JPS59575B2 - Manufacturing method for high-strength cold-rolled steel sheets with excellent formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength cold-rolled steel sheets with excellent formability.

As is well known, the manufacturing method for cold-rolled steel sheets involves the following processing steps: pickling, cold rolling, surface cleaning, annealing, etc. after rolling a steel sheet with a hot finishing mill and winding it up with a down coiler. However, in recent years, the material used has changed from rimmed steel to aluminum-killed steel.

Not only is this a high-strength steel material, there are few changes in properties due to component segregation, and it is resistant to aging, but the batch annealing method eliminates nitrogen N dissolved in the material. This is because AlN can be precipitated as fine AlN during the annealing process and can bring the Lankford value (hereinafter referred to as lower value), which is used as an index of formability (deep drawability), to a high level.

However, in the continuous annealing method, it is generally difficult to improve the lower value due to the precipitation of fine AlN, which is a feature of this aluminum killed steel.

Since the heating rate is fast, the order in which recrystallization proceeds after precipitation of fine AlN is reversed or a state similar to this occurs.

In the continuous annealing method, this does not provide any precipitation effect as AlN, and also inhibits recrystallized grain growth.

Therefore, in recent years, the following method has been proposed to improve this lower value in the continuous annealing method.

Namely, 1. The steel sheet after hot rolling is coiled at a high temperature to agglomerate carbides and precipitate large AlN precipitates, thereby improving the lower value and growing recrystallized grains.

2. Recrystallization annealing in a continuous furnace The humidity is raised above normal to improve the texture of the steel sheet by extending it to the ferrite + austenite deposit zone, with the aim of improving the bottom value.

3. There are some products in which the lower value is improved by adding titanium.

However, when high-temperature winding is performed as described in 1 above, descaling properties and surface properties deteriorate, coarsening of crystal grains, poor shape, etc. occur, and the material is often degraded as a cold-rolled base material.

Moreover, if the recrystallization annealing humidity in step 2 is increased, the fuel consumption rate required for the continuous furnace increases, resulting in an increase in cost.

Also, 3. When titanium is added, titanium combines with carbon, so when adding titanium, a vacuum decarburization treatment must also be performed at that time, and this treatment and the use of titanium have the disadvantage of increasing losses. Ta.

The present invention eliminates these methods and partially changes the heat pattern in the continuous annealing process, thereby eliminating the above-mentioned drawbacks.
This paper proposes a method for manufacturing mm2 class high tensile strength cold rolled steel sheets.

That is, this invention has the following properties: 1 Carbon: 0.15% or less, Silicon: 0.20% or less, Manganese: 0.40 to 3.0%, Phosphorus: 0.20% or less, Aluminum: 0.02 to 0.15%, Nitrogen: 0 .0025~0.
02%, the remainder substantially iron, is subjected to normal hot rolling and wound into a coil at a temperature of 600°C or lower and 300°C or higher,
After pickling, cold rolling is performed at a rolling ratio of 40% or more and 80 inches or less, and then a recrystallization depth of 350°C or higher and preferably 450 to 5
Preheating is applied to a humidity range of 50°C for 10 to 60 seconds, followed by recrystallization annealing by holding for a short time in a humidity range above recrystallization humidity and below 850°C, and then the temperature is lowered to a humidity range of 250 to 450°C. A high-strength cold-rolled steel sheet with excellent formability, which is characterized by being subjected to an over-aging treatment at a humidity of 250 to 450 degrees Celsius after being rapidly cooled to 200°C or less and then being reheated. manufacturing method.

2 In addition to the components in item 1 above, chromium 0, 5
% or less, or less than 0.3% of molybdenum,
The gist of the present invention is a method for manufacturing a high-strength cold-rolled steel sheet with excellent formability, characterized by containing the above-mentioned or both.

First, to explain this invention in detail, as shown in FIGS. 1 and 2, curves 1a and 2a show the heat pattern curves in the recrystallization annealing and overaging treatment process of the method of this invention, Curves 1b and 2b show the heat pattern curves in the recrystallization annealing and overaging treatment process of the conventional method, in which the coil is continuously annealed while passing through the annealing furnace continuously. However, first, at the initial stage of annealing, the annealing preheating humidity range PreR is performed in a humidity range of 350°C or higher to less than the recrystallization gain.
Hold A for a short period of about 10 to 60 seconds.

By applying this annealing preheating humidity range PreRA,
Nitrogen in the cold rolled steel sheet is finely precipitated as AlN.

That is, it is known that in aluminum killed steel, AlN tends to be finely precipitated in the initial stage of recrystallization annealing after cold rolling, and this improves the recrystallization texture and obtains higher and lower values.

Therefore, in the continuous annealing method, the inventors developed a method of keeping AlN in a humidity range where AlN tends to precipitate for a short time.
The aim is to improve the texture, improve the lower value, and improve the formability.

In the subsequent humidity process, heat treatment is performed in the same manner as in a normal heat pattern.

That is, the recrystallization annealing humidity range RA near the A1 transformation point
The material is held at the outer edge for about 20 to 120 seconds, during which time it is softened through the process of recrystallization and grain growth, thereby improving moldability.

Next, the temperature is lowered to the overaging treatment humidity range OA of 250 to 450 °C or cooled to 200 °C or less and then reheated for 2 to 4 hours.
This is a method in which the solid solution carbon, which causes aging, is reduced.

Further, FIG. 2 shows another embodiment of the present invention, and is a curve showing an example of a heat pattern when cooling after recrystallization annealing and reheating to perform an overaging treatment.

The reason why the above annealing preheating humidity range PreRA was set from 350°C or higher to below the recrystallization temperature is because below 350°C, the amount of heat necessary for AlN precipitation cannot be obtained, and if the recrystallization humidity is exceeded,
This is because it becomes difficult to achieve the object of the present invention, which is to "improve the texture by recrystallizing fine A[N after precipitation".

Also, if you hold it for more than 10 seconds, you can get enough A during that time.
IN can be precipitated.

Although it is preferable that the precipitation time of this 1N be as long as possible, since it leads to an increase in the length of the equipment, the practical upper limit is about 60 seconds.

The above recrystallization black annealing humidity range RA is below the recrystallization temperature 85
The temperature may be 0°C or lower, but it is better to keep it as low as possible considering the fuel consumption rate of the furnace.

In this regard, in the case of single-phase structure steel, the humidity range may be 650 to 750°C due to the improvement in the lower value described below.

Although the longer the holding time, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the longer the holding time is, the more the effect of the recursive black annealing treatment can be obtained.

On the other hand, in the case of so-called dual-phase steel that has a composite structure of ferrite and martensite, the re-black annealing humidity is maintained uniformly in the ferrite (α) + austenite (γ) region, and the component elements are concentrated into the γ phase. Therefore, a temperature of 750 to 850°C is required.

The longer the retention time, the better, but it is within the allowable range for the equipment.
The effect can be obtained in about 120 seconds.

In addition, the overaging treatment humidity OA is 250 to 450, which is the same as normal.
The humidity range suitable for aging treatment at .degree. C. is sufficient, and the aging treatment time of 2 to 4 minutes, which is the same as usual, is sufficient.

For the preheating operation for annealing in the annealing furnace, the annealing preheating zone of the present invention can be easily provided by adjusting the humidity inside the furnace.

In this way, by only partially changing the heat pattern during re-black annealing, the down value of the finished product is improved, the formability is excellent,
It is possible to produce high-quality products that suppress the occurrence of strain aging.

Along with this improvement in the lower value, various problems described below will also be resolved.

In other words, it is possible to reliably improve the value of 7 even if the winding humidity of the coil is 600° C. or lower without requiring high winding at around 750° C. in a down coiler.

Therefore, it is possible to suppress descaling properties, deterioration of surface properties, coarsening of crystal grains, poor shape, etc. caused by high-temperature winding, and it is possible to obtain an optimal cold-rolled base material.

In addition, the reconsolidation black annealing humidity range RA is usually 700~700 within a short time so that the lower value can be improved by improving the texture.
However, in the method of this invention, a new preheating step is provided to improve the lower value, so in the case of ferritic phase steel, the re-solidification black annealing humidity range R
A can be lowered to about 650 to 750°C.

Therefore, the fuel consumption rate in the continuous reactor can be reliably reduced.

Moreover, there is no need for addition of titanium or vacuum decarburization, and it is possible to obtain an improvement in the value of 7 very easily and reliably.

Further, the cold-rolled aluminum-killed steel sheet of the present invention is applicable to, for example, a high-strength cold-rolled steel sheet manufactured by a continuous casting method or an ingot-forming method, a high-strength cold-rolled steel sheet with a two-phase structure, and the like.

In this invention, the chemical composition of the steel is limited for the following reason.

Carbon is an element necessary to obtain strength, but if carbon exceeds 0.15%, weldability deteriorates, so 0.1%
It was set as 5 or less.

If silicon exceeds 0.20%, it will color the surface of the steel sheet during annealing and cause surface defects due to scale.
．． It was set to 20% or less.

Manganese is an element necessary to obtain strength, but 0
．． If it is less than 40%, it is not sufficient to obtain the target strength, and in the case of ferrite-phase steel, the amount of manganese is 0.40 to 1.
．． 50% is suitable, but on the other hand, in the case of Ferrai 1 + Martensi V dual-phase steel, the manganese content must be 1.5% or more, but if it exceeds 3.0%, it is difficult to melt and the cost is high. Therefore, it was set to 0.40 to 3.0%.

Phosphorus is necessary to increase the tensile strength, but if it exceeds 0.20%, there is a risk of secondary work embrittlement and spot weldability deteriorates, so it is set to 0.20% or less.

Aluminum is necessary for the precipitation of AIN, and if it is less than 0.02%, it will have little effect, and if it exceeds 0.15%, the solid solution of AlN will be incomplete when heating the slab, and the ductility will decrease due to the refinement of crystal grains. Therefore, the ratio was set at 0.02 to 0.15.

It is better to have less nitrogen in order to improve elongation, but
If it is less than 0.0025%, precipitation of AlN is insufficient;
If it exceeds 0.02%, the elongation decreases, and together with aluminum, the solid solution of AlN becomes incomplete during heating of the slab, so the ratio was set at 0.0025 to 0.02.

Chromium is effective in reducing the amount of manganese in order to improve strength and form a two-phase structure, but if it exceeds 0.5%,
The increase in these effects is small in spite of its addition, and it actually deteriorates the texture, so the coefficient was set to 0.50 or less.

Moreover, molybdenum was also set to 0.30% or less for the same reason.

Coiling humidity after hot finish rolling is 600℃ or lower.
Since N precipitates and it becomes impossible to precipitate fine AlN in the preheating temperature range PreRA, which is the original purpose, the temperature was set to 600°C or less.

Further, at temperatures below 300° C., the strength of the copper strip during winding is strong, making winding difficult, resulting in production defects, an increase in the amount of water for cooling, or a decrease in efficiency.

On the other hand, even if the temperature is 300°C or lower, the effect on the drawing property** remains the same.

In addition, the reason why the cold rolling reduction ratio is set to 40% or more and 80% or less is to improve the recrystallization texture as well as improve the dimensional accuracy and shape of the product, as is the case with ordinary cold rolled steel sheets. A reduction of more than 40% increases the total rolling load, resulting in decreased workability and deterioration of plate thickness accuracy.In addition, in order to ensure the plate thickness accuracy and shape required for cold-rolled steel sheets, it is necessary to This is because a rolling reduction ratio is necessary, and if the rolling reduction ratio is less than 40 factors, good drawing properties cannot be obtained.

[Example 1] Next, this invention method will be explained by taking as examples the manufacturing processes of high-tensile cold-rolled steel sheet samples AI, 2, 3.8 and dual-phase structure high-tensile cold-rolled steel sheet samples A4, 5, 6, 7゜9. Table 1 shows the results of a comparison between the conventional method and the conventional method, and also shows the composition of the product and annealing treatment conditions.

As can be seen from Table 1 above, it is recognized that samples A1 to A7 produced by the method of the present invention all have significantly improved lower values compared to samples A8 and 9 produced by the conventional method.

This is because the preheating annealing operation of precipitating AlN in the method of this invention significantly contributed to the improvement of the 7 value.

As mentioned above, this invention is capable of producing a high-strength cold-rolled steel sheet made of alkymild steel by performing a reconsolidation black annealing treatment after a preheating process without rapidly heating to a predetermined high annealing temperature in the continuous annealing method. A high tensile strength cold rolled steel sheet with high strength and excellent formability can be easily manufactured at low cost using the reconsolidation black annealing method most suitable for manufacturing.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a heat pattern of the reconsolidation black annealing process of the present invention, and FIG. 2 is a diagram showing an example of the heat pattern of another reconsolidation black annealing process of the present invention. PreRA...Annealing preheating humidity range, RA...
・・・Reconsolidation black annealing humidity range, OA・・・Overaging treatment temperature range, 1a, 2a・・・Curve flJ of heat pattern in the reconsolidation black annealing process of this invention method
, 1b, 2b...Examples of heat pattern curves in the conventional method of reconsolidation black annealing process.

Claims (1)

[Claims] 1 Carbon 0.15% or less, silicon 0.20% or less, manganese 0.40 to 30%, phosphorus 0.620% or less, aluminum 0.02 to 0.15%, nitrogen 0.0025% to 0.0
2%, the remainder being substantially iron, is subjected to normal hot rolling, wound into a coil at a temperature of 600°C or lower and 300°C or higher, and after pickling, cold rolled at a rolling rate of 40 or more and 80 or less. After that, preheating is held for 10 to 60 seconds in a temperature range of 350°C or higher and below the recrystallization temperature, followed by recrystallization annealing by holding it for a short time in a temperature range of 850°C or higher, above the recrystallization temperature, and then te2
It is characterized by lowering the temperature to an overaging treatment temperature of 50 to 450°C and performing an overaging treatment, and once rapidly cooling to 200°C or less, reheating, and then performing an overaging treatment at a temperature range of 250 to 450°C. A method for manufacturing high-strength cold-rolled steel sheets with excellent formability. 2 Carbon 0.15% or less, silicon o, 2o% or less, manganese 0.40 to 3.0%, phosphorus 0.20% or less, aluminum 0.02 to 0.15%, nitrogen 0.0025 to 0.
0.2% plus 0.5% or less chromium and 0.3% molybdenum
A steel containing one or two of the following, with the remainder substantially made of iron, is subjected to normal hot rolling and wound into a coil at a temperature of 600°C or lower and 300°C or higher, and after pickling, the rolling rate is After cold rolling at 40 to 80 modulus, preheat and hold for 10 to 60 seconds in a temperature range of 350°C or higher and below recrystallization humidity, and then hold for a short time in a temperature range of 850°C or higher than recrystallization temperature. Then, the temperature is lowered to an overaging treatment temperature of 250 to 450°C, and an overaging treatment is performed, and after being rapidly cooled to 200°C or less and reheated, the temperature is 250 to 450°C.
A method for producing a high-strength cold-rolled steel sheet with excellent formability, which is characterized by performing an over-aging treatment in a temperature range of 0°C.