JPH0559491A - High tensile thin steel sheet for press working and its manufacture - Google Patents

Abstract

PURPOSE:To provide a high tensile thin steel sheet good in ductility, deep drawability and secondary working brittleness resistance. CONSTITUTION:A dead soft steel is mixed with substituting solid soln. elements such as Si, P and Mn and is compositely mixed with ferrite stabilizing elements and austenite stabilizing elements in the range of the following formula; (Mn+Ni+o,5Cr)/(Si+0.7Al+7P+3Mo)= 0.5 to 2(Mn+Ni+0.5Cr)(Si+0.7Al+7P+3Mo)= -1 to +1%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel sheet used by being pressed into various shapes, particularly a high-strength steel sheet having good ductility, deep drawability and resistance to secondary work brittleness, and a method for producing the same. Regarding

[0002]

2. Description of the Related Art When the industrial technology field is highly divided into labor as in today's world, the requirements for materials used in each technology field are also specialized and sophisticated, and, for example, high strength is required for press-formed steel sheets. As a result, it has been attempted to use a high-strength thin steel sheet. However, ductility
It is considered that the press formability such as deep drawability and the strengthening of the steel sheet are contradictory properties, including the secondary work brittleness resistance.
It is difficult to realize these characteristics to the extent that they are practically satisfied.

Until now, deep drawing in which Si, Mn, Cr, and P have been added to increase the strength based on ultra-low carbon steel to which Ti or Nb has been added after sufficient decarburization at the steelmaking stage There are many proposals for high-strength steel sheets for business. However,
They cannot both improve strength and press formability to the extent particularly recently required.

For example, Japanese Examined Patent Publication No. 57-57945 discloses a method for producing a high-strength cold-rolled steel sheet in which a large amount of P is added to the ultra-low carbon Ti-added steel. Furthermore, no mention is made of secondary work brittleness. The maximum tensile strength of the high-strength steel sheet produced thereby is about 50 kgf / mm ² , and the r value is limited to 1.6 to 1.9.

Further, in Japanese Examined Patent Publication No. 59-42742, the ultra-low carbon Ti-added steel is mainly composed of P, and Mn and S are further added.
Disclosed is a method for producing a high-strength cold-rolled steel sheet, which is characterized by adding a strengthening element such as i, Mo, or Cr and adding B to prevent secondary work embrittlement peculiar to IF steel. But,
Also in this case, the maximum tensile strength is about 50 kgf / mm ² , and the r value is
There is only about 1.6.

[0006]

[Problems to be Solved by the Invention] Si, P,
It is conventional wisdom that ductility and deep drawability deteriorate as the strength of a steel sheet is increased by adding a substitutional solid solution element such as Mn. Therefore, assuming a tensile strength of 50 kgf / mm ² , the r value is
The current state of the art is that it can only be improved to about 1.6.

Here, a general object of the present invention is to provide a high-strength steel sheet having a ductility superior to that of a conventional high-strength steel sheet and maintaining the deep drawability comparable to that of ultra-low carbon Ti-added mild steel, and a method for producing the same. Is to provide. Here, the specific object of the present invention is to maintain the deep drawability of ultra-low carbon Ti-added mild steel and the tensile strength of 45
kgf / mm ² or more, r value is 1.3 or more, TS × El
= 1600 kgf / mm ² ·% or more high tensile strength thin steel sheet and its manufacturing method.

[0008]

[Means for Solving the Problems] As a result of research to clarify the cause of deterioration of deep drawability when only Si or Mn is added as a strengthening element to ultra-low carbon steel, the following has been found. (1) When a ferrite stabilizing element such as Si is added alone, the α → γ transformation point of the steel increases, and a part or the whole of the steel strip is rolled in the ferrite region during the hot rolling process. Therefore, the Goss orientation ({110} <011
>) A nearby texture develops. This orientation disappears when cold-rolled, but reappears at the time of subsequent recrystallization and significantly impairs deep drawability.

(2) When an austenite-stabilizing element such as Mn is added alone, the α → γ transformation point decreases, and under normal hot rolling conditions, the temperature between the hot rolling completion temperature and the transformation point is reduced. Since the temperature difference is large, the austenite crystal grain size is large, and the ferrite crystal grain size of the hot-rolled sheet is large. Further, the temperature difference between the Ar ₃ transformation point and the Ar ₁ transformation point becomes large, and the transformation itself proceeds slowly, so that the ferrite crystal grain size after transformation is further increased. The presence of such coarse-grained ferrite crystal grains hinders the development of a recrystallization texture that is favorable for deep drawability after cold rolling and annealing.

(3) When adding substitutional solid solution elements such as Si, P, and Mn to ultra-low carbon steel to strengthen the steel sheet, a ferrite stabilizing element and an austenite stabilizing element are added together,
By controlling the ratio and difference of the addition amount, it becomes possible to control the transformation point and to increase the strength of the steel without impairing the deep drawability.

(4) Furthermore, if a large amount of P or Mn is added to the ultra-low carbon Ti-added IF steel, the reason is not clear, but Ti
Precipitation of C is suppressed, a trace amount of solute C remains in the steel sheet,
Improves secondary work brittleness resistance.

Therefore, the gist of the present invention is, by weight%, C: 0.0005 to 0.0050%, N: 0.0005 to 0.0100.
%, S: 0.010% or less, acid-soluble Al: more than 0.1% and 4.0% or less, Ti
≧ 48 (N / 14 + C / 12), Si: 3% or less, P: 0.01 to 0.15
%, Mn: more than 0.50% and 4% or less, Ni: 4% or less and / or Cr: 5% or less, if necessary, Mo: 10% or less, and (Mn + Ni + 0.5Cr) / (Si + 0.7Al + 7P + 3M
o) is 0.5 to 2, and (Mn + Ni + 0.5 Cr)-(Si + 0.7Al)
+ 7P + 3Mo) satisfies the relationship of -1 to + 1%, and the balance Fe
It is a high-strength thin steel sheet having a steel composition composed of and unavoidable impurities and having good deep-drawing and good secondary work embrittlement resistance.

According to another aspect of the present invention, a steel slab having the above-mentioned steel composition is cooled without cooling to below the Ar ₃ transformation point.
_{It is characterized in} that hot rolling is performed at ₃ transformation points or more, coil is wound at room temperature to 700 ° C., then descaled, cold rolling is performed at a reduction rate of 50 to 90%, and further recrystallization annealing is performed.
It is a method for producing a high-strength thin steel sheet having excellent deep drawability and secondary working brittleness resistance.

[0014] From yet another aspect, the present invention, among the above-mentioned steel composition, a steel piece having a Si + Mn ≧ 2% of the steel composition, without cooling below Ar ₃ transformation point, Ar ₃ transformation point Hot-rolled as above, coiled at room temperature to 700 ° C, then descaled, cold-rolled at a rolling reduction of 50 to 90%, re-crystallized and annealed, and then the highest in the continuous hot-dip galvanizing line. This is a method for producing a high-strength thin steel sheet having good deep drawability and resistance to secondary work embrittlement, which is characterized by plating at a heating temperature of 550 ° C or lower. As described above, according to the present invention, it is possible to simultaneously achieve high strength and improvement of deep drawability, ductility, and secondary work embrittlement resistance.

[0015]

Next, the reason why the steel composition of the steel sheet according to the present invention is limited as described above will be explained. In this specification, "%" is "% by weight" unless otherwise specified.

C: One of the important constituent factors of the present invention. It is inevitably contained in steel, and the smaller the amount, the better. However, less than 0.0005% cannot be easily and stably achieved by current steelmaking technology. On the other hand, if it exceeds 0.0050%, the required addition amount of Ti increases, which not only leads to an increase in cost, but also the precipitation amount of TiC increases and the ductility is impaired. It is preferably 0.0030% or less.

The lower N: N, the more preferable. However, like C, it is inevitably contained in steel, and current steelmaking technology cannot easily and stably make it less than 0.0005%. On the other hand, if it exceeds 0.0100%, not only the required addition amount of Ti increases and the cost increases, but also the precipitation amount of TiN increases and the ductility deteriorates.

S: Inevitably contained in steel, the smaller the better. If the S content is large, the amount of Mn added is increased correspondingly and the cost is increased, so the content was made 0.010% or less.

Ti: One of the important constituent elements of the thin steel sheet according to the present invention. In order to fix C and N as TiC and TiN, respectively, and to make the steel so-called IF steel (Interstitial Free), 48 × (C / 12 + N / 14)% or more is required. Generally, 0.004 to 0.055% is sufficient. Preferably
It is 0.03 to 0.05%.

Si, P, acid-soluble Al: one of the important constituent factors of the present invention. Si and P are inevitably contained in the steel, and the acid-soluble Al is contained as a part of Al added as a deoxidizing agent in the steelmaking stage in an amount of more than 0.1% and 4.0% or less. In the present invention, any may be selected as the strengthening element, but Si of 3.0% or less and P of 0.010 to 0.150% are particularly effective. The addition amount is restricted by the relationship given by the equations (1) and (2) with the following addition amount of the austenite stabilizing element. If it is out of this range, the variation of Ar ₃ and Ar ₁ transformation points is large, and a recrystallization texture suitable for deep drawability cannot be obtained after cold rolling and annealing. On the other hand, when the respective upper limits are exceeded, the amount of precipitates increases and the composition hardens too much, which impairs ductility.

Mn, Ni, Cr: One of the important constituent factors of the present invention. Mn combines with S to form MnS.
It is necessary to add more than 50%. If it is less than this, TiS precipitates and solid solution C remains in the steel, and it may not be possible to obtain a recrystallization texture suitable for ductility and deep drawability after annealing. The upper limit of Mn addition is 4.0%. Other than 4%
Ni, 5% or less of Cr also has the effect of stabilizing austenite and strengthening steel at the same time.

In the present invention, either Ni or Cr may be selected as a strengthening element. Mn is particularly effective as a strengthening element. The addition amount is restricted by the relationship given by the equations 1 and 2 with the addition amount of the ferrite stabilizing element. If it is out of this range, the variation of Ar ₃ and Ar ₁ transformation points is large, and a recrystallization texture suitable for deep drawability after cold rolling / annealing cannot be obtained. On the other hand, when the respective upper limits are exceeded, the amount of precipitates increases, the composition hardens too much, and ductility is impaired.

[0023]

[Formula 1] (Mn + Ni + 0.5Cr) / (Si + 0.7Al + 7P + 3Mo) = 0.5 to 2

[0024]

[Formula 2] (Mn + Ni + 0.5Cr)-(Si + 0.7Al + 7P + 3Mo) =-1 to + 1% FIG. 1 is a graph showing the range of the present invention represented by Formula 1 and Formula 2. The high-strength steel sheet according to the present invention is particularly for press forming, and therefore has a thickness of, for example, 4 mm.
It is used in the form of the following thin steel sheets. Therefore, although not particularly limited thereto, a thin steel plate of the present invention having a thickness of 2 mm or less is sufficient.

Further, the reason for the limitation and the function of the method for producing a high-strength steel sheet according to the present invention will be described in more detail. After the steel having the above-mentioned steel composition is melted, it is usually made into a billet by a continuous casting method. The billet thus obtained is made of Ar
_It is sent to the hot rolling process of the next process without cooling to ₃ points or less, but even in that case, if necessary, even if the steel slab or its end is heated by a heating furnace or an end heating device, Good.

If necessary, the steel ingot obtained by the ingot-making method may be subjected to slab-rolling to form a steel slab. In that case, after slab-rolling and heating to an Ar ₃ point or higher, the slab is then dropped. It is sent to the hot rolling process without being cooled. The hot rolling step is not particularly limited as long as the rolling end temperature is Ar ₃ points or more, but preferably, from the viewpoint of achieving grain refinement, rolling such that final rolling is performed immediately above Ar ₃ points. Conditions are preferred.

The coiling temperature after hot rolling is set to 700 ° C. or lower because coarse crystal grains are likely to be formed when coiled at a temperature higher than 700 ° C., which leads to deterioration of deep drawability of the annealed sheet. is there. In the case of the steel of the present invention, when rolled at a low temperature, the grain size of the hot-rolled sheet becomes finer, which leads to the improvement of the deep drawability of the annealed sheet. Therefore, it is preferably 500 ° C or lower. Prior to cold rolling, scale removal is carried out. Generally, this may be carried out by pickling, but other conventional means such as mechanical means may be appropriately used.

The reason why the reduction ratio of cold working is 50% or more and 90% or less is to form a recrystallized texture preferable for improving deep drawability after annealing. On the other hand, when the reduction ratio exceeds 90%. Cold rolling at such a high reduction ratio is not normally performed because rolling becomes difficult. Within the above range, it is basically desirable that the rolling reduction is higher. The steel sheet cold-rolled in this way is then subjected to recrystallization annealing treatment, which may be box annealing, continuous annealing or continuous hot dip galvanizing.

The annealing temperature at this time is higher than the recrystallization temperature. In order to obtain deep drawability in an annealed sheet, the annealing temperature should be Ac ₃
Below the transformation point.

Since Si and Mn have a strong tendency to oxidize, Si +
When producing a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, etc. according to the present invention for cold rolled steel sheet with Mn ≧ 2.0%, if it is carried out by an ordinary continuous hot-dip galvanizing method, Si, Mn on the plate surface during the heating process Oxide is formed, resulting in non-plating and insufficient alloying. Therefore, prior to the plating step, after recrystallization annealing in a low dew point annealing furnace such as a hydrogen annealing furnace, or a continuous annealing furnace having steam cooling or water quenching type cooling and pickling equipment, in a continuous hot dip galvanizing line. A stable plating layer may be formed by plating at a maximum heating temperature of 550 ° C or lower.

Of course, a cold-rolled steel sheet of Si + Mn <2.0% may be subjected to ordinary Zn plating to obtain a Zn-plated steel sheet. Further, alloying treatment may be performed as necessary to obtain a stable alloyed plating layer.

After that, the annealed steel sheet is temper-rolled, if necessary, and further subjected to surface treatment such as electroplating,
Shipped. Next, examples of the present invention will be shown, which are merely examples of the present invention, and do not unduly limit the present invention.

[0033]

EXAMPLE Steels having the compositions shown in Table 1 were melted into a steel ingot in a vacuum melting furnace. These steel ingots were hot forged into 25 mm thick steel slabs (slabs). Next, after heating each slab in an electric furnace at 1250 ° C for 1 hour, it was rolled in a temperature range of 1150 ° C to 930 ° C at a temperature of Ar ₃ points or more by a hot rolling mill for 3 passes, and then heated to a thickness of 5 mm. A rolled steel plate was obtained.

As a simulation of winding, the hot-rolled steel sheet is cooled immediately after hot rolling by forced air cooling or water spray cooling to a temperature of 450 to 800 ° C., and then inserted into an electric furnace maintained at that temperature. Further, after holding at that temperature for 1 hour, the furnace was cooled at 20 ° C./hr. Next, the hot-rolled steel sheet was subjected to surface grinding to form a cold-rolled base metal having a thickness of 3.2 mm, and cold-rolled to a thickness of 0.8 mm at a rolling reduction of 75%. The cold-rolled steel sheet obtained is heated to 820 ° C at 10 ° C / s in an infrared heating furnace,
After holding for s, it was gradually cooled to 700 ° C at 3 ° C / s, and then cooled to room temperature at 50 ° C / s.

After annealing, temper rolling was performed at an elongation of 1.2%, and then a JIS No. 5 tensile test piece was formed and subjected to a tensile test. In addition, a pre-strain of 2% was applied by tensile deformation, and after holding at 170 ° C. for 20 minutes, it was again subjected to a tensile test and the bake hardening amount was measured as the difference in deformation stress before and after heat treatment. Table 2 shows the results of the tensile test and the bake hardening test. In order to compare the ductility of steels with different strengths, tensile strength x elongation at break was calculated.

Comparative Example N deviating from the composition of the present invention
o.9, 11, 13, and 16 have low r-values, and steel Nos. 9 and 15 have poor strength-ductility balance. It can be seen that all the steel sheets having a composition within the range of the present invention have TS × El = 1600 or more, strength of 45 kgf / mm ² or more and r value of 1.3 or more, and are excellent in both strength and formability. ..

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

As described above, according to the present invention, the strength of the steel sheet can be increased without impairing the deep drawability and the ductility. In particular, in recent years, when the social demand for improving the fuel efficiency of automobiles is increasing due to global environmental problems, the steel of the present invention greatly contributes to the weight reduction of the vehicle body and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a graph showing the scope of the present invention with respect to a weighted total amount of a ferrite stabilizing element and an austenite stabilizing element.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical indication C22C 38/18

Claims (4)

[Claims] 1. By weight%, C: 0.0005 to 0.0050%, N: 0.0005 to 0.0100%, S: 0.0
10% or less, acid-soluble Al: more than 0.1% and 4.0% or less, Ti ≧ 48 (N / 14 + C / 12)
, Si: 3% or less, P: 0.010 to 0.150%, Mn: more than 0.50% 4.0
% Or less, and (Mn + Ni + 0.5Cr) / (Si + 0.7Al + 7P) = 0.5 ~
2. Furthermore, (Mn + Ni + 0.5Cr)-(Si + 0.7Al + 7P) = -1 to
Satisfying the + 1% relationship, and having a steel composition consisting of the balance Fe and unavoidable impurities,
A high-strength thin steel sheet with good deep-drawing and secondary working brittleness resistance. 2. The high-strength thin steel sheet according to claim 1, further comprising one or two of Ni: 4.0% or less and Cr: 5.0% or less. 3. A steel slab having the steel composition according to claim 1,
Without cooling below Ar ₃ transformation point, and hot rolled at Ar ₃ transformation point or higher, wound into a coil at room temperature to 700 ° C., then after descaling, cold rolling at a reduction rate of 50-90% ,
A method for producing a high-strength thin steel sheet having good deep drawability and good secondary work embrittlement resistance, which is characterized by further performing recrystallization annealing. 4. The steel composition according to claim 1, wherein Si + Mn ≧
The steel slab with 2% of the steel composition, without cooling below Ar ₃ transformation point, and hot rolled at Ar ₃ transformation point or higher, cold 700
After coiling at ℃, then descaling, cold rolling at a reduction rate of 50-90%, further recrystallization annealing,
Maximum heating temperature of 550 in continuous hot dip galvanizing line
A method for producing a high-strength thin steel sheet having good deep drawability and good secondary work embrittlement resistance, which comprises plating at a temperature of ℃ or less.