JP5408585B2 - Grain refinement method for steel products - Google Patents

Description

  The present invention relates to a method for refining crystal grains of steel products, and more particularly to a method for refining crystal grains of steel products using a fusing mechanism of dendrites.

  In general, the smaller the crystal grains and the more equiaxed the steel products, the lower the segregation and the better the mechanical properties and corrosion resistance. However, since the formation mechanism of crystal grain refinement and equiaxed crystallization is not necessarily clear, based on empirical knowledge, crystal grain refinement is controlled by various techniques as described below. Things have been done.

  For example, as a technique for containing and controlling inclusions serving as nucleation sites for crystal grains, a technique for containing a metal oxide containing Ca as inclusions in the production of a steel slab is disclosed (Patent Document 1). .

  In addition, a technique such as control using TiN or the like as an inclusion serving as a nucleation site for crystal grains has been put into practical use.

  In addition, as a technology to control by adding flow, there is a technology to prevent center segregation and internal defects of the slab as well as refinement of crystal grains and equiaxed crystallization by electromagnetic stirring and reduction in continuous casting. It is disclosed (Patent Document 2).

  In addition, a technique for miniaturization by vibration generated by electromagnetic force is also disclosed (Non-Patent Document 1).

Incidentally, in dissolved Fe-C system, in the cooling of the following steel materials 0.5 mass% C, bcc from the liquidus temperature of the delta phase (body-centered cubic lattice, usually called delta phase) grows, follicle The fcc (face-centered cubic lattice, usually called gamma phase) formed at the crystal temperature (1493 ° C. in the Fe—C system) grows and solidification is completed. In the solidification of the delta phase and the gamma phase, it becomes a dendrite (dendritic crystal) form. In addition, even in the cooling of steel materials having a carbon concentration higher than 0.5 mass% C in the Fe-C system, if the gamma phase does not nucleate even when cooled to below the liquidus temperature of the delta phase, the delta phase nucleates. Thus, a gamma phase is formed below the peritectic temperature , and a solidification mechanism similar to cooling of a steel material of 0.5 mass% C or less is obtained. In actual steelmaking, it is important from the viewpoint of controlling the structure of rolling in the subsequent process that the delta phase and gamma phase crystal grains be uniform and refined.

JP 2008-127599 A JP-A-8-224650

M.M. Usui and two others, “Solidified Structure Comparison under Impression of Oscillating Electromagnetic Force and DC Electromagnetic Force”, ISI International V. 47 (2007), no. 11 pp1571-1574

  However, the conventional technology based on empirical knowledge has certain limitations and limitations. Therefore, with the advancement of industry and technology in recent years, the segregation of steel materials has been reduced and the mechanical properties and corrosion resistance have been improved. However, the reality is that it cannot be said that the demands of users that are becoming increasingly severe are sufficiently satisfied.

  For example, the above-described method of including inclusions serving as nucleation sites for crystal grains not only has a limited effect, but the inclusions may adversely affect product characteristics.

  And the method using the electromagnetic stirring and electromagnetic vibration of the molten steel is difficult to apply in many cases when trying to apply it to production (casting) using an ingot.

  For this reason, it is possible to provide a steel material in which the segregation is further reduced and the mechanical properties and corrosion resistance are further improved by making the crystal grains of the steel material finer and equiaxed without any restrictions on application. The development of a technology that can do this has been desired.

The present inventors, in the intensive study the result of the cooling process of the molten steel material for the purpose of solving the above problems, dendrite delta phase occurs, in yet process of the gamma phase is formed, follicle By controlling the cooling near the crystal temperature , that is, by controlling the peritectic transformation, the boundary between the delta phase and the gamma phase is dissolved and the dendrites are found by X-ray observation. Thus, the present invention has been completed which can be miniaturized and at the same time further equiaxed. Hereinafter, each technique related to the present invention will be described.

The first technique related to the present invention is:
In the cooling process of steel materials, by controlling the cooling rate, stirring, blending of predetermined elements, etc.
A transformation from delta phase to gamma phase is formed near the solidification interface,
Furthermore, the temperature at the boundary between the formed delta phase and the gamma phase is within a predetermined range from the peritectic temperature ,
Similarly, the solid phase ratio at the boundary between the delta phase and the gamma phase is less than a predetermined critical value,
It is a method for refining crystal grains of a steel product, characterized by having a phase boundary melting step for melting a boundary between a delta phase and a gamma phase.

In the present technology , the boundary between the delta phase and the gamma phase is dissolved in the cooling process of the steel material, and crystals of the delta phase or the gamma phase are formed from the separated dendrites, so that the crystal grains can be reduced. For this reason, the crystal grains of the steel material can be further refined and equiaxed.

Unlike conventional methods such as electromagnetic fluid addition, this technology has few restrictions on application and can be widely applied. Therefore, it provides a wide range of steel materials with reduced segregation and improved mechanical properties and corrosion resistance. can do. Further, the present technology does not exclude implementation in combination with other conventional miniaturization methods, and may be combined with other methods.

In addition, the meaning of “etc.” in “control of cooling rate, stirring, blending of predetermined elements, etc.” is that “the transformation from delta phase to gamma phase is formed near the solidification interface, and the formed delta phase and gamma The temperature of the phase boundary is within a predetermined range from the peritectic temperature , and the solid phase ratio is less than a predetermined critical value at the boundary between the delta phase and the gamma phase. The purpose is to specifically illustrate the method of “dissolving the boundary between the gamma phases”. For example, other methods such as the addition of vibration are also included in the present technology if “dissolve the boundary between the delta phase and the gamma phase”. It is.

  The term “near the solidification interface” refers to the boundary surface between the liquid phase and the solid phase or the vicinity thereof. The liquid phase includes the case where the liquid phase is once solidified and then dissolved again. It is a delta phase or a gamma phase formed as a result.

  In addition, the “peritectic transformation” is not only the general peritectic transformation in which the gamma phase, which is the peritectic phase, surrounds the primary delta phase, but also the dendritic (dendritic) delta phase in the arm. Along with the phenomenon of transformation to gamma phase.

Further, “within a predetermined range from the peritectic temperature ” means a peritectic temperature of ± 15 ° C., preferably ± 10 ° C. However, since it has a fine structure including a liquid phase, a delta phase, and a gamma phase, it may be locally out of range.

  In the solidified shell and the steel material in the mold as a whole, in the region that is not subject to melting at the boundary between the delta phase and the gamma phase, for example, the temperature near the mold wall is lower than that, and the liquid phase part in the center is It may be a higher temperature.

  Further, “the solid phase ratio is equal to or less than a predetermined critical value” indicates that the solid phase ratio is about 40% or less, but may be slightly increased depending on the composition of the material.

The second technique related to the present invention is:
The phase boundary dissolution step includes
In the vicinity of the solidification interface, a delta phase dendrite growth step for growing a dendrite composed of a delta phase;
A gamma phase growth step in the dendrite that transforms the cooling side of the growing dendrite into a gamma phase and shortens the distance between the boundary between the delta phase and the gamma phase and the tip of the dendrite as the cooling time elapses;
When the distance from the tip of the dendrite becomes smaller than a predetermined distance as the gamma phase grows, the boundary between the delta phase and the gamma phase is dissolved, and the dendrite on the tip side made of the delta phase is divided to divide the inside of the liquid phase. The dendrite cutting step to be moved to
The method for refining crystal grains of a steel product according to the first technique , characterized by comprising:

By controlling the cooling rate in the cooling process of steel materials, delta phase dendrites are generated in the liquid phase near the peritectic temperature , and the cooling side is transformed to gamma phase by cooling. By shortening the distance between the boundary and the tip of the dendrite as the cooling time elapses and by dissolving the boundary between the delta phase and the gamma phase when the distance decreases, segregation is reduced more than the present, and mechanical properties and corrosion resistance are reduced. Can provide a wide range of steel materials.

The “predetermined distance” in which “the distance from the tip of the dendrite is a predetermined distance” generally refers to 5 mm or less. However, it is the distance obtained by dividing the temperature difference between the liquidus temperature of the delta phase and the peritectic temperature minus 15 ° C. by the temperature gradient at that position, and may be somewhat longer depending on the composition of the material, temperature distribution, and the like.

  Further, when “splitting the dendrites on the tip side made of the delta phase”, the solid phase ratio at the boundary position or in the vicinity thereof is preferably about 40% or less.

The third technique related to the present invention is:
The phase boundary dissolution step further includes
Based on the tip side dendrite consisting of the delta phase moved into the liquid phase, a split dendrite growth step for newly growing a dendrite consisting of the delta phase,
The method for refining crystal grains of a steel product according to the second technique , characterized by comprising

Based on the distal end side of the dendrite consisting delta phase which has moved to the liquid phase, by growing dendrite consisting newly delta phase, since the division and growth of new dendrites is repeated, the effect of the second technique It is further demonstrated.

  In addition, electromagnetic stirring etc. can be considered as a means to move the dendrite of the front end side actively into the liquid phase.

The fourth technique related to the present invention is:
The phase boundary dissolution step is performed by controlling the cooling rate,
The cooling rate in the gamma phase growth step in the dendrite and the dendrite fragmentation step is 70% or less of the cooling rate in the delta phase dendrite growth step, according to the second technique or the third technique , This is a method for refining crystal grains of steel products.

By setting the cooling rate in the dendrite gamma phase growth step and the dendrite fragmentation step to 70% or less of the cooling rate in the delta phase dendrite growth step, the boundary between the delta phase and the gamma phase of the dendrite is appropriately dissolved. Therefore, the effects of the second technique or the third technique are further exhibited.

  The cooling rate of “70% or less” described above is preferably 60% or more from the viewpoint of production efficiency. More preferably, it is 50% or less.

The fifth technique related to the present invention is:
The cooling method in the dendrite gamma phase growth step has a cooling rate of 150 ° C./s or less, which is the method for refining crystal grains of steel products according to the fourth technique .

In the present technology , since the cooling rate in the in-dendrite gamma phase growth step is 150 ° C./s or less, the effect of the second technology or the third technology is further exhibited.

  When the cooling rate exceeds 150 ° C./s, the boundary between the delta phase and the gamma phase of the dendrite is not properly dissolved. These problems do not occur and are preferably 150 ° C./s or less. More preferably, it is 1 ° C./s or less.

The sixth technique related to the present invention is:
The steel material is an ordinary steel, a first technique to a fifth grain refinement method for steel product of any crab described technique, characterized in that carbon is 0.4～0.9Mass% It is.

In ordinary steel, carbon in the case of using the steel material is 0.4～0.9Mass%, can be preferably to no first technique to satisfactorily exhibit the effect of the fifth technique.

The seventh technique related to the present invention is:
The phase boundary dissolution step includes
A part of the solidified shell transformed from the delta phase to the gamma phase is heated to a peritectic temperature or higher within a predetermined time after the transformation to the gamma phase to dissolve the part of the solidified shell and form a delta phase. A gamma phase heating step,
The method for refining crystal grains of a steel product according to the first technique , characterized by having a melting step for dissolving a boundary between the formed delta phase and gamma phase.

A part of the solidified shell once transformed from the delta phase to the gamma phase is heated to a peritectic temperature or higher within a predetermined time after the transformation to the gamma phase to dissolve the part of the solidified shell, By forming and dissolving the boundary between the delta phase and the gamma phase, the first technique can be applied to the solidified shell, and the application range is expanded.

  The “predetermined time” refers to a time during which microsegregation does not disappear, and specifically, it is preferably within 30 seconds, and more preferably as soon as possible. A few seconds is desirable.

  The term “solidified shell” refers to a region where dendrites and solid phases in which dendrites are coarsened are in contact with each other and mechanically behave like solid phases.

The eighth technique related to the present invention is:
The gamma phase heating step includes:
The heat of the liquid phase in the solidified shell is transferred from the delta phase to the gamma phase transformed from the delta phase by electromagnetic stirring, and the delta phase at that location is heated to a peritectic temperature or higher . It is the refinement | miniaturization method of the crystal grain of the steel product described in this technique .

In this technology , as a means of “heating above the peritectic temperature ”, electromagnetic stirring, which is a simple means, is given to the liquid phase to transfer heat from a liquid phase having a high temperature to a solid phase having a low temperature. Since the solid phase is heated, the effect of the seventh technique can be easily exhibited.

The ninth technique related to the present invention is:
The method for refining crystal grains of steel products according to the seventh or eighth technique , wherein the steel material is plain steel and carbon is 0.1 to 0.5 mass%.

The use of a steel material of ordinary steel and carbon of 0.1 to 0.5 mass% is preferable because the effects of the seventh technique or the eighth technique can be satisfactorily exhibited.

The tenth technique related to the present invention is:
In the cooling process of steel materials, the transformation from the delta phase to the gamma phase is formed near the solidification interface by controlling the cooling rate, stirring, mixing of certain elements, etc., and the boundary between the formed delta phase and gamma phase The temperature is set within a predetermined range from the peritectic temperature , and the solid phase ratio is also below a predetermined critical value at the boundary between the delta phase and the gamma phase, and as a result, the boundary between the delta phase and the gamma phase is dissolved. Observe in real time the 18 to 30 keV monochromatic X-ray or white X-ray including its energy region, and the temperature and composition conditions where the boundary between the delta phase and the gamma phase dissolves near the peritectic temperature A data acquisition process for measuring and acquiring conditions such as the critical solid phase ratio,
Based on the temperature conditions obtained in the data acquisition step, the conditions of the composition, the conditions such as the critical solid fraction, the temperature conditions in the production of the steel material, the conditions of the composition, the conditions such as the critical solid fraction are controlled, A phase boundary dissolution process in which the boundary between the delta phase and the gamma phase is dissolved near the peritectic temperature and divided.
It is the refinement | miniaturization method of the crystal grain of the steel products characterized by having.

In this technique, for setting various conditions in the phase boundary dissolution process based on the data obtained by the data acquisition process, a large, the boundaries of reliably delta phase and the gamma phase to a variety of steel materials peritectic It becomes possible to dissolve and divide in the vicinity of the temperature . For this reason, since the crystal grains formed when the gamma phase is formed in the delta phase and further around or part of the circumference with cooling can be reliably and efficiently reduced, the crystal grains of the steel material can be reduced. It becomes possible to further refine and equiax the crystal.

Note that the “data acquisition process” and “phase boundary melting process” do not have to be the same steel mill or steel company, and for this reason, phase boundary melting based on already obtained data and published data. It is included in this technique if it has a process or a phase boundary dissolution process based on numerical values estimated from those data.

The eleventh technology related to the present invention is:
The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.4 to 0.9 mass%.
The growth of the gamma phase in the dendrite is performed under a cooling rate of 150 ° C./s or less,
The cooling rate when the delta phase is grown and then transformed into the gamma phase, and the boundary between the formed delta phase and gamma phase is dissolved is 70% or less of the cooling rate when the delta phase dendrite grows. The method for refining crystal grains of a steel product according to the tenth technique , characterized in that:

The growth rate of the gamma phase in the dendrite has a cooling rate of 150 ° C./s or less, and the cooling rate when the boundary between the formed delta phase and the gamma phase is dissolved is the cooling rate when the dendrites of the delta phase grow. By controlling to 70% or less, the effect of the tenth technique is further exhibited when producing a steel material of 0.4 to 0.9 mass% of ordinary steel and carbon.

The twelfth technique related to the present invention is:
The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.1 to 0.5 mass%.
After the transformation from the delta phase to the gamma phase in the solidified shell, a part of the gamma phase is transformed to the gamma phase and heated to a temperature higher than the peritectic temperature within a predetermined time and dissolved, and then the delta phase is formed. The method for refining crystal grains of a steel product according to the tenth technique , wherein the boundary between the delta phase and the gamma phase is dissolved.

The portion transformed from the delta phase to the gamma phase in the solidified shell is then heated to a peritectic temperature or higher within a predetermined time to dissolve a part of the gamma phase immediately after the transformation and to form a delta phase, and then to form The effect of the tenth technique is further exhibited in producing a steel material of 0.1 to 0.5 mass% of ordinary steel and carbon by dissolving the boundary between the produced delta phase and gamma phase.

The thirteenth technique related to the present invention is:
The phase boundary dissolution step includes
Because the boundary between the delta and gamma phases dissolves and the delta phase is further separated,
The boundary temperature between the delta phase and the gamma phase is near the peritectic temperature ,
The refinement of crystal grains of the steel product according to any one of the tenth to twelfth techniques , wherein the solid phase ratio at the boundary between the delta phase and the gamma phase is set to a predetermined value or less. Is the method.

By appropriately controlling the temperature and the solid phase ratio to separate the delta phase, the crystal particles can be further refined, and the effect of the technique described in any one of the tenth to twelfth techniques can be achieved. It is further demonstrated.

The fourteenth technology related to the present invention is:
The phase boundary dissolution step includes
The method has a divided dendrite growth step in which the temperature of the location where the separated dendrite is located is adjusted to be below the liquidus, and a new crystal is grown using the separated delta phase dendrite as a seed . A method for refining crystal grains of a steel product according to any one of the tenth to thirteenth techniques .

By newly growing crystals using the separated delta phase dendrites as seeds, crystal grains can be further refined, and the effects described in any one of the tenth to thirteenth techniques can be further exhibited. The

The fifteenth technology related to the present invention is:
The phase boundary dissolution step includes
Any one of the first to fourteenth techniques characterized by having an element blending temperature adjusting step for blending a predetermined element and adjusting at least one of the liquidus temperature and the peritectic temperature . The method for refining crystal grains of steel products as described in 1. above.

Since at least one of the liquidus temperature and the peritectic temperature (particularly the peritectic temperature ) is adjusted by blending a predetermined element, the applicable range is expanded.

In general, examples of the element that increases the peritectic temperature (equilibrium temperature) include Mn, Co, Cu, Nb, Ni, and the like, and examples of the elements that decrease include Si, other Al, and Cr. , Mo, P, Sn, Ti, V, W and the like. Note that the liquidus temperature generally decreases as the formulation increases.

The present invention is based on the above knowledge, and the invention according to claim 1
In the cooling process of steel materials,
A transformation from delta phase to gamma phase is formed near the solidification interface,
Furthermore, the temperature at the boundary between the formed delta phase and the gamma phase is set to a peritectic temperature ± 15 ° C.,
Similarly, the solid phase ratio should be 40% or less at the boundary between the delta phase and the gamma phase.
It is a method for refining crystal grains of a steel product, characterized by having a phase boundary melting step for melting a boundary between a delta phase and a gamma phase.

The invention described in claim 2
The phase boundary dissolution step includes
In the vicinity of the solidification interface, a delta phase dendrite growth step for growing a dendrite composed of a delta phase;
A gamma phase growth step in the dendrite that transforms the cooling side of the growing dendrite into a gamma phase and shortens the distance between the boundary between the delta phase and the gamma phase and the tip of the dendrite as the cooling time elapses;
As the gamma phase grows, the distance from the tip of the dendrite becomes smaller than the distance obtained by dividing the temperature difference between the liquidus temperature of the delta phase and the peritectic temperature minus 15 ° C. by the temperature gradient at that position. A dendrite cutting step in which the boundary between the phase and the gamma phase is dissolved, and the dendrites on the tip side consisting of the delta phase are cut and moved into the liquid phase.
The method for refining crystal grains of a steel product according to claim 1, characterized by comprising:

The invention according to claim 3
The phase boundary dissolution step further includes
Based on the tip side dendrite consisting of the delta phase moved into the liquid phase, a split dendrite growth step for newly growing a dendrite consisting of the delta phase,
The method for refining crystal grains of a steel product according to claim 2, characterized by comprising:

The invention according to claim 4
The phase boundary dissolution step is performed by controlling the cooling rate,
The steel product according to claim 2 or 3, wherein a cooling rate in the gamma phase growth step in the dendrite and the cooling rate in the dendrite fragmentation step is 70% or less of the cooling rate in the delta phase dendrite growth step. This is a crystal grain refinement method.

The invention described in claim 5
The method for refining crystal grains of a steel product according to claim 4, wherein a cooling rate in the in-dendritic gamma phase growth step is 150 ° C / s or less.

The invention described in claim 6
The method for refining crystal grains of a steel product according to any one of claims 1 to 5, wherein the steel material is plain steel and carbon is 0.4 to 0.9 mass%. It is.

The invention described in claim 7
The phase boundary dissolution step includes
A part of the solidified shell transformed from the delta phase to the gamma phase is heated to a peritectic temperature or higher within a predetermined time after the transformation to the gamma phase to dissolve the part of the solidified shell and form a delta phase. A gamma phase heating step,
A dissolution step for dissolving the formed boundary between the delta phase and the gamma phase.
The method for refining crystal grains of a steel product according to claim 1, characterized by comprising:

The invention according to claim 8 provides:
The gamma phase heating step includes:
The heat of the liquid phase in the solidified shell is transferred from the delta phase to the gamma phase transformed from the delta phase by electromagnetic stirring, and the delta phase at the location is heated to the peritectic temperature or higher. 7 is a method for refining crystal grains of a steel product according to item 7.

The invention according to claim 9 is:
The method for refining crystal grains of a steel product according to claim 7 or 8, wherein the steel material is plain steel and carbon is 0.1 to 0.5 mass%.

The invention according to claim 10 is:
In the cooling process of the steel material, a transformation from the delta phase to the gamma phase is formed near the solidification interface, and the temperature at the boundary between the formed delta phase and the gamma phase is set to a peritectic temperature ± 15 ° C. The solid phase ratio is set to 40% or less at the boundary between the phase and the gamma phase, and as a result, the state where the boundary between the delta phase and the gamma phase is dissolved is expressed by white color including 18 to 30 keV monochromatic X-ray or its energy region. A data acquisition step of observing in real time with X-rays, measuring and acquiring conditions under which the boundary between the delta phase and the gamma phase dissolves near the peritectic temperature;
Based on the conditions obtained in the data acquisition step, the phase boundary dissolution step of controlling the conditions in the production of the steel material and dissolving and dividing the boundary between the delta phase and the gamma phase in the vicinity of the peritectic temperature,
It is the refinement | miniaturization method of the crystal grain of the steel products characterized by having.

The invention according to claim 11
The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.4 to 0.9 mass%.
The growth of the gamma phase in the dendrite is performed under a cooling rate of 150 ° C./s or less,
The cooling rate when the delta phase is grown and then transformed into the gamma phase, and the boundary between the formed delta phase and gamma phase is dissolved is 70% or less of the cooling rate when the delta phase dendrite grows. The method for refining crystal grains of a steel product according to claim 10.

The invention according to claim 12
The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.1 to 0.5 mass%.
After the transformation from the delta phase to the gamma phase in the solidified shell, a part of the gamma phase is transformed to the gamma phase and heated to a temperature higher than the peritectic temperature within a predetermined time and dissolved, and then the delta phase is formed. The method for refining crystal grains of a steel product according to claim 10, wherein the boundary between the delta phase and the gamma phase is dissolved.

The invention according to claim 13
The phase boundary dissolution step includes
Because the boundary between the delta and gamma phases dissolves and the delta phase is further separated,
The boundary temperature between the delta phase and the gamma phase is near the peritectic temperature,
The method for refining crystal grains of a steel product according to any one of claims 10 to 12, wherein the solid phase ratio at the boundary between the delta phase and the gamma phase is 40% or less. It is.

The invention according to claim 14
The phase boundary dissolution step includes
It has a divided dendrite growth step in which the temperature of the location where the separated dendrite is located is adjusted to below the liquidus, and a new crystal is grown using the separated delta phase dendrite as a seed. Item 14. The method for refining crystal grains of steel products according to any one of Items 10 to 13.

The invention according to claim 15 is:
The phase boundary dissolution step includes
By blending an element selected from Mn, Co, Cu, Nb, Ni, Si, Al, Cr, Mo, P, Sn, Ti, V, and W, at least one of the liquidus temperature and the peritectic temperature is obtained. 15. The method for refining crystal grains of a steel product according to any one of claims 1 to 14, further comprising an element blending temperature adjusting step for adjusting.

  According to the present invention, the steel material is further improved in mechanical properties and corrosion resistance by reducing the segregation by making the crystal grains of the steel material finer and equiaxed without any restrictions on application. Can be provided.

It is a figure which shows notionally the structure of the observation apparatus which observes the solidification process of steel materials in real time. It is an X-ray transmission image which shows a mode that the dendrite melt | fused in the phase boundary of (delta) phase and (gamma) phase in the solidification process of the carbon steel of 0.58 mass% C, 0.6 mass% Mn, and 0.3 mass% Si. 3 is an X-ray transmission image showing a state 3 seconds after the δ phase and δ phase generated in the solidification process of 0.44 mass% C carbon steel are transformed into a γ phase. FIG. 3 conceptually shows the cooling rate in the temperature range near the peritectic transformation point from the liquidus temperature (MP) at the position where dendrite fusing occurs in the solidification process of the steel material in the first embodiment of the present invention. It is a graph.

  Hereinafter, the present invention will be described based on embodiments. Note that the present invention is not limited to the following embodiments. Various modifications can be made to the following embodiments within the same and equivalent scope as the present invention.

1. Acquiring data on the solidification process of steel materials The composition of steel materials varies widely, and the melting point and transformation points vary depending on the composition. Therefore, in order to appropriately carry out the present invention, a solid crystal or phase transformation in the vicinity of the temperature at which peritectic transformation is performed in advance on a sample having the same composition as the material to be implemented is observed, and columnar crystals are observed. It is preferable to acquire or confirm necessary data on the temperature, cooling rate, temperature gradient, etc. from the transition to the equiaxed crystal. In addition, it is preferable to obtain necessary data regarding the influence on the equilibrium temperature of the δ phase and the γ phase when various elements are mixed in the steel material. Hereinafter, a means for acquiring data will be described.

(1) Observation device for observation of coagulation process and sample for observation a. Observation Device FIG. 1 is a diagram conceptually showing the configuration of an observation device that observes the solidification process of a steel material in real time. In FIG. 1, 19 is a sample, 51 is an X-ray slit, 52 is a chamber, 53 is a graphite heater, 54 is an X-ray detector (two-dimensional), and 55 is an X-ray (not shown). This is an X-ray beam emitted from a generation source.

  This observation apparatus has a function of cooling by controlling the cooling rate when solidifying the molten sample 19 within a range of 0.1 to 2 K / s. The X-ray beam to be used is a monochromatic X-ray of 18 to 30 keV or white X-ray including its energy region. In addition to the above, it is equipped with a sample holder, a temperature sensor, a graphite heater power supply, a vacuum pump for the chamber, a device for controlling the temperature and cooling rate of the sample, etc., but these are directly in the spirit of the present invention. This is not shown because it is self-evident and complicated. This also applies to other drawings.

B. Sample for Observation The thickness of the sample is appropriately set according to the magnitude of the energy of the X-ray beam to be used. For example, when a 20 keV X-ray beam is used, it is about 50 to 200 μm.

(2) Control of sample melting method and solidification process a. Melting method First, the sample 19 was heated by a graphite heater so that the temperature was raised from 0.1 ° C./s (seconds) to 1 ° C./s, and once the temperature was raised to a temperature slightly higher than the melting point, the sample was melted and melted. Hold the state for 1 minute or more.

B. Control of solidification process Next, cooling is performed at a speed in the range of 0.1 to 2 K / s to form δ-phase dendrites. Next, a further decrease in temperature forms a γ phase on the cooling side of the dendrite, but at this point, the cooling rate is reduced by 30% or more, or is maintained at a constant temperature, whereby the δ phase and the γ phase in the dendrite are reduced. Make the phase boundary approach the tip of the dendrite. It can be confirmed that the phase boundary approaches within a certain distance from the tip of the dendrite, and that the temperature of the phase boundary rises in the vicinity of the peritectic temperature, and the dissolution of the phase boundary. The temperature gradient in the sample at this time was 1 to 3 K / mm, and the growth rate of the δ-phase dendrite was in the range of 5 to 200 μm / s.

  The sample in the coagulation process controlled as described above is observed using the observation device, and data such as temperature necessary for implementation is also acquired. In addition, regarding the influence of various elements, necessary data is obtained by performing similar experiments on a plurality of samples containing these elements at a predetermined concentration.

(3) Data Acquisition Example Hereinafter, an example of data acquisition for two types of steel materials will be described.

A. First Example This example is an example of acquiring data in the solidification process of carbon steel of 0.58 mass% C, 0.6 mass% Mn, and 0.3 mass% Si. FIG. 2 is an X-ray transmission image showing a state in which the dendrite is melted at the phase boundary between the δ phase and the γ phase in the solidification process of the carbon steel of this example. In the case of this example, by cooling at a rate of 1.7 K / s, a δ phase dendrite grows and a γ phase is generated on the low temperature side, and the cooling is stopped when this phenomenon occurs. Shows that the dendrite is melted (divided) at the phase boundary between the δ phase and the γ phase, and as a result, the dendrite of the δ phase is separated (bent). Note that a branched columnar crystal-like structure is formed when the fragmented dendrite stays in place or when the solid phase ratio at the fragmentation position of the dendrite cannot move. For example, when dendrites that are melted and bent in FIG. 2 do not move to the liquid phase, branched columnar crystals are formed.

  Further, at a cooling rate in the range of 0.1 to 2 K / s possible with the apparatus shown in FIG. 1, the dendrite dimensions (average arm diameter) vary from 50 to 10 μm, but the growth forms are the same and similar. Generation of peritectic transformation and dendrite fusing are expected.

  Even when the cooling is not stopped and the speed is reduced by 30% or more, the growth rate of the dendrite is lowered and the distance (length) of the phase boundary between the tip and the δ phase and the γ phase is shortened. It was also confirmed that fusing occurred at the phase boundary.

  In addition, according to the knowledge based on the conventional equilibrium diagram, the composition range in which the C content is 0.5 mass% C or more is a composition range in which δ-phase dendrite is not formed, but from 0.4 to 0.9 mass% C. In the composition range, δ-phase dendrite growth and a phenomenon that the phase boundary between the δ-phase and the γ-phase moved to the vicinity of the tip of the dendrite (within about 5 mm from the tip of the dendrite) were observed. In this way, in this example, the dendrite melts and separates regardless of the conditions under which the dendrite can be continuously grown in one direction, that is, the conditions under which columnar crystals can be formed. As a result, there is an effect of suppressing generation of defects such as segregation.

B. Second Example In a carbon steel having a C content of 0.45 mass% C or less, transformation from the δ phase to the γ phase may occur in a non-equilibrium manner. In the state where the δ phase is lower than the peritectic temperature and the solid phase ratio is high, it transforms to the γ phase within a few seconds, but in this state, no fusing occurs at the phase boundary between the δ phase and the γ phase. That is, FIG. 3 is an X-ray transmission image showing a state 3 seconds after the δ phase and the δ phase generated in the solidification process of 0.44 mass% C carbon steel (S45C) are transformed into the γ phase. In the case of the present carbon steel, the δ phase is supercooled to a temperature below the peritectic temperature , and is transformed into the γ phase in a supercooled state and a high solid phase ratio. In such a transformation mode, dissolution does not occur at the phase boundary between the δ phase and the γ phase.

However, when the γ phase generated by transformation is heated to the vicinity of the peritectic temperature immediately after transformation, a liquid phase and a δ phase are formed, and the solid phase ratio is 40% or more. A boundary between the δ phase and the γ phase is formed in the vicinity of the tip (within 5 mm), and this phase boundary dissolves in the same way as the cooling process in the first example, so that a dendrite-like δ phase can be separated. It becomes.

Thus, in the carbon steel of 0.4 mass% C or less in which the difference between the liquidus temperature and the peritectic temperature is 20 K or more, that is, the phase boundary between the δ phase and the γ phase is not formed near the tip of the δ phase dendrite. Can dissolve the phase boundary between the δ phase and the γ phase by means of heating the γ phase to near the peritectic temperature and separate the dendrite-like δ phase.

2. Explanation of Fusing Mechanism and Fusing Conditions Fusing of the phase boundary between the δ phase and the γ phase is a phenomenon that could not be predicted by conventional knowledge, and is related to the principle and points of the present invention. The fusing mechanism and the conditions under which fusing occurs will be described below based on the observation results such as the data acquisition example.

(1) When the δ phase is transformed into a γ phase and a phase boundary is formed, excessive energy (grain boundary energy between the δ phase and the γ phase) is generated there.
(2) Under the condition that the phase boundary between the δ phase and the γ phase is maintained at the peritectic temperature , the bulk free energy does not increase even if a small amount of liquid phase is formed.

(3) the sum of the interfacial energy between the surface energy and γ-phase and a liquid phase of the δ phase and the liquid phase is lower than the surface energy of the δ-phase and γ-phase, at the peritectic temperature or temperature in the vicinity thereof and δ phase When the liquid phase is present at the interface with the γ phase, the free energy becomes lower and the phase boundary dissolves.

(4) Since the liquid phase can exist in an equilibrium state at a temperature higher than the peritectic temperature , dissolution of the phase boundary between the δ phase and the γ phase becomes easier.

(5) When the temperature is lower than the peritectic temperature , dissolution becomes difficult because the bulk free energy increases due to the formation of a liquid phase, but dissolution occurs in a certain temperature range. However, the extent to which dissolution occurs depends on the interfacial energy between the δ phase and the liquid phase, the interfacial energy between the γ phase and the liquid phase, the interfacial energy between the δ phase and the γ phase, as well as the δ phase, the γ phase, Depends on the relative relationship of the free energy of the liquid phase. For this reason, in implementation, it is necessary to acquire the data of the steel material which is the object of implementation with the apparatus shown in FIG.

(6) The range in which the phase boundary between the δ phase and the γ phase is dissolved is about the peritectic temperature ± 10K. In the present invention, this temperature range is close to the peritectic temperature .

3. Use of acquired data For steel materials of each composition, data such as appropriate cooling conditions and peritectic temperature acquired by the above method will be used for actual continuous casting and casting.

  In general, in continuous casting of steel materials, the range of cooling conditions is wider than in the case of the above-described sample. That is, the cooling rate during solidification from the surface of the slab to the inside changes from 200 K / s to several K / s, and the temperature gradient also changes from several tens of K / mm to almost 0 K / mm. However, the growth rate, which is a main factor for determining the shape and properties of the dendrite, is in the range of 10 to 200 μm / s for both the sample of the apparatus shown in FIG. 1 and continuous casting, which is the same as the growth rate in continuous casting. In addition, the solid fraction distribution can be easily obtained from the temperature gradient. For this reason, it is possible to evaluate the growth of the δ phase dendrite during continuous casting and the peritectic transformation from the δ phase to the γ phase without losing scientific rationality based on the data obtained from the sample. . In particular, the observation of the solidification process using the observation apparatus shown in FIG. 1 is a technique that can empirically obtain information on the actual solidification process than any conventional technique, and during continuous casting based on the observation. The growth of δ phase dendrite and the evaluation of peritectic transformation are highly reliable.

In general, the dendrite growth rate V, the temperature gradient G at the tip of the dendrite, and the cooling rate R are:
R = G x V
The relationship holds. The conditions for fusing dendrites obtained using the observation apparatus shown in FIG. 1 can be used for evaluating the conditions in actual continuous casting using the above equation.

In the case of continuous casting, a position where a transition from a columnar crystal to an equiaxed crystal is desired is determined, and the cooling rate and temperature gradient at the solidification start temperature and the growth rate of the δ phase dendrite at that position are estimated. Then, (1) heat transfer between the mold flux and the mold is adjusted so that the cooling rate when the position reaches the peritectic temperature is 30% or more lower than the cooling rate at the start of solidification. (2) Adjust the cooling water amount of the mold. (3) Adjust the spray amount of cooling water outside the mold. (4) An operation such as increasing the flow rate of the liquid phase by an electromagnetic stirring device or the like to promote heat transport from the liquid phase to the solidified shell is performed.

  If necessary, an element that increases or decreases the equilibrium temperature of the δ phase and the γ phase may be added. In general, Mn, Co, Cu, Nb, Ni, etc. are used as the elements to be raised, and especially Si, Al, Cr, Mo, P, Sn, Ti, V, W are used as the elements to be lowered. Etc. are used. At the time of addition, an element to be added is appropriately selected in consideration of the interaction with other elements present in or mixed with the steel material.

4). Embodiment (1) First Embodiment In this embodiment, in the process where dendrite is generated and grows, when the temperature reaches a temperature near the peritectic transformation point as compared to the liquidus temperature. This is related to slowing the cooling at a low temperature and controlling the peritectic transformation to melt the dendrite.

  4 is a graph conceptually showing the specific contents of the data, that is, the cooling rate of the steel material in the vicinity of the peritectic transformation point (peritectic temperature) from the liquidus temperature (MP). In this graph, the vertical axis is the temperature, and the horizontal axis is the elapsed cooling time. In FIG. 4, the broken line is the conventional cooling rate, and the solid line is the cooling rate of the present embodiment.

In the present embodiment, in carbon steel having a C content in the range of 0.4 to 0.9 mass% C, δ phase dendrite is grown by cooling at a rate of 150 ° C./s or less, and then cooled. By reducing the speed by 30% or more, the phase boundary between the δ phase and the γ phase is brought to the vicinity of the peritectic temperature , the γ phase is generated in the dendrite, and the dendrite is melted. The δ phase newly grows from the tip of the divided dendrites, and the transformation from the divided portion, that is, from the cooling side to the γ phase occurs, so that the phase boundary is blown again.

  Through the above cooling process, not only equiaxed crystals but also branched columnar crystals are formed. For these reasons, columnar crystals from the mold side toward the inside of the slab, and at the time of casting, branch crystals are formed from the position where the phase boundary between the δ phase and the γ phase is blown to the position of the dendrite tip, and the tip of the separated dendrite is inside. The seeded equiaxed crystal has a distributed structure. In this state, rolling and further heat treatment are performed.

(2) Second Embodiment In this embodiment, the solidified shell transformed from the δ phase to the γ phase is heated to form a δ phase near the solidification tip, and the phase boundary between the δ phase and the γ phase. Is dissolved to melt the solid phase.

  Under the above conditions, in a steel material having a C content of 0.1 to 0.5 mass% C, the solidified shell transformed without melting from the δ phase to the γ phase is subjected to microsegregation after the γ phase is transformed. Stir electromagnetically within 30 seconds, which does not occur, thereby transferring heat from the liquid phase and heating, re-dissolving part of the solidified shell or part of the γ phase, and cooling the δ phase. The solidified shell is melted by melting the phase boundary between the δ phase and the γ phase.

  Also in this embodiment, not only equiaxed crystals but also branched crystals are formed. For these reasons, columnar crystals from the mold side toward the inside of the slab, and at the time of casting, branch crystals are formed from the position where the phase boundary between the δ phase and the γ phase is melted to the position of the dendrite tip, and the tip of the separated dendrite inside. Becomes a structure in which equiaxed crystals with seeds are distributed. In this state, processing such as rolling and further cooling is performed.

19 Sample 51 X-ray slit 52 Chamber 53 Graphite heater 54 X-ray detector 55 X-ray beam

Claims (15)

In the cooling process of steel materials ,
A transformation from delta phase to gamma phase is formed near the solidification interface,
Furthermore, the temperature at the boundary between the formed delta phase and the gamma phase is set to a peritectic temperature ± 15 ° C. ,
Similarly, the solid phase ratio should be 40% or less at the boundary between the delta phase and the gamma phase.
A method for refining crystal grains of a steel product, comprising a phase boundary melting step for melting a boundary between a delta phase and a gamma phase. The phase boundary dissolution step includes
In the vicinity of the solidification interface, a delta phase dendrite growth step for growing a dendrite composed of a delta phase;
A gamma phase growth step in the dendrite that transforms the cooling side of the growing dendrite into a gamma phase and shortens the distance between the boundary between the delta phase and the gamma phase and the tip of the dendrite as the cooling time elapses;
As the gamma phase grows, the distance from the tip of the dendrite becomes smaller than the distance obtained by dividing the temperature difference between the liquidus temperature of the delta phase and the peritectic temperature minus 15 ° C. by the temperature gradient at that position. dissolved phase boundary and gamma phase, dendrite cutting steps to be divided the tip side of the dendrite consisting delta phase is moved to the liquid phase
The method for refining crystal grains of a steel product according to claim 1, wherein: The phase boundary dissolution step further includes
Based on the tip side dendrite consisting of the delta phase moved into the liquid phase, a split dendrite growth step for newly growing a dendrite consisting of the delta phase,
The method for refining crystal grains of a steel product according to claim 2, comprising: The phase boundary dissolution step is performed by controlling the cooling rate,
The steel product according to claim 2 or 3, wherein a cooling rate in the gamma phase growth step in the dendrite and the cooling rate in the dendrite fragmentation step is 70% or less of the cooling rate in the delta phase dendrite growth step. Crystal grain refinement method.   The method for refining crystal grains of a steel product according to claim 4, wherein a cooling rate in the step of gamma phase growth in the dendrite is 150 ° C / s or less.   The method for refining crystal grains of a steel product according to any one of claims 1 to 5, wherein the steel material is plain steel and carbon is 0.4 to 0.9 mass%. . The phase boundary dissolution step includes
A part of the solidified shell transformed from the delta phase to the gamma phase is heated to a peritectic temperature or higher within a predetermined time after the transformation to the gamma phase to dissolve the part of the solidified shell and form a delta phase. A gamma phase heating step,
2. The method for refining crystal grains of a steel product according to claim 1, further comprising a melting step for dissolving the formed boundary between the delta phase and the gamma phase. The gamma phase heating step includes:
The heat of the liquid phase in the solidified shell is transferred from the delta phase to the gamma phase transformed from the delta phase by electromagnetic stirring, and the delta phase at the location is heated to the peritectic temperature or higher. 8. A method for refining crystal grains of a steel product according to 7.   The method for refining crystal grains of a steel product according to claim 7 or 8, wherein the steel material is plain steel and carbon is 0.1 to 0.5 mass%. In the course of cooling the steel material, the transformation from the delta phase into gamma phase is formed on the solidified near the interface, yet the temperature of the boundaries of the formed delta phase and the gamma phase is the manner is peritectic temperature ± 15 ° C., again The solid phase ratio is set to 40% or less at the boundary between the delta phase and the gamma phase, and as a result, the state where the boundary between the delta phase and the gamma phase is dissolved includes a monochromatic X-ray of 18 to 30 keV or its energy region. was observed in real time a white X-ray, and a data acquisition step of the boundary of the delta phase and gamma phase measures the condition you dissolve in the vicinity of the peritectic temperature, to obtain,
Wherein based on conditions obtained in the data obtaining step, by controlling the conditions that put in the manufacture of iron and steel materials, phase boundary dissolution of dividing by dissolving the boundaries of the delta phase and gamma phase in the vicinity of the peritectic temperature Process
A method for refining crystal grains of steel products, comprising: The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.4 to 0.9 mass%.
The growth of the gamma phase in the dendrite is performed under a cooling rate of 150 ° C./s or less,
The cooling rate when the delta phase is grown and then transformed into the gamma phase, and the boundary between the formed delta phase and gamma phase is dissolved is 70% or less of the cooling rate when the delta phase dendrite grows. The method for refining crystal grains of a steel product according to claim 10. The phase boundary dissolution step includes
Normal steel and carbon are used to produce a steel material of 0.1 to 0.5 mass%.
After the transformation from the delta phase to the gamma phase in the solidified shell, a part of the gamma phase is transformed to the gamma phase and heated to a temperature higher than the peritectic temperature within a predetermined time and dissolved, and then the delta phase is formed. The method for refining crystal grains of a steel product according to claim 10, wherein the boundary between the delta phase and the gamma phase is dissolved. The phase boundary dissolution step includes
Because the boundary between the delta and gamma phases dissolves and the delta phase is further separated,
The boundary temperature between the delta phase and the gamma phase is near the peritectic temperature ,
The method for refining crystal grains of a steel product according to any one of claims 10 to 12, wherein the solid phase ratio at the boundary between the delta phase and the gamma phase is 40% or less. . The phase boundary dissolution step includes
It has a divided dendrite growth step in which the temperature of the location where the separated dendrite is located is adjusted to below the liquidus, and a new crystal is grown using the separated delta phase dendrite as a seed. The method for refining crystal grains of a steel product according to any one of claims 10 to 13. The phase boundary dissolution step includes
By blending an element selected from Mn, Co, Cu, Nb, Ni, Si, Al, Cr, Mo, P, Sn, Ti, V, and W , at least one of the liquidus temperature and the peritectic temperature is obtained. The method for refining crystal grains of a steel product according to any one of claims 1 to 14, further comprising an element mixing temperature adjusting step for adjusting.