JP6347164B2 - Low carbon aluminum killed steel manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a low-carbon aluminum killed steel having excellent surface properties, and particularly to a method for producing a low-carbon aluminum killed steel having a high Al content.

  In general, surface cracks that occur in continuously cast low carbon aluminum killed steel slabs tend to occur mainly during low strain rate deformation during straightening or unbending with a vertical bending continuous casting machine or curved continuous casting machine. strong.

  The surface cracks generated by the slab bending stress are the effects of grain boundary embrittlement due to the precipitation of carbides and nitrides at the γ grain boundaries of the surface layer structure, and propagate along the austenite grain boundaries.

  The surface crack of the low-carbon aluminum killed steel slab is a temperature at which the surface temperature of the slab transforms from an austenite phase to a ferrite phase that reduces high-temperature ductility (embrittlement temperature range: about 600) when the slab is bent and straightened. It is known that it occurs at ˜850 ° C.).

  Conventionally, as a countermeasure against slab surface cracking that develops in this embrittlement temperature range, the surface temperature when bending the slab is changed from a temperature range (embrittlement temperature range) that reduces hot ductility to a low temperature side or a high temperature side A way to avoid it has been implemented. Thus, if the surface temperature at the time of straightening the slab can be controlled to avoid the embrittlement temperature region, surface cracking can be prevented.

  However, at present, it is difficult to completely avoid the brittle region by controlling the surface temperature of the slab, so it is difficult to completely prevent the surface cracking of the low carbon aluminum killed steel slab.

  Therefore, in Patent Document 1, for example, when continuously casting a material having a brittle temperature range of 820 to 950 ° C., the surface temperature of the upper part of the slab in the upper part of the secondary cooling zone is reduced to 650 to 700 ° C. A method is disclosed in which transformation is caused, and thereafter, the surface temperature at the bending straightening point is maintained in the range of 700 to 800 ° C. by slowly reheating.

  Moreover, in patent document 2, surface cracking is prevented by cooling and holding the surface layer part of the slab at a temperature of 350 to 500 ° C. for 1 minute or more before charging into the heating furnace during the ingot rolling of the slab. A method is disclosed. In the method disclosed in Patent Document 2, the surface temperature of the slab is temporarily lowered to transform most or all of the slab from the γ phase to the α phase. This is a method of reducing cracking sensitivity.

  The techniques disclosed in Patent Documents 1 and 2 both reduce the surface temperature of the slab, thereby causing a phase transformation in the surface of the slab, giving the metal structure strength and cracking sensitivity. It is a method of lowering.

  However, as in the method disclosed in Patent Document 1, when the surface temperature of the upper part of the slab is lowered to 700 ° C. or lower during continuous casting, the deformation resistance of the slab increases and the force required for bending correction increases. The equipment becomes large. Further, in the method disclosed in Patent Document 1, ferrite bands and epidermal cracks are likely to be generated.

  In the method disclosed in Patent Document 2, one or more of Cr: 0.75 to 1.5% by mass, Mo: 0.15 to 0.35% by mass, and Ni: 0.4 to 2.0% by mass. It is intended for an aluminum killed steel that has improved hardenability and easily forms aluminum nitride (AlN).

  On the other hand, in Patent Document 3, in order to improve the cold workability while maintaining the hot rolling even if the spheroidizing annealing treatment is omitted, the ferrite crystal grain refinement element such as Ti is reduced as much as possible, and the ferrite aggregated grains are reduced. A technique of adding B or Zr for increasing the diameter is disclosed.

  The technique disclosed in Patent Document 3 aims to reduce the deformation resistance by coarsening the ferrite crystal grain size in order to reduce the deformation resistance at room temperature and the deformation resistance in the processing heat generation region.

Japanese Patent Publication No.58-3790 JP-A-5-329505 JP 2001-342544 A

  The problems to be solved by the present invention are that the methods disclosed in Patent Documents 1 and 2 both reduce the surface temperature of the slab and cause a phase transformation in the surface of the slab, resulting in a metallographic strength. This reduces the susceptibility to cracking and requires a large amount of force for bending correction, resulting in a large facility. In addition, in the method of Patent Document 1, ferrite bands and subepidermal cracks are likely to be generated, while Patent Document 2 is directed to an aluminum killed steel that easily forms AlN, resulting in a large amount of Al added. This is a point different from the present invention aiming at suppression of generated surface cracks.

  In addition, when the technique disclosed in Patent Document 3 is applied to a hot slab that is the subject of the present application, the ferrite crystal grain size becomes coarse, and a slab surface crack is likely to be generated.

  The present invention relates to a slab surface crack that occurs during the straightening or straightening of a bending bend in a vertical bending continuous casting machine or a curved continuous casting machine without controlling the surface temperature during continuous casting of a low carbon aluminum killed steel slab. The purpose is to suppress.

That is, the present invention
C: 0.01-0. 09 %, Mn: 0.25 to 0.40%, Si: 0.03% or less, P: 0.005 to 0.030%, S: 0.01 to 0.03%, Al: 0.063 to 0. 093 %, N: 0.0060% or less, in addition, a method of continuously casting low carbon aluminum killed steel composed of Fe and impurities,
Addition of 0.010% <Ti ≦ 0.025% to the low carbon aluminum killed steel, and ALN precipitation starts at the surface temperature of the slab at the upper part of the secondary cooling zone where the solidified shell thickness of the slab is 10 mm to 30 mm The most important feature is that the temperature (1050 ° C.) or higher.

In the present invention, 0.010% <Ti ≦ 0.025% is added, and the surface temperature of the slab in the upper part of the secondary cooling zone having a solidified shell thickness of 10 mm to 30 mm is set to the ALN precipitation start temperature (1050 ° C) or more, so that Al is 0.063-0. Precipitation of AlN can also be suppressed in continuous casting of low carbon aluminum killed steel containing 093 % by mass.

In the present invention, from the viewpoint of suppressing the precipitation of AlN, the surface temperature of the slab, the precipitation start temperature of TiN (1400 ° C.) is preferably set to below.

  In the present invention, since precipitation of AlN can be suppressed, it is possible to prevent slab surface cracking of the low carbon aluminum killed steel that is caused by increasing the amount of Al added.

  The present invention is required to suppress age hardening in low-carbon aluminum killed steel products, and in steel types where the amount of Al added must be increased, surface cracks caused by increasing the amount of Al added. It aims at suppressing.

  Precipitation of AlN has been confirmed at the grain boundaries where surface cracks have occurred in low carbon aluminum killed steel slabs, and surface cracks can occur due to concentration of bending straightening stress on AlN precipitated at the γ grain boundaries. It is considered that the nature is high.

  On the other hand, TiN (titanium nitride) is preferentially precipitated with respect to AlN by adding Ti, which has a higher affinity with N to low carbon aluminum killed steel than Al.

  The inventors suppress the precipitation of AlN, which is the main cause of slab surface cracking that occurs during bending correction or bending correction in a vertical bending type continuous casting machine or a curved type continuous casting machine. It was found that it is effective in suppressing the occurrence.

  Further, the surface temperature of the slab at the upper part of the secondary cooling zone with the thickness of the solidified shell ranging from 10 mm to 30 mm is between the TiN precipitation start temperature (about 1400 ° C.) and the ALN precipitation start temperature (about 1050 ° C.). It has been found that by controlling, it is possible to preferentially precipitate TiN in the slab surface layer portion to suppress the precipitation of ALN, and to obtain the effect of adding Ti more.

  That is, the present invention achieves the above object by adding an appropriate amount of Ti as a component control and controlling the slab temperature in the upper part of the secondary cooling zone.

Specifically, the present invention provides:
C: 0.01-0. 09 %, Mn: 0.25 to 0.40%, Si: 0.03% or less, P: 0.005 to 0.030%, S: 0.01 to 0.03%, Al: 0.063 to 0. 093 %, N: 0.0060% or less, in addition, a method of continuously casting low carbon aluminum killed steel composed of Fe and impurities,
Addition of 0.010% <Ti ≦ 0.025% to the low carbon aluminum killed steel, and ALN precipitation starts at the surface temperature of the slab at the upper part of the secondary cooling zone where the solidified shell thickness of the slab is 10 mm to 30 mm The temperature is set to be equal to or higher than 1050 ° C.

From the viewpoint of suppressing the precipitation of AlN, the surface temperature of the slab described above, deposition starting temperature of TiN (1400 ° C.) is preferably set to below. According to the inventors' tests, the more desirable range of the surface temperature of the slab is 1055 ° C to 1082 ° C.

  In the present invention, the reason why the low-carbon aluminum killed steel having the above chemical composition range is targeted and the reason for defining the addition amount of Ti will be described below. Hereinafter, “mass%” is simply referred to as “%”.

C: 0.01-0. 09 %
C is an element essential for imparting the necessary strength of the steel material. However, if the content is less than 0.01%, the required strength cannot be obtained, and if it is attempted to control the concentration to a low concentration of less than 0.01%, the cost increases and it is not economical. On the other hand, 0. This is because if the content exceeds 09 %, the strength of the steel increases and the workability of the product decreases, making it impossible to satisfy the properties required for the product.
Therefore, in the present invention, the C content is set to 0.01-0. It was defined as 09 %.

・ Mn: 0.25 to 0.40%
Mn is not only effective for deoxidation and desulfurization, but is also an element effective for improving the quenching and tempering softening resistance during the heat treatment after cold working. In order to effectively exhibit the above action, it is necessary to add 0.25% or more. However, if added over 0.40%, bainite that inhibits cold workability is easily generated. Therefore, in the present invention, the Mn content is specified to be 0.25 to 0.40%.

-Si: 0.03% or less Si is an element effective as a deoxidizing agent, but if added over 0.03%, there is a possibility that the deformability is lowered due to solid solution strengthening, and the content is as low as possible It is desirable. Therefore, in the present invention, the Si content is set to 0.03% or less.

・ P: 0.005 to 0.030%
P is effective as an element for improving hardenability. However, if added over 0.030%, microsegregation occurs during solidification, grain boundary segregation occurs during hot rolling, embrittles the grain boundary, and cracks may occur during cold working. On the other hand, when the addition amount is less than 0.005%, the load of the de-P treatment process is high, leading to cost deterioration. Therefore, in this invention, content of P was prescribed | regulated as 0.005-0.030%.

・ S: 0.01-0.03%
S mainly forms sulfide inclusions of MnS. And when the content exceeds 0.03%, it may segregate in the ingot at the time of hot rolling, embrittle the ingot, and may break at the time of cold working. On the other hand, if the content is less than 0.01%, MnS sulfide inclusions are difficult to precipitate, and as a result, the machinability of the product may be reduced. Therefore, in the present invention, the S content is defined as 0.01 to 0.03%.

-Al: 0.063-0. 093 %
Al is an element effective for deoxidation, but nitride (AlN) is easily generated, and when AlN increases, crystal grains become finer. Moreover, Al combines with N described below as an element for fixing N, thereby solving the problem of age hardening caused by free N in steel. However, if stress such as slab pressure is applied while AlN is precipitated at the grain boundaries during continuous casting, stress concentration occurs at the grain boundaries, causing surface cracks. Therefore, in the present invention, it is better not to generate AlN as much as possible. Therefore, in order to prevent the generation of AlN as much as possible, the aim is 0.063% or more with a target lower than the median of the composition range. 093 % or less.

-N: 0.0060% or less N is an element that causes age hardening of products when present in a free state in steel, and should be as low as possible in products that require workability, such as low-carbon steel. Is desirable. However, when N is to be lowered, it is necessary to perform excessive sealing in order to suppress N pickup from the atmosphere, leading to a reduction in operational costs. Therefore, in the present invention, the aim of N is set to 0.0060% or less.

Ti: more than 0.010% and 0.025% or less Ti binds to N in steel and precipitates as TiN, thereby suppressing precipitation of AlN and slab surface at austenite grain boundaries Suppresses cracking. In the present invention in which the Al content is 0.063 to 0.12%, the addition of 0.025% Ti is sufficient for bonding with N in the steel. Moreover, even if added over 0.025%, there is no change in the effect, and the load in terms of cost increases, so the upper limit was made 0.025%. If the content is excessive, TiN becomes coarse and causes a decrease in fatigue strength. On the other hand, when the addition amount of Ti is 0.010% or less, the effect of suppressing the precipitation of AlN cannot be expected, so the lower limit was set to 0.010%.

  According to the present invention, in continuous casting of low carbon aluminum killed steel, it is possible to stably suppress precipitation of AlN, without installing equipment remodeling and low temperature slab straightening device for controlling the surface temperature of the slab, Surface cracks occurring in the slab can be suppressed.

Hereinafter, the results of tests conducted to confirm the effects of the present invention will be described.
The test is performed by continuously casting a slab having a thickness of 339 mm and a width of 465 mm from a low carbon aluminum killed steel having the chemical composition shown in Table 1 below using a curved continuous casting machine, and investigating surface cracks generated on the surface. went. Continuous casting was performed at a speed of 0.60 to 0.70 m / min, and the specific amount of secondary cooling water was controlled to be 0.18 to 0.35 liter / kg-steel.

Table 2 below shows test conditions and test results.
The light care rate in Table 2 below is based on the number of cracks and the number of man-hours required for maintenance after the surface slab of the steel slab after the slab cast by the continuous casting machine is transported to the slab and rolled. This is an evaluated surface quality index.

  The larger the light care rate, the better the surface quality. If this light care rate is less than 100%, it is necessary to grind the steel slab surface with a grinder, which not only reduces the yield, but also cannot be commercialized if the grinding amount is excessive. means. In Table 2 below, the case where the light care rate was 100% was evaluated as ◯, the case where it was less than 100 and 90% or more was evaluated as Δ, and the case where it was less than 90% was evaluated as ×.

  In Comparative Example 1, the Al content was 0.062%, which is less than the range specified in the present invention. Therefore, even if the addition amount of Ti is 0.001%, which is less than the amount specified in the present invention. The amount of precipitated AlN was small and it was not necessary to care for it, the light care rate was 100%, and the evaluation was ◯.

  However, in Comparative Example 2, although the Al content is 0.083%, which is within the range defined by the present invention, the amount of Ti added is 0.002%, which is less than the amount defined by the present invention. Therefore, surface cracking occurred due to the precipitation of AlN, and the light maintenance rate was 76%, and the evaluation was x.

  Further, in Comparative Example 3 as well as Comparative Example 2, although the Al content is 0.080% which is within the range defined by the present invention, the amount of Ti added is less than the amount defined by the present invention. Since it was .010%, surface cracking occurred due to precipitation of AlN, and the evaluation was x with a light care rate of 71%.

  In the case of these comparative examples 1 to 3, the surface temperature of the slab at the upper part of the secondary cooling zone where the solidified shell thickness of the slab was 10 mm to 30 mm was 1050 ° C. or more, which is the ALN precipitation start temperature. .

  On the other hand, in Comparative Example 4, although the Al content is 0.083%, which is within the range defined by the present invention, and the addition amount of Ti is 0.023%, which is within the range defined by the present invention, the casting speed Even when Vc is reduced to 0.60 m / min, the specific water volume of the cooling water at the upper part of the secondary cooling zone is kept constant at 0.35 liter / kg, so that the secondary shell having a solidified shell thickness of 10 mm to 30 mm is obtained. The surface temperature of the slab at the upper part of the cooling zone was lowered to 980 ° C., which was lower than the ALN precipitation start temperature. Therefore, the light care rate was 91%, not 100%, and the evaluation was Δ.

  In contrast, Invention Example 1 in which the Al content is 0.084% and the Ti addition amount is 0.025%, and the Al content is 0.093% and the Ti addition amount is 0.023%. % Of Invention Example 2 was obtained by adding Ti within the range specified in the present invention to the low-carbon aluminum killed steel targeted in the present invention, so that the light care rate was 100% and the evaluation was “good”.

  In addition, in Invention Example 3 in which the Al content is 0.073% and the Ti addition amount is 0.011%, Ti within the range defined in the present invention is added to the low-carbon aluminum killed steel targeted by the present invention. Therefore, the light care rate was 100%, and the evaluation was ◯.

  In the case of these inventive examples 1 to 3, the surface temperature of the slab at the upper part of the secondary cooling zone where the solidified shell thickness of the slab is 10 mm to 30 mm was 1050 ° C. or more, which is the ALN precipitation start temperature. .

  On the other hand, in the case of Invention Example 4 in which the Al content is 0.082% and the Ti addition amount is 0.022%, the secondary cooling zone is particularly effective when the casting speed Vc is reduced to 0.60 m / min. The specific surface water temperature of the slab in the upper part of the secondary cooling zone with a solidified shell thickness of 10 mm to 30 mm is reduced to 0.18 liter / kg, which is lower than that of Invention Examples 1 to 3. Was set to 1050 ° C. or more, which is the ALN deposition start temperature, and the light care rate was 100%, and the evaluation was “good”.

  It goes without saying that the present invention is not limited to the above-described examples, and the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

  The method of the present invention can be applied to continuous casting of slabs and billets regardless of the slab size described in the above embodiments. Moreover, since casting speed and secondary cooling have casting restrictions for each continuous casting machine, the casting speed and the specific water amount of the secondary cooling water are not limited to the ranges described in the above embodiment, and there are no upper and lower limit values in particular. Conceivable.

Claims (2)

C: 0.01-0. 09 %, Mn: 0.25 to 0.40%, Si: 0.03% or less, P: 0.005 to 0.030%, S: 0.01 to 0.03%, Al: 0.063 to 0. 093 %, N: 0.0060% or less, in addition, a method of continuously casting low carbon aluminum killed steel composed of Fe and impurities,
Addition of 0.010% <Ti ≦ 0.025% to the low carbon aluminum killed steel, and ALN precipitation starts at the surface temperature of the slab at the upper part of the secondary cooling zone where the solidified shell thickness of the slab is 10 mm to 30 mm A method for producing a low-carbon aluminum killed steel, characterized by having a temperature (1050 ° C.) or higher. Method for producing a low-carbon aluminum-killed steel according to claim 1, characterized in that to make the surface temperature of the slab described above, the deposition starting temperature (1400 ° C.) hereinafter of TiN.