KR101149121B1 - High strength hot rolled steel sheet and the method of producing the same - Google Patents

Abstract

The present invention relates to manufacturing a high strength hot rolled steel sheet by reheating and extracting a steel slab to a predetermined extraction temperature, performing hot finish rolling at a predetermined finishing finish temperature above an Ar3 transformation point, and then cooling and winding at a predetermined winding temperature. , The steel slab in weight% C: 0.05 ~ 0.07wt%, Si: 0.10 ~ 0.25wt%, Mn: 1.0 ~ 1.8wt%, S: 0.01wt% or less, P: 0.01wt% or less, Nb: 0.01 ~ 0.03wt%, V: 0.06 ~ 0.13wt%, Cu: 0.01 ~ 0.09wt%, Sn: 0.01 ~ 0.09wt%, balance Fe and other unavoidable impurity alloy composition, the predetermined winding temperature is 600 ~ 630 ℃. .

Accordingly, the present invention has the advantage that the load applied to the hot-rolled steel sheet is more uniformly distributed, so that plate breakage does not occur, and even if six rolls are applied, the rolling reduction is compared with the case of seven rolls of the finishing mill. This is not very disadvantageous, there is an advantage that it is possible to manufacture a high-strength hot-rolled steel sheet reinforced with strength by satisfying the lower limit of the average tensile strength specification.

Description

High strength hot rolled steel sheet and the method of manufacturing the same

The present invention relates to a high-strength hot-rolled steel sheet and a method for manufacturing the same. More particularly, the high-strength hot-rolled steel sheet and its production are improved by optimizing the content of Nb and V in the alloy composition and optimizing the winding temperature after hot rolling It is about a method.

The main manufacturing factors that determine the material of the coiled steel sheet are known as the composition (component) of the steel, hot rolling finish temperature and winding temperature. The component system used for the production of hot rolled steel sheets requiring ductility, formability, and processability is about 0.03 to 0.06 wt% of C and 0.10 to 0.30 wt% of Mn. After that, cooling is performed.

1 is a photograph showing that plate break occurs in the hot rolled steel sheet.

In general, when breaking 60Kg / mm ² high-strength hot-rolled steel sheet is produced in the filament rolling process as shown in Figure 1, the breakage occurs frequently, which is 900 ~ 1000 ℃ during the filament rolling by adding a large amount of Nb when manufacturing a high strength steel sheet This is because hot deformation resistance increases because Nb is deposited therebetween.

That is, when Nb is precipitated between 900 ~ 1000 ℃ during finishing rolling, the hot deformation resistance is increased by the precipitated Nb, causing the roll force of the finishing mill roll to rise, resulting in uneven load on the hot rolled steel sheet. It is distributed to one side and occurs to one side. In particular, when there are six rolls of the finishing mill, the rolling reduction is disadvantageous compared to the finishing mill having seven rolls, and plate breaking occurs more frequently.

The present invention is to solve the conventional problems as described above, the present invention is to optimize the content of Nb and V in the alloy composition, the high-strength hot-rolled steel sheet with improved rolling plateability by optimizing the coiling temperature after hot rolling and its Its purpose is to provide a method of manufacture.

High-strength hot-rolled steel sheet of the present invention, by weight% C: 0.05 ~ 0.07wt%, Si: 0.10 ~ 0.25wt%, Mn: 1.0 ~ 1.8wt%, S: 0.01wt% or less, P: 0.01wt% or less, Nb : 0.01 ~ 0.03wt%, V: 0.06 ~ 0.13wt%, balance Fe and alloy composition of other unavoidable impurities.

In addition, the high-strength hot-rolled steel sheet may further include Cu: 0.01 to 0.09 wt%, Sn: 0.01 to 0.09 wt% by weight.

According to another aspect of the present invention, the present invention is a high-strength to re-heat the steel slab to a predetermined extraction temperature to extract, hot finish rolling at a predetermined finishing rolling temperature above the Ar3 transformation point, and then cooled and wound up at a predetermined winding temperature. In the method for producing a hot rolled steel sheet, the steel slab is C: 0.05 to 0.07 wt%, Si: 0.10 to 0.25 wt%, Mn: 1.0 to 1.8 wt%, S: 0.01 wt% or less, P: 0.01 wt % Or less, Nb: 0.01 to 0.03 wt%, V: 0.06 to 0.13 wt%, Cu: 0.01 to 0.09 wt%, Sn: 0.01 to 0.09 wt%, balance Fe and other unavoidable impurity alloy compositions, and the predetermined winding Temperature is 600-630 degreeC.

In addition, the predetermined finishing rolling temperature may be 850 ~ 900 ℃.

The present invention optimizes the content of Nb and V in alloy composition of high strength hot rolled steel and optimizes the winding temperature after hot rolling, thereby significantly reducing the amount of Nb precipitated during finishing rolling, resulting in hot deformation resistance due to precipitated Nb. Will decrease. Therefore, the increase in force of the finishing mill roll is reduced, and the load applied to the hot-rolled steel sheet is more uniformly distributed than the conventional one, so that plate breaking does not occur.

In addition, even if the six-roll finishing mill is applied, the rolling reduction is not significantly disadvantageous compared to the case of the finishing roll of seven finishing mills. There is an advantage to losing.

Hereinafter, preferred embodiments of the present invention will be described in detail.

High-strength hot rolled steel sheet according to the present invention, in weight% C: 0.05 ~ 0.07wt%, Si: 0.10 ~ 0.25wt%, Mn: 1.0 ~ 1.8wt%, S: 0.01wt% or less, P: 0.01wt% or less, Nb: 0.01 ~ 0.03wt%, V: 0.06 ~ 0.13wt%, Cu: 0.01 ~ 0.09wt%, Sn: 0.01 ~ 0.09wt%, balance Fe and other unavoidable impurities.

The content and the respective properties of the alloying elements which are the basic components of the present invention are as follows.

C: 0.05 ~ 0.07wt%

Carbon (C) is an essential element for giving high strength to steel. It is necessary to limit the content of carbon in order to obtain the appropriate strength and ductility in manufacturing hot rolling for processing.

When the content of C is less than 0.05 wt%, the second fraction of the second phase decreases in strength, and if it is more than 0.07 wt%, the crack sensitivity is increased during the production process, and cracks are generated during slab production. Limit to 0.07 wt%.

Si: 0.10 ~ 0.25wt%

Si is effective as a deoxidizer in the steelmaking process, and it is known that solid solution strengthening is caused by lattice deformation between a solid solution and Fe.

Si has a problem that the tensile strength is reduced when the content is less than 0.10wt%, the strength is increased but toughness worsens when it is more than 0.25wt%, the content is limited to 0.10 ~ 0.25wt%.

Mn: 1.0-1.8 wt%

Mn is known as a solid solution strengthening element that precipitates sulfur (S) dissolved in steel as MnS and prevents hot shortness caused by solid solution sulfur.It is effective in securing strength with carbon and lowers Ar3 transformation point. It is effective for preventing mixed tissues.

If the content is less than 1.0wt%, it is difficult to secure high strength, and if it is more than 1.8wt%, the carbon equivalent value which has a significant effect on the weldability becomes high and the weldability becomes worse, so the content is limited to 1.0 to 1.8wt%.

S: 0.01wt% or less

Although S is inevitably present as an impurity in steel, it segregates into FeS, MnS, etc. type, and is elongated in the rolling direction during hot rolling to promote anisotropy of steel sheet properties, as well as surface cracks, internal cracks, and center segregation during playing. It causes the impact toughness to deteriorate, so the content is limited to 0.01 wt% or less.

P: 0.01wt% or less

P is a representative central segregation element with Mn, S, etc., and it is segregated into Fe3P type at the center and grain boundaries when playing, and impairs the toughness greatly, and deteriorates internal quality. Therefore, it is desirable to limit P as much as the steelmaking operation technology allows. However, due to manufacturing cost and processing time, the content is limited to 0.01 wt% or less.

Nb: 0.01 ~ 0.03wt%

Nb precipitates as an Nb [C, N] type in the austenite and ferrite phases at about 700 to 1000 ° C to inhibit initial austenite growth and recrystallization, and also enlarges the microcrystalline region to increase the cumulative reduction rate, thereby miniaturizing tissue. It is an element that improves tensile strength and impact toughness through.

If the content is less than 0.01wt%, it is difficult to secure high strength due to insufficient strength increase effect. If the content is more than 0.03wt%, hot rolling load increases due to precipitation and microstructure of Nb during finishing rolling. Therefore, the content is limited to 0.01 to 0.03 wt%.

V: 0.06 ~ 0.13wt%

V is a solid solution strengthening and precipitation strengthening element, but the strength increase effect is less than that of Nb, but the precipitation temperature is 500 ~ 700 ℃ to precipitate after the finishing rolling. Therefore, the rise of the hot rolling load does not occur during the finishing rolling, it is possible to prevent the plate breaking.

However, if the content is less than 0.06wt%, it is difficult to secure high strength, and if the content is more than 0.13wt%, there is no further increase in strength, so the content is limited to 0.06 to 0.13wt%.

Cu: 0.01 ~ 0.09wt%, Sn: 0.01 ~ 0.09wt%

Cu and Sn are components that are inevitably included in an electric furnace steelmaking process using scrap iron as a main raw material.

Cu and Sn are added in small amounts to help improve the strength, but are also a strong element tending to generate surface defects of the slab. If the content is less than 0.01wt%, it is difficult to secure high strength due to insufficient strength increase effect, and if it exceeds 0.09wt%, a problem occurs in the surface defects of the slab, so the content is limited to 0.01 to 0.09wt%, respectively.

The present invention includes the components of the alloy steel, the remainder is iron (Fe) and unavoidable elements, and fine incorporation of unavoidable impurities may be allowed as elements contained according to the situation of raw materials, materials, manufacturing facilities, and the like.

The steel slab having the same component as described above is obtained through molten steel through a steelmaking process, and then manufactured through a continuous casting process. The hot slab is reheated and extracted to a predetermined extraction temperature through a heating furnace, and then hot rolled to roll to a desired thickness. High strength hot rolled steel sheet is manufactured through a cooling process and a winding process of cooling the steel sheet at room temperature.

Hereinafter, the manufacturing process of the hot rolled steel sheet of the present invention will be described in detail.

[Heating furnace process]

In the heating process of the steel slab, reheating is performed in the furnace to reclaim segregated components during casting.If the reheating temperature is low, the segregated components are not reusable. As it coarsens, its strength decreases.

In addition, it is necessary to adjust the reheating temperature holding time according to the thickness of the steel slab, that is, it is necessary to keep the reheating time longer as the thickness is thick, and to shorten the holding time as the thickness is thinner. Therefore, in one embodiment of the present invention, the substrate is reheated for a predetermined time, for example, 150 to 250 minutes, and extracted at a predetermined extraction temperature for the homogenization treatment.

Here, the predetermined extraction temperature is 1200 ~ 1300 ℃. If the extraction temperature is lower than 1200 ℃, coarse precipitates produced during continuous casting remain completely insoluble, leaving a large amount of coarse precipitates even after hot rolling, resulting in a large rolling load. This is because the coarsening is difficult to secure strength.

[Hot Rolling Process]

The reheated slab is finished hot rolling at a predetermined finishing rolling temperature above the Ar3 transformation point.

As described above, filament rolling is a process of manufacturing slab worked in rough rolling to the final size required by the customer. It secures materials, secures a good surface and shape according to the temperature of each application, and improves productivity. The tandem rolling mill is equipped with six to seven rolls in series.

In addition, the latest facilities such as AGC (Automatic Gauge Control) to secure the accuracy of thickness, Pair Cross and Shift Mill for shape manufacturing, and measuring machine facilities to check the size and shape of the manufactured coil are installed. The material of hot rolled products is very influenced by rolling temperature together with chemical composition.

Therefore, it is important to roll to a temperature suitable for the application, in the present invention, the predetermined finishing rolling finishing temperature is limited to 850 ~ 900 ℃.

When the finishing rolling finish temperature is lower than 850 ℃, excessive dislocations are introduced into the ferrite to grow during the subsequent cooling or winding process to form coarse grains on the surface, and if the finishing rolling finish temperature exceeds 900 ℃ There is a fear that the grain size of the ferrite increases and the decarburization depth of the surface layer increases, thereby deteriorating mechanical properties such as strength.

[Cooling, winding process]

After performing hot finish rolling on the steel slab to which the alloying element is added at a predetermined finishing rolling finish temperature, the steel slab is wound in a coil form at a predetermined winding temperature so as to be easily cooled and stored and moved. Here, it is preferable to control the said predetermined winding temperature to 600-630 degreeC.

The coiling temperature is an important control factor that determines the final structure of the hot rolled steel sheet and has a great influence on the tensile and impact toughness. If it exceeds 630 ° C, the oxidation scale of the hot rolled steel sheet becomes thick, making it difficult to remove the scale in the pickling process. In addition, the transformation strengthening effect is reduced, if less than 600 ℃ precipitation strengthening effect and winding shape is poor, so the temperature is limited to 600 ~ 630 ℃.

Hereinafter, with reference to the embodiment of the high strength hot rolled steel sheet according to the present invention will be described in detail.

Table 1 below shows the inventive steels and comparative steels in which each component element is different.

(Far Fe, Unit: wt%) division C Si Mn P S Cu Sn V Nb Comparative Steel 1 0.055 0.14 1.57 0.015 0.003 0.05 0.09 - 0.055 Comparative Steel 2 0.058 0.13 1.30 0.015 0.002 0.04 0.05 0.095 0.02 Invention steel 0.058 0.06 1.50 0.009 0.001 0.05 0.05 0.095 0.03

Compared with the alloy composition of the hot rolled steel sheet of this invention, the comparative steel 2 has a difference in content of P.

Invented steels and comparative steels having the composition shown in Table 1 were subjected to steel smelting in a ladle through electrical steelmaking to desulfurization, and to control the inclusions. In order to prevent the front scarfing.

Then, after reheating for 150 to 250 minutes through a heating furnace was the extraction temperature to 1200 ~ 1300 ℃. Thereafter, in the hot rolling process, the finishing rolling finish temperature was 880 ° C and the winding temperature was 580 ° C.

Table 2 shows the test results of the inventive steels and the comparative steels having the composition shown in Table 1 under the above manufacturing conditions, and the average value of various mechanical properties was calculated through ten tests.

division
Mechanical properties Tensile Strength (MPa) Yield strength (MPa) Elongation (%) Number of breaks The lower limit of the specification 590 420 21 Comparative Steel 1 Min 553 492 21
5
Max 623 563 30 Avg 598 538 25 Comparative Steel 2 Min 566 548 29
0
Max 600 555 31 Avg 584 551 30 Invention steel Min 592 547 23
0
Max 593 557 25 Avg 592 554 24

As a result, as shown in Table 2, in the case of 0.055 wt% Nb-added steel as in Comparative Steel 1, a plurality of plate breaks occurred. It can be seen that the mail order is not good.

However, Comparative Steel 2 has a number of failures that do not satisfy the lower limit of the specification of 584MPa, the average tensile strength of 584MPa.

On the other hand, in the case of the invention steel, not only the sheeting property is good, but the average tensile strength is 592 MPa, and it can be confirmed that the lower limit of the specification, 590 MPa, is satisfied.

Like Comparative Steel 2, the plate breakage does not occur, so the flowability is good, but the average tensile strength is 584 MPa, which does not satisfy the lower limit of the specification, 590 MPa. This is because V [C, N] is not easy to be precipitated at a low temperature.

Therefore, in order to examine the difference in tensile strength according to the change in the coiling temperature, rolling was again performed under the condition of adjusting the coiling temperature to 600 to 630 ° C under the test conditions as the composition of the inventive steel.

Figure 2 is a graph showing the results of rolling by dividing the case of the winding temperature to 580 ℃ and the case of upward adjustment to 600 ~ 630 ℃ as a component composition of the invention steel.

In FIG. 2, the part indicated by X indicates the result when the coiling temperature is 580 ° C., and the part indicated by the bold dot indicates the result of adjusting the coiling temperature upward to 600 to 630 ° C. FIG.

Referring to FIG. 2, when the winding temperature is 580 ° C. under the test conditions of the inventive steel, the average tensile strength does not reach 600 MPa. However, when the winding temperature is adjusted to 600 to 630 ° C. under the test conditions of the invention steel, the average tensile strength is 610 MPa. It can be seen that the strength increases. This is because the precipitation temperature of V [C, N] increases when the winding temperature is increased from 580 ° C to 600 ~ 630 ° C, thereby increasing the strength.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

1 is a photograph showing that plate breaking occurs in the hot rolled steel sheet,

Figure 2 is a graph showing the results of rolling by dividing the case of the winding temperature to 580 ℃ and the case of upward adjustment to 600 ~ 630 ℃ as a component composition of the invention steel.

Claims (4)

By weight% C: 0.05 ~ 0.07wt%, Si: 0.10 ~ 0.25wt%, Mn: 1.0 ~ 1.8wt%, S: 0.01wt% or less, P: 0.01wt% or less, Nb: 0.01 ~ 0.03wt%, V : 0.06 ~ 0.13wt%, balance Fe and alloy composition of other unavoidable impurities, High-strength hot rolled steel sheet, characterized in that it further comprises by weight: 0.01% to 0.01% by weight, Sn: 0.01% to 0.01% by weight. delete In the manufacturing method of high strength hot rolled steel sheet which re-heats and extracts the steel slab to predetermined extraction temperature, and after performing hot finishing rolling at the predetermined finishing rolling finishing temperature of Ar3 transformation point or more, it cools and winds up at predetermined winding temperature, The steel slab is C: 0.05 ~ 0.07wt%, Si: 0.10 ~ 0.25wt%, Mn: 1.0 ~ 1.8wt%, S: 0.01wt% or less, P: 0.01wt% or less, Nb: 0.01 ~ 0.03 wt%, V: 0.06 ~ 0.13wt%, Cu: 0.01 ~ 0.09wt%, Sn: 0.01 ~ 0.09wt%, balance Fe and other unavoidable impurity alloy composition, The predetermined winding temperature is 600 ~ 630 ℃ manufacturing method of high strength hot rolled steel sheet. The method of claim 3, The predetermined finishing rolling finish temperature is a manufacturing method of high strength hot rolled steel sheet, characterized in that 850 ~ 900 ℃.

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