JP5640931B2 - Medium carbon cold-rolled steel sheet excellent in workability and hardenability and its manufacturing method - Google Patents

Description

The present invention relates to a medium carbon cold-rolled steel sheet excellent in cold workability, in particular, cold forgeability, and excellent in hardenability, and a manufacturing method thereof.

Medium carbon cold rolled steel sheets are widely used as materials for chains, gears, clutches, saws, blades and the like. When the medium carbon steel sheet is applied as such a material, it is formed and then hardened by a heat treatment such as quenching and tempering. Therefore, it is necessary to achieve both formability and hardenability.

  In particular, in recent years, the development of processing technology has led to the rise of social needs for energy saving due to the adoption of forming methods with a higher degree of processing than conventional methods, such as forging steel sheets, and due to environmental considerations. As a result, the quenching / tempering process is also being accelerated to change to shorter and lower temperatures.

  In this way, the two required characteristics are becoming more stringent and severe processing is performed mainly on sheet thickness materials of 2 mm or more, so extremely high sheet thickness accuracy is required over the entire width and length. It is getting to be.

In order to respond to the above changes in needs, a medium-carbon cold-rolled steel sheet that is soft, can withstand cold working, has excellent hardenability, and has excellent thickness accuracy is developed. Is an urgent need. Here, the processing includes bending, thickening, drawing, and the like.

  Conventionally, many investigations have been made on the relationship between workability and hardenability of medium carbon steel sheets (for example, see Patent Documents 1 to 5).

  For example, Patent Document 1 is made of hypoeutectoid steel with C: 0.1 to 0.8% by mass and S: 0.01% by mass or less, and carbides have a carbide spheroidization rate of 90% or more. A medium-high carbon steel sheet is disclosed which is dispersed in ferrite and has an average carbide particle size of 0.4 to 1.0 μm, and the ferrite crystal particle size is adjusted to 20 μm or more as necessary. .

  However, with regard to conventional steel sheets, the structural influence on local processing such as stretch flanges is mainly investigated and the relationship is organized. The specific YR required during cold working (Yield ratio) and TS (tensile strength) are not related to mechanical properties, and in addition, the manufacturing method has not been established as a manufacturing method using actual equipment. Is the actual situation.

Japanese Patent Laid-Open No. 11-80884 JP 2003-89846 A JP-A-9-268344 JP 2001-329333 A JP 2001-355047 A

In view of the above circumstances, an object of the present invention is to provide a medium carbon cold-rolled steel sheet excellent in cold workability and hardenability that can be applied to the automobile field, and a method for producing the same.

The inventors of the present invention have intensively studied a method for solving the above-described problems. As a result, what is most necessary at the time of cold working is to achieve softening. For that purpose, the steel sheet is made a low YR (yield ratio) type steel sheet, and the YR (yield ratio) of the material itself is lowered. , together with this, carbide flat Hitoshi径 is at 0.4μm or less, if more than 90% spheroidization ratio, it is possible to ensure the uniformity of strain propagation was found that it is possible to take a microscopic cracks measures.

Furthermore, steel sheets with improved workability, as a further feature, the carbide Mono径 has been controlled very small, particularly, in any conditions, and also finding that satisfactory hardenability.

  Furthermore, it is difficult to manufacture steel sheets that satisfy this requirement, simply by devising rolling conditions and annealing conditions, and can only be achieved by achieving optimization in an integrated process of hot rolling, cold rolling, and annealing. I also learned through the accumulation of various studies.

  This invention was made | formed based on the said knowledge, The summary is as follows.

(1) By mass%, C: 0.10 to 0.80%, Si: 0.01 to 0.35%, Mn: 0.3 to 2.0%, P: 0.005 to 0.03% , S: 0.0001 to 0.01%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, and Cr : 0.02 to 1.0% %, Ni: 0.01 to 0.5%, Cu: 0.05 to 0.5%, Mo: 0.01 to 0.5%, Nb: 0.01 to 0.5%, V: 0.0. Containing one or more of 01-0.5%, Ta: 0.01-0.5%, B: 0.001-0.01%, W: 0.01-0.5%, The balance consists of Fe and inevitable impurities, the average carbide diameter of the carbide is 0.4 μm or less (excluding 0.05 μm or less) , the spheroidization rate of the carbide is 90% or more, and the yield ratio is 60% or less. there is, cold Tensile strength before processing is not more than 500 MPa, furthermore, the carbon cold-rolled steel sheet in which excellent cold workability and hardenability, characterized in that it comprises a quench-hardening ability to cure over 500HV after quenching.

( 2 ) The medium-carbon cold-rolled steel sheet is further in mass%, Mg: 0.0005-0.003%, Ca: 0.0005-0.003%, Y: 0.001-0.03%, One or more of Zr: 0.001 to 0.03%, La: 0.001 to 0.03%, and Ce: 0.001 to 0.03% are contained (1) The medium carbon cold-rolled steel sheet having excellent cold workability and hardenability as described in 1 ) .

(3) the continuous casting slab having the component composition according to any one of (1) or (2), directly during the hot rolling, or hot to heat the said template piece over 1100 ° C. At the time of rolling, hot rolling is performed at Ae3 or more, and then an air cooling time of 2 to 10 seconds is secured, and then from the start to the end of strong cooling at a cooling rate of 10 to 80 ° C./second, 480 to 600. After cooling to a pearlite region of ℃, and then winding in a temperature range of 400 to 580 ℃, a hot rolled sheet having a hot rolled sheet structure of 5-10% proeutectoid ferrite and less than 50% bainite is obtained, and the heat The cold-rolled sheet is subjected to one cold rolling at a rolling reduction of less than 5 to 30% , and after the cold rolling, a single annealing treatment is performed to perform annealing at 650 to 720 ° C. for 40 hours or less. excellent (1) or cold workability and hardenability according to any one of (2) to Method for producing a medium-carbon cold-rolled steel sheet.

  According to the present invention, by applying strain to the structure obtained by hot rolling by cold rolling at a light reduction rate, grain growth and recrystallization are promoted, and the yield ratio is extremely low and extremely soft. By precipitating fine carbide with excellent workability and fine spheroidization rate, it is possible to provide a medium carbon steel sheet having sufficient quench hardening ability under any quenching condition.

  The medium carbon steel sheet excellent in cold workability and hardenability of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) is mass%, C: 0.10 to 0.80%, Si: 0. 0.01 to 0.35%, Mn: 0.3 to 2.0%, P: 0.005 to 0.03%, S: 0.0001 to 0.01%, Al: 0.005 to 0.10 %, And N: 0.001 to 0.01%, the balance is made of Fe and inevitable impurities, the average carbide diameter of the carbide is 0.4 μm or less, the spheroidization rate of the carbide is 90% or more, In addition, the yield ratio is 60% or less, and further, it has quench hardening ability to harden to 500 Hv or more after quenching.

  And the characteristics in the manufacturing method of this invention steel plate are as follows.

(I) Characteristics of hot-rolled sheet The heat-rolled sheet is devised in a cooling pattern, wound in a temperature range of 400 to 580 ° C., and heat of pro-eutectoid ferrite 5 to 10% and bainite less than 50% (excluding 0). It is made into a sheet, then pickled and cold-rolled.

(Ii) Characteristics of cold-rolled sheet Cold rolling is performed only once at a rolling reduction of less than 5 to 30%. Cold rolling is performed so that the accuracy of the plate thickness is better than that of the hot rolled plate.

(Iii) Characteristics of heat treatment The cold-rolled sheet is subjected to annealing in Ac1 or less for a relatively short time. With this annealing, spheroidization proceeds with finely dispersed carbides, and the target spheroidizing diameter and spheroidizing ratio are obtained. The precipitation effect of carbide promotes the precipitation of C at the grain boundaries and within the grains, thereby lowering the yield point. Utilizing this lowering of the yield point and the increase in particle size due to the application of light strain to the ferrite structure, it becomes soft and secures workability.

  The necessity of each requirement will be described below.

  The steel sheet of the present invention is based on the component system defined by JISG4051 (carbon steel for machine structural use), JISG4401 (carbon tool steel), or JISG4802 (cold rolled steel strip for springs), and conducts research related to products and manufacturing. It was discovered by repeating. Therefore, the basic components of the steel sheet of the present invention conform to JIS standards, but the reason for further limiting the component composition will be described. Hereinafter, “%” relating to the component composition means “% by mass”.

C: 0.10 to 0.80%
C is an important element in securing the quenching strength of the steel sheet. Add 0.10% or more to ensure the required strength. If it is less than 0.10%, the hardenability is lowered and the strength as a high-strength steel sheet for machine structures cannot be obtained, so the lower limit is made 0.10%. If it exceeds 0.80%, the proportion of carbide that becomes the starting point of fracture increases and the cold forgeability decreases, so the upper limit is made 0.80%. Preferably, it is 0.35 to 0.70%.

Si: 0.01 to 0.35%
Si acts as a deoxidizer and is an element effective for improving hardenability. If it is less than 0.01%, the effect of addition cannot be obtained, so the lower limit is made 0.01%. If it exceeds 0.35%, the surface properties are deteriorated due to scale wrinkling during hot rolling, so the upper limit is made 0.35%. Preferably, it is 0.15% or less.

Mn: 0.3 to 2.0%
Mn is an element effective for improving hardenability. If it is less than 0.3%, the effect of addition cannot be obtained, so the lower limit is made 0.3%. If it exceeds 2.0%, the impact properties after quenching and tempering will deteriorate, so the upper limit is made 2.0%. Preferably, it is 0.5 to 1.5%.

P: 0.005 to 0.03%
P is a solid solution strengthening element and is an element that effectively acts on the strength of the steel sheet at a relatively low cost. However, excessive content inhibits toughness, so the upper limit is made 0.03%. Reduction to less than 0.005% causes an increase in refining cost, so the lower limit is made 0.005%.

S: 0.0001 to 0.01%
Since S forms non-metallic inclusions such as MnS and becomes a cause of hindering workability and toughness after heat treatment, the upper limit is made 0.01%. Reducing to less than 0.0001% causes a significant increase in refining costs, so the lower limit is made 0.0001%.

Al: 0.005-0.10%
Al acts as a deoxidizing agent and is an element effective for fixing N. If it is less than 0.005%, the effect of addition cannot be sufficiently obtained, so the lower limit is made 0.005%. If it exceeds 0.10%, the effect of addition is saturated and surface defects are likely to occur, so the upper limit is made 0.10%. Preferably, it is 0.01 to 0.04%.

N: 0.001 to 0.01%
N is an element that forms nitrides. If nitride precipitates during slab bending correction in curved continuous casting, the slab may crack, so the upper limit is made 0.01%. N is preferably as small as possible, but reducing it to less than 0.001% leads to an increase in refining costs, so the lower limit is made 0.001%. Preferably, it is 0.001 to 0.006%.

  In order to enhance the mechanical properties of the steel sheet of the present invention, a required amount of one or more of Cr, Ni, Cu, and Mo may be added.

Cr: 0.02-1.0%
Cr is an element effective for improving hardenability. If it is less than 0.02%, there is no significant effect of addition, so the lower limit is made 0.02%. If it exceeds 1.0%, the effect of addition is saturated, so the upper limit is made 1.0%. Preferably, it is 0.05 to 0.6%.

Ni: 0.01 to 0.5%
Ni is an element effective for improving toughness and hardenability. If it is less than 0.01%, there is no effect of addition, so the lower limit is made 0.01%. If it exceeds 0.5%, the effect of addition is saturated and the cost is increased, so the upper limit is made 0.5%. Preferably, it is 0.05 to 0.3%.

Cu: 0.05 to 0.5%
Cu is an element effective for ensuring hardenability. If it is less than 0.05%, the effect of addition is insufficient, so the lower limit is made 0.05%. If it exceeds 0.5%, it becomes too hard and the cold workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.05 to 0.3%.

Mo: 0.01 to 0.5%
Mo is an element effective for improving hardenability and improving resistance to temper softening. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, the effect of addition is saturated, so the upper limit is made 0.5%. Preferably, it is 0.05 to 0.3%.

  In order to further enhance the mechanical properties of the steel sheet of the present invention, a required amount of one or more of Nb, V, Ta, B, W, Mg, Ca, Y, Ce, and La may be added. Good.

Nb: 0.01 to 0.5%
Nb is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is not sufficiently exhibited, so the lower limit is made 0.01%. If it exceeds 0.5%, the effect of addition is saturated, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

V: 0.01 to 0.5%
V, like Nb, is an element that forms carbonitride and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.07 to 0.2%.

Ta: 0.01 to 0.5%
Ta, like Nb and V, is an element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving toughness. If it is less than 0.01%, the effect of addition is small, so the lower limit is made 0.01%. If it exceeds 0.5%, carbides are generated and the quenching hardness is lowered, so the upper limit is made 0.5%. Preferably, it is 0.05 to 0.3%.

B: 0.001 to 0.01%
B is an element effective for enhancing hardenability by adding a small amount. If it is less than 0.001%, there is no effect of addition, so the lower limit is made 0.001%. If it exceeds 0.01%, the castability is reduced, and a B-based compound is generated and the toughness is reduced. Therefore, the upper limit is made 0.01%. Preferably, it is 0.0015 to 0.005%.
W: 0.01-0.5%
W is an element effective for strengthening the steel sheet. If it is less than 0.01%, the effect of addition does not appear, so the lower limit is made 0.01%. If it exceeds 0.5%, the workability deteriorates, so the upper limit is made 0.5%. Preferably, it is 0.05 to 0.3%.

Mg: 0.0005 to 0.003%
Mg is an element effective for controlling the form of oxides and sulfides with a small amount of addition. If it is less than 0.0005%, the effect of addition cannot be obtained, so the lower limit is made 0.0005%. If it exceeds 0.003%, the effect of addition is saturated, so the upper limit is made 0.003%. Preferably, it is 0.0007 to 0.002%.

Ca: 0.0005 to 0.003%
Ca, like Mg, is an element effective for controlling the form of oxides and sulfides by addition in a small amount. If it is less than 0.0005%, the effect of addition cannot be obtained, so the lower limit is made 0.0005%. If it exceeds 0.003%, the effect of addition is saturated, so the upper limit is made 0.003%. Preferably, it is 0.0010 to 0.003%.

Y: 0.001 to 0.03%
Y, like Ca and Mg, is an element that is effective for controlling the form of oxides and sulfides by adding a small amount. If it is less than 0.001%, the effect of addition cannot be obtained, so the lower limit is made 0.001%. If it exceeds 0.03%, the effect of addition is saturated, so the upper limit is made 0.03%. Preferably, it is 0.0015 to 0.004%.

Ce: 0.001 to 0.03%
Ce, like Mg, Ca, and Y, is an element effective for controlling the form of oxides and sulfides by adding a small amount. If it is less than 0.001%, the effect of addition cannot be obtained, so the lower limit is made 0.001%. If it exceeds 0.03%, the effect of addition is saturated, so the upper limit is made 0.03%. Preferably, it is 0.0015 to 0.004%.

La: 0.001 to 0.03%
La, like Mg, Ca, Y, and Ce, is an element that is effective for controlling the form of oxides and sulfides by adding a small amount. If it is less than 0.001%, the effect of addition cannot be obtained, so the lower limit is made 0.001%. If it exceeds 0.03%, the effect of addition is saturated, so the upper limit is made 0.03%.

Zr: 0.001 to 0.03%
Zr is an element effective for strengthening steel sheets. If it is less than 0.001%, the effect of addition does not appear, so the lower limit is made 0.001%. If it exceeds 0.03%, the workability deteriorates, so the upper limit is made 0.03%. Preferably, it is 0.005 to 0.01%.

  When scrap is used as a raw material for the steel sheet of the present invention, inevitably, one or more of Sn, Sb, and As are mixed in by 0.003% or more, both of which are 0.03% or less. If present, the hardenability of the steel sheet of the present invention is not hindered. Therefore, in the steel sheet of the present invention, Sn: 0.003 to 0.03%, Sb: 0.003 to 0.03%, and As: 0.003. The content of 1 type or 2 types or more of -0.03% is allowed.

  In the steel sheet of the present invention, the amount of O is not specified, but when the oxide aggregates and coarsens, the cold workability deteriorates, so O is preferably 0.0025% or less. A smaller amount of O is preferable, but since it is technically difficult to reduce it to less than 0.0001%, a content of 0.0001% or more is allowed.

  In addition to the component composition described above, the steel sheet of the present invention has a yield ratio (YR) of 60% or less and a tensile strength of 500 MPa or less before cold working, and an average sphere diameter of carbide: less than 0.4 μm, Spheroidization rate: 90% or more, characterized by having quenching and hardening ability to harden to 500 Hv or more after quenching.

  The strength before cold processing here refers to the strength before processing. In addition to the component composition, the tensile strength before cold working is 500 MPa or less, the yield ratio is 60% or less, the average carbide diameter of the carbide is less than 0.4 μm, and the spheroidization rate of the carbide is 90% or more. Thus, the cold workability of the steel sheet is improved, and both cold workability and hardenability can be achieved. This is a new finding found by the present inventors.

  The strength before cold working is 500 MPa or less in terms of tensile strength (TS). When the tensile strength (TS) exceeds 500 MPa, even if the yield ratio is less than 60%, the ductility is lowered, and a sufficient amount of molding during processing cannot be ensured.

  If the strength is lowered, ductility is improved and workability is improved, but sagging during punching may increase. In recent years, forging technology for steel sheets integrated with punching, bending, and thickening has become widespread, and it is preferable in some cases to have a tensile strength (TS) of 400 MPa or more depending on the manufacturing process.

  The reason for setting the yield ratio (YR) to 60% or less is to avoid stress strain concentration during processing. When processing steel sheets, in order for the steel sheets to be deformed while conforming to the mold, the deformation of the steel sheets is uniformly propagated and deformed in synchronization with the movement of the mold, or the plastic flow of the steel sheet is uniform. It is necessary to be.

  In particular, in recent years, press methods aiming at improving processing accuracy and reducing the time required to pressurize compressive loads from multiple directions at the same time are also appearing, realizing more uniform material deformation. In addition, it has become important not to generate a crack starting point.

  For this purpose, the present inventors have found that it is necessary to form a situation in which work hardening is easily started as a low yield point. When the yield ratio is more than 60%, the strain propagation property is inferior, which causes stress concentration in one place during cold working, resulting in inflow failure and cracking.

  It is necessary that the average particle size of the carbide is less than 0.4 μm and the spheroidization rate is 90% or more. Coarse carbides require time to complete dissolution, which not only impairs hardenability but also causes cracking. There has been no investigation and knowledge of carbide diameter and spheroidization rate in stretch flange processing. However, when the degree of processing was large, there was no knowledge that evaluated the carbide diameter and spheroidization rate when local deformation was not performed.

  The present inventors investigated the influence of carbides and examined means for improving the diameter and spheroidization rate of carbides. Normally, the steel is made to have a low yield ratio so that strain accumulation does not occur during processing, but since the amount of processing strain is very large, microvoids are generated. Since microvoids are generated in the vicinity of carbides, it has been found that strains do not concentrate when the starting carbide is finely dispersed.

  That is, the present inventors have found that carbides smaller than 0.4 μm, which are smaller than conventionally considered carbides, must be finely dispersed and spheroidized. However, in the vicinity of the needle-shaped carbide, stress is likely to be localized during cold working, and is likely to be a starting point of cracking.

  If the spheroidization rate is less than 90%, even if the carbide is less than 0.4 μm, it becomes a starting point of cracking due to local stress, and the cold forgeability may deteriorate. % Or more is preferable.

  Spherical carbide has a smaller surface area in contact with the base material than that of acicular carbide, and has a narrow carbon diffusion path from the carbide to the base material. In particular, when the spheroidization ratio is 90% or more, in order to obtain a quenching effect, the carbide needs to have a diameter smaller than that of the conventional carbide. In order to achieve both workability and hardenability, a carbide diameter of less than 0.4 μm and a spheroidization rate of 90% or more are essential conditions.

  Tissue observation is performed with a scanning electron microscope. Since the number of very finely precipitated carbides greatly affects the hardenability, a field of view containing 500 or more carbides on the structure observation surface at a magnification of 3000 to 10000 times, or in some cases about 30000 times 4 or more are selected, and the area of each carbide contained in the region is measured in detail. Thereafter, the diameter when the average area per piece is approximated by a circle is determined as the average carbide particle diameter.

  A case where the ratio of the major axis length to the minor axis length of the carbide is 3 or more is referred to as acicular carbide, and a case where the ratio is less than 3 is referred to as spherical carbide. A value obtained by dividing the number of spherical carbides by the number of total carbides is defined as a carbide spheroidization rate.

  The hardness after quenching is preferably 500 HV or more. When it is 500 HV or more, the wear resistance is improved as the strength of the hardened steel is increased. In particular, for members that require wear resistance as a main characteristic, such as clutch plates and gears for automobile parts, this characteristic is very important, so it is important to satisfy a hardness of 500 HV or higher after quenching. It is.

  After heating to 950 ° C. at a heating rate of 100 ° C./second, the hardness of the specimen that was held for 3 seconds and rapidly cooled at a cooling rate of 300 ° C./second or more was evaluated as the quenching hardness. As the rate increases, the carbide becomes difficult to dissolve, and the average sphere diameter for securing the quenching hardness must be reduced. On the other hand, the workability increases as the spheroidization rate increases.

  The steel sheet according to the present invention is characterized by satisfying both factors of conflicting workability and hardenability.

  Next, the manufacturing method of this invention steel plate is demonstrated. The technical idea of the present invention is to form the optimum hot-rolled sheet structure by devising the hot-rolling finish rolling and cooling conditions in the steel material having the above-described component composition, and then the light cold-rolling rate and low-temperature short-time annealing ( (1) Cold rolling and 1 annealing) Manufacturing the steel sheet of the present invention, in particular, excellent cold forgeability, and quenching that can ensure the target hardness even in a short time low temperature heating and quenching treatment It is to produce a medium carbon steel plate having excellent properties.

  Hereinafter, a method for producing the steel sheet of the present invention (hereinafter sometimes referred to as “the present invention production method”) will be specifically described.

[Hot rolling]
In the hot rolling, a continuous cast slab satisfying the component composition range is charged directly or after cooling the slab into a heating furnace, heated to 1100 ° C. or higher, and rolled at Ae 3 or higher. The cooling pattern at this time is characteristic. First, after finishing rolling, air cooling is performed for 2 seconds or more and 10 seconds or less, and thereafter cooling from the start to the end of strong cooling to a pearlite region of 480 to 600 ° C. at a cooling rate of 10 to 80 ° C./second, It is characterized by winding in a temperature range of 400 to 580 ° C.

  Below, the reason for prescribing the conditions in each hot rolling will be described.

[Finish rolling finish temperature and cooling start point]
The temperature of finish rolling is set to Ae3 or higher to promote recrystallization. Usually, Ar3 is used as a guideline for the end temperature of rolling, and the rolling is finished with an austenite structure. In this case, the steel sheet is in a supercooled state, recrystallization does not occur sufficiently, and the influence of the cooling rate is affected by the rolling end temperature, etc. Therefore, a great variation occurs in the structure obtained in the hot-rolled sheet.

  Therefore, in the manufacturing method of the present invention, the end temperature of continuous hot rolling is set to Ae3 or more, and the air cooling time of 2 to 10 seconds is taken in consideration of the cooling start temperature until the start of cooling, and then the cooling of the steel sheet is started.

  When the air cooling time exceeds 10 seconds, the temperature is remarkably lowered, the recrystallization behavior becomes slow, and the improvement effect is saturated. The air cooling time is preferably 7 seconds or less. The end temperature of continuous hot rolling is preferably 1000 ° C. or less. When the hot rolling end temperature exceeds 1000 ° C., the scale thickness increases, the pickling property becomes extremely poor, and industrial production becomes difficult.

  Further, after rough rolling, after forming a sheet bar, the sheet bar may be joined and continuous rolling may be performed. Even if the sheet bar is heated and rolled at the above temperature, there is no influence on the material and structure.

[Cooling rate and strong cooling end temperature]
The reason for defining the cooling rate and end temperature is to obtain the target tissue. If the structure control is not sufficient here, the target structure cannot be obtained even if the subsequent cold rolling and annealing are performed within the target range.

  After performing the air cooling after rolling described above, it is cooled to a pearlite region of 480 to 600 ° C. at a cooling rate of 10 to 80 ° C./second. If the cooling rate is less than 10 ° C./second, the transformation is completed when cooling to this temperature range, resulting in a coarse ferrite and coarse pearlite structure. After the subsequent cold rolling and annealing, the target Carbide that can not be obtained. Therefore, the cooling rate is set to 10 ° C./second or more. Preferably, it is 20 ° C./second or more.

  When the cooling rate is more than 80 ° C./second, uneven cooling occurs in the width direction of the steel sheet. In particular, the vicinity of the end of the steel sheet is supercooled, and the bainite ratio becomes too high in the subsequent processing until winding, making it impossible to obtain the target structure. If the end of the steel plate is removed by trim or the like, the yield is lowered. Therefore, in the manufacturing method of the present invention, the cooling rate is set to 80 ° C./second or less. Preferably it is 70 degrees C / sec or less.

[Taking temperature after cooling after strong cooling]
Winding after cooling is performed in a temperature range of 400 to 580 ° C. If the coiling temperature is less than 400 ° C., a part of martensitic transformation occurs, and there is a greater concern that cracking will occur during pickling and rewinding. It is absolutely necessary to avoid cracking. In addition, even if no cracking occurs, the material will vary, and from this point also, some occurrence of martensitic transformation should be avoided. The winding temperature is preferably 430 ° C. or higher.

  The reason why the coiling temperature is set to 580 ° C. or lower is to bainite a structure that has not been transformed after strong cooling. When the coiling temperature exceeds 580 ° C., the untransformed structure is transformed into a pearlite structure, so that the target structure cannot be obtained. The winding temperature is preferably 550 ° C. or lower.

  The hot-rolled structure obtained by these hot-rolling / cooling conditions is a structure of 5-10% pro-eutectoid ferrite and less than 50% bainite. The reason for controlling this structure is deeply related to cold rolling conditions and annealing conditions.

  The aim of the subsequent cold rolling and annealing treatments is to add cold rolling strain and utilize the difference in strain accumulation by structure to promote grain growth. Although strain is introduced into the structure of the hot-rolled sheet during cooling and transformation, the amount of strain differs depending on the structure, and the strain accumulation amount in the bainite structure is large.

  However, when the bainite ratio is 50% or more, there are too many bainite structures in the whole, so that a strain difference does not occur and recrystallization and grain growth become nonuniform, which is inappropriate. Further, in order to increase the strain difference and promote grain growth and recrystallization, it is necessary to use proeutectoid ferrite with a small amount of strain generated in the hot-rolled sheet, and this amount is controlled to 5 to 10%. There is a need.

  If the amount of pro-eutectoid ferrite is less than 5%, even if the bainite ratio is low, the grain growth cannot be sufficiently performed, and if it exceeds 10%, strain accumulation hardly occurs and grain growth hardly occurs. Therefore, the amount of pro-eutectoid ferrite needs to be controlled to 5 to 10%. Preferably, it is 6 to 9%.

  The wound hot-rolled sheet is subjected to cold rolling after removing surface oxide scale by pickling or shot blasting.

[Cold rolling ratio: Reduction ratio of less than 5-30%]
The hot-rolled sheet having a desired structure is cold-rolled at a reduction rate of less than 5 to 30% by the hot rolling method described above. This treatment is for imparting strain to the structure obtained with the hot-rolled sheet, thereby making the difference in strain for each structure more prominent and promoting grain growth and recrystallization. In order to obtain such an effect, the rolling reduction needs to be 5% or more. Preferably, it is 7% or more.

  Further, another meaning of the lower limit of the rolling reduction is to refine the carbide. If the rolling reduction is less than 5%, the carbide precipitated by hot rolling remains without being destroyed, and the carbide Since the average particle diameter of becomes large, it is not preferable. Further, when the rolling reduction is 30% or more, the ferrite after recrystallization becomes finer and the strength increases, so the rolling reduction is set to less than 30%. Preferably it is 27% or less.

[Annealing temperature and time for cold rolled sheet annealing]
The steel sheet subjected to hot rolling, pickling and cold rolling is annealed to obtain a target carbide structure and low YR type steel sheet. By annealing, the difference in strain accumulated in each structure is utilized, and pro-eutectoid ferrite is recrystallized and grain-grown to make coarse grains, thereby softening.

  Since the carbide is cold-rolled in addition to being dispersed at a relatively low coiling temperature, the carbide is very fine when the cold-rolling is completed. As a result, as soon as annealing is started, the carbide starts to dissolve and spheroidization proceeds.

  However, since the dispersion is very uniform and small due to the manufacturing conditions described above, spheroidization progresses at once, and a large number of very fine ones are generated simultaneously. As a result, the amount of C remaining in solid solution in the steel is extremely small.

  Up to now, due to the decrease in the amount of solid solution C that has increased the yield point (YP), YP is decreased, and at the same time, the target spheroidization rate and carbide diameter can be obtained. In the cold-rolled sheet annealing of this manufacturing method, a low temperature and a short time are preferable, and 650 to 720 ° C. and 40 hours or less are set. Preferably, it is 20 to 40 hours at 690 ° C.

  Note that annealing here refers to box annealing, and is performed in an atmosphere of 95% or more of hydrogen, a dew point of less than -20 ° C up to 400 ° C, and a dew point of more than 400 ° C of less than -40 ° C. However, it is preferable in terms of suppressing variation in characteristics in the width direction of the steel material, but it is possible to manufacture a steel material having target characteristics even in a nitrogen atmosphere.

  Next, examples will be described. The level of the embodiment is an example of execution conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  Using steel sheets having the composition shown in Table 1, the hot rolling conditions shown in Table 2 and the cold rolling conditions shown in Table 3 were combined, and the annealing conditions were two levels (690 ° C. × 35 hours, 710 ° C. × 35 hours). As described above, the hardness of the steel sheet and the diameter of the carbide in the steel sheet were controlled, and the workability of the steel sheet was evaluated by quenching hardness and cold bendability. Cooling after annealing was furnace cooling after completion of holding.

  In addition, the quenching test was performed by heating a test material having a thickness of 5 t at a frequency of 78 kHz at a heating rate of 100 ° C./second from room temperature, holding at 950 ° C. for 10 seconds, and immediately increasing the temperature to 100 ° C./second or more. The test was carried out under the condition of rapidly cooling to room temperature at the cooling rate, and the Vickers hardness of the quenched material was measured.

  The bending test was performed under the condition that a specimen with an annealing thickness of 5 t was bent in a 90 ° plate length direction with a bending radius of 5 mm, and the fine cracks and roughness of the outer bending portion, which were conventionally evaluated, were evaluated. In addition to the defects, considering the compression during processing, those in which fine cracks and roughness defects on the inner bending side were observed were judged to be inferior in cold workability and were evaluated as x. The evaluation results are shown in Tables 4 and 5.

  Serial numbers 1 to 6, 11, 12, 13, 24 to 29 in Tables 4 and 5 are developed steels (invention steels) that have both cold workability and hardenability. Serial numbers 7 to 10 are comparative steels whose hardness after quenching does not reach the target and inferior in hardenability. Serial numbers 30 and 31 are comparative steels that are soft but have poor hardenability due to the large average diameter of carbides. The serial number 34 is an inferior comparative steel whose YR is high, the cold workability does not reach the target, and the average diameter of the carbide is large, so the hardness after quenching does not reach the target.

  Serial numbers 17, 18, 20, 22, 23, 32, 33, 35, and 36 are comparative steels that do not reach the target for both TS and YR and have poor cold workability. Serial numbers 14 to 16, 19, and 21 are comparative steels with low cold workability because the spheroidization rate of carbide is low and both TS and YR are high.

As described above, according to the present invention, grain growth / recrystallization is promoted by applying strain to the structure obtained by hot rolling by cold rolling at a light reduction rate, and the yield ratio is low and extremely soft. Therefore, by providing carbide with excellent cold workability and fine and high spheroidization rate, we provide medium-carbon cold-rolled steel sheets with sufficient quench-hardening ability under any quenching conditions. can do. Therefore, the present invention has high applicability in the steel industry.

Claims (3)

In mass%, C: 0.10 to 0.80%, Si: 0.01 to 0.35%, Mn: 0.3 to 2.0%, P: 0.005 to 0.03%, S: 0.0001 to 0.01%, Al: 0.005 to 0.10%, and N: 0.001 to 0.01%, Cr : 0.02 to 1.0%, Ni : 0.01-0.5%, Cu: 0.05-0.5%, Mo: 0.01-0.5%, Nb: 0.01-0.5%, V: 0.01-0 0.5%, Ta: 0.01 to 0.5%, B: 0.001 to 0.01%, W: 0.01 to 0.5%, or one or more, and the balance is Fe And the average carbide diameter of the carbide is 0.4 μm or less (excluding 0.05 μm or less) , the spheroidization rate of the carbide is 90% or more, and the yield ratio is 60% or less. , cold working Tensile strength is not more than 500 MPa, furthermore, the carbon cold-rolled steel sheet in which excellent cold workability and hardenability, characterized in that it comprises a quench-hardening ability to cure after quenching above 500 HV. The medium carbon cold-rolled steel sheet is further mass%, Mg: 0.0005-0.003%, Ca: 0.0005-0.003%, Y: 0.001-0.03%, Zr: 0. It contains 1 type or 2 types or more of 0.001-0.03%, La: 0.001-0.03%, Ce: 0.001-0.03% of Claim 1 characterized by the above-mentioned. Medium carbon cold rolled steel sheet with excellent cold workability and hardenability . The continuous casting slab having the component composition of any crab according to claim 1 or 2, directly, when the hot rolling, or, when the hot rolling by heating the template pieces above 1100 ° C., hot Rolling is performed at Ae3 or higher, and then an air cooling time of 2 to 10 seconds is secured, and then from the start to the end of strong cooling to a pearlite region of 480 to 600 ° C. at a cooling rate of 10 to 80 ° C./second. and, then, by winding in a temperature range of from 400 to 580 ° C., the pro-eutectoid ferrite 5-10%, to obtain a hot rolled sheet having a hot-rolled sheet structure is less than 50% bainite, the heat-rolled plate, reduction ratio subjected to one cold rolling at less than 5-30%, the after cold-rolled, from 650 to 720 ° C., according to claim 1 or 2, characterized by applying the processing of single annealing subjected to annealing below 40 hours manufactured carbon cold-rolled steel sheet in which excellent cold workability and hardenability according to any one of Method.