HIGH-CARBON HOT-ROLLED STEEL SHEET HAVING EXCELLENT SURFACE QUALITY AND MANUFACTURING METHOD THEREFOR
The present invention relates to a hot-rolled steel sheet suitable for construction, tools, vehicle parts, and the like and, more particularly to a high-carbon hot-rolled steel sheet having an excellent surface quality and a manufacturing method therefor.
The present invention relates to a hot-rolled steel sheet suitable for construction, tools, vehicle parts, and the like and, more particularly, to a high-carbon hot-rolled steel sheet having an excellent surface quality and a manufacturing method therefor.
BACKGROUND ARTA high-carbon hot-rolled steel sheet, used variously in construction, tools, vehicle parts, and the like, is pickled and cold-rolled at a secondary customer company, and then the steel sheet is heat-treated and molded to form a component according to intended purpose at a final customer company.
However, if grain boundary oxidation is provided on a surface of the high-carbon hot-rolled steel sheet, cracking may occur during the processing described above, or cracking may easily occur while a product is used.
Thus, it is required to significantly reduce grain boundary oxidation in a product using high carbon steel.
However, in a hot rolling state, if a large amount of elements, having higher affinity to oxygen than that of Fe, such as Cr, Mn, Al, Si, and the like, are contained, grain boundary oxidation may easily occur. As described above, if the grain boundary oxidation is developed in the hot rolling state, additional processes are required at the secondary customer company in order to remove this, which may mainly increase manufacturing costs. In addition, there is a limit to removing all of the grain boundary oxidation by the additional processes.
Thus, it is necessary to manufacture a steel sheet so as to significantly reduce the grain boundary oxidation in a hot rolling state.
On the other hand, in order to impart a high degree of quenching properties, in the case of steel containing boron (B) as an essential element, the content of hardenability improving elements such as C, Mn, Cr, or the like, may be lowered. Accordingly, an amount of elements with a high affinity to oxygen is reduced, and thus, an effect of preventing grain boundary oxidation maybe obtained (Patent Document 1). However, the addition of B has a limitation in effectively preventing the grain boundary oxidation.
(Patent Document 1) Korean Patent Laid-Open Publication No. 2016-0018805
DISCLOSURE Technical ProblemAn aspect of the present disclosure may provide a high-carbon hot-rolled steel sheet having an excellent surface quality by significantly reducing the grain boundary oxidation of the high-carbon hot-rolled steel sheet by optimizing an alloy composition and manufacturing conditions, and a method for manufacturing the same.
Technical SolutionAccording to an aspect of the present disclosure, a high-carbon hot-rolled steel sheet having an excellent surface quality includes: carbon (C): 0.3 wt % to 1.3 wt %, silicon (Si): 0.01 wt % to 0.5 wt %, manganese (Mn): 0.3 wt % to 2.0 wt %, aluminum (Al): 0.1 wt % or less, excluding 0 wt %, chromium (Cr): 5.0 wt % or less, excluding 0 wt %, further includes: one or more selected from the group consisting of molybdenum (Mo): 2.0 wt % or less, antimony (Sb): 0.005 wt % to 0.1 wt %, vanadium (V): 0.5 wt % or less, copper (Cu): 0.5 wt % or less, and nickel (Ni): 2.0 wt % or less, and includes: a balance of iron (Fe) and other inevitable impurities, an HI value, represented by Relation 1, is 0 or more, and Relation 2 is satisfied.
(HI)=−5.69+(4.43×C)+(3.71×Mn)−(4.5×Si)+(1.77×Ni)+(6.18×Cr)+(12.0×Mo)−(43.6×Cu)+(48.1×V)≥0 [Relation 1]
Mo+(10×Sb)−(0.1×Cr)≥0.14 [Relation 2]
Here, the content of each element means weight %.
According to another aspect of the present disclosure, a method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality includes: reheating a steel slab, satisfying the alloy composition and Relations 1 and 2, described above, in a temperature range of 1100° C. to 1300° C.; producing a hot-rolled steel sheet by rough rolling and finish rolling the reheated steel slab; and coiling the hot-rolled steel sheet in a temperature range of 500° C. to 710° C. after cooling.
Advantageous EffectsAccording to an exemplary embodiment in the present disclosure, a high-carbon hot-rolled steel sheet having an excellent surface quality, in which grain boundary oxidation is significantly reduced, may be provided.
Therefore, additional processing costs at a secondary customer company may be reduced and the durability of a final product may be significantly improved.
The inventor of the present disclosure has studied in detail a method for significantly reducing grain boundary oxidation, in providing a high carbon steel hot-rolled steel sheet. As a result, it has been confirmed that grain boundary oxidation on a surface of a hot-rolled steel sheet could be significantly reduced, by thoroughly controlling an alloy composition of the hot-rolled steel sheet while optimizing coiling conditions among manufacturing conditions, thereby resulting in completion of the present disclosure.
Hereinafter, the present disclosure will be explained in detail.
A high-carbon hot-rolled steel sheet having an excellent surface quality according to an aspect of the present disclosure preferably includes C: 0.3% to 1.3%, Si: 0.01% to 0.5%, Mn: 0.3% to 2.0%, Al: 0.1% or less, Cr: 5.0% or less (excluding 0%).
Carbon (C): 0.3% to 1.3%
Carbon (C) is the most effective element for securing strength. In order to obtain excellent hardness in the present disclosure, it is preferable to add 0.3% or more of C. However, if the content of C exceeds 1.3%, it may cause a defect in a process due to significant hardness during hot rolling.
Thus, in the present disclosure, the content of C is preferably controlled to be 0.3% to 1.3%. More preferably, the content of C is controlled to be 0.35% to 1.25%.
Silicon (Si): 0.01% to 0.5%
Silicon (Si) is an element effective for the deoxidation effect. To this end, Si is preferably contained in an amount of 0.01% or more. However, if the content of Si exceeds 0.5%, it is not preferable since the possibility of causing grain boundary oxidation on a surface of a hot-rolled steel sheet is increased.
Thus, in the present disclosure, the content of Si is preferably controlled to be 0.01% to 0.5%. More preferably, the content of Si is controlled to be 0.1% to 0.4%.
Manganese (Mn): 0.3% to 2.0%
Manganese (Mn) is an element effective for securing strength together with the C. If the content of Mn is less than 0.3%, iron sulfide (FeS) may be formed, which may cause grain boundary brittleness at high temperature. On the other hand, if the content of Mn exceeds 2.0%, a quality of a hot-rolled steel sheet may be degraded due to grain boundary oxidation along with center segregation and inclusion formation.
Thus, in the present disclosure, the content of Mn is preferably controlled to be 0.3% to 2.0%. More preferably, the content of Mn is controlled to be 0.4% to 1.5%.
Aluminum (Al): 0.1% or less (excluding 0%)
Aluminum (Al) is an element added for not only the deoxidation effect but also the solid solution strengthening effect. If the content of Al is significant, for example, more than 0.1%, slab cracking may be caused in continuous casting, and grain boundary oxidation may be caused in a final product.
Thus, in the present disclosure, the content of Al is preferably controlled to be 0.1% or less, and 0% is excluded.
Chromium (Cr): 5.0% or less (excluding 0%)
Chromium (Cr) is an element added to enhance the hardenability of steel, and has the effect of inhibiting the generation of rust of iron by forming a passive film in the atmosphere. However, if the content of Cr exceeds 5.0%, it is not preferable since cracking at an edge of a hot-rolled sheet may be caused during cooling.
Thus, in the present disclosure, the content of Cr is preferably controlled to be 5.0% or less, and 0% is excluded. More preferably, the content of Cr is controlled to be 3.5% or less.
The hot-rolled steel sheet of the present disclosure may further include the following elements to improve physical properties in addition to the alloy composition described above.
In detail, it is preferable to further include at least one among molybdenum (Mo), antimony (Sb), vanadium (V), copper (Cu) and nickel (Ni).
Molybdenum (Mo): 2.0% or less
Molybdenum (Mo) is an element effective for improving hardenability of steel, and may be added for imparting thermal stability of a precipitation strengthening element. However, since Mo is a relatively expensive element, if the content of Mo exceeds 2.0%, manufacturing costs may be increased significantly.
Thus, in the present disclosure, when Mo is added, the content of Mo is preferably controlled to be 2.0% or less.
Antimony (Sb): 0.005% to 0.1%
Antimony (Sb) is an effective element concentrated at grain boundaries at high temperature to inhibit grain boundary oxidation. In detail, when a large amount of elements having higher oxygen affinity than that of Fe, such as Cr, Mn, Al, Si, and the like, are contained, Sb is an element effectively suppressing the grain boundary oxidation.
In order to sufficiently obtain the effect described above, Sb is preferably added in an amount of 0.005% or more. However, if the content of Sb exceeds 0.1%, it is not preferably since grain boundary embrittlement may be rather caused.
Thus, in the present disclosure, the content of Sb is preferably controlled to be 0.005% to 0.1%.
Vanadium (V): 0.5% or less
Vanadium (V) is an element added for improvement of strength. However, since V is a relatively expensive element, if the content of V exceeds 0.5%, manufacturing costs may be increased significantly.
Thus, in the present disclosure, when V is added, the content of V is preferably controlled to be 0.5% or less.
Copper (Cu): 0.5% or less
Copper (Cu) is an element added to increase strength and improve corrosion resistance. However, if the content of Cu exceeds 0.5%, it is not preferable since grain boundary brittleness may be caused at high temperature.
Thus, in the present disclosure, when Cu is added, the content of Cu is preferably controlled to be 0.5% or less.
Nickel (Ni): 2.0% or less
Nickel (Ni) is also an element added to increase the strength and to improve the corrosion resistance, and has an effect of preventing grain boundary embrittlement, caused by Cu, at high-temperature when added together with Cu. However, if the content of Ni exceeds 2.0%, an interface may be non-uniform and thus the descaling properties of the scale may be deteriorated at high temperature.
Thus, in the present disclosure, when Ni is added, the content of Ni is preferably controlled to be 2.0% or less.
The remaining elements of the present disclosure are iron (Fe). Merely, in a common manufacturing process, unintended impurities maybe inevitably mixed from surroundings, and thus, this may not be excluded. Since these impurities are known to a person having skill in the common manufacturing process, all contents will not be particularly described in the present specification, but P, S and N are preferably controlled as follows.
Phosphorus (P): 0.03% or less
Phosphorus (P) is an element which is inevitably added during a steel making process. Since P may cause brittleness due to segregation, the content of P is preferably controlled as low as possible.
On the other hand, since the content of P may be increased by adding scrap iron or the like in the production of molten iron, the content of P is preferably controlled up to 0.03%, more preferably 0.02% or less.
Sulfur (S): 0.02% or less
Sulfur (S), an element which is inevitably added during a steelmaking process, forms inclusions or forms sulfide such as iron sulfide FeS having a low melting point, which may cause grain boundary brittleness during hot rolling.
Therefore, the content of S is preferably controlled as low as possible, and the content of S is preferably controlled up to 0.02% since the content of S may be increased by the addition of scrap iron or the like in the production of molten iron. More preferably, the content of S is controlled to be 0.01% or less.
Nitrogen (N): 0.01% or less (excluding 0%)
Nitrogen (N) has a solid solution strengthening effect. However, if the content of N is excessive, a solid solution element may cause the yield point elongation to lower a surface quality. In addition, nitride is precipitated to deteriorate the workability.
Thus, in the present disclosure, the content of N is preferably controlled to be 0.01% or less, and 0% is excluded.
In the hot-rolled steel sheet of the present disclosure including the alloy composition described above, the relationship among elements may preferably satisfy Relations 1 and 2.
(HI)=−5.69+(4.43×C)+(3.71×Mn)−(4.5×Si)+(1.77×Ni)+(6.18×Cr)+(12.0×Mo)−(43.6×Cu)+(48.1×V)≥0 [Relation 1]
Mo+(10×Sb)−(0.1×Cr)≥0.14 [Relation 2]
Here, the content of each element means weight %.
In other words, when the HI value, represented by Relation 1, is 0 or more, it is preferable that the element relationship among Mo, Sb, and Cr satisfies Relation 2.
If the HI (Hardenability Index) value is less than 0, the grain boundary oxidation may not occur. However, if the HI value is 0 or more, the grain boundary oxidation may significantly occur. In this regard, in the present disclosure, when the HI value is 0 or more, the relationship among alloy elements (Mo, Sb, and Cr) is controlled by Relation 2, thereby significantly suppressing the grain boundary oxidation.
As described above, the hot-rolled steel sheet of the present disclosure satisfying the alloy composition and element relationship preferably includes a ferrite and pearlite composite structure as a microstructure.
In more detail, the hot-rolled steel sheet includes the ferrite in an area fraction of 2% to 70%, and the balance of pearlite. In this case, if the ferrite fraction is less than 2%, it is not preferable since a small amount of elements such as Cr for securing hardenability is contained.
On the other hand, if the ferrite fraction exceeds 70%, the transformation speed is fast or the transformation speed is significantly slow as the case in which a hardenability strengthening element such as Cr, Mo, or the like is excessively added.
As described above, in the hot-rolled steel sheet of the present disclosure satisfying an alloy composition, a component relationship, and a microstructure configuration, an area fraction of an oxide located within 10 μm in a thickness direction from a surface is 5% or less, and the grain boundary oxidation suppressing effect is excellent.
In other words, as an area fraction of an oxide located within 10 μm in a thickness direction from a surface of a steel sheet is low, a grain boundary oxidation thickness (a depth) formed on the surface of the steel sheet is thin. In the present disclosure, by forming the oxide in an area fraction of 5% or less in the corresponding region as described above, a thickness of the grain boundary oxidation could be obtained to 2 μm or less.
Hereinafter, a method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality according to the present disclosure will be described in detail.
Briefly, the high-carbon hot-rolled steel sheet of the present disclosure may be manufactured through a process of [steel slab reheating—hot rolling—cooling and coiling], and conditions for each process will be described in detail below.
[Steel Slab Reheating]
First, a steel slab according to the present disclosure, satisfying the alloy composition and component relationship (Relations 1 and 2) described above, is prepared, and then the steel slab is reheated in a temperature range of 1100° C. to 1300° C.
The reheating process is a process for homogenization of a slab. If a temperature of the reheating process is less than 1100° C., rolling load may be significantly increased during hot rolling, a subsequent process. On the other hand, if the temperature of the reheating process exceeds 1300° C., a surface defect maybe caused during rolling since a surface temperature is high during finish rolling of subsequent hot rolling and thus high temperature oxidation scale grows thicker on a surface, or a defect, in which scale is separated from a surface when a wound coil is uncoiling, may be caused.
[Hot Rolling]
The steel slab, reheated as described above, is hot rolled to produce a hot-rolled steel sheet, and the hot rolling may include rough rolling and finish rolling.
In this case, the finish rolling is preferably performed in a temperature range in which an inlet temperature is 900° C. to 1100° C. and an outlet temperature is 800° C. to 950° C.
If the outlet temperature is less than 800° C., rolling load may be significantly increased. In detail, in the case of both edge portions of a steel sheet, in which a temperature is significantly dropped, a pro-eutectoid ferrite phase is generated and a material may be non-uniform in a width direction. On the other hand, if the outlet temperature exceeds 950° C., a structure of a steel sheet may be coarsened, and scale may be thickened and a surface quality may be degraded.
[Cooling and Coiling]
The hot-rolled steel sheet, manufactured as described above, is preferably coiled after cooling.
The cooling is preferably performed at an average cooling rate of 30° C./s to 60° C./s using water cooling in a run-out table (ROT), by way of example.
The coiling is preferably performed in a temperature range of 500° C. to 710° C. If a temperature during the coiling is less than 500° C., it is not preferable since a shape defect may occur. On the other hand, if the temperature during the coiling exceeds 710° C., it is not preferable since a surface quality may be degraded due to scale peeling.
In the case of the high-carbon hot-rolled steel sheet according to the related art, when coiling is performed in the temperature range, described above, due to transformation heat, the grain boundary oxidation may easily occur. However, in the case of the present disclosure, when the relationship among the elements is satisfied with Relation 1 while an alloy composition, contained in a high-carbon hot-rolled steel sheet, is controlled, the content relationship among Mo, Sb, and Cr is controlled by Relation 2. Thus, since an amount of transformation heat is not significant during coiling in the temperature range described above, the grain boundary oxidation is could be suppressed.
Hereinafter, the present disclosure will be detailed through embodiments. However, these embodiments are provided so that this invention will be more completely understood, and are not intended to limit the scope of the invention. The scope of the invention is determined based on the matters claimed in the appended claims and modifications rationally derived therefrom.
Mode for Invention EXAMPLEThe steel slab having an alloy composition, illustrated in Table 1, was reheated in a temperature range of 1100° C. to 1300° C., and was then hot-rolled to manufacture a hot-rolled steel sheet. During the hot rolling, finish rolling was performed in a temperature range in which an outlet temperature is 800° C. to 950° C. Then, cooling was performed to perform coiling at a coiling temperature illustrated in Table 2.
A fraction (an area fraction) of an oxide, located within 10 μm in a thickness direction from a surface of each manufactured hot-rolled steel sheet, was observed, and a thickness of the grain boundary oxidation was measured together.
Regarding the fraction of the oxide and the thickness of the grain boundary oxidation, a cross section was measured using the scanning electron microscopy (SEM), a fraction of an oxide was measured using the oxide picture and image analysis, and a thickness of the grain boundary oxidation was measured.
Moreover, a shape of each hot-rolled steel sheet was observed with naked eyes to check for unevenness, and the like, and the presence or absence of scale peeling was checked to evaluate whether or not a defect occurred.
In this case, in the case of the shape of a steel sheet, when a difference between a high portion and a low portion of a wave of an edge portion is 10 mm or more, it was determined as a defective.
As illustrated in Tables 1 and 2, in the case of Inventive Examples 1 to 9, satisfying not only the alloy composition but also the component relationship (Relations 1 and 2), proposed in the present disclosure, it was confirmed that a depth of the grain boundary oxidation was within 2 μm, a shape was good, and a scale defect did not occur.
On the other hand, in the case of Comparative Examples 1 to 8, in which a hardenability index (HI) value, represented by Relation 1, is 0 or more, but a value of Relation 2 does not satisfy the present disclosure, it was confirmed that a depth of the grain boundary oxidation exceeded 2 μm, and a maximum depth was 21 μm. Moreover, in the case of Comparative Examples 1 to 8, as a depth of the grain boundary oxidation becomes great, it was confirmed that an area ratio of the oxide, located within 10 μm from a surface, is increased.
Further, regarding steel 11 in which a value of Relation 2 is outside of the present disclosure, a shape is poor in the case of Comparative Example 8 in which a coiling temperature is relatively low, while scale peeling occurs when a coil is uncoiled after coiling since a scale thickness is significantly great in the case of Comparative Example 7 in which the coiling temperature is relatively high.
On the other hand, in the case of Contrast Example, in which an HI value, represented by Relation 1, is less than 0, it was confirmed that occurrence of the grain boundary oxidation is not significant.
In the case of Comparative Example 6, it was confirmed that an oxide such as Cr, Mn, Si, Al, or the like, is formed inside a grain boundary and base metal. However, in the case of Inventive Example 7, it was confirmed that almost no oxide is generated in a grain boundary and base metal.
Claims
1. A high-carbon hot-rolled steel sheet having an excellent surface quality, comprising: 0.3 wt % to 1.3 wt % of carbon (C), 0.01 wt % to 0.5 wt % of silicon (Si), 0.3 wt % to 2.0 wt % of manganese (Mn), 0.1 wt % or less, excluding 0 wt % of aluminum (Al), 5.0 wt % or less, excluding 0 wt % of chromium (Cr), further comprising: one or more selected from the group consisting of 2.0 wt % or less of molybdenum (Mo), 0.005 wt % to 0.1 wt % of antimony (Sb), 0.5 wt % or less of vanadium (V), 0.5 wt % or less of copper (Cu), and 2.0 wt % or less of nickel (Ni), and comprising: the balance of iron (Fe) and other inevitable impurities,
- wherein a hardenability index (HI) value, represented by Relation 1, is 0 or more, and Relation 2 is satisfied, (HI)=−5.69+(4.43×C)+(3.71×Mn)−(4.5×Si)+(1.77×Ni)+(6.18×Cr)+(12.0×Mo)−(43.6×Cu)+(48.1×V)≥0 [Relation 1] Mo+(10×Sb)−(0.1×Cr)≥0.14 [Relation 2]
- where the content of each element means weight %.
2. The high-carbon hot-rolled steel sheet having an excellent surface quality of claim 1, wherein the hot-rolled steel sheet comprises 0.03 wt % or less of phosphorous (P), 0.02 wt % or less of sulfur (S), and 0.01 wt % or less of nitrogen (N).
3. The high-carbon hot-rolled steel sheet having an excellent surface quality of claim 1, wherein the hot-rolled steel sheet includes a ferrite and pearlite composite structure as a microstructure.
4. The high-carbon hot-rolled steel sheet having an excellent surface quality of claim 1, wherein the hot-rolled steel sheet has an area fraction of an oxide, located within 10 μm in a thickness direction from a surface, is 5% or less.
5. A method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality, comprising:
- reheating a steel slab in a temperature range of 1100° C. to 1300° C., the steel slab comprising: 0.3 wt % to 1.3 wt % of carbon (C), 0.01 wt % to 0.5 wt % of silicon (Si), 0.3 wt % to 2.0 wt % of manganese (Mn), 0.1 wt % or less, excluding 0 wt % of aluminum (Al), 5.0 wt % or less, excluding 0 wt % of chromium (Cr), further comprising: one or more selected from the group consisting of 2.0 wt % or less of molybdenum (Mo), 0.005 wt % to 0.1 wt % of antimony (Sb), 0.5 wt % or less of vanadium (V), 0.5 wt % or less of copper (Cu), and 2.0 wt % or less of nickel (Ni), and comprising: the balance of iron (Fe) and other inevitable impurities, and in which an HI value, represented by Relation 1, is 0 or more and Relation 2 is satisfied;
- producing a hot-rolled steel sheet by rough rolling and finish rolling the reheated steel slab; and
- coiling the hot-rolled steel sheet in a temperature range of 500° C. to 710° C. after cooling, (HI)=−5.69+(4.43×C)+(3.71×Mn)−(4.5×Si)+(1.77×Ni)+(6.18×Cr)+(12.0×Mo)−(43.6×Cu)+(48.1×V)≥0 [Relation 1] Mo+(10×Sb)−(0.1×Cr)≥0.14 [Relation 2]
- where the content of each element means weight %.
6. The method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality of claim 5, wherein the steel slab includes: 0.03 wt % or less of phosphorous (P), 0.02 wt % or less of sulfur (S), and 0.01 wt % or less of nitrogen (N).
7. The method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality of claim 5, wherein the finish rolling is performed in a temperature range, in which an inlet temperature is 900° C. to 1100° C. and an outlet temperature is 800° C. to 950° C.
8. The method for manufacturing a high-carbon hot-rolled steel sheet having an excellent surface quality of claim 5, wherein an area fraction of an oxide, located within 10 μm in a thickness direction from a surface of the hot-rolled steel sheet after the coiling, is 5% or less.
Type: Application
Filed: Dec 8, 2017
Publication Date: Mar 5, 2020
Inventor: Deuk-Jung Kim (Pohang-si, Gyeongsangbuk-do)
Application Number: 16/468,114
