JPH04247829A - Manufacture of hot rolled steel sheet excellent in surface property and formability - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 which has excellent surface properties and formability despite its high Si content. The hot-rolled steel sheet manufactured by the present invention is suitable for applications that require complex formability (burrability), such as wheel disks, automobile suspension parts, and the like.

0002

[Prior Art] Conventionally, in the production of high-strength steel materials with a tensile strength of 50 kgf/mm2 or more, which have been used for automobile parts, etc., hot rolling with excellent formability is required by controlling the rolling temperature after hot finish rolling. It manufactured steel plates. In addition, regarding the heating temperature of the steel ingot (slab) prior to hot rolling,
With emphasis on production volume, it was managed at the same level as other materials, or at a slightly higher price.

However, in recent years, the number of parts that require high-level forming has increased, and it has become difficult to meet the demands of steel materials that have formability at the level required for current products. By the way, it has long been known that increasing the amount of Si added to steel improves the formability of hot-rolled steel, but as the Si content in steel increases, so-called island-like scale defects occur. The surface roughness of the steel plate became severe, making it unsuitable for use as an exterior material. FeO-Fe-Si
"Island scale flaws" caused by scale mainly composed of O2 (Fayalite) are so-called primary scales generated during soaking, and residual scale that could not be removed by the descaler during hot rolling is removed during subsequent rolling. This is what is called red scale, which is pushed into the surface of the steel strip and stretched out to form islands. The presence of such island scales makes the surface rough after pickling, and even after painting (usually about 25 μm), an uneven surface remains.

[0004] Therefore, recently the amount of Si added has been limited (0.
10% or less), and steel plates are manufactured in such a way as to prevent island scale defects from occurring. However, as a result of reducing Si, C, Mn, and N other than Si are added to maintain the strength of the steel plate.
b, etc. had to be added, and deterioration of moldability was unavoidable. Specific component systems used in such conventional examples are shown in Table 1.

[0005]

[Table 1]

[0006] These conventional materials have a soaking temperature of 1220°C→
Rough rolling outlet temperature 1040℃→finish rolling outlet temperature 830℃
It is made into a hot rolled steel sheet under the manufacturing temperature conditions of 630° C. → cooling intermediate temperature after hot rolling → 420° C. coiling temperature. Although such conventional methods are effective in preventing island-like scale defects, they are not sufficient in terms of formability and cannot meet the specifications required today. by the way,
Other methods for removing island-like scale defects that have been tried in the past are as follows.

(1) Use a powerful descaling box. (2) Remove using a vertical scale breaker. (3) Apply antioxidant prior to heating to soaking temperature. (4) Use a mechanical descaling device (wire brush, etc.). (5) Use low temperature heating (≦1150℃) to prevent scale formation. However, these methods are extremely difficult to apply to actual operations due to cost, man-hours, location, or target materials. In addition, as a technique for preventing the formation of island-like scale defects, Japanese Patent Publication No. 58-1167 and Japanese Patent Publication No. 58-2
No. 1009 has been proposed.

[0008] The above-mentioned Japanese Patent Publication No. 58-1167 discloses that the rate of occurrence of island-like scale flaws becomes almost constant during rough rolling in a low temperature range of 1200°C or lower, and that hot rolling The entrance temperature of finish rolling is 930
℃ or less, and that if the outlet temperature is 780 ℃ or less, no island-like scale defects will occur. In particular, in Japanese Patent Publication No. 58-21009, after hot rolling, the Si content is set to 0.2% or less, the rough rolling exit temperature is set to 1000°C or higher, and the finishing rolling entrance temperature is adjusted to 940 to 980°C. , it is stated that a hot-rolled steel strip with excellent surface black scale properties can be produced by cooling after coiling at a rate of 5° C./hr or less.

[0009]

[Problem to be solved by the invention] However, with recent orders for small lots and a wide variety of products, the direct charge rate (ratio of operations in which steel billets are continuously cast and charged into a soaking furnace without being cooled to the total number of operations) is increasing. ), it becomes difficult to control the time that steel ingots remain in the furnace for more than 10 hours. In the case of steel ingots that have been in the furnace for a long time, island-like scale defects often occur even if they are treated under the conditions disclosed in the latter publication, and it is not necessarily possible to suppress island-like scale defects. Can not. The present invention solves the problems in the prior art, for example, by creating a hot-rolled steel sheet with excellent formability that does not cause island-like scale defects even when the steel slab remains in a soaking furnace for a long time. The purpose is to provide a manufacturing method. A more specific object of the present invention is to provide a method for producing a hot-rolled steel sheet with excellent surface quality and formability that does not cause island-like scale defects even when the Si content is high.

0010

[Means for Solving the Problems] In order to achieve the above object, the present inventors have conducted intensive research and found that (1) prevention of primary scale grain boundary oxidation by high-temperature heating of steel ingots;
Rough rolling is carried out in a temperature range where island-like scale defects due to secondary scales are difficult to occur, and (3) hot finishing rolling is carried out at a temperature below the freezing point of fayalite, and (4) variations in properties in the width direction are reduced. It was discovered that a combination of each stage of cooling at a cooling rate and winding is effective in preventing the occurrence of island-like scale defects, and the present invention was completed. In other words, the present invention is designed to prevent grain boundary oxidation, which causes island-like scale defects, during heating of a steel billet before rough rolling.
C: 0.20% or less, Si: 0.04-2.00
%, Mn: 2.0% or less, the balance containing Fe, 1
Descaling is performed by heating to an ultra-high temperature of 290°C or higher to prevent the formation of primary scale roots, and then 115°C.
By completing rough rolling in a temperature range of 0°C or higher, it is possible to suppress the generation of island-like scales that cause island-like scale defects, and also to
Next, after removing scale, hot finish rolling is started at 1100°C or lower, and the gist is a method for manufacturing a hot rolled steel sheet that prevents the formation of surface flaws due to island scale.

[0011] In the present invention, grain boundary oxidation of the primary scale is prevented by heating to a temperature of 1290°C or higher, and even during hot rolling, the
This is to avoid rolling at a temperature range of 50°C, where island scales are likely to occur. By limiting the air-fuel ratio to 6 or more during heating to 1290° C. or higher, the generation of so-called scale roots can be more effectively prevented. According to a preferred embodiment of the present invention, the hot-rolled steel sheet is further finished at 880° C. or higher, and the obtained hot-rolled steel sheet is coiled at once by controlling the cooling temperature.
A hot-rolled steel sheet with satisfactory forming performance and strength can be produced.

That is, when hot rolling is carried out as described above, the cooling rate of the steel strip having a high temperature of 880° C. or higher is further controlled so that the hot rolling finishing exit temperature is 630° C. to 670° C.
Even better moldability can be obtained by adjusting the temperature to an intermediate temperature of 400 DEG to 480 DEG C. and carrying out continuous winding at a line speed of 520 to 600 m/min. The steel sheet manufactured by the present invention may be used as is as a hot-rolled steel sheet, or may be further cold-rolled and used as a cold-rolled steel sheet, and further may be used as a hot-dip galvanized steel sheet. Surface treatment may also be performed.

[0013]

[Operation] The structure and operation of the present invention will be explained. 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 to maintain the strength of steel, but 0.20%
%, weldability deteriorates. Si: 0.04 to 2.00% If it is less than 0.04%, the problem of scratches due to scale caused by Si does not occur, but the processability decreases, making it impractical. Moreover, if it exceeds 2.00%, there is no practical use and it becomes difficult to manufacture the steel itself. It can be seen that there is a close relationship between the Si content and workability (e.g. elongation or hole expansion rate). In particular, when the Si content exceeds 0.40%, deterioration of workability and scale formation become noticeable in conventional methods, so the Si content in the present invention is preferably 0.40 to 2.0%.
It is 00%.

Mn: 2.0% or less If it exceeds 2.0%, the cost of a steel sheet for automobiles will increase, which is undesirable and less practical. The steel hot-rolled according to the present invention may contain other ordinary impurities such as P and S, but generally P:0
．． 0.012% or less, S: 0.0020% or less. Next, in the present invention, the steel ingot heating temperature is set to 129% before rough rolling.
The reason for setting the temperature to 0°C or higher is to sufficiently heat the center of the steel ingot. Preferably, the air-fuel ratio of the atmosphere in the soaking zone is set to 6 or higher to prevent the formation of primary scale roots due to grain boundary oxidation. This is to suppress the primary scale and make it easier to remove the primary scale.

FIGS. 1 and 2 are schematic illustrations of the state of scale formation during heating in the soaking zone. FIG. 1 is a schematic cross-sectional view according to the present invention. No roots have developed;
According to the present invention, descaling from a steel billet heated to a temperature of 1290° C. or higher can be easily performed. FIG. 2 illustrates the state of scale formation in a conventional example. Grain boundary oxidation causes so-called roots, which are difficult to remove. In addition, conventionally, the soaking temperature was at most 1220℃.
℃ is considerably lower than that of the present invention, so if waiting is performed before rough rolling, the oxidation of the primary scale will be promoted as shown in Figure 2, and the scale will penetrate deep into the surface due to grain boundary oxidation, making it difficult to remove it by descaling. It becomes difficult. Waiting before rough rolling then promotes the growth of secondary scale, necessitating strong descaling.In addition, by the time the rough rolling is finished, the temperature is unstable, where island-like scale defects are most likely to occur. This results in rolling at a temperature of around 1150°C.

In this regard, in 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 recover the heat even if the surface temperature decreases due to descaling. . In addition, in order to ensure a rough rolling outlet temperature of 1150°C or higher, a temperature of 1290°C or higher is required.
In addition to carrying out ultra-high temperature soaking at temperatures above ℃, if necessary, the descaling nozzle at the rough rolling stage is reduced to 1/2 of the normal one.
It is preferable to keep it at a certain level. In addition, in order to keep the rough rolling exit temperature of the steel billet at 1150° C. or higher, a bar heater, a heat insulating cover, etc. may be used to prevent the temperature from decreasing.

[0017] By completing the rough rolling under the above conditions, there is a tendency for the occurrence of island scale flaws to be reduced. Secondary scale is removed on the entry side of hot finish rolling. The entrance temperature of hot finish rolling at this time is 1100°C or less, which is below the freezing point of fayalite (FeO-FeO-SiO2). Between the rough rolling exit temperature and the hot finish rolling entry temperature of 1150 to 1100°C, the fayalite is semi-molten, so if rolling is performed in this temperature range, scales made of fayalite will form on the rolled surface. It is rolled up and rolled to form island-like scales, which causes surface flaws. Therefore, it is necessary to avoid rolling in the above temperature range.

[0018] Furthermore, the growth of secondary scale is promoted even while waiting for rolling on the entry side of hot finishing rolling, and the temperature of the steel material during finishing rolling of hot rolling is in an unstable temperature range (1150~1100°C).
℃), island-like scale defects will occur.
We must try 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 because the rough roll outlet temperature is high, so the temperature is inevitably high as described above. According to a preferred embodiment of the present invention, since it is disadvantageous to wait the steel material between rolls because it promotes the formation of island scales, the steel ingot that comes out of the soaking furnace is rolled at once and rolled up. is preferable. According to this, the cooling rate of the steel strip after the hot rolling finish rolling exit side is controlled, the finish exit temperature is set at 880°C or higher, the intermediate temperature is set at 630-670°C, the target is 640°C, and the coiling temperature is set at 400-480°C. By setting the target temperature to 460° C. and the winding line speed to 520 to 600 m/min, a steel plate with satisfactory forming performance and strength can be obtained. Outside these ranges, moldability and economical efficiency will not be sufficient.

[0019]

[Example] An example of the present invention will be explained.

[Example 1] The steel material used has a composition of C: 0.05
~0.10%, Si:0.04~2.0%, Mn:
A hot-rolled steel plate was produced under the basic manufacturing conditions of a slab heating temperature of 1290° C. or higher, a rough rolling finish of 1150° C. or higher, and a finish rolling start of 1100° C. or lower. S when varying the rough rolling exit temperature
FIG. 3 shows the relationship between the island-like scale flaw occurrence rate and the area ratio of i content. In other words, as shown in Fig. 3, island-like scale defects will occur if the rough rolling finishing temperature is 1150°C or higher, regardless of the Si% of the steel or the length of time the steel ingot is in the furnace. It is clear that the outbreak is significantly suppressed. FIG. 4 shows the relationship between the air-fuel ratio in the soaking zone and the incidence of island-like scale defects, and it was confirmed that the occurrence of island-like scale defects was effectively prevented at an air-fuel ratio of 6 or higher. From these results, it can be seen that no surface flaws due to island-like scales occur in the obtained hot rolled steel sheets within the range that satisfies the conditions of the present invention.

[0020]

[Example 2] Steel having the composition shown in Table 2 was hot rolled under the conditions shown in the same table, and the mechanical properties and island scale flaw occurrence rate of the obtained hot rolled steel were evaluated. The results are summarized in the same table.

[0021]

[Table 2]

[0022]

[Effects of the Invention] Since the present invention is constructed as described above, even if the amount of Si added to the steel is increased to improve formability, the occurrence of surface flaws in hot rolled steel sheets due to island scales can be suppressed. It is a high-strength hot-rolled product that not only can withstand applications that require complex molding such as automobile suspension parts, but also can be used as exterior products due to its excellent surface properties. It is possible to provide steel plates and is extremely useful industrially.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the state of scale during 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 soaking furnace time, rough rolling finishing temperature, Si content, and island scale flaw occurrence rate.

FIG. 4 is an explanatory diagram showing the relationship between the atmosphere of the soaking zone (air-fuel ratio) and the incidence of island scale.

Claims (2)

[Claims] [Claim 1] C: 0.20% or less, Si: 0.04 to 2.00 in weight %
%, Mn: 2.0% or less, a steel billet containing the balance Fe,
After heating to 1290°C or higher, rough rolling is carried out, the rough rolling finish temperature is set to 1150°C or higher, and then the hot finish rolling start temperature of the steel material is set to 1100°C or lower. A method for producing a hot-rolled steel sheet that has the same properties and prevents scale defects caused by Si oxidation. [Claim 2] 1290°C by limiting the air-fuel ratio to 6 or more.
2. The method for manufacturing a rolled steel sheet according to claim 1, wherein the heating is performed to a temperature higher than 100%.