JP3747982B2 - Manufacturing method of medium and high carbon steel sheet - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot rolled steel sheet of medium / high carbon hypoeutectoid steel.
[0002]
[Prior art]
So-called medium and high carbon steels with a C content of approximately 0.2 to 0.8% by mass in steel can be hardened and have a certain degree of workability in the annealed state before quenching. . For this reason, the hot-rolled steel sheet is widely used as a raw material for various machine parts and bearing parts including automobile parts. In manufacturing parts, generally, punching and bending are performed, and relatively mild drawing and stretch flange molding may be performed. Further, when the part shape is complicated, it is often produced by welding two or three parts. However, in recent years, integrated molding of parts and simplification of processing steps have been promoted in order to reduce the manufacturing cost of parts. This means that it must withstand processing with a higher processing rate (= a large amount of plastic deformation) when viewed from the material side. In other words, with the advancement of processing technology, higher ductility has been required for the medium and high carbon steel plates themselves.
[0003]
In order to impart high ductility to a steel material, it is fundamental to achieve further “softening”. Conventionally, it has been known that spheroidization of carbide is effective for softening medium- and high-carbon steel materials. For this purpose, for example, the following heat treatment method is adopted.
▲ 1 ▼ way to long held just below the A ₁ transformation point. This method is suitable for spheronization after cold working.
▲ 2 ▼ After a certain time maintained at a temperature between the A ₁ transformation point and the A ₃ transformation point, a method of gradually cooling to just below the A ₁ transformation point. This method is suitable for spheroidizing a pearlite structure and has an advantage that spheroidization is completed in a short time.
▲ 3 ▼ across the A ₁ transformation point, and heating at a temperature just below the A ₁ transformation point, a method of repeatedly performing the heating at temperatures between the A ₁ transformation point and the A ₃ transformation point. This method is suitable for uniformizing the carbide particle size and has the advantage that spheroidization can be completed in a short time, but requires strict temperature control and is not necessarily suitable for industrialization in terms of cost.
In general, it is known that if a heat treatment is performed on a steel material that has been cold worked, the steel material becomes even softer than if the heat treatment is performed without the cold work.
[0004]
[Problems to be solved by the invention]
However, sufficient softening cannot be achieved by the heat treatments (1) to (3) above in order to use hot rolled steel sheets of medium and high carbon steel for processing with a high degree of processing such as highly integrated forming. . Even when the cold working and the heat treatments (1) to (3) are combined, the softening level still cannot be satisfied.
Therefore, the present invention is to soften a hot rolled steel sheet of medium and high carbon steel that has been widely used so that it can be sufficiently used for integral forming with a high degree of workability while maintaining its hardenability. Objective.
[0005]
[Means for Solving the Problems]
The object is the invention of claim 1, that is, by mass%, C: 0.1 to 0.8%, Si: 0.15 to 0.40%, Mn: 0.3 to 1.0% , Cu: 0.30% or less, Ni: 0.25% or less , P is 0.03% or less, S is 0.01% or less, T.I. The hot-rolled steel sheet of hypoeutectoid steel, the content of which is limited to 0.1% or less with the balance being Fe and unavoidable impurities, is subjected to cold rolling under a light pressure of 20% to 30%, and then Ac. ₁ -50 ° C. to Ac 0.5 hours or more at ₁ below temperature range after the heat of the first stage to (but excluding or soaking 6 hours) held in a temperature range of Ac ₁ to Ac ₁ + 100 ° C. the heating of the third stage for holding 2-20 hours at a temperature range of the heating and Ar ₁ -50 ℃ ~Ar ₁ of the second stage to hold 0.5 to 20 hours is performed in succession, and, holding the second stage This can be achieved by a method for producing a softened medium- and high-carbon steel sheet that is subjected to three-stage annealing with a cooling rate from the temperature to the third stage holding temperature of 5 to 30 ° C./h.
Here, Ac ₁ means the A ₁ transformation point (° C.) of the steel in the temperature raising process, and Ar ₁ means the A ₁ transformation point (° C.) in the temperature lowering process.
[0008]
The invention of claim 2 is the hypoeutectoid steel according to the invention of claim 1 in particular in terms of mass%: C: 0.1-0.8%, Si: 0.15-0.40%, Mn: 0.3 -1.0%, Cu: 0 to 0.30% (including no addition), Ni: 0 to 0.25% (including no addition), Cr: 0 to 0.2% (including no addition) , P is 0.03% or less, S is 0.01% or less, T.I. The content of aluminum is limited to 0.1% or less, and the balance is defined as steel consisting of Fe and inevitable impurities.
Here, 0% of the lower limit of Cu, Ni and Cr means a case where the element is not added.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
According to the researches of the present inventors, a medium-high carbon steel hot-rolled steel sheet is subjected to “light rolling cold rolling” in a specific rolling reduction range, and then a specific condition sandwiching A ₁ point of the steel. As a result of the "three-stage annealing", it was possible to achieve ultra-softening that was difficult to achieve in the past without impairing the hardenability inherent to medium and high carbon steels. Hereinafter, matters for specifying the present invention will be described.
[0012]
In the present invention, hypoeutectoid steel containing C: 0.1 to 0.8% by mass is targeted. C is an alloy element which is the most basic in carbon steel, and the quenching hardness and the amount of carbide vary greatly depending on its content. In the hypoeutectoid steel having a C content of 0.1% by mass or less, sufficient quenching hardness cannot be obtained for application to various machine structural parts. On the other hand, if the C content exceeds 0.8% by mass, the toughness after hot rolling deteriorates and the manufacturability and handleability of the steel strip deteriorate, and sufficient ductility cannot be obtained even after annealing. This makes it difficult to apply to parts with a high degree of processing. Therefore, in the present invention, steel with a C content in the range of 0.1 to 0.8% by mass is targeted from the viewpoint of providing a raw steel plate having both appropriate quenching hardness and workability.
[0013]
S is an element that forms MnS inclusions. Since the local ductility deteriorates when the amount of inclusions increases, the S content in the steel is preferably reduced to 0.01% by mass or less particularly in applications used for stretch flange processing or the like. The local ductility depends on the C content in addition to S, and the local ductility becomes worse as the C content increases. In order to maintain good local ductility even when the C content increases to near 0.8% by mass, the S content is preferably further reduced to 0.005% by mass or less.
[0014]
Since P deteriorates ductility and toughness, the content is preferably 0.03% by mass or less.
Al is added as a deoxidizer for molten steel. If the amount of Al exceeds 0.1% by mass, the cleanliness of the steel is impaired and surface flaws are likely to occur, and the surface quality of the steel sheet is deteriorated. Therefore, T.W. Al is preferably 0.1% by mass or less.
[0015]
Si is desirably contained in an amount of 0.15% by mass or more for deoxidation of molten steel. However, Si hardens ferrite by a solid solution strengthening action, and causes a large amount of scale flaws on the surface of the steel sheet when contained in a large amount. Furthermore, it also causes a decrease in toughness. Therefore, Si is desirably contained in the range of 0.15 to 0.40 mass%.
Mn is an additive element that enhances the hardenability of the steel sheet and is effective for toughening. In order to obtain sufficient hardenability, the content is preferably 0.3% by mass or more. However, if it is contained in a large amount exceeding 1.0% by mass, the ferrite is cured and the workability is deteriorated. Therefore, it is desirable to contain Mn in the range of 0.3 to 1.0% by mass.
[0016]
Moreover, in this invention, it can apply to the steel which aimed at the improvement of each characteristic by adding elements, such as Cu and Ni, as needed. Cu improves the surface properties of the steel sheet because it improves the peelability of the oxide scale produced during hot rolling. However, if 0.3% by mass or more is contained, fine cracks are likely to be generated on the surface of the steel sheet due to the embrittlement of the molten metal, so Cu can be added in a range of 0.3% by mass or less. A preferable range of the Cu content is 0.10 to 0.15% by mass.
Ni is an alloy component that improves hardenability and prevents low temperature brittleness. In addition, since Ni has an action to counteract the adverse effect of molten metal embrittlement caused by Cu addition, particularly when adding about 0.2% or more of Cu, it is possible to add Ni equivalent to the Cu addition amount. It is extremely effective. However, the press formability and workability before quenching also aim to soften subjected to 3-stage annealing is contained is plenty of Ni comes to deteriorate. Therefore, when adding Ni, it is set as the range of 0.25 mass% or less.
[0018]
The present invention is characterized in that light rolling cold rolling is performed on the hot-rolled steel sheet prior to the three-stage annealing with the A ₁ point interposed therebetween. In general, steel that has been cold worked before annealing promotes recrystallization during annealing due to the introduced work strain, resulting in a softer steel than when not cold worked. It is done. However, medium- and high-carbon steels that are the subject of the present invention have high strength, so that the degree of softening obtained by combining general cold working and annealing seems to be compared to ordinary steels. Therefore, it was insufficient to be used for integral molding with a large degree of processing. However, only when the three-stage annealing specified in the present invention is performed, the hardness after annealing can be further remarkably reduced by performing light rolling cold rolling at a rolling reduction in a specific range in advance. It is.
[0019]
Specifically, when the rolling reduction of the cold rolling performed before the three-stage annealing exceeds 15%, the hardness after the annealing rapidly decreases and the conventional annealing method (the above-described circled numbers 1 to 1). The difference in hardness from the case of using method 3) is also remarkable. In other words, when the rolling reduction exceeds 15%, the effect of the three-stage annealing becomes prominent. Then, as the cold rolling rate increases, the fragmentation of the carbide progresses, and the hardness after the three-stage annealing becomes the lowest at a cold rolling rate of 20 to 30%. However, when the cold rolling rate exceeds 30%, the metal structure after the three-stage annealing becomes a so-called mixed grain structure in which the sizes of ferrite crystal grains are not uniform, and the hardness gradually increases .
[0020]
Among these, in particular, when the rolling reduction is in the range of 20 to 30%, it is possible to obtain a very soft steel having a Hv of 130 or less despite being a medium / high carbon steel. If the Hv value is 130 or less, many of the general integral forming processes performed on hot-rolled steel sheets of ordinary steel can be applied. In other words, relatively high-level machining that could only be applied to conventional steel can now be applied to medium- and high-carbon steel sheets, and there is a large degree of freedom in designing parts processing for medium- and high-carbon steel hot-rolled steel sheets. It will be improved.
[0021]
Next, three-stage annealing will be described. In medium- and high-carbon steels that are the subject of the present invention, it is not possible to sufficiently soften by merely promoting recrystallization, and it is important to control the dispersion form of carbides after annealing.
Generally, when steel is heated to a temperature of Ac ₁ point or higher, fine ones of carbides are dissolved in austenite, and then cooled to a temperature of Ar ₁ point or lower to precipitate again as carbides. At that time, if a large amount of undissolved carbide can remain in a temperature range of Ac ₁ point or higher, if the cooling rate is slowed, C dissolved in austenite does not generate pearlite, and undissolved carbide. Since it precipitates as a nucleus, the spheroidization rate of the carbide after annealing becomes high. Also, in this case, the number of carbides decreases by ₁ or more points of heating than before annealing, and when the cooling rate is slow, new nucleation does not occur during cooling, and as a result, the number of carbides after annealing decreases from before annealing. To do. A reduction in the number of carbides means that a metal structure containing carbides with a large particle size can be obtained because the total carbon content is constant. In particular, when the undissolved carbides serving as nuclei remain uniformly in places, the distance between carbides also increases. When such a metal structure is obtained, the ductility of steel is improved.
[0022]
However, the temperature range of Ac ₁ point or higher is a region where all the carbides of hypoeutectoid steel are in solid solution. For this reason, the number of undissolved carbides usually decreases when heated to a temperature range of Ac ₁ point or higher, and then C dissolved in austenite during the cooling process to a temperature of Ar ₁ temperature or lower is regenerated pearlite with a wide lamellar spacing. To be deposited. As a result, the spheroidization rate of the carbide is extremely low, and a steel sheet with high ductility cannot be obtained.
[0023]
Therefore, as a result of repeated studies, the present inventors have conducted hypoeutectoid steel if the steel sheet is heated to a specific temperature range lower than Ac ₁ point for a certain period of time in advance before heating the steel plate to Ac ₁ point or higher. Even in such a case, it has been found that an appropriate amount of undissolved carbide can remain in a temperature range of Ac ₁ point or higher. In addition, it has also been found that a carbide dispersion form optimum for softening can be obtained by holding for a specific time in a specific temperature range after cooling to ₁ point or less of Ar.
Hereinafter, the conditions for the three-stage annealing of the present invention will be described.
[0024]
[First stage heating]
The purpose of the first stage heating is to hold the steel plate at a temperature less than Ac ₁ point and to break up the pearlite produced by hot rolling to make the carbide (cementite) spherical. Although the divided carbide is relatively fine, the surface area per unit volume of carbide decreases as the spheroidization progresses. As a result, when heating at the second stage Ac ₁ point or more, the carbide / austenite interface area is reduced. The solid solution of carbide can be delayed. In order to promote the division and spheroidization reaction of hot-rolled pearlite, it is desirable that the temperature be as high as possible within a range of less than Ac ₁ point. Spheroidization does not proceed sufficiently at temperatures lower than Ac ₁ -50 ° C. On the other hand, when Ac is ₁ or more, hot-rolled pearlite having a large interfacial area easily dissolves in austenite, and the object cannot be achieved. Therefore the heating temperature in the first stage was a temperature range of less than Ac ₁ -50 ° C. to Ac _1. In addition, since the spheroidization cannot be sufficiently achieved when the holding time in the temperature range is less than 0.5 hours, the heating and holding time of the first stage is set to 0.5 hours or more (however, excluding soaking 6 hours or more) . .
After the first stage heating, the temperature may be raised as it is, and the second stage heating may be performed. Alternatively, after cooling to room temperature, the temperature is raised again and used for the second stage heating. May be. When the holding time of 0.5 hour or more cannot be ensured by one heating due to the convenience of the equipment, etc., the first stage heating may be divided into a plurality of times. In that case, the holding time within the above temperature range is set to be 0.5 hours or more in total.
[0025]
[Second stage heating]
The purpose of the second stage heating is to keep the steel sheet that has undergone the first stage heating at a temperature of Ac ₁ point or higher, so that fine carbides dissolve and disappear in the austenitized portion, and a relatively large spherical carbide is not removed. It is to leave it dissolved and, if ferrite is present, to make Ostwald growth of the carbide in that part. That is, it is a step of determining the number of undissolved carbides and the dispersion state to be the core of carbide precipitation by the third stage heating. If the heating temperature is less than Ac ₁ point, austenite is not generated. On the other hand, if the temperature of Ac ₁ + 100 ° C. is exceeded, even if the carbides are spheroidized by heating in the first stage, many of them dissolve / disappear in the austenite, and the number of undissolved carbides becomes too small. Or disappear. In this case, regenerated pearlite is generated in the cooling process to the third stage, and it cannot be sufficiently softened. If the heating and holding time is less than 0.5 hours, the solid carbide is not sufficiently dissolved in the austenite, and if the heating is continued for more than 20 hours, the number of undissolved carbides is reduced too much because it approaches an equilibrium state. Therefore, the second stage heating was to hold 0.5 to 20 hours at a temperature range of Ac ₁ ~Ac ₁ + 100 ℃.
[0026]
[3rd stage heating]
The purpose of the third stage heating is to maintain the steel sheet that has undergone the first to second stage heating at a temperature of Ar ₁ point or lower, and from the austenite to the ferrite transformation by cooling from the second stage temperature. It is to cause the carbon to be discharged to precipitate with undissolved carbides as nuclei and to perform Ostwald growth of these carbides. That is, the number of carbides is a step of maintaining the number of undissolved carbides left by the second stage heating almost as it is and increasing the spheroidization rate of the carbides. If the holding temperature is not lower than Ar ₁ point, austenite → ferrite transformation does not occur. Further, when the holding temperature is lower than Ar ₁ -50 ° C. or when the holding time is less than 2 hours, the Ostwald growth does not proceed sufficiently. However, even if the holding time exceeds 20 hours, the effect is saturated and there is no industrial merit. Thus, the third stage heating was keeping 2-20 hours at a temperature range of Ar ₁ -50 ℃ ~Ar _1.
[0027]
[Cooling rate from second stage holding temperature to third stage holding temperature]
When this cooling rate is high, the degree of supercooling of austenite increases and regenerated pearlite is easily generated. In order to sufficiently suppress the generation of regenerated pearlite, the cooling rate needs to be 30 ° C./h or less. On the other hand, even if the cooling rate is slower than 5 ° C./h, the reproduction pearlite suppressing effect is saturated and there is no industrial merit. Therefore, the said cooling rate was prescribed | regulated to 5-30 degrees C / h.
[0028]
【Example】
In mass%, C: 0.35%, Si: 0.22%, Mn: 0.72%, P: 0.012%, S: 0.011%, Cu: 0.1%, Ni: 0. 02%, T.W. A steel containing Al: 0.007% and the balance being Fe and inevitable impurities was melted to obtain a hot rolled steel sheet having a thickness of 4 mm. This hot-rolled steel sheet was cold-rolled at various rolling reductions and subjected to conventional annealing and three-stage annealing according to the present invention, respectively. The annealing conditions are as follows.
[Conventional annealing] 710 ° C. × 5 hr holding → cooling to 620 ° C. at a cooling rate of 10 ° C./hr→air cooling [3-step annealing] 690 ° C. × 4 hr holding → 730 ° C. × 4 hr holding → cooling rate of 10 ° C./hr at 690 ° C. Cooling to 690 ° C. × 4 hr holding → cooling to 620 ° C. at a cooling rate of 10 ° C./hr→air cooling Note that the Ac ₁ point of this steel is 727 ° C. and the Ar ₁ point is 738 ° C.
[0029]
The cross section hardness of each steel plate after annealing was measured. The result is shown in FIG. It can be seen that the one subjected to the three-stage annealing according to the present invention rapidly promotes softening when the cold rolling ratio before annealing exceeds 15%, and the degree of softening is remarkable as compared with the one subjected to conventional annealing. In addition, according to the three-stage annealing of the present invention, the hardness becomes Hv 130 or less when the cold rolling rate before annealing is in the range of 20 to 30%, and a remarkable softening of 10 or more in terms of Hv value is seen compared with the conventional annealing. It was. The metal structure of the steel sheet according to the present invention having an Hv value of 130 or less is a sized structure with ferrite crystals of FGS.No: 8 to 8.5, the carbide is sufficiently spheroidized, and the residual pearlite is I was not able to admit.
Moreover, all the steel plates obtained by the present invention maintained good hardenability.
[0030]
【The invention's effect】
According to the present invention, it is possible to remarkably reduce the hardness of a hot rolled steel sheet of medium / high carbon steel as compared with the conventional steel, and it is possible to manufacture a medium / high carbon steel having Hv of 130 or less. . As a result, advanced processing that could only be applied to ordinary steel can be applied to hot-rolled steel sheets of medium and high carbon steel. For example, parts with complex shapes can be produced at a low cost by integral forming. It became possible. Moreover, the hardenability after parts processing is maintained as usual. Furthermore, since light rolling under cold rolling is provided in the present invention, the steel sheet according to the present invention can be suitably used for applications requiring good surface skin and plate shape. Therefore, the present invention contributes to expanding the use of medium and high carbon steel and reducing the manufacturing cost.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the reduction ratio of cold rolling performed before annealing and the hardness after annealing.

Claims (1)

0.1~0.8%, Si:: C in mass% 0.15~0.40%, Mn: 0.3~1.0% , Cu: 0. 30% or less , Ni : 0 . 25% or less , P is 0.03% or less, S is 0.01% or less, T.I. The hot-rolled steel sheet of hypoeutectoid steel with Al being limited to a content of 0.1% or less and the balance being Fe and inevitable impurities is subjected to cold rolling under a light pressure of 20% to 30%, and then A c ₁ -50 ° C. in a temperature range of less than to a c ₁ 0.5 hour (excluding higher soaking 6 hours) after the heat of the first stage to _{hold, a c 1 ~A c 1 +} 100 ℃ subjected to heat in the second stage to hold 0.5 to 20 hours at a temperature range and a r ₁ -50 ° C. to a in the third stage heat holding in a temperature range from 2 to 20 hours of r ₁ in succession, and A method for producing a softened medium- and high-carbon steel sheet, which is subjected to three-stage annealing with a cooling rate from the second-stage holding temperature to the third-stage holding temperature of 5 to 30 ° C./h .

Images