JPH08283846A - Production of hot rolled steel sheet for deep drawing excellent in longitudinal crack resistance and roundness - Google Patents

Abstract

PURPOSE: To produce a hot rolled steel sheet for deep drawing excellent in longitudinal crack resistance and roundness by subjecting a steel having a specified componental compsn. in which the contents of P, S and Si are prescribed to hot rolling under specified conditions and coiling the same. CONSTITUTION: A steel having a compsn. contg., by weight, >=0.0040 to 0.0080% C, <=0.2% Mn, <=0.05% Si, <0.009% P so as to satisfy 8P+Si<=0.08%, <=0.005% S, 0.01 to 0.04% Al, <=0.0030% N, Ti; [(48/14)N(%)+(48+32)S(%)] or above and [(48/14)N(%)+(48+32)S(%)+48/12C(%) or below] and 0.001 to 0.005% B, and the balance Fe with inevitable impurities is prepd. This steel is subjected to hot rolling at 880 to 950 deg.C finishing temp. and is coiled in the temp. range of <=680 deg.C. Thus, the hot rolled steel sheet small in roundness at the time of deep drawing and free from brittle fracture even if being applied with impact after drawing can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a soft hot-rolled steel sheet, which is applied to a portion such as a compressor cover where deep drawing property is required. The present invention relates to a method for producing a hot-rolled steel sheet for deep drawing, which does not cause brittle fracture even if an impact is applied to the member after processing, that is, excellent in longitudinal crack resistance.

[0002]

2. Description of the Related Art Japanese Unexamined Patent Publication (Kokai) No. 61-295324 discloses a deep carbon steel in which Ti or Nb is added and solid solution C and N are fixed as carbonitrides of Ti or Nb to improve ductility. A method for manufacturing a hot rolled steel sheet for drawing is disclosed. In this method, even C and N, which can be present in the ferrite grain boundaries, are fixed as precipitates, so the grain boundary strength decreases and the longitudinal cracking resistance deteriorates.

In order to prevent the deterioration of longitudinal cracking resistance due to such a decrease in grain boundary strength, Japanese Patent Laid-Open No. 2-209424 discloses a method of adding B to an ultra low carbon steel.
B exists in the ferrite grain boundary and enhances the grain boundary strength, so it is effective in improving the vertical cracking resistance.
Since there is no element that exerts a solute drag effect such as Ti, not only is recrystallization and grain growth of austenite good at the hot rolling stage, but also ferrite grain growth property during runout cooling after finish rolling. Is also good, the ferrite grain size after the hot rolling is likely to be significantly increased. Longitudinal cracking resistance is closely related to the low temperature toughness of the steel sheet. If the steel sheet has a large grain size, the toughness decreases, and along with this, the longitudinal cracking resistance also deteriorates. Therefore, in this method, the grain boundary strengthening by the addition of B and the effect of increasing the ferrite grain size due to the ultra-low carbon steel cancel the longitudinal cracking resistance.

In contrast to these methods, in JP-A-64-73052, ductility is improved by adding Ti and B to ultra-low carbon steel and limiting finishing temperature and winding temperature in the hot rolling process. At the same time, a method for improving the vertical crack resistance is disclosed. However, the hot rolling conditions in this method are such that the finishing temperature range is narrow, the actual operation is difficult, and it is difficult to aim at the finishing temperature and the winding temperature. It is a very difficult method from the viewpoint of mass production.

Also, like a compressor chamber material
For round cylindrical deep-drawn products, the roundness of the product after molding is
Although it is an important form factor, the conventional method focuses on this factor.
There is no consideration of the elements added and the rolling conditions.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is a stable production with a high yield in the production of hot-rolled steel sheets excellent in deep drawability and longitudinal crack resistance. The method also provides vertical cracking resistance and roundness that improves the roundness, which is an important shape factor of the product after molding, for cylindrical deep-drawing molded product applications such as compressor chamber materials. The present invention provides an excellent hot-rolled steel sheet for deep drawing.

[0007]

The method for producing a hot-rolled steel sheet for deep drawing which is excellent in longitudinal cracking resistance according to the present invention is, by weight%, C: more than 0.0040% and 0.0080% or less Mn: 0.2% or less Si: 0.05% or less P: less than 0.009% 8P + Si ≦ 0.08% S: 0.005% or less Al: 0.01% or more and 0.04% or less N: 0.0030% Below Ti: {(48/14) N (%) + (48/32) S (%)} and above, {(4
8/14) N (%) + (48/32) S (%) +48/12 C (%) or less} B: 0.001% or more and 0.005% or less, with the balance being Fe and unavoidable impurities Steel having a finish temperature of 880 ° C to 950 ° C and then wound in a temperature range of 680 ° C or less, which is excellent in longitudinal cracking resistance and roundness It is a method of manufacturing a rolled steel sheet.

[0008]

The method for producing the hot-rolled steel sheet for deep drawing which is excellent in vertical cracking resistance of the present invention will be described in detail below. First, the reason for adding an additional element and the reason for limiting the range of addition will be described.

C: 0.0080 over 0.0040%
% Or less If the C content exceeds 0.0080%, the ductility of the steel deteriorates, the yield strength and the tensile strength increase, and not only the deep drawability deteriorates, but also the roundness after the cylinder deep drawing decreases. C has the effect of lowering the γ-α transformation point of steel, that is, the Ar3 point. However, when the amount of C is 0.0040% or less, the Ar3 point rises and finish rolling in the γ region over the entire length of the coil becomes difficult, which is not preferable from the viewpoint of improving the yield in actual operation.

Si: 0.05% or less When Si is added, not only the strength of the steel increases and the roundness decreases, but also the low temperature toughness of the steel decreases, which deteriorates the vertical cracking resistance. Therefore, the upper limit is 0.05%
And

Mn: 0.2% or less Mn is an austenite stabilizing element and is a useful element for securing the hot rolling finishing temperature, but addition of more than 0.2% increases the strength of the steel. This leads to a decrease in roundness. Therefore, the upper limit is set to 0.2%.

P: less than 0.009% P has an upper limit of 0.009% for exactly the same reason as Si. Even if 8P + Si ≦ 0.08 P: less than 0.009% and Si: 0.05% or less, there is a region where the roundness deteriorates when 8P + Si> 0.08. By setting 8P + Si ≦ 0.08, the roundness does not deteriorate. In other words, although the relationship between the roundness after cylindrical deep drawing and the mechanical properties of steel sheets has not been clarified yet, it is considered that elastic recovery behavior after deep drawing deformation, so-called springback, is an important factor. To be Therefore, it is considered that such elastic recovery behavior after deformation depends on the yield strength and work hardening behavior of the steel sheet.

The inventor of the present invention investigated the effect of the amounts of P and Si on the circularity, which significantly enhances the strength of steel by the addition of a small amount. In FIG. 4, the horizontal axis is Si weight% and the vertical axis is P weight%.
The effect of Si weight% and P weight% on the roundness (μm) is shown, and the circled numbers indicate the roundness. As a result, as shown in FIG. 4, P: less than 0.009%, S
i: There is a region where the roundness deteriorates even within the region limited to 0.05% or less, and P which affects such roundness
In consideration of the combined effect of the addition amount of Si and Si, it is understood that 8P + Si ≦ 0.08 must be added in addition to the range of P and Si added individually.

S: 0.005% or less If S is present as a solid solution S in the hot rolling stage, it deteriorates the hot ductility and is not preferable for rolling. In the present invention, S is Ti
However, since the amount of S added increases, the amount of Ti added also increases, which is economically disadvantageous, so the upper limit is 0.005.
%.

N: 0.0030% or less N is not preferable if it exists as solid solution N in the steel because not only the ductility of the steel sheet decreases but also the strength increases. In the steel of the present invention, Ti is added and fixed as TiN in the hot rolling process. However, as with S, when the amount of N increases, Ti
The upper limit is set to 0.0030% because the amount of addition increases and it becomes economically disadvantageous.

Al: 0.01% or more and 0.04% or less Al is required to be at least 0.01% in order to fix O in the steel and increase the yield of Ti in the steel during the steelmaking stage. However, if more than 0.04% is present in the steel, the solid solution increases the strength of the steel and reduces the ductility, so the upper limit is 0.04%.
And

Ti: {(48/14) N (%) + (48/32) S (%)
} Or more, {(48/14) N (%) + (48/32) S (%) +48/12
C (%) or Less} Ti is added to fix a part of the solid solution N, S, and C in the steel as a precipitate and to improve the workability of the steel sheet. Below the lower limit, the amount of Ti necessary to fix N and S in steel; {(48/14) N (%) + (48 /
32) S (%)} cannot be secured. Further, the solid solution C remaining in the steel improves the vertical cracking resistance, but when Ti is added in an amount exceeding the upper limit value, the C remaining in the steel is fixed as TiC, which deteriorates the vertical cracking resistance. I will end up.

B: 0.001% or more and 0.005% or less In the present invention, B not only improves the vertical cracking resistance of the steel sheet, but also improves the manufacturing stability in the hot rolling stage. In the amounts of Ti and N within the component range of the present invention,
B tends to segregate at the ferrite grain boundaries after hot rolling without forming precipitates, and B segregated at these grain boundaries improves the longitudinal crack resistance after deep drawing by increasing the grain boundary strength. . Further, B tends to segregate not only to the grain boundaries but also to the substructure of austenite processed by rolling during rolling in the austenite region during the hot rolling stage of production, similar to the effect in the ferrite region. It is in. B segregated in the substructure delays the rearrangement of dislocations in the processed austenite due to the effect of the solute drag, and thus delays the recrystallization reaction. The delay effect of such recrystallization is to shift the start time of the subsequent grain growth to the long time side, so when performing hot rolling by continuous rolling, each rolling stand passage time is limited and austenite grain refinement occurs. Promote. Further, γ-α transformation occurs in the cooling stage of the steel sheet after the rolling is completed in the austenite region. At the time of this γ-α transformation, B segregates at the austenite grain boundaries to delay the nucleation of ferrite from the austenite grain boundaries, which is the preferential nucleation site of ferrite grains, and consequently lowers the Ar3 point. There is.

In this case, if the amount of B is less than 0.001%,
The segregation of the steel sheet to the ferrite grain boundaries is insufficient, and the grain refining effect of B is small, so that the vertical crack resistance of the hot rolled sheet is not improved. On the other hand, when the amount of B exceeds 0.005%, not only the deformation resistance in the hot rolling stage increases and the rolling property deteriorates, but also the yield strength increases due to the solid solution strengthening by B and the roundness decreases. Resulting in.

Finishing temperature for hot rolling: 880 ° C to 950
℃ This hot rolling finishing temperature range indicates a temperature range in which good longitudinal crack resistance and good roundness after working can be ensured in the steel of the composition range of the present invention. Good vertical crack resistance or good roundness after processing cannot be secured. Further, in the steel outside the composition range of the present invention, the temperature range in which good longitudinal crack resistance and good roundness after working can be secured becomes narrow or does not exist.

That is, when manufacturing steel by hot rolling, the rolling end temperature (finishing temperature) is an important factor that determines the ferrite structure of the steel sheet. Generally, when finishing rolling is finished in the austenite region, if the hot rolling finishing temperature is high, only recrystallized austenite is generated from the austenite grains processed by rolling up to the γ-α transformation start temperature (Ar3 point). Instead, the grain growth continues,
The particle size of the austnast before the start of transformation becomes large. Therefore, along with this, the ferrite grain size after transformation also increases,
Longitudinal cracking resistance is also reduced. On the other hand, when the hot rolling finishing temperature is in the ferrite region, the recovery reaction of the worked ferrite proceeds preferentially over the austenite, so the frequency of nucleation during recrystallization decreases, and the grain size after recrystallization is the case of the austenite region finishing. Compared with, the resistance to vertical cracking is deteriorated.

Therefore, the present inventor examined the effect of the hot rolling finishing temperature on the vertical crack resistance by hot rolling using the sample steels shown in Table 1. In Table 1, A steel, B steel,
Steel C is a comparative steel and underline shows a composition outside the scope of the invention. Steel D, steel E, and steel F are steels of the present invention. In Fig. 1, the horizontal axis shows the hot rolling finish temperature, and the vertical axis shows the upper limit temperature of vertical cracking and the yield point (YP). It shows the effect of finishing temperature. The hot-rolled finished plate thickness is 3.2m
m, and the winding temperature after finish rolling was in the range of 550 to 680 ° C. As shown in FIG. 1, in the steels A, B and C having C content and B content lower than the steel composition range of the present invention, the sensitivity of the upper limit temperature of vertical cracking to the finishing temperature is strong and the vertical cracking resistance is good. It is impossible for steels A and B to keep the upper limit temperature of the occurrence of vertical cracks to be −80 ° C. or less. Also,
In Steel C as well, the finishing temperature region in which the upper limit temperature of vertical cracking is −80 ° C. or less is narrow, at 890 ° C. to 920 ° C. On the other hand, for steels D and E in the steel composition range of the present invention, 880 ° C
It is possible to stably obtain the upper limit temperature of vertical crack generation in a wide hot rolling finishing temperature range of 950 to 950 ° C and good vertical crack resistance of -80 ° C to -115 ° C. On the other hand, in the steel F in which the amounts of C and B exceeded the range of the present invention, in the hot rolling stage, the grain size was further refined and vertical crack resistance was secured in a wider finishing temperature range, but as shown in FIG. In addition, the yield strength is remarkably increased as the grain size is reduced, and the roundness after processing is reduced accordingly.

In the present invention, the main purpose is to secure stable vertical cracking resistance in the longitudinal direction of the hot rolled coil, and in order to secure good vertical cracking resistance over the entire length in the coil longitudinal direction, good longitudinal cracking resistance is ensured. It is important to optimize the steel components that allow a wide range of hot rolling finish temperatures to obtain cracking properties. As described above, in the composition range of the present invention, the hot rolling finishing temperature is 880 ° C.
Good vertical cracking resistance can be secured in a wide range from to 950 ° C.

The comparative steel G and the steel F of the present invention shown in Table 2 were hot-rolled to investigate the longitudinal change of longitudinal crack resistance in the finished hot-rolled coil having a plate thickness of 3.2 mm. At this time, the finishing temperature of both steels was in the range of 880 ° C to 950 ° C, and the winding temperature was 550 ° C to 680 ° C.
It was in the range of ℃. At each position in the longitudinal direction,
110mmφ at 5 points in the width direction from both edges to the center.
The blank was sampled, subjected to cylindrical deep-drawing cup molding with a punch of 50 mmφ, and edge trimming was performed. Then, the vertical cracking temperature was evaluated by the drop weight test at each test temperature depending on whether or not the cup cracked. .

FIG. 2 shows changes in the longitudinal cracking temperature of the hot-rolled coils of steel F and steel G in the longitudinal direction of the coil. The vertical crack generation temperature at each longitudinal position is indicated by the value at each position in the width direction at that position, and the maximum and minimum values. Steel F, which is a comparative steel, shows good vertical cracking resistance with a vertical cracking generation temperature of -80 ° C or less in the rolling longitudinal center portion, but has a vertical cracking generation temperature of -80 ° C at both ends of the hot rolled coil. Higher, defective parts of about 250 m are generated at both ends of the coil having a length of about 1200 m, and the yield of the product is remarkably low. On the other hand, in the steel of the present invention, the vertical crack generation temperature at both ends of the hot-rolled coil was found to rise under the same hot-rolling temperature conditions, but at a good level of -80 ° C or lower, and in comparison with Comparative Steel F. It can be seen that the amount of rise is small and good longitudinal crack resistance can be secured over the entire length of the coil.

Winding temperature: 680 ° C. or lower After finishing rolling, cooling is carried out on a runout table by a conventional method and winding is performed at 680 ° C. or lower. The reason why the upper limit of the coiling temperature is specified as 680 ° C. is as follows. After finish rolling, before transformation into a coiler, ferrite transformation occurs from the finely grained austenite, and ferrite grain growth occurs after transformation. Here, the winding temperature is 6
When the temperature is 80 ° C. or higher, ferrite grain growth becomes large, and the final ferrite grain size after winding and cooling becomes remarkably large. For this reason, the ferrite grain refinement due to the austenite grain refinement effect in the hot rolling finishing stage is offset, and the vertical cracking resistance deteriorates. Further, when the coiling temperature is 680 ° C. or higher, the scale generation amount increases in the coiling cooling stage, the pickling property deteriorates, and the surface quality of the steel sheet deteriorates.

[0027]

Embodiments of the present invention will be described below. After the steel having the chemical composition shown in Table 3 (1 to 6 of the present invention, 7 to 16 of the comparative steel) was melted, heated to a temperature range of 1150 ° C to 1280 ° C, and roughly rolled to a thickness of 35 mm. The 7-pass continuous rolling was performed under the hot rolling temperature conditions shown in Table 4. The finished plate thickness is 3.2 mm. After removing the scale from these hot-rolled sheets by pickling, the mechanical property values were measured by a tensile test after processing JIS 5 test pieces. The vertical cracking resistance and roundness of the steel sheet were also evaluated by the following methods. For vertical cracking resistance, a 110 mmφ blank is taken from a hot-rolled sheet, a cylindrical deep-drawing cup is formed with a 50 mmφ punch, and the edges are edge-cut, and then the cup is cracked by a drop weight test at each test temperature. Evaluation was made by using the temperature of vertical crack occurrence depending on the presence or absence of occurrence. On the other hand, the roundness is schematically shown in FIGS. 3 (a) and 3 (b) after a blank having a drawing ratio of 5.2 is sampled from a hot-rolled sheet and subjected to multistage deep drawing with a punch diameter of 220 mm. As described above, at each depth position of the cylindrical molded product, the diameters at four positions in the circumferential direction were measured, and the difference between the maximum value and the minimum value of the diameter was obtained and evaluated as the roundness. From Table 3, it can be seen that all of the steels of the present invention have a low vertical cracking temperature, a small difference between the maximum value and the minimum value of the diameter, and have good vertical cracking resistance and roundness. On the other hand, the winding temperature outside the range of the present invention is No. No. 1 * has a high vertical cracking generation temperature of -70 ° C and is a comparative steel No. 7 * ~ N
o. 11 *, No. 13 * -No. In the case of 16 * steel, the roundness is low even though the finishing temperature and the winding temperature are within the range of the present invention. 10 *, No. 12 *, N
o. It can be seen that the 16 * steel has a high vertical cracking temperature even though the finishing temperature and the winding temperature are within the range of the present invention.

[0028]

As described above, according to the present invention,
By limiting the chemical composition of the steel sheet as described above and specifying the hot rolling temperature conditions, it is possible to stably produce a hot-rolled steel sheet for deep drawing that is excellent in both longitudinal cracking resistance and roundness. .

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a hot rolling finish temperature, a yield strength, and an upper limit temperature for generating vertical cracks.

FIG. 2 is a diagram showing a change in vertical crack generation temperature in the longitudinal direction of the hot-rolled coil.

3A and 3B show positions for evaluating circularity after cylindrical deep drawing, where FIG. 3A is a schematic drawing after deep drawing and a round drawing, and FIG.
FIG. 4 is a view showing an inner diameter measurement position of a wheel cut surface.

FIG. 4 is a diagram showing the influence of the amounts of Si and P added on the roundness after cylindrical deep drawing.

Front page continuation (72) Inventor Masaki Omura 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Sao Oda 1-2 1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Inside Steel Pipe Co., Ltd.

Claims (1)

[Claims] 1. By weight%, C: more than 0.0040% and 0.0080% or less Mn: 0.2% or less Si: 0.05% or less P: less than 0.009% 8P + Si ≦ 0.08% S: 0.005% or less Al: 0.01% or more and 0.04% or less N: 0.0030% or less Ti: {(48/14) N (%) + (48/32) S (%)} or more ,{(Four
8/14) N (%) + (48/32) S (%) +48/12 C (%) or less} B: 0.001% or more and 0.005% or less, with the balance being Fe and unavoidable impurities Steel having a finish temperature of 880 ° C to 950 ° C and then wound in a temperature range of 680 ° C or less, which is excellent in longitudinal cracking resistance and roundness Manufacturing method of rolled steel sheet.