JP2013036080A - Method for producing soft hot-rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a soft hot-rolled steel sheet excellent in formability.SOLUTION: A steel material has composition containing, in mass%, 0.01-0.06% of C, 0.1% or less of Si, 0.1-0.5% of Mn, 0.03% or less of P, 0.03% or less of S, 0.005% or less of N, 0.02% or less of O, and further 0.002% or less of sol. Al, with a balance comprising Fe and unavoidable impurities. The steel material is heated and further subjected to finish rolling of which the finish rolling end temperature is in a temperature range of 750°C to Artransformation point, and then is rolled up at a roll-up temperature of at least 600°C. Preferably, temper rolling may be performed with an elongation rate of 0.5-5% after performing pickling. Thus, the soft hot-rolled steel sheet having high ductility with elongation of at least 40% while having low strength with yield strength of less than 210 MPa and excellent in formability is easily obtained.

Description

  The present invention relates to a method for producing a soft hot-rolled steel sheet suitable for a member such as an automobile or an electric product, or a material such as a container or a pipe, and particularly relates to improvement of formability.

Soft hot-rolled steel sheets have been conventionally used as materials for automobiles, electrical products, containers, pipes, and the like. Recently, there is a strong demand for cost reduction of products, and from the viewpoint of reducing processing costs, a soft hot-rolled steel sheet having excellent formability and a yield strength of less than 210 MPa is required.
In response to such demands, Ti, Nb, etc. are added to ultra-low carbon steel to fix interstitial elements C and N, to achieve IF (Intersticial free), and to improve ductility and formability. "Ultra-low coal IF method" has been proposed. For example, Patent Document 1 includes, by weight, C: 0.02% or less, Mn: 0.50% or less, Al: 0.01 to 0.05%, Ti: 0.01 to 0.20%, Nb: 0.005 to 0.05%, with the balance being iron. In addition, a method for producing a well-workable hot-rolled steel sheet is described in which a steel slab composed of inevitable impurities is hot-rolled at a cumulative reduction rate of 1000 ° C. or less at 80% or more and a finishing temperature of Ar ₃ points or more.

Patent Document 2 includes C: 0.0050% or less, Si: 0.03% or less, Mn: 0.15% or less, N: 0.0025% or less, solAl: 0.005-0.03%, Nb: 0.005-0.015%, and Nb / C: Soft hot rolling with excellent workability for hot rolling steel having a composition satisfying less than 7.75 at a finishing temperature of Ar ₃ points to (Ar ₃ points + 50 ° C.) and a coiling temperature of 600 to 650 ° C. A method for manufacturing a steel sheet is described. According to the techniques described in Patent Documents 1 and 2, the reduction of carbon and the inclusion of a small amount of Ti and Nb achieves a finer structure, ensuring low strength and high ductility of 50% or more. It is said that a hot-rolled steel sheet having excellent workability can be obtained.

There is also a technique called “γ rolling method” in which hot rolling is finished in the austenite (γ) region and then rolled up at a high temperature to improve ductility and formability as a ferrite coarse grain structure. For example, Patent Document 3 includes C: 0.025% or less, Mn: 0.05 to 0.35%, S: 0.015% or less, N: 0.0025% or less, solAl: 0.005 to 0.080%, N: 0.0025% or less, and B: A steel having a composition of 0.0005% by mass or more and N-14 / 11B: 0.0005% or less, B- (11/14) × N: 0.0015 or less, finish temperature: Ar ₃ points or more, coiling temperature: 550 to 750 A method for producing a soft hot-rolled steel sheet that is hot-rolled under the condition of ° C is described. According to the technique described in Patent Document 3, a hot and highly ductile hot-rolled steel sheet is obtained.

Also, the hot rolling is finished in the ferrite (α) region, and then subjected to a recrystallization process, whereby ferrite coarse grains are produced by recrystallization and grain growth of ferrite due to processing strain introduced in the α region. There is also a technique called “α rolling method” that obtains a structure and improves ductility and formability. For example, Patent Document 4 discloses a steel having a composition containing C: 0.08% or less, Si: 0.05% or less, Mn: 0.05 to 0.4%, sol. Al: 0.02 to 0.08%, N: 0.01% or less, and Ar _3. In the temperature range above the point, the primary rolling including the rolling reduction rate of 30% or more and the rolling time within 5 s within 2 s, and after the completion of the primary rolling, at a cooling rate of 30 ° C / s or more to Ar ₃ points or less A method of manufacturing a hot-rolled steel sheet having excellent workability after cooling, hot rolling consisting of secondary rolling with a total rolling reduction of 50% or more at a temperature of Ar ₃ points to 450 ° C., and then performing recrystallization treatment Is described. According to the technique described in Patent Document 4, a hot-rolled steel sheet having low yield strength, high elongation, and sufficient characteristics as a steel sheet for processing is obtained.

Patent Document 5 discloses steel having a composition including C: 0.10% or less, Mn: 0.05 to 0.4%, S: 0.005% or less, Cr: less than 0.10%, B: 0.0010 to 0.0050%, and N: 0.0020% or less. Describes a method for producing a hot-rolled steel sheet having excellent workability after hot rolling at an end temperature (Ar ₃ points −40 ° C.) to 700 ° C. and then winding at 500 ° C. or higher. According to the technique described in Patent Document 5, the rolling end temperature is set to (Ar ₃ points −40 ° C.) to 700 ° C., ferrite is generated, and the ferrite is processed to accumulate a lot of strain in the ferrite. B is contained, and hot rolling is performed in a temperature range where BN does not precipitate, so that a large amount of solid solution B remains, and recrystallization between rolling passes is suppressed to improve the workability by rolling. BN is precipitated to promote the grain growth of recrystallized ferrite to obtain a coarse recrystallized ferrite structure.

In Patent Document 6, the weight percentage is C: 0.01 to 0.05% or less, Si: 0.5% or less, Mn: 0.01 to 1.00%, Al: 0.005 to 0.07%, N: 0.0030 to 0.0100%, B: 0.0020. Slabs containing ~ 0.0070% and Ti: 0.001 ~ 0.04%, with B, Ti and N satisfying a specific relationship are heated to 950 ° C or higher and lower than 1120 ° C, and finished to 750 ° C or higher and Ar ₃ points or lower A method for producing a hot-rolled steel sheet that is hot-rolled at a temperature is described. According to the technique described in Patent Document 6, it is possible to lower the recrystallization end temperature by appropriately adjusting the component balance of B, Ti, and N, and it is stable even by hot rolling of low temperature heating-low temperature finishing. Thus, a hot-rolled steel sheet having excellent workability can be manufactured.

Japanese Examined Patent Publication No. 61-10532 Japanese Patent Publication No. 5-17285 JP-A-58-207335 JP-A-64-31934 Japanese Unexamined Patent Publication No. 08-134541 Japanese Patent No. 334037

  However, in the techniques described in Patent Documents 1 and 2, in addition to reducing the carbon content to an extremely low carbon range, it is necessary to contain an alloy element such as Ti and Nb, which increases the material cost. There was a problem to do. Further, in the process of reduction in the γ region and subsequent high-temperature winding adopted in the technology described in Patent Document 3, there is a limit to the coarsening of ferrite grains, and as a steel plate for processing, yield strength There is a problem that it cannot be said that the reduction of the thickness and the improvement of the ductility are sufficient.

Moreover, in the technique described in Patent Document 4, secondary rolling in the ferrite region and subsequent recrystallization treatment are combined. However, since recrystallization varies greatly depending on the component composition and manufacturing conditions, each steel plate or steel plate Among them, there was a problem that variation in yield strength and ductility became large.
Further, in the technique described in Patent Document 5, B is contained in an essential manner, and the inclusion of B having an action of suppressing recrystallization affects the recrystallization of ferrite and the growth of recrystallized grains, and yield strength. In addition, there is a problem that it tends to cause a variation in ductility, and there is also a problem that the inclusion of B tends to cause a material variation in relation to a variation in the N content.

Moreover, in the technique described in Patent Document 6, a slab (steel material) having a composition containing Ti and B in combination is used, and the contents of Ti and B having an action of suppressing recrystallization are those. There is a problem that material variations such as yield strength and ductility are likely to occur in relation to component content variations and N content variations.
The present invention advantageously solves such problems of the prior art, and manufactures a soft hot rolled steel sheet having a yield strength of less than 210 MPa that does not contain Ti or B and is excellent in formability at low cost and with high productivity. An object of the present invention is to provide a method for producing a soft hot-rolled steel sheet. The “steel plate” here includes a steel plate and a steel strip.

In order to achieve the above-described object, the present inventors paid attention to α-region rolling, and conducted extensive research on various factors affecting recrystallization of processed ferrite and grain growth of recrystallized grains. As a result, it was found that Al greatly affects the recrystallization of the processed ferrite obtained by α-region rolling.
First, the results of experiments conducted by the inventors and serving as the basis of the present invention will be described.
0.011 to 0.013% C-0.01% Si-0.16 to 0.17% Mn-0.010 to 0.011% P-0.005 to 0.006% S-0.0022 to 0.0033% N as a basic component and 0.001 to 0.27% Al content The steel material, which was changed in various ranges, was heated to 1250 ° C and subjected to finish rolling at 800 ° C, which is the temperature below the Ar ₃ transformation point, and finished at the finish rolling temperature. In this state, it was cooled (air-cooled) to obtain a hot-rolled steel sheet having a thickness of 3.2 mm. The obtained hot-rolled steel sheet was pickled and then subjected to temper rolling with an elongation of 1.0%.

In addition, a JIS No. 5 test piece (GL: 50 mm) is taken from a temper-rolled hot-rolled steel sheet so that the tensile direction is parallel to the rolling direction, and a tensile test is conducted in accordance with the provisions of JIS Z 2241. The yield strength YS and elongation El were determined. The obtained results are shown in FIG. 1 in relation to the Al content.
FIG. 1 shows that when the Al content is 0.003% by mass or more and less than 0.09% by mass, the yield strength YS increases, the elongation El decreases, and the formability decreases. That is, in order to obtain a steel sheet that is soft, has high elongation, and is excellent in formability, the steel composition is set to Al content of 0.002% or less, or 0.09% or more, and finish rolling is performed at an Ar ₃ transformation point or less. It is important to roll in the temperature range. Although the mechanism of this phenomenon has not been clarified so far, when Al is 0.002% or less, AlN is not formed, and when Al is 0.09% or more, AlN becomes coarse and inhibits recrystallization of ferrite. I think it will disappear.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) A method for producing a hot-rolled steel sheet, in which a steel material is heated and hot-rolled into a hot-rolled steel sheet, wherein the steel material is in% by mass, C: 0.01 to 0.06%, Si: 0.1 %: Mn: 0.1 to 0.5%, P: 0.03% or less, S: 0.03% or less, N: 0.005% or less, O: 0.02% or less, and sol.Al: 0.002% or less, the balance It is a steel material having a composition comprising Fe and inevitable impurities, and the finish rolling of the hot rolling is a rolling whose finish rolling finish temperature is in the temperature range of 750 ° C. to Ar ₃ transformation point, and the finish rolling is finished. Then, the manufacturing method of the soft hot-rolled steel sheet characterized by winding at winding temperature: 600 degreeC or more.
(2) A method for producing a soft hot-rolled steel sheet according to (1), wherein the wound hot-rolled steel sheet is subjected to pickling and temper rolling.
(3) The method for producing a soft hot-rolled steel sheet according to (2), wherein the temper rolling is rolling with an elongation ratio of 0.5 to 5%.

  According to the present invention, a soft hot-rolled steel sheet having a yield strength of less than 210 MPa and excellent in formability can be produced at low cost and with high productivity, and has a remarkable industrial effect.

It is a graph which shows the influence of Al content which gives to yield strength and elongation.

First, the reasons for limiting the composition of the steel material used in the present invention will be described. In the following, mass% is simply expressed as% unless otherwise specified.
C: 0.01-0.06%
C is an element that increases the yield strength through carbide formation, hardens the steel sheet, and lowers the Ar ₃ transformation point. In the present invention, which aims to soften the steel sheet and improve formability, Although it is preferable to reduce as much as possible, 0.06% or less is acceptable. On the other hand, excessive reduction to less than 0.01% leads to an increase in manufacturing cost. For this reason, C was limited to the range of 0.01 to 0.06%. In addition, Preferably it is 0.01 to 0.03%.

Si: 0.1% or less
Si is an element that increases the strength by forming a solid solution in steel, and the content exceeding 0.1% causes an increase in yield strength. For this reason, Si was limited to 0.1% or less.
Mn: 0.1-0.5%
Mn is an element having an action of binding to S to form MnS and preventing adverse effects (hot brittleness) due to S. In order to obtain such an effect, the content of 0.1% or more is required. Moreover, Mn has the effect | action which makes a solid solution in steel and increases an intensity | strength, and the content exceeding 0.5% accompanies the increase in intensity | strength and raises material cost. For this reason, Mn was limited to the range of 0.1 to 0.5%.

P: 0.03% or less P is an element that dissolves in steel and increases the strength of the steel sheet, and the content exceeding 0.03% causes excessive hardening. For this reason, P was limited to 0.03% or less. In addition, Preferably it is 0.015% or less.
S: 0.03% or less S forms sulfides in steel and decreases ductility and formability. For this reason, it is preferable to reduce S as much as possible, but 0.03% or less is acceptable. For these reasons, S is limited to 0.03% or less. In addition, Preferably it is 0.01% or less.

N: 0.005% or less N combines with Al to form fine AlN, and has an effect of inhibiting recrystallization of processed ferrite in ferrite region rolling. Therefore, in the present invention, it is desirable to reduce N as much as possible, but 0.005% or less is acceptable. Therefore, in the present invention, N is limited to 0.005% or less.

sol.Al: 0.002% or less
Al is the most important element in the present invention in order to obtain a steel plate that is soft and excellent in formability. When rolling is performed in the ferrite region, AlN is strain-induced precipitated due to the introduced work strain and is finely dispersed in the steel. The finely dispersed AlN suppresses the subsequent recrystallization of ferrite and the growth of recrystallized grains. Such an adverse effect of AlN is suppressed by Al: 0.002% or less or 0.09% or more of Al as shown in FIG. For this reason, in this invention, Al was limited to 0.002% or less. In addition, Preferably it is less than 0.002%.

O: 0.02% or less O exists as an oxide inclusion in steel, and has an adverse effect such as lowering cleanliness and lowering surface properties. For this reason, it is desirable to reduce O as much as possible, but when the Al content is low, the amount of inclusions tends to increase. Therefore, O is limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

The balance other than the above components is Fe and inevitable impurities.
The method for producing the steel material is not particularly limited, but the molten steel having the above composition is a conventional melting method using a converter, an electric furnace, or a refining furnace, and the molten steel having the above composition. It is preferable to use a steel material such as a slab by a conventional casting method such as a continuous casting method. It should be noted that there is no problem even if an ingot-bundling rolling method, a thin slab casting method, or the like is used.

In the present invention, first, a steel material having the above composition is subjected to a heat treatment for hot rolling.
The heating condition of the steel material is not particularly limited, but is preferably set to 1000 ° C. or higher from the viewpoint of performing desired α-region rolling by finish rolling. Note that if the heating temperature is too high, scale loss increases, so it is preferable to set the temperature to about 1300 ° C. or less.

The heated steel material is then subjected to hot rolling.
Hot rolling consists of rough rolling and finish rolling. The rough rolling is not particularly limited as long as it can be a sheet bar having a desired size and shape. The obtained sheet bar is further subjected to finish rolling.
In the finish rolling, the finish rolling finish temperature is 750 ° C. to the temperature range of Ar ₃ transformation point. If the finish rolling finish temperature is less than 750 ° C., the recrystallization after rolling and the growth of recrystallized grains are not sufficient, and the yield strength becomes high. On the other hand, when the finish rolling finish temperature becomes higher than the Ar ₃ transformation point, the yield strength increases due to the formation of relatively fine ferrite grains due to the austenite (γ) -ferrite (α) transformation after rolling. For this reason, finish rolling was limited to rolling in which the finish rolling finish temperature is in the temperature range of 750 ° C. to Ar ₃ transformation point.

After finishing rolling, the coil is wound at a coiling temperature of 600 ° C or higher.
In the present invention, due to rolling strain introduced in finish rolling and slow cooling after winding, recrystallization of ferrite and growth of recrystallized grains occur, and ferrite grains having a large grain size can be obtained. And it can be set as the hot-rolled steel plate excellent in a formability. When the coiling temperature is less than 600 ° C., ferrite recrystallization and ferrite recrystallized grain growth are not sufficient, and the yield strength increases. The winding temperature is preferably 650 ° C. or higher. In addition, when winding temperature is too high, scale production | generation will become remarkable and a scale defect will occur frequently. For this reason, the coiling temperature is desirably 750 ° C. or lower.

  The wound hot-rolled steel sheet may be used as it is, or may be used after being pickled and further subjected to temper rolling. For pickling, any conventional method can be applied. In addition, mechanical properties are stabilized by subjecting the hot-rolled steel sheet in a wound state to temper rolling. Moreover, it is preferable to make the elongation rate of temper rolling into the range of 0.5 to 5%. If the elongation rate is less than 0.5%, the above-mentioned effects cannot be expected. On the other hand, if the elongation exceeds 5%, the yield strength increases, which is not preferable. The temper rolling can be replaced with a scale breaker before pickling.

The hot-rolled steel sheet obtained under the above-described manufacturing conditions has the above-described composition, and has a structure in which the recrystallized ferrite grains are 90% or more, preferably 95% or more in terms of area ratio on the average of the entire cross section of the sheet thickness. The second phase other than the ferrite phase is preferably one or more selected from pearlite and cementite having an area ratio of 5% or less.
Hereinafter, based on an Example, this invention is demonstrated further.

A steel material (slab thickness: 30 mm) having the composition shown in Table 1 is subjected to heating and finish rolling under the conditions shown in Table 2 to form a hot rolled sheet having a thickness of 1.6 mm, and wound at the winding temperature shown in Table 2. It was. Incidentally, Ar ₃ transformation point of each steel material (℃), the following equation
Ar ₃ = 901−325C + 33Si−92Mn + 287P + 100Al
(Here, C, Si, Mn, P, Al: content of each element (mass%))
And was also shown in Table 1.

The obtained hot-rolled sheet was further subjected to pickling and temper rolling with an elongation of 1%. JIS No. 5 test piece (GL: 50mm) is taken from the hot rolled sheet after temper rolling so that the tensile direction is the rolling direction, and a tensile test is performed in accordance with the provisions of JIS Z 2241. Properties (yield strength YS, tensile strength TS, elongation El) were determined.
Moreover, the specimen for structure | tissue observation was extract | collected from each hot-rolled sheet after the obtained temper rolling. The thickness direction cross section parallel to the rolling direction of these specimens was polished, corroded with a corrosive liquid (Nital), and the structure was observed with an optical microscope (50 times).

  The obtained tensile test results are shown in Table 3.

  In all of the examples of the present invention, recrystallization from the rolling strain and growth of recrystallized grains are sufficient, and it has a structure mainly composed of recrystallized ferrite grains, yield strength YS: less than 200 MPa, elongation El: 40 % Or more, it is a hot-rolled steel sheet that is soft, highly ductile and excellent in formability. On the other hand, the comparative example out of the scope of the present invention has insufficient recrystallization and recrystallized grain growth, high YS, low El, low formability, cannot secure desired characteristics, and molding It is a hot-rolled steel sheet with reduced properties.

Claims (3)

A method for producing a hot-rolled steel sheet, in which a steel material is heated and hot-rolled into a hot-rolled steel sheet,
The steel material is mass%,
C: 0.01 to 0.06%, Si: 0.1% or less,
Mn: 0.1 to 0.5%, P: 0.03% or less,
S: 0.03% or less, N: 0.005% or less,
O: a steel material containing 0.02% or less, and further including sol.Al: 0.002% or less, having a composition composed of the balance Fe and inevitable impurities,
The hot rolling finish rolling is a rolling whose finishing rolling finish temperature is 750 ° C. to the temperature range of Ar ₃ transformation point,
After finishing the finish rolling, a method for producing a soft hot-rolled steel sheet, which is wound at a winding temperature of 600 ° C. or higher.   The method for producing a soft hot-rolled steel sheet according to claim 1, wherein pickling and temper rolling are performed on the wound hot-rolled steel sheet.   The method for producing a soft hot-rolled steel sheet according to claim 2, wherein the temper rolling is rolling with an elongation ratio of 0.5 to 5%.

Images