JP2848190B2 - Manufacturing method of high strength steel sheet with excellent surface properties - 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 high-strength steel sheet containing Si having excellent surface properties and having few island-like scale flaws which are easily generated during hot rolling.

[0002]

2. Description of the Related Art One of the means for improving the fuel efficiency of an automobile is to reduce the weight of a vehicle body. If a high-strength steel sheet is used, the strength required for the vehicle body can be satisfied with a thickness smaller than that of the conventional steel sheet, so that the weight of the vehicle body can be reduced. Since the steel sheets used in these applications are required to have good formability, it is desirable to increase the strength by solid-solution strengthening the ferrite structure.

In order to strengthen the ferrite structure by solid solution,
It is sufficient to add P, Si, Mn, etc., but P cannot be added so much because it causes embrittlement of steel, and Mn is expensive and reduces the ductility of the steel sheet. Although Si does not significantly reduce ductility and the raw material is inexpensive, it has the following problems.

1) Generation of scale flaws FIG. 3 is a schematic cross-sectional view of a part of the surface of a steel material for explaining how the scale generated in a Si-containing steel material changes before and after descaling and rolling. As shown in FIG. 3A, when a steel material containing Si is heated, a FeO—Fe ₂ SiO ₄ (firelite) eutectic compound layer is formed near the interface between FeO (wustite) and ground iron. Since this layer and the ground iron are intricately meshed with each other, they cannot be completely removed by descaling, and the eutectic compound and FeO that has complicatedly entered the eutectic compound remain on the steel surface (Fig. 3 (b)). In the subsequent rolling step, FeO comes into contact with air while being pulverized, and becomes Fe ₂ O ₃ (hematite) and is pushed into the surface of the steel material. Thus, what is called a red scale is generated on the surface of the steel material (FIG. 3 (c)). This red scale is called an island scale because it is generated unevenly (in an island shape) on the surface of the steel sheet. However, the appearance of the steel material is impaired, and even after pickling, the portion remains as a dented scale flaw. It is easy to be.

[0005] 2) Insufficient plating in hot-dip galvanizing Even in the case of a steel material having scale flaws as described above, the dents can be eliminated by performing cold rolling, but the black scale (FeO, Fe ₃ O ₄ in a dense scale) is generated portion and red scale has occurred portion and mainly, because there are differences in the chemical composition near the surface of the steel, the alloying during galvanizing is uneven And uneven plating occurs.

To add Si as a method of increasing the strength of a steel material, these problems must be solved. Conventionally, the following measures have been considered.

(A) Countermeasures by the components The inventions of JP-A-3-72031 and JP-A-3-79718 disclose increasing the Si content of the steel sheet and increasing the red scale over the entire surface of the steel sheet in order to make the surface properties uniform. Is to be generated. However, this measure alone cannot stably generate the red scale uniformly over the entire surface of the steel sheet. In particular, when the Si content of the steel sheet is relatively small, it is difficult to generate red scale over the entire surface of the steel sheet.

(B) Countermeasures by changing the oxidation state of the heating furnace According to Japanese Patent Application Laid-Open No. 1-246318, oxides such as CaO and MgO are applied to the slab surface before heating, and A method of changing the surface oxidation state to improve the state of the interface between the scale and the iron base is described. However, it is extremely difficult and troublesome to apply the oxide uniformly on the slab surface. Moreover, this method is difficult to apply to the back surface of the steel sheet.

The invention of Japanese Patent Publication No. 15682/1985 controls the air-fuel ratio in the atmosphere of the heating furnace to change the oxidation state of the slab surface. However, the air-fuel ratio can be controlled due to restrictions such as combustion efficiency. The range is limited and cannot be used generally.

(C) Countermeasures by descaling Attempts have been made to remove island scales using a mechanical brush or the like, but what is an effective method because the service life of the mechanical brush is short because it is intended for high-temperature steel materials? Hard to say.

The method using a high-pressure water jet disclosed in Japanese Patent Publication No. 60-1085 is a method in which high-pressure water is jetted after rolling to some extent to remove island scales. Nozzle wear is severe and maintenance is problematic.

[0012]

As described above, a number of technical improvements have been made to prevent the occurrence of scale flaws which are problematic when Si is added for the purpose of improving the strength of a steel sheet. However, no satisfactory solution has yet been found. An object of the present invention is to solve this problem and to provide a method for manufacturing a high-strength steel sheet to which Si is added and which has high strength and excellent surface properties.

[0013]

Means for Solving the Problems The present invention relates to a method for producing a semiconductor device comprising the steps of: "the content of Si is 0.2 to 3.0% by weight, and the sum of the contents of Ni and Cu is 0.
A steel slab having a weight ratio of 01% by weight or more and a Ni / Cu content ratio of Cu / Ni of 2 or less is heated to 1170 ° C. or more,
A method of producing a high-strength steel sheet having excellent surface properties, wherein the first descaling is performed in a temperature range of 1200 ° C. or less and hot rolling is performed.

[0014] A steel sheet suitable as an object of the present invention comprises, in addition to the above-mentioned Si, Ni and Cu, C: 0.2% by weight or less;
n: 0.1 to 3.0% by weight or less, P: 0.05% by weight or less, S:
0.02% by weight or less, Nb, V and Ti as necessary
Steel.

[0015]

The basic concept of the present invention is to generate a red scale, which is usually formed in an island shape, over the entire surface of a steel sheet.
The point is to make the surface of the steel sheet uniform.

This concept is common to the above-mentioned Japanese Patent Application Laid-Open Nos. 3-72031 and 3-79718. However, in the present invention, the contents of Ni and Cu in the steel sheet,
By properly selecting the heating temperature of the slab before hot rolling and the first descaling temperature after extraction from the heating roll, uniform red scale can be generated on the entire surface of the steel sheet. .

The present inventors have found that there are Ni and Cu in addition to Si as elements that greatly affect the generation of island scale. Normally, trace amounts of Ni and Cu derived from a raw material for ironmaking are mixed in any steel type. As will be described later, Ni and Cu are elements that promote the formation of red scale, but Ni and Cu are inevitably mixed into steel.
It is extremely difficult to reduce i and Cu. The present inventors have considered that if it is difficult to remove Ni and Cu in steel, on the contrary, they actively utilize the property of promoting the formation of island scales of Ni and Cu. And Ni
And a steel sheet containing a specific amount of Cu at a temperature of 1170 ° C or higher, and a first descaling temperature of 1200 ° C or lower. If hot rolling is performed, island scale is uniformly generated on the entire surface of the steel sheet. It was confirmed that the entire surface of the steel sheet could be covered with red scale.

If the entire surface of the steel sheet can be uniformly covered with the red scale, the problem of the island scale which locally occurs on the steel sheet surface and causes scale flaws and plating defects can be solved.

Ni and C for generation of island scale
The effect of u is as follows.

FIG. 1 is a schematic cross-sectional view of the interface between the steel (ground iron) and the scale (FIG. 3) to explain that the interface between the Si-containing steel and the scale generated on the surface becomes uneven. It has been further expanded). As described earlier with reference to FIG. 3, in the case of Si-containing steel, ground iron and wustite (FeO)
A eutectic compound layer of FeO—Fe ₂ SiO ₄ (firelite) is formed between the layers. At this time, the grain boundaries are selectively oxidized, and the eutectic compound enters the grain boundaries, so that irregularities are formed in the base iron.

On the other hand, as shown in FIG. 1 (a), along with the development of the firelite layer, oxidation proceeds even in the surface layer of the base iron, and an internal oxide is generated. These internal oxides grow and are taken into the upper eutectic compound layer as Mn ₂ SiO ₄ or the like, and the traces of the removal become the concave portions of the base iron. However, Ni and Cu, which have a low affinity for oxygen, remain in the base iron and become concentrated, so that even if the oxidation time is long, they are not oxidized and remain as projections of the base iron. In this way, the interface between the ground iron and the scale becomes uneven as shown in FIG. 1 (b).

Since the scale meshing with the highly irregular ground iron cannot be removed by ordinary descaling as described above, the FeO remaining on the upper part of the eutectic compound layer is removed during the subsequent rolling. While being crushed, it comes into contact with air to become hematite and is pushed into the surface of the steel material. In this way, red scale is generated on the surface of the billet. The island scale is one in which red scale is partially generated on the surface of the billet.

The amount of red scale generated depends not only on the Si content but also on the Ni and Cu contents. In other words, if the Ni content and the Cu content left in the ground iron are large, the roughening of the ground iron is severe, and since it occurs over the entire surface, the scale meshing with the ground iron also remains over almost the entire surface, The entire scale becomes red. When the Ni content and the Cu content are small, the unevenness of the base iron occurs locally, and the scale remaining thereon becomes a partial red scale, that is, an island scale.

In order to stably generate the entire red scale, the following conditions must be satisfied.

(1) Si Content Si is an indispensable element for increasing the strength of steel and promotes the formation of red scale. In order to generate red scale stably, the descalability must be reduced so that the eutectic compound engaged with the base iron is not removed by the descaling. For this purpose, it is desirable that the content of Si in the steel is large, but when coexisting with Ni and Cu described below, the object can be achieved if Si is 0.2% by weight or more. This Si is 0.2% by weight for ensuring the strength required for high-strength steel sheets.
The above is necessary. On the other hand, if too much Si is added, the weldability deteriorates and edge cracks easily occur during cold rolling. Therefore, the upper limit of the content was set to 3.0% by weight.

(2) Ni and Cu contents As described above, Ni and Cu increase the formation of red scale. The sum of the Ni content and the Cu content is 0.01 to 0.
Even at about 02% by weight, the Ni-Cu alloy is concentrated at the interface between the scale and the steel during the slab heating step, forming complex irregularities at the interface, thus helping to generate red scale on the entire surface of the steel sheet. However, in order to easily generate a red scale on the entire surface, the sum of the contents of Ni and Cu is desirably 0.02% by weight or more.

However, when the content of Cu is too large as compared with that of Ni, since the melting point of Cu (1085 ° C.) is low, local concentration at the interface does not occur and the Cu diffuses uniformly. Then, unevenness at the interface between the scale and the steel is less likely to occur, and red scale is less likely to appear. When the content of Cu is large, there is a possibility that the molten Cu may enter the grain boundary and embrittle the steel interface. An appropriate Cu to Ni content ratio (Cu / N
i) is 2 or less.

The upper limit of the sum of the contents of Ni and Cu is not particularly limited. However, the Si high-strength steel sheets to be used in the present invention are mass-produced products for automobile bodies and the like and are inexpensive. Therefore, it is not desirable to add a large amount of expensive Ni. Also, it is not desirable to add a large amount of Cu for the above reason. In practice, the upper limit of the sum of the contents of Ni and Cu will be about 0.5% by weight.

As described above, the contents of Ni and Cu have been described. However, it is not always necessary to contain both, and even when only Ni is contained, the above-mentioned function is performed. That is, as for the contents of Ni and Cu, the sum of the contents is 0.01% by weight or more, and the contents of Ni and Cu are Cu /
It is sufficient that Ni ≦ 2 is satisfied.

(3) Heating temperature (heating furnace extraction temperature) The reason for setting the lower limit of the heating temperature (heating furnace extraction temperature) to 1170 ° C. is that when heating at a temperature lower than this temperature, the firelite eutectic compound in the base iron And the interface between the scale and the steel does not enter into an uneven shape. Therefore, the scale is easily removed by the subsequent descaling, and a red scale is partially generated in the steel sheet after hot rolling. Further, when the heating temperature of the steel sheet is too low, another problem that the rolling load in hot rolling becomes too large also occurs.

The upper limit of the heating temperature does not need to be particularly limited. However, when the method of the present invention is carried out by an apparatus in which the first descaling temperature is necessarily determined by the heating furnace extraction temperature, the first method is used. Second descaling temperature is 12
The furnace extraction temperature must be below 00 ° C.
The first descaling temperature will be described below.

(4) First Descaling Temperature The first descaling is the first descaling performed before and during rolling after extraction from the heating furnace. This temperature is 1200 ° C. or less.

If the first descaling temperature is too high, the firelite eutectic compound (eutectic point: 1173 ° C.) at the interface between the scale and the steel becomes a molten state during the descaling, and is easily removed. At that time, FeO layer present on top of the eutectic compound layer is also removed together of course, its parts
The red scale formed as a result of oxidation of FeO after rolling does not occur in minutes . This phenomenon occurs uniformly over the entire surface
The eutectic compound and its
Remains on the surface, resulting in non-uniform red scale
It becomes an island scale. Therefore, the first descaling temperature is 1200
It should be below ° C.

The descaling after the second time is as follows.
Naturally, the temperature is lower than that of the first time, and the temperature is not particularly limited.

Next, components of a steel sheet suitable as an object of the present invention will be described.

Si, Ni and Cu are as described above.

(1) Regarding P P is the element having the greatest solid solution strengthening ability. However, if too much is added, it is concentrated on the scale side at the interface between the scale and the steel, so that the descaling property is improved and the generation of a red scale on the entire surface is prevented. If P is excessive, it causes grain boundary embrittlement, so that it is not preferable to contain P in excess of 0.05% by weight.

(2) About S S, like P, is concentrated at the interface between the scale and the steel, thereby improving the releasability of the scale and preventing the generation of red scale on the entire surface.
On the other hand, S is a harmful element that deteriorates the mechanical properties (particularly hot workability) of steel. S may be contained if used in combination with an amount of Mn sufficient to prevent hot embrittlement,
The amount is desirably not more than 0.02% by weight.

(3) About Mn If a large amount of Mn is contained, the steel is solid-solution-hardened and the ductility is reduced. Therefore, it is not preferable to contain more than 3.0% by weight. In order to suppress the hot embrittlement hardening effect of S, 0.1 wt%
It is better to contain the above. (3) Nb, V, and Ti Nb, V, and Ti all precipitate as carbides on ferrite ground and are effective elements for further increasing the strength of steel sheets. Therefore, if necessary, contain one or more of these elements. You may. However, if any of the elements is less than 0.005% by weight, the desired effects cannot be obtained. When these elements are contained, Ti and Nb are 0.005 to 0.10% by weight, and Vb is 0.0% by weight.
It is desirable to set the content to 05 to 0.20% by weight.

[0040]

Example 1 Table 1 shows the chemical compositions of the sample steel sheets used in the experiment. A steel scale of components A to P shown in Table 1 was melted to conduct a red scale reproduction experiment. Table 2 summarizes the test temperatures and results.

The test method will be described with reference to FIG. Using the heating furnace 1, each sample steel plate was heated in the air for 2 hours so as to have a heating furnace extraction temperature shown in Table 2 to generate a primary scale. Thereafter, the first descaling was performed using the descaler 2 shown in FIG. 2 at the descaling temperature shown in Table 2, and the steel was rolled by a F1 rolling mill (thickness of the billet 30 mm → 25 m).
m).

Further, the steel slab is returned to the F1 inlet side again and waited for 40 seconds to generate a secondary scale. After descaling is again performed by the descaler 2, rolling is performed by rolling mills F1, F2 and F3. Then, it was water-cooled. The descaling was performed by jetting high-pressure water having a nozzle outlet pressure of 120 kgf / cm ² . Thereafter, the area ratio of the red scale remaining on the steel sheet was observed and is shown in Table 2.

As shown in Table 2, the amounts of Si, Ni, Cu
If the amount, the heating furnace extraction temperature and the first descaling temperature are within the ranges specified in the present invention, the red scale area ratio becomes 60% or more, and the steel sheet is almost completely covered with the red scale. Was. On the other hand, under conditions outside the range defined by the present invention, the red scale area ratio was low, and non-uniform island scales occurred on the steel sheet surface. In addition, P with a large Cu content
In the material, surface flaws due to slab cracks occurred. In addition, steel type
In H, the red scale is substantially 0 (island scale)
Rolled because the furnace extraction temperature was too low
Trouble. This is due to the steel type H of Example 2 described below.
But it was the same.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

Second Embodiment In the first embodiment, since the number of times of descaling is small, the entire surface of the steel sheet tends to be red scale.
Therefore, it is necessary to actually perform hot rolling on a line of an actual machine (about 12 times of descaling) to check whether or not the entire surface of the steel sheet has a red scale. So A
-P material was hot-rolled in the hot-rolling line shown in FIG.

A steel having the chemical composition shown in Table 1 was melted,
The steel slab (slab) obtained by continuous casting is heated in a heating furnace, and 3.8 mm (thickness of 1600 mm) from a 300 mm thick slab in a hot rolling line.
mm) to investigate the occurrence of red scale.

The heating furnace extraction temperature and the first descaling temperature were adjusted to the temperatures shown in Table 3.
The descaling headers are provided at a total of 12 places (A to L in the figure) on the hot rolling line as shown in FIG. In the drawing, R1 to R6 are rough rolling mills, and F1 to F7 are finishing rolling mills.

As shown in Table 3, the amounts of Si, Ni, Cu
If the amount, the heating furnace extraction temperature and the first descaling temperature (the temperature of the steel sheet at the time of descaling by the descaling header A) are within the ranges defined in the present invention, the red scale area ratio is 90% or more and almost the entire surface. It became a red scale. On the other hand, under conditions outside the range defined by the present invention, the red scale area ratio was low, and non-uniform island scales occurred on the steel sheet surface. In the case of the P material having a large amount of Cu, surface defects due to slab cracks occurred.

A steel plate whose entire surface is covered with red scale is 14% HC
(1) When pickling was carried out under the conditions of an aqueous solution and a liquid temperature of 80 to 90 ° C., the red scale could be uniformly removed over the entire surface, and a steel sheet substantially free of scale flaws, which had conventionally been a problem, was obtained. As a result of cold rolling this steel sheet and galvanizing it, no problem of uneven plating occurred.

[0051]

[Table 3]

[0052]

EXAMPLE 3 Table 4 and FIG. 5 show the relationship between the contents of Ni and Cu in steel and the red scale area ratio. A steel slab in which the amount of Si in the steel was about 0.7% by weight and only the contents of Ni and Cu were changed, the heating furnace extraction temperature was 1220%.
Under the condition of 1 ° C. and the first descaling temperature of 1050 ° C., hot rolling was performed by the apparatus shown in FIG. 4 to examine the generation state of red scale.

As is clear from Table 4 and FIG. 5, the red scale area ratio increases as the content of Ni and Cu in the steel increases.

[0054]

[Table 4]

[0055]

According to the method of the present invention, it is possible to produce a high-strength Si-containing steel sheet which has few island scale flaws which are likely to occur during hot rolling and has excellent surface properties.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a surface layer portion of a steel sheet for explaining that an interface between a Si-containing steel and a scale generated on the surface thereof becomes uneven.

FIG. 2 is a schematic diagram of a hot rolling line used in the experiment.

FIG. 3 is a schematic cross-sectional view of a surface portion of a steel sheet for explaining how a scale generated in a Si-containing steel material changes before and after descaling and rolling.

FIG. 4 is a schematic view of an actual hot rolling line used in an example.

FIG. 5 is a diagram showing the relationship between the contents of Ni and Cu and the red scale area ratio.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-72031 (JP, A) JP-A-52-18419 (JP, A) JP-A-4-74899 (JP, A) JP-A-6-1994 279923 (JP, A) JP 60-1085 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8/20-8/04 C2 D 9/46-9/48 B21B 45/00

Claims (1)

(57) [Claims] 1. The method according to claim 1, wherein the content of Si is 0.2 to 3.0% by weight,
And the content of Cu is 0.01% by weight or more, and Ni and C
A steel slab having a u content ratio (Cu / Ni) of 2 or less
Heat to 70 ° C or higher and perform the first descaling at 1200 ° C
A method for producing a high-strength steel sheet having excellent surface properties, which is performed in the following temperature range and hot-rolled.