JP5087980B2 - High-strength hot-rolled steel sheet excellent in punching workability and manufacturing method thereof - Google Patents

Description

  The present invention is subjected to burring and stretch flange processing, for example, suitable for use in high-strength structural parts such as automobiles. The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same.

  In recent automobile parts, weight reduction is emphasized from the viewpoint of energy saving, and in addition to this, there is a tendency for safety and durability to be emphasized, and higher strength is progressing more rapidly than before. . As an example of such a tendency, a high-strength steel plate is applied not only to the outer panel of an automobile but also to a structural member. Steel sheets applied to such structural members are required to have workability such as hole expansibility in addition to press formability. For this reason, development of high-strength hot-rolled steel sheets having excellent workability such as burring and stretch flange processing has been promoted (see, for example, Patent Document 1).

  Furthermore, along with the expansion of application of these high-strength hot-rolled steel sheets, hot-rolled steel sheets having particularly excellent stretch flangeability with a tensile strength of 690 MPa or more have been proposed (see, for example, Patent Documents 2 and 3). However, along with the high strength of the hot-rolled steel sheet, there is a problem that peeling (peeling) or curling (meklet) defects occur on the end face of the hole formed by punching the steel sheet. These defects not only significantly impair the design of the end face of the product, but also have a risk of becoming a stress concentration part and affecting the fatigue strength.

In order to solve such a problem, a hot rolled steel sheet in which the area ratio of the hard second phase and cementite is limited and damage to the punched end face is suppressed has been proposed (see, for example, Patent Documents 4 and 5). However, even if the generation of hard second phase and cementite is suppressed, if the clearance of the punching process is the most severe condition for the damage of the end face, defects may occur on the end face of the hole. .
Japanese Patent Laid-Open No. 10-36917 JP 2001-172745 A JP 2006-152341 A JP 2004-315857 A JP 2005-298924 A

  The present invention has been made in order to solve the above-mentioned problems, and has both excellent stretch flangeability and ductility, and particularly has a high strength of a tensile strength of 690 MPa or more and is punched under extremely severe conditions. An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in punching workability and a method for producing the same, which can reliably prevent damage to the end face even when the work is performed.

As a result of examining the correlation between the frequency of occurrence of damage at the punched end face and the segregation element type and segregation amount at the grain boundaries, the present inventors have determined that the clearance of the punching process is the strictest condition. It has been found that the damage of the punched end face is reduced by segregating an appropriate amount of C and B at the large-angle crystal grain boundaries of more than 0 °.
The present invention has been made based on such findings, and the gist thereof is as follows.

(1) By mass%, C: 0.010 to 0.200%, Si: 0.01 to 1.50%, Mn: 0.25 to 3.00%, B: 0.0002 to 0.0030% And P: 0.05% or less, and Ti: 0.03 to 0.20%, Nb: 0.01 to 0.20%, V: 0.01 to 0.20% , Mo: contains any one or more of 0.01 to 0.20%, the balance consists of Fe and inevitable impurities, and the segregation amount of C to the large-angle grain boundaries of ferrite and B A high-strength hot-rolled steel sheet excellent in punching workability, characterized in that the total of the segregation amount is in the range of 4 to 10 atoms / nm ² .
(2) In mass%, P is limited to 0.02% or less, and the amount of segregation of P to the large-angle grain boundary of ferrite is 1 atoms / nm ² or less. High-strength hot-rolled steel sheet with excellent punchability.
(3) The high-strength hot-rolled steel sheet having excellent punchability as described in (1) or (2) above, wherein the segregation amount of C to the large-angle grain boundaries of ferrite is ² atoms / nm ² or more. .

(4) A method for producing the high-strength hot-rolled steel sheet according to (1) or (2), wherein the steel material having the component according to (1) or (2) is heated to 1200 ° C. or higher. Rolling is completed at a temperature of Ar ₃ or higher, then cooled to a range of 600 to 650 ° C. at a cooling rate of 50 ° C./s or higher, and further 350 to 600 ° C. at a cooling rate of 10 ° C./s or lower. A method for producing a high-strength hot-rolled steel sheet excellent in punching workability, characterized by cooling within a range and winding up.
(5) A method for producing the high-strength hot-rolled steel sheet according to (3) above, wherein the steel material having the component according to claim 1 or 2 is heated to 1200 ° C or higher, and Ar is at a temperature of ₃ points or higher. Rolling is completed, and then cooled to a range of 600 to 650 ° C. at a cooling rate of 50 ° C./s or more, and further cooled to a range of 350 to 550 ° C. at a cooling rate of 10 ° C./s or less. A method for producing a high-strength hot-rolled steel sheet excellent in punching workability, characterized in that it is taken.

  According to the high-strength hot-rolled steel sheet excellent in punching workability and the manufacturing method thereof of the present invention, the balance between stretch flangeability and ductility is good, and in particular, the tensile strength has a high strength of 690 MPa or more, In addition, it is possible to provide a hot-rolled high-strength steel sheet excellent in punching workability that suppresses the occurrence of damage to the end face during punching and a manufacturing method thereof, regardless of the clearance conditions of the punching process. It is remarkable.

  Hereinafter, an embodiment of a high-strength hot-rolled steel sheet excellent in punching workability and a method for producing the same according to the present invention will be described. The present embodiment will be described in detail in order to better understand the gist of the invention. Therefore, the present invention is not limited unless otherwise specified.

The high-strength hot-rolled steel sheet (hereinafter sometimes abbreviated as a rolled steel sheet) having excellent punchability according to the present invention is mass%, C: 0.010 to 0.200%, and Si: 0.01 to 1. .50%, Mn: 0.25 to 3.00%, B: 0.0002 to 0.0030%, P: 0.05% or less, and Ti: 0.03 to 0 .20%, Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, Mo: 0.01 to 0.20% The balance is composed of Fe and unavoidable impurities, and the sum of the segregation amount of C and the segregation amount of B to the large-angle crystal grain boundaries of ferrite is approximately 4 to 10 atoms / nm ² .

  The present inventors performed punching with various clearances using a high-strength hot-rolled steel sheet having a tensile strength of 690 MPa or more excellent in ductility and hole expansibility, and quantitatively investigated the end face properties. Specifically, a 10 mm diameter hole was punched by changing the clearance according to the method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996, and in the entire circumference of the end surface punched into a circle, damage was visually observed. The angles were measured and summed, and the value was divided by 360 ° to determine the damage occurrence ratio (referred to as the punched end face damage occurrence ratio) on the entire circumference of the punched end face.

The graph of FIG. 1 shows the correlation between the clearance at the time of punching and the occurrence ratio of the punched end face damage. As shown in Fig. 1, if the clearance is increased, peeling or twisting damage that cannot be confirmed when punched with a clearance of around 12.5% recommended in a normal hole expansion test will occur. 16% clearance was found to be the most severe condition.
Therefore, the following investigation was conducted by adopting this 16% clearance, the influence of the structure on the punching workability of the steel sheet, and the frequency of occurrence of damage to the punched end face, that is, the punched end face damage occurrence ratio and the large angle grain boundary. The relationship between the type of segregated elements and the amount of segregation was investigated.

  First, in mass%, C: 0.01-0.2%, Si: 0.01-1.5%, Mn: 0.25-3%, B: 0.0002-0.003% P: 0.05% or less, Ti: 0.03-0.2%, Nb: 0.01-0.2%, V: 0.01-0.2%, Mo: A steel piece containing any one or more of 0.01 to 0.2%, with the balance being Fe and inevitable impurities, and hot-rolling the steel plate under various heat treatment conditions. Manufactured. From these steel plates, No. 5 test piece of JIS Z 2201 was collected, and tensile properties were evaluated according to JIS Z 2241. Moreover, the hole expansion test was done in accordance with the test method described in Japan Iron and Steel Federation Standard JFS T 1001-1996, and the stretch flangeability of the steel sheet was evaluated. In addition, the punching end face damage occurrence ratio was evaluated after the punching process and before the hole expansion test.

  Next, the segregation amounts of B, C, and P at the large-angle grain boundaries of ferrite in five or more locations in each steel material were measured, and the average value was obtained. The large-angle crystal grain boundary described in the present invention is a crystal grain boundary having a grain boundary angle of 15 ° or more. Small angle grain boundaries with an angle of less than 15 ° show a tendency for the amount of segregation to decrease compared to large angle grain boundaries due to the difference in the number of segregated element trap sites, etc., and in the ferrite structure of the high strength hot rolled steel sheet of the present invention. Then, since the crystal grain boundaries are mostly large-angle grain boundaries, the amount of segregation at the large-angle grain boundaries was measured. The grain boundary angle was determined by analyzing the Kikuchi figure obtained from transmission electron microscope observation of the sample.

  Next, as a method for measuring the amount of segregated elements, in order to strictly compare the distribution of segregated elements in such a small region, a method for obtaining the amount of excision as follows using a three-dimensional atom probe method. Is suitable. That is, a needle-like sample is produced from the crystal grain boundary portion of the sample to be measured by cutting and electropolishing. At this time, a focused ion beam processing method may be used together with the electropolishing method. Then, the region including the grain boundary and the grain boundary angle are observed with a relatively wide field of view by FIM, and three-dimensional atom probe measurement is performed.

  In the three-dimensional atom probe measurement, the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space. At the grain boundary position, since the atomic plane is discontinuous, it can be recognized as a grain interface, and the appearance of segregation of various elements can be visually observed. Next, in order to estimate the amount of segregation of each element, a rectangular parallelepiped was cut out perpendicularly to the crystal grain boundary from the atomic distribution image including the crystal grain boundary to obtain a ladder chart. An example of observation of a crystal grain boundary and an example of ladder chart analysis are shown in the schematic diagram of FIG. 2 and the graph of FIG. From such ladder chart analysis, the amount of segregation of each atom was evaluated using the excess amount that is segregated, that is, the number of atoms added from the solid solution amount per unit grain interface area (non-patent document). 1: Takahashi et al., “Quantitative observation of grain boundary segregation carbon content in paint bake hardened steel sheet,” Nippon Steel Technical Report, No. 381, October 2004, p.26-30).

  The graph of FIG. 4 shows the relationship between the total amount of segregation of C and B and the punched end face damage occurrence ratio of the steel material. As shown in FIG. 4, segregation of C and B was observed at the ferrite large-angle grain boundaries of the steel sheet having a small punching end face damage occurrence ratio. In the high-strength hot-rolled steel sheet of the present invention, carbides of Ti, Nb, V, and Mo are partially dispersed and precipitated in crystal grains to ensure solid solution C in the crystal grains, and nitrides of Ti, Nb, and V By precipitating BN and suppressing the precipitation of BN and leaving the solid solution B in the crystal grains, the total amount of segregation of C and B at the grain boundaries can be within an appropriate range. Thereby, the damage resistance of the punching end surface at the time of punching a steel plate can be maintained satisfactorily. The reason why the end face damage resistance of the steel sheet is improved may be that the segregated C and B reinforce the crystal grain boundaries and suppress the crack growth at the grain boundaries during the punching process.

  Further, as shown in the graph of FIG. 5, it was found that the punching damage generation ratio can be reduced by setting the segregation amount of C and B to a certain level or more and suppressing the segregation amount of P at the grain boundaries. Here, FIG. 5 is a graph showing the relationship between the amount of segregation of P and the punched end face damage occurrence ratio.

  From the above results, when carbide and BN are excessively precipitated during cooling after hot rolling, solid solution C and solid solution B are reduced, and C and B segregating at the grain boundaries are reduced, resulting in damage to the punched end face. It became clear. Therefore, a further study was conducted on a method for improving the punching workability by segregating more C and B in the large-angle grain boundaries than in ordinary steel materials. As a result, the present inventors have found that when the precipitation of carbides and BN in the crystal grains is suppressed, damage to the punched end face is suppressed. On the other hand, it was also found that, unlike C and B, there is an element that reduces the grain boundary strengthening amount when segregating at the grain boundary.

[High-strength hot-rolled steel sheet]
Hereinafter, the high-strength hot-rolled steel sheet excellent in punching workability of the present invention will be described in detail.

`` Segregation amount ''
If the incidence of punching end face damage is within 0.3 under the strictest clearance conditions, it is an acceptable range for practical steel. In the study in the present invention, the clearance of 16% was the most severe condition, but this varies depending on the material of the steel plate and the tool, so the punching is performed by changing the clearance between 12.5 and 25%. Therefore, it is necessary to confirm the properties of the end face and the most severe clearance conditions. In order to keep the damage to the end face when the steel sheet is punched under the strictest clearance conditions within 0.3, it is necessary to optimize the amount of grain boundary segregation elements at the grain boundaries as follows. .

As shown in the graph of FIG. 4, when the total amount of C segregation and B segregation at the grain boundaries is 4 to 10 atoms / nm ² , the steel sheet was punched under the most severe clearance conditions. End face damage can be within 0.2. When the total amount of segregation of C and segregation of B is less than 4 atoms / nm ² , the grain boundary strengthening amount is insufficient, and punching end face damage becomes remarkable.

On the other hand, if the total amount of C segregation and B segregation at the grain boundary exceeds 10 atoms / nm ² , C concentrates at the grain boundary and the precipitation of cementite cannot be suppressed. This is a starting point for damage to the punched end face. A more preferable range of the total amount of C segregation and B segregation at the grain boundaries is 6 to 9 atoms / nm ² at which damage to the punched end face hardly occurs.

Furthermore, the segregation amount of C alone is preferably ² atoms / nm ² or more. The reason for this is that although B plays a role in supplementing the grain boundary strengthening, in order to achieve a segregation amount of 4 atoms / nm ^{2 with} only B, excessive addition of B is necessary, resulting in a decrease in ductility and workability. . Therefore, the segregation amount of C is preferably ² atoms / nm ² or more.

In addition, the total amount of the segregation amount of C and the segregation amount of B is made into the range of 4-10 atoms / nm < ² > by setting the coiling temperature of the hot rolling mentioned later for details in the range of 350-600 degreeC. Can do. Furthermore, by setting the upper limit of the coiling temperature to 550 ° C. or less, the amount of segregation of C can be set to ² atoms / nm ² or more.

On the other hand, as for P, it is preferable that the amount of segregation is small. This is probably because P has the effect of embrittlement of the grain boundaries. Moreover, when the amount of segregation of P increases, the progress of the crack at the time of stamping is promoted, and the occurrence rate of damage is increased. In addition, there is a concern that the amount of segregation of C and B may be reduced due to P occupying the segregation sites. For these reasons, the amount of segregated P is preferably 1 atoms / nm ² or less. Further, in order to set the amount of segregation of P to 1 atoms / nm ² or less, the P content may be limited to 0.02% or less.

"Chemical composition"
Below, the limitation conditions of the chemical component composition of the high-strength hot-rolled steel sheet of the present invention will be described in detail.
In the present invention, the steel sheet structure has the above grain boundary segregation amount, the steel sheet has an elongation of 20% or more, the hole expansion ratio is 80% or more, the tensile strength is 690 MPa or more, and the steel sheet is punched under the strictest clearance conditions. In order to make the end face damage when performed within 0.3, it is preferable to define the composition of the steel sheet as follows. In the following description, “%” means “% by mass” unless otherwise specified.
In addition, the basic components described below are sufficient to achieve the intended effect of the present invention, but contain other components as long as they do not impair the steel sheet characteristics targeted by the present invention. Is acceptable. For example, less than 0.2% Cr or less than 0.15% Cu may be contained.

(C: 0.010-0.200%)
C is an element that contributes to the improvement of strength and needs to be added in an amount of 0.010% or more. Moreover, in order to ensure the amount of C segregation to a grain boundary, 0.020% or more of addition is more preferable. On the other hand, when the content of C exceeds 0.200%, formation of cementite and formation of a transformation structure such as pearlite and martensite are promoted, and elongation and hole expansibility are lowered. Therefore, the C content is in the range of 0.010 to 0.200%.

(B: 0.0002 to 0.0030%)
B is an important element in the present invention, and even if the segregation of C at the grain boundary is insufficient due to the addition of B, damage to the punched end face is prevented. In order to obtain such an effect, it is necessary to add B in an amount of 0.0002% or more. On the other hand, when B is added in excess of 0.0030%, workability such as ductility is lowered. Therefore, the B content is in the range of 0.0002 to 0.0030%.

(Si: 0.01-1.50%)
Si is effective for improving the strength as a solid solution strengthening element, and 0.01% or more of addition is necessary to obtain such an effect. On the other hand, if the Si content exceeds 1.50%, workability deteriorates. Therefore, the Si content is in the range of 0.01 to 1.50%.

(Mn: 0.25 to 3.00%)
Mn is an element necessary for deoxidation and desulfurization, and is also effective as a solid solution strengthening element. In order to obtain such an effect, the Mn content needs to be 0.25% or more. On the other hand, if the Mn content exceeds 3.00%, segregation is likely to occur and stretch flangeability is deteriorated. Therefore, the Mn content needs to be in the range of 0.25 to 3.00%.

(P: 0.02% or less)
P is an impurity, and the content of P needs to be limited to 0.05% or less. Further, in order to suppress segregation of P to grain boundaries and prevent grain boundary cracking, it is more preferable to limit to 0.02% or less.

  Furthermore, in the present invention, in order to control the segregation amount of C, one or more of Ti, V, Nb, and Mo may be contained as a carbide precipitation element in the ferrite crystal grains of the steel sheet. is necessary. In order to promote grain boundary segregation of B, it is preferable to contain any one or more of Ti, V and Nb which are nitride precipitation elements to suppress the precipitation of BN. .

(Ti: 0.03-0.20%)
(V: 0.01-0.20%)
(Nb: 0.01-0.20%)
Ti, V, and Nb are elements that precipitate carbide and nitride in ferrite crystal grains and increase the strength of the steel sheet by precipitation strengthening. In order to sufficiently generate carbide and nitride, it is preferable that the addition amount of Ti is 0.03% or more, and the addition amounts of V and Nb are each 0.01% or more. On the other hand, if the added amounts of Ti, V, and Nb exceed 0.20%, carbides and nitrides may become coarse. Therefore, the Ti content is preferably 0.03 to 0.20%, and the V and Nb contents are preferably 0.01 to 0.20%, respectively.

(Mo: 0.01-0.20%)
Mo is a carbide forming element and can be contained for the purpose of fixing C which contributes to precipitation strengthening and contributes to the formation of cementite in the ferrite crystal grains. In order to sufficiently generate carbide, it is preferable to add Mo by 0.01% or more. On the other hand, when the addition amount of Mo exceeds 0.20%, coarse carbides may be generated. Therefore, the Mo content is preferably in the range of 0.01 to 0.20%.

  Furthermore, in the high-strength hot-rolled steel sheet of the present invention, it is preferable to limit the upper limit of the N, S, and Al contents in the chemical component composition as follows.

(N: 0.009% or less)
N forms nitrides and lowers the workability of the steel sheet, so the content is preferably limited to 0.009% or less.

(S: 0.005% or less)
S, as inclusions such as MnS, deteriorates stretch flangeability and further causes cracking during hot rolling, so the content is preferably reduced as much as possible. In particular, in order to prevent cracking during hot rolling and improve workability, it is preferable to limit the S content to 0.005% or less.

(Al: 0.002% or more)
Since Al forms precipitates such as nitrides and impairs the workability of the steel sheet, it is preferably limited to 0.5% or less. In addition, it is more preferable to add 0.002% or more for molten steel deoxidation.

  In the present invention, in addition to the above basic components, W may be added as a solid solution strengthening element for the purpose of improving the strength of the steel sheet.

[Method for producing high-strength hot-rolled steel sheet]
The method for producing a high-strength hot-rolled steel sheet having excellent punchability according to the present invention heats a steel material having the above-described components to 1200 ° C. or higher, completes rolling at a temperature of Ar ₃ points or higher, and then 50 ° C. This is a method of cooling in a range of 600 to 650 ° C. at a cooling rate of not less than / s, and further winding in a range of 350 to 600 ° C. at a cooling rate of not more than 10 ° C./s.
Moreover, the manufacturing method of the high-strength hot-rolled steel sheet of the present invention can cool the steel sheet at a speed of 10 ° C./s or less and set the temperature at the time of winding to a range of 350 to 550 ° C.

"Hot rolling temperature"
Steel material is melted and cast by a conventional method, and the obtained steel slab is hot-rolled. The steel slab is preferably manufactured by continuous casting equipment from the viewpoint of productivity. The heating temperature of the hot rolling is set to 1200 ° C. or higher in order to sufficiently decompose and dissolve the carbide forming element and carbon in the steel material. After casting, the steel slab may be cooled and rolling may be started at a temperature of 1200 ° C. or higher. When heating a steel piece cooled to 1200 ° C. or lower, it is preferable to hold for at least 1 hour.
The end temperature of hot rolling should be not less than the Ar ₃ transformation point in order to suppress variations in the characteristics of the steel sheet, and it is necessary to end hot rolling in the austenite region.

"Cooling rate and temperature after hot rolling"
After the hot rolling is completed, in order to suppress the ferrite transformation, pearlite transformation, and formation of coarse carbide as much as possible, it is necessary to set the cooling rate to 50 ° C./s or more and the cooling end temperature to 650 ° C. or less. . Moreover, in order to ensure the amount of segregation of B, it is necessary to make the end temperature of cooling 600 degreeC or more.

"Cooling speed and winding temperature before winding the steel sheet"
Subsequently, in order to realize the ferrite transformation and precipitation of fine carbides, it is necessary to cool the steel sheet to the coiling temperature at a rate of 10 ° C./s or less. When the cooling rate of the steel plate is faster than 10 ° C./s, carbide precipitation becomes insufficient and the amount of segregation of C increases. After finishing the hot rolling, cooling of the steel sheet to a temperature range of 600 to 650 ° C. is completed at a cooling rate of 50 ° C./s or more, and then 10 s at a cooling rate of 10 ° C./s or less before winding. The following cooling is performed. As a result, the ferrite transformation is partially performed and the carbide is finely precipitated partially, and the segregation amount of C can be secured. Further, the steel sheet may be held at a temperature at which cooling is completed, but in this case, the holding time is preferably 10 s or less from the viewpoint of productivity. If the cooling time at 10 ° C./s or less is longer than 10 s, the precipitation of carbides proceeds, the C to be segregated becomes insufficient, and it becomes difficult to obtain the C grain boundary segregation amount of the present invention.

  On the other hand, in order to achieve grain boundary segregation of C and B, the coiling temperature after cooling needs to be in the range of 350 to 600 ° C. When the coiling temperature is less than 350 ° C., the segregation amount of C and B is insufficient, and hard martensite may be generated to deteriorate stretch flangeability. On the other hand, if the upper limit of the coiling temperature exceeds 600 ° C., pearlite cementite that is harmful to stretch flangeability may be generated. Further, when the coiling temperature exceeds 550 ° C., the precipitation of carbide proceeds in the crystal grains. Therefore, in order to ensure the amount of segregation C to the grain boundary, it is preferable to set it as 550 degrees C or less.

  In addition, when water cooling is performed with a normal hot rolling mill and the end temperature is set to 600 ° C., the coiling temperature becomes less than 600 ° C. due to air cooling from the end of water cooling to winding, and the grain boundary of B during air cooling. Segregation can be achieved. When the winding temperature is 600 ° C., the end temperature may be determined in consideration of the temperature drop from the end of water cooling to the winding.

  As described above, according to the high-strength hot-rolled steel sheet excellent in punching workability and the manufacturing method thereof according to the present invention, the balance between stretch flangeability and ductility is good, and particularly, the tensile strength is as high as 690 MPa or more. Because it can provide a hot-rolled high-strength steel sheet that has strength and is excellent in punching workability that suppresses the occurrence of damage to the end face during punching, regardless of the clearance conditions of the punching process, and its manufacturing method. The industrial contribution is very remarkable.

  Hereinafter, examples of the high-strength hot-rolled steel sheet excellent in punching workability according to the present invention will be given and the present invention will be described more specifically, but the present invention is not limited to the following examples from the beginning, The present invention can be implemented with appropriate modifications within a range that can be adapted to the gist of the following, and these are all included in the technical scope of the present invention.

  First, steels having symbols A to I having chemical composition shown in Table 1 below were melted. The component values in Table 1 indicate mass% as chemical analysis values, and “0” in the component values in Table 1 below means that they are not intentionally added. Moreover, the underline in the following Table 1 means outside the scope of the present invention.

Next, hot rolling was performed under the production conditions shown in Table 2 below to produce a hot rolled steel sheet. Here, the hot rolling end temperatures in Table 2 below are all Ar ₃ or higher. The primary cooling is a cooling process immediately after the end of hot rolling, and the secondary cooling is a cooling process before winding. Moreover, the underline in the following Table 2 means outside the scope of the present invention.

From these steel plates, No. 5 test piece described in JIS Z 2201 was processed, and tensile properties were evaluated along the test method described in JIS Z 2241.
Further, as one of the stretch flangeability, the hole expansion test was evaluated according to the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.
In addition, the ratio of occurrence of damage to the punched end face is determined by punching a 10 mm diameter hole in the same manner as the hole expansion test, visually observing the shape of the end face, and measuring the angle within a range where damage is recognized in the circular punched end face. The ratio was obtained by

In addition, a columnar sample of 0.3 mm × 0.3 mm × 10 mm is cut out from each of the above steel plates, and the target grain boundary portion is sharpened by electrolytic polishing or focused ion beam processing, and three-dimensional atom probe measurement is performed. It was. First, in order to estimate the segregation amount of each element at the grain boundary, a rectangular parallelepiped was cut out from the atomic distribution image including the grain boundary perpendicular to the grain boundary to obtain a ladder chart. Next, from the ladder chart analysis, the segregation amount of each atom was evaluated using the Excess amount. And in each steel material, the segregation amount of each element was investigated about five or more grain boundaries, and the average value was made into each element segregation amount of each steel material.
A list of these evaluation results is shown in Table 3 below.

  In Table 3, test no. 2, 3, 5, 7-9, and 11 are examples in which the components and production conditions of the steel sheet are within the scope of the present invention. It is clear from the evaluation results shown in Table 3 that these steel plates of the present invention have high strength, good hole expansibility, and a small damage ratio on the punched end face.

On the other hand, no. Nos. 1 and 6 have high end temperatures for primary cooling. Nos. 4 and 10 have a slow primary cooling rate. No. 10 is an example in which the total temperature of the grain boundary segregation of C and B is insufficient due to the high winding temperature, and the punched end face is damaged.
No. No. 12 is an example in which the amount of addition of B is insufficient, the grain boundary segregation amount cannot be achieved, and end face damage occurs during punching. On the other hand, no. No. 13 is an example in which the amount of addition of B exceeds the range, the grain boundary segregation amount of C and B increases, damage to the punched end face occurs, and elongation decreases.
No. No. 14 is an example in which the P content is large, the elongation and the hole expansion rate are reduced, and damage to the punched end surface occurs.

  From the above results, when using a steel material having a chemical composition within the range specified by the present invention and producing a steel sheet under the manufacturing conditions within the range specified by the present invention, both excellent stretch flangeability and ductility are achieved. A high-strength hot-rolled steel sheet having a high strength of 690 MPa or more and capable of reliably preventing damage to the end face even when punching is performed under extremely severe conditions is obtained. It is clear.

It is a figure explaining an example of the high intensity | strength hot-rolled steel plate excellent in the punching property which concerns on this invention, and is a graph which shows the correlation with the clearance of a punching process, and a punching end surface damage incidence. It is a figure explaining an example of the high intensity | strength hot-rolled steel plate excellent in the punching property which concerns on this invention, and is a schematic diagram which shows the three-dimensional atomic distribution image of a crystal grain boundary position. It is a figure explaining an example of the high intensity | strength hot-rolled steel plate excellent in the punching property which concerns on this invention, and is a ladder chart analysis graph which analyzed the three-dimensional atomic distribution image of the crystal grain boundary position shown in FIG. It is a figure explaining an example of the high intensity | strength hot-rolled steel plate excellent in the punching property which concerns on this invention, and is a graph which shows the correlation of the segregation amount of C and B, and a punching end surface damage incidence. It is a figure explaining an example of the high intensity | strength hot-rolled steel plate excellent in the punching property which concerns on this invention, and is a graph which shows the correlation of the amount of P segregation and punching end surface damage incidence.

Claims (5)

% By mass
C: 0.010-0.200%
Si: 0.01 to 1.50%,
Mn: 0.25 to 3.00%,
B: 0.0002 to 0.0030%
Each containing
P: limited to 0.05% or less,
Ti: 0.03 to 0.20%,
Nb: 0.01-0.20%,
V: 0.01-0.20%,
Mo: 0.01-0.20%
And the balance is composed of Fe and inevitable impurities, and the sum of the segregation amount of C and the segregation amount of B to the large-angle grain boundaries of ferrite is 4 to 10 atoms. A high-strength hot-rolled steel sheet excellent in punching workability, characterized by being in the range of / nm ² . % By mass
^2. High strength excellent in punching workability according to claim 1, wherein P is limited to 0.02% or less, and the amount of segregation of P to the large-angle grain boundaries of ferrite is 1 atoms / nm ² or less. Hot rolled steel sheet. The high-strength hot-rolled steel sheet with excellent punchability according to claim 1 or 2 , wherein the amount of segregation of C to the large-angle grain boundaries of ferrite is ² atoms / nm ² or more. A method for producing the high-strength hot-rolled steel sheet according to claim 1 or 2,
The steel material having the component according to claim 1 or 2 is heated to 1200 ° C or higher, and rolling is completed at a temperature of Ar ₃ or higher, and then within a range of 600 to 650 ° C at a cooling rate of 50 ° C / s or higher. A method for producing a high-strength hot-rolled steel sheet excellent in punching workability, wherein the steel sheet is further cooled to a temperature of 350 to 600 ° C. and wound at a cooling rate of 10 ° C./s or less. A method for producing the high-strength hot-rolled steel sheet according to claim 3,
The steel material having the component according to claim 1 or 2 is heated to 1200 ° C or higher, and rolling is completed at a temperature of Ar ₃ or higher, and then within a range of 600 to 650 ° C at a cooling rate of 50 ° C / s or higher. A method for producing a high-strength hot-rolled steel sheet excellent in punching workability, wherein the steel sheet is further cooled to a temperature of 350 ° C./s at a cooling rate of 10 ° C./s or less and wound up.

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