JPH0143816B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a new method for producing heat-treated aluminum killed steel used in a low temperature range. In order to improve the low-temperature toughness of various low-temperature steels, ferrite crystal grains (in the case of martensitic steel,
It is well known that refinement of prior austenite crystal grains is an essential condition. Methods conventionally employed for this grain refinement are broadly classified into "controlled rolling method" and "direct quenching method." The controlled rolling method targets steel that contains grain refining elements such as Nb, V, and Zr, such as high-strength steel for line pipes, and heats the steel at a sufficiently high pre-rolling temperature to remove these elements. After being completely dissolved in the base material, recrystallized austenite grains are made as fine as possible by rolling in a high-temperature austenite recrystallization region, and then rolling is added in a low-temperature austenite non-recrystallization region to introduce a deformed structure into the austenite structure. In particular, it refines the ferrite grains produced during the subsequent cooling, and this refinement of the ferrite grains has the advantage of simultaneously achieving higher strength and lowering the Charpy fracture transition temperature. . however,
On the other hand, in order to make full use of the effects of the additive elements, it is necessary to heat them at high temperatures before rolling to completely dissolve the carbides of the additive elements, or to maintain a constant rolling reduction rate in the low-temperature austenite non-recrystallized region. Since it is necessary to apply a rolling reduction that exceeds 10,000 yen, a considerable waiting time must be provided between high-temperature area rolling and low-temperature area rolling, which results in manufacturing constraints such as a significant loss of productivity on the manufacturing line. In addition, because the refinement of the crystal grains overlaps with the increase in strength due to the added elemental carbide, there is a limit in terms of quality in that the shear pie absorbed energy (maximum absorbed energy) in the ductile fracture region is significantly impaired. . On the other hand, the direct quenching method omits heat treatment by applying water cooling and tempering treatment immediately after hot working, and has already been put into practical use under the name quenching for machine structural steel. However, since the austenite grains obtained by hot working are coarse unless special grain refining elements are added, the mechanical properties at room temperature of steel produced by this method are The quality is usually lower than that of other materials (which are then re-austenitized to adjust the grain size, etc.). The present invention was made to overcome the above-mentioned problems regarding low-temperature steel, and is made of low-carbon aluminized steel that does not contain any special elements, and has a slightly coarse, heterogeneous austenite structure obtained by hot working. By performing processing with a constant area reduction rate in the low temperature range, that is, the temperature range where the processed austenite does not transform into ferrite or recrystallize, the transformation phase that occurs during subsequent water cooling or cooling is reduced. This was completed based on the new knowledge that the crystal grains of (ferrite-bainite mixed structure or ferrite-pearlite structure) can be made substantially finer. That is, the present invention hot-works low-carbon aluminium-killed steel, including a processing step with an area reduction of 30% or more and 90% or less in a temperature range that is above the Ar ₃ transformation point and does not cause recrystallization of deformed austenite. Attached,
The present invention provides a method for producing low-temperature steel that is immediately water-cooled to room temperature after hot rolling and then tempered at a temperature below the Ac ₁ transformation point. , normalizing and tempering,
Alternatively, the re-austenitization process (quenching, normalizing) after rolling can be omitted from the existing manufacturing process of heat-treated low-temperature steel, which has been subjected to heat treatments such as quenching and tempering before use.
Without taking any special measures such as adding new special elements or changing the steel composition system, the existing composition system can be used to achieve the characteristics required for low-temperature steel, including the maximum absorption energy of Charpy and low-temperature toughness such as transition temperature. This has succeeded in greatly improving various properties. The method of the present invention includes C0.01-0.1%, Si0.1-0.9%,
Good results can be obtained when applied to so-called "heat-treated" low-temperature steels containing 0.5-2.0% Mn and 0.01-0.3% Al as essential components, with the balance being iron and unavoidable impurities. The steel subject to the present invention is different from conventional steels of this type in that it does not require the presence of special elements, but rather eliminates the addition of such elements. If C is less than 0.01%, it is difficult to obtain the high strength required for practical use, while if it exceeds 0.1%, it causes deterioration of toughness and weldability, so it is added in the range of 0.01 to 0.1%. Si is added as a deoxidizing element, but if it exceeds 0.9%, toughness deteriorates, so it is added in an amount of 0.1 to 0.9%. Mn is added in an amount of 0.5% or more in order to impart the required strength, but if added in a large amount, toughness will be impaired, so the upper limit is set at 2.0%. Al is added in a range of 0.01 to 0.3% for deoxidizing molten steel and fixing solid solution nitrogen. Grain refinement also plays an important role in improving the low-temperature toughness of tempered bainite or ferrite/pearlite structures, but grain refinement of Nb, etc., as in non-thermal high tensile strength steel for line pipes, is important. It is generally difficult to make the austenite crystal structure fine and uniform after rolling and before cooling unless special elements are added. Therefore, conventionally, as a means for uniformly refining the austenitic structure when stable low-temperature toughness is required, it has been usual to carry out re-austenitizing treatment after rolling. In contrast, in the present invention, the relatively coarse and non-uniform austenite structure obtained by hot working is processed in a temperature range in which neither transformation nor recrystallization occurs in the austenite that has undergone deformation due to working (hereinafter referred to as (referred to as "low-temperature processing"), and then subjected to rapid cooling such as water cooling. FIG. 1 is a graph showing the heat pattern of the manufacturing process according to the present invention, in which T ₁ is the heating temperature before hot working, T ₂ is the austenite recrystallization start temperature (the range of T ₁ to _{T 2} is austenite recrystallization region), _T3 is the rolling end temperature, _T4 is the transformation start temperature ( _T2 to _T4 is the austenite non-recrystallization region)
It is. After the rolled material is heated to a predetermined temperature _T1 , it is subjected to hot working in the austenite region, including low temperature working in the temperature range _T2 to _T3 . The degree of processing in this low-temperature processing is preferably an area reduction rate of 30% or more and 90% or less, as described later. In the process of hot working, the temperature of the workpiece decreases over time, so processing in a high temperature area (recrystallization area) is followed by low temperature processing in a low temperature area (non-recrystallization area). can be done. In this way, low-temperature processing can be performed continuously at the same time as natural cooling, so the heating temperature T ₁ before processing is determined by the low-temperature processing conditions (temperature and area reduction rate) and the capacity of the rolling mill. From the viewpoint of initial grain size refinement before processing, it is desirable to set the temperature to the lowest temperature calculated back from the processing pass schedule. In addition, in the above-mentioned hot working, processing in the recrystallization zone T ₁ to _{T 2} is not necessarily necessary; for example, the pre-processing heating temperature is set near the recrystallization start temperature T ₂ and low-temperature processing is started immediately from the same temperature. You may also do this. Also in this case,
As mentioned above, processing in the low temperature range requires an area reduction rate of 30% or more and 90% or less. Following the hot working, the material is rapidly cooled by water cooling or the like, that is, directly quenched, and cooled to room temperature. This rapid cooling treatment provides a mixed structure of ferrite and bainite or a ferrite-pearlite structure. In other words, for low carbon alkylated steel, the ferrite/pearlite nose and bainite nose in the continuous cooling curve (CCT curve) are located on the short time side, so it is virtually impossible to use any cooling method (on an industrial scale). However, it is difficult to obtain a martensite single-phase structure, and the water-cooled structure when cooled at a relatively high cooling rate is a mixed structure of bainite and ferrite (this ferrite is not polygonal ferrite (equiaxed ferrite)). When the cooling rate is relatively low, the structure becomes ferrite/pearlite. However, when the quenched/tempered material is cooled at a high cooling rate, the volume fraction of bainite or martensite is large, so the toughening effect is obtained through the mechanism described below.On the other hand, when the cooling rate is relatively low, In the ferrite-pearlite structure of , the ferrite structure is significantly refined by water cooling, so the effect of significantly reducing the transition temperature can be obtained. The refinement of the ferrite structure during low-temperature processing of rapidly cooled materials is due to the high frequency of ferrite formation during the phase transformation from austenite to ferrite, and the mechanism is that the conventional Nb-containing direct quenching high tension The same is true for steel. The cooling rate in the above-mentioned quenching treatment depends on the cooling condition of the cooling device used as well as the thickness of the steel material, and with normal quenching means, when the thickness of the steel material is less than 10 mm, the cooling rate will be reduced to ferrite due to the high cooling rate. A mixed structure of bainite is given, and if the plate thickness is larger than that, the cooling rate is low and the structure becomes a ferrite/pearlite structure. After the rapid cooling treatment is performed, the material is subjected to a tempering treatment at an Ac of ₁ point or less. This tempering treatment is preferably carried out at a temperature comparable to or about 50° C. higher than the tempering temperature in the conventional "reheating treatment and quenching/tempering treatment" method. This provides various properties such as required low-temperature toughness and high strength as described later. As mentioned above, the method of the present invention allows direct quenching and
By performing the tempering treatment, a stable and good charpy fracture surface transition temperature can be obtained regardless of the prior austenite grain size. That is, according to the conventional method (a) hot rolling (usually 1150 to 950°C) followed by air cooling;
After being heated to re-austenitizing temperature and quenched,
When tempering (usually 625℃ x 60 minutes),
(b) Hot processing that does not include low temperature processing (e.g. temperature 1200
℃), then directly quenched and then tempered (e.g. 620℃ x 60 minutes), the prior austenite grain size greatly affects the shear py fracture transition temperature, and in order to lower the transition temperature, The austenite crystal grain size is required to be fine. In contrast, according to the present invention, when a deformed structure is introduced into austenite by applying a certain low-temperature processing and then the transformation is performed by rapid cooling, the same old austenite grain size (the grain size immediately before low-temperature processing) is obtained. Said
It can be seen that a transition temperature lower than that obtained in (a) or (b) is given, and the lower the reduction rate in low temperature processing becomes, the more remarkable the decrease in the transition temperature becomes. In addition, the finer the crystal grains before low-temperature processing, the more advantageous it is.
Even with a coarse grain structure with grain size number 5, if the rolling reduction rate in low-temperature processing is increased and processing is performed with an area reduction rate of preferably 30% or more and 90% or less, fine prior austenite grains with grain size number 8 to 10 can be formed. It is possible to obtain a transition temperature as good as or better than that obtained by the conventional method (a) using the material, and it is substantially free from the influence of prior austenite grain size. In addition, as long as the temperature at which low-temperature processing is performed is within a range in which transformation to ferrite and recrystallization of austenite do not occur during or immediately after the processing deformation, the temperature within this range will not affect the transition temperature obtained. Therefore, a large low-temperature processing rate can be easily applied by accumulating many rolling passes, and no special measures are required in actual operation. Increasing the reduction rate during low-temperature processing is effective in raising the transition temperature, but from a practical perspective such as rolling mill capacity, the upper limit is 90%.
shall be. Furthermore, when low-temperature processing and rapid cooling/tempering are performed by the method of the present invention, the tensile strength and falling strength are superior to materials quenched/tempered using conventional methods, which are tempered at the same temperature. The mechanical properties of the steel change depending on the temperature setting for tempering, and the maximum absorbed energy, elongation, reduction of area, etc. improve as the tempering temperature increases, while the yield strength and tensile strength increase.
On the contrary, it tends to decrease. Therefore, by setting the tempering temperature appropriately, the required properties can be imparted, and the tempering temperature is such that the strength level is the same as that of conventional quenching and tempering [method (a)].
For example, if a temperature of about 630 to 670°C is selected, both the strength and the maximum absorbed energy value will be equivalent to that of quenched and tempered material. Moreover, in that case, the Shally pie fracture surface transition temperature tends to decrease as the tempering temperature is increased, so a good value can be given to the transition temperature as well. As mentioned above, the following mechanisms are thought to be responsible for imparting a good transition temperature without any loss of maximum absorption energy by adding low-temperature processing during the hot working process: Cr, Mo, Cu In low-C aluminum killed steels that do not particularly contain components that enhance hardenability, the transformed phase after cooling changes significantly after the finished rolling thickness reaches approximately 10 mm. For products with a thickness exceeding approximately 10 mm, it is difficult to obtain low-temperature transformation products such as martensite or bainite by water cooling from the austenite region, and generally the structure is equiaxed ferrite mixed with some acicular structure. Become. For this reason, even when directly quenched after rolling, a low-temperature transformed phase cannot be obtained. This is due to the reduction of the transition temperature due to rolling in a low temperature range and the refinement of the ferrite structure, as in the case of the controlled rolling method of Nb (or V)-containing high-strength steel. However, conventional Nb steel has the effect of lowering the transition temperature significantly due to the reduction in the maximum absorbed energy of shear py due to the increase in strength, or the indirect effect of lowering the transition temperature due to the reduction of the same energy due to the occurrence of separation. On the other hand, in the present invention, the excellent shear strength maximum absorbed energy, which is strongly required as well as the transition temperature, for low temperature steel (for example, in aluminum killed steel, vEmax > 20Kg-
m, whereas Nb steel generally has vEmax<10
Kg-m), making it possible to further reduce the transition temperature significantly. On the other hand, when the finished rolled plate thickness is relatively thin (for example, approximately 10 mm or less), the following mechanism is considered to be involved: "per area"
(also called "effective ferrite grain size") is a graph showing the relationship with the prior austenite grain size (in the martensitic structure,
The structure corresponding to ferrite grains is called a "packet" or "block" and is made up of a plurality of divided prior austenite grains, so the transition temperature of the martensitic structure is related to the packet or block. It is something that should be devised. However, in martensite transformed from deformed austenite grains, it is difficult to confirm microstructural factors such as packets or blocks through microscopic observation. there was). The fracture surface unit and prior austenite grain size are both expressed as the average intercept length.
In the figure, solid lines indicate equiaxed prior austenite grains,
The broken line indicates the case where low-temperature processing was performed. however,
The austenite grain size after low-temperature processing is
When observed on the Shallie fracture surface, it corresponds to a substantially smaller austenite grain size, so correction is performed using the following formula. d'=d(1-R) [In the formula, d' is the grain size after correction, d is the grain size before low-temperature processing, and R is the area reduction rate (%) during low-temperature processing] It is clear from the figure By adding low-temperature processing, the fracture surface unit (effective ferrite grain size)
It can be seen that the grains are refined more than the geometric refinement on the surface of a simple Shapey test piece.The introduction of such low-temperature processing results in effective grain refinement that significantly lowers the transition temperature. This is thought to be one of the reasons for this. Furthermore, Fig. 3 is a graph showing the relationship between the fracture surface unit and the fracture surface transition temperature (the marks in the figure are the same as in Fig. 2), and according to this,
For those subjected to low-temperature processing, the relationship between the fracture surface unit and the transition temperature deviates from the relationship (shown by the solid line) when austenite is equiaxed grains. It is said that the transition temperature is determined not only by the fracture surface unit but also by the magnitude relationship between the descending stress and the fracture stress. It is inferred that this unique result was obtained. Next, the various characteristics obtained by the treatment method of the present invention that involves low-temperature processing and direct quenching/tempering will be specifically explained in comparison with those obtained by other methods such as conventional methods. As sample materials, C0.08%, Si0.31%, Mn1.41
%, P0.013%, S0.0006%, and Al 0.032%. The processing conditions are as follows. (1) Test No. 1 material: "1150℃ heating" → "High temperature range rolling" → "Low temperature processing (applying rolling with an area reduction rate of 45% at a temperature of 850 to 830℃)"
→ “Direct quenching (water cooling)” → “Tempering (625℃×
1Hr)", (2) Test No. 2 material (comparative material) "1150℃ heating" → "High temperature range rolling (temperature 1150~
950℃) → ``Air cooling'' → ``Re-austenitization (900℃ x 1Hr)'' → ``Quenching (water cooling)'' → ``Tempering (625℃ x 1Hr)'' Among the above test materials, No. 1 was This method is based on the method of the present invention, and No. 2 is an example of a conventional method in which hot working is followed by reheating and quenching and tempering. Test pieces were taken from the obtained rolled plate with a thickness of 20 mm in the rolling direction, and a JIS No. 4 full size test piece was used for the impact test, and a test piece with a gauge length of 35 mm and a diameter of 10 mm was used for the tensile test. Table 1 shows the properties of each of the test materials. As shown in the table, according to the method of the present invention, the fracture surface transition temperature is significantly lowered without impairing strength, ductility, etc., compared to the conventional method (No. It can be seen that compared to .2), it has superior properties as a low temperature steel. Furthermore, in addition to the desired low-temperature transformation products, a considerable amount of ferrite (acicular ferrite) may be mixed in during direct quenching (water cooling) after rolling, but this does not impair the above-mentioned properties. do not have.

[Table] As described above, according to the method of the present invention, by performing a certain low-temperature processing, the reheating process is omitted, and there is no need to add special elements, and various types of existing low-temperature steels can be used. The properties can be significantly improved. In addition, since it does not contain special elements, there is no need for high-temperature heating to solidify these elements, and it is possible to lower the pre-rolling heating temperature as long as the rolling mill capacity and pass schedule allow.
Various effects such as energy saving and productivity improvement can also be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the heat pattern in the method of the present invention, FIG. 2 is a graph showing the relationship between prior austenite grain size and fracture surface unit, and FIG. 3 is a graph showing the relationship between fracture surface unit and Charpy fracture surface transition temperature. It is a graph showing a relationship.

Claims (1)

[Claims] 1 C0.01~0.1%, Si0.1~0.9%, Mn0.5~2.0%,
Low-temperature aluminum killed steel containing 0.01 to 0.3% Al, with the balance being iron and unavoidable impurities, has an area reduction of 30% or more in a temperature range that is above the Ar ₃ transformation point and does not cause recrystallization of deformed austenite grains. ,90%
A method for producing aluminum killed steel for low temperature use, which comprises subjecting it to hot rolling including the following processing, immediately water-cooling it to room temperature after hot rolling, and then tempering it at a temperature below the Ac ₁ transformation point.