JP4516924B2 - Thin steel plate with excellent surface crack resistance during hot rolling and its manufacturing method - Google Patents

Description

  The present invention relates to a steel sheet having excellent surface properties, particularly during direct rolling or hot charge rolling of a steel slab, following the melting and solidification, as it is or during the hot rolling in the process of reheating and hot rolling. The present invention relates to a hot-rolled steel sheet having excellent surface crack resistance and a method for producing the same, and a cold-rolled steel sheet, a surface-treated steel sheet and a steel pipe which are produced using the hot-rolled steel sheet as a raw material.

  In the steel plate manufacturing process, direct feed rolling, which is hot rolled immediately after continuous casting, or so-called hot charge rolling, in which the slab charging temperature to the heating furnace is set to the austenite temperature range after continuous casting, and then hot rolling is performed. The method of effectively using the thermal energy stored in the slab is actively used.

  In contrast to the normal rolling in which the slab is cooled to room temperature after casting and reheated and rolled after transformation from the γ phase to the α phase, in the direct feed rolling and hot charge rolling as described above, the slab surface undergoes transformation to the α phase. Without being heated, the coarse γ grains are heated and rolled. For this reason, a crack occurs along the γ grain boundary in the initial stage of hot rolling, and a so-called steel plate surface crack that reaches a large crack may occur due to the progress of the crack.

  As a method for preventing such a steel sheet surface crack in direct feed rolling or hot charge rolling, Patent Document 1 discloses that the range within 10 mm from the slab slab surface is Ar 3 to 100 ° C. or lower and 1000 ° C. or higher and 1250 ° C. or lower. A method of hot rolling after repeating heating and cooling more than once is described. In the disclosed technique shown in Patent Document 1, by repeating heating and cooling in the above temperature range, γ grains are refined, and carbonitrides precipitated at the grain boundaries during cooling are the positions of the old grain boundaries. And the above conditions were proposed by finding that surface defects can be suppressed by repeating heating and cooling at least twice or more. ing.

Patent Document 2 proposes a method in which a continuously cast slab is once cooled to the Ac1 point or less and then heated and rolled again. Patent Document 3 describes a method in which a surface layer portion of a slab is cooled and held at a temperature of 350 ° C. to 500 ° C. for 1 minute or longer, and then the surface layer portion is transformed and then reheated and rolled. These technologies need to be reheated after being cooled once for the purpose of transformation, and the advantages of direct feed rolling and hot charge rolling for the purpose of charging at high temperatures are impaired, which is disadvantageous in terms of thermal energy. In addition, there is a problem that productivity is remarkably impaired because it is necessary to cool the slab to a target temperature. Patent Document 4 discloses a method for preventing surface cracking by rolling twice or more at a rolling reduction of 15% or more in one pass in a temperature range of 1300 ° C. to 1150 ° C. to control the form of precipitates. ing. In the technique disclosed in Patent Document 4, it is necessary to set the slab extraction temperature to be very high in order to achieve rolling at a high temperature, which is similarly disadvantageous in terms of thermal energy, and the productivity is lowered due to high temperature extraction. It becomes a problem. Patent Document 5 describes a method of setting the hydrogen sulfide concentration of the fuel used in the heating furnace for hot rolling to 50 ppm or less. However, it has an effect other than significantly inhibiting the degree of freedom of the fuel used and cracking caused by sulfide. There is a problem that cannot be demonstrated. In Patent Document 6, among Ti, Mg, Ca, Ce, La, Ba, and Li, which have a stronger binding force with S than Mn, in order to suppress surface cracking due to grain boundary precipitation of (Fe, Mn) S. A method of suppressing the formation of (Fe, Mn) S by adding one or two or more to make sulfides has been proposed, but the effect cannot be exhibited except for cracks caused by sulfides. Moreover, since the coarse sulfide in molten steel reduces the workability of a product, there exists a problem that it cannot apply to the steel plate in which workability is calculated | required.
JP 7-290101 A JP 55-84202 A Japanese Patent Publication No.7-112563 JP 55-77901 A JP 2001-25801 A JP 2002-178007 A

  Therefore, the present invention has been devised in view of the above-described conventional problems, and the object of the present invention is to provide a hot-rolling heating furnace at the highest possible temperature in direct feed rolling or hot charge rolling of a steel slab. It is an object of the present invention to provide a thin steel sheet having excellent surface crack resistance at the time of hot rolling and a method for producing the same, in which surface cracking is unlikely to occur in the step of charging, as it is or reheating and hot rolling.

The present invention finds that the precipitation of nitrides is the main cause of hot rolling cracks, and in order to render the precipitation of the rows harmless, a grain boundary nitride having an equivalent circle diameter of 50 nm or less is converted to a grain boundary of 1 μm ^2. It was devised by paying attention to the fact that cracking in hot rolling can be suppressed by setting it to 140 or less per hit.

That is, the invention according to claim 1 is mass%, C: 0.06 to 0.30%, Si: 2.0% or less, Mn: 0.1 to 3.0%, P: 0.1% Hereinafter, S: 0.0005 to 0.01%, Al: 0.004 or more and less than 0.025%, N: 0.0005 to 0.010%, with the balance being Fe and inevitable impurities The number of grain boundary nitrides having an equivalent circle diameter of 50 nm or less is 140 or less per 1 μm ^{2 of} grain boundaries.

Further, the invention according to claim 2 further contains, by mass%, Ti: 0.01 to 0.20%, and the content of N [N] and the content of Ti [Ti] are expressed by the following formula (A The thin steel sheet according to claim 1, wherein:
[Ti] -3.4 × [N]> 0 (A)

  Further, the invention according to claim 3 is further mass%, Nb: 0.01 to 0.10%, V: 0.005 to 0.05%, B: 0.0003 to 0.010%, Ca: 0.0005-0.02%, Mg: 0.0005-0.02%, Zr: 0.0005-0.02%, REM: 0.0005-0.02%, Cu: 0.04-1. 4%, Ni: 0.02 to 0.8%, Mo: 0.02 to 0.5%, Cr: 0.02 to 1.0%, containing one or more It is a thin steel plate of Claim 1 or 2.

  The invention according to claim 4 is characterized in that the grain boundary nitride having an equivalent circle diameter of 50 nm or less includes one or more of AlN, NbN, and a composite nitride of Al and Nb. It is a thin steel plate of any one of 1-3.

The invention according to claim 5 is the method for producing a thin steel sheet according to any one of claims 1 to 4, wherein the steel slab is subjected to a cooling process subsequent to melting and solidification when performing direct feed rolling or hot charge rolling. The steel sheet is characterized in that the time from continuous casting to being heated in a hot rolling furnace is 2 to 10 hours without being reduced to a temperature of Ar1 or lower , and is subjected to hot rolling as it is or after reheating. It is a manufacturing method.

Further, the invention according to claim 6 is characterized in that rough rolling is further performed three times or more at a reduction rate of 10% or more per pass and a strain rate of 1 / s or more in a temperature range of 900 to 1200 ° C. a method for producing a thin steel sheet to claim 5, wherein.

  ADVANTAGE OF THE INVENTION According to this invention, the thin steel plate excellent in the surface property which does not produce a surface crack by hot rolling in the direct-feed rolling or hot charge rolling of a steel slab can be provided. In the present invention, the thermal energy stored in the slab can be effectively used to reduce the manufacturing cost, and the industrial value can be improved as the productivity can be improved by shortening the cooling and reheating time. It becomes possible. .

  Hereinafter, the best mode for carrying out the present invention will be described in detail by taking, as an example, a thin steel plate having excellent surface crack resistance during hot rolling. Hereinafter, the weight% in the composition is simply described as%.

  In general, it is known that cracks occurring near 1150 ° C. to 1000 ° C. are brittle in the II region, and because medium carbon steel has a high peritectic temperature, the γ grain size is large and stress concentration is likely to occur. It is said that the sulfides and oxides precipitated in the steel cause the grain boundary strength to decrease.

In order to obtain a thin steel sheet having surface crack resistance at the time of hot rolling, which is extremely difficult with the prior art, and having excellent manufacturing cost and productivity, the present inventors have obtained a hot rolling step of 1150 ° C. to Coils and slabs that cracked during rough rolling at 1000 ° C. were investigated in detail. As a result, in the slab where cracks occur, it is found that Al nitride, Nb nitride, or a composite nitride of Al and Nb as shown in FIG. 1 is precipitated in a row on the old γ grain boundary, It has been found that the above-mentioned precipitation of nitrides on the γ grain boundary is the main cause of hot rolling cracks. And in order to detoxify the columnar precipitation, the influence of the size and density of the precipitates is intensively studied, and the grain boundary nitride having an equivalent circle diameter of 50 nm or less is set to 140 or less per 1 μm ^{2 of} grain boundaries. It discovered that the crack by hot rolling can be suppressed. And as means to achieve this, the amount of Al addition is reduced, the driving force for precipitation of AlN is lowered, and the time and temperature conditions from continuous casting to heating in a hot-rolling heating furnace are specified. Thus, the inventors have found that it is effective to reduce the density of the nitride precipitates on the grain boundaries, and have come up with the present invention.

  Hereinafter, the reason for adding each component constituting the thin steel plate to which the present invention is applied and the reason for limiting the numerical value will be described.

C: 0.06-0.30%
When C is 0.12%, the peritectic temperature is the highest, and the γ grain size becomes large, so surface cracking is likely to occur during hot rolling. It is an element that must be added in order to produce NbC and obtain precipitation strengthening. In order to obtain these effects stably, addition of 0.06% or more is necessary. However, if it exceeds 0.30%, the weldability decreases. For this reason, in the present invention, from the viewpoint of maintaining weldability, the upper limit of the C content is set to 0.3%.

Si: 2.0% or less Si is an element effective for improving the elongation property by increasing the ferrite fraction suppressing generation of harmful carbides, effective for material strength ensured by solid solution strengthening Therefore, addition of 0.01% or more is desirable. However, the chemical conversion processability is deteriorated by excessive addition. Particularly, when the Si content exceeds 2.0%, the descaling property at the time of hot rolling is remarkably lowered, and Si scale is also generated. For this reason, the upper limit of Si content was set to 2.0%. In particular, in a steel plate where surface layer quality is a problem, the Si content is desirably 1.0% or less.

Mn: 0.1 to 3.0%
This Mn is an element necessary for ensuring the strength, and requires addition of 0.1% or more. However, if it is added in a large amount exceeding 3.0%, microsegregation and macrosegregation are liable to occur, which deteriorates the workability of the material and the chemical conversion treatment. Therefore, the Mn content is set to 0.1 to 3.0%.

P: 0.1% or less P is dissolved in ferrite to lower its ductility, so its content is 0.1% or less.

S: 0.0005 to 0.01%
S forms sulfides, significantly lowers the brittleness of the steel, and causes surface cracks. That is, since the lower content of S is desirable, the upper limit is made 0.01%. In addition, S forms sulfide inclusions such as MnS and becomes a starting point of cracking to deteriorate workability. Therefore, when workability is required, S is preferably 0.005% or less. However, even if the content is reduced to less than 0.0005%, the effect of improving the workability by reducing the content is saturated and the production cost is increased, so the lower limit was made 0.0005%.

Al: 0.004 or more and less than 0.025%
Al is one of the most important elements in the present invention. Al is an element necessary for deoxidation, and if the addition amount is small, the oxide in the steel increases, which deteriorates brittleness and promotes surface cracking during hot rolling. In order to perform sufficient deoxidation, addition of 0.004% or more is necessary. On the other hand, Al combines with N and precipitates at grain boundaries as AlN, and in steel sheets with Nb added, grain boundaries precipitate as Nb and Al composite nitrides. Reduces and causes surface cracking. In order to suppress this, Al needs to be less than 0.025%. In particular, in a steel sheet with strict surface properties, it is desirable that the Al content be 0.02% or less.

N: 0.0005 to 0.010%
When N is added too much, it binds to Al and precipitates Al nitride, Nb nitride, or a composite nitride of Al and Nb at the γ grain boundary, reducing brittleness and surface cracking during hot rolling. Producing the cause of. For this reason, the upper limit of the content of N is set to 0.010% or less. On the other hand, if the N concentration is less than 0.0005%, the manufacturing cost increases, so 0.0005% is made the lower limit.

Ti: 0.01-0.20%
Ti is one of the important elements in the present invention. Ti, like Nb, enables strengthening of the steel sheet by precipitation strengthening due to fine precipitation of carbides. Ti has a strong affinity for N, and forms a relatively large TiN with an equivalent circle diameter of more than 100 nm at a temperature of more than Ar1, and fixing N allows Al nitride, Nb nitride to the γ grain boundary, or There is an effect of suppressing row precipitation of the composite nitride of Al and Nb and suppressing surface cracks during hot rolling. In order to exhibit these results effectively, addition of at least 0.01% is necessary. However, if these additions become excessive, ductility deteriorates due to precipitation strengthening, so the upper limit is made Ti 0.20% or less. The equivalent circle diameter of TiN is preferably more than 100 nm and 5 μm or less. Further, if the TiN equivalent circle diameter is less than 100 nm, N cannot be fixed and the precipitation of the grain boundary nitride cannot be suppressed, and the grain boundary precipitate of TiN causes cracking on the contrary. . When the equivalent circle diameter of TiN exceeds 5 μm, it becomes too large, and workability such as hole expansibility deteriorates, so that it is preferably 5 μm or less. More preferably, it is more than 100 nm and 1 μm or less.

Further, the Ti content needs to satisfy the formula (A) with respect to the N content.
[Ti] -3.4 × [N]> 0 (A)

  If the formula (A) is not satisfied, TiN precipitation does not occur sufficiently, N cannot be fixed, and the precipitation of the grain boundary nitride cannot be suppressed, so that surface cracking in hot rolling cannot be suppressed. Further, since the present invention aims to suppress the nitride, the formula (A) is not changed even when the content of S is particularly high.

Nb: 0.01 to 0.10%
Nb makes it possible to increase the strength of the steel sheet by precipitation strengthening due to fine precipitation of carbides. Further, by suppressing the recrystallization of γ, the crystal grains of the steel sheet are refined and the fatigue strength is increased. For this purpose, it is necessary to add 0.01 or more of Nb. On the other hand, when a large amount is added, the precipitation strengthening ability reaches a peak, and Nb nitride and a composite nitride of Nb and Al are formed similarly to Al, which causes surface cracking in hot rolling. % Or less.

V: 0.005-0.05%, B: 0.0003-0.010%
V and B are both nitride-forming elements and, like Nb and Al, reduce brittleness and cause surface cracks during hot rolling, so it is desirable not to add them. However, V is added to strengthen the steel by forming fine carbides, and B is added to strengthen the steel by enhancing the hardenability of the steel. In order to obtain this effect, it is necessary to add 0.005% or more for V and 0.0003% or more for B. However, even if V exceeds 0.05% and B exceeds 0.01%, the effect is saturated, so this is the upper limit.

Ca: 0.0005 to 0.02%, Mg: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, REM: 0.0005 to 0.02%
Ca, Mg, Zr, and REM are effective for controlling the form of sulfide inclusions and improving local ductility. In order to make this form control effect effective, it is desirable to add 0.0005% or more of one or two of Ca, Zr, Mg, and REM. On the other hand, addition of a large amount leads to coarsening of sulfide inclusions, not only deteriorates the cleanliness and lowers the ductility, but also causes an increase in cost. 02%. In addition, as REM, it is an element of the element numbers 21, 39, 57-71, for example.

Cu: 0.04-1.4%, Ni: 0.02-0.8%, Mo: 0.02-0.5%, Cr: 0.02-1.0%
Cu, Ni, Mo, and Cr are used for controlling the microstructure and strength. If the addition amount is small, there is no effect of increasing the strength, and excessive addition deteriorates the ductility. Therefore, Cu is 0.04 to 1.4%, Ni is 0.02 to 0.8%, Mo is 0.02 to 0.5%, and Cr is 0.02 to 1.0%. It is necessary to add more seeds.

The precipitation state of the γ grain boundary is one of the most important factors in the present invention. The present inventors diligently investigated the relationship between the occurrence of surface cracks during hot rolling and the precipitation state on the γ grain boundaries. The following Al nitride 50nm on γ grain boundaries is precipitated in a row, when the total number is less than or equal to 140 per grain boundary 1 [mu] m ^2, the grain boundary precipitates are harmless, surfaces in hot rolling It was found that cracking can be suppressed. Further, in Nb-added steel, Al nitride, Nb nitride, and composite nitride of Al and Nb are precipitated in a row, and when the total number is similarly 140 or less per 1 μm ^{2 of} grain boundary, It has been found that the grain boundary precipitate is rendered harmless and surface cracking during hot rolling can be suppressed. If the grain boundary precipitate contains Al, the effect of the present invention is not changed even if it is a composite nitride of Nb, B, and V.

  Next, the manufacturing method of the thin steel plate to which this invention is applied is demonstrated.

  The rolling of the steel slab is performed by direct feed rolling or hot charge rolling. If the steel slab is dropped to a temperature of Ar1 or lower during the cooling process after melting and solidification, surface cracking during hot rolling can be suppressed, but in order to perform hot rolling, a large amount of thermal energy is required in the heating furnace, and the manufacturing cost In addition to the increase in productivity, productivity is significantly reduced due to cooling to Ar1 and subsequent heating. For this reason, in the present invention, in the cooling process, the steel slab is held at Ar1 or more and is subjected to hot rolling as it is or after reheating.

  After continuous casting, the time required for charging and heating in a hot-rolling heating furnace at a temperature exceeding Ar1 is an important factor in the present invention, and is 2 to 10 hours. Although the mechanism is not clear, in the case of 2 hours or less, in the Ti addition system, TiN precipitation growth is not sufficient, and in addition, Al nitride, Nb nitride, or composite nitride of Al and Nb precipitates at the grain boundary. Since the growth is not sufficient, the density of the grain boundary precipitates having an equivalent circle diameter of 50 nm or less increases. On the other hand, in the system to which Ti is not added, the Ostwald growth of the nitride that precipitates at the grain boundary does not sufficiently occur, so the density of the grain boundary precipitate with an equivalent circle diameter of 50 nm or less increases, and as a result, the surface during hot rolling. This is because cracks occur. On the other hand, if the above time exceeds 10 hours, the heat of the slab is released to the atmosphere, the steel slab becomes a temperature of Ar1 or lower, and the manufacturing cost increases.

  Surface cracking during hot rolling can be further prevented by removing rough rolling in the hot rolling process after heating from the II region embrittlement temperature. However, in order to avoid this on the high temperature side, it is necessary to increase the extraction temperature of the slab, in addition to increasing the manufacturing cost by heating, in order to make the finish rolling end temperature after that a predetermined temperature, It is necessary to wait for the temperature before finish rolling, which hinders productivity. On the other hand, in order to avoid the low temperature side, the finish rolling finish temperature cannot be secured, and the strength and elongation deteriorate. Therefore, in this invention, rough rolling is implemented at 900-1200 degreeC. In addition, in order to keep finishing temperature stably, it is preferable to implement rough rolling at 1000 degreeC or more.

  Rough rolling needs to be performed three times or more at a rolling reduction of at least 10% in the temperature range described above. The reason is that if the rolling reduction per pass is low, the number of rollings for obtaining a predetermined plate thickness increases, and it becomes difficult to lower the productivity and secure the finishing temperature. In addition, when the number of rolling is 3 times or less, the rolling rate per pass is increased, and the plate shape is easily deteriorated and end cracks are likely to occur. The strain rate in each rolling is 1 / s or more. The reason for this is that when the strain rate is low, productivity is lowered and it is difficult to ensure the plate temperature due to a decrease in the amount of heat generated by processing.

  The present invention suppresses surface cracks during hot rolling, and does not limit the finish rolling temperature after rolling, ROT cooling, or the winding temperature. Further, the effects of the present invention are not lost even when a cold-rolled steel sheet, a surface-treated steel sheet and a steel pipe are produced using this hot-rolled steel sheet as a raw material.

  Next, examples of the present invention will be described.

  Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. Table 2 shows the time until heating in the subsequent heating furnace, the charging temperature, and the like. The heating furnace charging temperature was measured with a radiation thermometer. The rough rolling was performed at a temperature of 1000 ° C. to 1200 ° C. for 6 passes at a rolling reduction rate of 10% or more and a strain rate of 1 / s (s means second) or more.

Table 2 shows the types of elements forming grain boundary nitrides present in these steels, the number of nitrides of 50 nm or less per 1 μm ² , and the occurrence of surface cracks in the steel sheet. In this test, the precipitate was prepared by using a transmission electron microscope (TEM) equipped with an EDS elemental analysis function for a sample prepared based on the extraction replica method (3rd edition, Steel Handbook IV, Steel Materials, Test and Analysis, see P397) Using this, observation of precipitates and elemental analysis, identification of precipitates from diffraction points, and measurement of equivalent circle diameter and the number of precipitates were performed. Precipitate density was determined by measuring the width, length and number of precipitate rows with TEM, and (precipitate density) = (number) / {(length) × (width)}.

  Steels A to O shown in Table 1 are steels whose components are limited in the present invention (hereinafter referred to as invention steels). On the other hand, the steels p to v are comparative steels deviating from the components limited in the present invention, the steel p is the addition amount of Nb, the steel q is the addition amount of N, and the steels u and v are Al. Is added to deviate from the upper limit range of the component defined in the present invention, and the nitride density is outside the range of the present invention. Further, the amount of Al added to the steels r and t deviates from the range of components specified in the present invention.

  Of Table 2, A3 and C3 are outside the scope of the present invention after casting and before heating, and the nitride density is outside the scope of the present invention.

  The hot-rolled steel sheet thus obtained was subjected to a surface crack observation test after hot rolling. X indicates that the surface crack was confirmed, and ○ indicates that the surface crack was not confirmed.

  It can be seen that surface cracks are observed in the comparative steels, whereas no surface cracks are observed in the inventive steels.

Row-like nitrides found at γ grain boundaries

Claims (6)

% By mass
C: 0.06 to 0.30%,
Si: 2.0% or less,
Mn: 0.1 to 3.0%
P: 0.1% or less,
S: 0.0005 to 0.01%,
Al: 0.004 or more and less than 0.025%,
N: 0.0005 to 0.010%,
And the balance is Fe and inevitable impurities, and the number of grain boundary nitrides having an equivalent circle diameter of 50 nm or less is 140 or less per 1 μm ^{2 of} grain boundaries. Steel sheet with excellent properties. In addition,
2. The heat according to claim 1, wherein Ti: 0.01 to 0.20%, and the N content [N] and the Ti content [Ti] satisfy the following formula (A): Thin steel plate with excellent surface crack resistance during hot rolling.
[Ti] -3.4 × [N]> 0 (A) In addition,
Nb: 0.01-0.10%,
V: 0.005-0.05%,
B: 0.0003 to 0.010%,
Ca: 0.0005 to 0.02%,
Mg: 0.0005 to 0.02%,
Zr: 0.0005 to 0.02%,
REM: 0.0005 to 0.02%,
Cu: 0.04 to 1.4%,
Ni: 0.02 to 0.8%,
Mo: 0.02 to 0.5%,
Cr: 0.02-1.0%,
The thin steel plate excellent in the surface crack resistance at the time of hot rolling of Claim 1 or 2 characterized by containing 1 type (s) or 2 or more types of these.   The grain boundary nitride having an equivalent circle diameter of 50 nm or less includes one or more of AlN, NbN, and a composite nitride of Al and Nb, according to any one of claims 1 to 3. A thin steel sheet with excellent surface crack resistance during hot rolling. In the method of manufacturing a thin steel sheet according to any one of claims 1 to 4, when direct rolling or hot charge rolling, without reducing the steel slab to a temperature of Ar1 or less in the cooling process subsequent to melting and solidification, The time from continuous casting to heating in a hot rolling furnace is 2 to 10 hours, and the surface cracking resistance at the time of hot rolling is excellent as it is or as it is reheated and subjected to hot rolling Manufacturing method of thin steel sheet. 6. The rolling resistance at the time of hot rolling according to claim 5 , wherein rough rolling is performed at least 3 times at a rolling reduction of 10% or more per pass and a strain rate of 1 / s or more in a temperature range of 900 to 1200 ° C. A method for producing a thin steel sheet having excellent surface cracking properties.