KR20030021490A - Method for manufacturing the pickled-oiled steel sheet having Si/Nb - Google Patents

Abstract

PURPOSE: A method for manufacturing a pickled-oiled steel sheet containing Si/Nb for use in the manufacture of wheel disk for automobile is provided to prevent surface defects due to red scale. CONSTITUTION: In a manufacture method of a pickled-oiled steel sheet containing Si/Nb by heating a slab comprising C 0.04 to 0.10 wt.%, Si 0.01 to 0.04 wt.%, Mn 0.10 to 0.17 wt.%, P 0.015 wt.% or less, Nb 0.04 wt.%, a balance of Fe and incidental impurities, followed by hot rolling, pickling and oil application, the method is characterized in that it includes the steps of preheating the slab in preheating zone of a heating furnace at 790 to 810 deg.C; resting the slab in a heating furnace of which atmosphere temperature is maintained between 1260 to 1280 deg.C for 50 to 70 minutes to raise the temperature of upper part of the slab to be in the temperature range of 1130 deg.C and 1150 deg.C; keeping the slab in soaking zone of the heating furnace for 45 to 55 minutes to regulate the temperature of the slab released from the heating furnace within 1165 and 1185 deg.C; rough rolling the slab at 990 to 1010 deg.C, followed by finish rolling; and coiling obtained steel sheet at 570 to 590 deg.C.

Description

Method for manufacturing the pickled-oiled steel sheet having Si / Nb}

The present invention relates to a method for producing a precipitation-reinforced high tensile PO steel sheet used in the manufacture of wheel discs, etc. of automobiles, and more particularly, when the steel sheet is heated in a heating furnace before performing hot rolling. It relates to a method for producing a Si / Nb-containing high-strength PO steel sheet that can prevent the surface deterioration and to ensure a sufficient tensile strength (TS).

In general, PO steel sheet is a product coated with an oil (oil) to the surface of the steel sheet to prevent oxidation after removing the scale (scale) of the hot-rolled steel sheet surface with hydrochloric acid. As shown in FIG. 1, the hot rolled steel sheet passes the slab 1a heated to the temperature required for rolling in the heating furnace 12 through the rough rolling roll 14 and the finishing rolling roll 16, and then removes the strip 1b. It is manufactured by winding in the form of a coil in the winding unit 18. At this time, the heating furnace 12 is divided into the regions of the preheating zone 12a, the heating zone 12b and the crack zone 12c as shown in Figure 2a, the atmosphere inside the heating furnace as shown in Figure 2b It is maintained in the atmosphere of the slab (1a) is to stay for about 75 to 80 minutes, 40 to 45 minutes and 30 minutes or more in each zone of the heating furnace.

On the other hand, automotive high-strength steel is the trend that more than 55kg class is used to reduce the weight and safety of the vehicle, and to be used as a wheel material, the formability at the time of press working, the processability and the durability in use after processing are required. As shown in Table 1, low-strength steels having a carbon content of 0.07% by weight or less are used to improve workability, and include 0.01% Si and 0.04% Nb as solid solutions to improve durability. do.

[Table 1] Component Composition

ingredient C Mn P SI Nb weight% 0.07 0.14 0.01 0.01 0.04

At this time, Si is an effective element as a deoxidizer in the steelmaking process, and is mainly dissolved in ferrite to increase the strength. When the Si-containing slab is heated to 1170 ° C. or higher in a furnace, low melting point compounds (Fayalite, 2FeO.SiO ₂ ) are formed on the scale and the ferrous interface as shown in FIG. 3A, and the low melting point compounds are shown in FIG. 3B. Since it is anchored to the steel, it creates an red scale (A) that cannot be completely removed by the current descaling pressure.

In addition, the processing of the strip on which the red scale is formed causes pollution due to the scattering of the scale, and the appearance of the processed product is poor, causing customer dissatisfaction. In addition, the red scale formed in the heating furnace acts as a defect that reduces pickling in a later step.

On the other hand, Nb is a precipitation strengthening element, which is precipitated as niobium nitride (NbN), which increases the strength of steel and increases the workability through refinement of grains. to be. Therefore, Nb is added to compensate for the decrease in strength caused by lowering the carbon content. On the other hand, as shown in Table 2, only when the heating temperature of the slab is maintained at more than 1180 ℃ 0.04% by weight of Nb can be completely solid so that the target tensile strength can be secured.

[Table 2] Nb solid solubility according to temperature

Temperature 1130 ℃ 1150 ℃ 1170 ℃ 1180 ℃ Design value Employment 0.023 0.029 0.031 0.039 0.04

As described above, when the slab containing Si and Nb is heated in a heating furnace, red scale does not occur when the heating temperature is 1170 ° C. or lower, but it is impossible to secure sufficient tensile strength due to a decrease in the solid solution of Nb. When the heating temperature is 1180 ° C. or more, Nb is completely dissolved to secure sufficient tensile strength, but a problem occurs that red scale occurs.

In order to solve this problem, Korean Patent Application No. 195-68439 discloses a technique for maintaining the composition of Si to 0.036 ~ 0.12%. However, this technique is available for hot rolled materials before pickling, but the surface quality after pickling has a problem that cannot be guaranteed. On the other hand, a technique for increasing the capacity of the pump to remove the scale to 300kg / cm ² is known, but this causes a problem of excessive cost of investment.

Therefore, in consideration of the existing components and the capacity of the pump for descaling (180kg / cm ² ), it is possible to optimize the rolling conditions from the heating furnace to the winding process to prevent the red scale and to secure the tensile strength of the material. need.

The present invention has been made to solve the conventional problems as described above, hot rolled to remove the red scale of the high-strength PO products for automobiles added with 0.01% by weight of Si, 0.04% by weight of Nb to low carbon steel It is possible to appropriately control the atmosphere temperature and heating time, slab temperature, rough rolling descale and backside temperature, finishing rolling descale and backside temperature, cooling start temperature (hereinafter referred to as winding temperature), etc. An object of the present invention is to provide a method for producing Si / Nb-containing PO steel sheet.

1 is a block diagram of a general hot rolled manufacturing apparatus.

Figure 2a is a block diagram of a heating furnace of the hot rolled manufacturing apparatus.

Figure 2b is a graph showing the temperature distribution inside the heating furnace.

3A is a state diagram of SiO ₂ —FeO—Fe ₂ O ₃ .

Figure 3b is a photograph of the red scale formed at the interface between the scale and the iron.

Figure 3c is an enlarged photograph of the red scale.

Figure 4 is a graph showing the distribution of tensile strength at the extraction temperature of 1170 ℃.

5 is a graph showing the frequency of occurrence of red scale according to the extraction temperature and the residence time in the heating zone and the crack zone.

Figure 6a is an experimental result showing the effect of hot rolling conditions on the tensile strength.

6B is a graph of FIG. 6A.

Figure 7 is a graph showing the distribution of tensile strength in accordance with the present invention.

In order to achieve the above object, according to the present invention in terms of weight%, carbon (C) 0.04 ~ 0.10%, silicon (Si) 0.01 ~ 0.04%, manganese (Mn) 0.10 ~ 0.17%, phosphorus (P) 0.015%, Nb 0.04%, the remainder of the slab containing iron (Fe) and irreversible impurities are heated in a furnace, hot-rolled, and then pickled and oiled to produce a PO steel sheet in the preheating zone of the furnace Preheating to a temperature of 790 ℃ ~ 810 ℃, and maintaining the temperature of the slab at 1130 ℃ ~ 1150 ℃ by holding the preheated slab for 50 minutes to 70 minutes in the heating table maintained at an upper temperature of 1260 ℃ ~ 1280 ℃ And maintaining the heated slab in the cracking zone of the heating furnace for 45 minutes to 55 minutes, and maintaining the extraction temperature of the slab extracted from the cracking zone of the heating furnace at 1165 ° C to 1185. And, roughly rolling the slab extracted from the cracks at 990 ℃ ~ 1010 ℃, A phase-rolled slab characterized by including the steps of winding at a temperature of 570 ℃ ~ 590 ℃.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings, and the same components as in the prior art are given the same reference numerals.

According to the present invention, there is a need to secure a material by selecting factors required for red scale formation and deriving items other than the selected factors. That is, the red scale is generated when the low melting point oxide is formed on the surface at a temperature of 1170 ° C. or more due to the steel grade characteristics when the Si-containing slab is heated as described above. In the case where the slab is kept at a high temperature for a long time, the low melting point oxide is fused to the interface between FeO and the iron, resulting in extremely poor scale removal. Therefore, the residence time of the slab staying in the furnace together with the temperature of the furnace is an important factor.

Based on this, as a result of the experiment according to the extraction temperature and the retention time of staying in the heating furnace 12 when the slab 1a is extracted from the heating furnace 12, The slab temperature is maintained at 1170 ° C. or lower, the residence time in a heating zone is maintained at 70 minutes or less, the residence time in a crack zone is maintained at 50 minutes or less, and the ambient temperature at the heating zone is maintained at about 1280 ° C. In this case, PO steel sheet with good surface quality with little red scale could be obtained.

In this way, the critical factors (extraction temperature, heating table / cracking table residence time, heating table atmosphere temperature) and the management level of the red scale were set, and in addition, due to the nature of the Nb-added strip, the optimum temperature was not obtained in the heating furnace 12. What is needed is a way to prevent the tensile strength from becoming poor. That is, as shown in Figure 4, when the extraction temperature of the slab (1a) is 1170 ℃ or less, it can be seen that the failure rate of tensile strength occurred more than 60%.

Therefore, in order to secure good tensile strength without generating red scale, it is necessary to select items other than the red scale influence factor.

To this end, reference is made to FIGS. 6a and 6b which show the influence of the tensile strength for each hot rolling condition as experimental data.

According to the data analysis, the slab temperature in the heating zone 12b, the slab temperature in the crack zone 12c, the rough rolling temperature (RDT), and the coiling temperature (CT) are selected as factors influencing the tensile strength, and the statistical regression equation is As follows.

TS = 42.7 + 0.0210 (heating slab temperature) + 0.0282 RDT-0.0567 CT. (One)

In order to secure 61 kg of tensile strength (TS) using Equation (1), it is possible to maintain slab temperature in the heating table by 1140 ° C, rough rolling temperature (RDT) at 1000 ° C and winding temperature (CT) at 580 ° C. I could see that.

At this time, since the slab temperature in the heating table is affected by the scale lethal factor in the heating furnace, that is, the atmosphere temperature, the residence time and the extraction temperature in the heating table, in order to ensure the slab temperature in the heating table at 1140 ° C, A regression equation related to slab temperature at different heating zones was required, and the following formula was obtained.

Heating slab temperature = -829 + 1.22 (ambient temperature in the heating zone) + 0.333 (slab temperature in the preheating zone) + 2.89 (time to stay in the heating zone). (2)

Based on the regression equation obtained in the above formula (2), the ambient temperature in the temperature-applied heating zone sensitive to red scale formation and the time of staying in the heating zone are fixed to 1270 ° C. or less and 65 minutes or less, which is the non-scale zone, respectively. In order to obtain the slab temperature in the heating table at 1140 ° C, it was necessary to secure the slab temperature in the preheating zone to 800 ° C or more.

According to the above results, in the condition of not generating red scale, the temperature immediately before extraction of the heating furnace is maintained at 1180 ° C. or less, the time for dwelling the slab in the heating zone is maintained at 65 minutes or less, and the slab is kept in the cracking zone. It was important to keep time at 50 minutes or less, and to maintain the atmosphere temperature in a heating stand at 1270 degreeC. That is, in order to secure the tensile strength TS of 61kg or more, the slab temperature in the heating table is maintained at 1140 ° C, the rough rolling temperature RDT is maintained at 1000 ° C, and the winding temperature CT is maintained at 580 ° C. It is important.

On the other hand, according to a preferred embodiment of the present invention, the slab temperature in the preheating zone is maintained at 790 ~ 810 ℃, the upper temperature in the heating zone is maintained at 1260 ~ 1280 ℃, the residence time to hold the slab in the heating zone 50 It is maintained at ˜70 minutes, the slab temperature in the heating zone is maintained at 1130 to 1150 ° C., the residence time for retaining the slab in the crack zone is maintained at 45 to 55 minutes, and the extraction temperature when the slab is extracted is 1165 ° C. It is maintained at ˜1175, the rough rolling temperature (RDT) is maintained at 990 to 1100 ° C., and the winding temperature (CT) is maintained at 570 to 590 ° C., as shown in FIG. The strength TS could be secured to 61 kg or more.

On the other hand, at least 21 sheets of the same material should be added in order to effectively perform the control described above due to the characteristics of the heating furnace.

Therefore, according to the present invention, by controlling the heating furnace atmosphere temperature and slab residence time, the slab temperature, the rough rolling temperature and the winding temperature, the hot rolled Si / Nb-containing slabs, pickling treatment and oil treatment reduce the formation of red scale. And PO steel sheet having good tensile strength can be produced.

The foregoing is merely illustrative of the preferred embodiments of the present invention and those skilled in the art to which the present invention pertains may make modifications and changes to the present invention without departing from the spirit and gist of the invention as set forth in the appended claims. It should be recognized.

Claims (1)

By weight%, carbon (C) 0.04 to 0.10%, silicon (Si) 0.01 to 0.04%, manganese (Mn) 0.10 to 0.17%, phosphorus (P) 0.015% or less, Nb 0.04%, the balance is iron (Fe) and In the method for producing a slab containing irreversible impurities in a PO steel sheet by heating in a heating furnace, hot rolling, followed by pickling and oiling treatment, Preheating the slab to a temperature of 790 ° C. to 810 ° C. in the preheating zone of the furnace; Maintaining the temperature of the slab at 1130 ° C. to 1150 ° C. by heating the preheated slab for 50 minutes to 70 minutes in a heating table where the upper temperature is maintained at 1260 ° C. to 1280 ° C .; Maintaining the heated slab in the cracking zone of the furnace for 45 to 55 minutes; Maintaining the extraction temperature of the slab extracted from the cracking zone of the heating furnace at 1165 ° C to 1185 ° C; Roughly rolling the slabs extracted from the cracks at 990 ° C.-1010 ° C., Si / Nb-containing PO product manufacturing method comprising the step of winding the finishing-rolled slab at a temperature of 570 ℃ ~ 590 ℃.