JP3047201B2 - Method for producing high-tensile cold-rolled steel sheet with excellent plating and formability - Google Patents

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-tensile cold-rolled steel sheet having excellent plating properties and press formability useful as a steel sheet for automobiles and the like.

[0002]

Recently, the demand for high-tensile steel plate is rapidly high pine as the body weight and its inner and outer plate members a demand for ensuring safety for improved fuel economy of automobiles. Needless to say, the steel sheet must satisfy not only the high tensile strength but also the high press formability required for an automotive steel sheet such as deep drawability. Good press formability of a high-strength steel sheet is indicated by characteristics such as a low yield ratio, a high elongation, a high rank ford value, and a high work hardening index. The conventional method for producing a high-tensile steel sheet having excellent press formability is to suppress the amount of C and N to an extremely small amount of about 150 ppm or less, and to fix C and N in the form of a carbonitride such as Ti or Nb. By reducing the amount of dissolved C and N as much as possible, good press formability is ensured, while P, S
It is common to add a solid solution strengthening element such as i, Mn, etc., and to obtain a required strength by the solid solution strengthening action (for example, Japanese Patent Application Laid-Open Nos. 2-149462 and 2-1732).
No. 42, Japanese Patent Application Laid-Open No. 2-175758, Japanese Patent Application Laid-Open No. 3-283
25, JP-B-56-34625, JP-B-57-58
427, JP-B-61-54089, JP-B-62-3
No. 5463, No. 2 15609, No. 2 15
612).

[0003]

As described above, Si, M
A high-strength steel sheet obtained by a conventional manufacturing method for increasing strength by adding a solid solution strengthening element such as n or P has the following problems. That is, when Si and Mn are added in large amounts,
That is, the wettability of the steel sheet to hot-dip galvanization is deteriorated, and the processability in alloying treatment for improving the corrosion resistance, weldability, paintability and the like of the galvanized layer is also reduced. The poor wettability of the plating is due to the formation of an oxide film of Si and Mn on the surface of the steel sheet in the annealing treatment of the steel sheet (cold rolled steel sheet) performed before the hot-dip coating, and the alloying property is reduced. This is because the oxide film acts as a barrier layer and prevents mutual diffusion between Fe atoms of the steel sheet and Zn atoms in the plating layer. Also, with the increase in the amount of P added, the interdiffusion between Fe atoms and Zn atoms in the alloying treatment is suppressed, resulting in a marked decrease in alloying treatment properties.

[0004] Hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets are steel sheets that have occupied a great demand as rust-preventive steel sheets having good corrosion resistance against corrosion caused by salt damage of automobiles. Alloying property) is an important property for steel sheets for automobiles. However, when the plating wettability is low as described above, it not only hinders the smooth operation of the hot-dip galvanizing line operation, but also makes it difficult to ensure the stable rust prevention function. Furthermore, in the case of steel sheets with low alloying property, a long time treatment is required to achieve a predetermined alloying reaction in the alloying furnace in the hot-dip galvanizing line, and the operability of the hot-dip galvanizing line is greatly reduced. In addition, the amount of heat supplied to the alloying furnace (fuel consumption) is significantly increased, and the cost is unavoidable.

Accordingly, the present invention is to solve the problems associated with the addition of a large amount of the above-mentioned elements by a strengthening mechanism different from that of the conventional high-strength steel sheet which relies on the solid solution strengthening effect by the large addition of Si, Mn, P and the like. A new method for manufacturing a high-tensile cold-rolled steel sheet that achieves the required high strength while avoiding it and has good plating properties (wetting properties and alloying treatment properties) and press formability such as deep drawability. What you want to do.

[0006]

The method for producing a high-tensile cold-rolled steel sheet according to the present invention is as follows: C: 0.0025 to 0.006%, Si: 0.2% or less, Mn: 0.25 to 0.8. %, P: 0.01% or more,
Less than 0.03%, S: 0.01% or less, Cu: 0.1 to
0.2%, Al: 0.02 to 0.1%, N: 0.002
0.005%, Ti: 0.01 to 0.02%, and Ti ≧ 3.42 × N (%), Nb: 0.025 to 0.0
6%, Nb ≧ 7.75 × C (%), the balance being Fe and unavoidable impurities, and the NbC precipitation start temperature is A ₃
Hot rolling of the steel below the transformation point is completed at 900 ° C. or more, and after descaling, cold rolling is performed at a rolling rate of 70 to 95%. Then, the recrystallization starting temperature to the temperature just below the A ₃ transformation point By annealing in the zone, almost all of N in steel is reduced to T
It is fixed as iN, and N in a temperature range below A ₃ transformation point
It is characterized in that substantially all of b and C are precipitated as NbC in the α phase. All percentages indicating the contents of the component elements are% by weight. The present invention relates to Ti, Nb, N and C
To fix the total amount of each element as TiN and NbC
The intention is that the mutual content and solubility product of each element
Reaction between C and Ti (generation of TiC)
Or the reaction between N and Nb (generation of NbN) is inevitable
May accompany. "Almost all N in steel is TiN
”And“ NbC almost as much as NbC ”
"Almost" refers to such a case.
Therefore, the generation of TiN and NbC is actively suppressed.
Or some of these elements remain intentionally in solid solution
It does not mean to let it.

[0007]

[Action] In the present invention method, Nb precipitates in a temperature range below A ₃ point of the steel (alpha-phase temperature range) as fine NbC.
A NbC precipitated in the temperature range exceeding ₃ points (γ phase temperature range)
Easily coarsened, whereas reinforcing action of the steel is less, A ₃
NbC precipitated in a temperature range below the point is fine, and greatly contributes to the strengthening of steel by its dispersion strengthening action.

The present invention, Nb in a temperature range exceeding the _three points A
Prevents C precipitation, in order to sufficiently precipitate fine NbC in the following α phase region ₃ points A, C content 0.0025 to 0.0
06%, the Nb amount is 0.025 to 0.06%, the atomic equivalent ratio to C is 7.75 × C (%) or more, and the Ti amount is 0.01 to 0.02%. In addition, the atomic equivalent ratio to N is set to 3.42 × N (%) or more.
The strength of the tendency to form carbonitrides (forming ability) of Ti and Nb is in the order of TiN>TiC> NbC, and the precipitation start temperature of NbC in steel adjusted to the above composition range is as follows:
A ₃ points or less (α phase temperature range). In the formation of carbonitrides in this steel, first, the precipitation reaction of TiN due to the bond between Ti and N proceeds preferentially, whereby N in the steel becomes almost completely T
Fixed as iN. Since most of Ti is also consumed in the reaction with N, the reaction with C (decrease of C due to generation of TiC) is very small, and C in the steel is mainly consumed in the reaction with Nb (formation of NbC). Is done. Because the precipitation starting temperature of NbC are the following _three points A, Nb is almost all is precipitated in α phase as fine NbC strengthen steel, and NbC
The fine precipitation of ferrite causes finer ferrite grains. That
Refinement of ferrite grains also contributes to the strengthening of the steel.

In addition, in the cold rolling step, the plastic working with a high rolling reduction of 70% or more is performed, whereby the crystal grains are sufficiently refined, and the effect of the refinement further increases the strength of the steel.

FIG . 3 shows fine NbC in the temperature range below the A ₃ point .
Amount of Nb precipitated as steel and steel due to its fine precipitation
This shows the relationship with the increase in the strength of the specimens.
Ultra low carbon steel with Nb and C added based on
Hot rolling finishing temperature: 910-940 ° C, cold rolling reduction: 80%,
Annealing temperature: 850 ° C]. From the figure, as fine NbC
Increases the precipitation amount of Nb and increases the strength improvement effect.
You can see it. The invention thus almost all the dispersion strengthening effect miniaturization effect of the crystal grains due to the precipitation as fine NbC at temperatures below ₃ points A of Nb in steel, you
And the strengthening effect associated with the refinement of the crystal grains in cold rolling enables the steel to sufficiently achieve high strength without depending on the addition of a large amount of solid solution strengthening elements such as Si, Mn, and P. S
Good plating properties (plating wettability, alloying treatment properties) are secured as an effect of reducing the contents of i, Mn, P, and the like.
As for the formability, as described above, N and C in the steel are almost completely fixed as TiN and NbC, respectively, thereby ensuring sufficient press formability to withstand high working such as deep drawing. ing.

Next, the reasons for limiting the chemical composition of steel in the present invention will be described in detail. C: 0.0025 to 0.006% C combines with Nb to form NbC. The lower limit of the C content was 0.0025 percent, when less than that, insufficient fine precipitation of NbC in A ₃ points below the temperature range (alpha phase region), it is possible to sufficiently enhance the steel Because it is gone. On the other hand, if it exceeds 0.006% deposition starting temperature of NbC exceeds the _three points A, it can not be prevented precipitation in γ-phase region. Precipitation of NbC in the γ-phase region is caused by steel in hot rolling.
Appropriate prediction of hot rolling conditions by greatly changing the rolling load of the material
And its control becomes difficult. Also, the amount of C is large.
In addition, the temperature at which NbC starts to precipitate reaches about 1200 ° C or more.
In such a case, in the slab soaking process performed before hot rolling, N
bC tends to precipitate as coarse particles. Deposition of coarse NbC
Not only does not contribute to the strengthening of the steel, but also contributes to the N
Nb at A ₃ or less effective for strengthening steel due to wear of b
The precipitation amount of C decreases, which causes insufficient strength.
Therefore, the upper limit is 0.006%. Although solid solution C in steel lowers deep drawability, in the present invention, since C is almost completely fixed in the form of NbC, there is no substantial influence on deep drawability.

Si: 0.2% or less Si is a deoxidizing element and is effective for strengthening steel as a solid solution strengthening element. Cause a drop in gender,
The upper limit is 0.2%.

Mn: 0.25 to 0.8% Mn has a desulfurizing action and converts S, which is a harmful impurity in steel, to M
Detoxifies as nS and is effective in preventing red-hot brittleness of steel.
Also, by fixing S as MnS, Ti
Consumption (formation of sulfide) is suppressed. To get these effects
To at least 0.25%. For increasing Mn
These effects are further increased, and the solid solution of Mn
Strength is also increased. However, when the amount becomes too large,
0.8% is made the upper limit because it deteriorates the wettability of galvanized and the alloying property.

P: 0.01 % or more and less than 0.03% P suppresses the alloying reaction of hot-dip galvanizing, lowers the operability of the plating line, and adversely affects spot welding. To avoid these adverse effects, the content is set to less than 0.03% . On the other hand, P is a solid solution strengthening element having a large strengthening ability of steel, and has a smaller effect on deep drawability than Si and Mn, and is useful for strengthening steel. did.

S: 0.01% or less S is a harmful impurity, and secures deep drawability ,
Low in terms of suppression and prevention of consumption (formation of sulfide)
However, if the content is 0.01% or less, a substantial adverse effect can be avoided, so that the upper limit of 0.01% is allowed to be mixed.

Cu: 0.1 to 0.2% Cu has an effect of suppressing high-temperature oxidation of steel, improves descalability after heating and hot rolling of a slab, and is annealed. This has the effect of improving the plating properties of the cold-rolled steel sheet. High content of Si and Mn
And easy to generate a large amount of scale.
Effect of Cu in the case where deterioration of copper becomes a problem
Is big. To achieve this effect, at least 0.1% must be added. The effect is increased by increasing the amount of addition, but if it exceeds 0.2%, the effect is almost saturated.
The upper limit is 0.2%.

Al: 0.02 to 0.1% Al needs to be added in an amount of 0.02% or more as a deoxidizing element. However, the addition of a large amount causes reduction in workability of steel, so the upper limit is 0.1%.

N: 0.002 to 0.005% The lower the N content, the more advantageous the steel in improving ductility and formability. In the present invention, the harmful effects of N are prevented by fixing N as TiN.
iIn order to reduce the amount of addition, the upper limit of the amount of N is 0.005%
And On the other hand, the reason why the lower limit is set to 0.002% is that the cost burden in the steel making stage required for setting the N content to be less than that significantly increases. Substantial effects on workability were to be prevented.

Ti: 0.01-0.02%, 3.42 ×
N (%) or more Ti is an element that improves the ductility and formability of steel by fixing N in steel as TiN and reducing the amount of solute N. The addition amount is set to 0.01% or more and the atomic equivalent ratio to N is set to 3.42 × N (%) or more because the total amount of solute N in steel is almost completely fixed as TiN. On the other hand, the upper limit was set to 0.02% because of the relationship with the allowable upper limit of the amount of N, when a large amount of Ti exceeding that is added, the amount necessary for fixing solid solution N in steel is reduced. Excess and excess Ti will combine with C, resulting in Nb
This is because the amount of C bonded to the steel decreases and the amount of NbC precipitated becomes insufficient, whereby the strengthening of the steel by the dispersion strengthening action of NbC becomes insufficient.

Nb: 0.025-0.06%, 7.75
× C (%) or more Nb is the most important element in the present invention, it has a role of increasing the strength of steel by fine precipitation as NbC in a temperature range of below ₃ points A. Was the Nb content is 0.025% or more, the amount of less, the precipitation amount of NbC to fine precipitation at a temperature range of less effective A ₃ points to strengthen the steel is insufficient, the reinforcing effect of the steel Because it is not enough.
In addition, the addition amount is expressed as an atomic equivalent ratio to the C amount,
Weighting the requirement of 7.75 x C (%) or more
This is because the entire amount of the solid solution C is almost completely fixed as NbC in order to prevent a decrease in ductility and formability of the steel due to the remaining solid solution C. On the other hand, the upper limit of the Nb amount is 0.06.
% That is given by is to the precipitation starting temperature of NbC than _three points A. FIG. 1 is a graph showing the relationship between the precipitation rate (%) of NbC and temperature with the amounts of Nb and C as parameters (test material: an ultra-low carbon steel to which Nb and C are added based on mild steel, A _3-point: 890~900 ℃). When a large amount of Nb and C are added as in the test material i, NbC
Deposition starting temperature is a temperature range exceeding the _three points A (gamma-phase region). As described above , precipitation of NbC in the γ-phase region is caused by hot strip.
Only make it difficult to properly predict and control
Not, because NbC in γ phase region to coarsely precipitate, do not contribute to strengthening of the steel, so it only a wasteful consumption of Nb. On the other hand, the test materials ii to iv having the Nb content of 0.06% or less are:
Not substantially NbC in a temperature range exceeding the _three points A precipitation, substantially the whole amount is precipitated by the following α phase region ₃ points A. That is, the added Nb precipitates as fine grains effective for strengthening the steel without waste. The Nb content exceeds 0.06%
As a result, the recrystallization temperature rises and the operability of continuous annealing
The present invention, in which the amount is reduced to 0.06% or less.
Does not cause such inconvenience.

Next, the hot rolling and the subsequent steps in the present invention will be described. The steel having the above component composition is formed into a slab by continuous casting or the like, and is subjected to hot rolling. Its hot no special conditions and restrictions on the rolling, terminates the hot rolling at γ phase temperature region exceeding the Supporting connexion A ₃ points a conventional method. The reason for performing the hot rolling in the γ phase temperature range is to ensure the uniformity of the material of the final product steel material, and the hot rolling is performed in the α + γ two phase temperature range in which the α phase is partially generated in the steel. And a mixed grain structure, resulting in unevenness in the material properties of the final product steel sheet, especially in formability. The A ₃ point of the steel in the present invention is 860 to 90
Since the temperature is 0 ° C., it is preferable to set the hot rolling end temperature to 900 ° C. or higher in order to ensure the homogeneity of the steel sheet of the final product.

The winding temperature of the steel sheet after the completion of hot rolling affects the amount of surface scale generated. If the coiling is performed at a high temperature exceeding 700 ° C., a large amount of scale is generated, and the pickling property in the subsequent pickling treatment step deteriorates.
Winding is preferably performed at a temperature of not more than ℃.

After the pickling treatment, cold rolling is performed. FIG.
It is the graph which showed the change of the tensile strength of the steel plate according to the rolling reduction in cold rolling.
Same as 1). As shown, the tensile strength of the steel increases with an increase in the cold rolling reduction, and the strength of the steel is increased. This strengthening of the steel occurs as a crystal grain refinement effect due to the high cold rolling reduction. The present invention sets the cold rolling reduction to 70% or more,
Improvement of the strength due to the effect of refining the crystal grains,
Under the effect of strengthening dispersion by fine precipitation of C, Si, M
By excluding a large amount of solid solution strengthening elements such as n and P, a sufficiently high tensile strength of the product steel sheet is achieved. Although the upper limit of the cold rolling reduction is not essentially required, it is appropriate to set the upper limit to about 95% from the viewpoint of facilities and operation of the cold rolling mill.

The steel sheet after cold rolling is subjected to an annealing treatment for the purpose of removing distortion, softening, non-aging, and the like. The annealing treatment is performed by heating and maintaining the temperature in the temperature range from the recrystallization initiation temperature (the temperature of the steel in the present invention is about 750 to 800 ° C.) to just below the A ₃ transformation point of the steel. The lower limit of the processing temperature as that recrystallization starting temperature, to achieve sufficiently soft reduction and non-aging of a steel sheet, and to secure a high degree of press formability, it was made the upper limit just below A ₃ transformation point Phase transformation from α phase to γ phase occurs at higher temperatures
However, this is because the material is deteriorated.

When performing galvanizing using the cold-rolled steel sheet of the present invention obtained through the above steps, the plating operation is as follows:
What is necessary is just to carry out according to a usual method, and a healthy plating quality can be ensured under high efficiency. Also, after the plating process,
If necessary, temper rolling (skin pass) for adjusting the plating surface, correcting the shape of the steel sheet, or eliminating the elongation at the yield point, etc. is appropriately performed, similarly to the normal manufacturing process.

[0026]

EXAMPLES [I] Production of test steel sheet Table 1 shows the chemical composition of the test steel. Nos. 1 to 4 are invention examples,
Nos. 101 to 105 are typical examples of conventional materials having high strength by solid solution strengthening elements, and No. 106 has a small amount of Si, Mn and P as in the invention example. However, this is an example in which the added amount of C, N, Ti, Nb, etc. is out of the range of the present invention. * In the table indicates that the content of the element is out of the range of the present invention.
In Table 1, the “a-precipitated Nb amount” refers to Ti, N,
Determined from the chemical equivalents of Mn, S, Nb, etc. and the amounts added
No. 102 and No. In 106,
Although it contains Nb, it is N and
Excess Ti after bonding combines with C, and the amount of C is small.
Because there is no, the total amount of C reacts with Ti (formation of TiC)
And there is no reaction between Nb and C as a result.
The amount of generation of fine NbC at this time is zero. For each sample steel having the chemical composition shown in Table 1, to exit the hot rolling at a temperature region exceeding the _three points A, to obtain a hot rolled steel sheet of the winding connexion thickness 3.2mm at 600 ° C., after pickling And cold-rolled (rolling ratio 75%) to obtain a cold-rolled sheet having a thickness of 0.8 mm, and then subjected to an annealing treatment (air atmosphere: 45% H ₂ -55% N ₂ mixed gas, processing temperature: 850 ° C.). Was given. The above cold-rolled steel sheet was used as a plating base sheet, and supplied to a continuous hot-dip galvanizing line, and zinc-plating (adhesion amount: 60 g / m) in a hot-dip zinc bath (free Al: 0.12%).
^2. One side), and then fed to the alloying furnace, adjusting the line speed and the burner fuel flow rate (fuel: natural gas) of the alloying furnace so that the Fe concentration in the plating layer becomes 10%. Alloying treatment, followed by temper rolling (elongation: 0.5%).

[II] Characteristics Table 2 shows the mechanical properties of each of the test steel sheets, and Table 3 shows the plating properties.
Shown in In Table 3, “plating surface” indicates the presence or absence of non-plating and the degree thereof (good or poor plating wettability). The “line speed (mpm)” and “fuel flow rate (Nm ³ / h)” of the “alloying conditions” indicate the quality of alloying processability. The higher the line speed and the lower the fuel flow rate, the higher the alloying processability. Means good.

(1) Mechanical Properties As shown in Table 2 (mechanical properties), the examples of the invention include Si,
It has high strength equal to or higher than No. 101 to 105, which is a conventional material to which a large amount of solid solution strengthening elements such as Mn and P are added, and also has high elongation and rank ford values, and is excellent in deep drawability and the like. It turns out that it has moldability. Its high strength is due to the fine precipitation of NbC in A ₃ points below the temperature range, also good press formability, N and C in the steel, due to which is fixed almost entirely as TiN and NbC Things. Comparative Example No. 10
No. 6 is an example in which a large amount of Si, Mn, and P was added in the same manner as the invention example, but the strength was insufficient because the contents of C, N, Nb, and Ti were out of the range of the present invention. ing.

(2) Plating property As shown in Table 3, the invention example has a sound plating layer without any non-plating, and has a Si, Mn, P It can be seen that the alloy has a good alloying treatment property which greatly exceeds No. 101 to 105, which is a conventional material to which a large amount of such elements are added. The reason why non-plating was remarkably generated in the steel sheet of No. 101 was that the wettability of the plating was reduced by the oxide film formed during annealing because of the large amount of Si and Mn, and the alloying was performed. The poor processability is also attributable to the fact that the oxide film acts as a barrier to hinder the interdiffusion between the Fe atoms of the steel sheet and the Zn atoms in the plating layer. No. 102 has a smaller amount of Si and Mn than No. 101, so the degree of unplating is
o. Lighter than 101, but contains a large amount of P,
Interdiffusion between Fe atoms of the steel sheet and Zn atoms in the plating layer is hindered, and as a result, a long time and a large amount of fuel consumption is required for the alloying treatment. For Nos. 103 to 105, the amounts of Si and Mn are the same as those of Nos. 101 and 101, respectively.
102, the same level as that of the invention example, the wettability of the plating is good and the plating surface is good, but since it contains a large amount of P, the alloying treatment property is poor compared to the invention example. , It takes a long time to process it. No.106
Has almost the same plating properties as the invention example as an effect of suppressing the added amounts of Si, Mn, and P, but is inferior to the invention example in that the strength is low as described above.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

According to the method of the present invention, Si, M can be obtained by the effect of fine precipitation of NbC and grain refinement by cold rolling.
It is possible to increase the strength of the steel sheet while minimizing the amount of the solid solution strengthening element such as n, P, etc., and to ensure a high degree of press formability. Good hot-dip galvanizing property (plating wettability, alloying property) can be secured as an effect of reducing the amount of addition of such as. The effect of improving the plating property is remarkable, and good plating wettability enables prevention of non-plating and guarantees the soundness of the plating surface. Significantly (reducing the speed of the line) and drastically reducing the fuel consumption in the alloying furnace, thus improving the quality as a manufacturing method for high-tensile cold-rolled steel sheets used as automotive steel sheets. -It has a great effect on cost reduction.

[Brief description of the drawings]

FIG. 1 shows the relationship between the deposition rate of NbC and temperature, Nb and C
4 is a graph showing an amount as a parameter.

FIG. 2 is a graph showing a relationship between a cold rolling reduction and a tensile strength of a very low carbon steel containing Ti and Nb.

3 is a graph showing the relationship between the increase of Nb content and the tensile strength of the fine precipitates as NbC in A ₃ points below the temperature range.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-246128 (JP, A) JP-A-59-143047 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C22C 38/00-38/60

Claims (1)

(57) [Claims] 1. C: 0.0025 to 0.006%, S
i: 0.2% or less, Mn: 0.25 to 0.8%, P:
0.01 % or more and less than 0.03% , S: 0.01% or less, Cu: 0.1 to 0.2%, Al: 0.02 to 0.1
%, N: 0.002 to 0.005%, Ti: 0.01 to
0.02% and Ti ≧ 3.42 × N (%), Nb:
0.025 to 0.06%, and Nb ≧ 7.75 × C
(%) Ri Do the balance Fe and unavoidable impurities, NbC
Deposition starting temperature of the A ₃ transformation point der Ru steel, 900 ° C.
The hot rolling is completed as described above, and after the descaling process, the rolling reduction is 7
0-95% cold rolling is performed and then the recrystallization starting temperature ~
By annealing at a temperature range just below the A ₃ transformation point, steel
Almost all of the solution of N is fixed as TiN, and A ₃ strange
Almost the entire amount of Nb and C in the temperature range below the state point as NbC
A method for producing a high-tensile cold-rolled steel sheet having excellent plating properties and formability, characterized by being precipitated in an α phase .