JP2674328B2 - Method for producing hot rolled steel sheet with excellent surface properties and formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet having excellent surface properties and forming performance despite having a high Si content. The hot-rolled steel sheet manufactured according to the present invention has a complicated formability such as a wheel disc, an automobile underbody part, etc.
Suitable for applications where (burring property) is required.

[0002]

^2. Description of the Related Art Conventionally, in the production of high-strength steel material having a tensile strength of 50 kgf / mm ² or more, which has been used for automobile parts, etc. Manufactured rolled steel sheets. Regarding the heating temperature of the steel ingot (slab) prior to hot rolling, the production amount was emphasized, and it was controlled at the same level as or slightly higher than other materials.

However, in recent years, the number of parts requiring a high degree of forming has increased, and it has become difficult to cope with steel materials having the level of formability required for current products. By the way, it has been known that the formability of hot-rolled steel is improved by increasing the amount of Si added to steel, but with the increase of Si content in steel, the occurrence of so-called island scale flaws occurs. It became remarkable, and the unevenness of the steel plate surface became severe, so it could not be said to be suitable as an exterior material. "Island scale flaws" due to FeO-Fe-SiO ₂ (Fayalite-Fayalite) -based scales are so-called primary scales generated during soaking and residual scale that could not be removed by the descaler during hot rolling. Is a red scale that is pressed into the surface of the steel strip during the subsequent rolling and stretched to form islands. The presence of such an island-shaped scale makes the surface rough after pickling and leaves a rough surface even after coating (usually about 25 μm).

Therefore, recently, steel sheets are manufactured by limiting the amount of Si added (0.10% or less) so that island scale flaws do not occur, but as a result of reducing Si, the strength of the steel sheet is maintained. Therefore, C, Mn, Nb, etc. other than Si must be added, and the deterioration of formability cannot be avoided. Table 1 shows specific components used in the conventional example.

[0005]

[Table 1]

These conventional materials are produced under the manufacturing temperature conditions of soaking temperature 1220 ° C. → rough rolling outlet temperature 1040 ° C. → finishing rolling outlet temperature 830 ° C. → cooling intermediate temperature after hot rolling 630 ° C. → winding temperature 420 ° C. It is a hot rolled steel sheet. Even with such a conventional method, the effect of preventing island scale flaws is observed, but it is not sufficient in terms of moldability, and the specifications required today cannot be satisfied. By the way, other methods of removing island scale flaws that have been conventionally tried are as follows.

(1) Use a powerful descaling box. (2) Remove with a vertical scale breaker. (3) Apply the antioxidant before heating to the soaking temperature. (4) Use a mechanical descaling device (wire brush, etc.). (5) Prevents scale generation by heating at low temperature (≤1150 ℃). However, these methods are very difficult to apply to the actual operation because of cost, man-hour, place or target material. In addition, Japanese Patent Publication No. 58-1167 and Japanese Patent Publication No. 58-21009 have been proposed as techniques for preventing the generation of island scale flaws.

The above Japanese Patent Publication No. 58-1167 discloses that the occurrence rate of island scale flaws becomes almost constant in rough rolling in a low temperature range of a steel slab (slab) heating temperature of 1200 ° C. or less, and hot rolling. It is stated that if the inlet temperature of finish rolling is 930 ° C or lower and the outlet temperature is 780 ° C or lower, island scale flaws do not occur at all. Particularly, in Japanese Examined Patent Publication No. 58-21009, the Si content is set to 0.2% or less, and the rough rolling outlet temperature is set to 1000
℃ or more, after adjusting the finish rolling inlet temperature to 940 ~ 980 ℃, hot rolling, and then cooling after winding at 5 ℃ / hr or less, hot rolled steel with excellent surface black scale properties It is stated that the strip is manufactured.

[0009]

However, the recent orders for small lots of a wide variety of products and the direct charge rate (ratio of the total number of operations of the operation example in which steel slabs are continuously cast and charged into the soaking furnace without cooling) ), It becomes difficult to control the ingot furnace time in the heating furnace, and many of them remain in the furnace for more than 10 hours. Thus, in the case of a steel ingot having a long in-furnace time, island scale flaws often occur even when treated under the conditions disclosed in the latter publication, and it is not always possible to suppress island scale flaws. Can not. The present invention is a problem in such a conventional technique, for example, even if the furnace time in the soaking furnace of the steel slab is long, no island scale flaws are generated, and the hot-rolled steel sheet having excellent formability is formed. It is intended to provide a manufacturing method. A more specific object of the present invention is Si
An object of the present invention is to provide a method for producing a hot-rolled steel sheet having excellent surface properties and formability in which island scale flaws do not occur even if the content is high.

[0010]

[Means for Solving the Problems] In order to achieve the above object, the inventors of the present invention have conducted extensive studies, and as a result, (1) prevention of primary-scale grain boundary oxidation by heating a steel ingot at high temperature, (2) 2 Rough rolling in a temperature range where island scale flaws are less likely to occur due to the next scale, and (3) hot finish rolling are performed at a temperature below the freezing point of Fayalite, and (4) cooling that reduces variation in characteristics in the width direction. The inventors have found that a combination of respective stages of cooling at a speed and winding is effective in preventing the occurrence of island scale flaws, and have completed the present invention. That is,
The present invention, in order to prevent the grain boundary oxidation that causes island scale flaws in the billet heating before rough rolling, C: 0.20% or less, Si: 0.04-2.00%, Mn: 2.0% or less, the balance Fe The slab containing the steel is heated to an ultrahigh temperature of 1290 ° C or higher, descaled without forming roots of the primary scale, and then
By finishing the rough rolling in the temperature range of 1150 ℃ or more, while suppressing the occurrence of island scale that causes island scale flaws,
The gist is a method for manufacturing a hot-rolled steel sheet in which hot finish rolling is started at 1100 ° C. or lower after removing the secondary scale to prevent the generation of surface flaws due to the island scale.

According to the present invention, the intergranular oxidation of the primary scale is prevented by heating at a temperature of 1290 ° C. or higher, and an island scale of 1100 to 1150 ° C. is likely to occur even during hot rolling. It avoids rolling in the temperature range. By limiting the air-fuel ratio to 6 or more during heating to 1290 ° C. or higher, so-called scale root generation can be more effectively prevented. According to a preferred embodiment of the present invention, further, finishing is carried out at 880 ° C. or higher, the cooling temperature of the obtained hot-rolled steel sheet is controlled, and winding is carried out at a stretch to produce a hot-rolled steel sheet at a level where the forming performance and strength are satisfactory. can do.

That is, when hot rolling is performed as described above, the cooling rate of the steel strip having a hot rolling finish outlet temperature of 880 ° C. or higher is controlled to control the intermediate temperature of 630 to 670 ° C. and 400 ° C. Adjust the coiling temperature to ~ 480 ℃ and 520
By performing continuous winding at a line speed of ~ 600 m / min, even better molding performance can be obtained. The steel sheet produced according to the present invention may be used as it is as a hot rolled steel sheet, or as a cold rolled steel sheet after cold rolling, and may be used as a hot dip galvanized steel sheet, if necessary, further appropriately. Surface treatment may be performed.

[0013]

The structure and operation of the present invention will be described. The reason for limiting the composition range of the steel used in the present invention is as follows. C: 0.20% or less It is an essential component for maintaining the strength of steel, but if it exceeds 0.20%, the weldability deteriorates. Si: 0.04 to 2.00% If it is less than 0.04%, the problem of flaws due to scale due to Si does not occur, but the workability deteriorates, which is not practical. Again 2.00
If it exceeds%, there is no practical use and it becomes difficult to manufacture the steel itself. Si content and workability (eg elongation or hole expansion rate)
It turns out that and are closely related. Especially Si 0.40%
Since the deterioration of the workability and the formation of scale become remarkable in the conventional method when the above is satisfied, the Si content in the present invention is preferably 0.40 to 2.00% of Si.

Mn: 2.0% or less If it exceeds 2.0%, the cost as a steel sheet for automobiles increases, which is not preferable and is not practical. The steel hot-rolled according to the present invention may contain other impurities such as P and S, but generally P: 0.012%.
Hereafter, S: 0.0020% or less is sufficient. Next, in the present invention, the steel ingot heating temperature before rough rolling is 1290 ° C. or higher,
This is to sufficiently heat the center of the steel ingot, and preferably, by setting the air-fuel ratio of the atmosphere in the soaking zone to 6 or more, generation of roots of primary scale due to grain boundary oxidation is suppressed, and removal of primary scale is performed. This is to make it easier.

FIG. 1 and FIG. 2 are schematic explanatory views of the scale generation state during heating in the soaking zone. FIG. 1 is a schematic sectional view according to the present invention. Roots are absent and descaling from billets heated to temperatures above 1290 ° C. according to the invention can easily be done. FIG.
Explains the state of scale formation in the conventional example, and so-called roots are generated by grain boundary oxidation, and their removal is difficult. Incidentally, the soaking temperature is 1220 ° C. at the most, which is considerably lower than that of the present invention, so when the waiting before rough rolling is performed, the oxidation of the primary scale is promoted as shown in FIG. Are difficult to remove by descaling. And by waiting before the rough rolling, the growth of the secondary scale will be promoted this time, so strong descaling is required, and in addition to the rough rolling finish,
An unstable temperature range (1150
The result is rolling at around (° C).

In this respect, according to the present invention, by ensuring the rough rolling outlet temperature at a sufficiently high level of 1150 ° C. or higher, it is possible to immediately reheat the surface temperature drop due to descaling. . In order to secure a rough rolling outlet temperature of 1150 ° C or higher, super high temperature soaking at 1290 ° C or higher is performed, and if necessary, the descaling nozzle at the rough rolling stage should be about 1/2 of the normal temperature. It is preferable. In addition, since the temperature of the rough rolling outlet of the steel slab is set to 1150 ° C. or higher, temperature reduction may be prevented by a bar heater, a heat insulating cover or the like.

By completing the rough rolling under the above conditions, the occurrence of island scale flaws tends to be small. Secondary scale is removed on the entry side of hot finish rolling. At this time, the inlet temperature of the hot finish rolling is 1100 ° C or lower, which is lower than the freezing point of Fayalite (FeO-FeO-SiO ₂ ). Rough rolling outlet temperature and this hot finish rolling inlet temperature 1150 to 1100 ℃
During the period, since the fayalite is semi-molten, when the rolling is performed in this temperature range, the scale made of fayalite is rolled into the rolling surface, and is rolled to form an island-shaped scale, which causes surface defects. Become. Therefore, it is necessary to avoid rolling in the above temperature range.

Also, during the rolling waiting on the entry side of the hot finish rolling, the growth of the secondary scale is promoted, and the steel material temperature is in the unstable temperature range (1150 to 1100 ° C) during the hot rolling finish rolling. And island scale flaws will occur, so it is necessary to avoid this as much as possible. Therefore, in the present invention, the hot rolling start temperature is set to 1100 ° C or lower. The exit temperature of hot finish rolling is preferably 900 ° C or higher. This is inevitably high in temperature as described above because the temperature on the outlet side of the rough roll is high. According to the preferred embodiment of the present invention, since it is disadvantageous to wait the steel material between the rolls because the generation of the island scale is promoted, the steel ingot discharged from the soaking furnace is rolled at once and wound up. Is preferred. According to it, the cooling rate of the steel strip after the hot rolling finish rolling side is controlled, the finish outlet temperature is 880 ℃ or higher, the intermediate temperature is 630 to 670 ℃, and the target 640
By setting the coiling temperature, the coiling temperature of 400 to 480 ° C, the target of 460 ° C, and the coiling line speed of 520 to 600 m / min, it is possible to finish a steel sheet of a level that satisfies the molding performance and strength. When the amount is out of these ranges, the moldability and the economical efficiency are insufficient.

[0019]

An embodiment of the present invention will be described.

[Example 1] The steel composition used had a composition of C: 0.05 to 0.10.
%, Si: 0.04 to 2.0%, Mn: 1.40 to 1.60%, slab heating temperature is 1290 ℃ or higher, rough rolling finish is 1150 ℃.
As described above, hot-rolled steel sheets were manufactured under the basic manufacturing conditions of finishing rolling starting at 1100 ° C or lower. FIG. 3 shows the relationship with the island scale flaw occurrence rate indicated by the Si content area ratio when the rough rolling outlet temperature was variously changed. That is, as shown in FIG.
Even if the Si% of the steel becomes high or the ingot furnace time of the steel ingot is long or short, if the rough rolling finishing temperature is 1150 ° C or higher, the occurrence of island scale defects is significantly suppressed. Is. FIG. 4 shows the relationship between the air-fuel ratio in the soaking zone and the island scale flaw occurrence rate. It was confirmed that the island scale flaw generation is effectively prevented when the air-fuel ratio is 6 or more. From these results, it is understood that the hot-rolled steel sheet obtained does not have surface defects due to the island scale in the range satisfying the conditions of the present invention.

[0020]

Example 2 Steels having the compositions shown in Table 2 were hot-rolled under the conditions shown in the table, and the mechanical properties and the island scale flaw occurrence rate of the obtained hot rolled steel materials were evaluated. The results are shown in the same table.

[0021]

[Table 2]

[0022]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, it is possible to suppress the occurrence of hot-rolled steel sheet surface flaws due to island scale even if the Si addition amount of steel is increased to improve formability. In addition to being able to fully withstand the use of complicated molding such as automobile suspension parts, it can also be used as an exterior part due to its excellent surface properties. A steel plate can be provided, which is extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a state of a scale in high temperature heating of a slab before rough rolling in the present invention.

FIG. 2 is an explanatory diagram showing primary-scale grain boundary oxidation in a conventional method.

FIG. 3 is a graph showing the relationship between the soaking furnace time, rough rolling finishing temperature, Si content, and island scale flaw generation rate.

FIG. 4 is an explanatory diagram showing a relationship between a uniform tropical atmosphere (air-fuel ratio) and an island scale occurrence rate.

Claims (2)

(57) [Claims] 1. C: 0.20% or less by weight%, Si: 0.04 to 2.00%, Mn: 2.0% or less,
Characteristic that the steel slab containing the balance Fe is heated to 1290 ° C or higher, and then rough rolling is performed, the rough rolling finishing temperature is set to 1150 ° C or higher, and the hot finishing rolling start temperature of the steel material is set to 1100 ° C or lower. And a method for producing a hot-rolled steel sheet having high formability and preventing scale flaws due to Si oxidation. 2. The method for producing a rolled steel sheet according to claim 1, wherein the air-fuel ratio is limited to 6 or more and heating to 1290 ° C. or more is performed.