MX2013010601A

MX2013010601A - Steel sheet for hot-stamped member and process for producing same.

Info

Publication number: MX2013010601A
Application number: MX2013010601A
Authority: MX
Inventors: Makijun; Hiroyuki Tanahashi
Original assignee: Nippon Steel & Sumitomo Metal Corp
Priority date: 2011-03-18
Filing date: 2012-03-16
Publication date: 2013-10-01
Also published as: KR20130126714A; WO2012128225A1; EP2687620A1; JPWO2012128225A1; CA2829327C; BR112013023792A2; RU2013146540A; US20140004378A1; JP5605503B2; CN103443317A; CA2829327A1; MX360240B; EP2687620A4; ZA201307377B; RU2560890C2

Abstract

Provided is a steel sheet for giving members by hot stamping, the members having excellent fatigue properties equal to those of ordinary high-strength steel sheets that have the same strength as the steel sheet. Also provided is a process for producing the steel sheet. The steel sheet for hot-stamped members contains, in terms of mass%, 0.15-0.35% C, 0.01-1.0% Si, 0.3-2.3% Mn, and 0.01-0.5% Al, with the remainder comprising Fe and incidental impurities, wherein the impurities comprise up to 0.03% P, up to 0.02% S, and up to 0.1% N as chemical components. The steel sheet is characterized in that the Vickers hardnesses of the steel sheet measured at sites located at a distance of 20 Âµm from a surface of the steel sheet toward the sheet thickness direction have a standard deviation of 20 or less. This steel sheet is produced through a recrystallization annealing step comprising: a first stage in which a cold-rolled steel sheet obtained by hot-rolling and then cold-rolling a steel that contains the chemical components is heated from room temperature to 600-700ÂºC at an average heating rate of 8-25 ÂºC/sec; and a second stage in which the steel sheet is successively heated to 720-820ÂºC at an average heating rate of 1-7 ÂºC/sec.

Description

STEEL PLATE FOR HOT STAMPED MEMBER AND METHOD TO PRODUCE IT Technical Field The present invention relates to a steel plate for a hot stamped member which is suitable for the hot stamping method, one of the forming methods provides a high strength member, and a production method thereof.

Background Technique In the field of automobiles, construction machinery, etc., vigorous efforts have been made to reduce weight through the use of high strength materials. For example, in automobiles, the amount of use of high strength steel plate has increased steadily for the purpose of nullifying the increase in vehicle weight that accompanies improvements in safety against impact and performance and further improves efficiency of fuel to reduce the amount of carbon dioxide emission.

In the trend towards the expanded use of such a high strength steel plate, the biggest problem, inevitable when the strength of the steel plate is raised, is the emergence of the phenomenon called "degradation of the Shape fixing capacity. "This phenomenon is the general term for the loss of ease to obtain an objective shape due to the increase in the amount of elastic recovery after shaping that accompanies the greater strength. They were necessary as low resistance materials (materials with fixation capabilities that are excellent or have no problem) (for example, straightening rectifier) have been made or the product forms have been changed.

As a method to deal with this situation, the hot forming method called the "hot stamping method" has been the subject of attention. This heats a steel plate (worked material) to a predetermined temperature (generally, the temperature that results in an austenite phase), to reduce the strength (ie, facilitate the shaping), then conforms it by a matrix of a temperature lower than the material worked (e.g., room temperature) to thereby easily impart a shape and simultaneously use the temperature between the two for heat treatment by rapid cooling (quenching) to ensure the strength of the product conformed.

Several techniques that relate to the steel plate suitable for such hot stamping method and method to conform it have been reported.

PLT1 shows the steel plate obtained by controlling the amounts of elements that contain the steel plate and the ratio between the amounts of elements with the predetermined margins to provide a member which is excellent in imparting characteristics and characteristics of delayed fracture after hot forming (synonymous with hot stamping).

PLT 2 in the same as the previous one, describes a method that comprises forming the quantities of elements that the steel plate contains and the ratio between the quantities of elements with predetermined margins and heating before the conformation of the steel plate in an atmosphere of nitriding or a carburation atmosphere to obtain a high strength part.

PLT 3 describes means for prescribing the composition and microstructure of the steel plate and limiting the heating conditions and forming conditions to obtain the hot stamped parts with high productivity.

Recently, the hot stamping method has been widely recognized for its utility. The members for whom its application has been studied have become much more diverse. Among these, for example, there are parts, such as parts of car bodies, where not only the Resistance of the parts, but also the characteristic of fatigue is an important, necessary feature.

The fatigue characteristic of the steel plate is improved together with the static resistance, so that the steel plate (product) formed with high strength by the hot stamping method can also be expected to show a commensurate fatigue characteristic, if it is compared with the steel plate of the same strength that does not use the hot stamping method (high strength steel plate produced when controlling the composition or production method of the resistant steel plate, hereinafter referred to as "steel plate" of high ordinary resistance "), it is clear that depending on the production conditions, the fatigue characteristics of the former were inferior to the later.

Studied in detail, it was discovered that compared to the hardness deviation of the most superficial part of the "ordinary high strength steel plate", the hardness deviation of the most superficial part of the steel plate (product) raised in Resistance when using the hot stamping method is greater. It was concluded that this deviation in hardness could be related to the characteristic of fatigue.

The relationship between deviation and hardness and the characteristic of fatigue is not necessarily clear, but in a high strength member which is produced by the hot stamping method (for example, a tensile strength of 1500 MPa or more), the effect of the rupture sensitivity on the fatigue characteristic is extremely large, So it is conjectured that this deviation in hardness can be an indicator comparable with the flatness of a surface layer.

Therefore, it has been studied in the art how to reduce as much as possible the deviation in the hardness after hot stamping and as a result it was discovered that the deviation in the hardness of the surface layer of the steel plate before stamping in hot it has an impact. No literature has been found which studies the steel plate for use in hot stamping from such a perspective.

PLT 1 discusses steel plate for the use of hot stamping where all Ni, Cu and Sn are essential, where the impact characteristics and the characteristic of delayed fracture are improved, but do not allude to the characteristic of fatigue or deviation in the hardness of the surface layer before hot stamping.

PLT 2 is related to the technique of heating in a carburizing atmosphere to raise the strength of a shaped part, but does not allude to the characteristic of fatigue or the deviation in the hardness of the surface layer before hot stamping Heating in a carburation atmosphere is essential. Compared to heating in air, production costs rise. In addition, when carbon monoxide is used as the carbon source, there is a concern that enormous costs may be required to ensure the safety of operations. It is believed that this technique is not easily feasible.

PLT 3 does not allude to the characteristic of fatigue and the deviation in the hardness of the surface layer before hot stamping.

In contrast to this, as a technique to obtain steel plate for use in hot stamping which has a fatigue characteristic of the same degree as "ordinary high strength steel plate", there is PLT 4. In addition, although as an inherent technique to the case of use of steel plate that has been galvanized PLT 5 is known as a technique to improve the fatigue characteristic of a member which is produced by the hot stamping method.

PLT 4 describes how to make fine particles containing Ce oxide dispersed slightly into the surface of the steel plate to improve the fatigue characteristic after hot stamping, but the advanced steel fabrication technique is required, So there is a problem that even a person with experience in the art can not necessarily find easy to work with this.

The PLT 5 technique is related to facilities for hot stamping technology. There is a problem that if a new capital investment, even a person with experience in the art could not enjoy those benefits. In this way, the steel plate for use in hot stamping to obtain the steel plate (product) manufactured in high strength by hot stamping, which allows fatigue characteristics of the same degree as "high steel plate". Ordinary resistance "of the same resistance is guaranteed relatively and easily, it has been sought, but no technique that solves this problem has been found.

List of Appointments Patent Literature PLT 1: Japanese Patent Publication No. 2005- 139485A PLT 2: Japanese Patent Publication No. 2005-200670A PLT 3: Japanese Patent Publication No. 2005- 205477A PLT 4: Japanese Patent Publication No. 2007- 247001A PLT 5: Japanese Patent Publication No. 2007- 182608A Compendium of the Invention Technical problem The present invention, in view of the above situation, has as its objective the provision of steel plate for a hot stamping member that allows the production of a product of high strength steel plate which has an excellent fatigue characteristic of the same grade as the high strength steel plate produced when controlling the composition of the steel plate or production method ("ordinary high strength steel plate") when a product is produced by applying the stamping method in hot to the steel plate and a method of producing it.

Solution to the problem The invention was involved in extensive research to solve this problem. As a result, he discovered that making the hardness deviation near the surface layer of the steel plate before hot stamping within a predetermined range is extremely effective in improving the fatigue characteristic of the steel plate after hot stamping (product) He discovered that such a steel plate can be obtained by controlling the conditions when the cold rolled steel plate is annealed by crystallization, carried out repeated tests, and therefore completed the present invention.

The essence of the invention is as follows: (1) Steel plate for a hot stamped member that includes the composition it contains,% by mass, C: 0.15 to 0.35% Yes: 0.01 to 1.0%, Mn: 0.3 to 2.3% Al: 0.01 to 0.5%, and the rest of FE and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, where a standard deviation of Vicker hardness at a position of 20 μp? from the surface of the steel plate in the thickness direction of the plate is 20 or less. (2) The steel plate for a hot stamped member as set forth in (1) which additionally contains, in% by mass, one or more of the selected elements of Cr: 0.01 to 2.0%, Ti 0.001 to 0.5% Nb: 0.001 to 0.5% B: 0.0005 to 0.01% ??: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0% (3) The steel plate for a hot stamped member as set out in (1) or (2) which has on the surface of the steel plate one of an Al coating layer of 5 μp? at 50 μp? of thickness, a galvanized layer of 5 μp? at 30 μ ?? of thickness or a layer of Zn-Fe alloy of 5μp? at 45 μp? of thickness. (4) A method of producing steel plate for a hot stamped member comprising recrystallizing annealing cold rolled steel plate including the composition containing% by mass, C: 0.15 to 0.35%, Yes: 0.01 to 1.0%, Mn: 0.3 to 2.3%, At 0.01 to 0.5%, and the rest of Faith and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, in which stage, it includes a first phase to heat by an average heating rate of 8 to 25 ° C / sec from the ambient temperature to a temperature M (° C) and after a second phase to heat through an average heating rate of 1 to 7 ° C / sec at a temperature S (° C), where the temperature M (° C) is 600 to 700 (° C) and the temperature S (° C) is 720 to 820 (° C). (5) The method of producing steel plate for a hot stamped member as set forth in (4) wherein the steel additionally contains in mass%, one or more of Cr: 0.01 to 2.0% Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to 0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0% (6) The method of producing steel plate for a hot stamped member as set forth in (4) or (5) wherein a ratio of hot rolling in the hot rolling step is 60 to 90%, while a cold rolling ratio of the cold rolling stage is 30 to 90%. (7) The method of producing steel plate for a hot stamped member as set forth in any of (4) to (6) which further includes, after the step of recrystallization annealing, a step of immersing the plate steel in an Al bath to form an Al coating layer on the surface. (8) The method of producing steel plate for a hot stamped member as set forth in any of (4) to (6) which further includes, after the recrystallization annealing step, a step for immersing the plate steel in a galvanization bath to form a galvanized layer on the surface. (9) The method of producing steel plate for a hot stamped member as set forth in any of (4) to (6) which further includes, after the annealing step by recrystallization, a step of immersing the plate of steel in a Zn bath to form a galvanized layer on the surface, then further heating to 600 ° C or less to form a layer of Zn-Fe alloy on the surface.

Advantageous Effects of the Invention The steel plate for a member stamped on The hot water of the present invention can be produced by a known steel manufacturing facility. In addition, a shaped part which is obtained using the steel plate for a hot stamped member of the present invention for forming by widely diffused hot stamping installations (hot stamped members) has a fatigue characteristic equal to "plate ordinary high strength steel "of the same strength, so as to have the effect of expanding the application range of the hot stamped members (parts).

Brief Description of the Invention FIGURE 1 is a perspective view which shows a plate stamp for hot stamping which is used for examples of the present invention.

FIGURE 2 is a view which shows the fatigue test pieces.

FIGURE 3 is a perspective view showing the hardness measuring locations in a test piece for hardness measurement use of the same dimensions as the crack growth region of the fatigue test piece which is shown in FIG. FIGURE 2 FIGURE 4 is a graph showing the correlation between the fatigue limit ratio and the standard deviation of the hardness before hot stamping of the steel plate for a hot stamped member of Example 1.

FIGURE 5 is a perspective view which schematically shows the steel plate (member) that is formed into a hat shape by the stamping method.

FIGURE 6 is a graph showing the correlation between the fatigue limit ratio and the standard deviation of the hardness before hot stamping of the steel plate for a hot stamped member of Example 2.

Description of Modalities The invention was responsible for the investigation using the steel plate containing, in% by mass, C: 0.23%, Si: 0.5%, and Mn: 1.6% to prepare a hot stamped member and evaluated its characteristics. He discovered that the characteristic of fatigue is one of the same but that there are hot stamped members that have the same composition of the steel plate and almost the same in tensile strength, but differ in the characteristic of fatigue. Therefore, he investigated the differences of these in detail, so he learned that there are differences in the hardness deviation near the surface layers of the hot stamped members. Therefore, it also changed the composition and recrystallization conditions of the cold-rolled steel plate over a wide range to investigate the fatigue characteristic of the hot-stamped members and found that there is a strong correlation between the fatigue characteristic of the hot-stamped members. hot stamping members and the deviation in the surface hardness thereof and that obtaining a hot stamped member which is excellent in fatigue characteristic, is effective to make the variations in the surface hardness of the steel plate before the hot stamping within a predetermined range and that in addition to obtaining the steel plate, it is possible to control the conditions when the cold-rolled steel plate anneals by recrystallization to a predetermined range.

Details will be explained in the examples, but the invention used these test findings as the basis for experimentally clarifying the appropriate range of deflection in hardness and annealing conditions and therefore completed the present invention.

Composition of the Steel Plate First, the composition of the steel plate will be explained. Here, the "%" in the composition means% in mass.

C: 0.15 to 0.35% C is the most important element to increase the strength of the steel plate by hot stamping. To obtain 1200 MPa or such resistance after hot stamping, 0.15% or more has to be included. On the other hand, if more than 0.35% is included, the deterioration of tenacity is a concern, so that 0.35% becomes the upper limit.

Yes: 0.01 to 1.0% If it is a solution reinforcing element. Until 1. 0% can be used effectively. However, if more than that is included, problems are likely to occur at the time of chemical treatment or coating after shaping, so that 1.0% becomes the upper limit. The lower limit is not particularly limited. The effect of the present invention can be obtained. However, the reduction of more than necessary only raises the steel fabrication load, so that the content becomes the level of inclusion due to deoxidation, that is, 0.01% or more.

Mn: 0.3 to 2.3% Mn is an element that works as a solution reinforcement element in the same way as Si and is also effective in raising the hardness capacity of the steel plate. This effect is recognized at 0.3% or more. Nevertheless, even if more than 2.3% is included, the effect becomes saturated, so that 2.0 becomes the upper limit.

P: 0.03% or less, S: 0.02% or less The two elements are both unavoidable impurities. They affect the hot forming capacity, so they have to be limited to the previous margins.

Al: 0.01 to 0.5% Al is suitable as a deoxidation element, so 0.01% or more must be included. However, if it is included in a large quantity, coarse oxides are formed and the anical properties of the steel plate are damaged, so that the upper limit becomes 0.5%.

N: 0.1% or less N is an inevitable impurity. It is easily linked to Ti or B, so it has to be controlled so as not to reduce the objective effect of these elements. 0.1% or less is admissible. The preferred content is between 0.1% or less. On the other hand, the reduction of more than necessary places a massive burden on the production process, so that 0.0010% must become the objective for the lower limit.

Cr: 0.01 to 2.0% Cr has the effect of raising the hardness capacity, so that it can be used properly. This effect becomes clear at 0.01% or more. On the other hand, even if more than 2.0% is added, this effect becomes saturated, so that 2.0% becomes the upper limit.

Ti: 0.001 to 0.5% Ti is an element that acts to stably remove the effect of B, explained later, through the formation of its nitride, so that it can be used effectively. For this reason, 0.001% or more has to be added, but if it is added excessively, the nitrides become excessive and the deterioration in the toughness or properties of the shear surface is invited, so that 0.5% becomes the upper limit.

Nb: 0.001 to 0.5% Nb is an element that forms carbonitrides and increases the resistance, so that it can be used effectively. This effect is recognized at 0.001% or more, but if more than 0.5% is included, the hot rolling control capacity is prone to damage, so that 0.5% becomes the upper limit.

B: 0.0005 to 0.01% B is an element that increases the hardness capacity.

The effect becomes clear at 0.0005% or more. On the other hand, an excessive addition leads to deterioration of the hot forming capacity and a fall in ductility, so that 0.01% becomes the upper limit.

Mo: 0.01 to 1.0%, W: 0.01 to 0.5%, V: 0.01 to 0.5% These elements will have the effect of raising the hardness capacity, so that they can be used appropriately. The effect becomes clear in each case at 0.01% or more. On the other hand, it is an expansive element, so that the concentration where the effect saturates becomes preferably the upper limit. For Mo, this is 1.0%, while for W and V, it is 0.5%.

Cu: 0.01 to 1.0% Cu has the effect of raising the strength of the steel plate by the addition of Cu at 0.01% or more. However, the excessive addition detracts from the surface quality of the hot-rolled steel plate, so that 1.0% becomes the upper limit.

Ni: 0.01 to 5.0% Nor is it an element that has the effect of raising the hardness capacity, so that it can be used effectively. The effect becomes clear at 0.01% or more. On the other hand, it is an expansive element, so that 5.0% where the effect becomes saturated becomes the upper limit. In addition, it also acts to suppress the drop in surface quality of the hot-rolled steel plate due to Cu, so that inclusion simultaneously with Cu is desirable.

Note that in the present invention, the composition other than the previous one consists of Fe, but Unavoidable impurities entering from scrap and other fusion materials or refractories, etc., are allowed.

Deviations in the Surface Hardness of the Steel Plate The deviations in the surface hardness of the steel plate will be explained.

First, the method to determine (measure) the hardness of the surface of the steel plate will be explained.

The hardness of the surface of the steel plate should ideally be measured by a hardness meter (for example, the Vicker hardness meter) with the surface of the steel plate facing upwards and with the direction of plate thickness correlated with the vertical direction but to clearly determine the indentations (measure the dimensions of the indentations accurately), the surface (measuring surface) has to be polished or another certain work is necessary. In such work (for example, mechanical polishing), at least several dozens of μp? or more are removed from the original surface. In addition, even if part of the surface is removed using acid, etc., to polish it chemically, there is no difference. In fact, smoothness is often degraded. Therefore, using such a technique to determine (measure) the hardness of the surface of the steel plate is not practical.

Therefore, the invention decided to determine the hardness in a cross section parallel to the direction of plate thickness of the steel plate. By doing this, the surface of the steel plate can be measured without working (without removing the surface of the steel plate). However, in this case too, the position is capable of being measured by a hardness meter so that a small amount in the direction of plate thickness is found within the surface. For this reason, as the next best solution, the invention attempted to obtain information in a portion close to the surface by performing an indentation for as low a load as possible.

Specifically, go to FIGURE 3. First, the measuring surface (cross-section of steel plate) was polished with a mirror finish. A Vicker hardness tester with a test load (load thrust in the indenter) of 10 gf, a push time of 15 seconds, and a measurement position in the direction of plate thickness of 20 μp was used? from the surface of the steel plate. The "hardness of the steel plate" as used in the Description indicates the hardness determined based on the prior art.

In addition, the hardness of the surface of the steel plate in the steel plate having as surface layer of the steel plate an Al coating layer, the galvanized layer, and the Zn-Fe alloy layer was measured in a position of 20 μp? from the limit (interconnection) between the coating layer and the steel plate.

For example, the Al facing plate of the steel plate which is used in the examples is considered comprised of an outer layer which has Al as its main composition and an inner layer (side of the steel plate) which is Cree is a reaction layer of Al and Fe, so that the hardness was measured at a position of 20 μt? from the limit of the inner plate and the steel plate in the direction of plate thickness and this was used as the surface hardness of the steel plate.

Then, the galvanized layer of the steel plate used in the examples is considered comprised of two layers of an outer layer which have Zn as their main composition and an inner layer which is an Al reaction layer that was added in a fine amount in the Zn and Fe bath, so that the hardness was measured at a position of 20 μ? from the boundary of the inner layer and the steel plate in the thickness direction of the plate and this was used as the surface hardness of the steel plate.

In addition, the Zn-Fe alloy layer of the steel plate used in the examples is considered to be comprised of a plurality of alloy layers comprised of Zn and Fe, so that the hardness was measured at a position of 20 μp? from the limit of the inner layer and the steel plate in The direction of plate thickness and this was used as the surface hardness of the steel plate.

For the purpose of finding the deviation in hardness, the previous measurement was made in the region corresponding to the fatigue crack growth region (21) of the fatigue test piece which is shown in FIGURE 2. FIGURE 3 is a perspective view showing the location of the hardness measurement. The indenter of the Vicker hardness tester was pushed into a position of 20 μp? from the surface of the steel plate or the interconnection of the steel plate and the coating layer in the direction of plate thickness. This operation, as shown in FIGURE 3, was performed at 0.1 mm indentation intervals in a direction parallel to the surface of the steel plate at 300 points per measurement sample (more than 30 mm per measurement length) ( first measuring surface). In addition, the same operation was performed in another location of 5 mm from the first measurement surface taken in advance (second measurement surface).

The hardness was found for the total of 600 points in this way. The standard deviation used by this as the population was calculated and used as an indicator of the deviation.

Note that the previous measurement length of 30 mm and the two 5 mm spacing locations were determined to coincide with the crack growth region of the fatigue test piece which is explained below.

In the experiment that is explained in the examples, samples with a fatigue limit ratio after hot stamping of 0.4 or more and those with a ratio below that which was compared for the deviation in hardness of the surface of steel plate, with which in the first, the standard deviation was 40 or less. Therefore, the invention proceeded with more detailed investigations, whereby it became clear that the deviation in hardness after hot stamping has a standard deviation of 40 or less when the hardness deviation of the steel plate before stamping In hot, determined by a similar technique, it has a standard deviation of 20 or less.

In the present invention, the standard deviation of the Vicker hardness at a position of 20 μp? from the steel plate surface in the direction of plate thickness was defined as 20 or less based on such experimental findings.

Production Method of Steel Plate for Hot Stamped Member Finally, the production method of the steel plate for a hot stamped member of the present invention will be explained.

The steel plate for a hot stamped member of the present invention is processed according to the usual methods by the steps of forming steel, casting, hot rolling, pickling, and cold rolling to obtain rolled steel plate cold The composition conforms to the mentioned scope of the present invention in the steelmaking stage, the steel is emptied into a slab in the continuous casting stage, then the slab begins to hot laminate for example at 1300 ° C or less. heating temperature. The lamination is finished around 900 ° C. The winding temperature can be selected for example as 600 ° C, etc. The hot rolling index can become from 60 to 90%. The cold rolling is carried out after the pickling step. The lamination index can be selected from 30 to 90% in the margin.

The annealing step for recrystallization of the cold rolled steel plate that was produced in this way is extremely important. The annealing step is carried out using a continuous annealing installation and is comprised of two phases of a first heating stage for an average heating rate of 8 to 25 ° C / seconds from room temperature to the temperature M (° C) and then a second heating phase for an average heating rate of 1 to 7 ° C / seconds at a temperature S (° C). Here, the temperature M has to be 600 to 700 (° C), and the temperature S has to be 720 to 820 (° C). These conditions are determined based on the results of the experiment explained in the examples described below.

The reason why, when annealing by recrystallization under these conditions, is the standard deviation of the Vicker hardness that was measured at a position of 20 μp? from the surface of steel plate in the direction of plate thickness is 20 or less, that is, the steel plate with a small deviation in hardness is obtained, it is not necessarily clear, but the crystal grain size distribution preferably it is as uniform as possible and the dimensions and distribution of the carbides are also preferably similar to as uniform as possible, so that the following can be conjectured from the point of view of recrystallized particle size distribution and the dimensions and distribution of carbides.

The recrystallization process of the cold rolled steel plate is complicated, so it is not suitable separate and independently discuss the meanings of the heating rate for the phenomenon called recrystallization and the higher heating temperature of that heating rate. Therefore, first, with respect to the first phase, for example, consider the case where the heating rate is small and where it is large with respect to a certain unique temperature M (° C). It is believed that in the first case, that is, when the heating rate is small, the density of recrystallization cores is low (relatively) and the individual recrystallized grains grow freely, but in the region of high temperature close to M ( ° C), recrystallized fine grains are produced from the region without remaining recrystallization and, in the phase where the temperature of the steel plate reaches M (° C), (relatively) large crystal grains and small crystal grains are they mix On the other hand, it is believed that in the case of the above, that is, when the heating index is large, the density of the recrystallized grains nuclei is high, a large number of recrystallized grains grows at a rapid rate, and the Grain boundaries get closer and closer, in the region of high temperature close to M (° C), the recrystallized grains compete in growth and as a result the crystallized grains that have specific crystal orientations grow while they are eaten away in crystallized grains that have other crystal orientations, so that the phase when they reach M (° C) it is believed that there are large crystallized grains and small crystallized grains mixed together. Therefore, a combination of the appropriate heating index and M (° C) so that the recrystallized grains approach the grain boundaries in the phase where the temperature reaches M (° C) becomes necessary to achieve a more even distribution of recrystallized particle sizes. At 8 to 25 ° C / second of the average heating index of the first phase and of 600 to 700 ° C of the temperature M (° C) it is believed that they correspond to these suitable conditions.

Next, to control the growth competition of the recrystallized grains after the temperature of the steel plate reaches (° C), the heating rate of the second phase has to become lower to the first phase. In addition, in the region of temperature temperature M (° C) to temperature S (° C), the reformation of carbides due to carbon diffusion is activated, so that the combination of the highest temperature setting S (° C) of the annealing step and the heating rate up to that temperature has an important significance.

When the heating rate is less than one S (° C), the carbides that occurred at the temperature M (° C) grows uniformly, so that a steel plate may result in the carbides of various dimensions that were present in the phase when they reach the temperature M (° C) are present in several shapes. On the other hand, when the heating rate is large, small carbides disappear and large carbides grow and therefore the dimensions of the carbides are closer to the uniform ones, speaking relatively, but the density becomes smaller. Therefore, the non-uniformity of the hardness of the steel plate occurs due to the carbides. In contrast to these, when the combination of the heating ratio and the temperature S (° C) of the second phase is adequate, the small carbides grow preferentially and it may be that a steel plate results in the carbides of relatively uniform dimension are dispersed to a suitable density, so that the non-uniformity of the hardness of the steel plate due to the carbides becomes uneven. From 1 to 7 ° C / seconds of the heating rate of the second phase and from 720 to 820 ° C of the temperature S (° C) corresponds to such suitable conditions.

After reaching the temperature S, the temperature S can be maintained for a short period or the next cooling step can be changed immediately. When the temperature S is maintained, starting from the point of view of thickening of the crystal grains, the retention time of preference is 180 seconds or less, more preferably 120 seconds or less.

The cooling rate of the temperature S in the cooling stage is not particularly limited, but 30 ° C / seconds or more rapid cooling is preferably avoided. Therefore, the cooling ratio of the temperature S is less than 30 ° C / sec, preferably 20 ° C or less, more preferably 10 ° C or less. The steel plate for hot stamping use is often cut to a predetermined shape and then used for hot stamping. This is because it is feared that rapid cooling will increase the shear load and reduce the production efficiency.

After annealing, the plate can be cooled to room temperature. During cooling, it can be immersed in a hot dip Al bath to form an Al coating layer.

The hot dip Al bath can contain 0.1 to 20% Si.

If it is contained in the Al coating plate, it affects the reaction of Al and Fe that occurs during heating before hot stamping. The excessive reaction is likely to detract from the patterning of the coating layer itself. On the other hand, Excessive control of the reaction is likely to invite Al adhesion in the stamping die. To avoid such a problem, the Si content in the Al coating layer is preferably 1 to 15%, more preferably 3 to 12%.

In addition, during cooling after annealing, the plate was immersed in a hot dip galvanization bath to form a galvanized layer.

In addition, the plate was immersed in a hot dip galvanization bath to form a galvanized layer, then heated to 600 ° C or less to form a Zn-Fe alloy layer.

The hot dip galvanization bath could contain 0.01 to 3% Al.

The existence of Al has a strong effect on the reaction of Zn and Fe. When a galvanized layer is formed, the reaction layer of Fe and Al becomes an obstacle and suppresses the mutual dispersion of Zn and Fe. On the other hand, a Zn-Fe alloy layer is comprised of a Zn-rich alloy layer (? -phase, d-phase) and the Fe-rich alloy layer (rifase, G-phase), but the former is rich in adhesion with base iron but the conformation capacity is degraded, while the latter is excellent in conformation capacity, but it is insufficient in adhesion. Therefore, it is necessary to adequately control the composition ratio of these four phases to meet the objective properties (giving preference to adhesion, giving preference to the capacity of conformation, or balancing the two). This can be done by including 0.01 to 3% Al in the hot dip galvanization bath to allow the control of Fe diffusion. What kind of concentration to use can be selected by the manufacturer according to the capacity or purpose of the installation of production.

The thicknesses of the Al coating layer, the galvanized layer, and the Zn-Fe alloy layer do not influence the fatigue characteristic of the steel plate after hot stamping or the fatigue characteristic of the parts, but if It is excessively thick, the ability to form by pressure is likely to be affected. As shown in the examples, when the thickness of the Al coating plate is greater than 50 μ? T ?, the scum phenomenon is recognized. When the thickness of the Zn coating plate exceeds 30 μ, the adhesion of Zn to the matrix occurs frequently. When the thickness of the Zn-Fe alloy layer is greater than 45 μP ?, dispersed cracking of the alloy layer is seen, and the productivity is damaged in a certain way. Therefore, the thicknesses of the layers preferably become the coating layer of Al: 50 μ ?? or less, galvanized layer: 30 μ? or less, and Zn-Fe alloy layer: 45 μp? or less.

When these coating layers are thin, there is no problem whatsoever in the forming ability, but from the point of view of the corrosion resistance, which aims to damage these coating layers, the lower limits of the coating layers are preferably formed as follows: that is, the limits are the coating layer of Al: preferably 5 μp? or more, more preferably 10 μp? or more, the galvanized layer: preferably 5 μp or more, more preferably 10 μp? or more and the Zn-Fe alloy layer: preferably 5 μ? a or more, more preferably 10 μ? or more.

Examples Next, examples will be used as a basis for explaining the present invention in detail.

Example 1 Steels "a" to "f" having the composition which is shown in Table 1 were produced and melted. The slabs were heated to 1250 ° C and supplied to a hot rolling stage where they were hot rolled at a final temperature of 900 ° C and a cooling temperature of 600 ° C to obtain 3.2 mm thick steel plates. . These hot rolled steel plates were pickled, then cold rolled to obtain steel plates 1.6 mm thick cold rolled.

The cold-rolled steel plates were recrystallized and annealed under the xviii conditions described in Table 2 to obtain steel plates for hot stamped members 1 to 32 shown in Table 3. From that part, the two test pieces for hardness measurement before hot stamping were obtained. The positions for sampling for the test pieces were made in the 5 mm positions spaced in the width direction of the steel plate obtained for the hot stamping member.

The average heating ratio 1 (first phase) and the average heating ratio 2 (second phase) in Table 2 respectively show the average heating ratios of ambient temperature at temperature M (° C) and the temperature M of heating ratio average (° C) at temperature S (° C).

These steel plates for the stamped members were held at 900 ° C for 10 minutes, then interspersed by the plate stamp of test use shown in FIGURE 1 and hot stamped. Each type of steel plate for a hot stamped member was used in 10 hot stamping parts. From one of these, two pieces of tensile test based on the provisions of JIS No. 5 and two test pieces for Hardness measurement (same procedure as with hot stamping) were obtained. Of the remaining nine, two pieces of fatigue test shown in FIGURE 2 each, for a total of 18, were obtained. The method to work for the obtaining of test pieces was machined by electro-discharge.

A tensile test was performed to find the tensile strength s? (Average value of two pieces of tensile test). On the other hand, 18 test pieces were used to execute a flat flexion fatigue test and determine the fatigue resistance CTw of cycle lxlO7. The test conditions were a voltage ratio of -1 and a repetition ratio of 5Hz.

The test pieces for hardness measurement were polished in a mirror finish in cross sections parallel to the rolling directions of the cold rolled steel plates before and after the hot stamping.

Hardness in 20 μ ?? Within the surfaces of these test pieces, in the direction of plate thickness was measured using a Vicker hardness tester (HM-2000 manufactured by Mitsutoyo). The thrust load was made at 10 gf, the push time was made at 15 seconds and the measurement interval in the direction parallel to the surface made 0.1 mm for 300 point measurement.

The test pieces were measured in the same way. The standard deviation of hardness was calculated from the Vicker hardness data of a total of 600 points.

Table 3 shows the steel number, the processing conditions, the standard deviation of the hardness before hot stamping, the tensile strength s? (average of two), resistance aw, ratio of fatigue limit ow / aB and standard of hardness after hot stamping. The correlation between the ratio of the fatigue limit aw / oB and the standard deviation of hardness before hot stamping is shown in FIGURE 4.

It was learned that the tensile strength s? of the steel plate after the hot stamping is almost not completely affected by the annealing conditions by recrystallization in the steel plate of the same composition (code "b"). On the other hand, the fatigue characteristics (aw / aB) were strongly affected by the annealing conditions by recrystallization.

In the steel plates using the annealing conditions i, iii, iv, vii, viii, xv, and xviii of the present invention, the relatively high fatigue characteristics, i.e., a fatigue limit ratio of 0.4 or more ( s "/ s?) could be obtained in the range of approximately 1200 to 1500 MPa in tensile strength. In in contrast to this, in steel plates that were annealed under conditions outside the scope of the present invention, the fatigue limit ratio obtained was a low level of about 0.3.

This difference is due to the fact that the fatigue limit ratio correlates with the standard deviation of the hardness after hot stamping. Simultaneously, it clearly depends on the standard deviation of the hardness before hot stamping. As shown in Nos. 1 to 6, 8, 9, 12, 13, 16, 17, 20, 21 and 23 to 28, it becomes clear that when the standard deviation of the hardness is 2 or less, a stamped member in hot which has an excellent characteristic of fatigue (high ratio of fatigue limit) is obtained.

In addition, since the conditions of annealing by recrystallization to obtain the steel plate with a standard deviation of hardness before hot stamping of 20 or less, there is a first heating phase for an average heating ratio of 15 to 25 ° C. / seconds of the room temperature at a temperature M (° C) and a second heating phase then by an average heating rate of 2 to 5 ° C / seconds at the temperature S (° C). It becomes clear that M is 620 to 680 (° C) and S is 780 to 820 (° C).

Table 1 The units are in% by mass.

Table 2 Table 3 The underlined figures indicate the external scope of the present invention.

Example 2 The steels 2a to 2h having the composition shown in Table 4 were produced and melted. The slabs were hot rolled under the same conditions as Example 1 to obtain 3.0 mm thick steel plates. These hot rolled steel plates were pickled, then cold rolled to 1.2 mm.

These steel plates were recrystallized and annealed under conditions i, ix, and xviii of Table 2 to obtain steel plates for hot stamped members.

From these steel plates, the test pieces for hardness measurement were obtained by the same procedure as in Example 1.

These steel plates for a hot stamped member were maintained at 900 ° C for 5 minutes, then formed in those configurations shown in FIGURE 5 by the hot stamping method. As shown in this figure, the fatigue test pieces shown in FIGURE 2 and the JIS No. 5 tensile test pieces were obtained from the tops of the hats.

These test pieces were used by the same procedure as in Example 1 to find the standard deviation of the hardness before hot stamping and the tensile strength s? (average of two) and a resistance to fatigue aw of cycle lxlO7 of the steel plate after hot stamping (member).

Table 5 shows these results. The correlation between the fatigue limit ratio aw / aB and the standard deviation of the hardness before hot stamping is shown in FIGURE 6.

In steel plates for a hot stamped member that were recrystallized and annealed using conditions i and xviii within the scope of the present invention, even if the steel plates containing Mo, W, V, Cu and Ni, the deviation in hardness of the surface layer before hot stamping had a standard deviation of 20 or less. Furthermore, if these are used, it became evident that a hot stamped member with a fatigue limit ratio of 0.4 or more, that is, excellent in fatigue characteristic, was obtained.

On the other hand, in steel plates which were recrystallized and annealed using condition ix which is outside the scope of the present invention, the hardness deviation of the surface layer before hot stamping had a standard deviation of more than 20. The fatigue limit ratio of the hot stamped members obtained when using these was from 0.26 to 0.31. It became evident that the fatigue characteristic was inferior.

Table 4 Table 5 The underlined figures indicate outside the scope of the present invention.

Example 3 The steels 3a to 3d having the composition shown in Table 6 were produced and melted. The slabs were hot rolled under the same conditions as Example 1 to obtain 2.5 mm thick steel plates. These hot rolled steel plates were pickled, then cold rolled to 1.2 mm.

These steel plates were heated by an average heating rate of 19 ° C / seconds to 655 ° C, then heated by an average heating rate of 2.5 ° C to 800 ° C, then immediately cooled by a cooling rate average of 6.5 ° C / seconds. In addition, they were immersed in a hot-dip Al bath at 670 ° C (containing 100% Si and unavoidable impurities), removed after 5 seconds, adjusted in amount of deposition by a gas scrubber, then cooled with air at room temperature.

From the steel plates obtained, the same procedure as in Example 1 was used to obtain test pieces for hardness measurement. To measure the hardness, the hardness in a position of 20 μp? from the limit of the inner layer of the Al coating layer (reaction layer of Al and Fe) and the steel plate was measured by the same procedure as in Example 1. At the time of this measurement, the thickness of the Al coating layer (total two layers) was also measured. The thickness measurement range was made the same length of 30 mm as the hardness measurement range. Seven points were measured at 5 mm measurement intervals on each of the first measurement surface and the second measurement surface for a total of 14 measurement positions. The average value was found.

These steel plates were hot stamped in hat shapes by the same procedure as in Example 2. The heating conditions were maintained at 900 ° C for 1 minute.

From the tops of the hats, the fatigue test pieces shown in FIGURE 2 and JIS No. 5, the tensile test pieces were obtained.

These test pieces were used to find the tensile strength s? (average of two) and fatigue resistance aw of cycle lxlO7. Table 7 shows the results.

In all the examples, an excellent steel plate was obtained for a hot stamped member with a fatigue limit ratio of 0.4 or more, but in Nos. 57, 62, and 72 where the thickness of the coating layer of When it exceeded 50 μp ?, a friction scouring phenomenon occurred at a high frequency in the long parts of the wall of the hat shape. In the examples of 50 μp? or less, no scorching phenomenon occurred at all by friction. Therefore, it was judged that the upper limit of the thickness when it is the coating of Al, the surface of the steel plate has 50 μp? or less.

Table 6 The units are in% by mass Table 7 The underlined figures indicate outside the scope of the present invention.

Example 4 The plates 3a to 3d having the composition that is shown in Table 6 were produced and melted. The slabs were hot rolled under the same conditions as Example 1 to obtain 2.5 mm thick steel plates. These hot rolled steel plates were pickled, then cold rolled to 1.2 mm.

These steel plates were heated by an average heating rate of 19 ° C / seconds to 655 ° C, then heated by an average heating rate of 2.5 ° C to 800 ° C, then immediately cooled by a cooling rate average of 6.5 ° C / seconds. In addition, they were immersed in a hot dip galvanization bath of 460 ° C (containing 0.15% Al and unavoidable impurities), removed after 3 seconds, adjusted in amount of deposition by a gas scrubber, then cooled with air at room temperature.

From the steel plates obtained, the same procedure as in Example 1 was used to obtain the test pieces for hardness measurement. To measure the hardness, the hardness in a position of 20 μp? from the boundary of the inner layer of the Zn coating layer (reaction layer of Al and Fe) and the steel plate was measured by the same procedure as in Example 1. At the time of this measurement, the thickness of only the coating layer of Zn could also be measured. The margin of thickness measurement had the same length 30 mm as the hardness measuring range. Seven points were measured at 5 mm measurement intervals on each of the first measurement surface and the second measurement surface for a total of 14 measurement positions. The average value was found.

These steel plates were hot-stamped in the shape of a hat by the same procedure as in Example 2. They were heated to 880 ° C and maintained for 5 seconds, then cooled with air at 700 ° C and stamped.

From the tops of the hats, the fatigue test pieces shown in FIGURE 2 and JIS No. 5, tensile test pieces were obtained.

These test pieces were used to find the tensile strength s? (average of two) and fatigue resistance aw of cycle lxlO7. Table 8 shows the results.

In all the examples, an excellent steel plate was obtained for a hot stamped member with a fatigue limit ratio of 0.4 or more, but in Nos. 77, 82, 87 and 92, where the thickness of the galvanized layer exceeded 30 μp? Zn adhesion was observed at a high frequency in the stamp. In the examples of 30 μp? or less, Zn adhesion did not occur at all. Therefore, it was judged that the upper limit of the thickness when galvanizing is carried out of the supeicie steel plate is 30 μt? or less.

Table 8 The underlined figures indicate outside the scope of the present invention.

Example 5 Plates 3a to 3d having the composition shown in Table 6 were produced and melted. The slabs were hot rolled under the same conditions as the Example 1 to obtain 2.5 mm thick steel plates. These hot rolled steel plates were pickled, then cold rolled to 1.2 mm.

These steel plates were heated by a average heating rate of 19 ° C / sec. to 655 ° C, then heated by an average heating rate of 2.5 ° C to 800 ° C, then cooled immediately by an average cooling ratio of 6.5 ° C / sec. In addition, they were immersed in a hot dip galvanization bath at 460 ° C (containing 0.13% Al, 0.03 Fe, and unavoidable impurities), removed after 3 seconds, adjusted in amount of deposition by a scrubber of gas, then heated to 480 ° C to form a layer of Zn-Fe alloy, then cooled with air at room temperature.

From the steel plates obtained, the same procedure as in Example 1 was used to obtain the test pieces for hardness measurement. To measure the hardness, the hardness in a position of 20 μ ?? from the boundary of the inner layer of the Zn-Fe alloy layer (reaction layer of Zn and Fe) and the steel plate was measured by the same procedure as in Example 1. At the time of this measurement, the thickness Total Zn-Fe alloy layer (which was comprised of four layers) was also measured. At the time of this measurement, the thickness of the Al coating layer (total of two layers) was also measured. The thickness measurement range was made the same length of 30 mm as the hardness measurement range. Seven points were measured at 5 mm measurement intervals in each of the first measuring surface and the second measuring surface for a total of 14 measurement positions. The average value was found.

These test pieces were used to find the tensile strength s? (average of two) and fatigue resistance aw of cycle lxlO7. Table 9 shows the results.

In all the examples, an excellent steel plate was obtained for a hot stamped member with a fatigue limit ratio of 0.4 or more, but in Nos. 97, 102, 107 and 112, where the thickness of the Zn-Fe alloy exceeded 45 μp ?, Fine cracks occurred in the alloy layer after stamping. In the examples of 45 μ ?? or less, fine cracks were not formed at all. Therefore, it was judged that the upper limit of the thickness when the Zn-Fe alloy layer was formed on the surface of the steel plate is 45 μp? or less.

Table 9 The underlined figures indicate outside the scope of the present invention.

List of Reference Signs lia superior picture 11b lower stamp 12 steel plate 21 fatigue crack growth region 51 test piece sampling position

Claims

1. The steel plate for a hot stamped member that includes the composition it contains, in% by mass, C: 0.15 to 0.35%, Yes: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and the rest of Faith and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, where a standard deviation of the Vicker hardness at a position of 20 μp? from the surface of the steel plate in the direction of the thickness of the plate is 20 or less.

2. The steel plate for a hot stamped member as set forth in claim 1, further containing, in% by mass, one or more of the selected elements of Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to 0.01%, ??: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0%

3. The steel plate for a hot stamped member as set forth in claim 1 having on the surface of the steel plate one of an Al coating layer of 5 μp? at 50 μp? of thickness, a galvanized layer of 5 μp? at 30 μp? of thickness, or a layer of Zn-Fe alloy of 5 μ ?? at 45 μ ?? of thickness.

4. A method of producing steel plate for a hot stamped member comprising recoating by recrystallization the cold-rolled steel plate including the composition containing% by mass, C: 0.15 to 0.35%, Yes: 0.01 to 1.0%, Mn: 0.3 to 2.3%, Al: 0.01 to 0.5%, and the rest of Faith and unavoidable impurities, and limit the impurities to P: 0.03% or less, S: 0.02% or less, and N: 0.1% or less, in which stage, it includes a first phase to heat by an average heating rate of 8 to 25 ° C / sec from the ambient temperature to a temperature M (° C) and then a second phase to heat by an average heating rate of 1 to 7 ° C / sec at a temperature S (° C), where the temperature M (° C) is 600 to 700 (° C) and the temperature S (° C) is 720 to 820 (° C).

5. The method of producing steel plate for a hot stamped member as set forth in claim 4, wherein the steel further contains in% by mass, one or more of Cr: 0.01 to 2.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5% B: 0.0005 to 0.01%, Mo: 0.01 to 1.0% W: 0.01 to 0.5%, V: 0.01 to 0.5%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 5.0%

6. The method of producing steel plate for a hot stamping member as set forth in claim 5, wherein a hot rolling ratio in the hot rolling step is 60 to 90%, while a cold rolling ratio of the cold rolling stage is 30 to 90%.

7. The method of producing steel plate for a hot stamped member as set forth in claim 4, further including, after the step of recrystallization annealing, a step of immersing the steel plate in an Al bath to form an Al coating layer on the surface.

8. The method of producing steel plate for a hot stamped member as set forth in claim 4, further including, after the step of recrystallization annealing, a step of immersing the steel plate in a Zn bath to form a galvanized layer on the surface.

9. The method of producing steel plate for a hot stamped member as set forth in claim 4, further including, after the step of recrystallization annealing, a step of submerging the steel plate in a Zn bath to form a galvanized layer on the surface, then further heating to 600 ° C or less to form a layer of Zn-Fe alloy on the surface.