KR101630548B1 - Hot rolled steel sheet used as material for cold rolling and method for producing the same - Google Patents

Abstract

A hot-rolled steel sheet as a raw material capable of producing a cold-rolled steel sheet or a surface-treated cold-rolled steel sheet as a raw material at low cost, and a method for producing the same. Wherein the steel sheet contains 0.016 to 0.07% of C, 0.1% or less of Si, 0.05 to 0.5% of Mn, 0.03% or less of P, 0.03% or less of S, 0.02 to 0.1% of Sol and 0.005% or less of N , 0.003 to 0.0030% of B, 0.004% or less of Ti, and 0.003% or less of Nb, the balance being Fe and inevitable impurities, the average crystal grain size is 13 μm or less, Hot - rolled steel sheet for cold - rolled materials of 10 MPa or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot rolled steel sheet for cold rolling,

The present invention relates to a cold rolled steel sheet or cold rolled steel sheet which is used as a material for cold rolling in the production of cold rolled steel sheets or surface treated cold rolled steel sheets of articles used in the fields of electricity, (Hot rolled steel sheet) and a method of manufacturing the same.

In recent years, along with the increase in the world population and the development of the economy, the demand for electric and structural steel sheets is increasing. Particularly, building materials such as outer walls and roofs of buildings are coated with cold-rolled steel sheets having a plate thickness of 0.5 mm or less or with a surface-treated cold-rolled steel sheet having been plated or painted thereon in a wave form Plate) is also used.

In order to make the cold-rolled steel sheet and the surface-treated cold-rolled steel sheet have a thinner gauge in order to lower the cost of the construction material, cold rolling is often carried out by cold rolling mills, There is a problem in that it can not be made into a thin gauge due to insufficient load at the time of cold rolling.

Therefore, there is a growing demand for a hot rolled steel sheet having a low cold rolling load as a material. For example, Patent Document 1 discloses a technique relating to a hot rolled steel sheet in which the C content in steel is extremely low to 0.010% Patent Document 2 proposes a technique relating to a hot rolled steel sheet in which the N content in the steel is reduced to 0.0020 mass% or less. In Patent Document 3, hot rolling (so-called alpha (ferrite) rolling) is performed at a C content of 0.01 to 0.10 mass% and an N content of 0.010 mass% or less and a finish temperature of 700 ° C or higher and an Ar ₃ transformation point or lower Thereby obtaining a hot rolled steel sheet having coarse crystal grains.

In addition, a technique relating to an interstitial free steel sheet to which Ti or Nb is added is known as a method of obtaining a soft hot-rolled steel sheet.

JP-A-3-79726 Japanese Patent Publication No. 63-30969 Japanese Laid-Open Patent Publication No. 2010-77482

However, in the technology relating to the hot-rolled steel sheet having a low gas component amount such as C or N described in Patent Document 1 or 2, it is necessary to perform the vacuum degassing process at the time of steelmaking, and the increase in the manufacturing cost can not be avoided. Further, in order to produce cold-rolled steel sheets or surface-treated cold-rolled steel sheets having a sheet thickness of 0.3 mm or less using a cold rolling mill having a small rolling ability, it is necessary to set the plate thickness of hot rolled steel sheets to 3 mm or less, When the N content is low, it is difficult to secure a finishing temperature equal to or higher than the Ar ₃ transformation point, and uneven microstructures tend to be formed in the thickness direction, thereby deteriorating the cold rolling property.

In the? Rolling technique described in Patent Document 3, the strength of the hot-rolled steel sheet is easily influenced by the finishing temperature and the coiling temperature, and stable thinning can not be achieved.

In the interstitial free steel sheet to which Ti or Nb is added, the addition of expensive Ti or Nb increases the component cost, increases the recrystallization temperature, and requires high-temperature annealing, which increases the manufacturing cost.

The present invention has been made to solve such a problem, and it is an object of the present invention to provide a hot-rolled steel sheet of a cold-rolled steel sheet or a surface-treated cold- The purpose.

As described above, the prior art has been focused on softening the hot rolled steel sheet as a material in order to reduce the cold rolling load. However, the inventors of the present invention have found that even if a soft hot-rolled steel sheet is used, the cold rolling load is increased, and the cause is a small amount of solid C or N that does not largely affect the strength of the hot-rolled steel sheet. Further, by adding B, which is an inexpensive element, and by setting the particle diameter of the hot-rolled steel sheet to a predetermined range or less and decreasing the aging index (AI), the winding temperature may fluctuate or the coil wound , It is possible to soften a steel sheet after cold rolling (as it is cold-rolled) with almost no solid solution C or N present, that is, to suppress an increase in strength due to cold rolling and to reduce a cold rolling load I found out.

Next, experimental results on which the present invention is based will be described.

And a steel containing 0.002 to 0.05% of C, 0.01% of Si, 0.15% of Mn, 0.012% of P, 0.008% of S, 0.035% of Sol.Al and 0.003% of N, Heating temperature: 1250 占 폚, followed by hot rolling at 920 占 폚, finishing temperature: 920 占 폚, winding at a coiling temperature of 650 占 폚, cooling, pickling, temper rolling at an elongation of 1% temper rolling was performed to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was cold-rolled at a reduction ratio of 78% to obtain a steel sheet as cold rolled steel sheet. From the obtained hot-rolled steel sheet and cold-rolled steel sheet, a tensile test specimen of JIS No. 5 in the rolling direction was taken and subjected to a tensile test in accordance with JIS Z 2241 to obtain a tensile strength TS. For the hot-rolled steel sheet, tensile test specimens of JIS No. 5 having a rolling direction in the longitudinal direction were used to impart 7.5% pre-strain by tensile processing and heat treatment at 100 ° C for 30 minutes And an aging index AI, which is defined as the difference in strength before and after the heat treatment, was obtained. Further, the microstructure of the hot-rolled steel sheet was observed, and the average grain size was determined by the cutting method described in JIS G 0552 (1998).

Fig. 1 shows the relationship between the average crystal grain size of the hot-rolled steel sheet and the TS of the hot-rolled steel sheet and the TS of the steel sheet as cold-rolled.

Fig. 2 shows the relationship between the AI of the hot-rolled steel sheet and the TS of the steel sheet as cold-rolled.

As is apparent from Fig. 1, the TS of the hot-rolled steel sheet depends on the average crystal grain size, the C content is as small as less than 0.016%, and becomes low when the average grain size is large. However, TS of cold-rolled steel sheet (cold-rolled steel sheet) after cold-rolling these hot-rolled steel sheets had a low value by setting the average crystal grain size of the hot-rolled steel sheet to 13 μm or less, contrary to TS of the hot-rolled steel sheet.

As is apparent from Fig. 2, in the case of C <0.016% in which cementite is not precipitated well due to low degree of supersaturation, the AI of the hot-rolled steel sheet is higher than that in the case of C ≥ 0.016% Is high.

Although the reason for this is not clear, the TS of the hot-rolled steel sheet is TS at a deformation of only about 0.3 which is obtained by the tensile test. On the other hand, since the TS of the cold- It is presumed that the work hardening ability is higher than that in the high strain region and the TS of the steel sheet as cold rolled is increased. That is, in order to lower the TS of the steel sheet as cold-rolled, it is advantageous to reduce the amount of solute C and N and to reduce the AI of the hot-rolled steel sheet to 10 MPa or less.

From the above results, it has been found that it is effective to reduce the solid content of C and N in the hot-rolled steel sheet. Therefore, it is preferable that 0.03% of C, 0.01% of Si, 0.15% of Mn, 0.012% of P, And N: 0.003%, B, and Ti, which have high affinity with C, N, etc., are added to the steel, and the steel is heated at a heating temperature of 1250 占 폚 and then hot rolled at a finishing temperature of 920 占 폚, : Rolled at 550 캜, cold-rolled, pickled, tempered and rolled at an elongation of 1% to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was cold-rolled at a reduction ratio of 79% After annealing for 2 seconds, temper rolling was performed at an elongation of 1% to obtain a cold-rolled steel sheet. From the obtained cold-rolled steel sheet, a tensile test specimen of JIS No. 5 in the rolling direction was taken and subjected to a tensile test in accordance with JIS Z 2241 to obtain a tensile strength TS.

Fig. 3 shows the relation between the annealing temperature by the amounts of B and Ti and the TS of the cold-rolled steel sheet.

As is apparent from Fig. 3, when B or Ti is added to reduce solids C and N, it is understood that the TS of the steel sheet as cold-rolled is lowered and the cold rolling load is lowered. However, in a steel to which 0.011% of Ti is added, the decrease accompanying the rise of the annealing temperature of TS is shifted to the higher temperature side, and the rise of the recrystallization temperature can be confirmed. For example, in order to have sufficient formability as a wafers, it is preferable that the TS is 400 MPa or less. It is necessary to anneal at a temperature as high as about 75 캜 as compared with a steel to which B or B is added (B <0.0001%, Ti <0.001%).

On the other hand, the steel to which B is added exhibits the same recrystallization behavior as the steel to which B and Ti are not added, although the TS of the steel sheet as cold-rolled decreases and the cold rolling load decreases. Therefore, in order to lower the TS of the steel sheet as cold rolled without raising the recrystallization temperature, addition of B is effective. Also, in the case of Nb forming C and N and carbonitride, the recrystallization temperature is raised in the same manner as the Ti addition steel.

The present invention has been made on the basis of the above findings, and has as its object the provision of a steel sheet comprising 0.016 to 0.07% of C, 0.1% or less of Si, 0.05 to 0.5% of Mn, 0.03% or less of P, And a balance of Fe and inevitable impurities, wherein the composition contains 0.02 to 0.1% of Sol.Al, 0.005% or less of N, 0.0003 to 0.0030% of B, 0.004% or less of Ti and 0.003% or less of Nb, A hot-rolled steel sheet for cold-rolled material having an average crystal grain size of 13 탆 or less and an aging index AI of 10 MPa or less.

The hot-rolled steel sheet of the present invention is subjected to hot rolling at a finishing temperature: Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C) and a reduction ratio of 20% or more in the final pass to a steel slab having the above- Cooling to 700 ° C or lower at a cooling rate of 10 ° C / sec or more, and winding in a coiled state at a coiling temperature of 450 to 650 ° C. At this time, it is preferable to cool the steel sheet on the coiled coil after winding.

According to the present invention, a cold rolled steel sheet or a surface-treated cold rolled steel sheet can be produced at low cost, even if it is a cold-rolled steel sheet having a low rolling ability. The cold-rolled steel sheet or the surface-treated cold-rolled steel sheet produced by using the hot-rolled steel sheet of the present invention can greatly contribute to lowering the cost of building materials such as wall materials and roofing materials.

Fig. 1 is a graph showing the relationship between the average crystal grain size of the hot-rolled steel sheet and the TS of the hot-rolled steel sheet and the TS of the steel sheet as cold rolled steel sheet.
Fig. 2 is a diagram showing the relationship between the AI of the hot-rolled steel sheet and the TS of the steel sheet as cold-rolled.
3 is a diagram showing the relationship between the annealing temperature by the amount of B and Ti and the TS of the cold-rolled steel sheet.

The details of the present invention are described below. The following "%" means "% by mass" unless otherwise specified.

1) chemical composition

C: 0.016 to 0.07%

If the amount of C exceeds 0.07%, a large amount of cementite is produced and the cold rolling load is increased. On the other hand, if the amount of C is less than 0.016%, the degree of supersaturation is low, so that cementite does not precipitate well, and C remains in a solid state to increase the cold rolling load. Therefore, the amount of C is 0.016 to 0.07%.

Si: 0.1% or less

When the amount of Si exceeds 0.1%, the strength is increased and the cold rolling load is increased. Therefore, the amount of Si should be 0.1% or less. The lower limit of the amount of Si does not need to be specially specified, but excessively lowering causes an increase in cost, so it is preferable to set the amount to 0.001%.

Mn: 0.05 to 0.5%

Since Mn has a function of fixing S as MnS and improving hot ductility, the content of Mn should be 0.05% or more. However, when the Mn content exceeds 0.5%, the steel is hardened and the cold rolling load is increased. Therefore, the amount of Mn is set to 0.05 to 0.5%.

P: not more than 0.03%

P is a solid solution strengthening element, and when it exceeds 0.03%, hardening of the steel is caused and the cold rolling load is increased. Therefore, the amount of P is 0.03% or less. The lower limit does not need to be specially specified, but it is preferable to set the lower limit to 0.001% because excessive reduction causes an increase in cost.

S: not more than 0.03%

S is an element which inhibits hot ductility. When the amount exceeds 0.03%, edge cracks occur at the coil edge. Therefore, the amount of S is 0.03% or less. The lower limit does not need to be specially specified, but it is preferable to set the lower limit to 0.001% because excessive reduction causes an increase in cost.

Sol.Al: 0.02 to 0.1%

Al has the effect of reducing the solid solution N by reducing N, which is not fixed to B, as AlN, thereby reducing the cold rolling load. In order to obtain such an effect, the amount of Sol.Al needs to be 0.02% or more. However, when the amount exceeds 0.1%, the production cost is increased. Therefore, the amount of Sol.Al is 0.02 to 0.1%.

N: 0.005% or less

N is likely to remain in the hot rolled steel sheet in a solid state, and the amount of N needs to be 0.005% or less in order to increase the cold rolling load. The lower limit does not need to be specially specified, but excessive reduction may cause an increase in cost, so it is preferably 0.001%, more preferably 0.002%.

B: 0.0003 to 0.0030%

As shown in Fig. 3, in order to lower the TS of the steel sheet as cold rolled, that is, to reduce the cold rolling load, B addition is effective. It is considered that this is because B is a strong nitride-forming element, and N is fixed as BN, and the solid solution N is reduced. In order to obtain such an effect, the amount of B needs to be 0.0003% or more. However, when the amount exceeds 0.0030%, a boride of iron is produced and the cold rolling load is increased. Therefore, the amount of B is 0.0003 to 0.0030%.

Ti: not more than 0.004%

Recently, production quantities of high tensile steels and high-strength forming IF steels are increasing. Ti, which is an indispensable element for these steels, is likely to remain as impurities in the steels and may significantly raise the recrystallization temperature as shown in Fig. Therefore, the amount of Ti needs to be 0.004% or less. The lower the amount of Ti is, the more preferable it is and it may be 0%.

Nb: not more than 0.003%

Like Nb, Nb is an essential element for high tensile strength steel and high-strength formed IF steel, and is likely to remain as an impurity in steel and may remarkably increase the recrystallization temperature. Therefore, the amount of Nb should be 0.003% or less. The smaller the amount of Nb is, the more desirable it is and it may be 0%.

The remainder is Fe and inevitable impurities.

2) Average crystal grain size: 13 占 퐉 or less (ferrite lips)

As shown in Fig. 1, in order to lower the TS of the steel sheet as cold-rolled, it is effective to set the average grain size of the hot-rolled steel sheet to 13 m or less. This is considered to be because the crystal grain boundaries which are precipitation sites of cementite are increased and the solid solution C is reduced by decreasing the average crystal grain size. Particularly, in order to precipitate cementite even when the steel sheet on the wound coil is cooled, it is effective to set the average crystal grain size to 13 μm or less. More preferably less than 12 mu m.

3) AI: 10 MPa or less

As shown in Fig. 2, AI is an index of the amount of solid solution C and solid N, and it is effective to lower the AI of the hot-rolled steel sheet to 10 MPa or less in order to lower the TS of the steel sheet as cold- MPa, it is considered that the amount of solid solution C and the amount of solid solution N become smaller.

4) Manufacturing method

The hot-rolled steel sheet of the present invention can be produced by hot-rolling the slab of the steel having the above chemical composition under the following conditions.

Finishing temperature: Ar ₃ transformation point - the temperature range of (Ar ₃ transformation point + 49 ℃), the final pass rolling reduction: 20%

If the outlet temperature of the finishing temperature of the final pass of the hot rolling less than the Ar ₃ transformation point is, in the sheet thickness direction and easily become a heterogeneous microphase tissue generation, because they are easy to plate thickness deviation occurs after the cold rolling, Ar ₃ transformation point or higher It is necessary to finish rolling. More preferably an Ar ₃ transformation point + 10 ° C or more. Here, the final pass means rolling in the final rolling stand in hot rolling. On the other hand, when the finishing temperature exceeds the (Ar ₃ transformation point + 49 ° C) or the reduction ratio of the final pass (also referred to as the rolling rate) is less than 20% And after the transformation, the hot rolled steel sheet having a structure with an average crystal grain size exceeding 13 탆 is formed, and the grain boundary which is the precipitation site of the cementite is decreased and the cold rolling load is increased. More preferably an Ar ₃ transformation point + 25 ° C or less. Accordingly, the finishing temperature to a temperature range of Ar ₃ transformation point ~ (Ar ₃ transformation point + 49 ℃), and it is necessary to the reduction ratio in the final pass to 20% or more.

Cooling after hot rolling: cooling to 700 ° C or less at a cooling rate of 10 ° C / sec or more within 0.4 seconds

When cold-rolling is not started immediately after the hot-rolling, that is, after the final pass in the above-mentioned hot rolling, the hot-rolled steel sheet has a structure with an average crystal grain size exceeding 13 탆 by crystal grain growth during cold- The precipitation site of the grain boundary is decreased and the cold rolling load is increased. Also, even when the cooling rate is slower than 10 ° C / second, the cold rolling load is likewise increased. Therefore, it is necessary to cool the steel sheet at a rate of 10 DEG C / sec or more immediately after the hot rolling, that is, within 0.4 seconds. The cooling needs to be performed until the crystal grain growth rate is 700 ° C or lower. On the other hand, although there is no upper limit on the cooling rate, a large-scale facility is required in order to achieve an excessive cooling rate, leading to an increase in cost.

Coiling temperature: 450 ~ 650 ℃

If the coiling temperature exceeds 650 DEG C, scale defects tend to occur. If the coiling temperature is less than 450 캜, the coil shape tends to be disturbed. Therefore, the coiling temperature is set to 450 to 650 ° C. Further, in the steel sheet in which the precipitation site of the cementite is increased by adding B to fix N and the average crystal grain size to 13 占 퐉 or less as in the present invention, even if the wound coil is water-cooled, C sufficiently precipitates , And the amount of solid solution C can be reduced. Therefore, it is preferable to cool the coil after winding from the viewpoint of improving the productivity.

The temperature in the heating prior to the hot rolling may be a temperature at which the finishing temperature can be secured, generally 1050 占 폚 or higher.

The hot-rolled steel sheet of the present invention does not change its properties even if it is acid preservative or black precipitate as it is. In addition, even if temper rolling and leveling are performed for the purpose of improvement in pickling performance and shape correction, or the like, or the temper rolling and leveling are not performed, the characteristics do not change.

The hot-rolled steel sheet of the present invention is subjected to cold rolling in order to form a cold-rolled steel sheet or a surface-treated steel sheet. Since the cold-rolled steel sheet of the present invention has a good cold-rolling property, even when a cold rolled steel sheet having a thickness of 0.5 mm or less is produced by making the reduction ratio of the cold- can do. In general, the rolling reduction of a cold rolled steel sheet is 60% or more.

The hot-rolled steel sheet of the present invention shows the same recrystallization behavior as in the case where B is not added as described above, even when recrystallization annealing is carried out with good cold rolling property. Therefore, it is possible to use not only a cold-rolled steel sheet as cold-rolled steel sheet or a cold-rolled steel sheet as cold rolled steel sheet but also a cold-rolled steel sheet produced by cold- It can be suitably used as a hot-rolled steel sheet for cold-rolled steel for processed steel sheets.

Example

The steel having the chemical compositions 1 to 10 of the chemical compositions shown in Table 1 was melted and turned into a slab and then heated to 1200 캜 and hot rolled under the hot rolling conditions shown in Table 2 to obtain hot rolled steel sheets A- P was prepared. Here, the Ar ₃ transformation point in Table 1 was determined by the following equation.

Ar ₃ transformation point (° C) = 901 - 325 x [C] + 33 x [Si] - 92 x [Mn] + 287 x [

Here, [M] represents the content (mass%) of the element M.

Next, the obtained hot-rolled steel sheet was subjected to temper rolling at an elongation of 1% after pickling, and the average grain size and AI were measured by the above-mentioned method. The hot rolled steel sheet after pickling was cold-rolled at a reduction ratio of 80% to prepare a cold rolled steel sheet (thickness: 0.36 mm), and TS of the steel sheet as cold rolled was measured by the above method. Here, the plate thickness at the center position in the width direction of the steel sheet as cold-rolled was measured at 30 points at regular intervals from the entire length of the steel plate, and the standard deviation was obtained. The steel sheet subjected to cold rolling was subjected to heat treatment at a pitch of 25 占 폚 from 450 占 폚 to 700 占 폚 at a temperature of 25 占 폚 for 30 seconds at each temperature and TS was measured to determine the heat treatment temperature And the lowest temperature among these temperatures was set as the recrystallization temperature.

The results are shown in Table 2.

The hot-rolled steel sheets D, I, N, and O of the present invention are soft and have a TS of 725 MPa or less as cold-rolled, can reduce the cold rolling load and have a recrystallization temperature of 550 deg. It can be understood that a cold-rolled steel sheet or a surface-treated cold-rolled steel sheet can be produced at low cost.

On the other hand, in the hot-rolled steel sheets A, E, F, G and H of Comparative Examples, the average grain size was large and the cementite was not sufficiently precipitated, AI became high and hot- As a result, the AI is increased, the TS of the cold-rolled steel is 725 MPa or more and hard, and the cold rolling load can not be reduced. The hot-rolled steel sheets J, L, and M having a content of Ti and Nb exceeding 0.004% require a high-temperature annealing at a recrystallization temperature of 650 ° C or higher, resulting in an increase in the manufacturing cost of the cold-rolled steel sheet or the surface-treated cold-rolled steel sheet. The hot-rolled steel sheet P having the final pass temperature of 805 ° C lower than the Ar ₃ transformation point has a large gauge fluctuation of the steel sheet after cold-rolling due to the material deviation of the hot-rolled sheet,

Claims (3)

0.001% or more and 0.03% or less of S, 0.001% or more and 0.03% or less of S, 0.02% or less of S, And the balance of Fe and inevitable impurities, wherein the content of N is 0.001 to 0.005%, the content of B is 0.0003 to 0.0030%, the content of Ti is 0.004% or less and the content of Nb is 0.003% A hot-rolled steel sheet for cold-rolled material having a grain size of less than 12 μm and an aging index AI of 10 MPa or less. 0.001% or more and 0.03% or less of S, 0.001% or more and 0.03% or less of S, 0.02% or less of S, Of steel having a chemical composition containing at least 0.001% of N, at least 0.001% of N, at least 0.003% of B, at least 0.003% of B, at most 0.004% of Ti, at most 0.003% of Nb and the balance of Fe and inevitable impurities, Prepare,
The slab is subjected to hot rolling at a finishing temperature: Ar ₃ transformation point to (Ar ₃ transformation point + 49 ° C) and a reduction ratio in the final pass: 20% or more. Thereafter, a cooling rate of 10 ° C / Wherein the rolled steel sheet is cooled to 700 DEG C or lower and rolled up into a coil shape at a coiling temperature of 450 to 650 DEG C. 3. The method of claim 2,
Further, a method for manufacturing a hot-rolled steel sheet for cold-rolled material, which rolls a coiled steel sheet after winding.

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