% Mn, 0.001 to 0.15% P, 0.001 to 0.02% S, 0.001 to 0.1% Al, 0.002 to 0.01% N, one or more kinds among W, Cr and Mo by 0.05 to 3.0%, and the balance iron is prepd. This steel is subjected to hot rolling, is galvanized without executing cold rolling and is thereafter subject to heating alloying treatment. The steel may further be added with one or more kinds among Ti, Nb and V by prescribed amounts. It is preferable that the microstructure of the steel is consisting of ferrite or essentially of ferrite, and the structure of the balance which is not composed of ferrite in the structure consisting essentially of ferrite is composed of pearlite and/or bainite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention is applicable to parts requiring structural strength, particularly strength and / or rigidity at the time of deformation, such as construction members, mechanical structural parts, and structural parts of automobiles. The present invention relates to a method for manufacturing a galvannealed steel sheet which is preferably used as a base steel sheet of a formed body subjected to a heat treatment for increasing the strength in a predetermined temperature range after working and forming by a press or the like.

[0002]

2. Description of the Related Art In manufacturing a press-formed body made of a thin steel sheet, there is a method of increasing the strength of parts by softening before press-forming and easy to press-forming, and hardening after press-forming.
There is a method of baking at a temperature lower than 200 ° C. A BH steel plate has been developed as a steel plate for coating and baking.

For example, in an Nb-added steel as disclosed in JP-A-55-141526 and JP-A-55-141555, Nb is added in accordance with the content of C, N and AI in the steel, and the steel is added in at%. Nb / (Solute C + Solute N) is limited to a specific range, and the cooling rate after annealing is controlled to adjust the solute C and solute N in the steel sheet. No. 45689 discloses that bake hardenability is improved by adding Ti and Nb in combination.

[0004] However, since the above-mentioned steel sheet is made of a material having excellent deep drawability, the strength of the steel sheet is low, and is not always sufficient as a structural material.

[0005] Further, as disclosed in Japanese Patent Application Laid-Open No. H5-25549, it is disclosed that the bake hardenability is improved by adding W, Cr and Mo alone or in combination to steel. However, the increase in strength due to bake hardening is due to the use of solid solution C and solid solution N contained in the steel sheet. For example, as schematically shown in the stress-strain curve of the dashed BH steel sheet in FIG. It only increases the yield strength of the material (the stress increase for BH in FIG. 1), but does not increase the tensile strength (tensile strength). For this reason, there is only an effect of increasing the stress at which the deformation of the member or component is started, and the effect of increasing the stress required for deformation throughout the entire region from the start of deformation of the member or component to the end of deformation (hereinafter, referred to as deformation strength characteristic) is as follows. Not always enough.

As a curing method other than the baking of the press-formed body, there is a method of soft nitriding after press-forming.
For example, as disclosed in Japanese Patent Application Laid-Open No. 2-80539, a method in which a nitride-forming element such as Cr, Al, or V is contained in steel so as to increase the strength by nitriding,
As disclosed in Japanese Patent Application Laid-Open No. 5 (1999) -2005, a method is disclosed in which the heat of nitriding treatment is used to precipitate and harden Cu to increase the hardness of a member.
However, these methods have the drawback that the heating temperature is higher than 450 ° C., and when ordinary zinc plating is used to increase the corrosion resistance, the plating layer evaporates and it is difficult to obtain a material having good corrosion resistance.

[0007]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention has a strength level of 3 before working.
00Mpa class, 400Mpa class, 500Mpa class, 60
It is easy to process and form by press forming etc. with relatively soft high-strength steel of 0Mpa class and 700Mpa class, and by performing a short-time heat treatment at a relatively low temperature for the purpose of increasing the strength after forming such as press forming, An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet as an elemental steel sheet capable of increasing the tensile strength or hardness to enhance the deformation strength characteristics of a member or component, or increasing the rigidity.

[0008]

Means for Solving the Problems The present inventors have developed a workability in forming various molding materials and parts made of thin steel sheets subjected to alloyed hot-dip galvanizing, and a heat treatment of members and parts. The inventors conducted intensive research on the heat treatment method for hardening at a temperature and the deformation strength characteristics of a press-formed body as a part made of the steel sheet. As a result, appropriate amounts of C and W in the steel composition,
By adding Cr and Mo in combination, further specifying the microstructure of the steel, and performing hot-dip galvanizing and heat alloying treatment, it is possible to shorten the time when heated to a temperature range of 200 to 450 ° C after forming. It has been newly found that a high tensile strength (or an increase in deformation strength characteristics) can be obtained by the method of the present invention. The present invention has been accomplished based on this finding.

The gist of the invention is as follows.

(1) C: 0.01 to 0.1% by weight
08%, Si: 0.005 to 1.0%, Mn: 0.01
To 3.0%, P: 0.001 to 0.15%, S:
0.001 to 0.02%, Al: 0.001 to 0.1
%, N: 0.0002 to 0.01%, and W, C
r, one or more of Mo, the total amount is 0.05 to
After hot-rolling a steel containing 3.0% and the balance consisting of iron and unavoidable impurities, hot-dip galvanizing was performed without performing cold rolling, followed by heat-alloying. Of manufacturing an alloyed hot-dip galvanized steel sheet having a post-forming strength increase heat treatment performance.

(2) As a steel composition, T
i: 0.005 to 0.1%, Nb: 0.005 to 0.1
%, V: production of an alloyed hot-dip galvanized steel sheet having a post-forming strength increase heat treatment performance according to (1), wherein one or more of 0.005 to 0.1% of V is contained. Method, (3) C: 0.01 to 0.1% by weight.
08%, Si: 0.005 to 1.0%, Mn: 0.01
To 3.0%, P: 0.001 to 0.15%, S:
0.001 to 0.02%, Al: 0.001 to 0.1
%, N: 0.0002 to 0.01% and W, Cr,
Mo, or two or more, in a total amount of 0.05 to 3.
After hot-rolling a steel containing 0% and a balance of iron and unavoidable impurities, temper rolling or cold rolling is performed, and then annealing is performed at a temperature of 650 to 900 ° C. A method for producing an alloyed hot-dip galvanized steel sheet having a post-forming strength increase heat treatment performance, characterized by performing plating and heat alloying treatment.

(4) As the steel composition, T
i: 0.005 to 0.1%, Nb: 0.005 to 0.1
%, V: production of an alloyed hot-dip galvanized steel sheet having a post-forming strength increase heat treatment performance according to (3), wherein one or more of 0.005 to 0.1% of V is contained. The way.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have proposed a method of imparting deformation strength characteristics to members and parts while ensuring workability such as press formability of members and parts, by using a steel sheet composition, a steel sheet manufacturing method, and the like.
After extensive research on the heat treatment method and formability (especially press formability), steel sheets containing Cr, W, and Mo, which are carbide-forming elements with low affinity for C, were subjected to hot-dip galvanizing and heat alloying. After that, it is processed by a press molding method giving a strain of 2% or more, and thereafter, 200 to 45%
It has been found that by performing a heat treatment at 0 ° C., the deformation strength characteristics of the alloyed galvanized steel sheet are significantly improved without impairing the corrosion resistance. Further, as a steel composition, Ti,
It has been found that the combined addition of Nb and V significantly increases the amount of increase in the deformation strength characteristics (ΔTS).

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the components of steel will be described below.

C is an element that affects the workability of steel, and the higher the content, the worse the workability. Therefore, it is set to 0.08% or less. On the other hand, if less than 0.01%, 2
The lower limit is 0.01% because the amount of carbide precipitated during the heat treatment at 00 to 450 ° C. is small and the effect of increasing the strength during the heat treatment is small.

If the content of Si is less than 0.005%, the production cost is dramatically increased and it is not economical.
% Is defined as the lower limit, and if it exceeds 1.0%, workability is deteriorated, zinc is hardly adhered and the adhesion of zinc plating is impaired, so the upper limit is 1.0%.

If the Mn content is less than 0.01%, the production cost rises dramatically and becomes uneconomical, so the lower limit is 0.01%, and if it exceeds 3.0%, the workability deteriorates.
3.0% is the upper limit.

P is an element capable of increasing the strength without impairing the deep drawability, and is added according to the strength level.
If the content is less than 001%, the production cost is dramatically increased and it is not economical.
If it exceeds 5%, the problem of secondary working brittleness occurs, so the upper limit is 0.15%.

S is essentially an element that is meaningless to be present in steel, so it is better to reduce it. However, if it is less than 0.001%, the production cost will increase dramatically and it will not be economical. % Is defined as a lower limit, and if it exceeds 0.02%, red hot brittleness occurs during hot rolling, and so-called hot brittleness occurs at the surface, so 0.02% is set as an upper limit.

Al is usually added as a deoxidizing component to prevent the occurrence of defects such as blowholes.
% Must be added. If it is less than 0.001%, it cannot be sufficiently deoxidized, so the lower limit is 0.001%. Also,
If it exceeds 0.1%, the effect of deoxidation is saturated.
% As the upper limit. Further, in order to reduce press cracking by further controlling the inclusion by adding Ti, Al is added in an amount of 0.1%.
001 to 0.005% is preferred.

N is preferably as small as possible in order to ensure workability, but if it is less than 0.0002%, the production cost will increase dramatically and it will not be economical. %, The workability deteriorates, so the upper limit is 0.01%.

In order to exhibit the effect of increasing the tensile strength at the time of a predetermined low-temperature heat treatment after working and forming, one or more of Mo, W, and Cr as essential elements in addition to the above-mentioned elements are further required. , The total amount of Mo, W, and Cr is 0.05 to 3.
It is contained in 0% steel. If the total amount is less than 0.05%,
Even if the above-mentioned heat treatment is performed, an increase in tensile strength cannot be expected sufficiently. When the total amount exceeds 3.0%, Mo,
W and Cr strengthen the steel too much, and the strength before working and forming becomes too high, impairing the workability, and the effect of increasing the tensile strength even if the heat treatment is performed after the working is small or saturated. This is economically inconvenient.

When a steel sheet containing Cr, W, and Mo is subjected to processing such as press forming to give a strain of 2% or more and subjected to a heat treatment at 200 to 450 ° C., the tensile strength of the steel sheet increases. . The reason for the increase in tensile strength when a predetermined amount of Cr, W, and Mo is added to steel and this heat treatment is performed is not clear. However, by applying strain of 2% or more, a considerable amount of ferrite portion in the steel sheet is added. Dislocations are introduced, C, Cr, W, M
The diffusion of o in steel is dramatically increased even at low temperatures, and Cr, W, and Mo
The present inventors believe that since carbide is precipitated, carbide is precipitated in a short time at low temperature to increase the tensile strength of members and components.

In addition, in order to increase the tensile strength during the heat treatment after forming, it is possible to add Ti, Nb and V as selective elements in addition to Cr, W and Mo described above. It is not clear why the effect of increasing the tensile strength and hardness of the member or component containing Ti, Nb, V is increased, but when Ti, Nb, V is added, the aforementioned Cr,
It exerts a synergistic effect with the action of the low-temperature heat treatment by the addition of W and Mo, and forms fine carbides in the steel sheet. The present inventors consider that the effect of increasing the amount appears. In particular, V is added, and Ti,
It is preferable to add one or two types of Nb since the post-working treatment can be shortened.

Ti is an element that enhances the effect of increasing the strength during heat treatment. If the content is less than 0.005%, the effect is small, so the lower limit is 0.005%. Also, T
i is an element that increases the strength of the steel. If it exceeds 0.1%, the workability deteriorates, so the upper limit is 0.1%.

Nb is an element that enhances the effect of increasing the strength during heat treatment. If the content is less than 0.005%, the effect is small, so the lower limit is 0.005%. Also, N
b is an element that increases the strength of steel, and if it exceeds 0.1%, the workability deteriorates. Therefore, the upper limit is set to 0.1%.

V is an element that enhances the effect of increasing the strength during the heat treatment. If it is less than 0.005%, the effect is small, so the lower limit is 0.005%. Further, V is an element that increases the strength of steel, and if it exceeds 0.1%, the workability is deteriorated. Therefore, the upper limit is 0.1%.

Although the components are adjusted as described above, in order to enhance the effect of increasing the strength at the time of heat treatment after forming, it is desirable that the amount of C in the steel sheet be in a solid solution state at the heat treatment temperature. , T, Nb, and V, which are carbide forming elements,
ｉ (48/12) × C [%] + (48/14) ×
N [%]} or less, or Nb amount is {(93/12) ×
C [%] + (93/14) × N [%]} or less, or V amount = {(51 × 4/12/3) × C [%] + (51 /
14) × N [%] or less, or when Ti, Nb and V are added in combination, {Ti [%] × 12/48 + Nb
[%] × 12/93 + V [%] × 12 × 3 / 5５１
It is desirable to add so as to satisfy <C [%] + N [%] × 12/14.

Next, a preferred embodiment of the steel microstructure obtained by the production method of the present invention will be described.

The microstructure of steel is preferably composed mainly of ferrite or ferrite (the microstructure fraction of ferrite in the steel microstructure is 60% or more). Ferrite is soft and excellent in workability, and can accumulate many dislocations in crystal grains when processed. Further, when dislocations are applied, dislocations enter uniformly and the strength of the entire steel sheet can be increased uniformly, so that the microstructure of the steel is mainly ferrite or ferrite. Furthermore, when the remaining structure that is mainly ferrite and is not ferrite is pearlite and / or bainite, stress concentration increases at the interface between the ferrite and the remaining structure, and dislocation can be effectively imparted to the ferrite. In addition, since the pearlite and / or bainite structure is not as hard as the martensite structure, the pearlite and / or bainite structure itself is also deformed and the dislocation amount increases as a whole steel sheet, so that the strength of the steel during heat treatment is effectively increased.

The post-molding strength increase heat treatment performance of the present invention is as follows.
After performing a forming process in which a strain of 2% or more is applied at a plastic equivalent strain, 200 to 450 ° C. (more preferably 220 to 450 ° C.)
370 ° C) for a short time of 1 to 30 minutes (heating after cold forming or temperature holding after warm forming)
, A tensile strength ΔTS (= TS after heat treatment after processing minus TS before processing) comparing the tensile strength before and after processing and forming is 60 M
The strength can be improved by Pa or more (more preferably, 90 Mp or more in ΔTS) or the Vickers hardness ΔHv (= ΔHv after heat treatment after processing−ΔHv before processing) can be increased by 18 or more (more preferably 27 or more in ΔHv) after heat treatment. Shows heat treatment. However, this heat treatment does not require the hardening element (for example, nitrogen or the like) to enter the molded body from the outside positively unlike the nitriding treatment. The hot-rolled steel sheet or cold-rolled steel sheet may be used as the high-strength alloyed hot-dip galvanized steel sheet as the post-processing strength (tensile strength) increase heat treatment material of the present invention, and the sheet thickness is not limited. But 0.4-6mm
Is particularly effective.

In the production of the steel of the present invention, the molten steel adjusted to the above-mentioned components is made into a slab or a slab by continuous casting to form a slab by the ingot ingot method, without heating at high temperature or After the heating, hot rolling is performed. After hot rolling, descaling is performed, hot-dip galvanizing is performed as it is, and then heat alloying is performed to form a high-strength galvannealed steel sheet. There are no particular restrictions on the conditions for hot rolling and winding, and the hot rolling and winding are carried out according to ordinary methods.

Alternatively, after hot rolling, a descaling treatment is performed and cold rolling is performed to obtain a cold-rolled steel sheet. Thereafter, annealing and hot-dip galvanizing are performed, and thereafter, a heat-alloying treatment is performed to obtain a high-strength galvannealed steel sheet. The annealing temperature at this time is 650 to 900 ° C. If the temperature is lower than 650 ° C, recrystallization is not completed and sufficient elongation cannot be obtained.
Is given as the lower limit. If the temperature exceeds 900 ° C., the effect of improving elongation is not only saturated, but also coarse grains are generated and workability is deteriorated. The heating method of the heating alloying treatment is not particularly limited,
Direct heating by combustion gas, induction heating, direct current heating, and the like can be appropriately selected.

After being formed into a high-strength alloyed hot-dip galvanized steel sheet, a steel sheet (dull-finished steel sheet, bright-finished steel sheet, a specific shape Any steel sheet that has been subjected to surface treatment that is usually used as a thin steel sheet, such as a steel sheet having a pattern transferred thereon, a steel sheet having an oil film layer such as a rust-preventive oil or a lubricating oil on the surface, the component range of the present invention. With the steel plate described above, the effects of the present invention can be sufficiently enjoyed.

Then, press forming such as drawing is performed by using the steel plate having the above chemical composition. In performing the press working, in order to impart an appropriate amount of dislocation to the steel sheet, a portion where a strength (tensile strength) or hardness is required is formed by applying a plastic equivalent strain of 2% or more. If the amount of strain is small, the effect of increasing the strength of the present invention cannot be exerted even if post-heating is performed, so the amount of strain applied at the time of pressing is set to 2% or more, preferably 5% or more.
The press forming method is not particularly limited as long as it gives a strain of more than 2%, and there is no problem even if drawing, stretching, bending, ironing, punching or the like is added. FIG. 2 shows the amount of strain during press forming and the amount of increase in tensile strength (ΔT) after press forming and heat treatment.
S) shows the relationship. It can be seen that the amount of increase in tensile strength is remarkable at a strain of 2% or more, preferably 5% or more.

After press molding, heat treatment is performed at a low temperature. At this time, if the heat treatment temperature is less than 200 ° C., the effect of the present invention cannot be exhibited, so the lower limit is 200 ° C., and if it exceeds 450 ° C., the iron-zinc alloying reaction of the galvanized layer proceeds,
Since the iron concentration in the plating layer abnormally increases and the corrosion resistance of the finished component is impaired, the upper limit is set to 450 ° C.

The heat treatment method for heating to a temperature of 200 to 450 ° C. is not particularly limited, and at least the strain imparting portion is heated to 200 to 450 ° C., such as partial high-frequency heating, electric heating, warm bath heat treatment, infrared heating, and hot air heating. Any method may be used as long as it is heated to the above temperature. Figure 3 shows the heat treatment temperature and the increase in tensile strength after heat treatment (ΔTS).
Shows the relationship. ΔTS after molding is even more preferable in terms of rigidity.
The temperature of the post-molding heat treatment of not less than MPa is preferably in the range of 220 to 370 ° C.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Steels having the components shown in Table 2 were melted and continuously cast into slabs according to a conventional method. And 1200 ° C. in a heating furnace
And hot-rolled at a finishing temperature of 880 ° C., wound at 550 ° C., and then pickled to obtain a hot-rolled steel sheet. In addition, a part of the hot-rolled steel sheet had a rolling reduction of 2 shown in Table 2.
-20% temper rolling was performed. The steel plate to be plated has a thickness of 0%, including those not subjected to temper rolling (temper reduction ratio is 0%).
A hot-rolled steel sheet of 8 to 5.1 mm was produced.

Further, a part of the hot-rolled steel sheet was subjected to cold rolling at a rolling ratio of 63 to 85% as shown in Table 2, and a sheet thickness of 0.4 to 85%.
A 3.9 mm cold rolled steel sheet was manufactured. Before the hot dip galvanizing, both steel sheets were annealed at the temperature shown in Table 2 for 60 seconds, and then 830
Recrystallization annealing at 60 ° C. × 60 seconds was performed to obtain a cold-rolled steel sheet.

The obtained hot-rolled steel sheet and cold-rolled steel sheet were processed into JIS No. 5 tensile test pieces, and their mechanical property values (without heat treatment) were evaluated.

Separately, the steel sheet is formed by pressing,
This was a hat-shaped press-formed product shown in FIG. At this time, the wrinkle holding pressure was adjusted, and the vertical
%, And a plastic equivalent strain of 2% was applied to the flat portion B. The parts were placed in a furnace maintained at 250 ° C. for 10 minutes, then air cooled and heated. Vertical part A and vertical part B of the part
A tensile test piece was cut out from the sample and the tensile strength was measured. In the tensile test after press working, the true stress-strain relationship will be measured, so to see the rise in nominal stress,
The plate thickness before press working was converted to the test plate thickness, and the result was defined as the nominal stress.

The above results are also shown in Table 1.

As is evident from Table 1, it is understood that the steel sheet produced by the production method of the present invention has better heat treatment hardening properties.

[0045]

[Table 1]

[0046]

According to the present invention, the strength level is 300 Mpa class, 400 Mpa class, 500 Mpa before working and forming.
Grade, 600Mpa grade, or 700Mpa grade, relatively soft, high-strength steel that can be easily formed by press forming, etc.
Then, by performing a short-time heat treatment at a relatively low temperature after molding such as press molding, the tensile strength or the hardness increases, and the deformation strength of the member or component can be increased, or the rigidity can be increased. An alloyed hot-dip galvanized steel sheet can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a stress-strain curve of a steel sheet according to a method of the present invention and a conventional method.

FIG. 2 is a diagram showing the relationship between the amount of strain imparted by press forming to a steel sheet according to the method of the present invention and the amount of heat treatment hardening.

FIG. 3 is a diagram showing a relationship between a post-molding heat treatment temperature and an increase in tensile strength after heat treatment.

FIG. 4 is a schematic view showing the shape of a hat-shaped press-formed product.

[Explanation of symbols]

 A Wall part B Flat part

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/18 C22C 38/18

Claims (4)

[Claims] C: 0.01 to 0.08 by weight%
%, Si: 0.005 to 1.0%, Mn: 0.01 to
3.0%, P: 0.001 to 0.15%, S: 0.
001-0.02%, Al: 0.001-0.1%, N
: 0.0002 to 0.01%, and W, Cr, Mo
One or two or more of the total amount is 0.05 to 3.0%
Contained, the balance after hot rolling steel consisting of iron and unavoidable impurities, hot-dip galvanizing without cold rolling, and then subjected to heat alloying treatment, characterized by strength after forming A method for producing an alloyed hot-dip galvanized steel sheet having ascending heat treatment performance. 2. The steel composition further comprises, by weight%, Ti:
0.005 to 0.1%, Nb: 0.005 to 0.1%,
The method for producing an alloyed hot-dip galvanized steel sheet having post-forming strength increasing heat treatment performance according to claim 1, wherein one or more of V: 0.005 to 0.1% is contained. 3. C: 0.01 to 0.08 by weight%
%, Si: 0.005 to 1.0%, Mn: 0.01 to
3.0%, P: 0.001 to 0.15%, S: 0.
001-0.02%, Al: 0.001-0.1%, N
: 0.0002 to 0.01% and one or more of W, Cr and Mo in a total amount of 0.05 to 3.0%, with the balance having iron and unavoidable impurities Forming characterized by subjecting steel to hot rolling, temper rolling or cold rolling, then annealing at a temperature of 650 to 900 ° C, hot-dip galvanizing, and heat-alloying. A method for producing an alloyed hot-dip galvanized steel sheet having post-strength heat treatment performance. 4. The steel composition further comprises, by weight%, Ti:
0.005 to 0.1%, Nb: 0.005 to 0.1%,
The method for producing an alloyed hot-dip galvanized steel sheet having a post-forming strength increase heat treatment performance according to claim 3, wherein one or more of V: 0.005 to 0.1% is contained.