JPH0788535B2 - Method for producing hot rolled steel sheet with excellent deep drawability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a hot-rolled steel sheet having excellent deep drawability, specifically, a hot-rolled steel sheet having a high rank Ford value (r value). .

(Prior Art) Hot-rolled steel sheets are widely used as relatively inexpensive structural materials for automobiles, home appliances, building materials, various industrial crises, and the like. Usually, it is rarely used as it is for these members, and it is often used after being molded into a predetermined shape by a press. Therefore, the hot rolled steel sheet is required to have excellent workability, but the hot rolled steel sheet is inferior to the cold rolled steel sheet in workability. This is because the hot rolled steel sheet is inferior in deep drawability to the cold rolled steel sheet. For example, the r value used as an index indicating deep drawability is 1.5 for cold rolled steel sheets.
While it is ~ 1.8, it is as low as 0.5-0.9 for hot rolled steel.

As described above, the hot-rolled steel sheet is inferior in workability to the cold-rolled steel sheet. Therefore, for applications requiring workability such as the above-mentioned automobiles and home electric appliances, the hot-rolled steel sheet is more expensive than the inexpensive hot-rolled steel sheet. More expensive cold-rolled steel sheets are used.

Therefore, if it becomes possible to manufacture a hot-rolled steel sheet having a deep drawability equal to or higher than that of the cold-rolled steel sheet, it is industrially very preferable. In recent years, various methods for producing such hot-rolled steel sheets having excellent deep drawability have been investigated, and patents related thereto have been filed.

For example, JP-A-59-153836 discloses C, Mn, Sol.Al.
Aluminum-killed steel containing, after precipitation treatment of AlN, under certain conditions, hot rolling, then pickling, light reduction rolling and recrystallization treatment, a method of manufacturing a hot-rolled steel sheet excellent in deep drawability Is disclosed. However, in this method, since there is no stipulation regarding the specific conditions before the precipitation treatment, heating for the precipitation treatment requires a long time such as 100 minutes or more, and therefore the production efficiency is remarkably poor.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method capable of efficiently depositing a precipitate to efficiently produce a hot-rolled steel sheet having a high r value and excellent deep drawability. Especially.

(Means for Solving the Problems) The present inventors have used a hot-rolled steel sheet excellent in deep drawability as a raw material using an alkymird steel having a specific chemical composition, which is subjected to primary rolling including large reduction rolling and precipitation treatment. , Found that it can be produced by a method of secondary rolling and recrystallization, and filed a patent for it (Japanese Patent Laid-Open No. 25521/1990).

In the method of the invention of this prior application, the ferrite grains are refined in the primary rolling before the precipitation treatment, the work strain is accumulated in the steel and the precipitation site of the precipitate is introduced, and in the precipitation treatment, the precipitation of AlN and TiN Since the product is rapidly precipitated, the deep-drawability of the final product after the secondary rolling and recrystallization treatment is significantly improved.

However, the present inventors were not satisfied with these results, and as a result of further studying the production method of the invention of the prior application, as a result of starting precipitation treatment while the processing strain obtained in the primary rolling was not released, The inventors have found that precipitation is further promoted and deep drawability is further improved, and the present invention has been completed.

Here, the gist of the first invention of the present application is "% by weight, C: 0.05%
Below, Mn: 0.01-0.4%, Si: 0.3% or less, Sol.Al: 0.01-
A method for producing hot-rolled steel sheets with excellent deep drawability, which is characterized in that aluminum-killed steel containing 0.08% and N: 0.01% or less, with the balance being Fe and unavoidable impurities, is sequentially heat-treated in the following steps. is there.

Rolling the final pass in the temperature range below 1100 ℃ and above 900 ℃,
And a step of performing a primary rolling with a rolling reduction of 40% or more, a step of performing a precipitation treatment by immediately maintaining the temperature range of 1050 ° C or less and 900 ° C or more for 1 to 60 minutes immediately after the primary rolling, or after the primary rolling, At a cooling rate of 5 ° C / sec or more until the precipitation treatment start temperature of 1050 ° C.
The step of performing the precipitation treatment by holding for ~ 60 minutes, after the precipitation treatment, cooling to the next secondary rolling start temperature at a cooling rate of 5 ° C / sec or more, and the total reduction rate in the temperature range of 800 ° C or less and 450 ° C or more. A step of performing a secondary rolling to 50% or more, and a step of performing a recrystallization treatment after the secondary rolling.

The second invention of the present application is, in addition to the above alloy components, further Ti, N
Using aluminum-killed steel containing 0.001 to 0.150% in total of one or more components selected from b, Zr and V,
In the third invention, in addition to the above alloy components, B is added in the range of 0.0001-0.
The fourth invention uses an aluminum-killed steel containing 50%, and in addition to the above alloy components, a total of 0.001 to 0.150% of one or more components of Ti, Nb, Zr, and V is added. B
Using aluminum killed steel containing 0.0001-0.0050%,
The gist of each is a method for manufacturing a hot-rolled steel sheet having excellent deep drawability, which is characterized in that the thermo-mechanical treatment is sequentially performed in the steps 1 to 3.

(Operation) Hereinafter, each constituent element of the present invention and its operation and effect will be described.

First, the reason for limiting the composition of the aluminum killed steel which is the raw material steel used in the present invention as described above will be described.

In addition, "%" regarding a steel composition is "weight%."

C: Since C is an element that adversely affects deep drawability, it is desirable that its content be small. If the content of C exceeds 0.05%, the deep drawability is significantly deteriorated.

Mn: Mn is an effective element that fixes S mixed as impurities in steel as MnS and improves hot workability. However, if the content exceeds 0.4%, the deep drawability deteriorates remarkably. On the other hand, due to the recent progress in the S-removing technology, it becomes possible to manufacture ultra-low S steel, and even if the Mn content is 0.01%, S can be sufficiently fixed as MnS and hot work cracking can be prevented. For this reason, the Mn content is set to 0.01 to 0.4%.

Si: Si is preferably as small as possible in order to improve the deep drawability. If the Si content increases, not only the deep drawability deteriorates, but also the scale properties deteriorate and the product quality is impaired, so the content should be 0.3% or less.

Sol.Al: Sol.Al is an important element in the present invention. This element has the effect of fixing N in the steel as AlN and improving the deep drawability at the same time as it is used for deoxidation treatment. But 0.01%
If the content is less than 0.08, the above-mentioned effects are not sufficiently obtained, and 0.08
If it is contained in excess of%, the effect is saturated and it is economically disadvantageous, so the content is made 0.01 to 0.08%.

N: N forms a nitride with Al and Ti, Nb, Zr, and V described later, and contributes to miniaturization of ferrite. However, on the other hand, it is also an element that adversely affects the deep drawability. If the N content increases, the solid solution N remains and the deterioration of the deep drawability becomes more remarkable than the effect of refining the ferrite, so the content is made 0.01% or less.

One of the material-killed aluminum-killed steels used in the present invention is, in addition to the above components, the balance being Fe and inevitable impurities. In addition to these components, 0.001 to 0.150% of Ti, Nb, Zr, V in total of 1 type or 2 types,
Alternatively, and / or aluminum killed steel containing 0.0001 to 0.0050% B can also be used as the base steel.

The effects of the above-mentioned additional elements are as follows.

Ti, Nb, Zr, V: These elements are expensive, but by forming carbonitrides, sulfides and carbosulfides, solid solution C, S, N are reduced and before secondary rolling. Is an extremely preferable element that has the effect of making the ferrite grain size finer and improving the deep drawability.
However, its content is 0.001% in total of one or more kinds.
If it is less, the above effect cannot be obtained, and even if the content exceeds 0.150%, the effect is saturated, which is economically disadvantageous.

B: B is an effective element that has the effect of preventing "vertical cracking" which is a problem in drawn parts. However, if the content is less than 0.0001%, the above effect is not sufficiently obtained, and if the content is more than 0.0050%, the effect is saturated and it is economically disadvantageous.

Using aluminum killed steel with such a chemical composition as the raw material steel,
This is sequentially subjected to thermomechanical treatment according to the above steps (1) to (3) to obtain a hot rolled steel sheet. FIG. 1 attached herewith is a schematic view showing an example of the manufacturing process.

The process of thermomechanical treatment will be described below with reference to this drawing.

[Primary rolling]

The raw material steel (slab) of the aluminum-killed steel having the above-mentioned chemical composition to be subjected to the primary rolling may be either a high temperature that is directly sent from continuous casting, or one that is once cooled after casting and then reheated.

The purpose of primary rolling is to introduce a precipitation site for rapidly precipitating precipitates in the next precipitation treatment, and to obtain fine ferrite grains. For that purpose, the primary rolling needs to be performed in the final pass in a temperature range of 1100 ° C or lower and 900 ° C or higher with a large reduction rate of 40% or higher.

If the rolling reduction of the final pass is less than 40%, the resulting processing strain is small, so the precipitation sites of precipitates are not sufficiently introduced,
1 in the temperature range of 1050 ℃ or less and 900 or less in the next precipitation treatment
It is difficult to deposit precipitates efficiently even after holding for ~ 60 minutes. If the final pass temperature is higher than 1100 ° C, the grain refining effect of ferrite cannot be obtained, and if it is lower than 900 ° C, it becomes difficult to secure the precipitation treatment temperature in the next step.

[Treatment after primary rolling]

What is important in the treatment after the primary rolling is that the rough rolled material is immediately sent to the next precipitation treatment step before the work strain is released. When the processing strain between the primary rolling and the precipitation treatment is released, the precipitation of the precipitate is reduced in the precipitation treatment and the improvement of the deep drawability is small. Therefore, after the primary rolling, it is preferable to perform the following processing to suppress the release of processing strain.

That is, when the finishing temperature of the primary rolling is a predetermined precipitation treatment temperature, it is immediately sent to the precipitation treatment step as it is, and when the finishing temperature after the primary rolling is higher than the predetermined precipitation treatment temperature, as shown in FIG. After the primary rolling, the material is rapidly cooled at a cooling rate of 5 ° C./sec or more, and the temperature of the rough rolled material is lowered to the precipitation treatment temperature before being sent to the precipitation treatment step. In this way, the processing strain accumulated in the primary rolling can be used even in the precipitation process without being released during cooling.
In the precipitation treatment step, precipitation of the precipitate due to processing strain is sufficiently promoted, the precipitate becomes coarse, and the deep drawability is improved. Further, when the finishing temperature is higher than the predetermined precipitation treatment temperature,
If the rough rolled material is rapidly cooled at a cooling rate of 5 ° C / sec or more,
The manufacturing time can be shortened.

[Precipitation treatment]

The purpose of this treatment is to improve the deep drawability by precipitating C, S and N in the steel as precipitates such as carbonitrides, sulfides and carbosulfides. To do so, without rolling the rough rolled material after primary rolling to room temperature, immediately after the rolling trace, 1050 ℃
It is necessary to keep the temperature below 900 ° C for 1 to 60 minutes. It is preferable to perform the treatment in a temperature range of 1050 ° C or lower and 950 ° C or higher. A precipitation nose exists in this temperature range in the austenite region. Therefore, if held at a temperature higher than 1050 ° C, not only the precipitation of the precipitate does not proceed rapidly due to the large solubility, but also the austenite grains grow and become coarse, and the ferrite grains before the secondary rolling become coarse. As a result, the deep drawability of the final product does not improve. On the other hand, 900 ℃
When held at a lower temperature in the austenite region, the precipitation rate is remarkably slow, and similarly, precipitation of precipitates does not proceed rapidly, and improvement in deep drawability cannot be obtained. If the holding time is less than 1 minute, the amount of precipitates deposited will be small, while if it is longer than 60 minutes, the precipitation of precipitates will be saturated, resulting in an increase in manufacturing cost.

It is desirable to keep the rough rolled material after the primary rolling within the above precipitation treatment temperature range within the rolling line. For example, the coil box developed in recent years may be used, and the roughly rolled material may be wound into a coil in a line.

[Cooling treatment after precipitation treatment]

After the precipitation treatment, it is necessary to cool the rough rolled material up to the secondary rolling start temperature to lower the temperature. This cooling is also preferably performed at a cooling rate of 5 ° C./second or more. By cooling at a cooling rate of 5 ° C / sec or more, the manufacturing time can be shortened, and the transformation from austenite to ferrite can proceed rapidly, so coarsening of ferrite grains can be prevented. Is improved.

[Secondary rolling]

The purpose of the secondary rolling is to work the rough rolled material to the final plate thickness and to give the working strain necessary for recrystallization of ferrite. For that purpose, in the secondary rolling, it is necessary to perform the rolling under the condition that the total rolling reduction is 50% or more in the temperature range of 800 ° C or lower and 450 ° C or higher without cooling the rough rolled material after the precipitation treatment to room temperature. . If the temperature exceeds 800 ° C. and the temperature or the total reduction ratio of less than 50%, the work strain required for recrystallization of ferrite is not sufficiently accumulated, and good deep drawability cannot be obtained after the recrystallization treatment. On the other hand, when the temperature is lower than 450 ° C., the deformation resistance becomes extremely high, which makes secondary rolling practically difficult.

If this secondary rolling is performed using a rolling lubricating oil, the work deformation in the plate thickness direction is made uniform, so that the r value is improved including the plate surface layer portion. Therefore, there is an effect that the r value of the entire plate is improved.

[Recrystallization treatment]

The hot-rolled steel sheet processed to a predetermined thickness in the above steps is wound on a coil, annealed by its own heat (self-annealing), and proceeds to recrystallization. This recrystallization is extremely important for improving the deep drawability of the steel sheet, but if it is a hot rolled steel sheet that has undergone the steps described above, a recrystallized texture preferable for improving the deep drawability is sufficient even in the above-mentioned self-annealing. To be selected. Of course, a separate annealing step may be provided to promote recrystallization. In this case, 550
It is recommended to carry out in the temperature range of ~ 900 ° C.

If hot-rolled and then galvanized to obtain a final product, it is also advantageous to recrystallize it in a continuous annealing process of a hot dip galvanizing line.

The present invention will be further described with reference to examples.

(Example) Aluminum-killed steel having a chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace and cast into a 60 mm thick slab. However, with regard to the steel type C, a part thereof has a thickness of 8 mm.

These slabs were subjected to primary rolling, cooling, precipitation treatment, cooling, and secondary rolling under the conditions shown in Table 2, wound on a coil, and then recrystallized under any of the conditions below. It was

"Recrystallization treatment condition" After being wound around a coil, recrystallization treatment (treatment A) is carried out by its own heat during slow cooling.

After winding, cool it to room temperature and then 800 ℃ × 2min
The recrystallization treatment (treatment B) is performed by applying a heat history corresponding to the continuous annealing of.

Similarly, after cooling to room temperature, a recrystallization treatment (treatment C) is performed by applying a heat history corresponding to continuous annealing in a hot dip galvanizing line at 850 ° C for 10 seconds.

Similarly, after being cooled to room temperature, a recrystallization treatment (treatment D) is performed by applying a heat history corresponding to batch annealing at 700 ° C. for 5 hours.

The total reduction ratio of primary rolling is all except No. 19 and No. 20.
80%.

The finished plate thickness after the secondary rolling was 3 mm, and the secondary rolling was performed by using lubricating oil rolling (coefficient of friction μ: 0.10 to 0.15).

Specimens were taken from the steel sheet thus obtained, and the yield strength (YP), elongation (El), Rankford (r value), and aging crack transition temperature were examined. The results are shown in Table 3.

The warp resistance transition temperature means the brittle crack stop temperature of the cup drawn at a drawing ratio of 2.0.
Further, the processes A to D in the recrystallization process in the table are the processes corresponding to the above-mentioned.

As is clear from Table 3, as in No. 14 to No. 21 of Comparative Example, the hot-rolled steel sheet produced under the condition that any of the primary rolling, precipitation treatment and secondary rolling deviates from the range defined by the present invention. Have low r-values, and have the same resistance to warp cracking transition temperature in the same steel type and the same recrystallization treatment.
High compared to No.2 or No.5.

On the other hand, all of No. 1 to No. 12 of the present invention have high ductility and a higher r value, and among them, No. 10 to No. 10 using aluminum killed steel containing B among them. No. 12 has a remarkably low transition temperature for vertical cracking.

(Effects of the Invention) As described above, according to the method of the present invention, it is possible to manufacture a hot-rolled steel sheet having an even higher r value and excellent deep drawability. Therefore, it greatly contributes to the expanded use of the hot-rolled steel sheet in the fields of automobiles, home appliances, building materials, etc. of the present invention.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an example of the manufacturing process of the present invention.

Claims (4)

[Claims] 1. By weight%, C: 0.05% or less, Mn: 0.01 to 0.4
%, Si: 0.3% or less, Sol.Al: 0.01 to 0.08%, N: 0.01% or less, and the balance is Fe and inevitable impurities. A method for manufacturing a hot rolled steel sheet having excellent deep drawability. Rolling the final pass in the temperature range below 1100 ℃ and above 900 ℃,
And a step of performing a primary rolling with a rolling reduction of 40% or more, a step of performing a precipitation treatment by immediately maintaining the temperature range of 1050 ° C or lower and 900 ° C or higher for 1 to 60 minutes after the primary rolling, or after the primary rolling, Cooling process to a precipitation treatment start temperature of 5 ° C / sec or more, and maintaining the temperature range of 1050 ° C or less and 900 ° C or more for 1 to 60 minutes to perform the precipitation treatment. After the precipitation treatment, 5 ° C / sec or more Cooling at the following secondary rolling start temperature at the cooling rate of 800 ° C, and performing a secondary rolling at a total reduction of 50 ° C or more in a temperature range of 800 ° C or less and 450 ° C or more, and performing recrystallization treatment after the secondary rolling. Process. 2. Weight%, C: 0.05% or less, Mn: 0.01 to 0.4
%, Si: 0.3% or less, Sol.Al: 0.01 to 0.08%, N: 0.01% or less, and 0.001 to 0.150 in total of one or more selected from Ti, Nb, Zr, and V. % Of the aluminum-killed steel, the balance of which is Fe and unavoidable impurities, is sequentially subjected to thermomechanical treatment according to the steps (1) to (3) of claim 1 to obtain a hot-rolled steel sheet having excellent deep drawability. Production method. 3. Weight%, C: 0.05% or less, Mn: 0.01 to 0.4
%, Si: 0.3% or less, Sol.Al: 0.01 to 0.08%, N: 0.01% or less, and 0.0001 to 0.0050% of B, and the balance of aluminum-killed steel consisting of Fe and unavoidable impurities. A method for producing a hot-rolled steel sheet having excellent deep drawability, which comprises sequentially performing thermomechanical treatment according to the steps (1) to (3). 4. By weight%, C: 0.05% or less, Mn: 0.01 to 0.4
%, Si: 0.3% or less, Sol.Al: 0.01 to 0.08%, N: 0.01% or less, and one or more selected from Ti, Nb, Zr, and V in a total of 0.001 to 0.150. % And B 0.0001 to 0.0050
% Of the aluminum-killed steel, the balance of which is Fe and unavoidable impurities, is sequentially subjected to thermomechanical treatment according to the steps (1) to (3) of claim 1 to obtain a hot-rolled steel sheet having excellent deep drawability. Production method.