JPS6171101A - Hot rolling method of billet prevented from surface cracking - Google Patents

Abstract

PURPOSE:To prevent effectively and supply a billet from surface cracking by making a billet, containing a specific amount of Mn, contain a specific amount of Ti and rolling it under specific rolling conditions. CONSTITUTION:An ingot or a billet just after continuous casting, containing <0.50% Mn, is rolled by direct-feed rolling or hot-charge rolling. In this case, the billet is made to contain Ti of the amount of 2-25 Ti/N, and is rolled within a temperature range of cooling stage and under the rolling conditions of >=1 rolling shape factor (m) defined by the equation; where R: radius of rolling roll, h1: thickness of material at inlet side of roll, h2: thickness of material at outlet side of roll.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is directed to steels containing less than 0.50% Mn, especially steel plates for automobiles such as aluminum killed, aluminum semi-killed or aluminum silicon killed steels, steel plates for general construction, and steel plates for shipbuilding. The present invention relates to a hot rolling method that prevents surface cracking during hot rolling of carbon steel and low alloy steel containing Nb, ■, etc., which are used for industrial steel sheets, mechanical structural steel sheets, etc., and in particular, The steel billet is immediately hot rolled after ingot making or continuous casting (direct rolling), or the billet is charged into a heat retention furnace as it is after ingot making or continuous casting and then hot rolled (hot charge rolling). ) process, it relates to a method for preventing cracks from occurring on the surface of a steel billet.

(Conventional technology) Without heating the solidified slab or steel ingot midway,
Using the retained heat, the material is sent directly to the rolling process for hot rolling (hereinafter simply referred to as "direct rolling").

Alternatively, similarly, hot rolling is performed after charging into a heat retention furnace without heating during the process (hereinafter simply referred to as "honto charge rolling", and "direct rolling" and "hot yardage rolling" are collectively referred to as "honto charge rolling"). Although direct rolling (also referred to as "direct rolling") is the most desirable operation form from the perspective of energy conservation, there are various problems in realizing it, such as the surface properties of the slab and the layout of equipment. However, as these technologies have been improved since then, studies on direct rolling have become more active.

As a result, the direct hot rolling method, that is, the direct rolling method, has many metallurgical phenomena that are different from those seen in the conventional method (a method in which the method involves cooling once below the transformation point, close to room temperature, and then reheating and rolling). Found out. Among these, when directly hot rolling the material, the hot workability of the material decreases markedly.In other words, direct rolling can cause cracks on the surface of the steel piece during hot rolling, even for steel types that did not cause any problems with conventional methods. It was found that this occurs.

Generally, the hot workability of steel is determined by the austenite grain size (hereinafter referred to as
In general, the finer the γ grain size and the less precipitation of sulfides, carbonitrides, etc. at the γ grain boundaries, the stronger the heat resistance. Machinability is improved.

In the conventional method, the material is repeatedly cooled and reheated to undergo T (austenite)-α (ferrite) transformation, thereby refining the γ grains and fixing most of the precipitates within the grains. Hot workability was improved by reducing the amount of precipitation in the interface.

On the other hand, in the case of the direct rolling method, the retained heat of the steel billet is utilized to the fullest, and the steel slab is rolled without going through the γ-α transformation, so the single grain size is very large, and the γ-grain boundary There is also a large amount of precipitation, resulting in a decrease in hot workability, and therefore, such a difference in thermal history is considered to be the cause of cracking during hot rolling.

Several proposals have already been made regarding the prevention of cracks during hot rolling that occur in direct rolling, but the common idea among them is described in JP-A-55-84201 or JP-A-55-84203. As typically seen, by slowing down the cooling rate of the slab after solidification (or by holding it at a predetermined temperature for a certain period of time during cooling, the precipitates coagulate and become coarser, reducing the number of precipitates at one grain boundary). In other words, all of the conventional techniques aim to control the precipitation form of sulfides, carbonitrides, etc., which are one of the causes of cracks mentioned above. Considerable effects were observed.

However, on the other hand, problems with these conventional techniques have also become more apparent. In other words, satisfying the cooling rate or temperature maintenance conditions necessary to prevent cracking will lead to a significant drop in productivity in actual operations, and as long as such conditions are followed, application to the continuous casting and rolling method currently under development will be difficult. This is extremely difficult.

(Problem to be solved by the invention) The effects of component elements and temperature on hot workability have been clarified through high-temperature tensile tests, etc.
It is known that Mn decreases hot workability and causes cracking during hot rolling, while addition of Mn is effective in preventing cracking. However, the influence of S or Mn differs depending on the thermal history during the test, and it is only by the conventional method that the influence of S and Mn can be summarized by a value such as Mn/S. This is just a so-called reheating tensile method that involves repeated cooling and reheating.

As a result of our research, the present inventors found that in the melt-solidification tensile method, which involves hot rolling during the solidification process, such as direct rolling, the negative effects of S only occur when the amount of Mn is low, and the amount of Mn is 0.50. % or more, good ductility can be obtained regardless of the Si content.

Based on this knowledge, C: 0.12%, Si: 0
．． 20%, n: 0.85%, P: 0.018
%, S: 0.010%, Sol, A1: 0.0
33%, N: 35 ppm, thickness 26 dmm
A continuously cast slab with a width of 1,350 mm was directly rolled to a thickness of 130 mm at 900'C with a rolling roll of 1,300 mm in diameter at 900'C. Contrary to expectations from the high temperature tensile test results, cracking occurred.

From the above, it has become clear that the high temperature tensile test results and the actual rolling results do not necessarily match, that is, it is not possible to evaluate hot workability in actual rolling based only on temperature, composition, and even thermal history. Ta.

The present inventors presume that the stress state inside the material in actual rolling is not as simple as in a tensile test, and that the cause lies in the difference in the deformation mode between the two. In order to simulate this, we directly reproduced rolling in the laboratory and conducted various research.

(Means for solving the problem) In a direct rolling experiment that adopted a method that simulates not only the thermal history but also the deformation mode, we conducted stop rolling in order to understand the occurrence of surface cracks during hot rolling by direct rolling. As a result of observing the occurrence of cracks, it was found that cracks had already occurred just before the material was rolled down by the rolls.

FIG. 1 is a schematic explanatory view schematically showing the flow of the material of the slab 1 in this case. 3 (
(shown as the shaded area in the figure), so in order to accommodate this, the center layer is subjected to compressive stress as shown by the dotted line arrow in the figure, and the surface layer is subjected to tensile stress as shown by the solid line arrow in the figure. Each receives stress. The cracks 4 that occur immediately before rolling are caused by such tensile stress in the surface layer.

From the above, it became necessary to consider the deformation mode in evaluating and examining hot workability.

The present inventors have discovered that this stress state depends on the degree of non-uniform deformation of the material, and that the degree of non-uniform deformation and hot workability are determined by rolling conditions such as the diameter of the rolling rolls, the shape of the slab, and the amount of reduction. It has been found that there is a strong correlation with the rolling shape ratio (ml) determined by the following formula.

n1+n, where R: rolling roll radius hl: material thickness on roll entry side h2 material thickness on roll exit side (thickness), and the larger m is, the more the reduction is '/+S' to the inside, meaning that uniform deformation is achieved.In other words, this means that the larger the roll contact arc length is, the more uniform the deformation is. This means that it will become deformed.

Therefore, we created a series of steel ingots or slabs with the shapes shown in Table 2 from molten steel with a wide range of compositions shown in Table 1, and then rolled them using a laboratory-scale rolling mill and a rolling mill on an actual production line. 3
Direct rolling was performed under the rolling conditions shown in the table, and the occurrence of cracks was investigated. In this case, the number of steel ingots or slabs subjected to direct rolling was about 300, and the rolling shape ratio ((6) was 0.3 to
It was changed to 9.7. The results of these series of tests are
The figures are summarized in a graph.

Table 2 Shape of test steel Table 3 From the examination of the test results above the rolling conditions, although there is a correlation between the occurrence of cracking and various factors such as composition, temperature, and range of rolling conditions, let us take a more comprehensive look. It was found that the presence or absence of cracking can be distinguished by the numerical range of Mn content and rolling shape ratio (ml).

In other words, as shown in No. 2rXJ, in direct rolling of steels containing 0.50% or more Mn, cracking can be prevented by rolling under rolling conditions such that the rolling shape ratio m during hot rolling is 1 or more. It has been found that this can be prevented.

However, the test results indicate that cracks occur in direct rolling when the Mn content is less than 0.5% and when the rolling shape ratio (ml) is less than 1. We further investigated ways to prevent cracking under such conditions and found that the cause of cracking under conditions where Mnffi is less than 0.5% or the rolled shape ratio (ml) is less than 1 is due to the presence of sulfides at the γ grain boundaries. It was concluded that either the amount of precipitation was large or the amount of precipitation was small (but this was due to the material being rolled under unfavorable stress conditions).

Therefore, as a result of research to further improve the precipitation form of precipitates that are the starting point of such cracks, we found that the addition of Ti is very effective, and we also investigated the range of the amount of Ti added that is effective in preventing cracks. I also gained some knowledge.

In addition to sulfides, carbonitrides are thought to be the precipitates that become the starting point of cracks, and based on the assumption that Ti affects the form of these precipitations, we investigated the relationship between the amounts of C, S, and -N. .

Here, based on the steel composition in the composition range shown in Table 1 above, up to 0.30% of Ti is added, while Mn1l
As a result of testing the occurrence of cracks during direct rolling on 80 slabs or steel ingots under the condition that the rolling shape ratio (ml) is less than 0.50%, the present inventors found that It has been discovered that the effective range of Ti addition amount is determined by the correlation with N.

This is because among C, S, and N, N has the strongest affinity for Ti, and by adding Ti, Ti first combines with N to form TiN, and further, sulfides are precipitated using this as a nucleus. This is thought to be due to this.

Moreover, as a result of precipitating a large amount within each grain, hot workability is also improved.

On the other hand, if the amount of Ti added is too large, TiC will precipitate at the γ grain boundaries, resulting in a decrease in hot workability.

Figure 3 organizes the above results in terms of the relationship between Ti and N. The presence or absence of cracking is determined by two straight lines with different slopes, and the boundary condition is that the Ti/N ratio is 2. and 2
It is 5. In other words, even if the Mn content is less than 0.50%, if the rolled shape ratio (ml is 1 or more, 2
It has been found that cracking can be prevented by adding ~25 times as much Ti.

Regarding the influence of the rolling shape ratio, as already mentioned, as the rolling shape ratio increases, the degree of non-uniformity of material deformation during rolling becomes smaller, and therefore the tensile stress generated on the material surface phase at the roll entry side becomes smaller, leading to cracking. It becomes difficult to do.

As a result of integrating the above, the present invention has been completed, that is, the present invention provides a method for directly rolling or hot charge rolling a steel billet containing less than 0.50% Mn immediately after ingot formation or continuous casting. The steel piece contains Ti and T.
It is characterized by containing an amount such that t/N is 2 to 25, and rolling under rolling conditions such that the rolling shape ratio (ml) defined by the following formula is 1 or more in the temperature range of the cooling process following melting and solidification. This is a hot rolling method that prevents surface cracking of steel slabs.

Formula: However, R: Roll radius h: Material thickness on the roll entry side h2: Material thickness on the roll exit side In the present invention, the rolling shape ratio Fml is 1 in each rolling pass. It is desirable that the rolling shape ratio (ml) be less than 1, but as described in l&, it is possible to apply a rolling pass with a rolling shape ratio (ml) of less than 1 under the condition that at least one pass of rolling with a rolling shape ratio (m) of 1 or more is applied. In such cases, it is preferable to limit the cumulative reduction amount to less than 20%.

However, while Ti addition has the above effects, it also reduces weldability, so Ti is added to steel types for such applications.
It is preferable to set the upper limit of the addition amount to 0.050%, and in that case, N should be further reduced accordingly.

(Function) Regarding the effect of the rolling shape ratio, which is an important point of the present invention, as mentioned above, the larger the rolling shape ratio (ml), the smaller the non-uniformity of material deformation during rolling. Since the tensile stress generated in the surface layer is reduced, cracks are less likely to occur.

As for the effect of h, based on the results of the investigation on the texture of sulfide precipitation, as the amount of Mn increases, the amount of sulfide precipitated within the γ grains increases, and the amount of sulfide precipitated at the γ grain boundaries decreases. It was found that the processability was improved.

In other words, the cause of cracking under conditions where the hn content is less than 0.50% or the rolling shape ratio is less than 1 is due to a large amount of sulfide precipitated at grain boundaries, or even if the amount of precipitation is small, it is due to the amount of sulfide precipitated during rolling. This can be said to be caused by the material being under unfavorable stress conditions.

Therefore, when Mni is less than 0.50%, it is necessary to suppress the precipitation of sulfide at the γ grain boundaries, and for this reason Ti is added, but its action is due to the extremely strong affinity between Ti and N. Therefore, it first combines with N to produce TiN, which acts as a nucleus and causes a large amount of sulfide to precipitate within the γ grains instead of at one grain boundary, resulting in improved hot workability.

However, as the amount of Tl added increased, the Ti/N ratio (weight) increased to 25
If the value exceeds 2, hot workability decreases because TiC precipitates at the γ grain boundaries, and the amount of Ti added decreases, resulting in a Ti/N ratio of 2.
If it is less than that, the amount of TiN produced will be small (and the function as a nucleus for precipitating sulfides in one grain will be insufficient, so hot workability will also be reduced in this case. Preferably, Ti/N The ratio (weight) is 3-17.

Slabs or steel ingots of various steels containing Rin J1 and Mn were directly rolled or hot charge rolled under various rolling conditions, and the conditions are summarized in Table 4. Test cases 6 to 9 in the cough table are examples belonging to the present invention, and test cases 1 to 5 and 10 to 13 are comparative examples outside the present invention, but from these data,
Even in steel containing less than 50% MnO, when the Ti and N contents are set to a Ti/N ratio of 2 to 25 and the rolled shape ratio is set to 1 or more, almost no cracking is observed. I can understand.

Note that all values of rolling shape ratio +ml in the table are those for the first bus of the pass schedule.

As a result of a series of experiments, cracks occur only when the total reduction is 2.
It has been confirmed that if the total rolling reduction is less than 20%, no cracking will occur even if rolling is performed with a pass schedule such that the rolling shape ratio (ml) is less than 1. Therefore, according to the present invention, even if a light reduction (m=less than 1) is applied in at least one pass, it is preferable to limit the cumulative reduction amount to less than 20%.

(Effects of the invention) As described above, even when a steel billet with Mn content of less than 0.5% is subjected to direct rolling or hot charge rolling, the Ti content is
According to the method of the present invention, in which the Ti/N ratio is controlled to be 2 to 25, and the rolling shape ratio (ml) is directly rolled, the decrease in energy efficiency and To effectively and reliably prevent the occurrence of cracks and to facilitate the application of direct hot rolling without the problem of requiring a troublesome processing process such as once cooling a steel billet to near room temperature and then reheating it. The effect of the present invention is very large in that it can do the following.

As mentioned above, when evaluating hot workability, ingredients, temperature,
It is clear how important it is to consider not only the thermal history but also the deformation mode, ie the various rolling conditions and the stress state determined therefrom. However, it is difficult to simulate the deformation mode using conventional evaluation methods such as tension or torsion, and it was only through the investigation method of the present inventors that the relationship between them was elucidated and the present invention was completed. be.

(Comparison with conventional technology) In addition to the technology examples listed in the (Conventional technology) section, Ti
The technology of improving hot workability by adding
428-430 (referred to as publicly known example A) and Special Publication No. 58-98
No. 12 (referred to as known example B) and the like.

In the above-mentioned known example A, the negative effect of S on hot workability can be eliminated not only by adding Mn but also by adding Ti, but by applying Ti, which forms a stable compound with S, instead of Mn. Imparting hot workability, 0
．． 02~0.05%S, 0.05%in steel with Mn+Ti
If the sum of T is 6 times S, hot embrittlement will not occur.
If the i/S ratio is 2 or more, practical high-temperature brittleness does not appear, and furthermore, there are large and fine TiN extrusions, and most of them are scattered in the ferrite ground. etc. are listed.

In addition, Publication B discloses that in a method of directly rolling a slab immediately after continuous casting, a high-strength steel plate is manufactured by adding Ti so that TiC precipitates after hot rolling is completed. has been done.

However, the above-mentioned known examples do not teach about adding Ti to prevent surface cracking during hot working, which is the basis of the technical idea of the present invention.
Regarding the strength of the correlation between Ti and N and setting a specific range of the ratio thereof, that is, setting the Ti/N ratio to 2 to 25,
The disclosure is completely absent.

In the present invention, Mn
When a slab containing less than 0.5% of C is directly rolled, surface cracking of the steel slab can be prevented.

Therefore, although the known examples include descriptions of adding Ti to improve the properties of steel materials and performing direct rolling,
In addition to the technical concept of preventing surface cracking during hot working by adding Ti, which is the basis of the present invention, the amount of Mn added and Ti
The technical idea of combining the /N content ratio and rolling shape ratio with specific values is not taught at all, and there is a significant difference between the present invention and those known techniques.

[Brief explanation of drawings]

Figure 1 is a schematic explanatory diagram showing the material flow during hot rolling seen in direct rolling; Figure 2 is a diagram showing the occurrence of cracking during hot rolling in direct rolling when the Mn content and rolling shape ratio of the steel billet are A graph showing the effect on
and Fig. 3 is a graph showing the effect of the content ratio of Ti and N on the occurrence of cracks in a steel billet when a steel billet with a Mn content of less than 0.50% is directly rolled at a rolling shape ratio of 1 or more. It is. 1: Steel billet 2; Roll 3: Rigid region 4: Cracking Applicant: Sumitomo Kinku Kogyo Co., Ltd. Agent Patent attorney: Akira Hirose - (1 other person) / Leap Figure 2 Rolled shape 0 length ratio <rn) Pan 3 Concave Ti+rx) Procedural Amendment October 9, 1985 Patent Application No. 160212 of 1988 2, Name of invention Hot rolling method for preventing surface cracking of steel billet 3, Relationship with the person making the amendment case Patent Applicant Address: 5-15 Kitahama, Higashi-ku, Osaka Name (211) Sumitomo Metal Notification Industry Co., Ltd. 4, Agent (Attachment) (1) The scope of the patent claims is amended as follows. `` Mn is 0.50% immediately after ingot formation or continuous casting.
Direct rolling or hot charge rolling for rolling tsubo containing less than "Revo" 1 person! - how to fix jl 54
The piece contains Ti in an amount such that Ti/N is 2 to 25,
And in the temperature range of the cooling process following melt solidification, the heat that prevents surface cracking of the steel billet, which is characterized by rolling under rolling conditions where the rolling shape ratio ((b) defined by the following formula is 1 or more) Inter-rolling method. Formula: Where, R: Roll radius hl: Material thickness on the roll entry side h2: Timber 4 thickness on the roll exit side." (2) Page 2 of the specification, lines 7-8, "or mass····
(6) In the specification, a slab with a surface temperature above the deafening point is once charged into a heating furnace or an insulating furnace and then subjected to gauge rolling. All descriptions in the following sections are corrected to "slabs." Wataru Oka Wataru Soil Point 5 Base 26112I Piece 6 2 Slab 7 4 Steel Piece 12 15.17 〃 The above is corrected to J. (3) Ibid., page 2, lines 15-17, ``Or similarly...
The description "...to be hot-rolled" has been corrected to "or, to hot-roll a slab that is still Ar or has a surface temperature above the transformation point, once placed in a heating furnace or insulating furnace." do. (4) On page 12, line 16 of the same book, the statement "to roll" is corrected to "J to manufacture steel billets by rolling." (5) The statement in the following parts of the specification is corrected as follows. The original description of the original description of the anus is 2 stars. 12
Or steel ingot -Deletion-34 transformation point A
r, transformation point -13 carbonide carbonitride 8 11 M lump or -deletion-l11
5 @ ingot or 1111 or steel ingot 15 8 or steel ingot

Claims (1)

[Claims] A method of directly rolling or hot charge rolling a steel billet containing less than 0.50% Mn immediately after ingot making or continuous casting, in which the steel billet has a Ti/N ratio of 2 to 2. 2
5, and is rolled under rolling conditions such that the rolling shape ratio (m) defined by the following formula is 1 or more in the temperature range of the cooling process following melting and solidification. Hot rolling method that prevents surface cracks. Formula: Rolling shape ratio (m) = {2√[R(h_1-h_2)]}
/(h_1+h_2) However, R: Roll radius h_1: Material thickness on the roll entry side h_2: Material thickness on the roll exit side