JP2825864B2 - Manufacturing method of cold rolled steel sheet with excellent ductility - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a cold-rolled steel with excellent ductility, which is produced by casting a thin cast strip directly from molten steel and then cold-rolling and annealing without passing through a hot-rolling step. The present invention relates to a method for manufacturing a cold-rolled steel sheet which is a rolled steel sheet and which does not cause surface roughness during forming.

(Prior art) In recent years, as a new sheet manufacturing process, a thin cast strip is cast directly from molten steel, and rough rolling in hot rolling or cold rolling is omitted, in order to greatly shorten the sheet manufacturing process. A method of casting a steel plate has been devised.

However, in these methods, due to insufficient precipitation of precipitates before cold rolling and coarse crystal grains, which were not a problem in the conventional manufacturing process, the ductility after cold rolling and annealing was the same as that of the conventional process. There is a disadvantage that it deteriorates as compared with.

Above all, in the step in which the hot rolling step is omitted, the structure of the thin cast strip is coarse, so that the cold-rolled steel sheet obtained in this step is roughened when formed, and the ductility is greatly deteriorated.
In order to prevent this roughening and to improve the ductility, it is necessary to make the structure after casting finer. As a method for this, a method already disclosed in JP-A-62-207828 is already known. . In this method, CaO and CaS formed by adding Ca are used as crystallization nuclei of δ ferrite to make the solidification structure fine.

However, such control of the solidification structure is difficult to cast because there are many restrictions such as the amounts of O and S in the steel, the temperature of the molten steel, and the timing of adding the additional elements. In addition, the structure of the slab is likely to be non-uniform, and as a result, the structure after cold rolling and annealing is made non-uniform, and ductility is deteriorated.

(Problems to be Solved by the Invention) The present invention provides a process in which a hot-rolling step is omitted for a slab cast directly into a thin cast strip in order to prevent surface roughness after molding and to improve ductility. However, it is an object of the present invention to provide a manufacturing method capable of homogenizing the structure after cold rolling and annealing and obtaining a cold rolled steel sheet having ductility equal to or higher than that obtained by a conventional process.

(Means for Solving the Problems) As a result of intensive studies in view of the above-described circumstances, the present inventors have found that
It has been found that when Mn, Ti, and B are added and the cooling conditions after solidification are properly taken, the structure after cold rolling and annealing becomes uniform. This means that at the slab stage, the solid solution S is fixed by Mn,
It is considered that the grain boundary segregation of B suppresses the precipitation of ferrite from the austenite grain boundaries during cooling, and the ferrite precipitates from within the grains, so that the structure of the slab is finely uniform. Can be

The present invention is intended for a cold-rolled steel sheet manufactured in a process without a hot rolling process, after relatively solid cooling from solidification to a low temperature region of austenite to precipitate MnS in an austenite region and fix solid solution S. By rapidly cooling to the ferrite region, ferrite precipitation from inside the grains is promoted. Thereby, the structure before cold rolling is made fine and uniform, the structure after cold rolling and annealing is made uniform, ductility is improved, and the occurrence of surface roughness after forming is prevented.

 That is, the gist of the present invention is as follows.

C: more than 0.02 to 0.20 wt%, Si: 2.0 wt% or less, Mn: 0.5 to 3.0 w
t%, S: 0.001 to 0.02 wt%, Ti: [N] × 48/14 to 0.05 wt
%, B: 0.0003 to 0.005 wt%, N: 0.01 wt% or less, molten steel consisting of the balance Fe and unavoidable impurity elements is made into a thin steel strip with a sheet thickness of 10 mm or less by continuous casting, from solidification to austenite range The average cooling rate is 1 ~ 30 ℃ / s, and the average cooling rate from the austenite zone to the ferrite transformation zone is 3
A method for producing a cold-rolled steel sheet having excellent ductility, comprising performing cold rolling without hot rolling after cooling at 0 ° C / s or more, continuous annealing, and temper rolling.

The reasons for limiting the chemical composition, casting thickness and cooling rate after solidification of the present invention will be described below.

C is an important element that determines the strength of steel, but excessive addition deteriorates weldability, so the upper limit is made 0.2 wt%. On the other hand, when the carbon content becomes extremely low, that is, 0.02 wt% or less, ferrite precipitation from austenite grain boundaries during cooling after solidification cannot be suppressed even by segregation of B to austenite grain boundaries as described later, and No ferrite precipitation occurs.

Si is added when increasing the strength without breaking the strength-ductility balance of the steel, and is also added to increase the transformation point and promote the precipitation of ferrite. However, excessive addition causes deterioration in ductility and surface properties, so the upper limit is made 2.0 wt%.

Mn is also added to strengthen the steel. It is another object of the present invention to precipitate Mn and S to fix solid solution S as described above. 0.5 wt% or more to precipitate MnS during cooling. Excessive addition not only raises costs, but also degrades workability, so the upper limit is 3.0w.
t%.

S should be small in order not to remain as solid solution S, but 0.001w in order not to increase the desulfurization cost unnecessarily.
The lower limit is t%. On the other hand, an excessive content indicates that solid solution S remains,
Alternatively, the workability is degraded due to the formation of a large amount of sulfide, so the upper limit is made 0.02 wt%.

In the case of the present invention, Ti is added to fix N and precipitate B as solid solution B at austenite grain boundaries without precipitating B as BN. Therefore,
([N] × 48/14] wt% or more is required. In order to increase the strength of steel, the precipitation strength of TiC can be used, but excessive addition causes large deterioration of ductility. , 0.
The upper limit is 05 wt%.

B segregates at austenite grain boundaries with a small amount of addition,
It has the effect of suppressing the precipitation of ferrite from the grain boundaries and relatively facilitating the transformation within the grains.
Add more than 03wt%. The upper limit is 0.005 wt% so as not to cause time cracking during transformation and large ductility deterioration.

N is preferably small in order to reduce the amount of Ti added and to efficiently segregate solid solution B to austenite grain boundaries.
Therefore, the content is set to 0.01 wt% or less.

Steel having the above chemical composition is a thin cast strip in continuous casting, but in the present invention, since the hot rolling step is omitted, the casting thickness is 10 mm or less in consideration of cold rolling. And If it exceeds 10 mm, hunting tends to occur during cold rolling, and stable cold rolling becomes difficult.

Next, in the present invention, regarding the most important cooling condition of the thin cast strip, the condition is that solid solution S is fixed as MnS, and the precipitation of ferrite from within the grain must be promoted. . Therefore, the temperature range from the solidification to the austenite range, preferably just above the Ar ₃ transformation point, needs to be cooled relatively slowly to sufficiently precipitate MnS. Therefore, the cooling rate in this range is set to 30 ° C./s or less.
If the cooling rate is higher than this, the precipitation of MnS becomes insufficient. As a result, since the solid solution S remains, the structure of the slab becomes non-uniform, the structure after cold rolling and annealing becomes non-uniform, and the occurrence of rough surface and the deterioration of ductility at the time of forming are caused. Also, plate thickness 10mm
For the following thin steel sheets, if the cooling rate in the above range is less than 1 ° C / s, heat insulation equipment is required, so the lower limit of the cooling rate is 1 ° C.
/ s.

Subsequently, the steel must be rapidly cooled to the ferrite transformation zone, and the cooling must be terminated in this temperature zone. If the cooling rate in this temperature range is less than 30 ° C./s, precipitation of ferrite from grain boundaries becomes remarkable. As a result, the structure of the cast slab is made non-uniform, and as described above, after cold rolling and annealing, it causes the occurrence of surface roughness at the time of forming and also causes deterioration of ductility. Therefore, in order to suppress ferrite precipitation from this grain boundary,
The cooling rate from the austenite region to the ferrite transformation region is 30 ° C / s or more. After cooling, it is not particularly limited, but it may be cooled by air cooling or holding if necessary so that the final cast slab has a uniform and fine structure.

In order to cold-roll directly the above slabs directly manufactured into thin cast strips without hot rolling, annealing, temper rolling, and final cold-rolled steel sheets, the conditions of cold rolling and continuous annealing are as follows. Although not particularly limited, it may be performed by a usual method of cold rolling and continuous annealing.

(Examples) Example 1 C: 0.080 wt%, Si: 0.25 wt%, Mn: 1.52 wt%, P: 0.007 wt%,
S: 0.010wt%, Al: 0.011wt%, N: 0.0052wt%, Ti: 0.02wt%,
B: A steel containing 0.0010 wt%, the balance being Fe and unavoidable impurities was output from a converter, and was cast into a thin steel strip of 6 mm by continuous casting.

Next, cooling was performed under cooling conditions as shown in Table 1, and the film was wound at a cooling end temperature of 700 ° C. After the pickling, cold rolling was performed at a rolling reduction of 80%, continuous annealing was performed at 750 ° C., and then temper rolling was performed at 1%. After that, in order to investigate the occurrence of rough skin, it was processed into JIS Z 2201, 5 test piece,
A tensile test was performed according to the test method described in 2241. The table shows the results.

No. 1, cooling conditions after solidification according to the scope of the present invention
In Nos. 2, 3 and 6, since the structure of the slab is fine and uniform, even if the slab is cold-rolled and annealed, the structure is uniform, and even if a tensile test is performed, no rough surface occurs and the ductility does not deteriorate.

However, in Nos. 4 and 5, since the cooling rate from the austenite region to the ferrite transformation region is lower than the range of the present invention, ferrite precipitates from the grain boundaries during cooling. Therefore, since the cast slab has a non-uniform structure as a whole or a structure in which coarse ferrite is formed on the entire surface, the slab has a non-uniform structure when cold-rolled and annealed, and a rough surface occurs when a tensile test is performed. At the same time, ductility is greatly deteriorated. In No. 7, the cooling rate from solidification to immediately above the Ar ₃ point was high, and the precipitation of MnS did not sufficiently occur, so that solid solution S remained and the structure of the slab was non-uniform. Therefore, similarly, the structure after cold rolling and annealing becomes non-uniform, the surface becomes rough, and the ductility is low.

Example 2 A steel having the chemical composition shown in Table 2 was output from a converter, cast into a thin cast strip of 3 mm by continuous casting, and then a product was obtained under the same conditions of cooling, cold rolling and annealing. That is, after solidification, the cooling rate up to the point immediately above the Ar ₃ point was 10 ° C./s, and the cooling rate up to the subsequent ferrite transformation region was 70 ° C./s. After cold rolling at a rolling reduction of 70% after pickling, continuous annealing was performed at 800 ° C., temper rolling was performed at 1%, and the presence or absence of occurrence of surface roughness was examined in the same manner as in Example 1.

The surface state after cold rolling / annealing was evaluated by examining the presence or absence of formation of Si scale, and was evaluated as ○: none, Δ: not causing a problem, ×: present. Table 3 summarizes the results.

In the steel according to the scope of the present invention (B, C, D and E steels), since the structure of the slab is uniform, a uniform structure is obtained by cold rolling and annealing the slab, and a tensile test is performed. No roughening occurs and ductility does not deteriorate.

Steel A has a low C content, and even if B is added to suppress the precipitation of ferrite from the grain boundaries, it cannot be suppressed, ferrite precipitates from the grain boundaries, and the structure of the slab becomes an uneven structure. Therefore, the structure after cold rolling and annealing becomes uneven, and when a tensile test is performed, the surface becomes rough and the ductility is low.

F steel with a large amount of Si has Si scale formed on the surface after cold rolling and annealing, and has poor surface properties. Since G steel has a small amount of Mn, it precipitates during cooling from solidification to the austenite region.
The precipitation amount of S is small, and the amount of solid solution S remaining in the slab stage increases. Therefore, the structure after cold rolling and annealing becomes uneven.
Therefore, when a tensile test is performed, skin roughness occurs and ductility is low.

Since H steel has a large amount of S, a large amount of solid solution S remains in the slab,
Like the G steel, the ductility is low and the workability is poor as compared with the B steel.

In the case of I and J steels, the structure after cold rolling and annealing is also non-uniform, and when a tensile test is performed, the surface becomes rough and the ductility is low. This is because in steel I where the amount of Ti added is small relative to the amount of N, B
Forms excess BN to form a binding BN, so that the amount of solute B segregated at the grain boundaries is reduced, and the effect of B for suppressing ferrite precipitation from the grain boundaries becomes insufficient. Therefore, it is considered that ferrite precipitates from the grain boundaries and the cast slab has an uneven structure. Also, it is considered that the structure of the cast slab of the J steel to which B is not added is also non-uniform for the same reason as that of the I steel.

(Effects of the Invention) As described above, according to the present invention, even after casting in a thin cast strip, the structure after cold rolling and annealing becomes uniform, and at the same time, it is possible to prevent rough surface generated at the time of forming processing, Ductility can be ensured. In other words, in a step in which the hot rolling step is omitted, it is possible to manufacture a cold-rolled steel sheet equal to or more than the material in the conventional step, thereby achieving a significant cost reduction as compared with the conventional step.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Yada 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Third Technical Research Institute (56) References JP-A-2-236228 ( JP, A) JP-A-60-77928 (JP, A) JP-A-63-290222 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8/02, 8/04 C21D 9 / 46,9 / 48

Claims (1)

(57) [Claims] (1) C: more than 0.02 to 0.20 wt%, Si: 2.0 wt% or less, M
n: 0.5 to 3.0 wt%, S: 0.001 to 0.02 wt%, Ti: [N] × 48/1
Molten steel containing 4 to 0.05 wt%, B: 0.0003 to 0.005 wt%, N: 0.01 wt% or less, the balance being Fe and unavoidable impurity elements is made into a thin steel strip with a thickness of 10 mm or less by continuous casting, and solidified. From the austenitic zone to the austenite zone at an average cooling rate of 1 to 30 ° C / s, and further from the austenitic zone to the ferrite transformation zone at an average cooling rate of 30 ° C / s or higher, and then cold-rolled without hot rolling. A method for producing a cold rolled steel sheet having excellent ductility, comprising performing continuous annealing and further temper rolling.