KR101510272B1 - Method for manufacturing hot rolled steel plate and hot rolled steel sheet - Google Patents

Abstract

According to one embodiment of the present invention, the hot-rolled steel sheet manufacturing method comprises the steps of: 0.04 to 0.07% of carbon (C), 0.01 to 0.8% of silicon (Si), 1.1 to 1.7% of manganese (Mn) (P): 0.005 to 0.015%, sulfur (S): 0.005% or less (excluding 0%), , Nitrogen (N): 0.005% or less (excluding 0), and the remaining Fe and other unavoidable impurities is cast into a thin slab having a thickness of 30 to 150 mm and the thin slab is subjected to rough rolling, heating, .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a hot rolled steel sheet and a hot rolled steel sheet using the same,

The present invention relates to a hot-rolled steel sheet manufacturing method and a hot-rolled steel sheet using the same, and more particularly, to a hot-rolled steel sheet making method capable of improving a material deviation and a hot-rolled steel sheet using the same.

In recent years, with the amplification of environmental problems in the automobile industry, there has been a demand for measures to improve fuel economy, such as weight reduction and integrated molding of parts. Accordingly, development of hot-rolled steel sheets having excellent press-

As a high-strength hot-rolled steel sheet for processing, there is known a ferrite and a martensite structure, a composite structure composed of ferrite and bainite structure, and a single-phase structure composed mainly of ferrite and bainite. However, the ferrite and martensite structure are unsuitable for use in rotating parts such as wheels of an automobile wheel due to the problem of poor stretch flangeability (hole expandability). Further, the material generally has a characteristic in which ductility (elongation) is decreased when the strength is increased. Therefore, it is difficult to satisfy both strength and elongation. In order to overcome this problem, there has been proposed a technique of reducing the hardness difference as means for improving the hole expandability of the ferrite-bainite structure.

As such techniques, Patent Documents 1 and 2 described below disclose a steel sheet mainly composed of bainite. Since a steel sheet has excellent elongation flangeability (hole expandability) but low softness ferrite, There are disadvantages. In addition, Patent Document 3 described below discloses a method of manufacturing a steel sheet that achieves both elongation flangeability (hole expandability) and ductility by using two-stage cooling. However, as the weight and complexity of parts are increased, It is demanding ductility.

However, all of the above prior art methods are based on a conventional milling process for producing slabs of 200 mm or more. Since the main phases constituting the microstructure are ferrite and bainite, It is difficult to control and it is difficult to prevent material deviation in the width direction or the length direction. In addition, since the final rolling speed of the conventional rolling mill is faster than 400 mpm, there is a problem that it is difficult to stably obtain the desired material due to the manufacturing characteristics of the high-burring steel which is to be rolled at a temperature lower than the Bs temperature in the case of producing the high- .

On the other hand, a manufacturing process (mini-milling process) using a so-called thin slab, which is a new steel manufacturing process which has recently attracted attention, has a small temperature deviation in the width direction and the longitudinal direction of the strip, Has attracted attention as a process having the potential to be manufactured. However, the development of high strength hot rolled steel steels with better material properties than existing steels has not been developed using the characteristics of the mini milling process using thin slabs.

1. Japanese Unexamined Patent Application Publication No. 3-180426 Aug. 6, 1991. 2. Japanese Unexamined Patent Application Publication No. 04-088125, March 23, 1992. 3. Japanese Patent Application Laid-Open No. 6-293910 Oct. 21, 1994.

It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet which can improve a material deviation using a thin slab-making method and a hot-rolled steel sheet using the same.

Another object of the present invention is to provide a hot rolled steel sheet production method capable of securing stretch flangeability and a hot rolled steel sheet using the same.

Other objects of the present invention will become more apparent from the following detailed description and the accompanying drawings.

According to one embodiment of the present invention, the hot-rolled steel sheet manufacturing method comprises the steps of: 0.04 to 0.07% of carbon (C), 0.01 to 0.8% of silicon (Si), 1.1 to 1.7% of manganese (Mn) (P): 0.005 to 0.015%, sulfur (S): 0.005% or less (excluding 0%), , Nitrogen (N): 0.005% or less (excluding 0), and the remaining Fe and other unavoidable impurities is cast into a thin slab having a thickness of 30 to 150 mm and the slab is subjected to rough rolling, heating, .

The finishing rolling step may have a rolling speed difference of 15% or less for one strip.

The finish rolling step may be performed at a temperature immediately above the Ar3 transformation point.

In the winding step, the strip subjected to the finish rolling step may be cooled to 630 to 690 ° C, air-cooled for 3 to 9 seconds, and then wound at a temperature of 420 to 445 ° C.

The casting step may be a continuous casting with a casting speed of 4.5 mpm or more.

In the rough rolling step, the surface temperature of the thin slab is 950 to 1100 ° C at the inlet of the roughing mill, and the cumulative rolling reduction rate may be 65 to 90%.

The heating step may heat the rough-rolled strip to 950 to 1100 占 폚.

(C / 12) / (Ti / 48) + (Mo / 96) + (V / Ti) (Nb / 93) + (V / 51)} < / = 1.5.

According to one embodiment of the present invention, the hot-rolled steel sheet comprises 0.04 to 0.07% of carbon (C), 0.01 to 0.8% of silicon (Si), 1.1 to 1.7% of manganese (Mn) : 0.01 to 0.05%, titanium (Ti): 0.09 to 0.13%, molybdenum (Mo): 0.09 to 0.13%, phosphorus (P): 0.005 to 0.015%, sulfur (S) (N): 0.005% or less (excluding 0), and the remaining Fe and other unavoidable impurities are cast into a thin slab having a thickness of 30 to 150 mm, and the thin slab is subjected to rough rolling, do.

The fine precipitates of the hot-rolled steel sheet may be 1 x 10 ⁵ / m ³ .

According to one embodiment of the present invention, it is possible to manufacture a hot-rolled steel sheet having improved stretch flangeability and material deviation using a thin slab method. In addition, it is possible to omit the reheating process in the existing mill through the thin slab method, thereby saving energy and improving the productivity. In addition, it is possible to use the steel in which scrap of scrap iron is melted in the electric furnace through the thin slab-making method, thereby raising the recyclability of resources.

1 is a schematic view of a mini-mill process facility according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to explain the present invention to a person having ordinary skill in the art to which the present invention belongs. Accordingly, the shape of each element shown in the drawings may be exaggerated to emphasize a clearer description.

First, the present invention can produce a hot-rolled gobling steel through a mini-mill process using a thin slab-making method. 1 is a schematic view of a mini-mill process facility according to an embodiment of the present invention.

The continuous casting machine 10 produces a thin slab a having a thickness of 30 to 150 mm. The thin slab (a) is compared to a slab of 200 mm or more produced in a continuous casting machine of conventional mill. Conventionally, slabs of 200 mm or more are completely cooled in a yard, so it is necessary to reheat sufficiently the surface temperature to 1100 DEG C or more in the reheating furnace before hot rolling. On the other hand, since the thin slabs (a) are directly fed to the rough rolling mill 20 without passing through the reheating furnace, the performance heat can be used as it is, thereby saving energy and improving productivity.

The rough rolling mill 20 rolls the thin slab a into a hot rolled strip of a predetermined thickness or less and in this process the temperature of the lowered strip is compensated through the heating means (or induction heater) 30, The strip b is rolled to a desired final thickness at the finishing mill 50 and cooled through a run out table 60 and then wound up at a constant temperature in a winder 70 to produce the desired material Hot-rolled steel sheets are manufactured.

At this time, in order to compensate for the difference between the performance speed and the rolling speed, a coil box 40 is provided in front of the finish rolling mill 50, and the hot-rolled strip b having passed through the heating means 30 can be firstly wound. Recently, as a high-speed play method of 6mpm or more is implemented, a true continuous continuous rolling process without using the coil box 40 is being developed.

As described above, the composition of the hot-rolled steel sheet produced through the thin slab process is 0.04 to 0.07% of carbon (C), 0.01 to 0.8% of silicon (Si), 1.1 to 1.7% of manganese (Mn) , The aluminum (Al) of 0.01 to 0.05%, the titanium (Ti) of 0.09 to 0.13%, the molybdenum (Mo) of 0.09 to 0.13%, the niobium (Nb) of 0.01 to 0.02%, the vanadium (V) 0.005 to 0.015% of phosphorus (P), 0.005% or less (excluding 0) of sulfur (S), 0.005% or less of nitrogen (N), and the balance of Fe and other unavoidable impurities. The function and content range of each element will be briefly described.

Carbon (C): 0.04 to 0.07 wt%

Carbon (C) is an important element for increasing the strength of a steel sheet and securing a composite structure of ferrite and bainite. If the content is less than 0.04 wt%, the desired strength can not be secured in the present invention. On the other hand, if the content exceeds 0.07 wt%, the risk of formation of martensite and weldability is lowered, The probability of occurrence of surface defects increases. Therefore, the content of C is preferably limited to 0.04 to 0.07 wt%.

Silicon (Si): 0.01 to 0.8 wt%

Silicon (Si) is a useful element that can secure strength without deteriorating the ductility of the steel sheet. When the content is less than 0.01 wt%, it is difficult to secure the above effect. When the content is more than 0.8 wt%, precipitation of C from ferrite is promoted, and coarse Fe carbide precipitates easily in the grain boundaries. In addition, there is a high possibility that surface properties and weldability are lowered. Therefore, the content of Si is preferably limited to 0.01 to 0.8 wt%.

Manganese (Mn): 1.1 to 1.7 wt%

Manganese (Mn) is an essential element for securing the strength. For this purpose, 1.1 wt% or more of Mn should be added. However, if it exceeds 1.7 wt%, segregation tends to occur, which deteriorates stretch flangeability (hole expandability). Particularly, in order to secure a strength of 780 MPa or more, addition of Mn is very effective, but in order to have high stretch flangeability (hole expandability) and ductility, it is preferable that the Mn content is 1.7% or less.

Aluminum (Al): 0.01 to 0.05 wt%

Aluminum (Al) is used as a de-oxidation material, and it is an element that, like silicon (Si), suppresses cementite precipitation and stabilizes austenite by slowing the progress of transformation. Since it segregates in grain boundaries in the high-temperature region and forms fine carbides in the hot-rolled steel sheet grain, unnecessary dissolved nitrogen (N) in the steel is precipitated as AlN by adding Al to the austenite stabilization minimum effect limit of 0.01% or more. However, when the content exceeds 0.05%, nozzle clogging occurs during continuous casting, and hot brittleness and ductility are remarkably lowered by Al oxide or the like during casting, and surface defects are likely to occur. Therefore, in order to remove quality defects due to Al segregated at grain boundaries in the high temperature region. Aluminum is strictly limited to 0.01% to 0.05%. Aluminum may be soluable Al, S.Al.

Phosphorus (P): 0.005 to 0.015 wt%

Phosphorus (P) increases strength by solid solution strengthening, and when added together with Si inhibits cementite precipitation while maintaining at 300 to 580 ° C and promotes carbon enrichment with austenite, so it is added in an amount of 0.015% or less. If the concentration of phosphorus is more than 0.015%, it is disadvantageous to secondary processing brittleness and it lowers the adhesion of zinc plating and deteriorates the alloying property, so that the content is limited to 0.015% or less.

Sulfur (S): Not more than 0.005%

Sulfur (S) is an impurity which is inevitably contained, and forms FeS by binding with Fe, thereby causing hot brittleness. Therefore, it is preferable to suppress its content to the maximum. In theory, it is advantageous to limit the content of S to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the S content is preferably limited to 0.005%.

Mo: 0.09 to 0.13% and Ti: 0.09 to 0.13%

Molybdenum (Mo) and titanium are also very important elements in the present invention, and are effective elements for securing strength by precipitating fine carbides such as MoC and TiC. For this purpose, it is necessary to add 0.09 to 0.13 wt% of molybdenum (Mo) and 0.09 to 0.13 wt% of titanium (Ti). When Mo is less than 0.09% and Ti is less than 0.09% %, And when Ti exceeds 0.13%, precipitates are excessively generated and ductility is deteriorated.

Further, in the case of a graded steel sheet having a tensile strength of 780 MPa or more, at least one selected from Nb: 0.01 to 0.02% and V: 0.03 to 0.06% can be included. These elements are not factors which have a decisive influence on securing the basic properties of the hot-rolled steel sheet according to an embodiment of the present invention, but they are preferably added for improving the properties of the product.

When the structure of a steel sheet having both of an elongation flangeability (hole expandability) and a ductility is made of a ferrite-bonnetite structure and each structure fraction is composed of 1 to 5% of bainite and ferrite, Can be rapidly improved.

The hot-rolled steel sheet satisfying the above-mentioned component system and internal structure may have MoC and TiC precipitates. The ferrite particles are sufficiently grown by the precipitates, thereby improving the ductility without lowering the hole expandability. (C / 12) / {(Ti / 48) + (Mo / 96) + (Nb / 93) + (V / 51)} 1.5 wherein C, Mo, Ti, , It is possible to obtain a tensile strength of 780 MPa or more because the fine precipitates have a density of 1 x ^{10 5} / μm ³ or more.

Hereinafter, a method of manufacturing a hot-rolled steel sheet satisfying the above-described steel component will be described. As described above, the mini-milling process includes continuous casting, rough rolling, heating, finish rolling, cooling, and winding. The present invention newly defines the operating conditions of each step, Can be prepared.

First, it is preferable that the casting speed of the continuous casting step is 4.5 mpm or more. Generally, steel having a tensile strength of 780 MPa or more has a higher content of elements added for the purpose of securing the strength of C, Mn, Si, etc., so that there is a risk of segregation from the casting as the casting speed is slower. The strength is difficult to secure and the speed of the material is limited to 4.5mpm or more because there is a great risk of material variation in the width direction or the length direction.

In the rough rolling step, the continuously cast thin slabs are rough-rolled by a roughing machine consisting of 2 to 4 stands. At this time, it is preferable that the surface temperature of the thin slab at the inlet side of the rough rolling mill is 950 to 1100 캜, and the cumulative rolling reduction during rough rolling is 65 to 90%.

If the surface temperature of the slab at the inlet side of the rough rolling mill is less than 950 ° C, the risk of edge cracking increases as well as the rough rolling load increases significantly. If the surface temperature exceeds 1100 ° C, Is limited to 950 to 1100 占 폚.

In addition, cumulative rolling reduction during rough rolling plays an important role in obtaining a uniform target material in the present invention. That is, the microscopic distribution of Mo, Ti, Nb, and V, which are important elements in the production of goburning steel, becomes uniform as the reduction rate in rough rolling increases, and the temperature gradient in the width direction and thickness direction of the strip becomes smaller. It is very effective in obtaining materials. However, if the cumulative rolling reduction is less than 65%, the above effects can not be sufficiently exhibited. If the cumulative rolling reduction exceeds 90%, the rolling deformation resistance increases greatly and the manufacturing cost rises. .

In the heating step, the coarsely rolled strip is heated again and heated to a temperature of 950 to 1100 ° C. When the surface temperature of the rough-rolled strip is less than 950 캜, the rolling load during finish rolling is large. When the surface temperature exceeds 1100 캜, the energy cost for temperature increase is increased and the tendency of surface- It is preferable to limit the heating temperature to 950 to 1100 占 폚.

It is preferable that the finishing rolling step is such that the rolling speed difference in one strip is 15% or less. Since the high-strength hot-rolled gobling steel of the 780 MPa class which is the object of the present invention uses the formation of the transformed structure as the strengthening mechanism, there is a high possibility that the material characteristics are changed according to the rolling speed during the finish rolling. That is, when the difference in the rolling speed is greater than 15% in a finishing mill composed of a plurality of stands, it becomes difficult to obtain a uniform cooling rate and a target coiling temperature in the runout table, .

It is preferable that the finish rolling step is finishing rolling at a temperature immediately above the Ar3 transformation point because if the finishing rolling temperature is lower than the Ar3 transformation point, the possibility that the ferrite structure is mixed is increased and the stretch flangeability is lowered.

Meanwhile, in the winding step, it is preferable that the finish rolled strip is continuously cooled to a temperature of 630 to 690 캜 in a run-out table, air-cooled for 3 to 9 seconds, and then wound at a temperature of 420 to 445 캜. The reason for first cooling the finished rolled strip to a temperature of 630 to 690 ° C is to transform some of the finishing rolled strip in the austenite region into ferrite. Therefore, when the cooling temperature is lower than 630 ° C, the possibility of carbide precipitation such as cementite increases. When the cooling temperature exceeds 690 ° C, the fraction of ferrite is low and it becomes difficult to effectively obtain bainite. Limit.

The continuously cooled strip is subjected to an air cooling process for 3 to 9 seconds on the runout table. If the time is less than 3 seconds, the C enrichment to the retained austenite is insufficient and the time for the ferrite transformation is insufficient, There is a risk that the stretch flangeability is lowered due to the deposition of carbide even when the time exceeds 9 seconds. In addition, the length of the equipment or the productivity may be deteriorated. Therefore, Sec.

Finally, when the air-cooled intermediate strip is wound, the risk of increasing the martensite increases when the coiling temperature is lower than 420 ° C. If the coiling temperature is higher than 445 ° C, pearlite may be formed, So that the coiling temperature is limited to 420 to 445 ° C.

The above-described finish rolling step and winding step are characterizing technical features of the present invention. By combining two or more of them, a high-strength hot-rolled gibbering steel having a material deviation of 780 MPa in tensile strength required in the present invention can be produced.

The following experiment was conducted to examine the technical effect of the present invention. Hot-rolled strips were produced using the steel produced as shown in Table 1 under the process conditions such as the slab thickness, peripheral speed and rolling speed difference shown in Table 2, and the respective materials (tensile strength, elongation, hole expansion rate and material deviation) The presence or absence of the surface scale was measured and shown together in Table 2.

In Table 1, Examples 1 to 10 show the case where the hot-rolled strip is produced by the thin-slab method (slab thickness: 84 mm), and the hot-rolled strip according to the comparative example (slab thickness: 230, 250 mm) .

In Table 2, the slab surface temperature refers to the surface temperature measured immediately before rough rolling. The difference in rolling speed is obtained by dividing the difference between the maximum sheet passing speed and the minimum sheet passing speed in one strip by the average sheet passing speed, , And the smaller the value, the smaller the fluctuation amount of the rolling speed. The ROT intermediate temperature is the surface temperature of the strip measured by ROT shear cooling immediately after finish rolling.

On the other hand, in Table 2, all the heating temperatures of the strips after rough rolling were all applied at 1080 ° C, and the finishing rolling temperature (FDT) was carried out directly on the Ar3 transformation point determined for each steel species. The reheating temperatures in Table 2 were all applied at 1200 ° C. The final thickness of hot-rolled strips in all grades was the same as 3.2 mm. CT means coiling temperature.

The tensile strength (TS) and elongation (T-EI) of the hot-rolled steel sheet prepared in the above manner were measured and the microstructure was observed. The results are shown in Table 2 below. In Table 2, TS = HER (elongation planarity) is 45,000 MPa% or more, material deviation is 20 MPa or less based on? TS (Mpa), and arithmetic scale generation occurs. Is satisfied, and when it is not satisfactory, it is marked as " X ".

The tensile strength (TS) and hole expandability (HER) of the above Table 2 are values measured by taking JIS No. 5 specimens in a direction perpendicular to the rolling direction at a width of w / 4, And the minimum value is subtracted from the maximum value among the material values measured in the direction and the width direction. On the other hand, the hole extensibility test is a calculation of a hole having a diameter of 10.8 mm which is pushed up to the cone after punching the hole, and the expanded hole is measured as a percentage of the initial hole (10.8 mm) until a crack is generated in the circumference portion. As can be seen from the experimental results shown in Table 2, according to the present invention, it is possible to manufacture a high strength hot rolled steel having excellent hole expandability and very small material deviation, and to produce a high strength hot rolled steel sheet having a tensile strength of 780 MPa or more.

Although the present invention has been described in detail by way of preferred embodiments thereof, other forms of embodiment are possible. Therefore, the technical idea and scope of the claims set forth below are not limited to the preferred embodiments.

10: Continuous casting machine
20: rough rolling mill
30: Heating means (induction heater)
40: Coil box
50: Finishing mill
60: Runout table
70: Winder

Claims (11)

(Si): 0.01 to 0.8%, manganese (Mn): 1.1 to 1.7%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.09 (S): 0.005% or less (excluding 0), and nitrogen (N): 0.005% or less (excluding 0) to 0.13%, molybdenum (Mo): 0.09 to 0.13% Wherein the steel comprises at least one selected from the group consisting of niobium (Nb): 0.01 to 0.02% and vanadium (V): 0.03 to 0.06%, and the remaining Fe and other unavoidable impurities are mixed at a casting speed of 4.5 to 6.1 mpm A continuous casting step of casting into a thin slab having a thickness of 30 to 150 mm by continuous casting;
A rough rolling step of subjecting the thin slab to rough rolling at a cumulative rolling reduction of 65 to 90%;
A heating step of heating the rough-rolled strip to a temperature of 950 to 1100 캜;
A finish rolling step of finishing rolling the heated strip at a temperature of 875 to 895 占 폚; And
And a winding step of cooling the finish-rolled strip to 630 to 690 占 폚, air-cooling it for 3 to 9 seconds, and then winding it at a temperature of 420 to 445 占 폚,
The steel,
(C / 12) / {Ti / 48 + Mo / 96 + Nb / 93 + V / 51}
The hot-
A tensile strength (TS) of 780 MPa or more,
The tensile strength (TS) x hole expandability (HER) is 45,000 MPa% or more,
Wherein the material deviation (? TS) is 18 MPa or less. The method according to claim 1,
Wherein the finishing rolling step has a rolling speed difference of 15% or less with respect to one strip. delete delete delete delete The method according to claim 1,
Wherein the rough rolling step is such that the surface temperature of the thin slab is 950 to 1100 DEG C at the side of the roughing mill. delete delete A hot-rolled steel sheet produced by the method of any one of claims 1, 2, and 7. delete

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