JP2013209725A - Cold rolled steel sheet excellent in bendability and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having excellent bendability and to provide an advantageous manufacturing method thereof.SOLUTION: A cold rolled steel sheet has a composition containing, by mass, 0.005-0.030% C, 0.05% or less Si, 0.10-0.35% Mn, 0.025% or less P, 0.015% or less S, 0.0050% or less N, and 0.07% or less Al, and satisfying the relation of [%Si]/[%Mn]<0.5, with the remainder comprising Fe and unavoidable impurities. The cold rolled steel sheet is characterized in that the ferrite grain size in the steel is 15 μm or less, and 50% or more of the precipitating cementite precipitates in the ferrite grain.

Description

  The present invention relates to a cold-rolled steel sheet excellent in bending workability and a method for producing the same, which is suitable as a material for a structural member such as an automobile part or a structural body such as a house, furniture, desk, or home appliance.

  Cold-rolled steel sheets are used as materials for a wide variety of structures because of their good formability. Normally, cold-rolled steel sheets are formed by pressing into a two-dimensional plate shape to form a three-dimensional structure, and these are joined to form a more complex three-dimensional structure, so excellent press workability is required. Is done.

  There are several types of pressing in press working, but bending is the only working method that must consider the strain distribution in the thickness direction. This bending process has not received much attention in the past, but the degree of bending process has become severe as the press-molded shape becomes more complicated. For this reason, there have been cases where press cracks occur during bending.

Therefore, although not intended to improve the direct bending workability, for example, in Patent Document 1, the amount of C, Mn, Al, N is reduced, and cold rolling is performed at a rolling rate of 50% or more. Thereafter, a method for producing a highly workable cold-rolled steel sheet having excellent aging resistance by prescribing the cooling condition and the overaging condition after annealing and the temper rolling ratio is disclosed.
However, although this method can produce a steel plate with good aging resistance, since the surface is strained by temper rolling, cracking tends to occur on the surface when bending is performed.

In Patent Document 2, a steel having a specified amount of C, Mn, S, O, and B is continuously cast under specified conditions, and then hot-rolled, cold-rolled, and continuously annealed to provide a cold work with excellent bending workability. A method of manufacturing a rolled steel sheet is disclosed.
However, in this method, although the size of MnS is controlled by oxide inclusions, oxygen must be contained at 60 ppm or more, and a large amount of oxide inclusions are generated. Thus, there was a problem that cracking occurred in the bending process.

Patent Document 3 discloses aging resistance and workability by rapidly heating and rapidly cooling a steel in which the amounts of C, Si, Mn, P, Al, and N are specified after hot rolling and cold rolling. The manufacturing technology of the excellent cold-rolled steel sheet is disclosed.
However, this method has a problem that the structure in the thickness direction is not uniform because heating and cooling are too fast, and is not suitable for bending.

Patent Document 4 discloses a thin steel sheet excellent in bending workability, characterized in that the amounts of C, Mn, Si, N, B, and S are defined and Mn is added in excess of S. . According to this technique, the segregation of Mn and Si on the surface is suppressed, and the yield point is lowered as non-aging, so that the bending workability of the steel sheet is improved to some extent by avoiding the buckling of the steel sheet.
However, since the C content is large, there is a problem that cracking is likely to occur at the cementite / ferrite interface, and sufficient bending workability cannot be obtained for severe bending work.

JP 61-124533 A JP-A-2-267227 JP-A-7-216459 JP 56-136956 A

As described above, with the conventional technology, it has been difficult to industrially stably provide a cold-rolled steel sheet having good bending workability.
The present invention advantageously solves the above-described problems of the prior art, and an object thereof is to provide a cold-rolled steel sheet having excellent bending workability together with its advantageous manufacturing method.

Conventionally, the workability of cold-rolled steel sheets has been developed with a focus on reducing the amount of dissolved C. For this reason, workability has been improved by overaging conditions and temper rolling conditions.
However, there is a limit in improving the bending workability only by reducing solute C.

  Therefore, the inventors sought other methods for improving bending workability rather than controlling overaging conditions and temper rolling conditions for the purpose of reducing the amount of solute C, and as a result, improved bending workability. Therefore, it is necessary to control the ferrite grain size and the cementite precipitation position at the same time, and the steel components suitable for this are found to have low Si and Mn contents and suitable content ratios for each. It was.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
1. In mass%, C: 0.005 to 0.030%, Si: 0.05% or less, Mn: 0.10 to 0.35%, P: 0.025% or less, S: 0.015% or less, N: 0.01% or less, Al: 0.07% or less And satisfying the relationship of [% Si] / [% Mn] <0.5, the balance is composed of Fe and inevitable impurities,
A cold-rolled steel sheet having excellent bending workability, characterized in that the ferrite grain size in steel is 20 μm or less, and 50% or more of precipitated cementite is precipitated in ferrite grains.
Here, [% M] represents the content (mass%) of M element in steel.

2. The cold-rolled steel sheet having excellent bending workability as described in 1 above, further comprising B: 0.0050% or less by mass%.

3. Furthermore, by mass%, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Ti, Pb, Ta, REM, V, Cs, Zr, Hf The cold-rolled steel sheet having excellent bending workability as described in 1 or 2 above, wherein the total content of

4). The cold-rolled steel sheet excellent in bending workability according to any one of the above 1 to 3, further comprising a plating film on the surface of the steel sheet.

5. The steel material having the composition described in any one of 1 to 3 above is hot-rolled, and after finishing rolling, wound on a coil, then pickled, and then cold-rolled and then continuously annealed. Furthermore, when producing cold-rolled steel sheets by further overaging treatment,
After heating to the austenite single-phase region, finish rolling temperature: 820 ° C or more and less than 950 ° C, after finishing hot rolling, winding at a temperature of 780 ° C or less, and then removing the scale on the steel sheet surface, then rolling rate of 85% or less Cold rolled steel sheet with excellent bending workability, characterized by annealing at 800 ° C or lower after cold rolling, cooling to the overaging temperature, and then overaging at a temperature range below 420 ° C Method.

6). 6. The method for producing a cold-rolled steel sheet having excellent bending workability according to 5 above, wherein water cooling is performed after the annealing prior to the overaging treatment.

7). The method for producing a cold-rolled steel sheet having excellent bending workability according to 5 or 6 above, wherein a plating treatment is performed after the overaging treatment.

  According to the present invention, it is possible to provide a cold-rolled steel sheet having significantly improved bending workability as compared with the prior art together with its advantageous manufacturing method.

Hereinafter, the present invention will be specifically described.
(Cold rolled steel plate with excellent bending workability)
First, the reason why the component composition of the cold rolled steel sheet is limited to the above range in the present invention will be described. In addition, unless otherwise indicated,% and ppm which represent the following component composition shall mean the mass% and the mass ppm.

C: 0.005-0.030%
C forms cementite in steel. If the amount of C is less than 0.005%, the driving force for precipitation of solute C decreases and the amount of solute C increases, and strain aging is likely to occur, so bending workability deteriorates. For this reason, the lower limit of C amount is 0.005% or more, preferably 0.010% or more.
On the other hand, if the content exceeds 0.030%, the amount of cementite increases, resulting in an increase in void generation sites at the interface between cementite and ferrite, which lowers the bending workability of the steel sheet. Therefore, the upper limit of the C amount is 0.030% or less, preferably 0.025% or less.

Si: 0.05% or less
Si is an element that suppresses the formation of cementite, and also suppresses the cementation of C. Therefore, when the Si content exceeds 0.05%, the control of the cementite precipitation position is lost, and as a result of the cementite being likely to come out to the ferrite grain boundary, voids may be generated during bending, so the Si content is 0.05%. The following.

Mn: 0.10 to 0.35%
Mn does not form a compound with C, but dissolves in cementite and keeps cementite fine. This cementite precipitates in the ferrite grains. If the Mn content is less than 0.10%, the effect of refining cementite is small, and desired characteristics cannot be obtained. For this reason, the lower limit of the amount of Mn is made 0.10% or more. On the other hand, if the amount of Mn exceeds 0.35%, MnS segregates with the segregation of Mn, resulting in deterioration of bending workability. Therefore, the upper limit of Mn is set to 0.35% or less.

P: 0.025% or less
Since P segregates at the ferrite grain boundary and promotes the generation of voids at the ferrite grain boundary during bending, the lower content is desirable. Therefore, the P content is 0.025% or less, preferably 0.020% or less.

S: 0.015% or less
In the present invention, S is an element that combines with Mn to form MnS. If this amount of S is large, a large amount of MnS is formed, which promotes fracture at the ferrite grain boundary during bending. Therefore, in the present invention, the S amount is set to 0.015% or less.

N: 0.01% or less
N combines with Al to form AlN, or when B is added, forms BN. If the N content is large, a large amount of AlN is generated, and voids are likely to be generated during bending, so that bending workability deteriorates. Therefore, in the present invention, the N content is 0.01% or less, preferably 0.0040% or less.

Al: 0.07% or less
Al is used as a deoxidizer. The content of Al exceeding 0.07% combines with fine AlN and O, which is an inevitable impurity, to produce fine oxides. As a result, void formation at the ferrite grain boundaries is promoted during bending. Therefore, the upper limit is made 0.07% or less.

Although the basic components have been described above, in the present invention, other elements described below can be appropriately contained as necessary.
B: 0.0050% or less
When B is added, it can combine with N to form BN and suppress the precipitation of fine AlN. Further, since BN precipitates with MnS as a nucleus, the amount of fine MnS can also be reduced, and ferrite grains take these precipitates into the grains. As a result, the number of nitrides at the ferrite grain boundary, which is a generation source of voids during bending, is reduced, so that bending workability can be further improved.
However, when the B content exceeds 0.0050%, fine Fe ₂₃ (CB) ₆ is generated at the grain boundary, and the bending workability is deteriorated. From the above, the B content is 0.0050% or less.

One or more selected from Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Sb, W, Mo, Pb, Ti, Nb, Ta, REM, V, Cs, Zr and Hf 1% or less in total
Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Sb, W, Mo, Pb, Ti, Nb, Ta, REM, V, Cs, Zr and Hf are all useful elements for improving corrosion resistance. However, if the total amount exceeds 1%, it segregates at the grain boundary and promotes the generation of voids from the grain boundary during bending. For this reason, it is made to contain at 1% or less in any case of single addition or compound addition. Preferably it is 0.5% or less.
Components other than those described above are Fe and inevitable impurities.

As described above, the component composition of the steel sheet has been described. However, in order to obtain the effect expected in the present invention, it is not sufficient to adjust the component composition within the above range, the ratio of the Mn amount and the Si amount, the ferrite grain size, It is important to control the precipitation rate of cementite in the ferrite grains.
That is, in the present invention, [% Si] / [% Mn] <0.5 (where [% M] represents the content (mass%) of M element in the steel) and the ferrite particle size is determined. It is necessary to set the ratio of precipitated cementite in the ferrite grains to 50 μm or less.

[% Si] / [% Mn] <0.5
(Here, [% M] represents the content (mass%) of M element in steel.)
This is an extremely important component definition in the present invention. Mn is an austenite-forming element and tries to keep C in the ferrite grains, but Si has an effect of expelling C from the ferrite grains. In other words, Si inhibits the effect of solidifying the cementite in the ferrite grains of Mn to refine cementite. Therefore, in order to obtain the effect of the present invention, it is necessary to define the [% Si] / [% Mn] ratio.
If this [% Si] / [% Mn] exceeds 0.5, the amount of cementite in the ferrite grains decreases, so in the present invention it is defined as 0.5 or less.

Ferrite grain size: 20 μm or less When the ferrite grain size exceeds 20 μm, strain in bending is concentrated on the ferrite grain boundary, cracking is likely to occur at the ferrite grain boundary, and bending workability deteriorates. For this reason, the ferrite grain size was set to 20 μm or less. Preferably, it is 15 μm or less.

Precipitated cementite: 50% or more is precipitated in ferrite grains In the present invention, the position of cementite precipitation is important. If a large amount of cementite precipitates at the ferrite grain boundary, voids are generated around the cementite at the ferrite grain boundary during bending, which significantly deteriorates the bending workability. That is, when the amount of cementite in the ferrite grains is less than 50% of the whole cementite, the bending workability is lowered. Therefore, it is necessary that 50% or more of the cementite volume is precipitated in the ferrite grains.
The amount of cementite precipitated in the ferrite grains can be determined from the cross-sectional structure as follows. The structure is observed in a plate thickness section parallel to the rolling direction. After the mirror polishing, the cementite appears in the Picral corrosive solution, and then the cementite is observed with a scanning electron microscope (1000 times). At this time, the ratio of the area of cementite present in the ferrite grains to the total cementite area is defined as the ratio of cementite present in the grains.

  The steel sheet of the present invention may have a plating film on the surface. By forming a plating film on the steel sheet surface, the corrosion resistance of the cold-rolled steel sheet is improved. In addition, as a plating film, electrogalvanization, for example, Zn-Ni electroalloy plating, etc. other than a hot dip galvanization film and an alloying hot dip galvanization film, etc. are mentioned, for example.

(Method for producing cold-rolled steel sheet with excellent bending workability)
Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
In the present invention, the slab obtained by continuous casting is preferably a steel material, hot-rolled, and after finishing rolling, cooled and wound into a coil, then pickled, cold-rolled, continuously A cold-rolled steel sheet is obtained by annealing and further overaging.

In the present invention, the method for melting the steel material is not particularly limited, and any known melting method such as a converter, an electric furnace or an induction furnace is suitable. The casting method is not particularly limited, but the continuous casting method is suitable. In addition, when hot-rolling the slab, it may be hot-rolled after reheating the slab in a heating furnace, or may be subjected to hot-rolling after being heated in the heating furnace for a short time for the purpose of temperature compensation. . Further, after cooling to room temperature once after casting, rolling may be performed by heating to 1200 ° C. or higher, more preferably 1250 ° C. or higher.
The steel material obtained as described above is hot-rolled. However, even hot rolling by rough rolling and finish rolling may be performed by omitting rough rolling and performing only finish rolling. The slab heating temperature and finish rolling temperature are important.

Slab heating temperature: Temperature range in which the austenite single phase is formed. If the slab heating temperature is in the ferrite-austenite two-phase region, which is less than the austenite single phase region, only ferrite expands during hot rolling, resulting in coarse ferrite grains. Therefore, when the slab is heated, it is necessary to heat to the austenite single phase region (Ac ₃ points or more).

Finishing rolling temperature: 820 ° C. or more and less than 950 ° C. When the finishing rolling temperature is 950 ° C. or more, coarse grains are produced in part, and the ferrite grain size varies and becomes unstable. As a result, bending workability deteriorates, so the finish rolling temperature is set to less than 950 ° C. The lower limit of the finish rolling temperature is set to 820 ° C. or higher so that coarse grains are not generated in the ferrite region rolling.

After the above hot rolling, the coil is cooled and wound around a coil, and this winding temperature is also important.
Winding temperature: 780 ° C or less When the winding temperature exceeds 780 ° C, ferrite grains become coarse and bending workability deteriorates. Therefore, the winding temperature is 780 ° C. or lower, preferably 700 ° C. or lower. In addition, when the lower limit of the coiling temperature is less than 550 ° C., the precipitation of nitride into the hot rolled sheet is suppressed, and a fine grain precipitates at the ferrite grain boundary during annealing after cold rolling, resulting in a mixed grain structure. The winding temperature is preferably 550 ° C. or higher in order to easily deteriorate the bending workability.

Rolling ratio in cold rolling: 85% or less When the rolling reduction ratio in cold rolling exceeds 85%, ferrite grains extend and cracks are likely to occur on the surface during bending. Therefore, the upper limit of the cold rolling rate is set to 85% or less.

Annealing temperature: 800 ° C. or less When the annealing temperature exceeds 800 ° C., coarse ferrite grains are randomly generated and tend to deteriorate the bending workability. Therefore, an annealing temperature shall be 800 degrees C or less. In addition, since an unrecrystallized structure tends to remain when the annealing temperature is less than 650 ° C., the temperature is preferably set to 650 ° C. or more, and more preferably 680 ° C.

  The cooling rate from the annealing temperature to the overaging temperature is preferably 50 ° C./s or more. When it is less than 50 ° C./s, cementite is not sufficiently precipitated in the ferrite grains, and the bending workability may be deteriorated. The upper limit of the cooling rate is not particularly limited, but about 500 ° C./s is sufficient.

Overaging temperature: less than 420 ° C. When the overaging temperature is 420 ° C. or more, cementite tends to precipitate at ferrite grain boundaries. In order to precipitate cementite in the ferrite grains, the overaging temperature was set to less than 420 ° C. Further, if the overaging time is too short, cementite cannot be sufficiently precipitated. In addition, although there is no problem in effect for a long portion, the length that can be industrially realized is 10 minutes or less due to restrictions on the production line.

  Moreover, it is preferable to perform water cooling prior to the overaging treatment. This is because precipitation of cementite into ferrite grains can be promoted, and as a result, better bending workability can be obtained.

  Moreover, in this invention, you may form a plating film on the steel plate surface by performing the plating process with respect to the cold-rolled steel plate manufactured as mentioned above. For example, as a plating process, a hot dip galvanizing process may be performed to form a hot dip galvanized film on the surface of the steel sheet. May be. At this time, hot dip galvanizing and annealing may be performed in one line. In addition, the plating film may be formed by electroplating such as Zn-Ni electroalloy plating.

The molten steel having the composition shown in Table 1 was continuously cast into a slab (steel material) having a thickness of 250 mm. Next, after heating the obtained slab to the austenite single-phase region at 1250 ° C. and finishing rolling at the temperature shown in Table 2, it was wound up at the temperature shown in Table 2 to obtain hot-rolled steel sheets having various thicknesses. It was. Next, after removing the scale on the surface of the steel sheet by pickling, the steel sheet was cold-rolled to a thickness of 1 mm at a rolling rate shown in Table 2. Thereafter, continuous annealing, cooling treatment and overaging treatment were performed under the conditions shown in Table 2. The temper rolling ratio was 0.8%.
Moreover, about the cold rolled steel plates of No. 17-19 of Table 2, it water-cooled before the overaging after the said annealing. Further, for the cold rolled steel sheets Nos. 8 to 11 in Table 2, an electrogalvanized film having an adhesion amount after drying of 30 g / m ² (both sides) was formed.

A specimen was taken from the cold-rolled steel sheet obtained as described above, and a tensile test was performed.
Furthermore, the workability of the obtained cold-rolled steel sheet was investigated.

The test method and measurement method are as follows.
(I) Structure observation The thickness cross section parallel to the rolling direction of the obtained cold-rolled steel sheet was polished to a mirror surface, the structure was revealed with a nital structure revealing solution, and the ferrite particle size was measured. Optical micrographs were taken at a magnification of 100, and 10 lines were drawn in the thickness direction and in the rolling direction at an actual length of 100 μm or more, respectively, and the number of intersections between the ferrite grain boundaries and the lines was counted. The total line length was divided by the number of intersections to obtain the line segment length per ferrite grain, which was multiplied by 1.13 to determine the ASTM ferrite grain size.

(Ii) Cementite Precipitation Position After a plate thickness cross section parallel to the rolling direction of the obtained cold-rolled steel sheet was polished to a mirror surface, cementite was exposed with a picral structure revealing solution, and the ratio of cementite precipitation positions was measured. Scanning electron micrographs were taken at a magnification of 1000, and the cementite deposition positions were observed for 10 fields of view. By dividing the area of cementite precipitated in the ferrite grains by the area of total cementite, the ratio of cementite precipitated in the grains was determined.

(Iii) Tensile test From the obtained cold-rolled steel sheet, a JIS No. 5 tensile test piece (JIS Z 2201) with the direction parallel to the rolling direction is taken, and a tensile test in accordance with the provisions of JIS Z 2241 The tensile strength was measured.

(Iv) Bending test A bending test jig having an apex angle of 90 degrees was prepared and tested. From the obtained cold-rolled steel sheet, a strip-shaped test piece having a longitudinal direction of 100 mm and a width direction of 35 mm was collected, and a bending test was performed so that the bending ridge line bends in the direction perpendicular to the rolling direction at the longitudinal center of the test piece. At this time, the radius of curvature of the apex angle of the bending test jig is changed to derive the minimum test jig tip radius (R) in which no crack is observed on the surface of the test piece, and the obtained radius is divided by the plate thickness (t). Thus, the critical bending radius (R / t) was calculated. The smaller this value, the better the bending workability.
In addition, for the steel that was not cracked by 90 ° bending, a 90 ° bent test piece was sandwiched in a vise and subjected to 180 ° complete adhesion bending. At this time, the limit bend radius was set to 0 when no cracking occurred even in close contact bending.

  As shown in Table 2, it can be seen that all the cold-rolled steel sheets obtained according to the present invention have a bending radius of 1 or less and are excellent in bending formability.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the cold-rolled steel plate which the bending workability improved significantly compared with the past, and it is very useful industrially.

Claims (7)

In mass%, C: 0.005 to 0.030%, Si: 0.05% or less, Mn: 0.10 to 0.35%, P: 0.025% or less, S: 0.015% or less, N: 0.01% or less, Al: 0.07% or less And satisfying the relationship of [% Si] / [% Mn] <0.5, the balance is composed of Fe and inevitable impurities,
A cold-rolled steel sheet having excellent bending workability, characterized in that the ferrite grain size in steel is 20 μm or less, and 50% or more of precipitated cementite is precipitated in ferrite grains.
Here, [% M] represents the content (mass%) of M element in steel.   The cold-rolled steel sheet having excellent bending workability according to claim 1, further comprising B: 0.0050% or less in terms of mass%.   Furthermore, by mass%, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, Mo, Sb, W, Nb, Ti, Pb, Ta, REM, V, Cs, Zr, Hf The cold-rolled steel sheet having excellent bending workability according to claim 1 or 2, characterized in that a total of 1% or less is contained.   The cold-rolled steel sheet excellent in bending workability according to any one of claims 1 to 3, further comprising a plating film on the surface of the steel sheet. A steel material comprising the composition according to any one of claims 1 to 3 is hot-rolled, and after finishing rolling, wound on a coil, then pickled, and then cold-rolled and then continuously annealed. In addition, when producing a cold-rolled steel sheet by performing overaging treatment,
After heating to the austenite single-phase region, finish rolling temperature: 820 ° C or more and less than 950 ° C, after finishing hot rolling, winding at a temperature of 780 ° C or less, and then removing the scale on the steel sheet surface, then rolling rate of 85% or less Cold rolled steel sheet with excellent bending workability, characterized by annealing at 800 ° C or lower after cold rolling, cooling to the overaging temperature, and then overaging at a temperature range below 420 ° C Method.   6. The method for producing a cold-rolled steel sheet having excellent bending workability according to claim 5, wherein water cooling is performed after the annealing prior to the overaging treatment. The method for producing a cold-rolled steel sheet having excellent bending workability according to claim 5 or 6, wherein a plating treatment is performed after the overaging treatment.