JP4140573B2 - Hot rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a method for producing the same, and in particular, is excellent in press formability suitable as a material for automobile undercarriage parts, compressor shells and covers such as air conditioners and refrigerators, and various pressure vessels. Both relate to a hot-rolled steel sheet having excellent secondary work brittleness resistance and fatigue resistance and a method for producing the same.

  In recent years, demands for improving the fuel efficiency of automobiles have increased rapidly from the viewpoint of saving energy and preventing environmental pollution. For this reason, in order to reduce the weight of the vehicle body, the number of parts is reduced, and switching to a thin high-strength steel sheet is being considered.

  However, the reduction in the number of parts leads to a complicated shape of individual parts. For this reason, recently, there is a demand for improving the press formability of hot-rolled steel sheets, which are materials that have not been considered much in the past, to deal with the complexity of part shapes. Increasingly, so-called “ultra-low carbon IF steel sheets” having a C content reduced to 0.0080% or less have been put to practical use in lower arms of some types of vehicles. The “IF steel” is a matrix in which the content of C and N, which are interstitial solid solution elements in Fe, is reduced as much as possible, and Ti or Nb or the like that forms a compound with C and N is added in an equivalent amount or more. It is a high-purity steel in which solid solution C and solid solution N are completely removed from the steel.

  The above “ultra-low carbon IF steel sheet” has large elongation and excellent stretch flangeability. However, from the viewpoint of imparting press formability, the coil is usually wound at a relatively high temperature of 650 ° C. or higher, so that C in the steel is fixed as a precipitate of Ti or Nb, and as a result, the grain boundary strength of the steel is increased. When severely pressed and severely pressed, brittle fracture (hereinafter referred to as “longitudinal crack”) is applied to the steel sheet itself or to the part when the processed part is used in a cold region. May occur. Furthermore, since a repetitive load is applied to the undercarriage parts of the automobile due to vibration during running or vibration from the engine, high fatigue strength is required.

  On the other hand, as in the case of steel plates used for automobile undercarriage parts, steel plates used for compressor shells and covers such as air conditioners and refrigerators, and also steel plates used for various pressure vessels, Formability and low-temperature toughness after processing are required, and containers having a fluctuating pressure are also required to have fatigue resistance.

  For this reason, “Patent Document 1” describes “a good burring high-tensile steel plate excellent in fatigue resistance”, and “Patent Document 2” describes a “manufacturing method of a high-ductility hot-rolled steel plate excellent in longitudinal crack resistance”. Document 3 proposes “a hot-rolled steel sheet for ultra deep drawing excellent in secondary work brittleness resistance and a manufacturing method”.

JP-A-7-90483 JP-A-6-122923 JP 2000-239790 A

  The object of the present invention is excellent in press formability, secondary work brittleness resistance and fatigue resistance, which are suitable as materials for automobile undercarriage parts, compressor shells and covers for air conditioners and refrigerators, and various pressure vessels. It is providing the hot-rolled steel plate excellent also in the characteristic, and its manufacturing method.

  The specific goals of the press formability, secondary work brittleness resistance and fatigue resistance characteristics of the present invention are as follows.

(B) Press formability:
Three discs each having a diameter of 200 mm are collected from a steel plate, and no cracks are generated when each of the discs is deep-drawn into a cylinder having a diameter of 100 mm with a drawing ratio of 2.0.

(B) Secondary processing brittleness resistance:
The embrittlement temperature is −40 ° C. or lower. Here, the “embrittlement temperature” means that a cylinder having a diameter of 100 mm obtained by deep drawing from a steel sheet is cooled to various temperatures, and then a frustoconical portion is formed at the mouth of the cylinder. When a punch having a shape is placed, a weight having a mass of 20 kg is dropped from a height of 5 m on the punch, and stress corresponding to shocking stretch flange processing is applied to the mouth edge of the cylinder. The lower limit cooling temperature at which no brittle longitudinal cracks occur in the part.

(C) Fatigue resistance:
The durability ratio in the plane bending fatigue test of a steel sheet is 0.52 or more. Here, the above-mentioned “durability ratio” means a double-bending plane bending fatigue using a No. 1 test piece (symbol 1-20) described in JIS Z 2275 (1978) taken from a steel plate, with a stress ratio of −1. the number of repetitions and the stress to investigate the relationship between the amplitude value, determine the resulting stress amplitude value that was broken at 10 ⁶ times of repeated cycles from fatigue curve, the tensile strength of the stress amplitude value of said at break when tested The value divided by (TS).

  The technique proposed in the above-mentioned Patent Document 1 disperses nitrides of a specific size in the vicinity of the steel sheet surface layer after adding either one or both of Ti and Nb to ultra-low carbon steel to form IF steel. By precipitating, it is intended to obtain a steel sheet having excellent workability and fatigue resistance. However, the optimum structure is only described as a ferrite structure, among which bainitic ferrite having excellent fatigue resistance and polygonal ferrite having excellent workability and secondary work brittleness resistance, and an optimal area ratio and crystal The particle size is not disclosed, and it is not necessarily excellent in fatigue resistance. Furthermore, no consideration has been given to secondary work embrittlement resistance, and no suggestion has been made.

  The technique proposed in Patent Document 2 is that after adding Ti to ultra-low carbon steel to make IF steel, or after adding Ti and Nb in combination to make IF steel, winding after hot rolling A steel plate with excellent workability and secondary work brittleness resistance is obtained by reducing the Ti carbide precipitation or Ti carbide and Nb carbide precipitation by setting the taking temperature to 600 to 710 ° C. and the high temperature side. It is about to try. However, no consideration has been given to fatigue resistance, and no suggestion has been made.

  The technique proposed in Patent Document 3 is to improve the workability and the resistance to secondary work brittleness by making the ferrite crystal grain size of the steel sheet after hot rolling less than 25 μm on average for the steel to which Ti and B are added in combination. It is intended to obtain an excellent steel sheet. However, in the case of Patent Document 3, the structure is simply described as a ferrite structure as in Patent Document 1, and bainitic ferrite having excellent fatigue resistance characteristics and workability and secondary processing resistance are included therein. The optimum area ratio and crystal grain size of polygonal ferrite having excellent brittleness are not disclosed, and it is not necessarily excellent in fatigue resistance characteristics, and is not even suggested for fatigue resistance characteristics.

  Therefore, as a result of intensive research, in order to obtain a hot-rolled steel sheet having the press formability, secondary work brittleness resistance and fatigue resistance characteristics described in (a) to (c) above, the following (a) to (d) ) Was found to be important.

  (A) The structure is a two-phase structure of bainitic ferrite having a high dislocation density and polygonal ferrite having a low dislocation density, and the area ratio is optimized.

  (B) Refine bainitic ferrite and polygonal ferrite.

  (C) To suppress the hardness of polygonal ferrite.

  (D) The precipitates of (Ti, Nb) (N, C) should not be coarsened in the solidification stage and the hot rolling stage. The precipitates of (Ti, Nb) (N, C) are TiN, TiC, Ti (C, N), NbN, NbC, Nb (C, N), (Ti, Nb) C, ( Ti, Nb) N, (Ti, Nb) (C, N).

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in the hot-rolled steel sheets shown in the following (1) to (4) and the hot-rolled steel sheets shown in (5) to (6).

  (1) By mass%, C: less than 0.0030%, Si: 0.1% or less, Mn: 1.0% or less, P: 0.02% or less, S: 0.015% or less, Ti: 0 .015 to 0.10%, Al: 0.10% or less, N: 0.005% or less, and B: 0.0010 to 0.0050%, with the balance having a chemical composition of Fe and impurities The structure is composed of bainitic ferrite having an average crystal grain size of 5 to 30 μm and polygonal ferrite having an average crystal grain size of 5 to 30 μm, and the area ratio of the bainitic ferrite is 1 to 80%, A hot-rolled steel sheet characterized in that the hardness of polygonal ferrite is 120 or less in terms of Vickers hardness.

  (2) The hot-rolled steel sheet according to the above (1), which contains Nb: 0.002 to 0.04% in mass% instead of part of Fe.

  (3) In place of a part of Fe, in mass%, one or two of Ni: 0.005 to 1.0% and Cr: 0.005 to 1.0% are contained (1 ) Or (2).

  (4) Instead of a part of Fe, by mass%, Ca: 0.0005 to 0.050% and REM (rare earth element): 0.0005 to 0.050% of one or two of them The hot rolled steel sheet according to any one of (1) to (3) above.

  (5) A method for producing a hot-rolled steel sheet in which a molten steel having the chemical composition according to any one of (1) to (4) above is cast and rolled, wherein the casting step is performed from a liquidus temperature of the molten steel. The average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range up to the phase wire temperature is 0.3 to 1.0 ° C./second, and the steel ingot is hot-rolled and wound up Up to 1100 ° C., the ingot having a temperature of 1100 ° C. or higher is hot-rolled to complete rolling at a temperature of 950 to 880 ° C., and primary cooling is started 0.5 to 2 seconds after the completion of rolling. After cooling to 770 ° C. to 630 ° C. at an average cooling rate of ˜200 ° C./second, then intermediate cooling for 1 to 15 seconds, and then secondary cooling to 620 to 400 ° C. at an average cooling rate of 5 ° C./second or more A method for producing a hot-rolled steel sheet, which is wound up at a temperature.

  (6) After hot rolling, the steel ingot is roughly rolled into a sheet bar, the sheet bar is heated, the average temperature in the longitudinal direction of the sheet bar is 950 ° C. or more, and the maximum and minimum temperatures in the longitudinal direction The method for producing a hot-rolled steel sheet according to (5) above, wherein the difference is made 150 ° C. or less and then finish rolling.

  The term “bainitic ferrite” as used in the present invention refers to a material in which lath ferrite is grown from an austenite grain boundary, and looks like a structure in which a prior austenite grain boundary remains by observation with an optical microscope. And when this "bainitic ferrite" is observed with an electron microscope, the dislocation density is high.

  On the other hand, “polygonal ferrite” refers to a crystal that does not retain the prior austenite grain boundary when observed with an optical microscope, and particularly when observed with an electron microscope, the dislocation density is low. It can be easily distinguished from “Bainitic Ferrite”.

  The “average crystal grain size” in the present invention refers to a value obtained by multiplying the average intercept length obtained by the so-called “intercept method” by 1.13.

  “REM (rare earth element)” is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements.

  The “steel ingot” as used in the present invention means one containing “slab” as defined in JIS G 0203 (1984).

  “Average cooling rate of the cross section perpendicular to the casting direction of the steel ingot” means the steel ingot when it is called a steel ingot including the case where a solidified shell is formed in the mold or continuous casting machine and the inside is in a molten state. The average value of the cooling rate in the whole area | region from the surface part in the cross section perpendicular | vertical to the casting direction of this to the center part.

  The “average cooling rate” of the primary cooling after the completion of rolling and the secondary cooling after the intermediate air cooling is obtained by dividing the temperature difference before and after cooling the steel sheet surface by the cooling time.

  “Air cooling” refers to air cooling and forced air cooling.

  In addition, the average temperature in the longitudinal direction of the sheet bar refers to the average value of the surface temperatures from the front end to the tail end in the rolling direction of the sheet bar at the center of the width of the sheet bar.

  Hereinafter, the invention relating to the hot-rolled steel sheet of the above (1) to (4) and the invention relating to the manufacturing method of the hot-rolled steel sheet of (5) to (6) are respectively referred to as “the invention of (1)” to “(6). "Invention of". Also, it may be collectively referred to as “the present invention”.

  The hot-rolled steel sheet of the present invention has excellent workability such as elongation and drawability, as well as excellent secondary work brittleness resistance and fatigue resistance, so that it can be used in automobile undercarriage parts, air conditioners, refrigerators, etc. It can be used as a material for compressor shells and covers, and various pressure vessels. This hot-rolled steel sheet can be manufactured relatively easily by the method of the present invention.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of steel C: less than 0.0030% In order to make the structure of a hot-rolled steel sheet a two-phase structure of bainitic ferrite and polygonal ferrite, the C content is made less than 0.0030%. There is a need to. When the C content is less than 0.0030%, the elongation characteristics are improved, the deep drawing formability is improved, and the effect that the deep drawing forming process is further facilitated is also obtained. When the C content is 0.0030% or more, not only the desired two-phase structure of bainitic ferrite and polygonal ferrite cannot be obtained, but also coarse TiC, NbC and (Ti, Nb) C are formed. In some cases, the secondary work embrittlement resistance and fatigue resistance are significantly reduced. Therefore, the C content is less than 0.0030%. The lower limit of the C content is not particularly limited, but is preferably 0.0005% from the viewpoint of refining costs.

Si: 0.1% or less Si causes embrittlement of steel and causes deterioration of workability and secondary work brittleness resistance. In particular, when the content exceeds 0.1%, workability and secondary work brittleness resistance are significantly reduced. Therefore, the Si content is set to 0.1% or less. Although the Si content is desirably reduced as much as possible, the lower limit is preferably set to 0.005% from the viewpoint of the balance between the cost for reducing the Si content and the improvement degree of the material.

Mn: 1.0% or less Mn increases the strength of the steel, but decreases the ductility. In particular, when the Mn content exceeds 1.0%, the ductility decreases and the elongation characteristics are significantly deteriorated. . Therefore, in the present invention where it is necessary to improve the elongation characteristics, the Mn content is set to 1.0% or less. When extremely good workability is required, the upper limit of the Mn content is desirably 0.5%. The lower limit of the Mn content is not particularly limited, but Mn is effective in refining the ferrite structure because it has an action of lowering the transformation point, and Mn also has an action of suppressing red heat brittleness. . For this reason, it is preferable to contain Mn 0.03% or more from a viewpoint of ensuring said effect | action.

P: 0.02% or less P is an element that embrittles steel, and is desirably reduced as much as possible to improve secondary work embrittlement resistance. However, if its content is 0.02% or less, The above-mentioned secondary processing brittleness resistance (b), that is, the brittle temperature of −40 ° C. or less can be secured. Therefore, the content of P is set to 0.02% or less. Although the P content is desirably reduced as much as possible, the lower limit is preferably set to 0.002% from the viewpoint of the balance between the cost for reducing the P content and the improvement degree of the material.

S: 0.015% or less Since S produces sulfide and lowers the elongation characteristics, it is necessary to reduce it. Particularly, when the S content exceeds 0.015%, the deterioration of the elongation characteristics becomes remarkable. . Therefore, the content of S is set to 0.015% or less. The S content is preferably 0.010% or less. Although the minimum of S content is not specifically limited, From a viewpoint of the refining cost for S content reduction, it is preferable to set it as 0.0005%.

Ti: 0.015 to 0.10%
Ti is an extremely important element for refinement of bainitic ferrite and polygonal ferrite. Further, Ti has an action of delaying the formation of polygonal ferrite and promoting the formation of bainitic ferrite, and is effective in producing bainitic ferrite in the structure of an extremely low C steel. These effects are obtained when the Ti content is 0.015%. On the other hand, if the content exceeds 0.10%, the above effect is saturated and the cost is increased. Therefore, the Ti content is set to 0.015 to 0.10%. Note that the Ti content is preferably 0.02 to 0.07%.

Al: 0.10% or less Al acts as a deoxidizing agent, but if it is contained in a large amount, surface defects caused by alumina or the like are generated, and the effect of improving the material commensurate with the content cannot be expected. In particular, when the content exceeds 0.10%, generation of surface defects becomes remarkable. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.08% or less. The lower limit of the Al content is not particularly specified, but is preferably 0.001% and more preferably 0.005% from the viewpoint of the stability of the steelmaking process.

N: 0.005% or less N is combined with Ti at the stage of the steel ingot to form coarse TiN, which significantly deteriorates the secondary work brittleness resistance and fatigue resistance of steel, so its content is reduced as much as possible. However, if it is 0.005% or less, it can be rendered harmless by optimizing the casting conditions of the steel ingot. Therefore, the N content is set to 0.005% or less. Although the N content is desirably reduced as much as possible, the lower limit is preferably set to 0.0010% from the viewpoint of the balance between the cost for reducing the N content and the improvement degree of the material.

B: 0.0010 to 0.0050%
B is an extremely important element in the present invention that improves the secondary work brittleness resistance. In the case of extremely low carbon steel, the bond strength at the ferrite grain boundary decreases, and secondary work embrittlement is likely to occur in the grain boundary fracture mode, but B segregates at the grain boundary, and the bond strength of the ferrite grain boundary is It has the effect of suppressing the decrease and increasing the secondary work brittleness resistance. This effect is obtained when the B content is 0.0010% or more. However, if B is contained in excess of 0.0050%, the hot workability deteriorates, and the effect of improving secondary work embrittlement resistance commensurate with the content cannot be obtained. Therefore, the content of B is set to 0.0010 to 0.0050%. The B content is preferably 0.0015 to 0.0045%.

  From the above, the chemical composition of the hot-rolled steel sheet according to the invention of (1) is defined as including the elements from C to B in the above-described range, with the balance being Fe and impurities.

  In addition to the above-described component elements, the hot-rolled steel sheet according to the present invention includes, as necessary, one or more elements selected from at least one of groups A to C described below as optional addition elements. It may be added and contained.

  Hereinafter, the optional additive element will be described.

Group A: Nb: 0.002 to 0.04%
Nb has the effect of refining crystal grains. In order to reliably obtain this effect, the Nb content is preferably 0.002% or more. However, even if Nb is contained in excess of 0.04%, the above effect is saturated and an effect commensurate with the content cannot be obtained, resulting in an increase in cost. Therefore, the content when Nb is added is preferably 0.002 to 0.04%. In addition, the more preferable content of Nb is 0.01 to 0.03%.

Group B: Ni: 0.005-1.0% and Cr: 0.005-1.0%
Both Ni and Cr have the effect of refining the structure and improving the secondary work brittleness resistance and fatigue resistance. In order to reliably obtain this effect, it is preferable to set at least one of the contents to 0.005% or more. However, even if Ni or Cr is contained in excess of 1.0%, the above effect is saturated and an effect commensurate with the content cannot be obtained, and the cost is increased. Therefore, the contents of Ni and Cr are preferably 0.005 to 1.0% for Ni and 0.005 to 1.0% for Cr. A more preferable content of Ni is 0.1 to 0.5%, and a more preferable content of Cr is 0.1 to 0.5%. In addition, Ni and Cr can be added only by any 1 type or 2 types of composite.

Group C: Ca: 0.0005% to 0.050%, REM (rare earth element): 0.0005% to 0.050%
Both Ca and REM have the effect of increasing secondary work brittleness resistance. In order to surely obtain this effect, it is preferable that at least one of the contents is 0.0005% or more. However, when Ca or REM is contained in excess of 0.050%, the cleanliness is significantly lowered. Therefore, the contents of Ca and REM when added are preferably 0.0005% to 0.050% for Ca and 0.0005% to 0.050% for REM. The more preferable content of Ca is 0.0010 to 0.010%, and the more preferable content of REM is 0.0010 to 0.010%. Said Ca and REM can be added only by any 1 type or 2 types of composite. REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and as described above, the REM content in the present invention refers to the total content of the above elements.

  Therefore, the chemical composition of the hot-rolled steel sheet according to the invention of (2) is defined as including Nb in the above-described range instead of a part of Fe of the invention of (1) described above.

  Further, regarding the chemical composition of the hot-rolled steel sheet according to the invention of (3), instead of a part of Fe of the invention of (1) or (2) described above, of the above-described ranges of Ni and Cr It was defined that 1 type or 2 types were included.

  Further, regarding the chemical composition of the hot-rolled steel sheet according to the invention of (4), Ca and REM in the above-described range are substituted for a part of Fe of any of the inventions of (1) to (3) described above. It was prescribed | regulated that 1 type or 2 types was included.

  Steel having the above-described chemical composition is produced by, for example, a converter or an electric furnace. For the production of the steel ingot, any means of “ingot-making method” or “continuous casting method” injected into the mold may be used.

(B) Structure of steel plate (B-1) Definition of phase The structure of the steel plate is an important element of the present invention, and the structure of the hot-rolled steel sheet according to the inventions of (1) to (4) is It consists of bainitic ferrite having an average crystal grain size of 5 to 30 μm and polygonal ferrite having an average crystal grain size of 5 to 30 μm, and the area ratio of the bainitic ferrite must be 1 to 80%.

  This is necessary to provide the hot-rolled steel sheet having the chemical composition described in the item (A) with all of the above-described press formability, secondary work brittleness resistance and fatigue resistance characteristics (a) to (c). It is a rule.

  First, when the structure of the hot-rolled steel sheet includes a structure other than bainitic ferrite and polygonal ferrite, at least the above-described press formability, secondary work brittleness resistance and fatigue resistance characteristics of (i) to (iii) are described. One cannot be achieved. Here, the phases other than bainitic ferrite and polygonal ferrite are, for example, a pearlite structure, a bainite structure, a martensite structure, and a retained austenite structure (a structure in which austenite remains untransformed). When the hot rolled steel sheet contains at least one of a pearlite structure, a bainite structure, a martensite structure, and a retained austenite structure, press formability deteriorates. This is because the pearlite structure, bainite structure, martensite structure, and retained austenite structure are extremely harder than bainitic ferrite and polygonal ferrite, and cracks from the structure interface during deep drawing due to the difference in hardness. It is because it becomes easy to generate | occur | produce.

  Even when the structure of the hot-rolled steel sheet is composed of bainitic ferrite and polygonal ferrite, if the average crystal grain size of at least one phase exceeds 30 μm, the crack propagation suppressing effect at the grain boundary is reduced, The secondary work embrittlement resistance and fatigue resistance are reduced, and the desired secondary work embrittlement resistance (that is, the embrittlement temperature of (b) below −40 ° C.) and the fatigue resistance (that is, 0 of (c) above). None of the durability ratio of .52 or higher). On the other hand, when the average crystal grain size of at least one phase is less than 5 μm, the yield point (YP) is increased, the moldability is lowered, and the desired press moldability cannot be obtained.

  Furthermore, even when the structure of the hot-rolled steel sheet is composed of bainitic ferrite having an average crystal grain size of 5 to 30 μm and polygonal ferrite having an average crystal grain size of 5 to 30 μm, the area ratio of bainitic ferrite is 1%. If it is less than the desired fatigue resistance characteristics cannot be obtained. This is because bainitic ferrite having a high dislocation density has an effect of suppressing the propagation of fatigue cracks in the crystal grains, and by further fixing B to the dislocations, the effect of suppressing the crack propagation in the crystal grains is further increased. This is because, when the area ratio of bainitic ferrite is less than 1%, the above effect cannot be obtained sufficiently.

  On the other hand, even when the structure of the hot-rolled steel sheet is composed of bainitic ferrite having an average crystal grain size of 5 to 30 μm and polygonal ferrite having an average crystal grain size of 5 to 30 μm, the area ratio of bainitic ferrite is 80%. If it exceeds 1, desired secondary work brittleness resistance cannot be obtained. When the area ratio of bainitic ferrite exceeds 80%, the desired secondary work brittleness resistance cannot be obtained because B is fixed to dislocations in bainitic ferrite and contributes to grain boundary strengthening. This is because grain boundary segregation is reduced.

  Accordingly, the hot-rolled steel sheet according to the inventions of (1) to (4) is composed of bainitic ferrite having an average crystal grain size of 5 to 30 μm and polygonal ferrite having an average crystal grain size of 5 to 30 μm. And the area ratio of the bainitic ferrite is 1 to 80%.

  In addition, the preferable range of the area ratio of bainitic ferrite is 5 to 80%.

  The term “bainitic ferrite” as used in the present invention refers to a structure in which lath ferrite is grown from an austenite grain boundary, and it looks like a structure in which an old austenite grain boundary remains by observation with an optical microscope. As described above, “polygonal ferrite” refers to a crystal in which a prior austenite grain boundary does not remain by observation with an optical microscope and looks circular. When observed with an electron microscope, the dislocation density of “bainitic ferrite” is high, whereas the dislocation density of “polygonal ferrite” is low, so both can be easily distinguished by observing with an electron microscope. it can.

  As described above, the “average crystal grain size” in the present invention refers to a value obtained by multiplying the average intercept length obtained by the so-called “intercept method” by 1.13.

(B-2) Hardness of polygonal ferrite Even when the structure of the hot-rolled steel sheet having the chemical composition described in (A) is composed of the phase described in (B-1), If the hardness of the ferrite exceeds 120 in terms of Vickers hardness (hereinafter, Vickers hardness is referred to as “HV hardness”), workability deteriorates and desired press formability cannot be obtained.

  Therefore, in the hot rolled steel sheet according to the inventions of (1) to (4), the hardness of the polygonal ferrite is HV hardness of 120 or less. In addition, when it is desired to place more emphasis on workability, the HV hardness of polygonal ferrite is preferably 110 or less.

  The lower limit of the HV hardness of the polygonal ferrite is not particularly specified, but in the case of the steel having the chemical composition described in the item (A), the lower limit of the HV hardness of the polygonal ferrite is about 90. become.

(C) Manufacturing method of hot-rolled steel sheet The hot-rolled steel sheet according to the invention of (1) to (4) having the chemical composition described in the item (A) and the structure described in the item (B) is, for example, The molten steel having the chemical composition described in the item (A) is cast into a steel ingot. “The casting process is perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature to the solidus temperature of the molten steel. The average cooling rate of a simple cross section is 0.3 to 1.0 ° C./second, and the step of hot rolling the steel ingot to wind it up is a steel ingot having a temperature of 1100 ° C. or higher. Hot rolling to complete the rolling at a temperature of 950 to 880 ° C., and starting the primary cooling 0.5 to 2 seconds after the completion of rolling, with an average cooling rate of 40 to 200 ° C./second, 770 ° C. to 630 ° C. Then, after intermediate cooling with air for 1 to 15 seconds, secondary cooling to 620 to 400 ° C. at an average cooling rate of 5 ° C./second or more According to the invention of (5), it can be manufactured relatively easily.

  That is, in the casting process of casting molten steel into a steel ingot, the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature to the solidus temperature of the molten steel is 0.3 to It is good to set it as 1.0 degree-C / sec.

  When the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature to the solidus temperature of the molten steel is less than 0.3 ° C / second, it is concentrated in the above temperature range. In some cases, Ti and N are combined to precipitate coarse TiN. Coarse TiN remains in the steel sheet after hot rolling and deteriorates the secondary work brittleness resistance and fatigue resistance, and does not contribute to the refinement of austenite grains. Does not have the effect of miniaturizing nittic ferrite. On the other hand, if the above average cooling rate exceeds 1.0 ° C./second, the cooling is too strong and the steel ingot becomes supercooled and steel ingot cracking may occur during casting. In some cases, the polygonal ferrite and bainitic ferrite are excessively refined.

  The average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature to the solidus temperature of the molten steel is 0.4 to 0.8 ° C./second. preferable.

  The cast ingot is preferably hot-rolled at a temperature of 1100 ° C. or higher.

  If the temperature of the steel ingot when it is subjected to hot rolling is less than 1100 ° C, the rolling completion temperature may be too low to obtain a desired structure, and scale removal may become insufficient and Occurrence may be noticeable. In addition, since the TiN precipitated in the steel ingot does not become small, the austenite particle size does not become small, and therefore, it is sometimes impossible to achieve miniaturization of polygonal ferrite. The steel ingot is more preferably hot-rolled in a state where the temperature is 1130 ° C or higher.

  When reheating is performed in order to suppress the coarsening of austenite grains and obtain fine bainitic ferrite and polygonal ferrite after completion of rolling, the upper limit of the reheating temperature is preferably 1300 ° C.

  In hot rolling, it is preferable to complete the rolling at a temperature of 950 to 880 ° C.

  When the completion temperature of hot rolling exceeds 950 ° C, austenite recrystallizes and coarsens, and fine bainitic ferrite and polygonal ferrite are not formed, and desired secondary work brittleness resistance and fatigue resistance are obtained. It may not be possible. On the other hand, when the completion temperature of hot rolling is less than 880 ° C., polygonal ferrite region rolling occurs and formability deteriorates, and desired press formability may not be obtained. The hot rolling is more preferably completed at 930 to 880 ° C.

  After completing the hot rolling, it is preferable to start primary cooling after 0.5 to 2 seconds and to cool to 770 to 630 ° C. at an average cooling rate of 40 to 200 ° C./second.

  After completing the hot rolling, when the primary cooling is started within 0.5 seconds, the average grain size of polygonal ferrite is less than 5 μm because there are many austenite grain boundaries and dislocations that become precipitation sites of polygonal ferrite. As a result, the formability deteriorates and the desired press formability may not be obtained. On the other hand, if the time from the completion of hot rolling to the start of primary cooling exceeds 2 seconds, the average crystal grain size of polygonal ferrite exceeds 30 μm, and the desired secondary work brittleness resistance and fatigue resistance characteristics cannot be obtained. There is a case. In addition, after completing the hot rolling, it is more preferable to start primary cooling after 0.5 to 1 second.

  Even if the primary cooling is started 0.5 to 2 seconds after the completion of the hot rolling, if the average cooling rate is less than 40 ° C./second, the formation of polygonal ferrite is fast, A desired structure may not be obtained due to a phase structure. On the other hand, when the average cooling rate exceeds 200 ° C./second, the formation of polygonal ferrite is slow, and bainitic ferrite may exceed 80% in area ratio and desired characteristics may not be obtained. The average cooling rate in the primary cooling is more preferably 50 to 150 ° C./second.

  After completion of hot rolling, if the cooling is stopped at a temperature exceeding 770 ° C. even after primary cooling at an average cooling rate of 40 to 200 ° C./second after 0.5 to 2 seconds, polygonal ferrite Is formed quickly, resulting in a single-phase structure of polygonal ferrite, and desired characteristics may not be obtained. On the other hand, when the cooling is performed to a temperature lower than 630 ° C., the formation of polygonal ferrite is slow, and bainitic ferrite may exceed 80% in area ratio and desired characteristics may not be obtained. In addition, it is more preferable that the primary cooling performed at an average cooling rate of 40 to 200 ° C./second after 0.5 to 2 seconds after the completion of the hot rolling is stopped at 750 to 650 ° C.

  After the hot rolling is completed, the above-described primary cooling performed from 770 ° C. to 630 ° C. may be followed by intermediate air cooling for 1 to 15 seconds.

  When the intermediate air cooling time is less than 1 second, B in the polygonal ferrite grains cannot be sufficiently diffused into the grain boundary, the HV hardness of the polygonal ferrite exceeds 120, and the moldability is lowered. There is a case where the press molding cannot be obtained. Moreover, since the segregation of B to the grain boundary is not sufficient, desired desired secondary work embrittlement resistance may not be obtained. On the other hand, when the intermediate air cooling time exceeds 15 seconds, the precipitation of carbides of Ti and Nb increases, the HV hardness of polygonal ferrite exceeds 120, the formability decreases, and the desired press molding is obtained. There may not be. The intermediate air cooling time is more preferably 2 to 13 seconds, whereby the HV hardness of polygonal ferrite becomes stable and easily becomes 110 or less.

  After performing the above-described intermediate air cooling, it is preferable to perform secondary cooling to 620 to 400 ° C. at an average cooling rate of 5 ° C./second or more and take up at that temperature.

  When the average cooling rate is less than 5 ° C./second, the amount of scale generated increases and scale flaws may occur on the surface of the steel sheet, and grain growth of polygonal ferrite and bainitic ferrite occurs. In some cases, the average crystal grain size becomes large and desired characteristics cannot be obtained. The average cooling rate is more preferably 20 ° C./second or more. The upper limit of the average cooling rate does not need to be specified in particular, and may be an upper limit value determined from the size of the steel plate or the equipment surface.

  Even after secondary air cooling at an average cooling rate of 5 ° C./sec or higher after intermediate air cooling, if the cooling is stopped at a temperature exceeding 620 ° C. and wound at that temperature, polygonal ferrite is generated even after winding. In some cases, bainitic ferrite is almost disappeared and desired fatigue resistance characteristics cannot be obtained. On the other hand, if cooling is stopped at a temperature lower than 400 ° C. and winding is performed at that temperature, the heat removal behavior of the steel plate changes, so that the flatness of the steel plate collapses and winding trouble occurs, or the product due to deterioration of the flatness of the steel plate In some cases, flaws may occur, and the area ratio of bainitic ferrite increases due to the progress of the generation of bainitic ferrite, and the desired press formability and secondary work brittleness resistance may not be obtained. . The secondary cooling at the average cooling rate of 5 ° C./second or more is preferably stopped at 580 to 450 and wound at that temperature.

  Therefore, in the invention of (5), the casting step, that is, the step of casting the molten steel into the steel ingot is performed in the casting direction of the steel ingot in the temperature range from the liquidus temperature to the solidus temperature of the molten steel. An ingot having an average cooling rate of a vertical cross section of 0.3 to 1.0 ° C./second and a step of hot rolling the steel ingot and winding it up at a temperature of 1100 ° C. or more The steel is hot-rolled to complete the rolling at a temperature of 950 to 880 ° C., and the primary cooling is started 0.5 to 2 seconds after completion of the rolling, and the average cooling rate of 40 to 200 ° C./second is set to 770 ° C. to 630 ° C. After cooling to 0 ° C. and then intermediate air cooling for 1 to 15 seconds, secondary cooling to 620 to 400 ° C. at an average cooling rate of 5 ° C./second or more and winding at that temperature was performed.

  The “steel ingot” as used in the present invention includes “slab” and “average cooling rate of the cross section perpendicular to the casting direction of the ingot” means that a solidified shell is formed in a mold or a continuous casting machine. The average value of the cooling rate in the entire region from the surface portion to the central portion in the cross section perpendicular to the casting direction of the steel ingot when it is called a steel ingot including the case where the inside is in a molten state, 1 after the completion of rolling The “average cooling rate” of secondary cooling after secondary cooling or intermediate air cooling refers to the difference in temperature before and after cooling divided by the cooling time, and “air cooling” refers to cooling in the atmosphere and forced air cooling. As already mentioned.

  In the invention of (5) above, when the hot rolling of the steel ingot includes rough rolling and finish rolling, after the rough rolling to obtain a sheet bar, the sheet bar is heated to increase the length of the sheet bar. It is preferable that the average temperature is 950 ° C. or more and the difference between the maximum temperature and the minimum temperature in the longitudinal direction is 150 ° C. or less, and then finish rolling is performed.

  When the average temperature in the longitudinal direction of the sheet bar is less than 950 ° C., the hot rolling completion temperature is lower than 880 ° C., the workability may be lowered, and the desired press formability may not be obtained. Further, if the difference between the maximum temperature and the minimum temperature in the longitudinal direction of the sheet bar exceeds 150 ° C., the temperature fluctuation during finish rolling becomes large, the rolling load fluctuates, and operation trouble may occur.

  In order to prevent the rolling completion temperature from exceeding 950 ° C., the average temperature in the longitudinal direction when heating the sheet bar is preferably 1250 ° C. or less. On the other hand, the smaller the difference between the maximum temperature and the minimum temperature in the longitudinal direction of the sheet bar, the better, and 0 ° C. is most preferable.

  Therefore, in the invention of (6), after hot rolling, the steel ingot is roughly rolled to form a sheet bar, and then the sheet bar is heated to set the average temperature in the longitudinal direction of the sheet bar to 950 ° C. or more and the longitudinal direction. It was defined that the difference between the maximum temperature and the minimum temperature in the direction was 150 ° C. or less, and then finish rolling.

  The average temperature in the longitudinal direction of the sheet bar refers to the average value of the surface temperature from the front end to the tail end in the rolling direction of the sheet bar at the center of the width of the sheet bar.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Each molten steel having the chemical composition shown in Table 1 is cast by a test continuous casting machine to form a slab having a thickness of 200 to 290 mm, and each slab is shown in Tables 2 and 3 using a test continuous rolling apparatus. It was hot-rolled under conditions to finish a hot-rolled steel sheet having a thickness of 3 mm.

  The average cooling rate of the cross section perpendicular to the casting direction of the slab in the temperature range from the liquidus temperature to the solidus temperature of the molten steel (hereinafter referred to as “average slab solidification rate”) is determined by the slab thickness and the continuous casting machine. It was adjusted by changing the amount of water in the secondary cooling zone spray. The “average slab solidification rate” in Tables 2 and 3 is obtained by cutting out a cross section of the slab cooled to room temperature, measuring the dendrite secondary arm interval from the slab surface to the center at a pitch of 10 mm, and solidifying at each slab cross section position. After calculating the speed, they are obtained by averaging them.

  The following methods are used to collect specimens from each hot-rolled steel sheet with a thickness of 3 mm, and to investigate the structure investigation, tensile test, deep drawability test, secondary work brittleness test, plane bending fatigue test, and product wrinkle occurrence rate. It carried out in.

  Regarding test number 30 in Table 3, because of a rolling trouble, a hot-rolled steel sheet having a thickness of 3 mm could not be obtained, and thus none of the above investigations were performed. Moreover, about the test numbers 16, 18, 21, 23, and 24 of Table 3, since the crack generate | occur | produced by deep drawing, the secondary work brittleness resistance test was not implemented.

(1) Organizational survey:
About the cross section parallel to the rolling direction of each steel plate, using an optical microscope or an electron microscope, the phase was determined, and the average section length determined by the “section method” was multiplied by 1.13 to bainitic ferrite and The average crystal grain size of polygonal ferrite was determined, and the area ratio was also determined. The phase determination and area ratio were performed for the entire region in the plate thickness direction, and the average crystal grain size was determined for the so-called “t / 4” position and “3t / 4” position, where the plate thickness was t.

(2) Tensile test:
The No. 5 tensile test described in JIS Z 2201 (1998) was taken from each steel plate in parallel with the rolling direction, the tensile test was conducted at a crosshead speed of 10 mm / min, yield point (YP), tensile strength (TS). And elongation (El) was measured.

(3) Deep drawability test:
Three discs each having a diameter of 200 mm are sampled from each steel plate, each of the discs is deep drawn into a cylinder having a diameter of 100 mm with a deep drawing ratio of 2.0, and then the presence of cracks is investigated and deep drawn. The properties (press formability) were evaluated. Those with no cracks were considered to have good deep drawability, while those with cracks were considered to have poor deep drawability.

  The evaluation with a deep drawing ratio of 2.0 was that the parts under the car's undercarriage, the compressor shell and the pressure vessel were subjected to severe processing with a drawing ratio of about 2.0. based on.

(4) Secondary processing brittleness test:
The above-mentioned cylinder having a diameter of 100 mm obtained by deep drawing from each steel plate with a deep drawing ratio of 2.0 is cooled to various temperatures, and then a truncated cone-shaped punch is placed on the edge of the cylinder. On the other hand, when a weight with a mass of 20 kg is dropped from a height of 5 m and a stress corresponding to shocking stretch flange processing is applied to the edge of the cylinder, a brittle vertical crack occurs in the edge. The secondary processing brittleness resistance was evaluated using the lower limit cooling temperature as the embrittlement temperature. The steel plate having a lower embrittlement temperature has better secondary work embrittlement resistance.

  Note that, as already described, the aim of the present invention is that the embrittlement temperature is −40 ° C. or lower. This is because when used outdoors such as undercar parts of automobiles, it may be exposed to a low temperature range of about −40 ° C.

(5) Plane bending fatigue test:
Sample No. 1 (symbol 1-20) described in JIS Z 2275 (1978) was collected from each steel plate, and a double swing plane bending fatigue test was conducted with a stress ratio of −1, and the number of repetitions and stress amplitude values at break. From the obtained fatigue curve, the stress amplitude value fractured in 10 ⁶ repeated cycles was obtained, and the fatigue resistance characteristics were obtained by dividing the stress amplitude value by the tensile strength (TS). Evaluated.

Note that the value of the number of repeated fractures is 10 ⁶ based on the fact that no repeated stress is applied up to 10 ⁷ times, which is generally used for the fatigue limit in practice.
(6) Survey of product defect rate:
Visually inspect the scale wrinkles on each steel sheet surface and the product wrinkles due to deterioration of flatness, and the total mass of the part that is cut off in the whole area of the width and length of the steel plate containing the scale wrinkles, that is, the product defects relative to the total mass. Was determined as the product defect rate (%).

Product wrinkle incidence (%) = [product defective part mass (kg) / total mass (kg)] × 100
Tables 4 and 5 collectively show the results of the above investigations. In addition, “◯” and “x” in the column of deep drawability in Table 4 and Table 5 indicate that “no cracking occurred” and “ Indicates that “cracking has occurred”. In addition, in the case of any of the test numbers, the structure was a two-phase structure of bainitic ferrite and polygonal ferrite. Accordingly, a value obtained by subtracting “area ratio of bainitic ferrite” from 100 indicates “area ratio of polygonal ferrite”.

  From Table 4, the hot rolled steel sheets of Test Nos. 1 to 14 having the chemical composition, structure and polygonal ferrite HV hardness defined in the present invention have an elongation of 46.4% or more, and their deep drawability. Is clearly “◯” and excellent in press formability. All of the above-mentioned hot-rolled steel sheets have an “embrittlement temperature” of −40 ° C. or less, and their “durability ratios” are both 0.52 or more, and are excellent in secondary work brittleness resistance and fatigue resistance. It is clear that

On the other hand, even if it has the chemical composition defined in the present invention, the HV hardness of its structure or polygonal ferrite deviates from the conditions defined in the present invention in the case of a hot rolled steel sheet of test numbers 15-29. The hot-rolled steel sheets of Test Nos. 15, 19 and 20 do not meet the target of at least one of press formability (deep drawability), secondary work brittleness resistance and fatigue resistance characteristics. Polygonal ferrite and bainitic ferrite Since the average crystal grain size of each exceeded 30 μm, the secondary work brittleness resistance and fatigue resistance were inferior. Of the above, the hot rolled steel sheet of test number 15 contained a large amount of coarse TiN. Therefore, the elongation was as low as 39%.

  In the hot rolled steel sheets of test numbers 16 and 21, since the average crystal grain sizes of polygonal ferrite and bainitic ferrite are both less than 5 μm, the yield point is increased and the formability is lowered, and the desired press formability ( Deep drawability) was not obtained. In addition, in the case of test number 16 among the above, since cracks occurred in the slab, the generation rate of product defects was as high as 30%.

  The hot rolled steel sheets of test numbers 17 and 26 were inferior in secondary work brittleness resistance and fatigue resistance because the average crystal grain size of polygonal ferrite exceeded 30 μm. In addition, since scale flaws were generated in the hot rolled steel sheets having the test numbers 17 and 26, the generation rate of product wrinkles was as large as 20%.

  The hot-rolled steel sheets of test numbers 18, 23 and 28 were inferior in fatigue resistance because the area ratio of bainitic ferrite was less than 1%.

  The hot-rolled steel sheets of test numbers 22 and 27 were inferior in secondary work brittleness resistance because the area ratio of bainitic ferrite exceeded 80%. In the case of test number 28 among the above, since the winding temperature was as low as 380 ° C., the flatness of the steel sheet deteriorated and surface flaws occurred during winding. Therefore, the incidence of product defects was as high as 30%.

  The hot-rolled steel sheets of test numbers 24 and 25 were inferior in press formability (deep drawability) because the hardness of polygonal ferrite exceeded 120 in HV hardness.

  The hot rolled steel sheet of test number 29 was inferior in fatigue resistance because the area ratio of bainitic ferrite was less than 1%. In the case of this test number 29, the completion temperature of hot rolling was as low as 870 ° C., and rolling was performed in the polygonal ferrite region, so that processed polygonal ferrite was obtained. As a result, since the hardness of polygonal ferrite exceeded 120 in terms of HV hardness, it was inferior in press formability (deep drawability).

The hot-rolled steel sheet of the present invention has excellent workability such as elongation and drawability, as well as excellent secondary work brittleness resistance and fatigue resistance, so that it can be used in automobile undercarriage parts, air conditioners, refrigerators, etc. It can be used as a material for compressor shells and covers, and various pressure vessels.

Claims (6)

  In mass%, C: less than 0.0030%, Si: 0.1% or less, Mn: 1.0% or less, P: 0.02% or less, S: 0.015% or less, Ti: 0.015 0.10%, Al: 0.10% or less, N: 0.005% or less, and B: 0.0010 to 0.0050%, with the balance having a chemical composition of Fe and impurities, It consists of bainitic ferrite with an average crystal grain size of 5-30 μm and polygonal ferrite with an average crystal grain size of 5-30 μm, and the area ratio of the bainitic ferrite is 1-80%. A hot-rolled steel sheet characterized by having a Vickers hardness of 120 or less.   The hot-rolled steel sheet according to claim 1, which contains Nb: 0.002 to 0.04% by mass% instead of part of Fe.   It replaces with a part of Fe, and contains 1 type or 2 types of Ni: 0.005-1.0% and Cr: 0.005-1.0% by the mass%. The hot-rolled steel sheet described.   It replaces with a part of Fe, and contains 1 type or 2 types of Ca: 0.0005-0.050% and REM (rare earth element): 0.0005-0.050% by the mass%. The hot-rolled steel sheet according to any one of 1 to 3.   It is a manufacturing method of the hot-rolled steel plate which rolls after casting the molten steel which has the chemical composition in any one of Claim 1 to 4, Comprising: A casting process is temperature from the liquidus temperature of a molten steel to a solidus temperature. The average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the range is 0.3 to 1.0 ° C./second, and the process until the steel ingot is hot-rolled and wound is 1100 A steel ingot having a temperature of ℃ or higher is hot-rolled to complete the rolling at a temperature of 950 to 880 ° C, and the primary cooling is started 0.5 to 2 seconds after the completion of the rolling to 40 to 200 ° C / second. It is cooled to 770 ° C. to 630 ° C. at an average cooling rate, and then air-cooled for 1 to 15 seconds, followed by secondary cooling to 620 to 400 ° C. at an average cooling rate of 5 ° C./second or more and winding at that temperature. A method for producing a hot-rolled steel sheet, comprising: After hot rolling, the steel ingot is roughly rolled into a sheet bar, and then the sheet bar is heated to set the average temperature in the longitudinal direction of the sheet bar to 950 ° C. or more and the difference between the maximum temperature and the minimum temperature in the longitudinal direction. The method for producing a hot-rolled steel sheet according to claim 5, wherein the temperature is set to 150 ° C or lower, and then finish rolling.